Ammonia Pages From Ullmann_s Enc. of Industrial Chemistry1

Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScienceMy Profile HOME ABOUT US CONTACT US HELP Home / Chemistry / Industrial Chemistry Lo Ullmann's Encyclopedia of Industrial Chemistry Recommend to Your Librarian Save title to My Profile Email this page Print this page BROWSE THIS TITLE Article Titles A–Z Topics Ammonia Standard Article Max Appl1 1Dannstadt-Schauernheim, Federal Republic of Germany SEARCH THIS TITLE Advanced Product Search Search All Content Acronym Finder Copyright © 2006 by Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved. DOI: 10.1002/14356007.a02_143.pub2 Article Online Posting Date: December 15, 2006 Abstract | Full Text: HTML Abstract The article contains sections titled: 1. 2. 3. 4. 4.1. 4.2. 4.3. 4.3.1. 4.3.2. 4.3.3. 4.4. 4.4.1. 4.4.1.1. 4.4.1.2. 4.4.1.3. 4.4.1.4. 4.4.1.5. 4.4.2. 4.4.2.1. 4.4.2.2. 4.4.2.3. 4.5. 4.5.1. 4.5.1.1. 4.5.1.2. 4.5.1.3. 4.5.2. 4.5.3. 4.5.3.1. 4.5.3.2. 4.5.3.3. 4.5.3.4. 4.5.3.5. 4.5.3.6. 4.5.3.7. 4.6. 4.6.1. 4.6.1.1. 4.6.1.2. 4.6.1.3. 4.6.1.4. 4.6.1.4.1. 4.6.1.4.2. 4.6.1.4.3. 4.6.1.4.4. 4.6.2. 4.6.2.1. 4.6.2.2. 4.6.3. 4.6.4. 4.7. 4.8. 5. 5.1. 5.1.1. 5.1.2. 5.1.3. Occurrence and History Physical Properties Chemical Properties Production Historical Development Thermodynamic Data Ammonia Synthesis Reaction General Aspects Catalyst Surface and Reaction Mechanism Kinetics Catalysts Classical Iron Catalysts Composition Particle Size and Shape Catalyst-Precursor Manufacture Catalyst Reduction Catalyst Poisons Other Catalysts General Aspects Metals with Catalytic Potential Commercial Ruthenium Catalysts Process Steps of Ammonia Production Synthesis Gas Production Feedstock Pretreatment and Raw Gas Production Carbon Monoxide Shift Conversion Gas Purification Compression Ammonia Synthesis Synthesis Loop Configurations Formation of Ammonia in the Converter Waste-Heat Utilization and Cooling Ammonia Recovery from the Ammonia Synthesis Loop Inert-Gas and Purge-Gas Management Influence of Pressure and Other Variables of the Synthesis Loop Example of an Industrial Synthesis Loop Complete Ammonia Production Plants Steam Reforming Ammonia Plants The Basic Concept of Single-Train Plants Further Developments Minimum Energy Requirement for Steam Reforming Processes Commercial Steam Reforming Ammonia Processes Advanced Conventional Processes Processes with Reduced Primary Reformer Firing Processes Without a Fired Primary Reformer (Exchanger Reformer) Processes Without a Secondary Reformer (Nitrogen from Air Separation) Ammonia Plants based on Partial Oxidation Ammonia Plants Based on Heavy Hydrocarbons Ammonia Plants Using Coal as Feedstock Waste-Heat Boilers Single-Train Capacity – Mega Ammonia Plants Modernization of Older Plants (Revamping) Material Considerations for Equipment Fabrication Storage and Shipping Storage Pressure Storage Low-Temperature Storage Underground Storage page 1 of 77 Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience 5.1.4. 5.2. 6. 7. 7.1. 7.2. 7.3. 8. 9. 10. Storage of Aqueous Ammonia Transportation Quality Specifications and Analysis Environmental, Safety, and Health Aspects Environmental Aspects of Ammonia Production and Handling Safety Features Health Aspects and Toxicity of Ammonia Uses Economic Aspects Future Perspectives [Top of Page] 1. Occurrence and History Occurrence. Ammonia, NH3, occurs in nature almost exclusively in the form of ammonium salts. Natural formation of ammonia is primarily by decomposition of nitrogen-containing organic materials or through volcanic activity. Ammonium chloride can deposit at the edges of smoldering, exposed coal beds, as observed in Persia before 900 A. D. Similar deposits can be found at volcanoes, for example, Vesuvius or Etna, in Italy. Ammonia and its oxidation products, which combine to form ammonium nitrate and nitrite, are produced from nitrogen and water vapor through electrical discharges in the atmosphere. These ammonium salts, as well as those arising from industrial and automotive exhausts, supply significant quantities of the nitrogen needed by growing plants when eventually deposited on the earth's surface. Ammonia and its salts are also byproducts of commercial processing (gasification, coking) of fossil fuels such as coal, lignite, and peat (see Fig. 1). Figure 1. The nitrogen cycle History. The name ammonia is derived from “sal ammoniacum” (Oasis Ammon in Egypt, today Siwa). Sal ammoniac was known to the ancient Egyptians; the Arabs were aware of ammonium carbonate. Free ammonia was prepared for the first time in 1774 by J. B. PRIESTLEY. In 1784, C. L. BERTHOLLET recognized that ammonia was composed of the elements nitrogen and hydrogen. W. HENRY, in 1809, determined the volumetric ratio of the elements as 1 : 3, corresponding to the chemical formula NH3. Following the discovery of the nature and value of mineral fertilization by LIEBIG in 1840, nitrogen compounds were used in increasing quantities as an ingredient of mineral fertilizers. At the end of the last century ammonia was recovered in coke oven plants and gas works as a byproduct of the destructive distillation of coal. The produced ammonium sulfate was used as fertilizer. Another nitrogen fertilizer was calcium cyanamide which was manufactured by the Frank – Caro process from 1898 onwards. Since both sources of nitrogen were limited in quantity they did not suffice for fertilization. Therefore, it was necessary to use saltpeter from natural deposits in Chile. In 1913, the first Haber – Bosch plant went on stream, representing the first commercial synthesis of ammonia from the elements. Subsequently, other ammonia plants were started up, and synthetic ammonia and/or its derivatives soon replaced Chile saltpeter. Today ammonia is a commodity product of the chemical industry. World production capacity in 2005 exceeded 168 × 106 t ammonia. [Top of Page] 2. Physical Properties Molecular Properties. Corresponding to its nuclear charge number, the nitrogen atom possesses seven shell electrons. One electron pair is in the ground state 1 s (K shell), and five electrons are distributed over the four orbitals with the principal quantum number 2 (L shell). Of these, one electron pair occupies the 2 s level and three unpaired electrons, respectively, a half of the remaining three levels, 2 px, 2 py, 2 pz. The unpaired electrons can enter into electron-pair bonds with the 1s electron of three hydrogen atoms. Thus, the three half-occupied orbitals of the L shell become about fully occupied (formation of an octet of the neon type in accordance with the octet theory of Lewis – Langmuir). The nitrogen atom is at the apex of a pyramid above the plane of the three hydrogen atoms, which are arranged in an equilateral triangle. The H-N-H bond angle is about 107° [7]. Although covalent, the nitrogen – hydrogen bonds have a polar contribution because of the stronger electronegativity of nitrogen relative to hydrogen. Because of polarization of the bonds and the unsymmetrical molecular arrangement, the ammonia molecule has a considerable dipole moment, 1.5 D [8]. As the ammonia molecule possesses the same electron configuration as water (isosterism) and similar bond angles (water vapor bond angle 105°, dipole moment 1.84 D), ammonia and water behave similarly in many reactions. Ammonia and water are diamagnetic. The dielectric constant of liquid ammonia is about 15 and greater than that of most condensed gases; therefore, liquid ammonia has a considerable ability to dissolve many substances. The ammonia molecule, with its free electron pair, can combine with a proton. In the resulting ammonium ion, the nitrogen atom is situated in the middle of a tetrahedron whose corners are occupied by hydrogen atoms. The four hydrogen atoms are equivalent in their behavior. According to LINUS PAULING, the positive excess charge is distributed about equally over all five atoms. Compilations of Physical Data. The results of comprehensive investigations of the physical properties of ammonia have been published in [9] and [12]. Both papers provide numerous equations for physical properties derived from published data, the laws of thermodynamics, and statistical evaluation. These equations are supplemented by lists and tables of thermodynamic quantities and an extensive collection of literature references. Moreover, data on physical properties and the complex systems important in synthesis may be found in [13-16], and of course, in the well-known tabulations LandoltBörnstein [17] and Handbuch der Kältetechnik [18], among others. The most important physical data are compiled in Table 1. Table 1. Properties of ammonia Mr 17.0312 page 2 of 77 Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Molecular volume (at 0 °C, 101.3 kPa) Specific gas constant R Liquid density (at 0 °C, 101.3 kPa) Gas density (at 0 °C, 101.3 kPa) Liquid density (at –33.43 °C, 101.3 kPa) 0.682 g/cm3 Gas density (at –33.43 °C, 101.3 kPa) 0.888 g/L Critical pressure Critical temperature Critical density Critical volume Critical compressibility Critical thermal conductivity Critical viscosity Melting point (triple point) Heat of fusion (at 101.3 kPa) Vapor pressure (triple point) Boiling point (at 101.3 kPa) Heat of vaporization (at 101.3 kPa) Standard enthalpy of formation (gas at 25 °C) Standard entropy (gas at 25 °C, 101.3 kPa) 192.731 J mol–1 K–1 Free enthalpy of formation (gas at 25 °C, 101.3 kPa) –16.391 kJ/mol Net heating value, LHV Gross heating value, HHV Electrical conductivity (at –35 °C), very pure Commercial Ignition temperature acc. to DIN 51794 Explosive limits NH3 – O2 mixture (at 20 °C, 101.3 kPa) NH3 –air mixture (at 0 °C, 101.3 kPa) (at 100 °C, 101.3 kPa) 16 – 27 vol % NH3 15.5 – 28 vol % NH3 15 – 79 vol % NH 3 651 °C 18.577 kJ/g 22.543 kJ/g 1 × 10–11 Ω –1 cm–1 3 × 10–5 Ω –1 cm–1 11.28 MPa 132.4 °C 0.235 g/cm 3 4.225 cm3/g 0.242 0.522 kJ K–1 h–1 m–1 23.90 × 10–3 mPa · s –77.71 °C 332.3 kJ/kg 6.077 kPa –33.43 °C 1370 kJ/kg –46.22 kJ/mol 0.7714 g/L 0.6386 g/cm3 22.08 L/mol 0.48818 kPa m3 kg–1 K–1 p – V – T Data. The p – V – T data in Table 2 are calculated from the equations in [12] and further data in [9], [13-18]. Measured p – V – T values for liquid ammonia in the pressure range from 7 to 180 MPa (70 – 1800 bar) and at temperatures from –20 °C to 40 °C may be found in [19]. Detailed information on compressibility gained from experimental data can be found in [24-29]. An equation of state for liquid ammonia is given in [20]; for a more simple form, see [16]. Further data may be found in references [21-23]. Properties of liquid ammonia from –50 to 65 °C and from saturation pressure up to 370 bar are reported in [30]. Table 2. Specific volume of ammonia in L/kg p, MPa Temperature, °C –33 0.1 0.5 1 2 3 4 5 6 7 8 10 15 20 25 30 35 –20 –10 0 10 20 50 100 150 200 250 300 1172.001 1235.492 1284.303 1333.094 1381.870 1430.634 1576.882 1820.589 2064.339 2308.163 1.467 1.503 1.533 1.566 257.613 269.524 303.259 356.206 407.353 457.602 1.466 1.503 1.533 1.565 1.600 1.638 144.981 173.883 200.728 226.667 1.465 1.501 1.531 1.563 1.598 1.636 64.713 82.476 97.322 111.121 1.464 1.500 1.530 1.562 1.596 1.634 1.772 51.740 62.785 72.589 1.463 1.499 1.528 1.560 1.594 1.631 1.768 36.088 45.448 53.305 1.462 1.498 1.527 1.558 1.592 1.629 1.764 26.361 34.981 41.713 1.461 1.497 1.526 1.557 1.590 1.627 1.760 19.379 27.944 33.968 1.460 1.495 1.524 1.555 1.589 1.625 1.756 2.171 22.859 28.422 1.459 1.494 1.523 1.554 1.587 1.623 1.752 2.151 18.986 24.251 1.458 1.492 1.520 1.551 1.583 1.618 1.745 2.115 13.375 18.385 1.453 1.486 1.514 1.543 1.574 1.608 1.727 2.048 4.321 10.478 1.448 1.481 1.508 1.536 1.566 1.599 1.712 1.999 2.780 6.534 1.443 1.475 1.501 1.529 1.558 1.590 1.697 1.959 2.516 4.474 1.438 1.470 1.496 1.522 1.551 1.581 1.684 1.926 2.376 3.540 1.434 1.465 1.490 1.516 1.544 1.573 1.672 1.898 2.282 3.094 2552.061 2796.020 507.299 556.610 252.117 277.272 124.391 137.352 81.818 90.721 60.532 67.414 47.754 53.432 39.229 44.109 33.137 37.449 28.565 32.455 22.163 25.467 13.632 16.176 9.408 11.574 6.957 8.863 5.443 7.113 4.494 5.922 page 3 of 77 among others.794 2.221 3.899 4.1826 6. Further data are given in [911] and [13-18].607 4.229 4.16 236.365 5.392 4.484 Enthalpy Heat of Entropy Liquid.479 1.108 2.432 4.301 4.413 100 2.498 1.157 5. [17].610 4.569 4.95 1. Enthalpy and entropy may be calculated by the equations in [12].069 3.3927 1.9390 5.174 2.361 0 2.328 2.6 1628.664 4.469 4.358 4.8 1144.543 4.653 2.850 1.08 1002.377 3.634 4.541 4.969 4. Enthalpy and entropy of solid ammonia are given in [13].31 1.388 4.427 4.31 293.561 4.683 4.456 4.7185 5.1 1642.462 4.727 3.203 3.9 1652.659 4.903 4.4 1.414 1.618 4.399 3. vaporization.628 2.385 2.202 4.19 1.53 202.367 4.724 3.4490 1551.4 1099.639 1.155 2.407 4.477 4.670 2. Table 3.497 4.975 5.1 1591.310 4.565 1.427 4.7299 1.039 5.496 4.530 1.370 50 2.369 4.981 3. Properties of saturated ammonia liquid and vapor t.413 4.6 1.502 4.536 1.121 3.535 4.15.865 5.028 2.1 1373.981 2.676 4.815 2.6 0.8 1388.5660 289.501 4.492 4.757 4.837 3.0 1279.1 1261.419 1.580 4.769 2.703 4.54 1.369 4.624 4.74 1.8267 5.673 4.255 3.643 5.572 4.14 119.618 4.385 –20 2.4892 770.607 3.442 4. [31] and [32].504 1.8479 0. Vapor.665 4.367 4.1 1358.046 4.4620 1215.5337 418. and [18].855 5.163 2.739 6. Assuming ammonia to be an ideal gas in the range 300 – 2000 K the following equation represents the specific heat: The values of the specific heat at constant pressure in Table 3 have been calculated according to [12] and have been extrapolated to 500 °C using data from [9].742 2.597 2.491 4.566 4.693 4.242 3.304 3.419 4.72 93.595 4.35 515.553 4.6194 174.0512 2.459 2. see [33].530 4.014 4.407 8.612 2.502 4.470 2.453 2.2488 6.626 4.6801 110.359 4.696 2.410 4.832 4.215 5.444 1.054 3.51 432.965 3.605 5.673 4.422 2.060 4.52 503.660 1.69 270.555 4.7505 72.450 4.527 4.65 614.741 4. Specific Heat.552 4.54 190.994 2.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience 40 45 50 55 60 1.861 4.460 1.651 4.456 2. L/kg L/kg –40 –35 –30 –25 –20 –15 –10 – 5 0 5 10 15 20 25 30 35 40 45 71.839 2.528 4.556 150 200 250 300 400 500 2.484 1.376 4.6 1350.966 2.330 6.9 1633.060 2.01 455.3 1648.454 4. Specific heat of ammonia at constant pressure in kJ kg–1 K–1 p.342 3.5636 1.463 3.7 1166.526 3.620 1.159 2. [15].175 4.244 4.736 4.6590 128.5 1584. Liquid.0571 5.812 3.701 2.542 2.458 4.361 4.67 479.364 4.468 4.699 1. °C –33 0.695 3.9 1343.067 3.470 3.830 1.4637 6.048 7.442 4.923 2.520 4.886 3.5 1 2 3 4 5 6 7 8 10 15 20 25 30 35 40 45 50 55 60 2. [34].636 2.330 3.785 5.365 20 2.24 290.645 4.546 4.551 6.714 4.594 4.557 1.7 1.811 1.5184 508.6656 page 4 of 77 .383 2.020 2.387 4.187 5.150 1.128 5.439 1.991 4.734 3.2166 1.1295 2.363 4. kPa Specific volume Liquid.31 1.8116 1.776 3.992 5.3 1312.050 2.86 728. °C p.4787 1.697 2.088 4.7722 5.6009 205.693 4. In the range of – 45 °C to 50 °C the specific heat (in kJ kg–1 K–1) of liquid ammonia can be calculated using the equation: where T = + 273.644 3.734 4.916 2.49 1.436 4.26 1.159 4.57 527.3 1651.9 1618.882 3.077 2.538 4.24 857.435 4.826 6.563 4.893 3.91 338.3175 6.400 4.0 1623.3053 1.5035 623.77 429.6387 149.8 1225.7 1576.96 385.033 5.7023 95.477 4.668 4.374 4.511 1.501 4.7105 6.550 1.190 2.889 4.652 5.86 522.530 4.6 1605.3890 6.409 1.197 4.600 4.415 4.444 4.362 10 2.642 4.8922 1.826 8.404 4.6241 6.080 6.387 4.426 5.074 5.033 2.418 4.574 2.475 4.103 20.827 6.339 2.40 577.432 4.587 4.967 4.611 2.377 4.446 4.526 4.518 2.5496 346. MPa Temperature.476 Caloric Data. For calculations from reduced data.379 4.788 2.510 1.7257 83.7 1638.630 1.236 2.818 5. Table 4 is derived from [12].523 1.49 151.806 4.387 4.603 4.77 354.6473 1.463 4.168 3. Further data can be found in [13-17].424 1.379 4.474 4.1 0.614 6.4754 962.502 4.484 4.462 1.776 6.486 1.449 1.19 247.454 4.1 1186.032 4.091 3.725 4.578 4.873 1.396 5.360 –10 2. Vapor.9 1.615 4.736 4.994 2.68 1.5 1122.5 1598.2849 6.787 3.87 361.22 1568.492 1.504 2. Vapor.511 4.3 1165.1188 6.8 1328.9972 5.873 4.17 408.650 1.557 4.5419 6.074 4.216 3.517 1.7 1243.454 1.805 2.521 4.240 2.0 1645.492 4.543 1.824 2.454 4.927 4.155 3.838 2.359 Properties of Saturated Ammonia Liquid and Vapor.104 3.8017 6.0 1554.567 4.939 4.473 1.607 4.402 4.0352 1.429 1.412 4.840 2.887 3.506 4.395 4.4 1611.4 1296.532 4.1 1650.2075 2.55 1.391 4.9721 2.324 4.934 3.211 2.537 1.497 3.516 4.980 3.238 3.466 4.05 315.788 3.16 1.2 1206.814 4.6 1781.450 4.068 2.1266 1. Table 4.440 4.854 6.028 3. Further data may be found in [9].912 4.998 2. kJ/kg kJ/kg kJ/kg kJ kg–1 K–1 kJ kg–1 K–1 180.894 4.701 5.439 4.468 1.397 5.39 1.539 4.574 4.5831 243.9423 1.707 3. An enthalpy log p diagram can be found in [18].947 4.996 5.80 225.8822 5.399 4.236 4.797 2.5 1075.547 2. 418 9.863 10 N2 1.1 MPa (121 bar) can be found in [57].568 3.647 10.461 3.623 11.75 2.632 1.088 2.853 H2 1. [46].50 6. The surface tension of aqueous ammonia solutions over the ranges 0.287 –20 N2 0.315 4.5 4. The basis for the saturation concentration (Table 5) and solubilities in liquid ammonia (Table 6) are unpublished BASF calculations produced with the help of the Lee – Kesler equation of state [47] and publications [37-46].5 10 1.9 – 49 MPa (49 to 490 bar) are reported.811 5.532 11.0 1050.272 32.10 4. Table 5.761 13. Hydrogen.954 18.036 2.036 2.874 4.6 11.343 40.967 13.36 1653.461 41.360 11.528 H2 3.80 9.0 Table 6.226 2.2 20.2 MPa (1 – 12 bar) and 20 – 100 °C can be found in [54].36 7.15 1. Generalized equations for viscosity calculations on refrigerants are presented in [58].9 7.028 39.922 1.452 4. MPa 5 –30 –20 –10 0 10 20 30 40 2. The ammonia content of the gas may be calculated with the formulas from [40].745 18.03 1.9 0.157 7.619 13. The interaction with methane is pronounced. from [52]. MPa 5 10 15 20 30 40 50 100 –30 N2 0. and methane : nitrogen in liquid ammonia (E).490 8. The new efforts are characterized above all by consideration of thermodynamic relationships in combination with equations of state.3 1.027 8.584 3.84 3. A survey on dynamic viscosity data for the ranges 30 – 250 °C and 0.743 3.55 5. Because of the great importance of absorption processes in synthesis loop engineering (see Section Inert-Gas and Purge-Gas Management).0 1.557 11.8 10. This interdependence is treated in detail in [44] and [49-51].056 27.488 3.564 6.127 49.7766 63. °C Total pressure.444 14. these binary and multicomponent systems have come under experimental and theoretical reexamination. °C p.248 9.1 – 1.80 5.40 9.6131 Vapor – Liquid Equilibria for the Ammonia.163 13. Methane System (Fig.956 9.317 H2 1.05 3. nitrogen (C).583 4. The following.625 12.1 – 80 MPa (1 – 800 bar) appears in [15].438 2.349 33. Experimental data are given in [53]. further data occur in [55] and [56].220 22.733 H2 2.6 2.310 63.590 22.63 3.142 26.80 8.87 0. among others.0 20 1.396 79.740 18.25 4.22 3.40 8.70 4.5 15.372 2.869 5.598 3.731 33.954 6.968 22.719 15.20 10.338 20.169 11.342 7.50 6.3619 5.457 7.0 30 0.207 6.4 15.2 2.800 9. in vol % t. F according to [44]. hydrogen : nitrogen 3 : 1 (D).50 7.654 8.70 9.6 6.195 4.0 19.0 15 1.783 20 N2 1.945 –10 N2 0.0 15.218 H2 2. Dynamic Viscosity.754 5.505 3.80 14.195 8.06 5.373 602.75 2.530 30 N2 1. Investigations of binary systems are reported in [36-44] and equations and compilations in [45].637 2.970 13.518 15.585 5.34 2.25 11. Ammonia concentration at saturation in 1 : 3 mixtures of nitrogen and hydrogen. Solubility of hydrogen and nitrogen in liquid ammonia for 1 : 3 mixtures of nitrogen and hydrogen in cm3 gas per gram NH3 (STP) t.35 9. Equilibrium solubilities (K factors) for argon (A). Surface Tension.759 6.94 1.8 1. D was developed from data reported in [46] In [35] various publications are compared with one another and equilibrium methods for the ranges 273 – 323 K and 4.085 H2 2. The following equation.285 17.969 6. represents the surface tension of ammonia ( in Nm–1) over the range –50 to 50 °C.081 2.5 2.994 3.864 1. Argon.345 40 N2 1.58 2. Figure 2.380 22. from [52].38 1.423 9.631 8.9 4. 2).303 11.291 3.581 3. Experimental viscosity data and correlating equations for the range 448 – 598 K and for pressures up to 12.190 14.916 28.178 16.90 2.294 12.767 7.6 20.7 2. hydrogen (B).167 4. xCH4.291 2.560 6.161 1.10 6.129 4.554 8.896 5.845 3.95 1. F shows the dependence of the methane partial pressure (p · yCH4) on the concentration of methane dissolved in the liquid.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience 50 2033.852 28. Nitrogen.527 H2 3.20 4.0 26.415 7.05 8.414 5.258 5.954 16.711 24. is a formula for interpolation along the vapor pressure curve for liquid ammonia (viscosity in N s m–2): page 5 of 77 .215 4.55 5.852 H2 1.945 0 N2 1.05 3.730 2.253 A mutual interaction of solubilities exists in all multicomponent systems.300 7.75 40 50 100 0.451 97.300 51.717 5. for three different gas mixtures A – C and E according to [48].219 33.756 7.269 6. and [399]. and acetates are very soluble in liquid ammonia.60 Table 8. page 6 of 77 .965 0.33 30.985 0. A compilation of the solubilities of organic compounds in liquid ammonia shows notable solubility of saccharoses [76].18 21.91 33.09 27.890 0.91 11.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience An interpolation formula for ammonia vapor for the temperature range –20 to 150 °C at 0.64 28. in general are insoluble. Because of its dipole moment.75 23. Ammonium salts are especially soluble.32 12. A nomogram for these data appears in [63].1 – 49 MPa (1 – 490 bar) and 20 – 177 °C are reported.995 0. thiocyanides. °C Mixture ratio (moles of water per mole ammonia) 1 0 20 40 2.35 24. In [62]. Solutions in liquid ammonia show significant electrical conductance. and methane are reported in [60] and [65-67]. Table for determining the percentage ammonia content of aqueous solutions from the density at 15 °C Density. such as oxides and sulfides.5 wt % water at pressures in the range 113 – 221 kPa.92 24.955 0.67 35. Heat of solution of ammonia in water. itself has a conductivity that. General physicochemical and chemical engineering handbooks can be consulted for further data.000 0.885 0.83 34.66 34.925 0.67 34. nitrites.3 kPa and –20 to 150 °C: The thermal conductivities of gas mixtures of ammonia with argon. Thermal Conductivity.38 32.69 34. and ethylene – ammonia.87 33.945 0.00 1.87 33.920 0. For the liquid phase the following formula may be used to interpolate in the range from –10 to 100 °C [52] (thermal conductivity in W m–1 K–1): Also from [52] is the formula for ammonia vapor at 101.62 33.975 0. cyanides.910 0. hydrogen.895 0.05 7. methane – ammonia.935 0. Pure ammonia.80 36. Ammonia partial pressure in aqueous ammonia solutions Figure 4.970 0.25 35. fluorides.880 17. and [61] for argon – ammonia.73 33. neon.99 32.63 0. Liquid ammonia is a good solvent for many salts as well as for some metals and nonmetals. Ammonia Density.33 4 9 19 49 99 30.990 0.80 6. A third-degree equation for the range 358 – 925 K can be found in [64].31 3. in kJ/mol NH3 t.59 9. Total vapor pressure and composition of the liquid (——) and vapor (– – –) in the system NH3 –H2O Isotherms Reference [68] gives p – V – T values for ammonia containing up to 0.12 18. is based on dissociation according to: Inorganic salts such as nitrates.17 15. The hydroxides.17 35.47 36.99 26.980 0.950 0. The anomaly in the thermal conductivity at the critical point is discussed in full detail. although limited. [60] for nitrogen – ammonia.55 4.22 36. Tables 7 and 8 and Figures 3 and 4 show miscellaneous information. Solubility in Water. like water. the ammonia molecule interacts with these ions to form solvates in a manner analogous to the water molecule in aqueous solutions.098 MPa is given in [52]: Reference [59] reports viscosity measurements for the hydrogen – ammonia system.900 0. Ammonia g/cm3 content g/cm3 content 1.53 31. Table 7.930 0. oxygen – ammonia.905 0.960 0. Solubility in Liquid Ammonia.95 34 Figure 3.915 0.64 20.940 0. iodides.14 2.50 34. and salts with diand trivalent anions. experimental data and calculation methods for ammonia liquid and vapor for the ranges 0.03 31.31 8.74 14. or molybdenum-containing catalysts. SO3 . it is better to obtain nitrides through the action of ammonia on the metals or metal compounds at higher temperature.or propanolamine. At higher temperatures. with formation of the corresponding metal amine salt and evolution of hydrogen: Correspondingly. is obtained with evolution of hydrogen by passing ammonia vapor over metallic sodium. RbO2. such as lithium or magnesium nitride. At relatively low temperatures. alkali and alkaline-earth metals. especially employing corona electrical discharges. halogens react with ammonia to form nitrogen – halogen compounds and ammonium halides. By reaction of olefins with ammonia. from their solutions. The magnetic properties and the electrical conductivity. Reactions in liquid ammonia [72] sometimes proceed differently from those in water because of the solubility difference of many salts between water and ammonia. In liquid ammonia. Liquid ammonia is a good solvent for white phosphorus. and in ammonia. amines. and free radicals. The reaction of ammonium salt with metal amide in liquid ammonia is analogous to the neutralization of acid and base in water.g.77 – at 18 °C. An ammonium azide. With organic acid halides. In the more dilute blue solutions. depending on the reaction conditions. such as copper. so-called ammonoxidation. these compounds may be prepared by decomposing the acid amide or by reacting ammonia with alkyl halides in multistage processes [70]. reacting triphenylchloromethane with dissolved sodium produces the triphenylmethyl radical Ph3C•. for example. NaC≡CH. sulfuric acid ( Sulfuric Acid and Sulfur Trioxide). For example. result with carbon monoxide [74]. Adding ammonia to aldehydes and ketones and splitting off water leads through unstable. Li2NH. and titanium. This ammonium ion is similar to the alkali metal ions in its alkaline and salt-forming The free radical NH4 has been isolated only in the form of its amalgam (DAVY. lanthanum. cobalt. K2O2. hydrobenzamide. water. to precipitation of barium chloride. and CsO2. Li3N or Mg3N2. and in the case of benzaldehyde. the equilibrium is very strongly shifted to the side of free ammonia. results in economic yields of the commercially important acid nitriles. 1811) which precipitates the more noble metals. confirm this. alkylamines can be obtained. may be prepared also. formamide from methyl formate. With CO2 . NaNH2 . in the infusible precipitate HgNH 2Cl from HgCl2. nickel. The imide. calcium. polystannides. methane. Ammonia is oxidized by oxygen or air. results from replacing a further hydrogen atom by metals. results from addition of a proton to the ammonia molecule. with acetaldehyde. which can again be dissociated hydrolytically. there are the nitrides. Chemical Properties The ionic complex attributes. On alkaline surfaces. A nitride also may result from thermal decomposition of an amide. acrylonitrile from propylene [71]. Examples are the sodium selenides. Ammonium salts result from reaction with acids in aqueous solution or in the gas phase ( Ammonium Compounds). methanol forms mono. The catalytic oxidation of methane in the presence of ammonia is employed for the industrial production of hydrogen cyanide. White explosive salts of the hypothetical acetylenediols. hexamethylenetetramine (urotropine) is obtained. manganese. For example.through trimethylamine. and zinc. The ammoniacal solutions allow preparation of many compounds otherwise unobtainable or obtainable only with difficulty. or the acids themselves (above 100 °C). Likewise. Na 2Se through Na2Se6. A survey of organic reactions in liquid ammonia appears in [75]. Nitrous Acid. intermediate amino compounds to three different forms of stable end products: for example. which is comparable to that of mercury. NH3 forms amides of carbonic acid ( Urea). cerium. as may be seen from the equilibrium constant at 18 °C: A 1 N solution of ammonium hydroxide has a pH of 11. or 15 % of 1-aminopropane. KPH 2. Gaseous ammonia reacts very violently or even explosively with nitrogen oxides to form nitrogen. cyanide. aluminum. arsenides. with elimination of hydrogen chloride. at lower temperature a blue solution results in which partial ammonolysis of sulfur to sulfur nitride and ammonium sulfides or polysulfides takes place. and P2O5 . ammonia – air mixtures are oxidized readily at 350 °C to nitrite and further to nitrate. such as KO–C≡C–OK. and thiocyanide. ammonia forms hydrogen cyanide with carbon monoxide. for example. acid amides result from acylating ammonia with esters. ammono acetaldehyde. the metals are completely dissociated to positive metal ions and solvated electrons [73]. The reaction with N2O requires ignition. The heats of neutralization in ammonia are even larger than in water. tellurides up to Na2Te4.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Liquid ammonia in combination with water as antisolvent and methylamine as prosolvent is also an excellent extraction medium for all types of petroleum fractions [81]. In concentrated solutions. Rhombic sulfur is dissolved by liquid ammonia at –11. Ammonoxidation of o-xylene yields phthalodinitrile in a single reaction step.5 °C. for example. the reaction leads in water to precipitation of silver chloride. with acetylene. as well as dioxides. At higher temperatures the products are nitrogen and hydrogen halides. [Top of Page] 4. Introducing oxygen into the solution of alkali metals forms peroxides. Finally. antimonides. NH4N3 . The controlled catalytic oxidation of ammonia to produce nitric acid or pure NO for hydroxylamine production is Hydroxylamine – Catalytic Hydrogenation of Nitric Oxide. NO2. such as quicklime or soda lime. KO2. With sodium hypochlorite. the metals largely remain in the metallic state. Metals may replace one or all of the hydrogen atoms of ammonia. and iron and its alloys are soluble in solutions of ammonium bromide. lithium imide. sodium amide. described in Nitric Acid. phosphorus. Sulfur. H2NCl. ammonia forms hydrazine hydrate (Raschig synthesis) with chloramine. such as lithium. Otherwise. Aqueous solutions of ammonia react as weak bases because hydroxyl ions result from the addition of a proton or H 3O+ from the water to the ammonia molecule. aqueous ammonia reacts to form ethanol. the reaction leads to the acid amide. The reaction rate can be increased by adding traces of nickel and cobalt oxides to the catalyst surfaces [69]. ammonium salts have an “acid” character. Solutions of ammonium in liquid ammonia are blue and usually exhibit properties similar to liquid ammonia solutions of sodium and potassium. Na2O2. The process of hydrolysis corresponds to ammonolysis in ammonia. cadmium. The amide. Once dissolved. or charcoal. For example. and polyplumbides. [Top of Page] 3. as an intermediate.. This results in ammonobasic compounds. for example. up to 10 % yield of ethylamine. on reaction with formaldehyde. and halogens also can take the place of the hydrogen atoms in ammonia. Ammonia undergoes numerous industrially important reactions with organic compounds. Sodium amide reacts with N2O to form highly explosive sodium azide. However. Reaction with alkyl halides or alcohols serves to manufacture amines or imines. to NO. In many cases. Catalytic hydrogenation converts the acid amides to primary amines. iodide. magnesium. are formed directly from the elements at elevated temperature. and Nitrogen Oxides – Industrial Production. or N2O or to nitrogen and water. for example. Production page 7 of 77 . ammonium nitrate or nitrite. and phosphoric acid ( Phosphoric Acid and Phosphates). e. and with phosphine. especially in the presence of surface-active materials. Reactions of ammoniacal metal solutions with halogen-containing organic compounds can be used for the synthesis of higher hydrocarbons. Reactions in Liquid Ammonia. The catalytic gas-phase oxidation of olefins in the presence of ammonia on vanadium. metal amides in liquid ammonia have a “basic” character. With ethylene and propylene oxides. acid anhydrides. The nitrides of reactive metals. Liquid ammonia's ability to dissolve alkali and alkaline-earth metals has been well known for a long time. For example. dichloromethane yields ethylene imine in the presence of calcium oxide. With growing availability of cheap petrochemical feedstocks and novel cost-saving gasification processes (steam reforming and partial oxidation) a new age dawned in the ammonia industry. developed by FRANK and CARO in 1898. coal or coke is used only under special economic and geographical conditions (China. became the basis for the industrial manufacture of ammonia and since then the same principle has found widespread application in numerous high-pressure processes. 4. So. partial oxidation of heavy oil fractions was used in several plants. since at this time pilot-plant operation of ammonia synthesis was already successful. [77-79].012 %).. finally reaching 240 000 t/a. from which they can be separated by physical methods and/or chemical reactions using almost exclusively fossil energy. enabled the chemical industry for the first time to compete successfully against a cheap natural bulk product. In an unprecedented effort. an industrial process was developed and a commercial plant was built. Around 1900 FRITZ HABER began to investigate the ammonia equilibrium [85] at atmospheric pressure and found minimal ammonia concentrations at around 1000 °C (0. After BERTHOLLET proved in 1784 that ammonia consists of nitrogen and hydrogen and was also able to establish the approximate ratio between these elements. BOSCH was already well aware that the production of a pure hydrogen – nitrogen mixture is the largest single contributor to the total production cost of ammonia [93]. which is the first process since 1913 to use a non-iron synthesis catalyst. SCHOENHERR at BASF developed a different electric arc furnace in 1905. 11 % of world nitrogen production (about 2 × 106 t/a) [80] was still based on the cyanamide process. namely. Expansions increased the capacity to about 250 t/d in 1916/17 and a second plant with a capacity of 36 000 t/a went on stream in 1917 in Leuna. where nitrogen and oxygen combine to give nitric oxide. To combine the atmospheric elements nitrogen and oxygen directly to form nitric oxides 2. The elements nitrogen and hydrogen are abundantly available in the form of air and water. described in the patent DRP 235 421 (1908). coal liquefaction. The driving force in the search for methods of nitrogen fixation. BASF withdrew from this joint venture soon after.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Ammonia is the second largest synthetic chemical product. liquefied petroleum gas (LPG). more than 90 % of world consumption is manufactured from the elements nitrogen and hydrogen in a catalytic process originally developed by FRITZ HABER and CARL BOSCH using a promoted iron catalyst discovered by ALWIN MITTASCH. and ammonia synthesis. Nevertheless. which finally displaced the other two and rendered them obsolete. Had this electricity been generated from fossil fuels this figure would correspond to about 600 GJ per tonne nitrogen. together with a team of dedicated and highly skilled co-workers. After a controversy with NERNST. The next revolution in the ammonia industry was the advent of the single-train steam reforming ammonia plants. some of the hydrogen comes also from the hydrocarbons themselves (methane has the highest content). Fischer – Tropsch Process. while NERNST concluded that commercial ammonia synthesis was not feasible in view of the low conversion he found when he first measured the equilibrium at 50 – 70 bar [257].1. This process. The predominant fossil fuels are natural gas. In 1904 CHRISTIAN BIRKELAND performed successful experiments and. many experiments in the 1800s were aimed at its direct synthesis. HABER repeated his measurements at atmospheric pressure and subsequently at higher pressures [87-90]. The specific energy consumption was tremendous: 60 000 kW per tonne of fixed nitrogen. Since the early days there has been no fundamental change in this process. which can be decomposed with water to yield ammonia. but a series of mistakes and misunderstandings occurred during the research. India. To combine nitrogen and hydrogen to give ammonia 3. The first plant started production at Oppau in September 1913 and had a daily capacity of 30 t of ammonia. and some plants even continued to operate after World War II. This process not only solved a fundamental problem in securing our food supply by production of fertilizers but also opened a new phase of industrial chemistry by laying the foundations for subsequent high-pressure processes like methanol synthesis. France. Only through this innovative plant concept with its drastic reduction in feedstock consumption and investment costs. and the carbon acts as a reducing agent for water and in some processes may also facilitate the separation of oxygen from nitrogen by formation of carbon dioxide which can be removed by various operations. but using the same catalyst. pioneered by M. and many other countries based on a BASF license or own process developments. Working in parallel. and the company's board decided to pursue the technical development of the process with all available resources. was to produce fertilizers. Further stepwise expansions. Historical Development The catalytic synthesis of ammonia from its elements is one of the greatest achievements of industrial chemistry. Continuous ammonia production with high space yields on large scale combined with the ammonia oxidation process for nitric acid. where steam reforming of natural gas was used for synthesis gas production. CARL BOSCH. was commercially established by 1910. succeeded in developing a commercial process in less than five years [91-98]. or using the formation of titanium nitride were investigated in Ludwigshafen by BOSCH and MITTASCH but did not appear promising. The LCA process of ICI (ICI LCA Process. After World War I ammonia plants were built in England. Recovery of ammonia as byproduct of other production processes. sodium nitrate imported from Chile. Even today the synthesis section of practically every ammonia plant has the same basic configuration as the first plants. HABER also anticipated the preheating of the synthesis gas to reaction temperature (at that time 600 °C ) by heat exchange with the hot exhaust gas from the reactor. the Norwegian plants operated throughout World War I and had total production of 28 000 t/a of fixed nitrogen with a power consumption of 210 000 kW [33]. In principle there are three ways of breaking the bond of the nitrogen molecule and fixing the element in a compound: 1. consuming 190 GJ per tonne of ammonia. This process was originally developed by BASF and greatly improved by ICI. However. Before natural gas became available in large quantities in Europe. The amount of ammonia formed in a single pass of the synthesis gas over the catalyst is much too small to be of interest for an economic production. His successful demonstration in April 1909 of a small laboratory scale ammonia plant having all the features described above finally convinced the BASF representatives. It was the new science of physical chemistry. which was developed immediately after ammonia synthesis. The design philosophy was to use a single-train for large capacities (no parallel lines) and to be as far as possible energetically self-sufficient (no energy import) by having a high degree of energy integration (process steps in surplus supplying those in deficit).) and the KRES/KAAP process. that enabled chemists to investigate ammonia formation more systematically. perhaps more importantly. HABER concluded that much higher pressures had to be employed and that. KELLOGG and others. Up to the end of World War II. formed from coke and lime in a carbide furnace ( Calcium Carbide). Increasing competition and rising feedstock prices in the 1970s and 1980s forced industry and engineering companies to improve the processes further. W. End of the 1990s a ruthenium-based catalyst was introduced which allows slightly lower synthesis pressure and is used in a few world-size plants (KAAP process of Kellogg – Braun – Root [1392-1394]. A hydrogen – nitrogen mixture reacts over the iron catalyst (today's formulation differs little from the original) at elevated temperature in the range of 400 – 500 °C (originally up to 600 °C ) and pressures above 100 bar with recycle of the unconverted part of the synthesis gas and separation of the ammonia product under high pressure. The development started in the USA. which by 1908 was already producing 7000 t of fixed nitrogen. Apart from HABER. and technical ammonia processes differ today mainly with respect to synthesis-gas preparation and purification. overcoming his colleague's preoccupation with the unfavorable equilibrium concentrations. South Africa). The availability of cheap hydrolelectric power in Norway and the United States stimulated the development of the electric arc process. For example. the gas was recirculated by means of a circulation compressor to the catalyst-containing reactor. dramatic changes happened over the years in the technology of synthesis-gas generation. oxo synthesis. A vast amount of research in all three directions led to commercial processes for each of them: the electric arc process.g. [86]. and synthesis-gas generation continued to be based on coal until the 1950s. Italy. One of the reasons for the lack of success was the limited knowledge of thermodynamics and the incomplete understanding of the law of mass action and chemical equilibrium. with modified process parameters. and higher petroleum fractions. The Norwegians and BASF combined forces in 1912 to build a new commercial plant in Norway. the cyanamid process. OSTWALD and NERNST were also closely involved in the ammonia synthesis problem. naphtha. in contrast to the synthesis reaction. coke and nitrogen. plant capacities were expanded by installing parallel lines of 70 – 120-t/d units. In 1934. came in full production only after World War I [77]. which is about 17 times the consumption of an advanced steam-reforming ammonia plant in 1996. After separating the ammonia by condensation under synthesis pressure and supplementing with fresh synthesis gas to make-up for the portion converted to ammonia. OSTWALD withdrew a patent application for an iron ammonia synthesis catalyst because of an erroneous experiment. a recycle process was necessary. Some other routes via barium cyanide produced from barytes. who extended it to naphtha feedstocks. already decided in 1916. Air was passed through an electric arc which raised its temperature to 3000 °C. HABER therefore recycled the unconverted synthesis gas. In 1908 HABER approached BASF (Badische Anilin & Soda Fabrik at that time) to seek support for his work and to discuss the possibilities for the realization of an industrial process. reacts with nitrogen to give calcium cyanamide. could the enormous increase in world capacity in the following years became possible. [33]. and Reppe reactions. The cyanamide process. but remained unsuccessful [82-84]. is no longer of great importance. together with SAM EYDE. Calcium carbide. To use compounds capable of fixing nitrogen in their structure under certain reaction conditions. the temperature of which would be raised by the exothermic ammonia formation reaction sufficiently (about 18 °C temperature rise for a 1 % increase of the ammonia concentration in converted synthesis gas). The process was energetically very inefficient. coke ovens. e. page 8 of 77 . Of course. are recent advances that make a radical breakaway from established technology. of course. which developed rapidly in the late 1800s. 47 2.96 53.40 11.43 73.22 –15.43 65.59 84.84 31.95 68.69 71.77 –117.74 10.29 73.99 61.11 43.99 73.52 8.72 40. with literature data and many tables.07 29. About 3.84 22.95 21.35 22.80 29.02 37.27 MJ of heat is evolved.25 45. Thermodynamic data for the reaction 0.31 page 9 of 77 .4 m3 (STP) of hydrogen and 0.95 32.75 69.26 29.69 30 74.20 58.47 49.05 4.97 78.61 17.34 84.92 39.52 cp. N2 : H2=1 : 3 t.49 83. [406].88 26.04 25.94 43.55 27.54 82.42 89.78 60.68 27.43 63.90 24.53 77.13 43.59 4.52 90.26 37.02 50.278 29.48 –50.59 63.12 8.67 11.80 32.60 56. [1259].57 33.43 74.69 40 79.05 60.46 50.5 mol % CH4).83 91.74 19.63 –110.87 15.01 78. GILLESPIE and BEATTIE [101] (see also [14]) were by far the most successful experimentally in establishing a firm basis for an analytical expression of the equilibrium constant in the range of industrial interest.86 45.23 14.45 40.98 29. The values in Tables 10 and 11 were calculated using their equation.132 38.36 31.53 74.29 89.85 35.85 4.81 69.08 61.46 51.79 49.52 50 83.85 60. A detailed description. MPa 5 10 20 30 40 50 60 70 80 90 100 300 32.67 81.49 29.49 3.52 79.79 70.64 63. H2.59 13.51 43.51 79.60 34.06 52.70 5.74 16.49 77.868 29.88 10.41 42.56 5.52 72.06 52.48 47.58 –52.72 68.91 47.84 17.69 65.60 46.29 51.10 9.93 56.601 26.68 42.46 6.61 55.39 21.79 20. MPa 5 300 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600 39.25 76.93 65.89 41.40 67.17 48.80 29.80 49.88 48.15 25.48 20 66.28 75.16 19.22 30.63 46.34 18.47 46.69 67.634 31.05 46.61 80.777 48.88 21.20 87.98 81.36 8.89 –116.41 62.15 82.75 30.20 60 70 80 90 100 86.724 cp.270 8.22 28. [1318].38 39.20 27.65 39.83 37.74 12.74 52.19 50. N2 : H2=1 : 3 t.24 20. Equilibrium ammonia content (in mol %) in the presence of inert gases (2. [100].50 47.10 54.21 29.20 62.37 50.12 72.64 18.64 16.26 4.81 35.03 43.22 71.12 70.89 35.39 55.04 –53.8 m3 (STP) of nitrogen.28 63.36 37.15 23.86 26.35 –105.58 49.53 68.57 12.61 58.92 55.96 33.92 42.77 13.38 36.12 22.35 75. J 28.23 13.54 23.03 –112.38 81. Ammonia content (in mol %) in equilibrium synthesis gas. 4.62 87.89 32.89 53.80 15.49 22.65 31.20 2. Thermodynamic Data Ammonia synthesis proceeds according to the following reaction: To fix a kilogram of nitrogen in ammonia requires reacting 2.04 70.22 48.20 55.24 15.20 35.95 48.64 25.48 40.57 92. °C pabs.72 8. 7.10 18.09 62. Table 10.35 – 48. Table 9 is a compilation of the most important thermodynamic data for the reaction at atmospheric pressure.48 –55.35 88. The reaction equilibrium has been investigated experimentally and theoretically many times. J mol–1 K–1 35. K 300 H.62 38.97 31.06 34.80 59. Values for the equilibrium constant are available for pressures up to 350 MPa (3500 bar).68 77. kJ S.71 27.68 –99.55 47.61 31.50 62.42 57.71 79.97 41.39 64.36 69.19 30.5 H2 t.02 34.65 14.21 23.19 55.75 51.28 10 52.216 32.42 60.39 44.341 30.79 30.144 cp.41 11.98 85.02 16. Table 9.20 71.87 53.43 46.86 39.96 65.28 –55.82 3.117 61.94 85.63 53.71 37.66 38.26 42. [99].80 76.37 88.61 84.24 7.14 80.19 72.26 57.69 20.63 60.454 30.14 93.03 75.21 33.19 10.48 37.04 52.34 25.38 17.27 60. °C pabs.5 N2+1.88 41.23 66.55 –115.54 72.67 41.13 18.06 –55.02 86.12 5.03 92.39 91.13 35.25 44.56 72.50 20.5 mol % Ar.97 29.34 6.77 44.79 41.42 77.04 29.41 60.95 77.09 86.58 75.54 25.94 46.28 76.37 32.96 14.32 45.57 70.56 53.199 29.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience A short survey of the history of ammonia process technology can be found in [95].81 37.92 41.99 66.12 38.12 88.81 56.68 12.00 Table 11.49 20.08 21.79 57.71 33. J K–1 mol–1 K–1 400 500 600 700 800 900 1000 NH3 at atmospheric pressure –46.33 84. [96]. N2.66 55.87 35. NH3.20 5.33 27.39 9.85 59.76 27. A description of the equilibrium using the REDLICH – KWONG equation of state is given in [400].16 37. J mol–1 K–1 Chemical Equilibrium.04 33.72 68.41 44.81 53. [1409].26 27.33 65.62 64.35 82.93 2.18 82.75 6.72 90.46 34.67 16.50 40.89 63.13 84.91 58.66 70.69 2.46 75.33 46.53 36.53 43.36 25.31 58.74 6.52 79.23 75.39 67. appears in [14].2.74 87.80 28.84 13.36 10.06 55. J K–1 G/T.13 24.00 7.60 12.73 87.17 49.26 23.451 55.71 –114.55 81.09 9.87 33.26 –54.97 69.48 58.62 39.16 37.33 54.49 74.37 29.03 30.84 32.60 29.60 64.19 3. 86 51.85 53.24 15.13 37.24 8.50 58.98 22.55 57.06 5.46 33.94 3.35 53.31 20.39 53.84 32.38 74.79 55.51 53.10 17. 4.66 79.21 60.00 64.24 69.06 41.16 71.03 63. Ammonia Synthesis Reaction 4. see [106].46 7.76 53.44 29.49 65.76 61.38 100.12 57.68 56.49 32.92 11.02 51.02 29.57 54.34 66.74 59.34 51.18 61.28 59.22 71.89 34.24 32.50 18.66 64.47 87.74 152.20 53.45 59.46 60.48 29.46 23.20 61.48 15.69 61.01 53.56 16.78 58.28 53.44 45.05 73.16 68.70 70.84 70.68 57.42 57.50 18.26 57.15 73.78 94.02 52.75 47.86 43.38 51.44 27.48 57.48 67. Numerous authors have estimated the pressure dependence under various assumptions.62 75.83 45. General Aspects Usually.63 50.92 56.14 51.22 7.64 39.08 51.63 56.07 13.22 63.90 32.32 61.98 63.63 16.55 67. According to estimates [109].52 59.86 51.91 61.37 49.87 69.59 31.63 70.92 53.01 1.49 25.19 13.20 57. [107].54 29.16 72.37 63.58 59.72 12.68 28.38 59.87 31.21 7.73 13.57 21.73 45.69 51.82 58. For further data see.31 35.17 60.43 14.31 86.48 32.79 14.03 30.58 70.59 83.74 68.00 75.28 68.86 53.23 63.64 27.88 47.23 63.79 17.88 66.85 22. [14].18 55.43 Heat of Reaction.92 45.36 20.14 60.24 46.26 104.70 6.52 66.01 36.70 55.06 61.12 61.31 72.62 64.10 2.93 32.58 52.25 36.08 58.66 24.28 56.68 35.41 56.29 25.48 57.80 55.77 33.12 54.70 40 87.46 47.59 2.48 41.51 73.27 66.18 66.40 59.85 40.86 53.37 38.49 4.98 49.37 63. Additional literature can be found in [105] Table 12.01 73.60 64.80 114.04 79.48 63.76 53.06 60.23 19.39 59.18 57.34 58.28 52.78 63.96 20.72 64.12 10.39 44.96 27.03 56.88 20 66.51 55.54 57. HABER investigated the heat of reaction at atmospheric pressure [102].10 3.91 59.62 40.96 46.76 17.79 45.50 56.96 56.27 64. significant energy input is required for the nitrogen molecule to achieve the activated state.22 12.64 54.86 62.81 57.85 61.66 58.33 9.47 21.73 3.69 23.53 67.29 120.97 47.18 2.39 63.5 N2+1.67 41.73 52.40 61.88 49.38 2.80 53.01 34.65 31.06 51.97 79.38 49.62 62.77 52.71 43.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600 29.67 64.20 39.86 59.82 59.59 65.81 14.02 93.71 16.54 51. This applies especially for high pressures and low temperatures.86 61.02 62.52 54.29 39.27 3.24 90 100 H (in kJ) for the reaction 0.73 97.40 44. For the specific heat.68 64.64 51. However.07 27. This is because of its high dissociation energy of 941 kJ/mol.17 30.44 58.57 60.36 62.49 62.11 9.92 30.97 41.42 26.05 65.25 53.84 53.77 64.81 21.5 H2 NH3.47 58.59 50.53 65.09 11.55 61.03 61.71 58.46 18.20 57.09 74.48 5.50 22.28 31.69 63.81 57.32 62. thermal conductivity.80 33.90 45.36 67.60 59.01 27.00 55.60 16.97 43.37 75.04 4.52 65.17 20.31 54. most people use the Gillespie – Beattie equation [103].56 34.14 56.77 33.11 54.09 62.94 71.97 43.26 39.86 65.93 31.11 47.63 7.41 62.36 51.44 60.96 15.35 25.99 51.10 8.85 49.79 30 76.86 54.84 29.73 12.12 29.08 57.06 48.28 26. Heat of reaction t.26 56.74 55.49 55.72 40.17 42.19 75. It is to be noted that heats of mixing must be considered for synthesis gas.43 58.01 35.05 24.81 30. to form ammonia from hydrogen and nitrogen molecules.05 18.04 23.88 91.96 35.44 56.88 63.50 5.09 53.51 49.89 61.89 72.59 59.06 39.27 24.07 52.29 57.46 51.33 28.54 68.28 51. from [101] 109.64 62.38 59.39 59.43 58.69 55.36 34.08 9.30 50.89 19.53 53.64 60.30 38.87 49.55 52. Reference [108] gives viscosities of hydrogen – nitrogen and hydrogen – ammonia mixtures.46 54.26 47.77 36.72 68.35 59.79 53.02 42.40 52.17 51.83 67.00 65.48 103.18 50.68 60.93 67.41 15.48 58.99 67. Various authors have measured the p – V – T behavior of nitrogen.94 88.65 60.92 22.56 42.30 22.67 55.72 63.52 54.09 54.44 13.19 57.29 58.63 72.04 62.76 36.48 52.57 8.10 55.42 48.82 55.95 25.66 55. MPa 0 (ideal) 10 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 575 600 44.86 64.43 58.56 23.76 68.1.23 57.83 70.59 19.94 43.51 19.47 69.65 37.04 71.83 38.98 55.83 75.23 62.45 60.33 61.12 74.09 27.05 11.65 69.14 63.64 4.41 6.32 44.57 54.02 12.38 56.95 48.09 55.19 16.68 38.97 5.27 69.48 60.76 50.81 47.16 10.94 79.61 69.55 58. °C pabs.75 57.45 13.74 7.61 50 98.34 80.04 59.90 39.03 75.16 38.10 65. which is considerably higher than that of hydrogen.84 61.20 89. initiation of ammonia synthesis homogeneously in the gas phase page 10 of 77 .87 8.69 43.41 40.57 47.57 60.30 57.87 24.81 63.39 20.97 4.45 52.93 130.71 54.63 66. for example.82 53.47 46.3.78 32.65 26.06 Physical Properties of the Reactants.44 68.02 25.81 17.68 49.65 30.79 11.68 71.52 60 70 80 130.53 63. hydrogen.84 55. Today.50 62.33 49. Reference [104] contains test results for the range 120 – 200 MPa (1200 – 2000 bar) and 450 – 525 °C.23 27.08 41.96 18.72 59.65 52.62 23.59 64.69 67.03 59.91 40.92 45.14 62.19 54.3.67 27.82 87.61 36.29 61.71 22.52 27.61 53.96 60.17 53.51 65.66 47.05 10.52 59.26 55.51 4.09 34.34 141.64 47.03 61.90 64.06 58.02 65.11 96.95 80.60 56.08 30.75 62.84 63.42 63.19 14.97 29.98 54.56 69.78 72. a system having an exothermic heat of reaction under operating conditions should react spontaneously.97 55.40 3.96 42.49 37.35 57.70 80.52 60.29 64. and viscosity of the reactants.09 37.82 55.09 6.58 63.52 55.35 55.76 45.31 73.37 57.59 66.93 71.43 60.28 54.37 51.76 84.80 15.39 61. and 3 : 1 hydrogen – nitrogen mixtures.79 67.51 25.51 20.44 61.34 26.95 64.56 121.38 35.15 63. Reference [14] contains a survey.27 71.83 2.51 37.18 10.87 66.45 58.87 50.39 11.92 113.10 66.75 34.28 63.44 83.49 107.79 70.14 45.33 22.71 53. This equation was used in calculating the values in Table 12. for example. Transport of the reactants by diffusion and convection out of the bulk gas stream. temperature-programmed adsorption and desorption. In fact. The transport processes occurring in the pores of the catalyst in accordance with the classical laws of diffusion are of importance in industrial synthesis (see also Sections Particle Size and Shape and Kinetics). and further through the pore system to the inner surface (pore walls) 2. This drastically reduces the required energy of activation. is that the energy supplied is useful only in part for ammonia formation. The reaction may then proceed in the temperature range 250 – 400 °C. [172]. Ca. In the case of nitrogen the first step seems to be an adsorption in molecular state followed by chemisorption in the atomic state. at such high temperatures and industrially reasonable pressures. A possible explanation for the high activity of faces (111) and (211) is that these are the only surfaces which expose C7 sites (iron with seven nearest neighbors) to the reactant gases. Isotopic experiments with 30N2 and 29N2 showed that the surface species resulting from lowtemperature adsorption was molecular. The principal difficulty with these so-called plasma processes. Desorption of the ammonia formed into the gas phase 5. the course of ammonia synthesis by the Haber – Bosch process can be divided into the following steps: 1. NH and NH2 species were identified [152]. In IR spectroscopic investigations. p. Transport of the ammonia through the pore system and the laminar boundary layer into the bulk gas stream Only the portion of the sequence that occurs on the catalyst surface is significant for the intrinsic catalytic reaction. X-ray photoelectron spectroscopy. 157]. [173]: Ammonia synthesis activity: (111) > (211) > (100) > (210) > (110) Activation energy of nitrogen adsorption: (111) > (100) > (110) Work function: (210) > (111) > (211) > (100) > (110) Surface roughness: (210) > (111) > (211) > (100) > (110) Ultrahigh vacuum (UHV) experiments with single crystals show that the activation energy of the nitrogen adsorption at zero coverage increases from about zero for Fe (111) to 21 kJ/mol for Fe(110) [115]. References [111-114] give reviews of the older and some of the newer literature. such as electrical energy or ionizing radiation. Many of these methods are based on interaction of slow electrons. Measurements on defined single crystal surfaces of pure iron performed under ultrahigh vacuum [153-155] clearly showed that Fe(111) is the most active surface. Ammonia synthesis is highly sensitive to the orientation of the different crystal planes of iron in the catalyst [174-178]. In singlecrystal experiments it was found that the activation energy for dissociative nitrogen adsorption is not constant. In 1972. through a laminar boundary layer. From experimental results it was concluded that for hydrogen a direct transition from the gaseous H2 molecule into the chemisorbed Had is most likely. 151]. 141. to form activated intermediate compounds 4. Catalyst Surface and Reaction Mechanism Earlier studies [142. the formation of ammonia may be forced by employing other forms of energy. adhesion coefficient (the probability that a nitrogen molecule striking the surface will be adsorbed dissociatively) and work function show a clear dependence on surface orientation [123]. For purely thermal energy supply with a favorable collision yield. work-function measurements. all older attempts to combine molecular nitrogen purely thermally with atomic or molecular hydrogen failed. reactivity. [118]. scanning tunnelling microscopy. the synthesis of ammonia proceeds even in the absence of specific catalysts. this activation barrier requires temperatures well above 800 – 1200 K to achieve measurable reaction rates. and evidence for the stepwise hydrogenation of surface nitrogen atomic species was found. Figure 5. Since the 1980s a large variety of surface science techniques involving Auger electron spectroscopy. Prereduced catalysts are also commercially available. Activation is usually accomplished in situ by reduction with synthesis gas. Promotion with potassium of single iron crystals enhances the sticking probability for nitrogen dissociation much more on the Fe(100) and (110) than on the Fe(111) page 11 of 77 .2. the molecules lose their translational degrees of freedom by fixation on the catalyst surface. Reaction of the adsorbed species. 129. It was further shown that at lower temperatures nitrogen becomes adsorbed only in the molecular state. to 103 kJ/mol on iron [110]. and it was also possible to identify adsorbed intermediates. [143]. to the outer surface of the catalyst particles. it was discovered that electron donor – acceptor (EDA) complexes permit making ammonia with measurable reaction rate at room temperature. A greater part is transformed in primary collision and exothermic secondary processes into undesirable heat or unusable incidental radiation. whereas that from high-temperature adsorption was atomic [182]. In the homogeneous phase under thermodynamically favorable temperature conditions. This argument might probably be the key for the special role of C7 sites. The results of these experiments allow the mechanism of ammonia synthesis in the pressure range of industrial interest to be elucidated [115. which also impedes their economic use. Adsorption of the reactants (and catalyst poisons) on the inner surface 3. both ammonia and hydrazine result from reacting atomic nitrogen with atomic hydrogen (see for example [109. Adsorption and desorption under higher pressure on finely dispersed catalyst indicate that the reaction is highly activated under these conditions with high coverage. 116. the theoretically achievable ammonia yield is extremely small because of the unfavorable position of the thermodynamic equilibrium. about 100 kJ/mol [142]. 144-150. This was also demonstrated by the rate of ammonia formation on five different crystallographic planes for unpromoted iron at 20 bar. These values increase significantly for higher coverage [171]. On the other hand. if need be with participation by hydrogen from the gas phase. [170-172]. or neutral particles and exhibit high sensitivity to surface structures. This has now been fully confirmed by microkinetic simulations based on the results of surface science studies [115. but subsequently dissociates when the temperature is raised. Iron catalysts which are generally used until today in commercial production units are composed in unreduced form of iron oxides (mainly magnetite) and a few percent of Al. 144. [151] Activation energy.3. Other reasons discussed are based on charge transfer and interaction of iron d-bands with antibonding 2 * orbitals of nitrogen [156]. At pressures above 200 MPa (2000 bar). other elements such as Mg and Si may also be present in small amounts. According to [170]. [171] adsorption on the planes Fe(111) and Fe(110) is associated with a regrouping of the surface atoms. Of special importance is the adsorption of nitrogen. This assumption is decisive in representing the synthesis reaction kinetics. In the catalytic combination of nitrogen and hydrogen. and others have been developed [117]. [144]. as shown in Figure 5. 119]. 186] ). ions. 4.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience requires an activation energy of 230 – 420 kJ/mol. it increases as the surface becomes increasingly covered with adsorbed nitrogen species. Numerous investigations have been performed to elucidate the mechanism of catalytic reaction of nitrogen and hydrogen to form ammonia. and K. As with every catalytic gas-phase reaction. However. 145] which are typical for ammonia synthesis. Ammonia formation rate on different iron surfaces [123]. Under reaction conditions it can attain values of 63 – 84 kJ/mol [119. This section concentrates mainly on the ammonia synthesis reaction over iron catalysts and refers only briefly to reactions with non-iron catalysts. [272] had already suggested that on iron catalysts nitrogen adsorption and dissociation can be regarded as the rate-determining step for the intrinsic reaction. and secondary ion emissions detecting NH+ also confirmed the presence of NH species on the surface [180]. At such extreme pressures the vessel walls appear to catalyze the formation of ammonia. With these powerful tools the kinetics of nitrogen and hydrogen adsorption and desorption could be investigated. There are theoretical arguments [160] that highly coordinated surface atoms should show increased catalytic activity due to low-energy charge fluctuations in the d-bands of these highly coordinated atoms. tungsten [201]. In more recent investigations of the adsorbed nitrogen species [166-169] a second molecularly adsorbed species was detected. Based on these experimental results a reaction scheme for the ammonia synthesis may be formulated comprising the following sequence of individual steps [115]: (1) (2) (3) (4) (5) (6) (7) The progress of the reaction may be described in the form of an energy profile. x ranging from 0. This new surface then may serve as a template on which iron grows with (111) and (211) orientation upon exposure to the synthesis-gas mixture in the reaction [159]. in the rate-determining step (see 111. Other reaction mechanisms have been debated for reaction temperatures below 330 °C [126]. [195-197]. This was indeed the case with the new AmoMax-10 catalyst [1321]. and for computer control of ammonia plants. This even holds for tungsten. the question whether the active industrial catalyst exposes mostly (111) faces remains unresolved. With still (111) faces present in the reduced state the catalyst has a higher specific surface area and an improved pore structure. which has no significant activation energy and a high adsorption enthalpy for nitrogen. but the energy differences involved can easily be overcome at the temperatures (ca. whereas the state is regarded as a terminally bound species. Pt (no nitrides) – Mn. Thus the following picture for the nitrogen adsorption emerges: where S* denotes a surface atom. chemisorbed hydrogen blocks the catalyst surface [192]. 7). Wustite is a nonstoichiometric iron oxide(Fe1–xO) with a cubic crystal structure.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience (factors 280. The reaction over a catalyst avoids this problem since the energy gain associated with the surface atom bonds overcompensates these dissociation energies and the first steps have actually become exothermic. They found evidence that a different reaction mechanism predominates below and above 330 °C [191]. [177]. It is. For industrial catalysts made by careful reduction of magnetite fused with nonreducible oxide promoters the important role of the (111) face seems to be confirmed [122]. This is somewhat surprising in view of the marked differences in the heats of adsorption of nitrogen and the adsorption activation energy. A proposed mechanism for these findings — backed by X-ray photoelectron spectroscopy. However. Co. at low temperatures. or of adsorption complexes containing diatomic nitrogen. temperature programmed desorption and electron microscopy — assumes that first alumina forms an iron aluminate FeAl2O4 on the surface. and the face (110) was found to be inactive with and without potassium [123]. activity and reaction mechanism of non-iron catalysts is given in [112]. investigated ammonia synthesis on a commercial Topsøe catalyst.3. This catalyst in its oxidic form is based on wustite instead of magnetite and has a significantly better activity at lower temperature than magnetite. NIELSEN et al. An attempt to explain these differences is given in [164]. 4. 1000 and 8. Non-iron systems which exhibit some potential to catalyze ammonia synthesis can be divided into the following groups [195]: – Platinum group metals such as Ru. for determining the optimal operating conditions. KM IR. Schematic energy profile of the progress of ammonia synthesis on Fe (energies in kJ/mol) [115] Dissociative nitrogen adsorption remains nevertheless the rate-determinating step. Also. high ammonia concentration in the synthesis gas (recycle gas) restricts it (Fig. V (present as nitrides under the reaction conditions) In the non-iron systems the rate-determining step is also dissociative adsorption of nitrogen and the catalyst effectivity seems to be primarily dictated by the activation energy of the dissociation reaction [195]. Further information on structure. A similar effect was not found for the ammonia synthesis at 20 bar and catalyst temperature of 400 °C: only a two-fold increase of the ammonia formation rate was measured for Fe(111) and Fe(100). 700 K) used in industrial ammonia synthesis. molybdenum [198-200].15. so that hydrogenation of adsorbed atomic nitrogen could be expected to be the rate-determining step. Industrial ammonia synthesis in the homogeneous gas phase is not feasible because of the high dissociation energies for the initial steps. The only system which seems to be promising for industrial application is ruthenium promoted with rubidium on graphite as carrier (see Section Commercial Ruthenium Catalysts). The arguments are based on an analysis of activation energies from early measurements of the nitrogen adsorption kinetics on singly (Al2O3) promoted catalysts and on the results of experiments with supersonic molecular beams [162]. These propose participation of diatomic nitrogen. The latter finding is in agreement with the observations of ERTL's group [141]. the rate of formation at first page 12 of 77 . Reaction Mechanism on Non-Iron Catalysts. 112 for further literature). [163]. Some critics [161] of the above energy diagram question the low net activation barrier from the gas phase. A critical evaluation of the present knowledge of the mechanism of the synthesis reaction was made by SCHLÖGL [172]. Os Ir. further improvements of the catalyst are at least theoretically possible [165]. If not. [1322] of Südchemie. Rh and their alloys (no nitride formation under synthesis conditions) – Mn. Therefore. ca. [202]. however. This so-called state was interpreted as a bridge-bonded species with electron donation from the surface to the antibonding levels of N2. respectively) to the effect that the differences in surface orientation disappear [1316].03 to 0. High pressure promotes a high rate of ammonia formation. [179-190]. over a wide temperature range. High temperatures accelerate ammonia formation but imply a lower value of the equilibrium ammonia concentration and so a lower driving force. Ni. not so much on account of its activation barrier but rather because of the very unfavorable pre-exponential factor in its rate constant.3. Figure 6. This means predicting the technical dependence on operating variables of the rate of formation of ammonia in an integral catalyst volume element of a converter. Specific references: vanadium [193]. Tc. Other experiments [158] show that even the least active face Fe(110) becomes as active for the synthesis as Fe(111) after addition of alumina with subsequent annealing with oxygen and water vapor. uranium [194]. Kinetics Knowledge of the reaction kinetics is important for designing industrial ammonia synthesis reactors. quite apparent that the rate-controlling step would switch from dissociative nitrogen adsorption to hydrogenation of adsorbed atomic nitrogen species if the temperature were lowered sufficiently because of these differences in activation energy. as shown in Figure 6. The factor common with the iron catalyst is the structure sensitivity. The subsequent hydrogenation steps are energetically uphill. Mo. where the converter feed has always a certain ammonia content) was the fact that for zero ammonia content. has prompted some research groups. To avoid this. [126]. [210]. and the rate constants on pressure [191]. Potential energy diagrams for the various intermediate steps and species. the equation gives an infinitely high reaction rate. with lower temperature. 9). Figure 7. Additionally.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience increases with rising temperature but then goes through a maximum as the system approaches thermodynamic equilibrium (Fig.5 mm to avoid pore diffusion restriction and heat-transfer resistance in the catalyst particles. 215] and the type of promoter [216]. Classical Expressions for Ammonia Synthesis Kinetics. Values for the factors between 0. to attempt the generation of a kinetic expression for ammonia synthesis from a detailed microscopic model of the reaction mechanism consisting of a number of discrete steps at molecular and atomic level. [116]. a high ratio of bed length to bed diameter is chosen. [210-214]. In some investigations. which are significant in industrial ammonia synthesis. [227] and STOLTZE and NØRSKOW [228]. Surface science based ammonia kinetics [115]. It was also shown later that the original Equation (1) is a simplified form of a more general model which can be derived from the concept of energetically homogeneous (Langmuir – Hinshelwood adsorption isotherm) as well as for heterogeneous surfaces (Elovich-type isotherm). usually made with very fine particles. To exclude any poisoning by the synthesis gas. in accordance with an equilibrium depending on the temperature and the water vapor – hydrogen partial pressure ratio. The best results have been achieved so far by calculating the overall rate from the rate of dissociation of the adsorbed nitrogen and equilibrium constants for all other reaction steps. reversible poisoning of the adsorbing areas. in some circumstances may reflect a lack of attention to the influence of impurities in the gas. A discussion of kinetics proposed up to 1970. very pure reactants were prepared by decomposition of anhydrous ammonia [127]. Ammonia synthesis rate constant dependence on hydrogen – nitrogen ratio Equations for describing ammonia synthesis under industrial operating conditions must represent the influence of the temperature. the validity of which is tested experimentally by their possibility for extrapolation to other reaction conditions. the maximum rate shifts to a lower hydrogen – nitrogen ratio (Fig. It is based on the assumption that dissociative adsorption is the rate-determining step. The data show a sharp drop in reaction rate with declining temperature at H2 /N2 = 3 : 1 ratio in contrast to a H2 /N2 = 1 : 1 ratio. The reaction expressions discussed in the following model the intrinsic reaction on the catalyst surface. An important modification was made by TEMKIN [206] who incorporated hydrogen addition to the adsorbed nitrogen as a second rate-determining step (Eq. A problem with this equation was that the values (reaction order) were dependent on temperature. [220]. Figure 9 presents data obtained using a commercial iron catalyst. is a combination of these model equations. [229]. [205]. Dependence on the temperature at various pressures. used by NIELSEN et al.5 [207209] and 0. The first expression useful for engineering purposes was the Temkin – Pyzhev Equation (1) proposed in 1940 [204]. especially in the earlier literature. This may be attributed to a hindering effect of adsorbed hydrogen at low temperature [192].5 – 1. differential reactors were used. were set up and Arrhenius expressions for each single step with known or estimated values for all pre-factors and activation energies were formulated. Sometimes the catalyst is also diluted with inert material. and the equilibrium composition. Reaction rate for NH3 synthesis. acquired with all the modern spectroscopic techniques. Dependence on the ammonia concentration at various pressures. insofar as they have been based on measurements in the operating range of commercial interest. More serious (not so much for industrial purposes. the gas composition. that uses fugacities instead of partial pressures. Equation (3). A similar situation exists for the dependence of the reaction rate on the ratio of the hydrogen and nitrogen partial pressures. A critical review of the data and equations published up to 1959 appears in [203]. The applicability of a particular equation resulting from this concept also depends on the state of reduction of the catalyst [214. that hydrogen and ammonia have no significant influence on nitrogen adsorption. 8). Topsøe KMIR. If oxygen compounds are present in the synthesis gas. [213]. a large number of different kinetic equations have been suggested. Reaction rate for NH3 synthesis. [226]. are described by theoretically deduced formulas.75 [14]. [127]. emanating in each case from a proposed reaction mechanism or from empirical evaluations. Near the thermodynamical equilibrium. and that the kinetics of nitrogen adsorption and desorption can be described adequately by Elovich-type adsorption on an energetically inhomogeneous surface. To exclude maldistribution effects and back mixing. [219]. Equation (3) transforms into Equation (1) [191]. Since the beginning of commercial ammonia synthesis. they must also take into consideration the dependence of the ammonia formation rate on the concentration of catalyst poisons and the influence of mass-transfer resistances. The large amount of available data on nitrogen and hydrogen adsorption from ultrahigh-vacuum studies on clean iron surfaces. A similar equation is found in [217]. Experimental measurements. the pressure. developed by OZAKI et al. [211] were used. Contradictory data on the kinetics of ammonia synthesis. a simpler expression (Eq. Figure 8. ICI demonstrated that this equation gives a better fit with experimental data [218]. such as BOWKER et al. [128] are presently still viewed as an academic exercise rather than as a practical tool for engineering. [127-129]. An evaluation of present knowledge is given in [115]. 2) was often used [212]. Commonly the isothermal integral reactor is used with catalyst crushed to a size of 0. must be taken into account when developing rate equations (see also Section Catalyst Reduction). For many years this kinetic expression was the basic design equation for ammonia converters. free of mass-transfer restrictions. Figure 9. Moreover. Experimental Measurements of Reaction Kinetics. a number of modified equations were proposed and tested with existing experimental data and industrial plant results [221-224]. as in some laboratory measurements. The adsorption – desorption equilibria were treated with approximation of page 13 of 77 . can be found in [126]. 4). Transport Phenomena.4 429. the following phenomena characteristically hold true in industrial ammonia synthesis [232]: in the temperature range in which transport limitation is operative. it determines the operating temperature range. The high velocity of the gas passing through the converter creates sufficient turbulence to keep the film thickness rather small in relation to the catalyst grain size. 12 % inerts.5 483.300 10. As a result. [242-248]. [238-240]. [231]. SV = 15000 h–1. Exact numerical integration procedures.1 481. decrease with increasing catalyst particle size [225][233-235].2 506. The ratio of the actual reaction rate to the intrinsic reaction rate (absence of mass transport restriction) has been termed as pore effectiveness factor E. As can be seen from Table 13 the differences in temperature and ammonia concentration between the bulk gas stream and the external catalyst surface are small.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience competitive Langmuir-type adsorption and by evaluation of the partition functions for the gaseous and adsorbed species. they are means of checking whether our knowledge and understanding of the elementary steps correspond to the reality of catalysts under industrial synthesis conditions. [238]. For this reason the largest concentration gradient (with respect to the concentration in the bulk gas stream) is within the catalyst particles. 11) [126].960 7.5 456. and energy consumption for synthesis gas production and purification. Comparison of ammonia concentrations calculated from surface science kinetics with experimentally measured values [229]. It also indirectly influences the make-up gas purity requirement. as adopted by various research groups [126]. particle diameter.6 523.4.3 For the particle sizes used in industrial reactors (≤ 1. Table 13. E (3) 21.0 428.5 R) (2) and pore effectiveness factor. For practical application. For a given operating pressure and desired production. Comparison of the calculated ammonia yields with those determined experimentally on a commercial Topsøe KM1 catalyst showed an agreement within a factor better than 2. A simplification [241] can be used which can be integrated analytically when the ammonia kinetics are approximated by a pseudo-first-order reaction [234-236]. For pore diffusion resistances in reactions having moderate heat evolution. as well as the ammonia production per unit volume of catalyst. Mass and heat transfer effects at the external surface of catalyst particle Position in catalyst bed. One aspect is interparticle mass transfer and heat transfer through the stagnant film which surrounds the catalyst particles. it directly fixes vessel and exchanger design in the synthesis loop.7 Temperature at catalyst surface. The data from single-crystal experiments for potassium-promoted Fe(111) surface were used for the rate of the dissociative nitrogen adsorption. recycle gas flow.7 mm Mathematical models for calculating these effectiveness factors involve simultaneous differential equations. Figure 11. the major temperature difference is in the external gas film. while the catalyst particles themselves operate under approximately isothermal conditions. Rather. For example at the gas inlet to a TVA converter. intra particle transport of the reactants and ammonia to and from the active inner catalyst surface may be slower than the intrinsic reaction rate and therefore cannot be neglected. It also appears that the effects are oppositely directed and will partly compensate each other. the effective rate of formation of ammonia on 5. and refrigeration requirement. the pore effectiveness factor E is a function of the so-called Thiele modulus m [237]: The Thiele modulus m is defined by where deff = effective particle diameter Deff = effective diffusion coefficient of ammonia in the catalyst particle kv = reaction rate constant referred to a unit of particle volume tnh = hyperbolic tangent The practical application of kinetic equations to the mathematical calculation of ammonia synthesis converters is described in [218]. the apparent activation energy and reaction order.7-mm particles is only about a quarter of the rate measured on very much smaller grains (Fig. The overall reaction can in this way be considerably limited by ammonia diffusion through the pores within the catalysts [225]. vol% from inlet 0 20 40 60 80 100 NH3 concentration in bulk gas.4 MPa.5 mm). mol% 2. These conclusions have been confirmed by calculations results of another independent group [129]. °C 400. 4. Catalysts The ammonia synthesis catalyst may be viewed as the heart of an ammonia plant. are rather troublesome and time consuming even for a fast computer. Ammonia content in the bulk stream (1) and in the catalyst pores (at r = 0. which on account of the complex kinetics of ammonia synthesis cannot be solved analytically.288 6.045 7.778 9. Since the thermal conductivity of the iron catalysts is much higher than that of the synthesis gas. This is often used as a correction factor for the rate equation constants in the engineering design of ammonia converters.5 522.000 9. the apparent energy of activation falls to about half its value at low temperatures.536 Temperature in bulk gas. Figure 10 demonstrates these encouraging results. Figure 10.9 505. [230] To compete with the empirical models (Temkin and improved expressions) for the best fit to experimental data cannot be the prime objective of the microkinetic approaches. mol% 2. So it can be concluded that their combined influence on the reaction rate is negligible compared to inaccuracies of the experimental data for the intrinsic catalyst activity [127].500 NH3 concentration at catalyst surface. °C 401. and so the operating pressure. the economics of the total page 14 of 77 . according to the equation: For this case.1 455.500 4. Although the proportionate cost of catalysts compared to the total cost of a modern ammonia synthesis plant is negligible.361 10.500 5. and capital cost. the above kinetic equations have to be modified to make allowance for mass and energy transfer since the reaction rates actually observed in a commercial converter are lower. 2 R = 5.592 4. in 1910. In principle. so the magnetite-based catalyst proved suitable for industrial use. Se. in which up to 1911 more than 2500 different formulations were testet in more than 6500 runs. Insufficient pressure and abrasion resistance may lead to an excessive increase in converter pressure drop. Th V (V) Nb. Zn B. Mechanical Strength. The experiments were finally brought to an end in 1922 after a total of 22 000 tests. [254]. BASF had secured almost the total world supply [250]. denotes molecule. Pb. 4. Cs II Be. 3. Mg. may become more severe as temperature declines (see Fig. With catalysts that would function at a reaction temperature about 100 K lower. In modern single-train ammonia plants. 2. Hg. Tl Sn. (Mol. a ruthenium-based catalyst promoted with rubidium has found industrial application. denotes molecule. The ammonia synthesis catalyst problem has been more intensively studied than the catalysis of any other industrial reaction.and chlorine-containing catalyst poisons. For example. metals or metal alloys are suitable as ammonia catalysts. because the catalysts in these plants showed a markedly reduced life owing to the severe operating conditions. U Cr. C P. replacing. and reducing the catalyst had a considerable effect on the ammonia manufacturing cost. La. Ru. F. They tested almost all elements of the periodic table for their suitability as ammonia catalysts [82]. La. Ce. S. (Zr) Si. Nd. Li. or catalyst poisons Catalysts Promoters Poisons I Li. 12. In older highpressure plants (60 – 100 MPa). Sb. or the pressure may be reduced to 15 MPa (compare Tables 9 and 10). Os. for example. Ca. a reliable primary raw material source. I Figure 12. Ti. [251]. Te F. U. Cd. Co. (Rh). could be maintained for longer operating periods only with extremely pure synthesis gas. Ir.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience process are determined considerably by the performance of the ammonia catalyst [249]. RAMSAY and YOUNG used metallic iron for decomposing ammonia. Effectiveness of various elements as catalysts. Na. which was planned as the first industrial catalyst. NERNST [257] used elevated pressures of 5 – 7 MPa. Table 14. The iron catalysts promoted with aluminum and potassium oxides proved to be most serviceable [258]. At BASF A. K. is so scarce that. Cu.4. and Gas Purification). Effect of various elements or their oxides on the activity of iron catalysts in ammonia synthesis a) positive: Al. Pure iron showed noticeable initial activity which. Table 15. conducted the first catalytic synthesis experiments with iron at atmospheric pressure. The ammonia synthesis catalyst problem could be considered solved when the catalytic effectiveness of iron in conversion and its onstream life were successfully and substantially improved by adding reduction-resistant metal oxides [251] (Table 15). Th. In assessing the newly developed catalyst systems recommended for operation at very low temperatures (see Section Other Catalysts). Classical Iron Catalysts From the early days of ammonia production to the present. PERMAN [255]. The basic composition of iron catalysts is still very similar to that of the first catalyst developed by BASF. Bi O. Na. Recently. 5. Because of the high and increasing world demand for ammonia. Ni. W. Te. Ca. Rb. catalyst life was a big issue. C.1. for example. Ni. Br. Zr. Mn. Later. Zr b) negative: As. Ta VI (Cr). W. which is determined essentially by resistance to thermal degradation and to irreversible poisoning (see Section Catalyst Reduction). B) The rate of ammonia synthesis at 523 K. Development work in the United States from 1922 can be found in [259]. For example. it must be kept in mind that the effect of poisons. 80 kPa of total pressure . started a tremendous program. which corresponds to a 535 °C equilibrium temperature. or relating to the mechanism of opposing activation or inactivation in doubly promoted systems. a yield of 45 vol % ammonia can be expected with the same approach to equilibrium. Sr. Si page 15 of 77 . mol denotes mole). The highest possible insensitivity to oxygen. however. Os. Br. V. conventional iron catalysts achieve service lifetimes up to 14 years. Y. High catalyst activity at the lowest possible reaction temperatures in order to take advantage of the favorable thermodynamic equilibrium situation at low temperatures. Industrial catalysts for ammonia synthesis must satisfy the following requirements: 1. concerning the relationships between catalytic effectiveness and the strength of the nitrogen bond and reducibility. Tl. Er. U VII (Mn). Ba. the only catalysts that have been used have been iron catalysts promoted with nonreducible oxides. Cl. Mg. promoters. Mo. K. Rb. Metals or metal compounds for which the chemisorption energy of nitrogen is neither too high nor too low show the greatest effectiveness (Figs. Cl. The catalytic activity of iron was already known well before the advent of industrial ammonia synthesis. above all those from the transition-metal group [252] (Table 14). which may be present in even the very effectively purified synthesis gas of a modern process (see Sections Catalyst Poisons. 20 kPa of pN2 (Mol. mechanical disintegration during operation along with oxygen sensitivity thwarted the industrial application of uranium carbide catalysts [250]. as well as HABER and OØRDT [256]. Average commercial catalysts yield about 25 vol % ammonia when operating at 40 MPa (400 bar) and 480 °C catalyst end temperature. Mo. mol Figure 13. Be. Pb. Re VIII Fe. Ti. Ba. 13). 23). 4. From these experiments came a series of technical findings. Co. and so to a premature plant shutdown. Nb. W. J. oxygen compounds. (Ir) Cd. osmium. Pd. Bi. Mo. as a precautionary measure for this option. S. Ta. B. the necessary downtime for removing. Zn c) doubtful: Au. Cr. The rate constants of ammonia decomposition (A) on and the ammonia synthesis capacities (B) of metals as a function of denotes mole) . Catalytic activity of carbon-supported metals promoted by metallic potassium as a function of A) The rate of isotopic equilibration of N2 at 623 K. Sn. Y. Sr III Ce and rare earths Al. Sm. Ce and rare earths IV (Ti). Long life. Pt. As. P. MITTASCH et al. [253]. calcium was added as the third activator. 10 0.59 0.5 0.35 0.7 0.0 0.47 0.57 0.84 1. 15) generally shows a peak at 25 – 50 nm that is formed on reduction of the wustite phase [126].00 0.10 2. the catalysts contain varying quantities of the oxides of aluminum.10 – 0.4 70.10 33.58 – 0. as has Grande Paroisse [271]. Pore size distribution of a commercial catalyst after reduction at various temperatures [130] The novel AmoMax catalyst [1321]. careful control of the manufacturing process. the highest ammonia yields are observed with an Fe(II) – Fe (III) ratio of 0.2 66.0 38. mm 1 2 normal 2 2 HT 68. uranium [307]. individual catalyst manufacturers now offer several catalyst types in various particle size distributions.7 Cr2O3 6.58 35.3 3.8 11 normal (1964) 66. the spaces between them form an interconnecting system of pores.43 4.30 0.94 3.53 2.17 0. and silicon as promoters. Examples of commercial ammonia catalysts from the years 1964 – 1966.63 0.85 2.11 The degree of oxidation of industrial catalysts has a considerable influence on their catalytic properties.65 0.16 3.66 4.55 2.65 0.40 1.34 38. the so-called inner surface.26 0.27 38. type Fe total FeO Fe2O3 Al2O3 MgO SiO2 CaO K2O Other Particle size. Origin. Thus.42 0.47 59.77 0.54 – 0. Freshly reduced commercial iron catalysts that contain aluminum.9 4 5 – 11 2.6. especially the melting conditions.40 0. Reference [267] describes cesium-containing catalysts. in oxidic and prereduced states.22 52.95 3.9 g/cm3.8 0.73 3.52 1 – – – – – – – – – 2–4 4 – 10 6 – 10 6 – 10 6 – 10 2–4 kg/L 2.2 11 normal (1964) 11 (1966) 69. The principal component of oxidic catalysts is more or less stoichiometric magnetite.7 11 (1966) 68.75 2.86 2 prereduced 88.85 1. The surface of the walls of the pores.85 2.5 23.29 0.3 0.27 1. The high-purity gas of modern processes and the trend to lower synthesis pressures especially favor the development of more active and easily reducible types of catalysts.01 – 3–9 5 – 10 5/5 2–4 2–4 2–4 2.80 2. magnesium.38 2.35 32.07 57. the pore distribution curve (Fig. about the degree of oxidation of stoichiometrically composed magnetite [260-262] (Fig. Auger electron spectroscopic (AES) measurements on an industrial catalyst (BASF S 6-10) have shown that a significant enrichment of the promoters into the surface results using the unreduced as well page 16 of 77 .35 3.32 – 0. For industrial catalysts.0 10 70.43 1. ammonia plant operating conditions and types of converters (Section Formation of Ammonia in the Converter) can differ greatly one from another.1 3 4 71.00 2.1.09 – 0.58 1.8 – 4.80 0.94 2.40 2.09 0. Nature of the Surface of Commercial Iron Catalysts.3 11 prereduced 84. at some sacrifice in temperature stability and resistance to poisons.35 2.9 7 prereduced 90.45 3. beryllium [264]. Values for the composition in weight %.70 – 0. [1322] of Südchemie is iron-based but uses wustite instead of magnetite—which previously was considered undispensible for the production of industrial ammonia catalysts—has an improved pore structure and higher specific surface area. with an apparent density of 4. Bulk density.2 22. 14). The composition of the outermost atomic layers of the pore walls deviates considerably from the overall average concentrations.28 0.5 66.4.18 2. Table 16. which determine the oxygen content.95 0.08 1. potassium.20 56. Figure 15. or platinum [266]. amounts to about 15 m2/g. ICI [270] has developed a cobalt-containing catalyst.30 62.62 1.57 0.85 3.64 0.37 0.67 0.14 0.73 2.0 1. conditions of preparation (Section Catalyst-Precursor Manufacture).9 23.5 11 HT 66.60 2.4 8 9 68.70 2.55 0.1 cm3/g.35 1.85 2.15 – 1.61 2. MITTASCH in 1909 established that catalysts manufactured by reducing a magnetite phase were superior to those prepared from other oxides.80 0.12 3 1.90 32.58 0.20 0.18 2.13 1.8 0. Catalysts patented by Lummus [268] and Ammonia Casale [269] contain cerium as additional promoter. 4. accordingly.81 2.86 1.50 0.70 54.94 0. the pores represent 44 – 46 % of the volume of a catalyst granule [14].Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Modern catalysts additionally contain other promoters that were present in the older catalysts only as natural impurities from the raw materials. Onstream life and performance were enhanced considerably by optimizing the component ratios (Section Composition).1.46 0.54 0.18 0.30 0.70 39. vanadium [265]. potassium.3 66.42 3. To obtain optimal catalyst composition. which transforms after reduction into the catalytically active form of -iron.37 2.65 0.0 1.96 3.6 5 6 (1964) 6 (1966) 71. Dependence of the ammonia yield on the degree of oxidation of the iron in the unreduced catalyst In general.9 12 13 14 33. Figure 14. The pore volume measures 0.90 31.95 56. The numbers at the beginning of the lines are keyed to the catalyst origin.71 2.5 69.30 2. and catalyst particle size and form (Section Particle Size and Shape). is necessary.50 3.55 0.97 3.15 2.28 0.23 0.1 1.30 32.97 – 1.25 0. Composition Table 16 gives a composition survey of commercial ammonia catalysts in the years 1964 – 1966. Besides a maximum at a pore radius of about 10 nm that originates on reduction of the Fe3O4 (magnetite) phase of the nonporous oxidic catalyst. Patents recommend adding sodium [263]. calcium.56 0.66 0.43 0.14 3.12 35.27 49.50 – 0.0 68.9 36.55 – 0.4 MnO 0.43 0.3 4 prereduced 90.68 1. [130].0 65.60 0.56 0. Fe3O4 .6 60.57 0.89 – 4.13 – 0.05 Cr2O3 1.15 2.0 0. To some extent even today.62 3.70 2.5 – 0.91 60.22 58.80 2. and calcium oxides as basic promoters consist of approximately 30-nm primary crystallites.67 3. 2 % SiO2 (Fig. classical Casale plants. In the alkali-metal series. The ICI catalyst with composition 5. 16 B). Numerical values in atomic % [119] Fe K Al Ca O Volume composition 40.0 % Al2O3 .5 – 4.2 Surface composition before reduction 8.5 0. The effect of a given promoter depends on concentration and on the type of promoter combination and the operating conditions. Less active but more poison. and in the reduction process a further subdivision of the slabs into even smaller platelets might occur.8 % K2O. a correlation exists between the distribution of the potassium and that of aluminum and/or silicon. Promoter oxides that are reduced to the metal during the activation process and form an alloy with the iron (see also Section Other Catalysts) are a special group.0 % Al2O3.0 The aluminum oxide promoter exists partly in the form of larger crystallites and.8 % K2O. for example. [132-138]. Auger spectroscopic investigations on reduced BASF and Topsøe catalysts reveal large local differences in composition [119]. 0 – 1. [289] and Ammonia Casale [269] cerium-containing catalysts. into separate regions. An industrial catalyst for operating temperatures up to 550 °C is stabilized against deterioration by 2 – 5 % V2O5 besides 3. 0.7 % K2O [265]. such as Al2O3. Silicon dioxide additions shift the optimum potassium oxide concentration to higher values [275]. they reduce the inner surface or lower the temperature stability and the resistance to oxygen-containing catalyst poisons [275]. the promoter effect increases with increasing atomic radius. 10 000 – 20 000 m3 m–3 h–1 (STP)].1 10. [262]. moreover. and 0 – 1. Calcium oxide segregates. 1 % SiO2 ) catalyst changes with varying promoter concentration and operating conditions (Fig.35 2. essentially at the grain boundaries. [283]. 2. apparently homogeneous regions that have originated from reduction of Fe3O4 crystallites alternate with nonhomogeneous regions that are formed by the reduction of FeO crystals or consist of amorphous phases [130]. 0. the optimum composition shifts to higher Al2O3 and SiO2 and lower K2O and CaO concentrations. [277]. the optimal activity corresponds to a composition of 2.7 53. and with highly purified synthesis gas. According to [130]. 1 % K2O. For example.14 % SiO2 and 1. All published experience appears to demonstrate that it is not possible to combine in a catalyst high thermal stability with easy reducibility and high activity at low temperatures. is relatively homogeneously distributed over the iron area of the surface.0 17. lower the rate of reduction [124]. [131]. Early work [296] referred to the rough parallels between the reducibility and the thermal stability of catalysts.5 – 3. Table 17. [279]. [124]. such as the alkali oxides. An older Norsk Hydro catalyst.0 % MgO. According to [119] the potassium. Influence of the Promoters. 0. cobalt is of special interest [282]. such as CaFe3O5 and FeO [293-295]. catalyst end temperatures of 520 – 530 °C maximum. However. [290]. is closely associated with their solubilities in the iron oxide matrix of the unreduced catalyst or with the capability of the regular crystallizing magnetite to form solid solutions with iron – aluminum spinels [14].5 % CaO. [291]. Figure 16.5 % CaO. as could be deduced from the results of prior investigations [126].3 – 5.4 % MgO. in the form of a K + O adsorbed layer. 16 C) or poisoned with 2000 cm3 water vapor per cubic meter of gas at 550 °C (Fig.5 – 4.2 % K2O. Under normal operating conditions [14 – 45 MPa. 1 % CaO. although with low concentration [119]. Macroscopic particles in the reduced catalyst are confined by fracture lines running through a system of blocks consisting of stacks of slabs in parallel orientation.6 % magnesium oxide were recommended for older plants. and 0. and Er2O3 [281]. Reducing the synthesis pressure and/or the synthesis temperature should enable application of the Lummus [268]. and the destructive effect with decreasing atomic radius [277].Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience as the reduced catalyst [119] (see Table 17).0 – 2. is substantially more active than the conventional cobalt-free catalysts [270].2 % CoO. and 1. A graphic picture is conveyed in [277] of how the activity of a quadruply promoted (4 % Al2O3 . D) Poisoning with water at increasing promoter concentrations For ammonia plants operating at pressures up to 35 MPa (350 bar). Magnesium oxide addition now shows a favorable effect.0 27. C) Overheating to 700 °C with increasing promoter concentrations. such as Sm2O3 [280]. Large. 16). B) Concentration of promoters. [288]. which operated at up to 80 MPa (800 bar) pressure and in which catalyst end temperature reached 650 °C (see for example [14] ). 0. 2.5 – 1. [276].8 % SiO2.6 36. Composition by volume of an industrial ammonia catalyst in comparison to the surface composition before and after reduction (an approximately 10–4 cm2 size typical surface). about 1 wt % of the alumina also remains statistically distributed in the form of FeAl2O4 molecular groups built into the -iron lattice of the reduced catalyst [139].8 – 1.0 Surface composition after reduction 11. Ho2O3.0 – 3. besides the usual additives. still higher V2O5 contents are recommended.9 % Al2O3. 0. 380 – 550 °C. the Bulgarian catalyst K-31 contains 3. Mechanism of the Promoter Effect. a multitude of structural and electronic promoters has been investigated. page 17 of 77 . 0.and temperature-resistant catalysts containing up to 3. 0. Greater differences in reducibility have also been observed in commercial catalysts with similar chemical composition [293] and in connection with particular oxide phases. In striving to improve the activity or stability of iron catalysts. and a natural content of about 0. Hence it may be advantageous to use a combination of active and thermally-resistant catalysts in the same converter. 2. [130]. Extensive studies in the last decade have provided a more refined picture of the morphology of the active catalyst (reduced state) and its precursor (oxidic state).7 4. [297-299]. Dy2O3. For higher operating temperatures.3 % CaO.0 % Al2O3. Magnesium oxide-activated iron.0 1.5 % SiO2. produce a high inner surface during reduction and stabilize it under thermal stress by restraining iron crystallite growth [158]. enhance the specific activity (based on a unit surface) of iron – alumina catalysts. [284-287].57 % K2O. 16 D). The ability of the various metal oxides to create a high specific surface decreases in the following order [274]: So-called electronic promoters. This structure is already preformed in the preparation of the catalyst precursor.0 41. The structural promoters. [275]. remainder Fe3O4. said to be thermally stable up to 650 °C. such as Al2O3. Promoters can be arranged in different groups according to the specific action of the metal oxides: Structural stabilizers.5 % Al2O3 . especially the reaction temperature and the synthesis gas purity [265]. [131]. among them rare-earth oxides [278].2 – 0. A review is given in [124]. 1. needs a higher reduction temperature than aluminum oxide-promoted iron catalyst [292]. contained 1. The action of the so-called structural promoters (stabilizers). Dependence of the catalyst activity on various factors A) Concentration of impurities. The free iron surface of the reduced BASF catalyst [119] and Topsøe catalyst KM-I [131] comprises only a fraction of the total surface.” separating neighboring platelets and thus providing voids for the interconnection of the pore system.2 % MgO. the preferred catalysts contain 2.5 % CaO. If before the test the catalyst is overheated at 700 °C for 72 h (Fig.2 % K2O. [140]. With methods such as scanning transmission electron microscopy (TEM) and electron microdiffraction a textural hierarchy has been modeled. Adding magnesium oxide decreases the catalyst performance.7 40. [273]. Raising or lowering the concentration of a particular oxide causes a reduction in activity.0 4. This texture is stabilized by structural promoters. 2.9 % CaO. After reduction. There is also evidence that the basal plane of many platelets has the Fe(111) orientation [122]. such as Al2O3. probably as a mixture of the silicate and ferrite [121]. which act as spacers and “glue. Among those in use industrially. Changes in the potassium and aluminum oxide concentrations have an especially strong influence.5 % SiO2 [262]. 2. The effect of the promoters on the rate of reduction and the temperature required for reducing the iron oxide phase is also significant in industrial practice. covers about 20 – 50 % of the iron surface. Higher alumina contents lead to separation into two phases. with decreasing particle size. In the reduced catalysts. more potassium is made available for activating the iron [218]. forms poorly reducible intermediate layers of calcium ferrite and silicates [130]. The aluminum probably remains partly in the iron crystallite in the form of very small FeAl2O4 areas statistically distributed over the lattice [139]. with formation of water vapor and release of electrons 2. the pressure drop and the risk of destructive fluidization of the catalyst increase (Fig. it separates at the grain boundaries as CaFe3O5 (at very rapid cooling rates) [293] and. gas composition (vol %): N2 20. This will increase the bond strength to the metal and further weaken the N – N bond. in the presence of SiO2. 4. from [14].06. According to [302]. the recrystallization-promoting effect of higher K2O concentrations may be attributed to a lowering of the portion of alumina dissolved in the magnetite phase [308]. The other effect consists of lowering the adsorption energy of ammonia. One mechanism is the lowering of the activation energy for the dissociative adsorption of nitrogen [111]. In the reduced catalysts.55. there exists a close connection with the mechanism of the reduction which consists schematically of the following partial steps [303]: 1.1 MPa. Ar 5. [306] or in small islands [131]. Depth of the catalyst bed. which creates a M – – K + dipole. [137] and so possibly prevents sintering together at high operating temperatures. [165].2. [1282]. [119]. about 30 nm. [294]. SV = 12 000 m3 m–3 h–1 (STP). reaction temperature 450 °C For processes operating at pressures of 25 – 45 MPa (250 – 450 bar) and at space velocities of 8000 – 20 000 m3 m–3 h–1 (STP) a grain size of 6 – 10 mm is page 18 of 77 . 316]. i. [313]. potassium oxide ought to improve the catalyst performance less at low than at higher operating pressures [210 p. the diffusion rate of the iron ions in the magnetite lattice is low. and in part it may also contribute to stabilizing the Fe(111) faces [125]. The explanation is based on an electrostatic model [115].and iron-rich regions were found [119]. as can be seen from a further reduction of the N – N stretching frequency [314]. This “patchy” monolayer of alumina acts like a “spacer” between iron atoms of neighboring crystallites and prevents sintering by means of a “skin” effect [140]. In the unreduced catalysts. It tends to stretch the magnetite lattice [112].5 % FeAl2O4 . Pressure drop in the catalyst bed for various catalyst particle size ranges. One may also presume that by partial neutralization of the “acid” components by calcium. however. (See also [112]. CH4 10 Figure 18. hence.59. 7 m. has been proved [308]. Since potassium oxide prevents the formation of solid solutions between alumina and magnetite to a certain extent [316]. [139]. Phase-boundary reaction of hydrogen with oxygen ions of the magnetite lattice.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience The solid solutions of Fe3O4 and the spinel FeAl2O4 have a miscibility gap below 850 °C [300]. it is desirable to use the finest possible particle. In this way. Particle Size and Shape The choice of particle size and shape of commercial ammonia catalysts is determined mainly by two factors: 1. partly as KOH with iron [1282].. an analogous effect is to be expected with Cr2O3.) Insofar as small crystals of nonreducible oxides dispersed on the internal interfaces of the basic structural units (platelets) will stabilize the active catalyst surface Fe (111). [302]. [140]. In the main. reaction pressure 27.1. 25 MPa. it is distributed over the iron phase. which. Hence. But the assumption that this will happen on a molecular level on each basic structural unit is not true. the stabilizer function of alumina reduces to paracrystalline lattice defects. Calcium oxide. [156]. The negative effect of K2O concentrations higher than about 0. Formation of metallic iron nuclei by combination the electrons with Fe2+ ions 3. Diffusion of Fe2+ ions and electrons to the nuclei and growth of the nuclei to iron crystals of various size A more detailed discussion of the reduction mechanism is given in Section Catalyst Reduction. H2 60. Thus the primary function of alumina is to prevent sintering by acting as a spacer. small iron crystallites. referred to iron. Catalyst performance 2. Remarks in [309] and [317] reveal another possible interpretation: the K2O located at the phase boundary surface and not bound to acid or amphoteric oxides may be converted by water vapor concentrations over 10–2 ppm in the synthesis gas into potassium hydroxide or by hydrogen into potassium and potassium hydroxide. [308]. nucleus formation proceeds rapidly relative to crystal growth. whereby only the portion dissolved in the magnetite phase appears to be responsible for the specific promoter action of alumina [132]. Potassium is likewise scarcely soluble in magnetite because of its ionic size [112]. Influence of the particle size on the ammonia production (BASF catalyst). [130]. at 500 °C. 17). The appearance of a K2Fe22O34 phase besides an unidentified phase. Pressure drop From the standpoint of space – time yield. separate potassium. is about 1 – 2 mm (Fig. the paracrystallinity hypothesis will probably hold true. 18). NH3 2.4. during reduction. Therefore. etc. The unique texture and anisotropy of the ammonia catalyst is a thermodynamically metastable state. While doing this it reacts with the more or less homogeneously distributed aluminum (silicon) compounds. however. the emerging K 2O migrates to the iron crystallite surface. [305]. 234].e. The promoting effect of potassium seems to be based on two factors which probably act simultaneously. there is a considerable electronic charge transfer to the substrate. An extensive review on promoters can be found in [116]. potassium associates partly with the alumina in the surface. According to [309]. the solubility limit in the magnetite mixed crystal is a maximum of 7. [133]. With increasing potassium concentration. [119]. [310]. In the presence of dissolved aluminum ions. Impurity stabilization (structural promotion) kinetically prevents the transformation of platelet iron into isotropic crystals by Ostwald ripening [122]. the potassium exists as a K + O adsorption layer that covers about 20 – 50 % of the iron surface [119]. practically speaking. A nitrogen molecule adsorbed near such a site will experience a more pronounced back-bonding effect from the metal to its antibonding orbitals. It was found that the enhancement of the catalysts' specific activity by potassium oxide is accompanied by a decrease in the electron work function [310-312]. which also acts as a structural promoter [130] has a limited solubility in magnetite. which avoids hindering of nitrogen adsorption by blocking (poisoning) of the catalyst surface by adsorbed ammonia molecules [109. p. As indicated by the strong decrease in the work function upon potassium adsorption. [121]. Figure 17. form with correspondingly large specific surfaces. Another theory is based on the observation that during reduction part of the alumina precipitates with other promoters into the surface of the iron crystallite in a molecularly dispersed distribution [135]. Sc2O3 . where an FeAl2O4 molecule occupies seven Fe lattice positions [112]. 3 % Al2O3. this manifests itself by the increasing size of the average iron crystallite or the decreasing specific surface in the reduced catalyst [275]. at not too high a temperature.58 % has not been explained unequivocally [158]. [301]. According to [136]. which would exist in molten form at operating conditions [309]. [305]. it segregates between the iron crystallites [120]. on cooling the magnetite melt. [315]. According to this concept. the catalyst particle ought to be 2 – 20 times as long as it is broad. with subsequent calcining and reduction [342-345]. however. 8 – 15 mm or 14 – 20 mm. The reduction step is very important for catalyst performance. only a surface layer of the catalyst grains.5 – 4. After water is added and.2 1. With magnesium oxide as support.3. In 1996 the prices of commercial ammonia catalysts were about 2 $/lb (3. The pellets subsequently are dried and sintered in an inert atmosphere at higher temperatures (about 1350 °C). as nominal size.5 4. have not gained industrial importance. The so-called Mont Cenis process employed such catalysts [341].5 – 3 1 – 1. for example. This is activated in the well-known manner by reduction-resistant metal oxides.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience preferred.5 – 8 3–6 1. a binding agent. together with the additives. Repeatedly described as a means of manufacture. claims catalysts manufactured by impregnating the reduced catalysts with cerium salts. The first effective catalysts were made by the oxygen-melt process. On the other hand. and its electrical conductivity allows the use of economical electrical melting processes. 19) [401]. To allow for the influence of various particle shapes and size distributions within a defined sieve fraction. they are among the least expensive catalysts. Therefore. which operate at comparatively low gas velocities [318]. 52). which. has a more favorable effective activity for the individual particle and for radial intermixing of mass and heat in an industrial converter [327] than a more cubic or spherical shape. mm mm % 6 – 10 4. the inner surface decreases from 11 – 16 m2/g to 3 – 8 m2/g [326]. At the high reaction rate typical for the converter inlet layer. Complex iron cyanides were decomposed thermally in the presence of hydrogen. allow the use of small granulations (1. [320]. Table 18. for example. Swedish charcoal iron). together with the activators. Table 18 shows the relation between the catalyst size classification. mechanical treatment [351].7 2. because this proceeds only by slow diffusion through the pore system.5 5. alternating-field heating [352] has been attempted also.1. The cooled melt is ground to the proper granulation and reduced with hydrogen – nitrogen mixtures. is equal to the diameter of a sphere with a volume equal to the (average) volume of the particles. The purest possible iron (e. The application of macroporous catalysts ought to be especially useful for very low synthesis pressures and in plants in which large catalyst particles must be used for reasons of low pressure drop. As an example. and the percentage saving in catalyst or converter volume relative to the 6 – 10 mm standard size [288]. it may be advantageous to use coarse particles for suppressing the reaction. or high-frequency. Granulation and sintering techniques have been used for the preparation of shaped macroporous iron catalysts. Effect of catalyst size on catalyst volume Catalyst size Approximate Relative classification. preferably 5 – 10 mm long and 1 – 2 mm thick (broad). usually in situ. [353]. have so far been unsuccessful [319]. magnetite is melted with the additives at high temperature (> 1600 °C) and the melt is cooled.4. The hydrodecomposition proceeds via the carbide and nitride. In fact this is only the catalyst precursor which is transformed into the active catalyst composed of -iron and promoters by reduction with synthesis gas. from aqueous solutions of the metal salts. finally to iron. page 19 of 77 . such as bentonite [337]. [346].5 7. In catalyst zones in which the ammonia formation rate is so high that the allowable temperature limits are exceeded. such as cerium nitrate [269]. deff. in lay-out calculations it is customary to employ an effective particle diameter.2 – 1. Radial-flow converters and the horizontal crossflow Kellog converter (Fig. [350].3 100 92 – 95 88 – 90 80 – 82 77 – 79 An irregular grain shape. Catalyst-Precursor Manufacture The term “ammonia catalyst” commonly refers to the oxidic form consisting of magnetite and oxidic promoters. and is the average ratio of the particle surface to the surface of a sphere of equal volume. are used only in plants where the lowest possible pressure drop is essential because of very high gas velocities. Fe3O4 . [340]. Improving the performance and life of industrial catalysts by radioactivity and X-rays [348].23 €/kg) for prereduced. participates in the reaction. A process developed by Farbenfabriken (formerly Friedrich Bayer) had only local significance. is coprecipitation of the catalyst components. For example. The diameter deff is defined as the ratio of equivalent diameter and a form factor .. about 1 – 2 mm thick. Figure 19. synthetic magnetite. is the thermodynamically stable oxide phase of iron [82]. the powder is pelletized. although only in the scientific literature. As an example. was melted electrically or in electric arc furnaces [261]. According to a patent by Chemie Linz AG [328].5 – 3 or 2 – 4 mm) with optimal use of the converter volume.58 €/kg) for oxidic and about 5. very small (under 10 nm) iron particles are obtained with high specific iron surfaces [345].5 $/lb (10.0 – 2. A United Kingdom patent [347]. treatment with ultrasound [349]. The effect is very significant. the equivalent particle diameter. Larger granulations. Impregnating the pore surface of prereduced passivated catalysts is a possibility for incorporating promoters into iron ammonia catalysts. Slow ammonia diffusion inhibits the rate of reaction.5 – 3 mm and distinctly superior to KM I 6 – 10 mm [339]. Comparison of the Ammonia Casale spherical catalyst and irregularly shaped catalyst [288] The advantages of regular catalyst shapes and the need to compensate for the above-described negative effects of larger grain size by a system of macropores in the oxidic and reduced catalyst stimulated various attempts to manufacture shaped. or a promoter salt. if the particle size increases from about 1 to 8 mm. 4. if required. broken. large grain size retards transport of the ammonia from the particle interior into the bulk gas stream. which were explored especially in the Soviet Union. was burned to Fe3O4 in a stream of oxygen. for example with a shape factor of 1. Two effects cause the low production capacity of coarse-grained catalyst: first. particle diameter. similar to those obtained by exchange of magnesium ions by iron ions in the surfaces of magnesium hydroxy carbonate crystals [345].g. the performance of the macroporous Topsøe catalyst KMG 6 mm at 5 MPa synthesis pressure is said to be at least equivalent to KM I 1.5 – 6. macroporous catalysts. Magnetite leads to especially efficient catalysts. with a shape factor close to one. by way of example. magnetite. The superiority of the catalyst manufacturing processes that use a molten iron oxide stage is mainly due to the fact that above 1000 °C in air. this induces a severe recrystallization [14]. less frequently. Various manufacturing techniques have been proposed [329338]. equivalent catalyst volume. and ground to powder. Fluidized-bed processes.5. The second effect is a consequence of the fact that a single catalyst grain in the oxidic state is reduced from the outside to the interior of the particle [321]: the water vapor produced in the grain interior by reduction comes into contact with already reduced catalyst on its way to the particle outer surface. This process was largely replaced by melt processes in which natural or. regular shapes have the advantages of greater abrasion resistance and lower pressure drop (see Fig. The FeO (wustite) phase is reduced faster and at lower temperatures than the Fe3O4 (magnetite) phase [293]. a summary is given in [124].) in magnetite and the rate of adjustment to the new phase equilibrium decline. Therefore. 65 kJ/mol) and in the final stage by diffusion processes involving hydrogen and water on the reaction site: 2. The progress of the reduction is controlled according to the catalyst temperature and the water concentration by means of the synthesis gas flow. although a slow induction period starts somewhat lower. Very slow cooling results in a more cubic chip. as far as possible. 20) [249]. Figure 20. Catalyst Reduction The reduction of oxidic catalyst is generally effected with synthesis gas. the reduction temperature is raised stepwise by using the exothermic heat of ammonia formation. slow cooling is avoided. 25 – 30 MPa in high-pressure plants) to hold the exothermic formation of ammonia under better control during the reduction. Catalysts conventionally employed in medium-pressure plants may be reduced from about 340 to 390 °C. and at not too high pressures (7 – 12 MPa in low-pressure. The walls of these furnaces should consist of a weakly basic tamping material indifferent to the melt. lime.1. 21). even for short melt times. On the atomic scale the reaction is controlled by two processes: 1. nitrogen content in the hydrogen in the range 0 – 100 %) on the production capacity of an ammonia catalyst has been investigated [368]. the appearance of certain phases. highly catalytically active form of -iron. temperatures above 440 °C are required to complete the reduction. which can be lowered with magnetic separators [354].4. Another important factor in catalyst manufacture is the melt cooling rate. Commonly. which is affected by the design and dimensions of the ingot molds. The precipitation of further wustite nuclei on the magnetite/wustite reaction interface seems to be effected by ion/electron diffusion processes rather than by direct contact of magnetite with hydrogen [367]: page 20 of 77 . To ensure maximum effectiveness of the catalyst. high surface area. by rapid cooling of the melt the activator oxide distribution can be frozen in a condition corresponding to that of a higher temperature [358]. High temperature and high water vapor partial pressure markedly accelerate premature catalyst aging by recrystallization. natural magnetite. sharp-edged chip after crushing. Melting is accomplished in electrical resistance or induction furnaces (arc furnaces in the past) operating at 1600 – 2000 °C. there may exist relationships between the melt cooling rate. A homogeneous distribution of the promoters in the magnetite melt and a degree of oxidation at or under that of stoichiometric magnetite should be obtained. Generally. The promoters. As a rough guideline. are not reduced [14]. a defined reduction procedure must be followed. According to [293]. 4. The gas/solid reaction between magnetite and hydrogen has been studied in great detail by rate measurements. where they are reduced and precipitated as iron nuclei. the water content of the gas effluent from the catalyst should not exceed 2 – 3 g/m3 (STP). Ammonia catalyst manufacture The raw materials — usually. the manufacturers hold these in strict secrecy. Metallic iron is formed from wustite by direct chemical reaction controlled in the initial phase by the reaction rate (activation energy ca. the reduction should be carried out at high gas velocities [about 5000 – 15 000 m3 m–3 h–1 (STP)]. with the exception of cobalt. It is claimed to be advantageous to bring the promoters into the melt as common chemical compounds that are isomorphous with magnetite [357]. This gradient leads to a rapid diffusion of iron(II) ions from magnetite through wustite to the chemical reaction interface. Many ores have too high a content of free or bound silica. 21).Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Operating conditions in the individual manufacturing steps — proportioning and mixing the raw materials. The chemical reaction creates an iron(II) ion concentration gradient in the solid. Since the melt is held in constant turbulent motion by the magnetic field produced in the primary coil. and X-ray diffraction [363-366]. In practice. cooling. long reduction times are needed at low temperatures and low pressures with a consequential loss of production. This is made possible by the structural defects of the wustite. both the solubility of the activator oxides (see Mechanism of the Promoter Effect. The influence of the hydrogen/nitrogen ratio during reduction on the catalyst performance after reduction is discussed in [362]. melting. plant cost and power consumption are higher. and reduction — influence the quality of the finished catalyst (Fig. Above all. With falling temperature. In comparison to the most frequently used resistance furnaces. it is important to hold the partial pressure of the resulting water vapor as low as possible and to insure that the water vapor does not come into contact with regions that have already been reduced. which is also an activating component of the catalyst [355]. and the reducibility of the catalysts. it is well mixed. and alumina — must. In practice. Figure 21. The influence of the reduction conditions (gas flow rate. Under these conditions. The minimum temperatures necessary for reduction are somewhat different for the various catalyst types. Quenching or fast cooling in thin sheets leads to a less abrasion-resistant. The magnetite is converted into a highly porous.and coarse-grained catalyst appear in [361]. Effect of the melting temperature and rate of cooling of the melt on the activity of ammonia catalysts a) Melt overheated to 3500 °C (rapid cooling). This is said to be promoted by initially overheating to temperatures up to 3500 °C [356] (Fig. Their good temperature control permits accurate adjustment of the degree of oxidation. depending on its size and operating pressure. the rather considerable differences in the reduction rates of commercial catalysts with similar compositions may be attributed to differences in manufacturing methods or operating conditions. potash. The reducibility of industrial catalysts is dependent on both the combination of promoters and the degree of oxidation. According to [293]. When the reduction of the oxidic catalyst is carried out in the production plants. at the lowest temperatures sufficient for complete reduction. b) Melt temperature 1800 – 2000 °C (rapid cooling). but with inferior catalyst quality (Fig. Therefore. be free of catalyst poisons (see Section Catalyst Poisons). Directions for reduction in multibed converters that combine fine. temperature in the range 300 – 600 °C. a synthesis converter with a fresh load of oxidic catalyst attains its full production capacity in 4 – 10 d.4. microscopy. such as magnesium oxide. c) Melt temperature 1800 – 2000 °C (slow cooling) Induction furnaces are optimal for the melting operation. Some older publications deal with the influence of catalyst granulation on the optimum reducing conditions [360]. crushing or if necessary grinding and preforming. 28 0. Figure 23. have gained a considerable market share. Oxygen compounds have a reversible effect on iron catalysts at not too high temperatures. For gaseous poisons in the synthesis gas. [376] proposed multiplying the rate equation by a correction factor 1 – . as well as the roughly 20 % lower bulk weight (which reduces the design loadings and costs of the internals of large converter units). [124]. reduction. °C 400 425 450 475 500 Fe + Al2O3 0. This concept was developed for single-crystal magnetite grains. commonly referred to as poisons.63 0. presumably because as soon as they enter the catalyst bed they rapidly and completely transform into H2O [14]. including a discussion of the most important literature in this field. a distinction can be made between permanent poisons that cause irreversible damage to the catalyst and temporary poisons which lower the activity while present in the synthesis gas. b) Core and shell structure of catalyst. for example.24 0. When removed from the hydrogen – nitrogen atmosphere at low to moderate pressure in the producing furnace. can be found in [122]. On account of the simplifications involved this model should be regarded as a formalistic approach to describe the reaction kinetics rather than a mechanistic approximation. including industrial catalysts.1 – 0. General measures to avoid this sort of contamination include selecting a rather pure magnetite. The somewhat inferior mechanical strength is a disadvantage that requires special care when a charge is being loaded into the converter. Figure 22 (b) shows the wustite/magnetite interface as observed by electron microscopy [124]. Prereduced catalysts have the full pore structure of active catalysts. such as reducing the risk of damaging the catalyst during activation by too high a local concentration of water. Catalyst Poisons The activity of an ammonia synthesis catalyst may be lowered by certain substances.74 0.12 Fe + Al2O3 + K2O 0. The damage [371]. passivating. Detailed data on the manufacturing steps most important to the catalyst performance. Reactivating such catalysts usually takes only 30 – 40 h. Part of this is only loosely bound and is removed in reactivation even below 200 – 300 °C. These latter should not play a major role with catalysts from manufacturers of repute and are not discussed in detail in this section because of the proprietary nature of the production processes. the method recommended by BURNETT is used. permanent poisons can be detected by chemical analysis. prereduction. which is proportional to adsorption equilibrium. The reducible oxygen content of the prereduced catalyst ranges between 2 and 7 %. Table 19. [375]. the degree of poisoning therefore rises with growing partial pressure ratio. The equivalence of H2O. Oxygen-containing compounds such as H2O. C values for the ammonia synthesis rate equation Catalyst Temperature. pH2O / pH2 and falls with increasing temperature. Usually. especially in single-train plants. the prereduced catalysts are pyrophoric and must be stabilized before transport and installation by passivating the surface. These substances can be minor gaseous constituents of the synthesis gas or solids introduced into the catalysts during the manufacturing procedure. Figure 22. Under the assumption of a displacement equilibrium in accordance with I. Some authors assume a different route for the formation of adsorbed atomic oxygen [377]. so that altogether the downtime of a production unit is reduced markedly. SMIRNOV et al. the quantitatively lower yield of aqueous ammonia solution (which can be added to the production). and O2 are the most common temporary poisons encountered in ammonia synthesis.46 0. 100 ppm of O 2 or CO2 and 200 ppm CO or H2O. [321]. the application of pretreatment processes. That is. This and further advantages. The experimentally determined effect of water vapor concentrations up to about 30 ppm on the activity of a commercial catalyst (BASF S 6–10) at 30 MPa is evident from Figure 23. appear in [14]. [373] that takes the effect of water vapor into consideration over a wide range of temperature and pressure: and found the values of C listed in Table 19. make using prereduced catalysts increasingly attractive. The reduced charge is treated with nitrogen containing 100 – 1000 ppm oxygen at 50 – 70 °C (maximum 95 °C) and a pressure of 0. derived from impurities in the natural magnetite from which the catalyst is made.5.2 MPa (1 – 2 bar) and up [359]. The melting process itself may also contribute to minimizing the content of some minor impurities.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience The topotactic reduction process leads to a core and shell structure which is visible under the optical microscope and is shown schematically in Figure 22 (a). Equivalent concentrations of oxygen compounds. According to this concept the active catalyst should not grow as a shell around a precursor core but as a core within an oxidic matrix. and the use of high-purity promoters. Corresponding to the depends approximately linearly on the quantity of adsorbed water taken up by the catalyst [370]. the activity of a damaged catalyst may be practically completely restored by reduction with clean synthesis gas. In contrast to temporary poisons. CO.39 0. CO2 and O2 [379] with respect to their poisoning effect has been confirmed.17 0. but the shell and core model is also valid for commercial polycrystalline catalysts [369]. Reversible effect of increasing water vapor concentrations in the synthesis gas on the activity of industrial ammonia catalysts page 21 of 77 . Information on the reduction kinetics. CO.20 0. and reactivation. 4. [378]: and is the where * denotes a surface site. Ammonia formation begins at substantially lower temperatures. A more detailed atomistic picture leads to the assumption that there is no homogeneous topotactic reaction interface. in spite of higher prices.1. introduced on the market some years ago. although the pore surface has been oxidized to a depth of a few atomic layers to make these catalysts nonpyrophoric. CO2. where molar fraction of H2O. stabilized catalyst types. set up a rate equation for ammonia synthesis [372].14 A more recent investigation [374]. A. Newer findings seem to question the shell and core model with the topotactic reaction interface [172].4. c) Reaction interface Prereduced. lead to the same degree of poisoning. Mechanism of catalyst reduction [117] a) Reducibility of catalyst under standard conditions as a function of its porosity. [322-325]. although in a few cases a pressure as low as 80 bar has been used. Further information on catalyst poisoning is given in [375]. In a catalyst bed. However. [376]. Chlorine compounds. high temperatures (400 – 500 °C) and large reactor volumes (more than 60 m3 for a capacity of 1500 t/d) to achieve good economics.and alkaline earth-containing industrial catalysts adsorb more H2S. a value under 0. With similar kinetic characteristics its volumetric activity is about twice that of the standard iron catalyst.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience With continuing exposure oxygen compounds also cause irreversible damage to the catalyst activity that is causally linked with growth of the iron primary crystallite [376]. In industrial plants. [251]. If the temperature is kept at this level. at high temperatures. The influence of the operating temperature is evident from the data.5 – 1 µg S/m3 (STP)). With regard to the sulfur bound on the catalyst surface. most of the sulfur already has been taken up in the gas inlet layer. the use of a support material is the only viable option. First of all iron is a cheap material and secondly by the incorporation of alumina a surface area similar to that attained in highly dispersed supported catalysts can be obtained. 0. Activity of BASF ammonia catalyst S 6–10 in a commercial converter after 14 years of operation • fresh catalyst. sulfur promotes recrystallization of the primary iron particle. the production declined by about 15 % per month. traces of H 2S and COS may not be removed in the upstream purification steps and so may enter the converter with the make-up gas. established that at 30 MPa and a water-vapor content of 250 ppm. Otherwise. thus cancelling the energy saving from synthesis gas compression. Sulfur. However.1.2. There are indications that these permanent poisons exert the most detrimental effect when present as hydrogen compounds and are less harmful in higher oxidation states [382]. For total poisoning. pioneering studies in the BASF laboratories [250]. To reach this goal a synthesis catalyst with a volumetric efficiency some two orders of magnitude greater than magnetite would be necessary. alkali.1 ppm are viewed as the uppermost allowable limit in order not to affect adversely the life of ammonia catalysts [385]. The classical commercial iron catalyst is an unsupported catalyst. then even at CO concentrations of 20 ppm in the recycle loop gas. Phosphorus.16 – 0. The properties of the supported catalyst will be influenced by several factors [195] – Adequate surface of the carrier to achieve a reasonable metal loading. especially those still using reciprocating compressors. in terms of activity. and major progress was achieved over the years. With the highly purified synthesis gas of modern synthesis processes (with final gas purification by methanation the carbon monoxide level is lowered to below 5 ppm). Substantial energy savings would require lowering the synthesis pressure considerably. Process studies show an energy saving potential of about 1 GJ per tonne NH3 [1279]. generally recommended for modern plants. especially between those containing or free of alkali and alkaline earths.1 % is recommended [385].4. If after mixing with the recycle gas the make-up gas first runs through an ammonia condensation stage in which the H2S and also to a certain extent COS are very effectively washed out by condensing ammonia. For example. In modern plants designed with centrifugal compressors and in which the sulfur content of the synthesis gas is extremely low because of very effective purification (about 0. is more tightly bound on iron catalysts than oxygen. temperatures above 500 – 520 °C must be avoided in order to achieve a converter charge operating period of more than 2 – 3 years. and porosity must also be taken into account. most metals have been tested. to compensate for the less favorable equilibrium situation much lower operating temperatures would be necessary. but there has been no substantial improvement in the catalyst since the 1920s. the sulfur contained in the compressor oil constitutes the main danger. for an expensive material such as the platinum group metals. A catalyst sulfur content of several 100 ppm suffices to impair its activity [14]. Of course. [384]. down to the synthesis gas production level. the greater is the harmful effect of oxygen compounds [381]. Corresponding to the data. however. they are more stable toward the action of sulfur and are partially regenerable [384]. occasionally present in synthesis gases from coal or heavy fuel oil. The only other catalyst system which exhibits a promising potential for industrial application is based on ruthenium [195]. From the vast amount of experimental and theoretical studies of the iron catalyst one can conclude that there is only limited potential for further improvement. whereas FeS formed by treatment above 300 °C with high H2S concentrations is reducible as far as the monolayer. and additional energy would be needed for recovery. This damage depends on the water-vapor partial pressure and is especially serious. The standard commercial iron catalyst still requires relatively high pressures (usually in excess of 130 bar). For a carbon monoxide content of 5 ppm. the higher the temperature. These new efforts to find improved catalysts could use methods and knowledge of modern surface science as developed on the example of the magnetite catalyst. For the overall performance of potential catalysts in practical application additional factors. Like oxygen. The performance of a catalyst operated at 570 °C is about 35 – 40 % under that of a catalyst charge operated at 520 °C. It is therefore very important to use an oil with low sulfur content in ammonia plants. a sulfur content of the oil of 0. page 22 of 77 . The first development which found commercial application was a cobalt-modified magnetite catalyst introduced in 1984 by ICI. Under similar poisoning conditions. Pure iron and catalysts activated simply with alumina chemisorb S2N2 or thiophene when treated with concentrations of H2S that lie below the equilibrium for the FeS bond. Rising energy costs since the mid-1960s have given a new incentive to the search for other catalyst systems with improved performance. the concentration and combination of promoters affect the degree of irreversible damage. say.1 – 0. Österreichische Stickstoffwerke (ÖSW). operating periods of up to 14 years can be achieved for a converter charge without significant loss of activity (Fig. such as number of active sites. sulfur may reach the ammonia converter in various forms. In the search for an alternative catalyst. for example. Compounds of phosphorus and arsenic are poisons but are not generally present in industrial syngas. after 14 years operation As already mentioned in Section Composition. That is. sulfur is freed as H2S.25 mg S/m2 is sufficient. a maximum of 0. General Aspects For a long time efforts to improve the efficiency of industrial ammonia production concentrated on synthesis gas production. [380]. differences exist between the various types of ammonia catalysts.2.4. Figure 24. either as primary components or as promoters. This must be considered in the choice of catalyst for a particular plant. The deactivation effect is based at least in part on the formation of alkali chlorides that are volatile at the upper synthesis temperatures. On cracking the oil to lower molecular mass hydrocarbons. Sulfur.5 mg of sulfur per m2 of inner surface or free iron surface. 4. 24). In spite of this. physical form. Concentrations of about 0. sulfur poisoning is of lesser importance than carbon monoxide and chlorine poisoning. Linz. Structural sensitivity and nature of the nitrogen adsorption and dissociation steps could serve as guidelines [196]. no serious deterioration of the catalyst performance is observed after a month of operation. In ammonia synthesis itself considerable progress was made in converter design and recovery of the reaction energy at high temperature. In a pilot plant. In some plants. Other Catalysts 4. The monolayer is also preserved on reduction with hydrogen at 620 °C. catalysts partially poisoned with hydrogen sulfide cannot be regenerated under the conditions of industrial ammonia synthesis. and Arsenic Compounds. This is probably one of the main causes of the decline in converter performance over the course of the catalyst operating life.2 wt % ought to be sufficiently low. Much of this work was performed in the early. Most of the studies in the following years concentrated on the magnetite system in the sense of more fundamental and general catalytic research. they determined about a 4 – 5 % decrease in activity per year. this corresponds to monomolecular coverage [383]. The permanent poisoning effect of chlorine compounds is two orders of magnitude worse than that of oxygen compounds. in contrast to reversible poisoning. they established that above all. because otherwise too low an ammonia concentration would result. Of the group of metals forming stable nitrides. Os. Molybdenum also seems to exhibit activity in biological nitrogen fixation [392] and is synthetically active at ambient conditions in the air-sensitive Glemser compounds [392].Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience – Dispersion stability by using a more active support with strong interaction between the support phase and the metal precursor. – Gas transport effects will be governed by pore size. use the KAAP ruthenium catalyst [1317]. Ammonia synthesis activity of metals supported on carbon with potassium metal promotion (mL NH3/ mL catalyst. A major advantage of graphite is its ability to stabilize high loadings of alkali metals.2 Rh 0. the AmonMax-10 of Südchemie which uses wustite instead of magnetite. Re) 3.52 Pd 0 Re 0. 4. Tc. The other common factor for the ammonia catalysis is the structure sensitivity of molecular nitrogen adsorption. its application for ammonia synthesis catalyst in [1323].36 Os 5. Pt) 2. which have to be considered in handling these catalysts see [406]. promoter concentrations. pore distribution. Indeed. Fe. The interest therefore shifted to ruthenium. which would lead to destruction of the support. it should contain 5 wt % Ru and 10 wt % Rb. Too strong an interaction may cause difficulties in the metal reducing. Metals forming nitrides unstable under reaction conditions (Mn. and other factors such as hydrogen and ammonia inhibition do not play a major role. [393]. This catalyst is already used in seven ammonia plants with a total capacity of 5600 t/d [1320-1322]. For a literature report on promoted iron catalysts see [1325].2. irrespective of the catalyst. because of its structure similar to graphite. Effect of H/N ratio on activity of Ru and Fe3O4 catalysts [195] Figure 26. the promoted ruthenium catalyst works best at a lower than stoichoimetric H/N ratio of the feed gas as shown in Figure 25. Co. kinetic investigations are reported in [196]. this metal never became an industrial catalyst because of its limited availability and its dangerous properties. The cobalt-enhanced catalyst formula was first used in an ammonia plant in Canada using ICI Catalco's AMV process (later also in other AMV license plants) and is also successfully applied in ICI's LCA plants in Severnside. Commercially successful is a new type of iron-based catalyst. Fe. but it can be avoided by careful heat treatment of the support above 1500 °C [389]. 4. It was attempted to improve the activity of the classical magnetite catalyst by alloying with nickel or cobalt.2. Metals likely to be present as nitrides under synthesis conditions (groups 3 – 6 of the periodic table) Of the platinum group metals only ruthenium and osmium show an activity superior to iron. occasionally called “white graphite”. A Ba-Ru/BN catalyst proved completely stable in a 5000 h test at 100 bar and 550 °C using a 3:1 H2 – N2 mixture [1326].008 Mo 0. The available data show that these conditions are fulfilled only for a limited number of metals: iron. Commercial Ruthenium Catalysts Since the early days of industrial ammonia synthesis only minor improvements have been achieved for the magnetite system: optimization of manufacturing procedures. – Anion retention capability. each with a nameplate capacity of 1850 t/d.68 Pt 0. Kellogg a new catalyst composed of ruthenium on a graphite support [395]. [396]. ruthenium. [1410]. Ni. The group of metals forming low-stability or unstable nitrides includes Mn. With this modified carbon material lifetimes of at least six years are expected [386]. The most active ruthenium catalysts use a graphite support and alkali metal promotion. Platinum group metals: no stable nitrides (Ru. [196]. the others with the ruthenium – graphite catalyst. Boron nitride. Only if both conditions are favorable. It is unlikely that the promoter is in metallic state as its high vapor pressure would probably lead to substantial losses under synthesis conditions.04 Ru 22. Ir. It is also less susceptible to self-inhibition by NH3 (Figure 26) and has excellent low-pressure activity.313 kPa. H:N = 3:1) Fe 0. and particle size to give somewhat higher activity and longer service life. The only commercial catalyst is a cobalt containing magnetite [390]. On account of its very strong ammonia inhibition rhenium is not an option. As in the case of iron a clear structural sensitivity was found for rhenium but the role of promoters remains the subject of discussion. There is no industrial page 23 of 77 .3. as may be seen from Table 20[386]. Extensive literature on non-iron catalysts is given in [195]. A special limitation until recently was the chemical reactivity of carbon supports. which is claimed to be 10 – 20 times as active as the traditional iron catalyst: The KAAP catalyst. Ni.4 Ni 0. 128. There are also indications of structure sensitivity for cobalt and nickel. Production of synthetic wustite is described in [1324]. In 1979 BP disclosed to M. this phenomenon has been observed [388]. Tc. after a ten-year test program. according to the patent example. and Re. A notable development for the magnetite system was the introduction of cobalt as an additional component by ICI in 1984 [395]. and osmium. 573 K. Table 20. [406] is loaded with conventional magnetite catalyst. Under synthesis conditions it is present as a nitride with some ammonia formation activity and structural sensitivity [391]. In October 1990. For the special properties of ruthenium. Besides having a substantially higher volumetric activity.72 Co 0.W. though only in presence of alkali metal promoters. Topsøe has developed a ruthenum catalyst which uses boron nitride instead of graphite as support. the catalyst may also catalyze the methanization of carbon.4. preferentially with rubidium or cesium [394]. According to the patent [390] the new catalyst is prepared by subliming ruthenium-carbonyl [Ru3(CO)12] onto a carbon-containing support which is impregnated with rubidium nitrate. In a typical ammonia synthesis environment with high hydrogen partial pressure.6 Ir 0. can a sufficient overall reaction rate be expected.6 Although osmium was the first active catalyst used by HABER in 1909 [387] in his laboratory-scale unit to demonstrate the technical viability of the high-pressure recycle process. – Promoter localization with respect to metal sites and support sites.4. Figure 25. Metals with Catalytic Potential Materials that show significant ammonia synthesis activity can be divided into three categories according to their ability to form nitrides: 1. [398]. The catalyst has a considerably higher surface area than the conventional catalyst and. The first layer of the four-bed reactor [1318]. [397]. the rate-determining step in the ammonia synthesis reaction is the dissociation of the nitrogen molecule and the catalyst effectivity is determined in the first instance by the activation energy of the dissociation reaction. only molybdenum is of some interest. Co. Ammonia inhibition of Ru and Fe3O4 catalysts [195] Three ammonia plants in Trinidad. is completely stable towards hydrogenation under all conditions relevant to technical ammonia synthesis. The results of the intensive research in this field over the last decades demonstrate that. It is assumed that a charge transfer complex M+···C– is formed between the metal and the graphite. which plays a role in the catalyst preparation by impregnation of a support with metal impregnation of the support. Kellogg started the commercialization of the Kellogg Advanced Ammonia Process (KAAP) using this catalyst [397]. and tortuosity.2. natural gas from the Slochteren field in the Netherlands contains 14 % nitrogen. Feedstock pretreatment and gas generation 2. Table 22.1. Process Steps of Ammonia Production The term “ammonia synthesis” is increasingly used when referring to the total ammonia production process. where air is used. Naturally. which provides all of the nitrogen. Ruthenium prices increased by a factor of four in 2000 to 170 $/oz. Synthesis gas production 1.1. In the EU. If water electrolysis (4. A literature summary of ammonia production is contained in [402] and [403]. some progress has been made in converter design and optimization of heat recovery. Therefore. and water. for example. CH4 . In case of electricty generation from fossil energy with 40 % efficiency the consumption figure is 85 GJ per tonne NH3. feed and fuel. now named Nitrogen + Syngas. which is 300 % of the consumption of a modern steam reforming plant [1364]. with 95 % of capacity. In common with the iron catalyst ruthenium will also be poisoned by oxygen compounds. (5) page 24 of 77 .g. Carbon monoxide conversion 3. The raw materials are water.0 1.15 1. and hydrogen produced by electrolysis. and a carbon-containing reducing medium. Table 22 shows the relative capital cost and the relative energy requirement for a plant with a capacity of 1800 t/d ammonia.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience application so far. for its part. a Stokes Engineering Publication. due to new applications in the electronic industry. In North America. 4. Reference [409] covers coke oven gas as a feedstock for ammonia synthesis and references [410-413] describe producing hydrogen by water electrolysis for ammonia production.5. These include solid fuels.6 4. Feedstock Pretreatment and Raw Gas Production The chemical reaction of hydrocarbons with water. the United States patent literature in the field from 1972 to 1980 is covered in [304]. ≈ CH) and nitrogen. Gas Production.e. 4. Synthesis conditions are no longer viewed in isolation. Even with some further potential improvements it seems unlikely to reach an activity level which is sufficiently high at low temperature to allow operation of the ammonia synthesis loop at the pressure level of the syngas generation. Gas purification B. Synthesis gas preparation. The capital cost and the specific energy requirement (i. that. Table 21. some nitrogen. Usually only the carbon-containing materials and hydrogen from other sources are regarded as raw materials in the narrow sense because of the abundance of air.5 1. Ruthenium price development is occasionaly published in FINDS. The potential for ruthenium to displace in the long run iron in new plants will depend on whether the benefits of its use are sufficient to compensate the higher costs.. [1132]. This trend is also expected to continue in the near future. published by British Sulphur. Table 21 provides an overview of the raw material sources (apart from water and air) for world ammonia capacity. which generally supplies most of the hydrogen. In the synthesis section itself. It yields a gas mixture made up of CO and H2 in various proportions along with carbon dioxide and.5. they are an important consideration in the total process but can be determined properly only in relation to the total plant integration (see Section Complete Ammonia Production Plants).5. coke oven gas. Of course. and so the manufacturing cost) largely depend on the raw material employed [414]. Feedstock distribution of world ammonia production capacity 1962 103 t N Coke oven gas and coal 2800 Natural gas 7800 Naphtha 2050 Other petroleum products 2950 Total 15600 1972 % 103 t N 1983 % 103 t N 1998 % 103 t N % 18 4600 9 7200 8 16500 14 50 32100 63 66850 74 94300 77 13 10700 21 9050 10 7300 6 19 3600 7 7200 8 4400 3 100 51000 100 90300 100 122500 100 Table 21 indicates that new ammonia plants are based almost exclusively on natural gas and naphtha. The complete process of industrial ammonia production may be subdivided into the following sections: A. For the natural gas based plant the current best value of 28 GJ per tonne NH3 is used. The term feedstock is often applied to the total consumption of fossil fuel.. and gas purification for ammonia synthesis are treated in detail in the article only a short abstract is presented here. [1318].3 2. Certain raw materials for synthesis gas production that were once of primary importance currently are used only under special economic and geographical circumstances (e. natural gas dominates.1 1. for example. More modern and comprehensive reviews of ammonia production technology can be found in [404-406]. A discussion on ruthenium as ammonia synthesis catalyst with an extensive literature review is found in [1389]. Synthesis Gas Production The goal is preparing a pure mixture of nitrogen and hydrogen in the stoichiometric ratio of 1 : 3. Relative ammonia plant investment and relative energy requirement for 1800 t/d NH3 Natural gas Naphtha Fuel oil Coal Relative investment 1. China. Synthesis and purge gas management The most fundamental changes over the years have occurred in synthesis gas production and gas compression. carbon monoxide conversion.1.5 1. where 66 % of production is based on coal). may contain hydrogen (natural gas. naphtha. Any carbon containing feedstock will undergo a reaction according to Equation (5) or (6) or both simultaneously. air. presents an update of the state of the art from time to time. oxygen. [415]. although strictly speaking a distinction should be made between gasification feed and fuel for energy generation.0 Relative specific energy requirement (based on lower heating values) 1. Compression C. 86 % of capacity is based on natural gas and 8 % on naphtha [502]. An overview over ammonia synthesis catalyst development history and newer research is given in [1326]. The investment could according to a rather rough estimate be about three times the investment for a steam reforming plant with natural gas. or any combination of these is generally referred to as gasification. the regional distribution is diverse. The journal Nitrogen. ≈ CH2. which would be only justified when electric power is generated from water power. air.5 kWh/Nm3 H2) is used together with an air separation unit for the nitrogen supply the energy requirement amounts to 34 GJ per tonne NH3 when the electric energy is valuated just with the caloric equivalent. petroleum. Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience (6) Light hydrocarbons ranging from natural gas (methane) to naphtha (max. According to the disposition of the burners primary reformers can be classified as top-fired. the reaction proceeds in indirectly heated tubes filled with nickel-containing reforming catalyst and is controlled to achieve a partial conversion only [in conventional plants 65 % based on methane feed. In this way the outlet temperature is lowered to around 1000 °C. is capable of processing any type of hydrocarbon feedstock. this reaction is carried out in a separate step over a different catalyst at a lower temperature than the preceding gasification step (Section Carbon Monoxide Shift Conversion). but this is limited by the material. The primary reformer consists of a multitude of reformer tubes loaded with the nickel catalyst (15 – 25 % NiO on -aluminum oxide. A recent development which avoids a fired primary reformer is the exchanger reformer. When higher hydrocarbons (naphtha) are used. The time to rupture for a specific material depends on the tube-wall temperature and on the internal pressure. A special consideration is the lifetime of the expensive reformer tubes. The furnace box usually accommodates 200 – 400 tubes (depending on plant capacity). terraced-wall. and markedly higher for naphtha. Natural gas consists predominantly of methane and is therefore the most hydrogen-rich and energetically the best raw material for the steam-reforming route. Unlike to a secondary reformer. HP modified (32 – 35 Ni/23 – 27 Cr stabilized with about 1. These raw materials contain a substantial amount of sulfur and also minor quantities of heavy metals. three processes are commercially available: the British Gas Process [426]. because under the severe reaction conditions the material exhibits creep which finally leads to rupture [1327]. (8) (9) The general overall reaction can be formulated as (10) or more specifically for methane. which is fed with partially reformed gas. and the Japan Gasoline Process [430]. Nitrogen already present in the natural gas tends to cause a reduced specific air ratio in the secondary reformer and a reduced secondary reformer temperature rise. Steam Reforming Processes. or magnesium aluminum spinel support) [1318] in a furnace box in which the heat needed for the reaction is transferred to the tubes by radiation. The standard material for a long time was HK 40 (20 Ni/25 Cr) but for replacements and new plants. In the following secondary reformer — a refractory-lined vessel filled with nickel catalyst — the gas is mixed with a controlled amount of air introduced through a nozzle (burner). a so-called “rich gas stage” can be utilized upstream of the primary reformer. which to save energy in synthesis-gas compression should be as high as possible. Corresponding to Equation (6). with an inner diameter of 75 – 140 mm and a wall thickness of 11 – 18 mm. usually the major constituent of natural gas. The heat is generated in burners. The steam/carbon ratio used in modern commercial primary reformers for natural gas is between 2. for which an alkalized nickel catalyst has to be used to prevent carbon deposition. But for the different feedstocks there are some constraints on the applicability of the various gas generation processes. as: (11) Simultaneously to this equilibrium the water gas shift reaction (Eq. the primary reformer exit temperature must be increased. the autothermal reformer (ATR) is fed directly with the hydrocarbon feedstock. The various commercial coal gasification processes may also be classified as partial oxidations. which could be volatile under the hydrothermal conditions and deposit downstream of the secondary reformer at lower temperature. which with some simplification may be viewed as tubular heat exchanger with page 25 of 77 . Thus for heavy feedstocks the only choice is noncatalytic partial oxidation. The catalytic steam reforming technology can only be applied to light hydrocarbon feedstock (up to naphtha) but not for heavy hydrocarbons such as fuel oil or vacuum residue. With this latter tube material a reforming pressure of 40 bar is possible at outer tube-wall temperatures of around 900 °C . In this case it would be necessary to use oxygen or oxygen-enriched air instead of air [404]. [406]. Because of the higher heat of reaction in the internal combustion (temperature >2000 C). [1318]. The natural gas enters such a pre-reformer with a temperature of 530 °C instead of being fed to the primary reformer tubes with the same temperature. From Equation (5) it can be seen that in the steam reforming variant the proportion of hydrogen supplied by the feedstock itself increases with its hydrogen content.5 % Nb) is being increasingly used on account of its superior high-temperature properties [425]. Another possibility for reducing the load on the primary reformer is to transfer part of the conversion duty to the secondary reformer with application of an superstoichiometric amount of air. [459]. In addition cracking reactions will occur on the catalyst. causing fouling of the waste-heat recovery surfaces. and a residual methane content of 0. to maintain the same methane leak. Medium-grade heat is used to reheat the exit gas to the correct primary reformer entrance temperature. [1413. side-fired. bottom-fired reformers [406]. which gives rise to a simultaneous reaction according to Eq. which causes catalyst deactivation and local overheating of tubes (hot bands and hot spots). 7) proceeds. including flue gas. As the nickel-containing catalysts are sensitive to poisons. calcium aluminate. [1318]. 406]. in the furnace box. generally with a combination of cobalt – molybdenum and zinc oxide catalysts [560]. the primary reformer. The raw gas composition is thus strongly influenced by the feedstock and the technology applied. but this is economically unfavorable. Adsorption on activated carbon is an alternative when the feed is natural gas with a rather low sulfur content. which. Another possibility to compensate is a higher steam surplus (steam/carbon ratio). This is then converted in the primary reformer under normal reforming conditions. See also Gas Production. commonly known as partial oxidation. A special requirement for the catalyst is a low content of silica. it is installed up-stream of an existing tubular reformer [431-438]. This limits the reforming pressure. [639]. depositing carbon. The hydrogen – oxygen bond energy in water is higher than the hydrogen – carbon bond energy in the hydrocarbon. Therefore special considerations in the design of burner and reactor are necessary. [460]. process gas. generally gas-fired. An additional equilibrium reaction involved in any gasification process is the water gas shift reaction (7). As the reforming reaction is endothermic and proceeds with volume increase the negative effect of a pressure increase (lower conversion) has to be compensated by a higher reaction temperature and hence higher wall temperatures. (7) Although reaction in the right hand direction is favored by lower temperatures it is responsible for the initial carbon dioxide content of the raw synthesis gas. the reforming reaction is split into two sections. 5).5. having a substantial concentration of hydrogen. A further improvement are microalloys that additionally contain Ti and Zr [406]. less common. which would poison the sensitive reforming catalyst. 10 – 13 m long. [1318]. In the first section. It attains the theoretical maximum of 66 % with methane. A temperature drop of about 60 – 70 °C occurs in the catalyst bed due to the overall endothermic reaction. The positive enthalpy per mole of hydrogen therefore decreases as the proportion of hydrogen contributed from the feedstock itself increases. This process has recently become the subject of renewed interest for increasing the capacity of existing natural-gas-based ammonia plants in which the primary reformer has been identified as bottle-neck.5 % or lower (dry basis) is attained in conventional plants [404. made of highly alloyed chromium-nickel steel by centrifugal casting. leaving around 14 mol % methane (dry basis) in the effluent gas]. [446]. which not only blocks the catalyst pores but also restricts interparticle flow. In this. or gas turbine exhaust. [1376]. [1318]. In the extreme case the whole reforming reaction could be performed without a tubular reformer by autothermal catalytic reforming in a design similar to a secondary reformer. This compensation heat may be derived from a variety of sources. the BASF – Lurgi Process [427-429]. heat release characteristics and the risk of soot formation are very different from the situation of a normal secondary reformer. or. the higher hydrocarbon – steam mixture is transformed at relatively low temperature (400 – 500 °C) and steam – carbon ratios less than two into a methane-rich gas. [561]. all carbon-containing feedstocks can be processed in a noncatalytic reaction with oxygen (together with a minor amount of steam for process reasons. For this rich-gas stage. In the partialoxidation route less hydrogen is produced in the primary gasification step and the raw synthesis gas has a rather high CO content. By combustion of a quantity of the gas the temperature is raised sufficiently (to about 1200 °C) that the endothermic reforming reaction is completed with the gas adiabatically passing the catalyst layer. the flow conditions. To maximize the hydrogen yield.8 and 3. This requires removal of the surplus of nitrogen from the synthesis gas either ahead of the synthesis loop (by cryogenic methods or pressure swing adsorption) or by purge gas and hydrogen recovery [457]. [1390]. Therefore. however. [1373]. (see also Gas Production – Steam Reforming of Natural Gas and Other Hydrocarbons). Under the name pre-reforming. any sulfur compounds present in the hydrocarbon feedstock have to be removed by hydrodesulfurization. [458]. C11) undergo reaction with steam over a catalyst according to Equation (5) which is usually called steam reforming. 1414]. To introduce nitrogen to achieve the required stoichiometric hydrogen/nitrogen ratio for ammonia synthesis. An oververview of the steam reforming technology. So far the Koppers – Totzek. so that the following reactions proceed in parallel: (14) (15) As the overall reaction is exothermic no external heat supply is necessary. too. A concept developed by Uhde goes a step further in this direction: exchanger reforming and subsequent noncatalytic partial oxidation. called HTCR with a dedicated burner. Detailed discussion on exchanger reforming and autothermal reforming is found in [1328]. Depending on the process configuration the gas is either cooled by quenching or in a waste-heat boiler. Information on the status and the development in the gasification of coal can be obtained from [493-496]. [406]. l) Refractory lining ICI has come out with a modified design. World-scale ammonia plants based on partial oxidation exist in Germany. 450]. Owing to insufficient mixing with oxygen. The heat produced by the exothermic reaction of coal and air in the “blow phase” is stored in the fixed bed and provides the heat needed for reaction of coal with steam in the “run phase” [99]. Uhde combined autothermal reformer (CAR) a) Sandwich type tubesheet. This combined autothermal reformer (CAR) design. and process gas cooling. the AGHR. [455]. g) Shellside outlet. [487.g. [1318]. there are no mechanical or material limitations to raising the gasification pressure further. In some designs the tubes may be open at the lower end. W. Kellogg reforming exchanger System (KRES) [406] Because of the smaller size compared to a conventional fired reformer considerable investment savings can be achieved. In some configurations the exchanger reformer is partially by-passed. Figure 28. [442-448]. the bayonet tubes are replaced by normal tubes attached to a bottom tubesheet using a special seal to allow some expansion. soot removal and recirculation. [492]. [1318-1320]. which is removed from the gas by water scrubbing. along with a small amount of steam) are introduced through a nozzle at the top of the generator vessel. for steam reforming catalysts. h) Catalyst tube. operates at 2. The Lurgi process handles lump coal. vacuum residues. d) Bayonet tube. Partial Oxidation of Hydrocarbons. [451-453]. page 26 of 77 . they mainly differ in the nozzle design. k) Catalyst. The temperature in the generator is between 1200 and 1400 °C. [497]. Hydrocarbons or coal will react with an amount of oxygen insufficient for total combustion to CO2 according to: (12) (13) In practical operation some steam must always be added. crude oil. The nozzle consists of concentric pipes so that the reactants are fed separately and react only after mixing at the burner tip or in the space below.. with “A” standing for advanced. the latter has to take on a higher reforming duty. For literature on steam reforming see [404-408]. [456]. e) Refractory lining. d) Tubesheet. where a number of small plants with a capacity of 30 – 80 t/d still produce their synthesis gas with the today largely outdated water-gas process which operates at atmospheric pressure. India. Kellogg [466-470]. The seal which prevents leakage of methane-rich gas to the secondary reformer effluent flowing on the shell side has a unique design which is subject to patent applications of ICI. the Koppers – Totzek process operates on coal dust at atmospheric pressure. Further information is found for the TSGP in [421-424]. for the SGP in [471]. [454]. To close the heat balance between the exchanger reformer and secondary (autothermal) reformer. The reactants (oil and oxygen. the Lurgi Process) the reaction according to Equation (15) may proceed to a considerable extent. in which case the gas flow on the shell side consists of a mixture of the off-gases from the secondary reformer and from the reformer tubes. c) Scabbard tube. Figure 27. f) Shellside inlet. see also Gas Production – Partial Oxidation of Hydrocarbons. [449. Partial Oxidation of Coal. they are more often referred to as coal gasification rather than as partial oxidation. In this way the delicate double tubesheet of the GHR is avoided. and other countries [465]. reaction kinetics and thermodynamics [1318]. B) AGHR a) Tubeside inlet. [1331] for reformer design. [1318]. Similar concepts are offered by other licensors and contractors e. [420]. but this is just a matter of definition. present state of the art. [1398] (Fig. which may be achieved by using an over-stoichiometric amount of air or oxygen-enriched air. As shown in Figure 27 B. but the successful demonstration of the Shell process in other applications makes it a potential candidate for ammonia production. which provides the reaction heat. and probably the Winkler process in some smaller installations. The reaction pressures may be as high as 80 bar. 28). ICI gas heated reformer A) GHR. Commercially operating designs are the GHR of ICI (Figure 27) [406]. The Texaco and Shell processes can handle fuel oil. i) Seal. [477-485]. In both the reaction is performed in an empty pressure vessel lined with alumina. The AGHR will allow a single-line concept for worldscale plants whereas with the GHR several parallel units for large plants would be necessary [406]. about 2 % of the hydrocarbon feed is transformed into soot. [503]. [406]. are accommodated in a single vessel. and coal. b) Enveloping tube. [461-464] and the KRES of M. for steam reforming of naphtha. China. Partial Oxidation. Certain processes have achieved particular significance. shown in Figure 29 was operated in a demonstration unit producing 13 000 m3/h of synthesis gas [472-475]. but with respect to the overall ammonia process this might be beyond the energy optimum because of the increasing energy demand for nitrogen and oxygen compression. its historical development. 489-491]. part of the feed being fed directly into the autothermal reformer. [486]. The TSGP and SGP are rather similar. A special case is China. Figure 29. Additional processes in different stages of technical development are the HTW and the Dow process.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience the catalyst inside the tubes.. c) Reformer tubes. Texaco and Lurgi gasifiers. Haldor Topsøe has developed an exchanger reformer called HTER which may be used in various process configurations as described in [1328]. It reportedly can operate without upstream desulfurization and should be able to gasify naphtha. Since in some processes with coal feedstock (e. e) Sheath tube. the quantity depending on feedstock and process configuration.g. f) Water jacket A unique steam reforming process [439-441] has been developed in Japan to the pilot-plant stage. The design may be also used as a convective reformer. or Topsøe [406]. newer developments and future perspectives together with quotation of newest literature is given in [1327]. which are heated by the hot secondary reformer effluent flowing on the shell-side.5 MPa. b) Tubeside outlet. have been used in ammonia plants. Braun & Root. Maximum raw gas generation capacity of a single generator corresponds to about 1000 t/d ammonia. [498-501]. j) Tail pipe. Coke or anthracite is reacted intermittently with air and steam in a fixed bed. The two dominant processes are the Texaco Syngas Generation Process (TSGP) and the Shell Gasification Process (SGP). and atmospheric or vacuum residues. traditionally known as carbon monoxide shift conversion (Eq. [545]. Carbon Monoxide Shift Conversion As ammonia synthesis needs only nitrogen and hydrogen. they are treated separately here. but nowadays usually with LTS catalyst [558]. In the unreduced state the HTS catalyst is iron(III) oxide (Fe2O3) containing additionally 5 – 10 % chromic oxide (Cr2O3).1 ppm. for example. To overcome the disadvantage of atmospheric pressure operation. Shell has completely departed from the concept of its process for partial oxidation of heavy hydrocarbons.2 vol % at 220 °C and 0. the balance being alumina.1. New catalyst types with increased activity and higher selectivity have reduced the problem. The gas leaving the top of the reactor vessel is cooled in a waste-heat boiler.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience The Koppers – Totzek Process [504]. Even with a low excess of steam in the gas. The British Gas/Lurgi Slagging Gasifier [495]. but even with such low concentrations. supplied in pellets like the HTS catalyst. Changing the guard bed more frequently prolongs the service life of the main LTS catalyst bed. Quasiisothermal reactors have been developed in which cooling tubes run though the catalyst layers. The temperature in the lower section of the bed is around 1000 °C. is introduced immediately at the head of the burners. The poisoning process moves through the catalyst as a relatively sharp front and can be seen in the change of the catalyst temperature profile over time [548]. Figure 30.4. the carbon monoxide serves as reducing agent for water to yield hydrogen and carbon dioxide. [532]. When the ZnO is exhausted in a given layer of the catalyst. In the water gas shift reaction. An aqueous slurry containing 60 – 70 % coal is fed by reciprocating pumps to the generator at a pressure of 20 – 40 bar. Byproduct formation is higher with fresh catalyst and declines with operating time. and at the top where the raw gas exits about 600 °C. This Prenflow (pressurized entrained flow) process [511]. [549]. The classical HTS iron catalyst is resistant against sulfur compounds. followed by a water scrubber to remove carbon and ash traces from the raw gas and to effect further cooling. 20 – 30 % zinc oxide. Crushed coal with a particle size of 4 – 40 mm enters the top of the gasifier through a lock hopper and is evenly distributed over the cross-section of the coal bed surface by a distribution disk equipped with scraper arms. The process actually can use any sort of coal and can handle ash contents higher than 30 %. which may be introduced with the natural gas or more often with the process air to the secondary reformer. which are effective Fischer – Tropsch catalysts that lead to the formation of some methane and higher hydrocarbons [406]. with intermediate heat removal between the individual catalyst beds in which the reaction runs adiabatically.3 vol %.g. As chemical composition and formulation of the LTS catalyst are very similar to methanol production catalysts. This catalyst is active in the temperature rage of 300 – 500 °C. The Texaco Coal Gasification Process [495]. As the process configuration and catalysts are to some extent different for steam reforming and partial oxidation. After a temperature increase of around 50 – 70 °C (depending on initial CO concentration) and with a residual CO content of around 3 % the gas is then cooled to 200 – 210 °C for the low temperature shift (LTS). 7). The Lurgi Dry Gasifier [495]. together with a small amount of steam.5. Decreasing the steam surplus lowers the oxygen to carbon ratio in the HTS to such an extent that the atmosphere can reduce magnetite partially to metallic iron. small quantities of methanol are formed and are found in the process condensate after cooling the LTS effluent. Alternative process steps for generation and purification of synthesis gas 4. During operation. may also deactivate the LTS catalyst by accelerating the sintering of the copper particles. In the traditional plant concept. usually operating at 25 – 30 bar. The residence time is less than 1 s and the temperature is 1500 – 1600 °C. The tendency for methanol formation increases with decreasing steam/gas ratio [552]. Steam surplus is not only necessary for thermodynamic reasons but also to suppress undesirable side reactions.1 – 0. but this is of greater importance in partial oxidation processes and less for the practically sulfur-free steam reforming gas. Kynoch Ltd and Babcock Wilson. which is carried out on a copper – zinc – alumina catalyst in a downstream reaction vessel and achieves a carbon monoxide concentration of 0. To keep the temperature low the heat of reaction must be removed in appropriate way and to achieve a sufficient reaction rate effective catalysts have to be applied. a version was developed capable of operating at 25 – 30 bar. formerly loaded with ZnO. Sulfur. The LTS catalyst is protected by a guard bed. usually present as H2S. Depending on feedstock and process technology. all carbon oxides must be removed from the raw synthesis gas of the gasification process. Dry coal dust is fed to the two (sometimes four) burners of the gasifier. Pulverized coal and oxygen are injected near the base of the gasifier through eight burners using a proprietary feed technique which employs a special pump. page 27 of 77 . The ZnO adsorbs the sulfur and it finally transforms into bulk ZnS. In this way not only is the carbon monoxide converted to readily removable carbon dioxide but also additional hydrogen is produced: (7) As no volume change is associated with this reaction. In some ammonia process schemes operating without a secondary reformer and applying pressure swing adsorption (PSA) for further purification (KTI PARC) only a HTS is used. and the mixture enters the reaction zone with high velocity..12 vol % at 200 °C for a steam/gas ratio of 0. consists of 40 – 55 % copper oxide. phenols. In a further development (HTW process) by Rheinbraun it has been tested in a demonstration plant with lignite at 10 bar [534]. Traces of chlorine compounds [557]. Oxygen.2. it is practically independent of pressure. is today still in use in some smaller plants (e. [528]. a new concept developed by AECI Engineering. and carbon scrubber are especially adapted to deal with the ash and slag introduced with the bituminous coal feed. These pollutants are removed from the process condensate by steam stripping and ion exchange. A block diagram showing steam reforming and partial oxidation together with the further steps needed to transform the raw gas into pure make-up gas for the synthesis is shown in Figure 30. amines (mainly methylamine) are formed from the methanol and traces of ammonia originating from the secondary reformer and the HTS. The Advanced Coal Gasification Process (ACGP) [537]. Newly introduced HTS catalysts with additional copper promotion suppress this side reaction [546] and are therefore less sensitive to lower steam-to-gas ratios. The gasifier is a slender column with a square cross-section. [535]. In a consecutive reaction. The catalyst properties are influenced far more by the formulation and manufacturing procedure [547] than by its chemical composition. it is favored by lower temperatures. With reversed flow pattern in the gasifier (from bottom to top). The combustion area is lined with heat resistant ceramic material. the equilibrium concentrations of CO are low. [531] operates without a grate with withdrawal of the liquid slag. Shift Conversion in Steam Reforming Plants. is an entrained gasification process at atmospheric pressure. it is reduced more or less stoichiometrically to the composition of magnetite (Fe3O4). The copper oxide is reduced in situ with hydrogen and a carrier gas (usually nitrogen) to form fine copper crystallites on which the activity depends. [512-517]. operates practically at atmospheric pressure. [1132] is being tested in a 48 t/d pilot plant. has to be below 0. and some higher hydrocarbons. chlorine is more diffusely distributed over the whole catalyst bed by migration as volatile zinc and copper chlorides. the catalyst is slowly poisoned. [510] used in several ammonia plants in China. Unlike sulfur poisoning. so that purification and conditioning of the raw gas is a rather elaborate task. In addition the Boudouard reaction can occur under these conditions. In its coal gasification process [495]. Waste-heat boiler. Liquid slag is withdrawn from the reactor bottom. [526-530]. The resulting carbon is deposited within the catalyst particles causing their disintegration. Ash with only 1 % of residual carbon is removed at the bottom of the gasifier by a revolving grid with slots through which steam and oxygen are introduced. which shift the equilibrium to the right-hand side. [506-509]. which operates at atmospheric pressure. The classic Winkler gasifier. enters the high-temperature shift (HTS) reactor loaded with an iron – chromium catalyst at 320 – 350 °C. So far no commercial installation exists but it is claimed that one unit can produce the synthesis gas for 750 t/d ammonia. India. this gas contains 10 – 50 % carbon monoxide and also varying amounts of carbon dioxide. the gas from the secondary reformer. [518-525] is rather similar to the Texaco partial oxidation process for heavy hydrocarbons. cooled by recovering the waste-heat for raising and superheating steam. The process can handle bituminous coal and lignite. As a result of this lower temperature the raw gas has an increased content of impurities such as tars. In addition the methane content is relatively high (up to 10 – 15 %). [533] performs the reaction in a moving bed. in China). [544]. 0. the H2S causes deactivation of the copper by sintering. and iron carbides will be formed. but as an exothermic process. dry coal dust is introduced via lock hoppers into the reactor vessel operating at 20 – 40 bar. The process is therefore performed in steps. The LTS catalyst. and South Africa. The upper part is made of finned tubes which are welded together and are circulated with water by thermosyphon action. quencher. For this reason tertiary amines are commonly used today. This cobalt – molybdenum – alumina catalyst [417]. special process configurations are required for partial oxidation gases to recover separately a pure CO2 fraction and an H2S-rich fraction suitable for sulfur disposal. CO2 and H2S. Additional literature on shift conversion can be found in [406-408]. The potassium carbonate processes from the various licensors differ with respect to the activator. [603] Process MEA (without Amine Guard) MEA (with Amine Guard III) Benfield hot potash (1-stage)a Heat requirement. 4. At low carbon dioxide partial pressures. for example. The classical method for CO2 removal is to scrub the CO2 containing synthesis gas under pressure with a solvent capable of dissolving carbon dioxide in sufficient quantity and at sufficient rate. In a new plant using the spiral-wound Linde reactor [551]. As this is an essential part of the steam reforming process it is treated in Section Feedstock Pretreatment and Raw Gas Production. second. [634-637].Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience A relatively new process concept is the intermediate temperature shift (ITS) [550] which performs the shift conversion in a single step. [538-543].5. [418]. carbon dioxide. [566] amine borate. corrosion inhibitor and to some extent in the process configuration.. and sulfur compounds (only present in the synthesis gas from partial oxidation) have to be removed as they are not only a useless ballast but above all poisons for the ammonia synthesis catalyst. The catalyst is based on a copper – zinc – alumina formulation and optimized for operating in a wider temperature range than the standard LTS catalyst. with intermediate cooling.7 mol % (dry basis) is achieved. Figure 31 shows the loading characteristic for various solvents. the Benfield process licensed by UOP. inhibitors to limit or prevent corrosion processes. which allow a higher loading of the solvent. Triethanolamine does not achieve the required final CO2 concentration. which was about 14 % of the total energy consumption. the Linde spiral-wound reactor). Reaction temperatures are between 230 and 500 °C. the chemical solvents absorb substantially more carbon dioxide than the physical solvents. The sulfur compounds (see Section Gas Purification) can be removed ahead of the shift conversion to give a sulfur-free gas suitable for the classical iron HTS catalyst. in the Exxon Flexsorb HP process [573]. served as solvent in a once-through process without regeneration and recycling. in the Giammarco Vetrocoke process glycine (originally arsenic oxide) [575-578]. Table 23 demonstrates the progress made in energy consumption. As the standard iron catalyst can tolerate only a limited amount of sulfur compounds. the high initial carbon monoxide concentration means that the reaction must generally be performed in steps. Table 23. in the Carbosolvan process [580]. a methane slip of only 0. which thus has to deal with a high-sulfur gas. Over the years considerable progress has been achieved in improving the efficiency of the carbon dioxide removal systems. In another process variant the sulfur compounds are removed after shift conversion. The reaction heat can be removed by use of a tube-cooled reactor raising steam or heating water for gas saturation to supply process steam in the reforming section (Linde LAC. for example. page 28 of 77 . [584] sulfosolvan B. in some cases in excess of 50 %. The following activators are used: in Benfield [567-570] and Carsol [571]. [574] a sterically hindered amine. depending on the feedstock composition. in some cases. [588] (British Gas) have been introduced. for which the solubility approaches a saturation value. [572] process ethanolamines. There is also a trade-off between energy consumption and investment costs. Further purification is performed by PSA. Amine Guard introduced by Union Carbide) [603]. the gas may contain a rather high amount of sulfur compounds (mainly H2S with smaller quantities of COS). Gas Purification In further purification. and the spent scrubbing liquid is subsequently heated and regenerated in a stripping column before being recycled to the pressurized absorption column. differ in the type of activator used to increase the reaction rate between the CO2 and the solvent. often in steps. which needs heat for its decomposition.8 % in a single step in a large hydrogen plant by using a quasi-isothermal reactor (e. have good solubility in the applied solvents. but recently radial gas flow configurations have been chosen in some instances. usually in countercurrent in a column equipped with trays or packings. [586] and LRS-10 [587]. residual carbon monoxide. The energy consumption of the modern systems.g. Primary and secondary amines. Heat requirements for regeneration in CO2 removal systems [406].g.. All hot potash systems need corrosion inhibitors the concentration of which must carefully to be monitored. [418]. The raw synthesis gases from partial oxidation of heavy hydrocarbons and coal differ mainly in two aspects from that produced from light hydrocarbons by steam reforming. The CO2-laden solvent is flashed. In contrast steam reforming requires removal of sulfur from the natural gas and light hydrocarbon feedstocks upstream of gasification to avoid poisoning of the sensitive reforming catalysts. at higher partial pressures the physical solvents (according to Henry's Law. In the chemical solvents the carbon dioxide becomes fixed as a chemical compound. The first progress was made by addition of corrosion inhibitors (e. In the physical solvents the carbon dioxide dissolves without forming a chemical compound. and in the few cases where it was used it was followed by additional scrubbing with monoethanolamine (MEA). b Two-stage regeneration. [638] is present under reaction conditions in sulfidized form and requires for its performance a sulfur content in the gas in excess of 1 g S/m3. [554-556]. often river water. Irrespective of the catalyst type used. Generally the shift conversion reactors have an axial gas flow pattern. In the early days of ammonia production water. to around atmospheric pressure. [419]. Today a variety of solvents are used and they can be categorized as physical or chemical solvents.1. [404]. the so-called dirty shift catalyst is used in this case. methyldiethanolamine together with an activator. The first generation of single-train steam reforming ammonia plants used MEA and consumed about 5. Very suitable for the partial pressure range of CO2 in steam reforming plants (4 – 7 bar) are chemical solvents based on aqueous solutions of potassium carbonate or alkanolamines containing additional activators to enhance mass transfer and. Shift Conversion in Partial Oxidation Plants. New activators named ACT-1 (UOP) [585]. This is usually performed by hydrodesulfurization and adsorption of the H2S by ZnO. kJ/mol CO2 209 116 88 BASF aMDEA process (1-stage)a 73 Benfield LoHeat 28 – 35 BASF aMDEA process (2-stage)b 28 – 30 a Single-stage regeneration. CO2 loading characteristics of various solvents As both sour gases. [536]. Figure 31. monoethanolamine (MEA) and diethanolamine (DEA) exhibit a high mass-transfer rate for carbon dioxide but have a high energy demand for regeneration. the loading is approximately proportional to the CO2 partial pressure) have a higher loading capacity than the chemical solvents. [1318]. the CO content is much higher. ICI Catalco LCA). The activators enhance mass transfer and thus influence not only the regeneration energy demand (circulation rate of the solvent) but also the equipment dimensions. in the Catacarb system [565].3. for example. and this allows recovery simply by flashing. which are removed as described below. as shown in the last two lines of Table 23 depends largely on process configuration and required final purity which may range from 50 to 1000 ppm in the purified gas. First. The hot potash systems. The total sulfur contained in the coal and hydrocarbon feedstock is converted in gasification to H2S and a smaller amount of COS.8 GJ per tonne NH3. It has been reported that the CO content can be reduced from 50 to 0. Liquid Nitrogen Wash [656-658]. No corrosion inhibitors are necessary. too.2 – 0. the Sepasolv MPE process (BASF) [565]. [1396] a theoretical analysis is given in [562]. [1332]. [590]. but it is also environmentally undesirable because of copper-containing wastewater. Selectoxo Process. In this case the shift conversion is carried out in two stages with a special sulfur-tolerant shift catalyst followed by removal of hydrogen sulfide and carbon dioxide in an acid gas removal unit. preferably at higher pressure in an intermediate stage of synthesis gas compression. Methanation is the simplest method to reduce the concentrations of the carbon oxides well below 10 ppm and is widely used in steam reforming plants. [1396]. [603]. Methanation. Lurgi Purisol Process (Nmethylpyrrolidone) [581-583]. For this reason water and traces of carbon dioxide must be removed from the make-up gas downstream of methanation. Because of the potential danger of a sulfur break-through causing poisoning. The Selexol process (UOP) [565]. In the lower half of this partial pressure range chemical solvents based on tertiary amines (e. and process configurations include two. a substantial amount of carbon dioxide can be recovered simply by flashing to low pressure. methanol regeneration in the Rectisol process can be tailored to achieve a H2S-rich fraction for downstream processing and pure CO2. If a breakthrough of carbon monoxide from the low-temperature shift or carbon dioxide from the absorption system occurs. which in this case has to operate over dirty shift catalyst (see Section Carbon Monoxide Shift Conversion) with the consequence that the H2S is diluted by a large amount of CO2. removing pure CO2. [618-622]. This is accomplished by passing the make-up gas through molecular sieve adsorbers. the cooled raw gas is mixed with the stoichiometric quantity of air or oxygen needed to convert the carbon monoxide to carbon dioxide. A prominent example is the Braun Purifier process [457]. It converts the residual carbon oxides to methanol. The purifier is a cryogenic unit placed downstream of the methanator and its duty is to remove the nitrogen surplus introduced by the excess of air used in the secondary reformer of the Braun ammonia process (Processes with Reduced Primary Reformer Firing). introduces some complications for CO2 removal from partial oxidation gases. a first scrubbing stage. page 29 of 77 . [648] has been proposed for partially replacing methanation. the intensely exothermic methanation reaction can reach temperatures exceeding 500 °C very quickly [641]. After low-temperature shift conversion. Cryogenic methods are usually used for final purification of partial oxidation gases. but at higher values physical solvents become increasingly preferable. [584]. Methanolation [647]. Dryers. In this case.1-dioxide + diisopropanol) [579]. BASF's aMDEA [592-594]. no heat is necessary. After bulk removal of the carbon oxides has been accomplished by shift reaction and CO2 removal. with methanol as solvent operating at –15 to – 40 °C. The mixture is then passed through a precious-metal catalyst at 40 – 135 °C to accomplish this selective oxidation [643-646]. a clean up methanation unit must follow the methanolation section. including inert gases and is also the means for adding some or all of the nitrogen required for synthesis. The cooling energy is supplied by expansion of the raw gas over the turbo-expander (pressure loss about 2 bar) and expanding the removed waste gas to the pressure level of the reformer fuel. [624-627]. Liquid nitrogen wash delivers a gas to the synthesis loop that is free of all impurities. Examples of physical solvent processes are Selexol Process. only a high-temperature shift conversion is used. Final Purification. the sour gases are removed after shift conversion. permitting the H/N ratio in the synthesis loop to be set independent of the secondary reformer. [628-633]. This results in a carbon monoxide content of the gas after shift conversion in the range 3 – 5 vol %. Normally. for the partial oxidation processes. 1 % CO2 breakthrough leads to an adiabatic temperature rise of 60 ° C. Before the solvent is recycled it has to be vacuum treated and/or air stripped to remove traces of carbon dioxide.g. [623]. [1280].005 – 0. It is actually the reverse reaction of steam reforming of methane: (16) and (17) The advantages of simplicity and low cost more than outweigh the disadvantages of hydrogen consumption and production of additional inerts in the make-up gas to the synthesis loop. The methanol may be recycled to the steam reformer feed or recovered as product. Physical solvent based processes can also be applied in steam reforming ammonia plants. The carbon dioxide is recovered simply by flashing. the normal copper – zinc – alumina catalyst is usually not applied. UOP has also developed a number of other process configurations [573]. the typical synthesis gas still contains 0. but they have a rather high capacity for adsorbing water. a task difficult to fulfil with the other physical solvents mentioned. Methanol is removed from the gas by water scrubbing. Copper liquor scrubbing for carbon monoxide removal. This concept reduces the effort required to receive a highly concentrated H2S fraction suitable for further processing in a Claus plant or a sulfuric acid plant. Not only does it have a high energy demand. In one. and only a small amount has to be recovered by stripping. a rectifier column with an integrated condenser and turbo-expander. which is usually achieved by chilling and operating at low temperatures. Since the sour gases are both soluble in the solvents and a separate recovery in the regeneration stage is only partially possible (only pure CO2 can be obtained along with a CO2 fraction more or less rich in H2S) two principal plant flowsheets are possible. The carbon dioxide formed by the Selectoxo reaction adds only slightly to the load on the downstream carbon dioxide absorption system. and not very volatile. Sulfinol process (tetrahydrothiophene-1. [593]polyethylene glycol methyl isopropyl ether. It is energetically advantageous to add the purified synthesis gas at a point in the synthesis loop where it can flow directly to the synthesis converter (see Section Synthesis Loop Configurations). The purifier is a relatively simple unit composed of feed/effluent exchanger.and singlestage designs. has become obsolete and is now operated in only a few installations. As full conversion of the carbon oxides is not achieved. The carbon dioxide binds much less strongly to MDEA than to MEA. These solvents are stable. as all oxygen-containing substances are poisons for the ammonia synthesis catalyst [640]. and the solvent character is more like a hybrid between a strong chemical and a purely physical solvent. which is surprising as the same risk exists in partial oxidation based methanol plants for the similarly composed methanol catalyst. [605-613] uses polyethylene glycol dimethyl ether. In the other arrangement. [649-655]. and the Fluor Solvent Process [614-617] polypropylene carbonate. is located downstream of the shift reactors. commonly employed in early plants. the latter allowing old MEA units to be revamped [599-601] simply by swapping the solvent without changing the process equipment. is positioned upstream of the shift conversion. as well as the inert gas content of the purified synthesis gas fed to the synthesis loop. MDEA) might be suitable. These compounds and any water present have to be removed down to a very low ppm level. For this reason the raw gas has to be dry. Another advantage of the process is that it separates the frontend and the synthesis loop.. Additionally the inert level in the synthesis loop is reduced through this unit because methane is completely and argon is partially removed from the make-up gas. The process is very versatile: increasing the activator concentration shifts the character of the solvent more to the chemical side and vice versa. noncorrosive. The second stage. [591]. Owing to the low vapor pressure of MDEA and the activator. Raw synthesis gas produced by partial oxidation has a carbon dioxide partial pressure between 10 and 30 bar. Controls should be installed and other security measures taken to avoid these high temperatures because the catalyst may be damaged or the maximum allowable operating temperature of the pressure vessel wall exceeded. The required catalyst volume is relatively small. The reaction is carried out over a supported nickel catalyst at a pressure of 25 – 35 bar and a temperature of 250 – 350 °C. the Rectisol process (Linde and Lurgi) [565]. Sepasolv MPE process. depending mainly on feedstock and gasification pressure. The high thermal efficiency of this two-stage adsorption process using lean and semi-lean solvent is achieved by recompression of the flash steam with an injector or a mechanical vapor compressor. At –185 °C methane and argon are washed out. The presence of sulfur compounds. [602]. [595-598] process uses an aqueous solution of the tertiary amine methyldiethanolamine (MDEA) with a special amine activator. [1333]. there are no losses of the active solvent components. When comparing the energy consumption of physical and chemical solvent based processes for a given application not only the heat for solvent regeneration but also the mechanical energy for solvent circulation has to be taken into account. Extensive surveys and additional literature of the CO2 removal processes are found in [564]. The Selexol Process for example operates at 5 °C. On account of the relative weak binding forces. and unlike MEA no solvent degradation is observed. probably the most important in this connection. [604]. nontoxic. The Selectoxo process (Engelhard) reduces the hydrogen consumption of the methanation system. but may be also incorporated in steam reforming plants.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience A typical example of an efficient modern hot potash system is the Benfield LoHeat [589] system of UOP. For example. so recovery installations are not required.5 vol % CO and 0.2 vol % CO2. Methanation as final purification for the raw gas from partial oxidation was proposed by Topsøe [642]. mainly H2S along with a minor quantity of COS. which removes the sulfur compounds. and. Part of the oil returns to the reservoir. Kellogg's KAAP) a centrifugal compressor could be used down to 220 t/d. For this reason the overall efficiency of a plant with steam-driven centrifugal compressors is superior. Usually. some designs applied a common crankshaft for the piston rods of the gas machine cylinders and compressor cylinders. It uses molecular sieves as adsorbents in a series of vessels operated in a staggered cyclic mode changing between an adsorption phase and various stages of regeneration. which even for plants with 1800 t/d is between 155 and 190 bar [404]. in Claude and Casale units.. Today modern plant concepts for world-scale capacity plants tend to limit the number of compressor casings to two. liquid-film shaft seals with cylindrical bushings (floating rings) are applied [680]. natural gas. In these machines a good dynamic balance can readily be achieved. for example. [679]. From this time onwards application of centrifugal compressors became standard practice in most ammonia plants irrespective of the synthesis gas generation technology. This process can be used to replace the LT shift conversion. process air. Today. 4. with the unique feature that compressor and electric driver were completely enclosed in a common high-pressure shell. The process scheme for the ammonia plant may consist of production of pure hydrogen followed by separate addition of pure nitrogen from an air separation unit [665-667]. the Linde LAC process (Linde LAC Process. so-called mole pumps were also used. from 25 to 200 bar would require 18 – 20 impellers. It flows to the top of a wash column. whereby the CO2 removal section was usually installed between the 3rd and 4th stages. Depending on synthesis configuration. Since this technology has proven its reliability in rather large hydrogen plants for refineries it is now also used for world-scale ammonia plants . provides the actual sealing. All of the cold equipment is installed in an insulated “cold box. Centrifugal compressor for make-up and recycle gas compression of an ammonia plant (courtesy of Uhde) a) Air cooler. The gas was first compressed to the level of the CO2 removal section (usually 25 bar) and afterwards to around 300 bar for final purification (at that time usually copper liquor scrubbing) and synthesis. rendering the machine extremely ineffective. When gas engine drivers (two-stroke type) were used instead of electric motors. Kellogg in 1963 was the use of centrifugal compressors for the compression duties of synthesis gas and recycle. Unless the total volumetric gas flow has a reasonable relationship to the passage width of the last impeller and the pressure ratio. Pressure Swing Adsorption. with improved manufacturing techniques. although the centrifugal compressors are inherently less efficient than reciprocating units. [676]. Figure 32 shows an example of the synthesis gas compressor of a large ammonia plant.). Of course. process air. together with a small quantity of gas. Reciprocating compressors with as many as seven stages in linear arrangement with intermediate cooling were used. In old Casale plants.” The wash column temperature is about –190 °C. mixing of make-up gas and recycle can be performed inside the casing or outside (three or four-nozzle arrangement. The hydrogen recovery may be as high as 90 % depending on the number of adsorbers in one line. the various compression services. Manufacturing capabilities limit the minimum possible passage width (today about 2. In practically all modern ammonia plants. higher speeds are possible and also the use of asynchronous motors is possible. [661-664]. and this limits the possible length such that a compressor shaft cannot accommodate more than eight or nine impellers. In a special version the nitrogen can be added in the PSA unit itself to increase hydrogen recovery [668-671]. for today's world-scale capacities of 1200 – 2000 t/d these technical limitations have no influence on the synthesis pressure. the make-up gas was introduced into the high pressure recycle loop and acted as the driving fluid of an injector which compressed the recycle gas. However. Further information on reciprocating compressors is given in [673]. In some processes it may also remove the excess nitrogen introduced with the process air fed to the secondary reformer. for example. Along with the introduction of pressure gasification. These flexible couplings prevent possible compressor damage resulting from slight misalignment and shaft displacement. Machines with a suction volume up to 15 000 m3 (STP) for the first stage were not uncommon. In older plants which used a reciprocating compressor for the recycle. carbon dioxide removal.8 to 3. it is withdrawn through a reduction valve. In some instances gas engines were used as drivers. In most cases the centrifugal compressors in ammonia plants are directly driven by steam turbines. Higher pressures were used in a few installations. a compressor shaft must have sufficient rigidity to avoid excessive vibration. Compression Up to the mid-1960s reciprocating compressors were used to compress the synthesis gas to the level of the synthesis loop.g. [660]. Figure 32. d) Water cooler Geared or metal diaphragm couplings are used to connect the shafts of the individual casings (two in Figure 32). [677]. while the remainder flows against the gas pressure into a small chamber from which. Seal oil flows between both halves of the floating ring. methanation. In modern plants. and expansion. causing explosions [659].g. Liquid nitrogen wash systems are in operation at pressures up to 8 MPa corresponding to the highest gasification pressures. an oil film between the shaft and a floating ring. [678]. and refrigeration. The floating ring is usually sealed to the compressor casing by O-rings.. the air separation and the nitrogen wash frequently are closely integrated with one another so that economies can be realized in the refrigeration system. allows simpler piping connections and facilitates maintenance. Huge flywheels were designed as the rotors of synchronous motors (ca. The fundamental advantage of these machines are low investment (single machines even for very large capacities) and maintenance cost. As newer synthesis catalysts allow a pressure of 80 bar in the synthesis loop (ICI's LCA Process. 34). To overcome the pressure drop (5 – 20 bar) in the synthesis loop re-compression of the recycle gas is required. This avoids the losses associated with generation and transmission of electric power. A further advantage is that centrifugal compressors require only a fraction of the space needed for reciprocating compressors. with compression ratios from 1. [406]. and synthesis gas compression. This corresponds to a capacity of 400 t/d at 145 bar. Traces of nitric oxide (NO) may react with olefinic hydrocarbons. which limits the pressure increase attainable by each impeller. e. The first single-train ammonia plants with a capacity of 550 – 600 t/d had to lower the synthesis pressure to 145 – 150 bar to meet the required minimum gas flow condition. In smaller plants. One of the most important features of the energy integrated single-stream steam reforming ammonia plant pioneered by M.g. The rapid technical progress in the hydrocarbon based technologies of steam reforming and partial oxidation made it possible to generate the synthesis gas at a pressure level sufficient for the CO2 removal operation. It is therefore necessary to arrange several compressor casings in series. A pressure increase. a recycle cylinder was often mounted together with the other cylinders on the reciprocating frame. and even the secondary reformer as well [406]. e. The regeneration of the loaded adsorbent is achieved by stepwise depressurization and by using the gas from this operation to flush other adsorbers at a different pressure level in the regeneration cycle. c) Silencer. page 30 of 77 . the minimum gas flow from the last wheel is 350 m3 for synthesis gas with a molecular mass of about 9 and an efficiency of around 75 %. e. The low height of the arrangement has less severe requirements for foundations. where it countercurrently contacts precooled synthesis gas from which most of the methane and hydrocarbons have been condensed. cooling. Prior to about 1950 gas generation processes and shift conversion operated at essentially atmospheric pressure. an air separation plant is installed in conjunction with liquid nitrogen wash for economy in operation. were apportioned among the crankshaft throws in such a manner that a single compressor can perform all compression duties [672].2. with about 50 ppm argon and less than 10 ppm of other impurities. Water and carbon dioxide in the inlet gas will freeze. the shaft of the final casing also bears the impeller for the compression of the recycle gas. As gasification proceeds with a considerable volume increase and feedstocks such as natural gas are usually already available under pressure at battery limits. Sometimes special rotary compressors. the LCA Process of ICI (ICI LCA Process. and high reliability (low failure rate) [958]. b) Separator. Very high purity can be obtained. W.). [627]. which was around 300 bar in the majority of the plants at that time. [674]. causing operating difficulties.5. excessive pressure losses would occur within the passage and in the diffusers between the impellers. less frequent shutdowns for preventive maintenance. The high pressures and the high rotational speeds involved do not allow mechanical contact shaft seals. In a very few cases steam turbines with special speed reduction gears have been used.8 mm) at the outer circumference of a centrifugal compressor impeller and this imposes a limit on the minimum effective gas volume leaving the last impeller. Sealing of the rotating shaft against the atmosphere is an important and demanding task.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience The nitrogen is liquefied in a refrigeration cycle by compression. considerable savings in compression energy are achieved in this way. capable of rotation. Careful surveillance of the inlet gases is required. Figure 33 is a schematic diagram of a liquid film shaft seal. Normally. The tensile strength of the steels normally used to manufacture the compressor impellers allow a maximum wheel tip speed of about 330 m/s. [675]. In this concept. which may be as high as 10.2. horizontally balanced compressors in which the cylinders are in parallel configuration on both sides of a common crankshaft became the preferred design. Fig. 125 rpm) with two crankshafts on both sides connected over crossheads with the piston rod for the horizontally arranged stages. 5. 34 A). in stock. the seal ring is pressed tight against the seat by means of a spring and the differential gas pressure. a methanol synthesis compressor for 75 000 m3/h and a pressure of 75 bar. a steam turbine) drives a common gear to which the individual compression stages are connected. spare rotors. Integrated geared centrifugal compressors developed by DEMAG and GHH are a new development which might become of interest for plants of smaller capacity operating at lower pressure. Otherwise the compressor can enter a state of pulsating flow. Liquid film shaft seal with cylindrical bushing for a high-pressure centrifugal compressor a) Shaft. or as combustion air in the primary reformer [705-709]. 4. with oil reservoir. Often. which is achieved at the radial interface of rotating and stationary rings. They can be classified according to the location of ammonia condensation and the point at which the make-up gas is introduced. e) Recycle compressor page 31 of 77 . 25 000 rpm or higher. An increased vibrational amplitude is often an early indication of mechanical faults. An interesting concept is dry (oil-less) gas seals for centrifugal compressors. The driver (e. Each stage has a single impeller which runs with very high speed. after molecular sieve dehydration or liquid nitrogen wash). or misalignment. After the gas leaves the synthesis converter. called surge. g) Seal oil. and oxygen.3. kickback control) is designed to prevent this condition. which requires relatively low temperatures for reasonable efficiency. Evaluation criteria are energy consumption. ammonia is condensed by cooling and the recycle gas is referred to the recycle compressor. Compressors with three or four stages are in operation.g. boiler feed water pumps. it is necessary to seal the compressor shaft towards the atmosphere only at suction pressure level. e. in addition to measuring bearing temperatures. sometimes blade failures occurred. where several compressors for synthesis gas and refrigeration duty equipped with dry seals have been successfully placed in service. i) Drain to gas – oil separator. investment and reliability. such as water and carbon dioxide (for example. A new development. Gas turbines have also been used as drivers for compressors in ammonia plants. Since then they have been widely used in off-shore service [681-683].g. which is of interest in plants where the ammonia product is split between direct users at the site and cold storage in changing ratios. k) Drain to lube oil tank. Figure 34. d) Ambient side. Magnetic bearings promise a wider temperature range. so that only partial conversion of the synthesis gas (25 – 35 %) can be attained on its passage through the catalyst. for example. It is likely that this concept can be extended to a final pressure of 120 bar with ammonia synthesis gas. the process steam in steam reforming plants. some of the seal gas flows back to the suction side to be re-compressed. As the labyrinth section on the pressure side is connected with the suction side through an equalizing line. Figure 34 shows the principal possibilities. occurrence of friction. B) Product recovery after recycle compression. A combination of magnetic bearings and dry seals (dry/dry) could totally eliminate the oil system. and nitrogen compression in partial oxidation plants. Other compression duties in the plants. Also for the ammonia compression in the refrigeration section centrifugal compressors are normally in service. the axial position of the rotor and the radial vibrational deflections are continuously monitored by sensors. e) Floating seal rings. Synthesis Loop Configurations A number of different configurations are possible for the synthesis loop. are less prone to wear. and blowers. f) Seal oil surge vessel. 4. During stops. 704]. Only recently have they gained acceptance in the ammonia industry. such as rotor imbalance. A point of discussion is sometimes the minimum load at which the compressor can run without kickback... Any wear resulting in friction could lead instantly to severe damage. h) Lube oil. Usually a load of 75 % is possible. These basic features together with the properties of the synthesis catalyst and mechanical restrictions govern the design of the ammonia synthesis converter and the layout of the synthesis loop. when the shaft is not rotating. Ammonia Synthesis Under the conditions practical for an industrial process ammonia formation from hydrogen and nitrogen is limited by the unfavorable position of the thermodynamic equilibrium. d) Synthesis gas compressor. already in commercial service. The concentration of the inert gases (methane and argon) in the synthesis loop is controlled by withdrawing a small continuous purge gas stream. b) Bearing. filters and (in part) pumps in common. Anti-surge control (minimum by-pass control. In older steam turbines. as well as to minimize the incidence and degree of uneconomical recirculation. which means without loss of efficiency. In modern plants the synthesis gas compressors. and for other drivers. and a small amount from the suction side may go to the atmospheric side and is sent to the flare on account of its content of combustibles. In this way normally no oil should enter into the synthesis gas.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Figure 33. for example. which eliminates an emission source. and with special impeller design. These are generally extraction turbines with a condensing section. C) Product recovery before recycle compression (four-nozzle compressor design).3. are almost exclusively driven by a steam turbines. The high rotational speeds and the relatively large masses of the compressor rotors place high demands on the performance of the bearings. Development dates back to 1969 and the first commercial application was in a naturalgas compressor in 1973. it is necessary to re-circulate gas through individual stages or through the whole machine. for example. Nitrogen is used as an inert fluid for the seal. Steam is extracted at suitable pressure levels (e. seal oil supply is combined with the lubricating oil system. but so far not used in ammonia plants. The exhaust may be used for steam production.g. but this is no longer a problem due to improved blade design and shroud bands. b) Ammonia recovery by chilling and condensation. It results in the lowest ammonia content at the entrance to the converter and the highest ammonia concentration for condensation. Schematic flow diagrams of typical ammonia synthesis loops A) Synthesis loop for pure and dry make-up gas. If the volumetric flows through the machine at start-up or during reduced load operation deviate too far from the design values. air compressor. Dry gas seals in combination with oil-lubricated bearings (dry/wet) have the advantage that a much smaller oil system is required and that there is no contact between oil and gas. During operation the seal is not completely tight. Criteria for compressor selection and economic comparisons are discussed in [685-692. Ammonia is separated from the unreacted gas by condensation.5. As failures and breakdowns of these large rotary machines could lead to long and expensive repairs and to a corresponding loss of production it is advisable to keep the essential spare parts. For these reasons. in modern plants often with the help of a distributed control system (DCS). including recycle. Compressor control is achieved basically by controlling the rotational speed of the driver. which are standard today. c) Ammonia recovery by condensation at ambient temperature. for preheating duties. for example.1. The unconverted gas is supplemented with fresh synthesis gas and recycled to the converter. are less prone to developing vibrations due to imbalance and require less maintenance. If the make-up gas is absolutely free of catalyst poisons. This represents the most favorable arrangement from a minimum energy point of view. which could cause damage. are also performed by centrifugal compressors. it can be fed directly to the synthesis converter (Fig. which must withstand high thrust forces. The minimum clearances necessary at the labyrinths and the impellers allow practically zero wear. bearing damage. ammonia compressor. Additional information is given in [693-703]. c) Pressure side. In some cases screw compressors have been used for this duty on account of their good efficiency and load flexibility. D) Two stages of product condensation a) Ammonia converter with heat exchangers. such as process air in steam reforming plants and air. is magnetic bearings [681]. [684]. the value may be lowered to 70 % but with a slight sacrifice of full-load efficiency. This is especially true for the thrust bearings. 45 – 55 bar) to provide. l) Elevation for seal oil head The seal oil pressure in the floating ring cavity must always be slightly higher than the gas pressure within the casing which is provided by static height difference of the oil level in the elevated oil buffer vessel. catalyst particle size is 6 – 10 mm Converter performance also diminishes (Fig. a level of 10 ppm in the make-up gas. Figure 38. Usually.0 –5 1 3. 37) with increasing oxygen content of the synthesis gas.6 Figure 35. there are plants that operate at about 8 MPa (80 bar). ammonia formation increases (Fig. Typical operating parameters for modern synthesis loops with different pressures are listed in Table 24.. Figure 36. advantage is taken of the fact that these materials are absorbed completely by condensing ammonia. Figure 37. mol % NH3 separator temperature. Typical operating parameters for modern synthesis loops at 140 and 220 bar (1000 t/d NH3) [404. The scheme shown in Figure 34 C. Usually. For a secondary loop based on purge gas.. Today. avoids this waste of energy. Table 24. In industrial practice. conversions near equilibrium are attained.5. a higher ammonia concentration exists at the inlet to the converter. The optimum conversion at high space velocity [SV = m3 (STP) gas h–1 · m–3 catalyst] lies close to an H2 /N2 ratio of 2 and approaches 3 at low space velocities. but also from the effect on the reaction rate itself. before diluting the recycle gas) and thereby to reduce the energy expenditure for refrigerated cooling. °C Relative catalyst volume 220 500 000 407 000 4.7 MPa pressure [1136] In practice. Also. it is possible to cool the recycle gas using cooling water or air immediately before admixing the make-up gas (i. In this configuration. the ratio is adjusted to 3. 35). but there are also those that operate at more than 40 MPa (400 bar). p. Nm3/h Inlet NH3 conc. 30 MPa pressure. 4.. Today. With increasing pressure. When ammonia-containing recycle gas and carbon dioxide containing make-up gas mix together under certain conditions of concentration and temperature. The reason is that equilibrium plays a greater role at low space velocities and has a maximum at a ratio of 3. except for small corrections [14] with regard to the behavior of real gases. precipitation of solid ammonium carbamate can result. Section Catalyst Poisons). 38). This applies especially at synthesis pressures above about 25 MPa (250 bar). 226] Parameters Inlet pressure.0 –5 0. In recent years. Figure 34 B shows the simplest such configuration. An additional drawback of this arrangement is that all the ammonia produced must be compressed with the recycle gas in the recycle compressor. Converter performance is determined by the reaction rate. inlet NH3 content is 3. At these pressures. a greater portion of the ammonia formed can be liquefied by cooling with cooling water or air (see Fig.9 12. mol % Outlet NH3 conc. which depends on the operating variables (cf. corresponding to about 3 ppm in the converter inlet gas. plants are built mainly for synthesis pressures of 150 – 250 bar. Splitting the cooling step for ammonia condensation also offers advantages when the recycle gas is compressed together with the make-up gas. Performance of a converter as a function of inlet inert gas (CH4 and Ar) content with space velocity (per hour) as parameter. Formation of Ammonia in the Converter The central part of the synthesis system is the converter. This results not only from the more favorable equilibrium situation for the reaction.8 19. Performance for a four-bed quench converter as a function of operating pressure with space velocity (per hour) as parameter. in which the conversion of synthesis gas to ammonia takes place. the frequently used “four-nozzle compressor”. is usually. This arrangement has the disadvantage that the ammonia concentration for condensation is reduced by dilution with the make-up gas. recycle compression follows directly after condensing and separating the ammonia.5 %.e.2. Ammonia conversion as a function of hydrogen/nitrogen ratios in the inlet synthesis gas with space velocity (per hour) as parameter. with increasing space page 32 of 77 . 36).Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience When the make-up gas contains water or carbon dioxide. space velocities vary from about 12 000 h–1 at about 15 MPa (150 bar) to about 35 000 h–1 at about 80 MPa (800 bar). Performance of a converter as a function of oxygen content (all oxygen-containing impurities) in the inlet synthesis gas In contrast to the above-mentioned variables. also as a retrofit in existing plants. at equal condensing temperature. This requires that the condensation stage be located partially or wholly between the make-up gas supply point and the converter. because in most plants. 10 % inerts in the inlet synthesis gas Converter performance decreases with increasing inert gas content (Fig. it can be 30 % or more (see Section Inert-Gas and Purge-Gas Management). Section Kinetics). 9. molecular sieve drying of make-up gas has increasingly been applied in order to realize the energy-saving arrangement of the synthesis loop corresponding to Figure 34 A. 34D). The usual range is 0 – 15 vol %. not exceeded (cf. The effect of these parameters is discussed briefly in the following.1 17. With this arrangement. mol % Inlet inert conc.1 8.3. bar 140 Inlet flow. the dependence of the converter performance on the H2 /N2 ratio shows a true maximum (Fig.. this latter feature is not always necessary. usually 530 °C (cf. The mixture is heated to about 200 °C in an internal exchanger located beneath the catalyst bed. other parameters have to be considered. such converters have the further disadvantage that temperature control is sluggish and temperature oscillations dampen out very slowly. temperature profiles. as ammonia formation progresses. that cooling predominates and the temperature of the reacting gas begins to fall. resulting in increasing heat removal (Fig. Section Kinetics). Relatively cool converter feed gas flows through the annular space between the outer pressure shell and the internal “basket”. and for some of them optimum values have to be found. it is possible to minimize the required catalyst volume by cooling the reacting synthesis gas such that. To remove the heat evolved in the synthesis reaction. a synthesis gas with about 3 % ammonia concentration entering the converter cannot be heated to the “ideal” temperature by heat exchange because the very high temperature required does not exist in the converter system. Today. The gas then enters the cooling tubes that run through the catalyst bed (Fig. Today the objective is to optimize the heat recovery (at the highest possible level) and to minimize the investment for the total synthesis loop. The second handles the temperature – position behavior of the reacting synthesis gas. Internally cooled with cooling tubes running through the catalyst bed or with catalyst inside the tubes and the cooling medium on the shell side. [740].. in turn requiring very careful control [710-712]. the first portion of the catalyst must initially operate adiabatically. a phenomenon called “hunting”. The line for zero reaction rate corresponds to the temperature – concentration dependence of the chemical equilibrium. The catalyst volume is divided into several beds in which the reaction proceeds adiabatically. To maintain maximum reaction rate. i. The reaction temperature profile is of particular importance because the reaction rate responds vigorously to temperature changes. a) Catalyst. Typical of such converters is the countercurrent design of the Tennessee Valley Authority (TVA) [710]. Nitrogen reaction rate v in m3 NH3 / (m3 catalyst · s) as a function of temperature and ammonia concentration at 20 MPa pressure and 11 vol % inerts in the inlet synthesis gas a) Locus of temperatures resulting in maximum reaction rate at a given ammonia concentration If the objective in design or operation were optimizing catalyst utilization. 2. The severe conditions of high pressure. As the reacting gas temperature rises. Part of the feed gas to this unit enters the converter at the top and flows down through the annular space between the pressure shell and the basket. Commercial converters can be classified into two main groups: 1. the heat is transferred to the converter feed gas to heat it to the reaction ignition temperature or to an external cooling medium. The reaction begins almost adiabatically in the catalyst bed. The first models the concentration – position relationship for transformation of the reactants to products. [737]. If the synthesis converter is to be operated in this region. With these tubes. This raises the question of the definition of optimum. The main gas flow enters the converter at the bottom and joins the shell cooling gas. 36. A converter temperature – concentration profile was often compared to the “ideal” for optimum usage of high-pressure vessel and catalyst volumes [731]. part of the converter feed – effluent heat-exchange system surface is placed within the converter pressure shell. For example. converters have been designed in which cooling tubes run through the catalyst bed. gas compositions. the operating point normally chosen means that the increase in gas flow rate more than compensates for the reduced ammonia concentration. For a discussion of modeling of different converter types. [716-718]. Thus. it was necessary to optimize catalyst utilization. By this means. The design of an ammonia converter is a demanding engineering and chemical engineering task. Increasing converter flow rate and declining synthesis catalyst activity can reach a point. and high hydrogen partial pressures place strict requirements on the construction materials and design for both groups. a still higher ammonia production rate is achieved. cross-flow or radial flow pattern. the temperature follows curve (a). The temperature profile depends not only on the rate of reaction heat evolution but also on the method and nature of the system for removing heat from the catalyst bed or beds. Converter Design. e) Vessel wall cooling gas inlet. 40 A). Models for multibed quench converters are described in [723-730]. Commercial Ammonia Converters. the reaction kinetic equation (cf.or cocurrently to the gas flow in the synthesis catalyst volume (tube-cooled converters). where the reaction “blows out” and production ceases. [723]. Figure 39. General information on converter calculations are given in [127]. if at all. up to about 530 °C. To reach the “ideal” temperature. In the days when converters were designed to operate at very high pressures and temperatures and before the advent of improved construction materials. However. Principal Reactor Configurations. As the reacting gas reaches the bottom of the catalyst bed. and this meant maximum use of the catalyst. permitting use of comparatively low-alloy chromium – molybdenum steels for its construction. g) Gas exit page 33 of 77 . the rate of reaction begins to decrease sufficiently. Curve (a) represents the temperature – concentration locus of maximum reaction rates. When designed to utilize the heat of reaction for heating the converter feed gas. the nozzle penetration through the pressure shell for the converter effluent gas is also maintained at relatively low temperature. 40 B). Section Catalyst Reduction). the temperature must decrease as ammonia concentration increases. f) Temperature control gas (cold-shot) inlet. see [713-717]. To calculate the parameters for the design. Figure 40 C shows the converter temperature – concentration profile. a suitable mathematical model is required.e. The form of the latter is characteristic of the type of converter. even with careful control. Often. Cold converter feed gas can be admitted at a point within the internal heat exchanger to control both the total system and reaction temperature profiles through bypassing. d) Main gas inlet. Achieving maximum ammonia production requires operating in the neighborhood of this “blow out” point. the state of the art in converter construction materials now allows design of exit nozzles for the maximum anticipated temperatures. such as gas distributors and heat exchangers. It is also obvious that in reality this “ideal” temperature – concentration profile cannot be achieved. To maximize converter output and plant capacity to achieve the most favorable overall manufacturing cost. [130]. almost all converters consist of an outer pressure vessel containing a separate inner vessel housing the catalyst and other internals. This occurs when the heat of reaction is no longer sufficient to provide the temperatures necessary for operation of the feed – effluent heat exchanger. too. Plant operation often takes advantage of these phenomena. then Figure 39 shows that the converter temperature – composition profile should follow curve (a). which corresponds to maximum reaction rate at all points. b) Heat exchanger. Tubular reactors are treated in [732-739]. The reaction temperature at first somewhat exceeds that for maximum reaction rate but eventually falls below curve (a). i. References [745] and [746] review some literature on ammonia converters. the converter design represented a real limitation on plant capacity.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience velocity. [740]. Design of ammonia synthesis reactors is not just the calculation of the required catalyst volume. Additional equations describe the behavior of the separate feed – effluent heat exchanger system [713-717]. this characteristic can be used to maintain ammonia production rate when the synthesis catalyst ages and its activity declines. which can flow counter.e. and the vessel internals. Consideration of the service life of the catalyst requires that this maximum initial temperature not exceed that recommended by the manufacturer. There it absorbs the heat released in the reaction and reaches the required reaction ignition temperature of about 400 °C. 38). This shields the outer shell from the high-temperature “basket”. 35. For example. The individual effects of the operational parameters are evaluated in [14]. and pressure drop. Computer programs and applications can be found in [719-722]. Tube-Cooled Converters. [757-759]. Between the individual catalyst beds heat is removed by injection of colder synthesis gas (quench converters) or by indirect cooling with synthesis gas or via boiler feed water heating or raising steam (indirectly cooled multibed converters). Figure 40. From Figure 39 it is apparent that there is a definite temperature at which the rate of reaction reaches a maximum for any given ammonia concentration. The different cooling methods can be combined in the same converter. Countercurrent tube-cooled converter (TVA converter) [740] A) Converter layout. because of high ammonia concentration. In the early days with more strict material and fabricationrelated limitations the converters were usually designed for minimum high-pressure volume. For example. c) Cooling tubes. Following this initial adiabatic temperature rise. the temperature difference between the reacting gas and the cooling tubes increases. Casale has employed a similar converter design concept [760]. The heat exchanger then fails to heat the cold converter feed gas to the required reaction “ignition” temperature. then it is advisable to oversize the converter feed – effluent heat exchanger system to attain a higher degree of control stability. The gas flow can have an axial. the catalyst. [724]. the ammonia concentration in the effluent synthesis gas from a given converter does indeed go down (Figs. [731]. Two differential equations describe mathematically the steady-state behavior of the reactor section of a converter. Figure 39 plots lines of constant reaction rate illustrating its dependence on temperature and ammonia concentration in the reacting synthesis gas. including dimensions and number of catalyst beds.. high temperature. The known designs for such converters are suited only for small production capacities and therefore currently of limited interest. [749-751]. The cooling medium is mostly the reactor feed gas. the synthesis gas reacts adiabatically. The Nitrogen Engineering Corporation (NEC) converter applies cocurrent flow by means of bayonet tubes [740]. The first commercial use of a converter of this type is operating successfully in a 1050 stpd (953 mtpd) plant in Trinidad [1336]. In this way the reaction profile describes a zig-zag path around the maximum reaction rate line. e) Gas exit B) Gas temperature profile through the converter. Kellogg three. maximum cooling performance — at the catalyst bed inlet. The plates are radially immersed in the axial – radial catalyst bed to remove the reaction heat while it is formed. [563]. C) Ammonia concentration versus temperature (cf. For example. The profile follows the line of the maximum reaction rate to obtain the highest possible conversion per pass from a given catalyst volume. the catalyst “basket” is not easily removable from the pressure vessel. In quench converters cooling is effected by injection of cooler. Fig. or be just defined by the location of cold gas injection tubes as for example in the ICI lozenge converter. In this type of converter only a fraction of the recycle gas enters the first catalyst layer at about 400 °C. The intent is to obtain closer approach to curve (a) (cf. 39) A disadvantage is that not all of the recycle gas passes over the entire catalyst volume so that considerable ammonia formation occurs at higher ammonia concentration and therefore at lower reaction rate. 39) Other tube-cooled converters with countercurrent flow are the Mont Cenis reactor [741-744]. and heat exchanger. Multibed Reactors with Direct Cooling (Quench Converters). and Uhde can be found in [746-748]. [751-758]. so that the total volume of the high-pressure vessel will remain about the same as that for the indirectly cooled variant [775]. 41) [1404]. The catalyst volume of the bed is chosen so that the gas leaves it at ca. c) Quench gas inlets. Figure 42. In this design. 39) Ammonia Casale developed a new converter concept which uses no tubes but arranged cooling plates. 44). However. Kellogg alone has installed more than 100 of its quench converters. catalyst bed. Though being increasingly replaced by the indirect-cooling concept by revamp or substitution they are still extensively used. W. W. f) Gas exit B) Gas temperature profile through the converter. Figure 41. c) Catalyst. Fig. [749]. The same is done in subsequent beds. M. As the quench concept was well suited to large capacity converters it had a triumphant success in the early generation of large single-train ammonia plants constructed in the 1960s and 1970s. 500 °C (catalyst suppliers specify a maximum catalyst temperature of 530 °C). 45). BASF. 42 C). the Claude converter [559].Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience B) Gas temperature profile through the converter. Figure 43 [1415] shows the temperature profile of this pseudo-isothermal reactor. and the older Fauser design [559]. d) Gas inlet. a) Catalyst. These converters were all used in relatively small plants and are now obsolete. Fig. Therefore a higher catalyst volume is needed compared to an indirectly cooled multibed converter. Descriptions of earlier designs of Kellogg. Figure 42 A shows the general arrangement of the converter. b) Heat exchanger. The catalyst beds may be separated by grids designed as mixing devices for main gas flow and cold shot. Figure 43. unconverted synthesis gas (cold shot) between the catalyst beds. The new Casale IAC ammonia converter is suited for very high capacities up to 4500 t/d for the the so-called mega-ammonia plants presently in discussion. Multibed converter with quench cooling [1137] A) Converter. Figure 45. ICI tube-cooled ammonia converter a) Top of catalyst bed. Fig. The use of plates allows a design without tubesheets and simplifies catalyst loading and unloading and the use of small size high active catalyst. the point where maximum reaction rate — therefore maximum rate of heat evolution — is taking place. This design places maximum heatexchange temperature difference — therefore. Internals of a Casale isothermal converter (courtesy of Ammonia Casale) Today. Before it enters the next catalyst bed. the gas is “quenched” by injection of cooler (125 – 200 °C) recycle gas. Fig. Mechanical simplicity and very good temperature control contributed to the widespread acceptance. no extra space is required for interbed heat exchangers. the temperature profile of the system. A schematic drawing of a quench converter together with its temperature/location and temperature/ammonia concentration profiles is presented in Figure 45. In a recent Casale company brochure the plate arrangement is shown taking a methanol converter as an example (Fig. [776-778] (Figure 46). b) Cooling tubes. Expected thermal profile of catalyst along the converter and comparison with plant data [1415] Figure 44. The catalyst can be changed by draining it at the bottom of the converter through “downcomers” that connect all catalyst beds with one page 34 of 77 . and direct or indirect cooling is provided between the catalyst beds for cooling the reacting mixture from a temperature above to a value below the “ideal” (curve (a). The most important example is the M. the great majority of synthesis converters are designed with catalyst distributed in several beds. In each bed. b) Heat exchanger. continued to apply such converters with only slight changes [760]. c) Cooling tubes. the original Haber – Bosch reactors [741].or four-bed converter [746]. a) Catalyst. d) Gas inlet. Figure 42 B. Chemico. Cocurrent-flow tube-cooled converter [740] A) Converter. An interesting rebirth of the countercurrent principle is the new ICI tube cooled converter used in the LCA process(Fig. e) Temperature control gas (cold-shot) inlet. within one or more reactor vessels. C) Ammonia concentration versus temperature (cf. C) Ammonia concentration versus temperature (cf. a derivative of NEC. [757]. Figure 47. The distributors consist of banks of transverse sparge pipes which deliver gas at regular intervals along their length. The gas flows vertically from the top to the bottom. For this reason it is justifiable to classify this converter as a multibed type. c) Catalyst discharge nozzle. Casale [746]. The uninterrupted catalyst bed is maintained in the opposed-flow converter. [746]. Radial gas flow in the converter avoids this dilemma. After the effluent from the first bed has been quenched with cooler recycle gas. [741]. The catalyst beds are arranged side by side in a removable cartridge which can be removed for catalyst loading and unloading through a full-bore closure of the horizontal pressure shell. e) Temperature control gas (cold-shot) inlet.5 – 3 mm. d) Gas inlet. a) Catalyst. suggested for very large capacities. f) Gas outlet. To compensate for the increasing pressure drop conventional axial flow converters have to use relatively large catalyst particles. thus needing no crane. As the cartridge is equipped with wheels it can be moved in and out on tracks. The temperature difference between the top and the bottom requires special design measures to prevent uneven circumferential warming of the pressure shell and to avoid bending. Multibed Converters with Indirect Cooling. Figure 48. Kellogg four-bed vertical quench converter a) Gas inlet. i) Outer annular space. [800]. An interesting variant in this group is the ICI lozenge converter [288]. g) Second catalyst chamber. especially for large capacities. Kellogg in their horizontal quench converter to obtain low pressure drop even with small catalyst particles [745]. 39) page 35 of 77 . [791]. 1271]. ICI [746]. even with small catalyst particle size. The cold gas enters through the bottom of the vessel and is mixed with the inlet gas to the first bed for temperature control. and others. The spargers are in a void space within horizontal mesh-covered structures. Axial – radial flow pattern was introduced by Ammonia Casale. Multibed converter with indirect cooling A) Converter. f) Catalyst support plate 1. Figure 48 shows a schematic of the converter [404]. c) First catalyst chamber. Figure 50. As shown in Figure 49 this is achieved by leaving the upper part of the catalyst cartridge at outlet side unperforated so that gas entering from the top is forced to undergo partially axial flow. It then passes through the feed – effluent heat exchanger and flows upwards through a central pipe to the first catalyst bed. g) Bypass Other designs were used by BASF [746]. b) Perforated wall Crossflow was chosen as a different approach by M. [789-794]. c) Cooling section. Converters with strictly radial gas flow require mechanical sealing of the top of each catalyst bed and dead catalyst volume with little or no flow to avoid bypassing of the catalyst. which may be cooler synthesis gas and/or boiler feed water heating and steam raising. Radial flow has been also applied in tube-cooled converters [787]. Österreichische Stickstoffwerke [797]. Radial flow quench converters have also been used by Chemoproject [796]. Haldor Topsøe S100 converter a) Outer internal lid. The heat exchanger may be installed together with the catalyst beds inside one pressure shell. m) Pressure shell. [790]. In this configuration the converted gas is collected and withdrawn from the middle of the catalyst bed. [780]. ICI lozenge converter a) Quench gas distributors. A special version of this reactor concept is the opposed flow design [779]. and with this concept it is possible to design converters for very large capacities without excessive diameters and with low pressure drop. [776]. d) Tube for thermocouples Converters with axial flow face a general problem: with increasing capacity the depth of the catalyst beds must be increased because for technical and economic reasons it is not possible to increase the bed diameter. Grand Paroisse [745]. and thus the pressure vessel diameter. Chemico. In the Casale concept there is no need for a dead catalyst zone as the annular catalyst bed is left open at the top to permit a portion of the gas to flow axially through the catalyst. is to accommodate the individual catalyst beds in separate vessels and use separate heat exchangers. b) Catalyst bed. and Lummus [798]. through which colder recycle gas is injected evenly across the whole cross section of the catalyst bed. 47) has a single catalyst bed that is divided into several zones (usually four) by quench gas distributors. j) Catalyst support plate 2. The advantages of radial flow are discussed in [783-786]. Fig. d) Inner annular space. In converters of this type cooling between the individual beds is effected by indirect heat exchange with a cooling medium.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience another. above a certain limit. c) Basket. it enters the second bed and passes through it in the inward direction. The converter feed – effluent exchanger. is designed for disassembly. C) Ammonia concentration versus temperature (cf. l) Refractory fiber. h) Transfer tube. attached to the top head. [801. f) Gas exit B) Gas temperature profile through the converter. which has two catalyst layers with a catalyst particle size of 1. e) Interchanger. [776]. [770]. W. The indirect cooling principle is applied today in most large new ammonia plants. This approach is especially suitable when using the reaction heat for raising high-pressure steam. The remainder of the gas flows radially through the bulk of the catalyst bed. A design similar to that of ICI with direct injection of the quench gas into the single catalyst bed has been proposed by Chemico [781]. [746]. with down-flow in the upper half and up-flow in the lower half of the catalyst bed. k) Heat exchanger. Figure 50 shows a schematic of the principle together with temperature/location and temperature/ammonia concentration profile. e) Perforated center tube. b) Heat exchanger. which is traversed from the inside to the outside. b) Heat exchanger. and also in revamps an increasing number of quench converters are modified to the indirect cooling mode. This design (Fig. d) Quench. [760]. but an attractive alternative. Figure 49. Uhde [746]. n) Refractory cement The major part of the gas enters the vessel at the top and flows down as shell cooling gas. The first radial flow converter introduced commercially and then widely used was the Topsøe S 100 converter [741]. Figure 46. Ammonia Casale axial – radial flow pattern a) Unperforated wall. whose cross section is lozenge shaped so that the catalyst particles can flow freely past them during loading and unloading. which have the disadvantage of lower activity compared to smaller particles mainly on account of diffusion restriction. b) Inner internal lid. The syngas with 110 bar from the low-pressure casing of the compressor is fed to a three-bed. the following ones with the new ruthenium catalyst. Braun.8 t of steam per tonne of ammonia at about 45 bar (ca. b) Interbed heat exchanger. The reactor contains two catalyst beds. c) 1st Catalyst bed. separate vessels were used for the two catalyst beds in the original version. designed for indirect cooling. [770]. f) Once-through ammonia converter Topsøe has now introduced a hot-wall converter with only one catalyst bed and no internal heat-exchange equipment. [469] a special converter design. Braun converters with interbed heat exchanger and waste-heat boiler a) Gas inlet. In the well-known Uhde – Chemie Linz converter with three catalyst beds — described in various versions [731]. Figure 54. f) Lower heat exchanger. The gas flow is axial and the mechanical design rather simple. Saudi Arabia (Safco IV). the gas is mixed with cold gas for temperature adjustment and passes through the first catalyst bed radially from the outside to the inside. the pressure vessel wall of the C. A horizontal design with indirect cooling is also proposed in [829]. This basic concept was already introduced in the heyday of the quench converters and prior to the first energy crisis. A cold shot is installed for adjusting the inlet temperature of the first catalyst bed. Figure 52. [765]. Contrary to most converter designs. 52) As in the quench version the pressure shell has a full-bore closure to remove the catalyst cartridge for loading and unloading. e) HP casing. suited for very large capacities. [757]. F. similar to C. h) Cold bypass pipe In the converter without a lower exchanger the feed gas enters at the bottom and flows as pressure wall cooling gas to the top of the converter. This Topsøe Series 50 converter. This is so far world-wide the largest capacity in a single synthesis (see also Single-train Capacity Limitations – Mega-Ammonia Plants). c) Refrigeration A new development is the Uhde dual pressure process [1343]. which discharges through a second external high-pressure boiler. f) Second bed. Two versions are shown in Figure 51. [807-811]. Figure 55 shows the Uhde dual pressure concept. [766]. c) Purge gas recovery unit. [745]. [746]. In a later version three catalyst vessels [822-826] with boilers after the second and the third were used. h) Steam drum. e) 2nd Catalyst bed. c) Bypass for temperature control. (Fig. c) Bypass. The gas flow is in radial mode in all three catalyst beds. Figure 51. but with radial flow. d) Annulus around catalyst bed. the Fauser – Montecatini reactor [741]. 54). which is possible with modern steels in compliance with the Nelson diagram (see Section Material Considerations for Equipment Fabrication). Figure 53 shows the two-bed version of the C. tube coils between catalyst beds transfer the reaction heat to a closed hot water cycle under pressure. d) First bed. [761-764]. [745]. Then a waste-heat boiler generating high-pressure steam cools the gas before it enters the second vessel containing the third bed. The gas passes downwards through the catalyst beds and the tube side of the interbed exchangers and the lower heat exchanger to leave the reactor vessel. Figure 55. i) Steam outlet. Braun converter. The first two are accommodated in a single pressure vessel together with an inlet – outlet exchanger. Three of these converters can be combined with an external heat exchanger and two high-pressure boilers to give an arrangement as described for Braun. C. The hot water releases the absorbed heat in an external steam boiler generating about 0. From the effluent about 85 % of the ammonia produced is removed from the gas before it is compressed in the high presure casing to 210 bar to enter the well-known Uhde two-vessel-three-bed combination as described above. once-through converter which can produce a third of the total ammonia yield. for example. F. [773]. and an inlet – outlet heat exchanger is located between first and second bed. C. g) waste-heat boiler (Borsig). After passing the centrally installed interbed exchanger on the tube side. which is crossed in the same direction as the first one. page 36 of 77 . [1351-1355]. [827]. b) Feed – first bed effluent exchanger. second and third bed. [819-822]. The first plant based on this concept with a capacity of 3300 t/d will come on stream 2006 in Al-Jubail. F. intercooled. with and without a lower internal heat exchanger. Figure 56 is a simplified sketch of the synthesis loop of the KAAP Figure 57 shows the internals of the KAAP converter [1375]. Feed gas enters at the top. The first bed is loaded with conventional iron catalyst. 250 °C). d) Bed 1. Braun reactors is at 400 °C. d) LP casing. Reactor feed gas passing between cartridge and shell is used to keep the pressure wall cool. b) Syngas compressor. Uhde has used three catalyst beds for capacities up to 2300 t/d (Fig. Kellogg has developed for its ruthenium catalyst based KAAP ammonia process [406]. Kellogg horizontal intercooled ammonia converter a) Inlet. passes down the annulus between basket and shell to cool the pressure wall. b) Interbed heat exchanger. F Braun ammonia synthesis concept [772]. The Topsøe Series 300 converter contains three catalyst beds and two central interbed exchangers in a single pressure shell. Casale [813-818] has also successfully commercialized converters based on the axial – radial flow concept with indirect cooling. Figure 53. Four radial flow beds are accommodated in a single pressure shell with intermediate heat exchangers after the first. Topsøe Series 200 converter a) Pressure shell. b) Second ammnia converter. Braun has devised a good solution by direct coupling the boilers and the exchanger to the converter. Further development of the radial flow concept used in the quench converter Topsøe Series 100 has led to the successful launch of the Topsøe Series 200 converter [552]. k) Gas outlet To withstand the high outlet temperature level (450 – 500 °C) needed for the high-pressure boilers. [776] — the indirect cooling is provided by converter feed gas. flows through the shell side of the lower feed – effluent heat exchanger. [771]. e) Bed 2. In this converter. [828]. g) Cold bypass. can also be combined with the Series 200 reactor and two external high-pressure boilers to attain the configuration as described for Uhde (Topsøe Series 250). and the second catalyst vessel was followed by a waste-heat boiler for high pressure steam. [746]. with the second one split into two parallel sections. The exit gas flows through the shell side of the interbed exchanger before it enters the second bed. f) Outlet In the C. [803-805]. in which the pressure shell is kept below 250 °C by means of insulation or by flushing with cooler gas. The feed – effluent heat exchanger was located between the first and second reactor vessels. Uhde dual pressure ammonia synthesis concept a) First ammonia converter. [783].F. Kellogg has re-engineered its horizontal cross-flow quench converter for indirect cooling [745]. Uhdes standard synloop configuration a) Ammonia converter. operating by natural draft. e) Line to second bed. and then via the center pipe and interbed exchangers to the top of the first catalyst bed. A “cold shot” ahead of the first catalyst bed is installed for temperature adjustment.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Converters with indirect cooling have been known since the early days of ammonia production. a special design is employed to keep the outlet nozzle cool. F. For trimming purposes a quench is foreseen. Water cooling was sufficient on account of the very high synthesis pressure.3.5 mol % by absorption in dilute aqueous ammonia. d) Cooling in feed – first bed effluent exchanger. With closer approach to equilibrium. which is recycled. 15. b) Purge gas recovery unit. 4. In modern plants. Initially there was practically no heat recovery. part of the heat must be recovered before the reaction is completed in the reactor system. because various technical problems were encountered with refrigeration at that time. The liquid ammonia of the high-pressure separator is flashed to about 20 bar.3.72 GJ per tonne NH3. recirculation compression.3. Reference [812] examines optimizing the temperature profile in a given four-bed quench converter. 802. Snam Progetti [288]. e) Temperature rise in second bed. Subsequently plants were modified to use this heat to some extent. The literature has frequently treated the problem of optimizing both the distribution of catalyst volume between a given number of beds and the bed inlet temperatures [713].22 kJ/mol at STP. Temperature versus enthalpy diagram for a two-bed system with steam generation a) Heating in main feed effluent exchanger. has now been raised to 125 bar. Advanced ammonia concepts produce as much as 1. [715]. 250 °C (ca. h) Cooling in main feed effluent exchanger Appropriate designs of waste-heat boilers are described in Waste-Heat Boilers for High-Pressure Steam Generation. In practice this corresponds to moving the lower heat exchanger (which in multibed converters exchanges feed to the first catalyst bed against effluent from the last bed) partially or completely to a position outside of the converter and downstream of the waste-heat recovery installation. The steam pressure. Figure 58 is a temperature – enthalpy diagram for a converter system corresponding to the original C. Optimizing the Temperature Profile and Bed Dimensions in Multibed Converters. The trend followed in newer plants is to increase conversion per pass with the result of higher ammonia outlet concentrations and lower outlet temperatures from the last bed. 799. At a loop pressure in the range 100 to 150 bar the quantity of reliquefied ammonia may be twice the ammonia product flow. F. In the early 1920s LUIGI CASALE successfully used condensation for his first plant. g) Temperature level of waste-heat boiler. [808]. Arrangement and location of the ammonia separator(s). [855] did not find widespread application. see also Figure 34). and nearly the total heat content of the gas down to ambient temperature and thus the reaction heat was transferred to cooling water. Early converters. Kellogg advanced ammonia process (KAAP) a) Compressor. which corresponds to 2. operating at about 300 bar. preferably after removal of ammonia vapor to avoid NOx formation in the combustion furnace. 350 °C .Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Figure 56. Waste-Heat Utilization and Cooling The reaction heat of ammonia synthesis is 46. [839]. In this way the temperature level at which heat could be recovered was increased. d) Bed no.5. [740]. whereby the majority of the dissolved gases are released in the let-down vessel. the cooling has to be supplemented by refrigeration. b) Further heating in feed – first bed effluent exchanger. [847]. Refrigeration down to – 25 °C is used. the first commercial units of BASF used ammonia absorption in water to remove the product from the cool synthesis loop gas. It was only in 1926 that ammonia condensation was introduced in the Haber – Bosch plants. [867] has proposed removing the ammonia from the loop gas at ambient temperature down to 0. formerly 100 bar. All ammonia vapors removed from flash gases and from purge gas by water scrubbing can be recovered as pure liquid product by distillation if there is no direct use for the aqueous ammonia. [845]. KAAP four bed ammonia converter a) Bed no. which means that the gas can be cooled in the boiler to ca. The amount of ammonia vaporized (and consequently reliquefied by compression and water or air cooling) can be substantial. c) Refrigeration Figure 57. page 37 of 77 . c) Temperature rise in first bed. [866]. [853]. see also Converter Design. the volume of catalyst required in the individual beds increases. 400 °C) . in further developments an additional heat exchanger for converter feed versus the converted gas was installed. This gas is normally used as a fuel. the remaining recoverable heat is used for boiler feed water heating. downstream of the above-mentioned heat recovery. As for any type of converter the outlet temperature rises with increasing inlet temperature ( T is determined by the degree of conversion). 20 bar). f) Cooling by steam generation. received the converter feed at about ambient temperature and therefore had outlet temperatures of ca. Absorption refrigeration with aqueous ammonia instead of a mechanical refrigeration system [832]. requiring greater total catalyst volume and appropriate changes in the catalyst bed dimensions. addition of makeup gas and extraction of purge gas are discussed in Section Synthesis Loop Configurations. Utilization of this heat at the highest possible temperature for generating high-pressure steam is a major contribution to the high total energy efficiency of modern ammonia plants [830]. For additional literature on this important issue in converter layout and design. 3 KAAP catalyst. but the low temperature level allowed only boiler feed water heating and generation of low-pressure steam (ca. with one or more temperature levels is generally used. as optimum energy efficiency of the whole ammonia plant requires maximum high-pressure steam generation. ultimately to the point where the inlet temperature to waste-heat recovery is equal to the outlet temperature of the last catalyst bed.5 °C per mol % of ammonia formed). Figure 58. 4 KAAP catalyst For revamps Kellogg has also proposed a two-bed version completely loaded with ruthenium catalyst to be placed downstream of a conventional converter [470]. Braun arrangement. High-pressure steam (113 bar. In some instances. 2 KAAP catalyst. which operate at moderate pressures. equipped with a lower heat exchanger for raising the inlet temperature for the (first) catalyst bed to the ignition temperature (ca. c) Bed no. water circulation was installed to use this heat in other plants or production steps. Although ammonia condensation was already used in HABER's lab-scale ammonia plant and in early pilot plants of BOSCH. Recovery of ammonia by water scrubbing offers the advantage of achieving a very low residual ammonia content. for which a mechanical ammonia refrigeration cycle. 806]. Ammonia Recovery from the Ammonia Synthesis Loop In all commercial plants ammonia is recovered from the synthesis loop by cooling the synthesis gas to condense the ammonia under synthesis pressure. but the drawback is that the whole recycle gas has to be dried afterwards and in addition distillation of aqueous ammonia is necessary to yield liquid ammonia. 4. b) Bed no.5 t of high-pressure steam per tonne of ammonia. In this way the waste-heat downstream of the synthesis converter in modern plants is available in the temperature range around 480 to 290 °C. [795. which correspond roughly to 90 % of the standard reaction enthalpy.. This can be accomplished [807]. 1 magnetite catalyst. The liquid ammonia product is separated from the gas. which with inclusion of the necessary temperature difference in the chiller requires ammonia evaporation at about atmospheric pressure. [831] by using three catalyst beds in separate pressure vessels with boilers after the second and the third vessel and an inlet – outlet heat exchanger for the first catalyst bed.5. However.4. Nevertheless the scrubbing route was again proposed for a synthesis loop to be operated under extremely low pressure (around 40 bar) [862]. 320 °C) is generated after the second catalyst bed. In older high-pressure synthesis loops (> 450 bar) cooling by water and or air is sufficient to obtain the required low residual ammonia concentration in the gas. The ammonia from the let-down vessel may be sent directly to downstream users or flashed further to atmospheric pressure for storage in a cold tank. [830]. Table 5 gives the ammonia concentration in a 1/3 nitrogen/hydrogen mixture at 20 bar total pressure as 9. 300 bar). for example. If the synthesis gas pressure is high.5 – 2. The gas recycle flow is increased by the amount of inert gas. methane. corresponding to 5. only a very small final purge stream is necessary and no recompression is needed. offers such a module under the name Serarex [859]. The main advantages of the Prism separator system are simplicity. c) Plate-fin exchanger. leaving a hydrogen-rich gas. Reference [946] compares membrane and cryogenic separation units for a large ammonia plant. At the same time expensive synthesis gas is also lost from the loop. The entire bundle is encased in a vertical shell (Fig. reducing the ammonia concentration to less than 200 ppm.8 mol %. These are removed in a separator. Molecular sieve adsorbers then eliminate moisture and traces of ammonia (Fig. Hydrogen Recovery by Cryogenic Units [848]. Up to 75 % of the hydrogen from the first-loop purge stream can be recovered.8 MPa (25 – 28 bar) and returned to the first-stage suction of the syngas compressor. d) Separator. Piping and equipment must correspondingly be increased in size. A portion of the inert gases dissolves in the liquid produced in the ammonia separator. g) Permeate gas outlet The purge gas is water scrubbed at 135 – 145 bar. which lowers the ammonia yield. Membrane technology is also offered by other licensors. 60). and the inert gas concentration in the synthesis loop make-up gas low enough. under 0. and L'Air Liquide. f) Feed stream of mixed gases. For this reason other methods have been developed. inerts have also to be removed from the gas phase by withdrawing a small purge-gas stream from the loop. [924]. [860]. Hydrogen permeates through the wall of the fibers. Of the recovered hydrogen. Hydrogen Recovery by Membrane Separation. for example.. Therefore.167 bar. This system has the advantage that nitrogen is also recovered. The raw gas flows in axial direction in the high pressure spacer and the permeate is withdrawn in the low pressure spacer. Reference [788] reports on the changes in operating conditions of an ammonia plant resulting from the operation of a cryogenic hydrogen recovery unit. nitrogen. less ammonia can be condensed from the recycle synthesis gas by less expensive air or water cooling or higher temperature level refrigeration.5. in addition to removal as dissolved gases (flash gas). then dissolution in the product ammonia suffices to remove the inerts from the synthesis loop.2 m) resembles a shell and tube heat exchanger. The remaining (nonpermeating) gases. are concentrated on the shell side. [870]. determining the appropriate inert gas concentration requires a precise economic calculation. Linde. see [854]. The scrubbed purge gas is heated to 35 °C and sent directly to the Prism separators. For further literature on gas separation by membranes. g) Hydrogen product to syngas compressor. b) Fiber bundle plug. With a higher inert gas content in the make-up gas this method is not applicable. The design of a single separator module (length. especially at plant startup. and argon.e. at 30 bar the ammonia vapor pressure according to Table 4 is 1. The remaining nonpermeate gas stream normally flows to primary reformer fuel. Figure 2 gives vapor – liquid equilibrium ratios for use in making precise calculations of the dissolved quantities of inerts. d) Separator (length. Trace concentrations of ammonia and water vapor in the gas stream pose no problem to the membrane.5 – 7 MPa). Overall hydrogen recovery is 90 – 95 %. Therefore. The gas stream enters the separator on the shell side. Ammonia is first removed from the purge gas by cooling or in a water wash operating at 7. In contrast. they tend to concentrate in the synthesis loop and must be removed to maintain the loop material balance. 3 – 6 m. diameter. Pressure swing adsorption on zeolite molecular sieves (PSA) (see Section Gas Purification) may be also used for hydrogen recovery from purge gas [663]. [936-941]. 0. Liquid ammonia from the ammonia plant may be used to provide additional refrigeration. with a high concentration of inerts. Therefore. [856]. an example is the Polysep Membrane System of UOP [858]. membrane modules have been developed in which the membrane is in the form of a sheet wrapped around a perforated center tube using spacers to separate the layers. Figure 59. serves as fuel for the primary reformer. because the required partial pressure of the inert gas at equilibrium in the loop gas would become so high that a synthesis under moderate pressure would be virtually impossible. Hydrogen-rich permeate gas flows down the bore of the fiber and through the tubesheet and is delivered at the bottom of the separator. There are several possibilities for reducing the losses associated with the purge gas.5 mm. ammonia-free purge gas from the adsorbers next enters the cold box. A bundle with many thousands of hollow fibers is sealed at one end and embedded in a tubesheet at the other. The hydrogen-rich gas also flows into the same exchanger (in separate passages) where the vaporizing liquid and the hydrogen are warmed by cooling the entering purge gas. but it is too expensive for use in modern plants and revamps. 4. Partial condensation liquefies methane and argon as well as some of the nitrogen and helium. [849-851].5 MPa (75 bar). and the associated power consumption for recycle gas increases. the outside of the hollow fibers. for example.5. i. a dryer system is not required. f) Fuel gas to reformer burners. a second bank of separators with lower pressure on the tube side is used to increase the hydrogen recovery. The most capital-intensive method consists of feeding the purge gas to a second synthesis loop operating at a slightly lower pressure [869]. Because of the dilution. The rate of permeation decreases across a bank of separators as the hydrogen partial pressure differential across the membrane approaches zero. This principle has been applied to separating hydrogen from other gases [942-945] ( are hollow fibers with diameters of about 0. e) Ammonia-free purge gas. These include methane (from gas generation). As this loop can operate at a very high inert level (40 % or more). and into a brazed aluminum (plate-fin or core-type) heat exchanger. For example. [947-949].2 vol % [868]. [950]. ease of operation. argon (from the process air).84 mol % at the total pressure of 20 bar. The fiber is a composite membrane consisting of an asymmetric polymer substrate and a polymer coating. among others. About 90 – 95 % of the hydrogen and 30 % of the nitrogen in the purge gas can be recovered. originally developed by Union Carbide under the name HYSIV. Heat exchange with cold product hydrogen fraction and with gas rejected to fuel cools the purge gas to a temperature of about –188 °C (85 K).Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience The extent to which the ammonia concentration in the gas can deviate from that expected for ideal behavior can be seen from Tables 4 and 5. The remaining gas. 59). there is an unfavorable effect on condensation of the ammonia product. diameter. 40 – 70 % leaves the first bank of separators at 7 MPa (70 bar) and is returned to the second-stage suction of the syngas compressor. Hydrogen Recovery by Pressure Swing Adsorption. Table 6 gives a rough approximation of the extent to which these inerts are dissolved. the fresh make-up gas supplied to the synthesis loop usually contains small quantities of inert gases. After heating in a preheater. Cryogenic hydrogen recovery units are supplied by firms such as Costain Engineering (formerly Petrocarbon Development) [782]. The Monsanto Prism membrane separator system uses selective gas permeation through membranes to separate Membranes and Membrane Separation Processes). The process. Because they are inert . and helium (from the natural gas). e) Coated carbon steel shell. Several separators operate in series. It decreases the specific converter performance by reducing the hydrogen and nitrogen partial pressures. Simplified flow diagram of cryogenic hydrogen recovery unit a) Molecular sieve adsorbers. The membranes gases.1 – 0. [857]. a portion serves to regenerate the molecular sieves and then likewise flows to reformer fuel. Gas separator module a) Nonpermeate gas outlet. and low maintenance. The dry. Moreover. In addition to the systems based on hollow fibers. Figure 60. The second bank permeate hydrogen is recovered at 2. Linde [852]. h) NH3 refrigerant The liquid flows through a control valve. reducing its pressure.3. A high inert gas level has various drawbacks. The warm hydrogen-rich gas flows back to the suction side of the second stage of the synthesis gas compressor (6. b) Heater. recovered through the top and pass to the next separator module. is now marketed as Polybed PSA by page 38 of 77 . So. c) Hollow fiber. and number of separators determined by ammonia process). Inert-Gas and Purge-Gas Management Apart from nitrogen and hydrogen. In principle it is possible to calculate the minimum amount of mechanical work needed in the synthesis loop.15 9. such diagrams should be interpreted with care.99 54.20 23. The ballast also is the binder for the pellets.40 16. In a plant based on partial oxidation with an operating pressure of 80 bar.00 1 2 3 4 5 6 7 8 9 10 11 12 28344 25368 25368 24672 21613 28344 3060 3678 320 7667 374 8041 The gas enters the converter (a) at 295 °C and is subsequently heated in the internal heat exchanger to 390 °C before it enters the first catalyst layer. A proprietary hydrogen separation process utilizing the reversible and selective adsorption capability of mixed metal hydrides has been proposed.25 18. the consequence of higher recycle rate is a lower ammonia concentration. Argon Recovery from Ammonia Purge Gas.50 0.26 18.53 20.09 0.43 20. If required. PSA technology is also offered by Linde and other companies.5. after which the gas leaves the converter with 469 °C and page 39 of 77 .18 0. but increases total ammonia production. the temperature attainable at a given site by air. Figure 61. with a purity of 99.82 62. 231]: – – – – – Pressure: increasing pressure will increase conversion due to higher reaction rate and more favorable ammonia equilibrium. for which a value of 155 bar is reported in [863]..00 0. The centrifugal compressor is one of the most important features of single-train ammonia plants. Example of an Industrial Synthesis Loop Figure 61 is an example of modern ammonia synthesis loop (Krupp – Uhde) with two converter vessels and three indirectly cooled catalyst beds producing 1500 t/d NH3 at 188 bar. [864]. were built in the mid-1960s and the maximum attainable synthesis pressure was restricted to ca. [860]. but also the mechanical design and the associated investment costs. the temperature of the ammonia separator determines the ammonia concentration at the converter inlet.6.79 50. required return on investment. the process also offers the possibility to supply pure hydrogen from the purge gas for other uses.999 %. this is used to supply the heat of desorption. the type of front end can profoundly alter the result. Third. With the exception of some older processes which used extremely high pressures (Casale. Inert level: increasing the inert level lowers the reaction rate for kinetic and thermodynamic reasons (Section Kinetics). 1 separator.25 3. If plots of kilowatt hours versus synthesis pressure for make-up gas compression. However.43 19. Subsequently. selection of the metal hydride must be based on the analysis of the gas to be treated.54 CH4 Ar 8.51 4. i) No.51 0.e.88 18. the costs for energy (i. which translates into either a lower catalyst volume or a higher conversion. f) Heat exchanger. The hydride. The first of these plants. Typical argon recoveries are in excess of 90 %.7. Therefore. at lower temperatures the maximum lies at lower H/N ratios (Section Kinetics).21 0. less than half as much energy is needed to compress the make-up gas to 180 bar. b) Converter with one radial bed.07 2.02 46. Space velocity: increasing the space velocity normally lowers the outlet ammonia concentration. this type of plot is strongly influenced by the catalyst activity. MPa Gas composition.3.30 0. mol % kmol/h N2 295 469 401 336 21 0 21 20 38 35 38 35 18. No commercial installations are known to be in operation in ammonia production processes.20 9. 4. – Separator temperature: together with pressure and location of make-up gas addition.31 1.55 51.33 23. 2 separator. Thus the problem of choosing the best synthesis pressure is complex because not only does the entire energy balance have to be examined. is in the form of ballasted pellets.36 99.16 2.86 1. Hydrogen/nitrogen ratio: a true maximum reaction rate for a certain H/N ratio.93 H2 61.88 0. and refrigeration are superimposed the result will be a minimum. Each type of metal hydride is susceptible to certain contaminants.23 3. PSA units usually operate at adsorption pressures of 20 – 30 bar and achieve recovery rates higher than 82 % for hydrogen and 20 % for nitrogen. The ballast material serves as a heat sink to store the heat of adsorption. preventing attrition. Position of the maximum also depends on the space velocity values (Section Formation of Ammonia in the Converter).20 0. In this case the difference to the equilibrium concentration and thus the reaction rate increases with the result that less catalyst is required. Analysis of the effects of various parameters on the energy consumption of the synthesis loop are also reported in [807]. [952] and a process developed by Bergbau-Forschung [953] is offered by Costain.12 24.79 47. with a capacity of 600 t/d. such as grain size and minimum possible temperature for the first bed. larger heat exchanger surface areas become necessary and the cross sections of piping and equipment have to be enlarged on account of the higher gas flow. 150 bar by technical limitations of the centrifugal compressor. j) NH3 recovery.72 72. – Recycle rate: at constant pressure and production rate.71 61.23 3.60 22. for example.67 8. 4. [808]. and its impact on the energy balance of the complete ammonia plant. p. The waste gas from hydrogen recovery plants is more highly enriched in argon than the purge gas.67 8. Second. t/h 4. or Mg2Cu. Inlet temperature: there are two opposed effects as increasing temperature enhances reaction rate but decreases the adiabatic equilibrium concentration. e) Water cooling. [951-953].54 0.02 2. Influence of Pressure and Other Variables of the Synthesis Loop The influences of individual parameters can be summarized as given in [846.01 7. recycle.03 21.73 90.48 18. FeTi. °C p. – Catalyst particle size (Section Particle Size and Shape): smaller catalyst particles give higher conversion because of lower diffusion restrictions (higher pore efficiency) The question of the best synthesis pressure is rather a difficult one and the answer is extremely dependent on optimization parameters such as feedstock price. No ammonia removal step is required upstream of the unit.01 0. Carbon-based adsorbents for pressure swing adsorption have also been investigated [951]. and other studies came to higher values in the range 180 – 220 bar. its energy level.20 73. The system yields 99 mol % hydrogen product at a recovery efficiency of 90 – 93 % [954].18 16. However. Example of a synthesis loop for 1500 t/d NH3 (Krupp – Uhde) a) Converter with two radial beds and internal heat exchange.89 3. and site requirements. then it may be possible to supplement the hydrogen recovery plant with one for recovering argon. The front-end pressure determines the suction pressure of the synthesis gas machine.18 98.3.80 18. A lower temperature means lower ammonia concentration. And fourth.40 19.or water cooling will affect the refrigeration duty. h) Refrigerated cooling. k) H2 recovery No.15 9. c) Waste-heat recovery. Flow rate.31 3. Cryogenic argon recovery from ammonia purge gas is discussed in [955-957]. [861]. which needed a minimum gas flow (see also Section Compression). T.55 0. as in a steam reforming plant with a suction pressure of only 25 bar.38 18. d) Heat exchanger. and thus by any factors that affect it.34 1.55 25.72 0.50 0. These plants usually operate in the pressure range of 170 – 190 bar and it is a confirmation of the argumentation presented above that the different pressures are not reflected in the overall energy consumption of the complete ammonia plant. this approach considers only the mechanical energy and ignores completely the recovered energy of reaction.20 50. and especially the equilibrium temperature of the last bed [865].22 24.20 0. For an actual project. g) No. such as LaNi5. At constant equilibrium temperature for the effluent from the last bed (constant ammonia concentration) energy consumption of the loop was found to be almost independent of pressure in the range of 80 – 220 bar [865].02 17.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience UOP [858]. Hydrogen Recovery Using Mixed Metal Hydrides.5.89 5. First. If potential markets for argon exist. the feedstock) have to be weighed against investment. The result is strongly dependent on assumed boundary conditions.78 2. at current plant sizes of 1200 – 2000 t/d this constraint is no longer of importance.00 0. Of course. The outlet gas from the first layer then passes through the aforementioned heat exchanger and enters the second bed.23 3. Claude) a pressure of around 300 bar was common for the old multistream plants operated with reciprocating compressors.37 Production of NH3 NH3 liquid.43 10. the temperature level for waste-heat recovery decreases and with lower temperature differences. 6. which was integrated in the flue gas duct. many of the differences between various commercial ammonia processes lie in the way in which the process elements are integrated. No parallel lines. and blowers. [961-964]. with process steps in surplus supplying those in deficit. with several parallel trains in the synthesis gas preparation stage and the synthesis loop. The decrease in energy consumption compared to the older technology was dramatic. and synthesis loop). and other contractors were offering similar concepts.1. ammonia production at a developed industrial site may import and/or export steam and power. and the required catalysts are reviewed in [539]. which operates at near atmospheric pressure. and refrigeration. Steam Reforming Ammonia Plants 4. was a technical and an economical breakthrough and triggered a tremendous increase in world ammonia capacity. Figure 62 shows a block flow diagram and the gas temperature profile for a steam reforming ammonia plant [1377]. [406]. and with the low gas prices at that time it is understandable that greater emphasis was placed on low investment cost. The Basic Concept of Single-Train Plants The innovative single-train concept. 4. together with consumption and costs of feedstock and energy. This situation was partially caused by inadequate waste-heat recovery and low efficiency in some energy consumers. while heat is needed. for the process steam for the reforming reaction and in the solvent regenerator of the carbon dioxide removal system. [553]. With the advent of the single-train steam reforming plants. Because considerable mechanical energy is needed to drive compressors. Ammonia plant steam systems are described in [404]. Kellogg in 1963 with a capacity of 600 t/d. 4.6. 30 new Kellogg large single-train plants with capacities of 1000 t/d or more were in operation. The other important factor is the plant capacity. [995]. it became standard for licensors and engineering contractors to express the total net energy consumption per tonne of ammonia in terms of the lower heating value of the feedstock used. all modern gasification processes operate at elevated pressure. In addition site requirements can influence the layout considerably. [980]. With the exception of the Koppers – Totzek coal gasification process. [959]. The outcome was a rather complex steam system. Even after substitution of the smaller turbines by electric motors.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience passes a waste-heat boiler generating 125 bar steam. whereas today it is interpreted as the complete series of industrial operations leading from the primary feedstock to the final product ammonia. The total net energy consumption is the difference between all energy imports (mainly feedstock) and all energy exports (mostly steam and/or electric power) expressed as lower heating value of the consumed feedstock. Block diagram and gas temperature profile for a steam reforming ammonia plant High level surplus energy is available from the flue gas of the reformer and the process gas streams of various sections. the steam system in the modern plant is still a complex system as shown in Figure 64. pumps. 276]. The way in which these process components are combined with respect to mass and energy flow has a major influence on efficiency and reliability. 4. has a temperature of 401 °C and the outlet enters a further waste-heat boiler generating 125 bar steam. The new plant concept had a triumphant success story. since sufficient steam could be generated from the waste-heat. Steam reforming of light hydrocarbons at 30 – 40 bar and partial oxidation of heavy hydrocarbons at 40 – 90 bar are generally used. whereby electric power is converted with an efficiency of 25 – 30 % and steam is accounted for with its caloric value. [1318].1.2. and a highly efficient use of energy. converters. Formerly the term ammonia technology referred mostly to “ammonia synthesis technology” (catalyst. Development of the net energy consumption of natural gas based steam reforming ammonia plants (real plant data) in GJ per tonne NH3 page 40 of 77 . [988]. the furnace flue gas was discharged in the stack at rather high temperature because there was no air preheating and too much of the reaction heat in the synthesis loop was rejected to the cooling media (water or air). efficiency of the mechanical drivers was low and the heat demand for regenerating the solvent from the CO2 removal unit (at that time aqueous MEA) was high. and needed an auxiliary boiler. not only for the major machines such as synthesis gas. Table 25. which largely governs the mode of gas generation and purification. which. Maximum use was made of direct steam turbine drive.1. were the main features. first introduced for steam reforming based production by M. An important development was the concept of single-train plants. The major determinant for process configuration is the type of feedstock. engineering contractors and plant owners to obtain better energy efficiency. which affects the total energy consumption. [999]. [960]. introduced in 1963 by Kellogg. even for high capacity. Typically. Further Developments The significant changes in energy prices from 1973 onwards were a strong challenge to process licensors. Table 25 lists all significant energy sources and sinks within the process. In contrast to a stand-alone plant. Today world-scale plants have capacities of 1200 – 2000 t/d. A survey of the development of the steam reforming concept through 1972 can be found in [969]. Complete Ammonia Production Plants The previous sections mainly considered the individual process steps involved in the production of ammonia and the progress made in recent years. The first generation of the single-train steam reforming plants is discussed in [420]. The lowest capital cost and energy consumption result when steam reforming of natural gas is used. [979]. Figure 62. Other references which cover the development of the steam reforming before the introduction of the single-train concept (1940 to 1960) can be found in [404 p. Main energy sources and sinks in the steam reforming ammonia process Process section Origin Reforming primary reforming duty flue gas process gas Shift conversion heat of reaction CO2 removal heat of solvent regeneration Methanation Synthesis Machinery Unavoidable loss Balance heat of reaction heat of reaction drivers stack and general auxiliary boiler or import export Contribution demand surplus surplus surplus demand surplus surplus demand demand deficit surplus The earlier plants operated at deficit.6. although there was a considerable potential for further reducing the energy consumption. Apart from the feedstock.6. The inlet gas of the second vessel. for example.1.W. Before then maximum capacities had mostly been about 400 t/d. As the level was high enough to raise high-pressure steam (100 bar) it was possible to use the process steam first to generate mechanical energy in the synthesis gas compressor turbine before extracting it at the pressure level of the primary reformer. By 1969. which accommodates the third catalyst bed. In addition. it was most appropriate to use steam turbine drives. is decisive for the production economics. but even for relatively small pumps and blowers. The overall energy consumption was reduced from around 45 GJ per tonne NH3 for the first large single-train units to less than 29 GJ per tonne NH3 (Table 26). process air. Table 26. Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Year Plant 1966 1973 1977 1980 1991 A B C D E 23.32 23.48 23.36 22.65 9.21 7.35 5.56 5.90 5.02 3.06 1.17 0.55 37.55 33.89 30.18 28.00 Feed 23.90 Fuel, reformer 13.00 Fuel, auxiliary boiler 2.60 Export Total 39.50 Energy saving modifications are described in [288], [867], [970-978], [1000], [1001], [1005], [1018], [1025], [1030], [1254]. For catalyst improvements see [1395]. Some of the most important improvements compared to the first generation of plants are discussed below. Feedstock Purification. In feedstock purification, mainly desulfurization, adsorption on active carbon was replaced by catalytic hydrogenation over cobalt – molybdenum or nickel – molybdenum catalyst, followed by absorption of the H2S on ZnO pellets with formation of ZnS. By itself this measure has no direct influence on the energy consumption but is a prerequisite for other energy saving measures, especially in reforming and shift conversion. Reforming Section. In the reforming section energy savings were achieved by several, often interrelated, measures, of which the most important are the following: – – – – – – – – Reduction of the flue gas stack temperature to reduce heat losses to the atmosphere [552] Avoiding excessive heat loss by better insulation of the reformer furnace Introduction of combustion air preheating [1018] Preheating the reformer fuel Increased preheat temperatures for feed, process steam and process air Increased operating pressure (made possible by using improved alloys for the reformer tubes and improved catalysts) Lowering of the steam to carbon ratio [1032] Shifting some reformer duty from primary to secondary reformer with the use of excess air [457], [458] or oxygen-enriched air [1276] in the secondary reformer, including the possibility of partially bypassing the primary reformer [1033], [1036] – Installing a prereformer or rich-gas step is another possibility to reduce primary reformer duty and stack temperature of the flue gas [431-438], especially in LPGand naphtha-based plants A more recent development that breaks away from the usual plant configuration is to replace the traditional fired primary reformer with an exchanger reformer which uses the heat of the effluent of the secondary reformer [461-464], [466-469], [1035], [1036]. Also other applications have been reported in which flue gas [1035] heat from the fired reformer is used to perform a part of the reforming. Shift Conversion. Improved LT shift catalysts can operate at lower temperatures to achieve a very low residual CO content and low byproduct formation. A new generation of HT shift catalysts largely avoids hydrocarbon formation by Fischer – Tropsch reaction at low vapor partial pressure, thus allowing lower steam to carbon ratio in the reforming section (see Section Carbon Monoxide Shift Conversion). Carbon Dioxide Removal Section. In the carbon dioxide removal section the first generation of single-train plants often used MEA with a rather high demand of lowgrade heat for solvent regeneration. With additives such as UCAR Amine Guard [603], [637], solvent circulation could be reduced, saving heat and mechanical energy. Much greater reduction of energy consumption was achieved with new solvents and processes, for example BASF aMDEA or Benfield LoHeat. Other hot potash systems (Giammarco Vetrocoke, Catacarb) and physical solvents (Selexol) were introduced (Section Gas Purification). Final make-up gas purification was improved by removing the water and carbon dioxide traces to a very low level by using molecular sieves. Some concepts included cryogenic processes with the benefit of additional removal of methane and argon. Ammonia Synthesis Section. In the ammonia synthesis section conversion was increased by improved converter designs (see Section Formation of Ammonia in the Converter), larger catalyst volumes, and to some extent with improved catalysts. The main advances in converter design were the use of indirect cooling instead of quenching, which allowed the recovery of reaction heat high pressure steam. Radial or cross-flow pattern for the synthesis gas instead of axial flow was introduced. All modern plants include installations for hydrogen recovery (cryogenic, membrane, or PSA technology; see Section Inert-Gas and Purge-Gas Management). Machinery. Developments in compressor and turbine manufacturing have led to higher efficiencies. – Gas turbine drive for a compressor and/or an electric generator combined with the use of the hot exhaust as combustion air for the primary reformer or raising steam (combined cycle) [705-709], [1039]. – Employing electric motors instead of condensation turbines [1040]. – Application of liquid and gas expansion turbines can recover mechanical work (e.g., let-down of the CO 2-laden solvent, liquid ammonia, purge and fuel gas). Steam system and waste-heat recovery were improved by the following measures: increased pressure and superheating temperature of high-pressure steam; providing a part of the process steam by natural gas feed saturation [458], [603], [965-967]; inclusion of a steam superheater downstream of the secondary reformer [964], [968]. Process Control and Process Optimization. Progress in instrumentation and computer technology has led to increased use of advanced control systems and computerized plant optimization. Advanced control systems [981-987], [1133-1135] allow operating parameters to be kept constant in spite of variations in external factors such as feedstock composition or ambient or cooling water temperatures. These systems may be operated in open loop fashion (set values changed manually by the operator) or in closed fashion (set points automatically adjusted to optimum values by using a computer model with input of operational and economic data). Also plant simulation [989-992] is possible by using extensive computer models of complete plants. These models can simulate in real time the dynamic response to changes in operating parameters, plant upsets, etc. Such systems are used for off-line optimization studies and for operator training [993], [994]. The above list, by no means complete, is also a survey of options for plant revamps (Section Modernization of Older Plants (Revamping)). Quite a number of options can be found in papers presented at the “AIChE Annual Symposium Ammonia Plants and Related Facilities” giving practical experience and presenting case stories [1350]. Many of these elements are strongly interrelated with each other and may affect different sections of the plant concept. It is thus a demanding engineering task to arrive at an optimum plant concept, which can only defined by the conditions set by the feedstock price, the site influences, and the economic premises of the customer. An evaluation of the individual merits of the described measures in terms of investment and operational cost in a generalized form is not possible and can be done only from case to case in real project studies. To illustrate the forgoing discussion of the concept of the single-train steam reforming plant, Figure 63 presents a modern low-energy ammonia plant with flow sheet and process streams (UHDE process). Figure 63. Modern integrated single-train ammonia plant based on steam reforming of natural gas (Uhde process) a) Sulfur removal; b) Primary reformer; c) Steam superheater; d) Secondary reformer; e) waste-heat boiler; f) Convection section; g) Forced draft fan; h) Induced draft fan; i) Stack; k) HT and LT shift converters; l) Methanator; m) CO2 removal solvent boiler; n) Process condensate separator; o) CO2 absorber; p) Synthesis gas compressor; q) Process air compressor; r) Ammonia converter; s) High-pressure ammonia separator; t) Ammonia and hydrogen recovery from purge and flash gas page 41 of 77 Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience 1 CH4, mol % CnHm, mol % CO2, mol % CO, mol % Ar, mol % H2, mol % N2, mol % NH3, mol % 1.02 1.02 2 3 4 0.60 5 0.53 6 0.65 7 1.16 8 9 91.24 91.24 14.13 5.80 1.94 5.80 1.94 10.11 9.91 65.52 0.33 24.34 0.12 7.38 13.53 0.28 54.57 23.64 18.14 0.33 0.25 59.85 20.90 0.01 0.40 0.30 0.37 7.21 0.01 73.08 73.54 21.72 0.03 25.56 24.93 46.55 0.02 0.18 99.82 Drygas, kmol/h 1713.7 534.43 5296.4 8414.2 9520.7 7764.0 8041.4 319.9 3676.6 3520.6 4086.1 2979.6 22.8 13.3 0.2 0.9 H2O, kmol/h Total, kg/h p, MPa T, °C 30213 9422 5.00 0.25 25 25 121183 199555 199555 70084 71004 6496 62626 3.95 3.90 3.61 3.43 3.23 0.25 2.50 808 976 229 50 35 38 20 Figure 64 shows a simplified diagram of the steam system. Even in such an advanced plant the quantity of steam generated from waste-heat is as much as 3.4 times the weight of ammonia produced. Figure 64. Steam system of a modern steam reforming a) Steam drum, 125 bar; b) NH3 loop; c) Turbine for syngas compressor; d) Turbine for process air compressor and alternator; e) Surface condenser; f) Condensate treatment; g) BFW pump 4.6.1.3. Minimum Energy Requirement for Steam Reforming Processes The energy saving measures described in Section Further Developments have considerably reduced the demand side (e.g., CO2 removal, higher reforming pressure, lower steam to carbon ratio, etc.). On the supply side, the available energy has been increased by greater heat recovery. The combined effects on both sides have pushed the energy balance into surplus. Because there is no longer an auxiliary boiler which can be turned down to bring the energy situation into perfect balance, the overall savings usually could not be translated into further actual reduction of the gross energy input to the plant (mainly natural gas). In some cases this situation can be used advantageously. If the possibility exists to export a substantial amount of steam, it can be economically favorable (depending on feedstock price and value assigned to the steam) to deliberately increase the steam export by using additional fuel, because the net energy consumption of the plant is simultaneously reduced (Table 27). Table 27. Increase of plant efficiency by steam export (GJ per tonne NH3) Plant A Natural gas 27.1 Electric power 1.1 Steam export Total energy 28.2 B 32.6 1.1 – 6.4 27.3 + 5.5 – 6.4 – 0.9 Difference A reduction in gross energy demand, that is, a lower natural gas input to the plant, can only be achieved by reducing fuel consumption, because the actual feedstock requirement is determined by the stoichiometry. So the only way is to decrease the firing in the primary reformer, which means the extent of reaction there is reduced. This can be done by shifting some of the reforming duty to the secondary reformer with surplus air or oxygen-enriched air, although this makes an additional step for the removal of surplus nitrogen necessary. A more radical step in this direction is total elimination of the fired primary reformer by using exchanger reformers like the ICI GHR and the Kellogg KRES. Based on pure methane, it is possible to formulate a stoichiometric equation for ammonia production by steam reforming: From a mere thermodynamic point of view, in an ideal engine or fuel cell, heat and power should be obtainable from this reaction. Since real processes show a high degree of irreversibility, a considerable amount of energy is necessary to produce ammonia from methane, air and water. The stoichometric quantity of methane derived from the above reaction is 583 m3 per tonne of ammonia, corresponding to 20.9 GJ per tonne NH3 (LHV), which with some justification could be taken as minimum value. If full recovery of the reaction heat is assumed, then the minimum would be the lower heating value of ammonia, which is 18.6 GJ per tonne NH3. Table 28 compares the specific energy requirement for ammonia production by steam reforming with the theoretical minimum. Table 28. Specific energy requirement for ammonia production compared to the theoretical minimum GJ per tonne NH3 (LHV) Classical Haber – Bosch (coke) Reforming, 0.5 – 10 bar (1953 – 55) Reforming, 30 – 35 bar (1965 – 75) Low energy concepts (1975 – 84) Modern concepts (since 1991) Stoichiometric CH4 demand 80 – 90 47 – 53 33 – 45 29 – 33 < 28 20.9 % theory (338 – 431) 225 – 254 139 – 215 139 – 158 134 = 100 page 42 of 77 Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Comparison of energy consumption figures without precise knowledge of design and evaluation criteria can be misleading. First of all the state of the ammonia product should be noted. Relative to the delivery of liquid ammonia to battery limits at ambient temperature, the production of 3 bar ammonia vapor at the same temperature would save 0.6 GJ per tonne NH3, while delivery as liquid at – 33 °C would need an extra 0.3 GJ per tonne NH3. The temperature of the available cooling medium has a considerable influence. Increasing the cooling water temperature from 20 to 30 °C increases energy consumption by 0.7 GJ per tonne NH3 . A detailed energy balance, listing all imports and exports, together with the caloric conversion factors used for steam and power is needed for a fair comparison of plants. The beneficial effect of energy export to the net energy consumption is discussed above. Gas composition is also of some importance. Nitrogen content is marginally beneficial: 10 mol % nitrogen leads to a saving of about 0.1 GJ per tonne NH3, whereas a content of 10 mol % carbon dioxide would add 0.2 GJ per tonne NH3 to the total consumption value [ 996 p. 263]. Energy requirements and energy saving possibilities are also discussed in [288], [867], [1000], [1001], [1005], [1018], [1025], [1030], [1254]. The energy consumption figures discussed so far represent a thermodynamic analysis based on the first law of thermodynamics. The combination of the first and second laws of thermodynamics leads to the concept of ideal work, also called exergy. This concept can also be used to evaluate the efficiency of ammonia plants. Excellent studies using this approach are presented in [997 p. 258], [998]. Table 29 [997] compares the two methods. The analysis in Table 29 was based on pure methane, cooling water at 30 °C (both with required pressure at battery limits), steam/carbon ratio 2.5, synthesis at 140 bar in an indirectly cooled radial converter. Table 29. Energy analysis of a low energy ammonia plant (GJ per tonne NH3) HHV LHV Exergy Input Natural gas consumption Reformer feed Reformer fuel Auxiliary boiler fuel Total consumption Losses Reforming Steam generation Shift, CO2 removal, methanation Synthesis Turbines and compressors Others (including stack) Total losses NH3 product Efficiency, % 24.66 22.27 7.49 6.78 0.34 0.29 32.49 29.34 0,38 0,38 0.33 0.33 1.30 1.30 1.70 1.70 6.50 6.50 1.30 0.68 11.51 10.89 20.98 17,12 64,60 58,40 23.28 7.08 0.33 30.69 4,94 2.39 0.67 1.55 0.54 0.46 10.55 20.14 65,60 Almost 70 % of the exergy loss in the process occurs in the reforming section and in steam generation. From conventional first law analysis it can be seen that almost all of the losses are transferred to the cooling water. As the analysis assumes water in liquid state, the LHV analyses in Table 29 is not completely balanced. For a perfect balance the heat of evaporation of water (as a fictive heating value) would have to be included. 4.6.1.4. Commercial Steam Reforming Ammonia Processes Especially with ammonia processes based on steam reforming technology it has become a habit to differentiate between processes from various licensors and engineering contractors. This is not so much the case for partial oxidation plants (see Section Ammonia Plants based on Partial Oxidation). Strong competition together with increased plant size and the associated financial commitment has reduced the number of licensors and engineering contractors to a few companies capable of offering world-scale plants, often on a lump-sum turnkey basis. In some cases these companies sub-license their processes and special engineering knowhow to competent engineering companies possessing no knowledge of their own in the ammonia field. There are also several smaller companies with specific and sometimes proprietary know-how which specialize in revamps of existing plants or small plant concepts. In the following, each of the commercially most important processes is discussed in some detail and a shorter description of economically less important processes is given. The process configuration offered and finally constructed by a given contractor may vary considerably from case to case, depending on economic and site conditions and the clients' wishes. Thus plants from the same contractor and vintage often differ considerably. It is possible to categorize steam reforming plants according to their configuration in the reforming section: 1. 2. 3. 4. Advanced conventional processes with high duty primary reforming and stoichiometric process air in the secondary reformer Processes with reduced primary reformer firing and surplus process air Processes without a fired primary reformer (exchanger reformer) Processes without a secondary reformer using nitrogen from an air separation plant In principle the amount of flue gas emitted should be related to the extent of fired primary reforming, but generalizations are questionable, because sometimes the plant layout, as dictated by site requirements, may considerably change the picture for the specific flue gas value. 4.6.1.4.1. Advanced Conventional Processes Kellogg Low-Energy Ammonia Process [1042], [1043], [1254-1259], [1277]. The Kellogg process is along traditional lines, operating with steam/carbon ratio of about 3.3 and stoichiometric amount of process air and low methane slip from the secondary reformer. The synthesis pressure depends on plant size and is between 140 and 180 bar. Temperatures of the mixed feed entering the primary reformer and of the process air entering the secondary reformer are raised to the maximum extent possible with today's metallurgy. This allows reformer firing to be reduced and, conversely, the reforming pressure to be increased to some extent to save compression costs. An important contribution comes from Kellogg's proprietary cross-flow horizontal converter, which operates with catalyst of small particle size, low inlet ammonia concentration, and high conversion. Low-energy carbon removal systems (Benfield LoHeat, aMDEA, Selexol) contribute to the energy optimization. When possibilities to export steam or power are limited, part of the secondary reformer waste-heat is used, in addition to steam generation, for steam superheating, a feature in common with other modern concepts. Proprietary items in addition to the horizontal converter are the traditional Kellogg reformer, transfer line and secondary reformer arrangement, waste-heat boiler, and unitized chiller in the refrigeration section. According to Kellogg 27.9 GJ per tonne NH3 can be achieved in a natural gas based plant with minimum energy export, but with export of larger quantities of steam this value could probably be brought down to about 27 GJ per tonne NH3. Figure 65 shows a simplified flowsheet of the process [1259] with Selexol CO2 removal systems (other options are, e.g., Benfield or BASF aMDEA). Figure 65. M.W. Kellogg's low energy process page 43 of 77 Synthesis loop and converter are licensed by Haldor Topsøe A/S. k) CO2 absorber. [867]. and the synthesis pressure depends on plant size ranging between 140 and 220 bar when the proprietary Topsøe two-bed radial converter S 200 is used. f) Secondary reformer. t) Synthesis converters. [1041] uses the proprietary propylene carbonate based CO2 removal system with adsorption refrigeration using low level heat downstream of the low-temperature shift. [1047].3 and with rather high residual methane content from the secondary reformer. After the converters. Topsøe offers two process versions. i) CO2 removal solvent reboiler. [654]. u) Dryer.6 mol % (dry basis).and low-temperature catalysts. while the second has a third bed with small particle size catalyst and radial flow. CO2 removal is performed with a physical solvent. o) Stripper (other options are. which makes it easier to design a balanced plant with no energy export.2. With a value of 32 GJ per tonne NH3 [845] this is not really a low-energy concept. The Fluor process [288]. Uhde Process. which also removes the methane and part of the argon. both copper-based. y) Refrigeration exchanger. q) CO2 lean pump. in the ammonia business since 1928.1. [1018]. g) Gas turbine.3 GJ per tonne NH3 is claimed. up to 1. and there are no special features compared to other conventional process configurations. k) CO2 absorber. p) Stripper air blower. b) Desulfurization. x) Start-up heater. [1044]. Synthesis is performed at about 180 bar in Uhde's proprietary converter concept with two catalyst beds in the first pressure vessel and the third catalyst bed in the second vessel. For this most energy-efficient concept a figure of 27. markets a low-energy ammonia plant with classic process sequence and catalysts [1061-1068]. Each catalyst bed (of which three are now used in newer plants [1262]) is accommodated in a separate vessel with an inlet – outlet heat exchanger after the first and high-pressure steam boilers after the following. The excess nitrogen introduced in this way is removed after the methanation step in a cryogenic unit known as a purifier [1263]. i) Heat recovery. Uhde. Methanation and CO2 removal are placed between the compression stages and thus operate at higher pressure. In this variant a once-through converter at lower pressure (100 bar). l) CO2 flash drum. the use of waste-heat in the secondary reformer effluent is split between steam raising and steam superheating. [655]. the Benfield or Vetrokoke process is used for carbon dioxide removal. Integrating the ammonia and urea process into a single train was proposed by Snam Progetti to reduce investment and operating costs [1097]. Heat exchange between inlet and outlet of the first bed is performed in the first vessel.6 [867] to 33. s) Methanator. The result is a purer synthesis gas compared to conventional processes. 53). h) High temperature shift converter. LEAD Process (Humphreys & Glasgow. q) Expansion turbine. r) Purifier column. j) Converter An actual plant has reported a consumption of 29. d) Primary reformer.5 t per tonne NH3) is generated (Uhde also offers its own boiler design in cooperation with an experienced boiler maker). Steam to carbon ratio is around 3 and methane slip from the secondary reformer is about 0. Characteristic of the low-energy Braun purifier process (Fig. For the Lummus Process schemes [288]. Achieved net energy consumption is about 28 GJ per tonne NH3 and Uhde's engineers expect values of below 27 GJ per tonne NH3 when a gas turbine and large steam export is included [1066]. Figure 67. and partly for superheating high-pressure steam. [1262-1268].5 and shift conversion with medium. Haldor Topsøe's low energy process a) Desulfurization. In addition to technology supply. s) Synthesis gas compressor. z) Refrigeration compressor Haldor Topsøe Process. [1072]. and only minimal purge from the loop is required. b) Primary reformer. [1261]. is followed by Uhde's standard loop (Fig. Figure 66. [1048] a consumption of 29. 55) (see Section Formation of Ammonia in the Converter). page 44 of 77 . which is achieved by shifting a proportion of the reforming reaction to the secondary reformer by using about 150 % of the stoichiometric air flow. The Exxon Chemical process [1070]. 54) at around 200 bar [1343]. v) High-pressure ammonia separator. h) High. In energy balanced plants. p) Purifier heat exchanger. now Jacobs) [867]. e) Air compressor.. r) Methanator feed preheater. e. [1351-1355]. h) Recycle compressor. [867]. The energy consumption of a basically classic plant configuration has been reduced considerably by applying systematic analysis and processes engineering. g) Main compressor. The lower reforming temperature allows a reduction of the steam/carbon ratio to about 2. [1045]. n) Semi-lean Pump. j) Condensate stripper. Benfield or BASF aMDEA). v) Purge gas H2 recovery. Fluor Process. [1069].g. [1073]. Refrigeration uses screw compressors with high operational flexibility and efficiency. c) Convection section. The smaller furnace produces less flue gas and consequently less waste-heat. the first of which contains two catalyst beds with axial-flow quenching. 4. The LEAD process is a highly optimized conventional approach with synthesis at 125 bar and two converter vessels. i) Low temperature shift converter. Shift conversion is conventional. [845]. followed by a single-bed radial S 50 converter (S 250 configuration).4.5 GJ per tonne NH3 [1046] is quoted. The synthesis is performed at 140 bar with a Topsøe two-bed S 200 radial converter. Instead of the standard synloop Uhde offers the Dual Pressure Process for very large capacities (Fig. and gas flow in all beds is radial.and lowtemperature shift converters. was specifically designed for the company's own site in Canada and so far not built for third parties. t) Synthesis gas compressor. High plant reliability at competitive overall costs was a major objective. n) Methanator. For CO2 removal Selexol or aMDEA is chosen. A consumption of 29. Processes with Reduced Primary Reformer Firing Braun Purifier Process [457]. After each converter vessel high pressure steam (125 – 130 bar. w) Ammonia converter. [867]. Haldor Topsøe also produces the full catalyst range needed in ammonia plants. Exxon Chemical Process. w) Ammonia letdown vessel.2 GJ per tonne NH3 [1058]. also well proven in many installations for hydrogen and methanol service. An additional proprietary item is the side-fired reformer. and for CO2 removal BASF's aMDEA process (1340 kJ/Nm3 CO2) is preferred. [1049-1060]. In the synthesis section either an axial flow quench converter or a radial flow converter with indirect cooling is used.75 without adverse effects on the HT shift. b) Primary reformer. [1343]. The Braun purifier ammonia process a) Sulfur removal. m) Recycle compressor. l) CO2 desorber. because of the less reductive character of the raw gas on account of its higher CO2 content.9 GJ per tonne NH3 is claimed [1057]. The first operates at steam/carbon ratio of 3. d) Secondary reformer. When only a minimum of energy export (steam or power) is possible. The second version has a S/C ratio of 2. f) Process air compressor. It uses a proprietary bottom-fired primary reformer furnace and a proprietary hot potash carbon dioxide removal system with a sterically hindered amine activator. e) waste-heat boiler. high-pressure steam is generated. Key features are the high reforming pressure (up to 43 bar) to save compression energy. d) Shift conversion. A simplified flowsheet is presented in Figure 66. the process heat from the secondary reformer outlet is partly used to raise high-pressure steam. g) Heat recovery. u) waste-heat boiler. which produces about one third of the capacity. The temperature of the mixed feed was raised to 580 °C and that of the process air to 600 °C. 67) is the reduced primary reformer duty. A process flow diagram together with the main process stream is presented in Figure 63. x) Ammonia recovery from purge gas Synthesis is carried out in the proprietary Braun adiabatic hot-wall converter vessels (Fig. Descriptions and operational experience are given in [552]. c) Secondary reformer. f) Methanation. m) CO2 stripper. A typical flow diagram of this process is shown in Figure 67.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience a) Feed gas compressor. [433]. use of Uhde's proprietary reformer design [1062] with rigid connection of the reformer tubes to the outlet header. Synthesis is carried out at 140 bar in a Topsøe S-200 converter and total energy consumption is reported to be 29 GJ per tonne NH3. Lummus Process. e) CO2 removal.6. Shift conversion and methanation have a standard configuration. o) Driers. [651]. [1071]. All nitrogen. On the other hand. p) Flash vessel q) Syngas compressor Figure 70. o) Refrigeration system In the Canadian plant. f) High temperature shift. Montedison Low-Pressure Process. h2) Regenerator. supplies refrigeration. methane. A further key feature is that about half of the process steam is supplied by feed gas saturation. Other process steps are conventional. ICI LCA process (core unit) a) Process air compressor. electricity. falling to 540 °C at the exit of the GHR. recovers pure carbon dioxide. from which ammonia is then recovered by distillation using low-grade heat. The nitrogen surplus is allowed to enter the synthesis loop. It is possible. c) Hydrodesulfurization. which receives the total steam generated in the plant and has an electric generator on the same shaft. and 41 % through the tubes of an exchanger reformer heated by the effluent of a secondary reformer. There is no tubesheet at the bottom of the tubes. including synthesis gas compressor. All other consumers. but differs from the preceding concepts in that only 20 – 50 % of the total feedstock is treated in the tubular primary reformer. h) Selexol CO2 removal. The recovered hydrogen is returned to the circulator suction. A cryogenic unit operating at synthesis pressure rejects the nitrogen surplus from the loop. Separate machines are used for make-up gas and recycle compression. d2) Secondary Reformer. m) Two-stage flash cooling (one stage shown). d) Saturator. Overall steam to carbon ratio is 2. An energy consumption of 29.67 % respectively (dry basis). and for recovery of the ammonia from the loop a water wash with subsequent distillation was suggested.4. [1089] proposed a process which extends the basic idea of the concept described above even further. [1269] resembles the above-described processes in its principal process features: a considerable proportion of the feed is sent directly to the secondary reformer. d1) Primary reformer. e) Boiler. The C. b) Natural gas saturation. which receives the purge gas and part of the synthesis gas taken ahead of the methanation step. The Montedison low-pressure process [867]. and molecular sieve drying. As driver of the process air compressor the installation of a gas turbine is suggested with use of the hot exhaust as preheated combustion air for the fired primary reformer. In addition it is possible to include an electric generator to cover the plant demand and export the surplus. residual methane. [862]. i) PSA system. steam.7. while the balance is injected as steam. Kellogg's LEAP Process. In the late 1970s Kellogg [288]. [1074-1086]. but with steam export and realization of the available improvement possibilities a value of 27 GJ per tonne NH3 seems feasible.8) and a surplus of process air in the secondary reformer. The Foster Wheeler AM2 process [1033]. Heated in an inlet – outlet exchanger to 425 °C the mixed feed enters the ICI gas heated reformer (GHR ) [461-463] at 41 bar. g) Low temperature shift. [1404] also operates with reduced primary reforming (steam/carbon ratio 2. c) Process air compression. l) Circulator. [1090]. h) Desaturator. with a lower inerts content than the synthesis gas after methanation. and the majority of the argon. Demonstrated efficiency is 28. Kellogg KRES reformer. 4. for example. k) Gas dryer. The prototype was commissioned 1985 at Nitrogen Products (formerly CIL) in Canada. with only a small withdrawal of purge gas.1. installation of a cryogenic unit as last step of make-up gas production to remove excess nitrogen. g) Shift converter.6.5 – 2. enters a single-stage shift conversion reactor with a special copper – zinc – alumina-based catalyst that operates in quasi-isothermal fashion and is equipped with cooling tubes in which hot water circulates. Excess nitrogen and inerts (methane and argon) are removed by taking a purge gas stream from the circulator delivery and treating it in a cryogenic unit operating at loop pressure. The synthesis converter is a three-bed design with quench between the first two beds and an exchanger after the second bed to raise the gas temperature of the feed to the first bed. the catalyst being a cobalt-enhanced version of the classical iron catalyst. further purification is performed by Selexol CO2 removal. To produce a stoichiometric synthesis gas the surplus nitrogen has to be rejected in the final purification. The hot exhaust (about 500 °C) of the turbine contains 16 – 17 mol % of oxygen and can serve as preheated combustion air of the primary reformer. The process air flow is stoichiometric. The ICI AMV process [458]. The shell side entrance temperature of the GHR (secondary reformer exit) is 970 °C. The gas. followed by additional plants in China. This is done in a PSA unit. residual carbon oxides. l) Ammonia converter.75 % (dry basis). The combined streams flow on the shell side to heat the reformer tubes in a manner similar to that described for the M. even at the cost of importing some electric power. k) Cooling and drying. W. F. n) Chiller.5 GJ per tonne NH3. [1096] involves a split flow to two primary reformers. Hydrogen and the remainder of the synthesis gas downstream of methanation are mixed to achieve a 3:1 H2:N2 gas composition. m) Hydrogen recovery. Synthesis is performed at 60 bar in a proprietary three-bed indirectly cooled converter with ammonia separation by water. f) Secondary reformer. Figure 69. An enormous quantity of the classical ammonia synthesis catalyst would have been necessary.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience The concept shows great flexibility [1264] for design options.1 GJ per tonne NH3 is claimed [1036]. [1084]. h1) CO2 absorber. [1060]. An energy consumption as low as 28.8 GJ per tonne NH3.7 GJ per tonne NH3 is claimed [1270]. o) Catchpot. Processes Without a Fired Primary Reformer (Exchanger Reformer) ICI LCA Process. b) Start-up air heater. which has a methane leakage of around 1 %. Foster Wheeler AM2 Process. j) Methanator. are driven by electric motors. It was intended to operate the ammonia synthesis at the pressure of the front end by using no synthesis gas compression or only a small booster. with 47 % through catalyst tubes in the radiant section of the fired primary reformer. bypassing the fired primary reformer. The process is especially suited for revamps. Jacobs Plus Ammonia Technology [1037] is especially tailored for small capacities in the 300 to 450 t/d range. an energy consumption of 28. shown in Figure 70. and with substantial steam export a value of 26. Figure 69 shows a diagram of the so-called core unit which includes only the sections essential for ammonia production (up to 450 t/d). it is possible to improve the overall efficiency further by exporting a greater amount of energy. using low-level heat in an integrated absorption refrigerator. Both figures show the configuration of the first two plants built at Severnside (UK) with a capacity of 450 t/d each. cooled to 265 °C in the inlet/outlet exchanger. where the reformed gas mixes directly with the secondary reformer effluent. where it allows plant capacity to be increased. The make-up gas compressor is located upstream of the methanator to make use of the compression heat to warm up the cold gas coming from the Selexol carbon dioxide scrubber. if needed. includes cooling and water-demineralization system. Methane levels at the GHR exit and the secondary reformer are 25 % and 0. The Humphreys & Glasgow (now Jacobs) BYAS process [1034]. Humphreys & Glasgow BYAS Process. which has been tested in a 50 bar pilot plant. Braun process can attain 28 GJ per tonne NH3 in a balanced plant. About 65 % of the feed – steam mixture flows conventionally through the radiant tubes of a fired primary reformer followed by a secondary reformer. methanation. Arrangement of core units and utility section Feed gas is purified in a hydrodesulfurization unit operating at lower than usual temperatures and passes through a saturator to supply a part of the process steam. ICI AMV Process. e) GHR. ICI AMV process a) Desulfurization. n) Ammonia converter. which operates at the very low pressure of 90 bar with an unusually large catalyst volume. For this process. A separate utility section. with a load shift from primary to secondary reformer and use of excess process air. passing to the secondary reformer at 715 °C. j) Methanation. 12 % through catalyst tubes in the convection section. see Section Processes Without a Fired Primary Reformer (Exchanger Reformer) and Feedstock Pretreatment and Raw Gas Production). use of excess air in the secondary reformer. The balance of the feed – steam mixture passes through the tubes of a vertical exchanger reformer. as described page 45 of 77 . and argon are adsorbed to give a stream of pure hydrogen. [867]. The flow of the preheated gas stream mixture is split into three streams. [1087]. also belongs to the group of processes that shift load from the primary to the secondary reformer. only the air compressor is driven by a steam turbine. In this latter case it is advantageous to incorporate a gas turbine to drive the process air compressor.8 GJ per tonne NH3 is claimed. to aim for minimal natural gas consumption.3. whereby the absorbed heat is used for feed gas saturation. i) Single stage compression. [1036]. A flow sheet is shown in Figure 68 Figure 68. A consumption below 28 GJ per tonne NH3 was calculated. This exchanger reformer has a tubesheet for the catalyst tubes at the mixed feed inlet. As a consequence the inert level in the loop can be kept rather low. The remaining feed is directly sent to the secondary (autothermal) reformer which operates with a high surplus of process air (up to 200 %) and a rather high methane slip of 2. The ICI LCA process [1091-1095] is a radical breakaway from the design philosophy of the highly integrated large plant used successfully for the last 25 years. and. The consumption figure reported for a totally energy-balanced plant is 28. After conventional shift.3 GJ per tonne NH3 is claimed. power generation in a Rankine cycle with CFC to generate electric power (optional). 57) (hot-wall design) with interbed exchangers. [1379-1381] shown in Figure 71. Figure 71. The shell side gas leaves the vessel at 40 bar. [1048].6. its own proprietary version of the Rectisol process [1396]. respectively. and. The overall energy consumption claimed for this process can be as low as 27. The LCA and KAAPplusProcess are environmentally favorable because atmospheric emissions of both nitrogen oxides and carbon dioxide are dramatically reduced as there is no reforming furnace. n) Rectifier column. j) Dryer. Chiyoda Process [1038]. isothermal shift. Figure 72.3 GJ per tonne NH3. which differ in the gasification process. only when pure CO2 product is required).6.5 GJ per tonne NH3 or. First application was for a 1350 t/d plant in India. The other process steps are more along traditional lines. and pure CO2 is recovered from the PSA waste gas by an aMDEA wash.8 GJ per tonne NH3. Synthesis proceeds at about 90 bar in a four-bed radial-flow converter (Fig.6. If pure CO2 is needed. to achieve greater flexibility for start-up and reduced-load operation. page 46 of 77 . f) Condensate stripper. The smaller of the two enters the exchanger reformer at 550 – 550 °C. h) Methanator. Ammonia Plants based on Partial Oxidation 4. synthesis loop. d) Automatic reformer (ATR). this gasification technology can be an economic choice. probably with better energy efficiency. and synthesis) can in principal be applied to larger capacities. This concept has a configuration similar to the Linde LAC process. gas turbines are proposed as drivers for the process air compressor and synthesis gas compressor with the hot exhaust being used for steam generation and feed gas preheating. [1084]. The exchanger reformer and the autothermal reformer use conventional nickel-based primary and secondary reforming catalysts. commercial use of this technology is restricted to the processing of higher hydrocarbons.1. The Linde LAC process [1112-1116] consists essentially of a hydrogen plant with only a PSA unit to purify the synthesis gas. e) Reforming exchanger (KRES). The flow sheet energy consumption is 29. In principle it is similar to the PARC process. 29. Purge gas is recycled to the PSA unit.1. a separate auxiliary boiler is normally necessary to provide steam for mechanical energy and power generation. for which in older installations values of around 38 GJ per tonne NH3 were reported. later Braun and Root) KBR offers now the KAAPplusProcess [1329]. because in absence of a large fired furnace. Very little steam is generated in the synthesis loop and from waste gases and some natural gas in the utility boiler in the utility section (60 bar). [665]. followed by methanation and drying. c) Process heater. [1330]. not directly active in ammonia plant design may be used as general contractors to coordinate a number of subcontractors supplying the different technologies required. Independent. The first bed is charged with conventional iron-based catalyst for bulk conversion and the others with Kellogg's high activity ruthenium-based catalyst. The original intention was to design an ammonia plant which can compete with modern large capacity plants in consumption and specific investment. often containing as much as 7 % sulfur.8 GJ per tonne NH3. Comparison of Linde LAC process with a conventional process Humphreys & Glasgow MDF Process (now Jacobs) [288]. rather than listing all individual contractor design approaches. p) KAAP ammonia converter. 4. classical primary reformer at 29 bar. The required heat for the endothermic reaction in the tubes of the exchanger reformer comes from the gases on the shell side. [666]. and all drivers are electric. [1107-1110]. which has a proprietary configuration (UOP) in which nitrogen flushing enhances hydrogen recovery. g) CO2 absorber. Therefore. by installing the CO2 removal unit directly in the synthesis gas train ahead of the PSA system. a standard cryogenic nitrogen unit. Processes Without a Secondary Reformer (Nitrogen from Air Separation) KTI PARC Process. while the remainder is passed directly to the autothermal reformer at 600 – 640 °C. [1104]. experienced engineering companies. [1118]. [1260] uses the following process elements: air separation unit. According to Linde the process should consume about 28. q) Refrigeration compressor. and Topsøe's technology for synthesis converter and loop.5 GJ per tonne NH3. Ammonia Plants Based on Heavy Hydrocarbons Although partial oxidation processes can gasify any hydrocarbon feedstock. KBR KAAPplus Process a) Air compressor. The overall efficiency ranges from 29.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience above. for which most contractors have their own proprietary technology for the individual process steps.2. Linde LAC Process. Additionally. Synthesis operates at 82 bar [1404] in a proprietary tubular converter loaded with a cobalt-enhanced formulation of the classical iron catalyst. the engineering firms which offer ammonia plants based on heavy hydrocarbons have often to rely for the individual process stages on different licensors. o) Synthesis gas compressor. To satisfy the heat balance. some optimization has successfully reduced the overall energy consumption. 4. Where natural gas is unavailable or the heavy feedstock can be obtained at a competitive price. r) Refrigeration exchanger Desulfurized gas is mixed with steam and then split into two streams in approximate ratio 2:1. The synthesis converter and loop are based on ICI and Casale know-how. With including the Purifier (of C. but designed for world-scale capacities. today Kellogg Brown and Root (KBR). comprising a mixture of the effluent from the autothermal reformer and the the gas emerging from the tubes. it can be recovered by scrubbing the off-gas from the PSA unit at low pressure or. Linde's air separation and liquid nitrogen wash. [1103]. with inclusion of pure CO2 recovery. carbon dioxide removal (optional. More recent commercial bids quote values as low as 33. [1102].4. The arguments presented above suggest describing the two principal routes. In this concept four sections of the classical process sequence (secondary reforming. Energy consumption with inclusion of pure CO2 recovery (which is optional) is 32. k) Expander. The basic plant features (GHR. the autothermal reformer operates with surplus of air. and an ammonia synthesis loop. allowing an exit ammonia concentration in excess of 20 % to be obtained. [1119]. In this process the traditional fired primary reformer is also replaced by an exchanger reformer and the heat balance requires excess air in the secondary reformer with the consequence of a cryogenic unit as final step in the make-up gas preparation to remove the surplus of nitrogen. LT shift. needing minimum staffing. [1105]. There are two commercially proven partial oxidation routes for heavy feedstocks: the Shell process and the Texaco process. Topsøe has described the lay-out for an ammonia plant based on fully autothemic reforming [1328]. nitrogen addition.2 GJ per tonne NH3. for example. PSA. standard HT shift. Kellogg. [1046].3 to 31. In contrast to the steam reforming. i) CO2 stripper. Figure 73 shows the classical sequence of process steps for both cases. b) Sulfur removal. CO2 removal. The purifier removes the nitrogen surplus together with residual methane and part of the argon. m) Condenser.3 GJ per tonne NH3. offers several process schemes with use of its KRES exchanger reformer and its KAAP ammonia technology [467470].4. l) Feed/effluent exchanger. Figure 72 compares the LAC process to a conventional one. on account of its higher energy consumption and investment costs. by means of lower energy integration. The KTI PARC ammonia process [603]. methanation) can be replaced by the fully automatic high-efficiency PSA system.F.2. there is no large amount of flue gas and consequently less waste-heat is available. Nevertheless. Braun. These are separately heated in a fired heater. has built very large capacity ammonia plants that use Shell's gasification process. This is in line with the fact that the degree of energy integration is usually lower than in steam reforming technology. CO2 removal and further purification is effected by a PSA system. Shell and Texaco. Lurgi. c) CO2 absorption. i) Liquid N2 scrubber. formed because of insufficient mixing of the reactants. e) Stripper. The large amount of carbon dioxide formed in shift conversion lowers the H2S concentration in the sour gas. In newer installations the carbon – water slurry is filtered off in automatic filters. Plant descriptions are given in [1106. and synthesis loop have no special features.. [486]. [487-491]. quench is thought to be more economical when hydrogenrich gases are manufactured. [554]. g) Refrigerant.g. [555]. methanol or oxogas). h) Dryer. units with operating pressures up to 90 bar are in operation [422]. The temperature in the gasification vessel (generator) is between 1200 and 1400 °C. which is subsequently scrubbed first in a Venturi scrubber and then in a packed tower to remove the soot. which is adjusted by nitrogen addition to the stoichiometric ratio N2 : H2 = 1 : 3. Converter and synthesis loop configuration depend on the licensor chosen. A Shell gasifier with a waste-heat boiler or a Texaco generator with a quench are equally well suited to this process. and the moist filter cake is subjected to a controlled oxidation in a multiple-hearth furnace. and the hot water is recycled to the saturator. Some modern installations (e. The gas is heated subsequently by heat exchange. To remove residual CO a liquid nitrogen wash is applied for final purification with the advantage of also lowering the argon content in the make-up gas. Temperatures in the generator are similar to those in the Shell process. a proprietary item of the Shell process. and fly ash. compression.g. which is regenerated by flashing and stripping. 1117. An appropriate process (Rectisol or amine based) removes the sour gases H2S. 1120]. The anticipated energy consumption is 34.6 mol %. A waste-heat boiler.. [1122]. A basically similar synthesis gas preparation. A cryogenic air separation plant provides oxygen for the gasification and the nitrogen for the liquid nitrogen wash and for supplying the stoichiometric amount for the synthesis of ammonia.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Figure 73. Descriptions of plants using the Texaco process can be found in [416].1 %. Make-up gas drying. and the pressure between 35 and 65 bar. [418]. and for this reason a special concentration step is required for methanol regeneration to obtain pure CO2 and a fraction sufficiently rich in H2S for a Claus plant or a sulfuric acid plant. and CO2. Lurgi) [471]. The synthesis gas purification train comprises soot scrubbing followed by shift conversion. Ammonia production based on heavy fuel oil (Linde flow scheme with Texaco gasification) a) Air separation unit. but based on gasification with pure oxygen. [638] which is not only sulfur-tolerant but also requires a minimum sulfur content in the gas for proper performance. Linde) use pumps for liquid oxygen instead of oxygen compressors. The remaining process steps are identical with the Shell route. A substantial pressure drop is needed to ensure atomization of the oil and proper mixing with oxygen. and then fed to shift conversion. The shift reaction uses a cobalt – molybdenum – alumina catalyst [417]. In contrast to the Shell arrangement. and this is followed by methanation. The removed sulfur-rich fraction is sent to a Claus plant for recovery of elemental sulfur or converted to sulfuric acid.g.. acid gas removal (for page 47 of 77 . The conversion is subdivided into stages with intermediate cooling by raising steam. which proceeds stepwise with intermediate heat removal. Older installations used an elaborate technique to remove the soot from the water by extraction with naphtha and light oil to form soot pellets which could be burnt or recycled to the feed oil. oxygen enters the gasifier through the central nozzle of the burner. [492]. Flow diagrams of ammonia production from fuel oil or heavy residues by the Shell (A) and the Texaco (B) process (standard configuration) Processes Using Shell Gasification (e. The hot gas contains soot. The gas is cooled by a direct water cooler. is already used in large commercial plant in Japan [1274]. leaving a residual CO content of 0. Instead of a waste-heat boiler a direct water quench is applied for cooling the raw synthesis gas. Although this is preferable when producing CO-rich synthesis gases (e. raises 100 bar steam and cools the gas to 340 °C. and the naphtha is distilled off and recycled to the extraction stage. A second Rectisol wash stage follows to remove CO2 by absorption at –65 °C in methanol.g. Soot recovery from the water is performed by extraction with naphtha. [477-485]. Foster Wheeler Air Partial Oxidation Process [1033]. [556]. A concept using enriched air instead of pure oxygen and methanation instead of a liquid nitrogen wash was proposed by Topsøe [1121]. and oil is fed through the annular space between central nozzle and outer burner tube. d) Methanol/H2O distillation. supplied with steam in a saturator. Figure 74 gives an example of a Linde flow diagram. Figure 74. Foster Wheeler. The following Rectisol process has a somewhat more elaborate configuration than the version used in the Shell route. Texaco also offers a version operating with a waste-heat boiler instead of a water quench. After soot removal. 1111. A selective Rectisol unit with methanol of about –30 °C as solvent is used to remove H2S and COS (together with some CO2) to less than 0. The soot – naphtha suspension is mixed with feed oil. Oil enters the alumina-lined gasification vessel through a central jet in the burner nozzle. [418]. Uhde). j) Syngas compressor. Molecular sieve adsorption then removes residual traces of methanol and CO2. k) NH3 reactor Material Balance Topsøe Process. Linde. fed through the annulus between the jet and the outer case of the burner nozzle. Processes Using Texaco Gasification (e. f) Hot regenerator.. COS. Soot is removed from the raw gas in a two-stage water wash. It is intended to operate at 70 bar with highly preheated air (815 °C) instead of pure oxygen. (see also Gas Production – Types of Processes). shift conversion is performed on a sulfur-tolerant catalyst in several beds with intermediate cooling. [1123] is a proposed modification of the Texaco gasification process.8 GJ per tonne NH3. b) Soot extraction. An energy consumption of 44. which is lower than the 48. As much as 1. The next step is compression to about 30 bar. [526. and the final purification step is a liquid nitrogen wash. 1332. Shell. which poses some restriction on the tube length. The inherent flexibility of thin tubesheets assists in dispersing stresses and reduces the risk of fatigue failure of the tube-to-tubesheet welds and tubesheet-to-shell welds. is that debris in feed water (mainly magnetite particles spalling from the water side of the tubes) can collect at the bottom of the horizontally mounted vessel without creating diffficulties and are removed easily by blowdown. The cooled gas. Common practice for a long time was to use a boiler with water tube design. The risk is overheating and tube failure. carried out stepwise with intermediate heat removal and with a standard HTS catalyst.6 – 37. The operating conditions for the boilers are more severe than those normally encountered in power plants. including Borsigs's thin-tubesheet design. and the tube inlets are protected by ferrules. and Dow. which differ in boiling point by only 13 °C.3 GJ per tonne NH3 is stated. b) Tube. and some ammonia. A figure of 35. Commercially proven coal gasification processes are Lurgi (dry gasifier). In earlier plant generations two boilers were usually installed in series. According to [537]. g) Anchor In the synthesis loop boilers at the exit of the converter up to 50 % of the total steam is generated. In contrast fire-tube boilers are much better suited for natural circulation and the steam drum can sit in a piggyback fashion right on top of the boiler. free of coal dust and fly ash. Today coal or coke (including coke oven gas) are used as feed for only a smaller part of world-wide ammonia production. power generation facilities are usually separate. [1273] (see Section Feedstock Pretreatment and Raw Gas Production) originates from Texaco's partial oxidation process for heavy oil fractions and processes a coal – water slurry containing 60 – 70 % coal. Any of the well known converter and synthesis loop concepts may be used. [1339] is reinforced by stiffening plates on the back side (Fig. followed by sulfur removal at –38 °C with chilled methanol (Rectisol process). [1272]. But with the enormous increase of the natural gas prices in the USA the share of coal in the feedstock pattern of ammonia might become larger in the future (see Chapter Economic Aspects).2. Several plants are operating in South Africa [1124]. demonstrated commercially in another application with a capacity equivalent to a world-scale ammonia plant. [495]. c) TIG welded root pass. Several process steps treat the separated gas liquor to recover tar. contains about 60 % of CO. because the deposits may collect in the lowest and most intensively heated part of the tube. refer to Section Feedstock Pretreatment and Raw Gas Production and [504-510]. Besause of size limitations two parallel units were installed. In one concept (Uhde [840].2. Lurgi Process. [1128-1131]. which can handle any sort of coal (ash content may exceed 30 %). A famous example is the Kellogg bayonet-tube boiler. Rectisol process. Both solutions have full penetration tube-to-tubesheet welds. is also a potential candidate for ammonia production processes. this may restrict the water flow to the point where boiling occurs irregularly (film boiling). however. [498-501]. [493]. which is separately processed in a steam reformer. A similar variant also using air instead of pure oxygen is offered by Humphreys & Glasgow (now Jacobs) [1281]. This concept uses twisted tubes [1378]. and tars. which similarly tend not to have specific ammonia technology of their own. and the reformed gas rejoins the main stream at the Rectisol unit for purification.5 % of the world ammonia capacity used this raw material [502]. Steinmüller [1337]) the flexible tubesheet is anchored to and supported by the tubes to withstand the differential pressure. 1331. phenols. Texaco. 4. with a bypass around the second to control the inlet temperature for the HT shift. [505]. A critical analysis of the two concepts is given in [1340].3. which removes the surplus nitrogen by condensation together with methane. which helps to drive the air compressor. Ammonia Plants Using Coal as Feedstock In the early days the entire ammonia industry was based on coal feedstock. for example. the conditions and stress pattern are different. operates at 30 bar. Lurgi [494]. [528. 1127] offers a concept using its proprietary Lurgi dry bottom gasifier.5 GJ per tonne NH3 (LHV) has been reported. operating at –58 °C and 50 bar. which creates rather large temperature gradients and therefore high expansive stress. Apart from a few plants operating in India and South Africa.6. and methanation. After washing with process condensates to remove ash and dust. In an extreme case of scaling. as well as the power required for synthesis gas compression. without striving for completeness. removes the CO2. British Gas/Lurgi (slagging gasifier). the tubesheet is only 20 – 30 mm thick. In steam reforming plants. Reinforced tubesheet of the Borsig boiler a) Tubesheeet. the majority of coal-based ammonia plants are found in China. and the resultant stress on the tube-to-tubesheet welds can lead to cracks.7 for natural gas as feedstock. A major aspect in the design concept is the separation of nitrogen and oxygen. Again. In 1990. due to the practically inert-free make-up gas. the atmospheric ACGP gasifier could lower the consumption to 44 GJ per tonne NH3 (HHV). [1275]. [503]. In the cryogenic unit for the Foster Wheeler process a lesser quantity of nitrogen is separated from hydrogen with a much higher boiling point difference (57 °C). and residual carbon monoxide. e) Supporting ring. The atmospheric-pressure gasification is a considerable disadvantage of this process route. 530]. But compared to the secondary reformer service.5 GJ per tonne NH3 quoted for another Texaco coal gasification-based ammonia plant [494]. the gas is cooled further with recovery of waste-heat. and liquid nitrogen wash are the further operations in the production of make-up gas. 1334]. on account of the high pressure on both sides the heat transfer rates and thus the thermal stresses induced are much higher. troublesome phenolic material. The Shell process. According to Foster Wheeler this leads to considerable saving in capital investment and energy consumption compared to the traditional approach using pure oxygen from an air separation plant and a liquid nitrogen wash for gas purification. In all the various designs of this concept.5 t of steam per tonne ammonia. [494]. 75). Ube Industries commissioned a 1000 t/d ammonia plant in 1984 using Texaco's coal gasification process [1274]. only 13. This makes it possible to provide each boiler with its own separate steam drum. [1125] and India [1126]. [1387]. [495]. f) Stiffener plate. Texaco. An alternative tube-bundle design which can directly substitute the bayonet-tube internals was recently developed. Thick tubesheets in this kind of service are too rigid and have too high a temperature gradient.6. A positive feature of the design. [841]. with no purge or minimal purge. In the synthesis loop boiler the opposite is the case with the result that the tubes are subjected to longitudinal compression instead of beeing under tension. especially bayonet-types. Waste-Heat Boilers for High-Pressure Steam Generation Because of its great influence on reliability and efficiency of all ammonia plants a special section for the generation of high-pressure steam seems to be appropriate. The Koppers – Totzek Process gasifies coal dust with oxygen in the temperature range 1500 – 1600 °C at about atmospheric pressure. the temperature of the gas from the secondary reformer has to be reduced from 1000 °C to 350 °C before entering the HT shift vessel. In recent years little development work has been done on complete ammonia plant concepts based on coal. A conventional air separation plant is based on the fractional distillation of oxygen and nitrogen. Koppers – Totzek. The hot inlet channel and the tubesheet are shielded by a refractory layer. d) Shell. Steam is added for the shift conversion. argon. higher hydrocarbons. and steam generation consumes 18 – 22 GJ per tonne NH3.6 GJ per tonne NH3 is claimed for heavy oil feedstock and 31. [1250. The gas is dried and finally fed to a cryogenic unit. Figure 75. An energy input of 51.4 – 32. which substantially increases equipment dimensions and costs. [1338]. A lock hopper system removes ash and glassy slag as a suspension from the quench compartment of the generator. The gasification has a power consumption of 32 – 34 GJ per tonne NH3. applied in more than 100 plants. 529. The key factor which allowed the use of fire-tube boilers after the secondary reformer was the development of thin-tubesheet designs. Thus it is difficult to identify specific ammonia processes for the individual contractors and the following descriptions serve as examples. The further gas purification steps used to arrive at pure make-up gas correspond to those described for an ammonia plant using the Texaco partial oxidation of heavy oil fractions.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience example Selexol). In a fire-tube boiler. For sufficient natural water circulation these boilers needed a steam drum at a rather high elevation and a considerable number of downcomers (feed water) and risers (steam/water mixture). For a more detailed description of the gasification. The rejected nitrogen is heated and expanded in a turbine. which allows a greater flexibility in the plot plan. For this service also fire-tube versions have been used. So far only the Koppers – Totzek. compared to a steam reforming plant. The liquid nitrogen wash produces a methane-rich fraction. are very sensitive in this respect. for example. The process can handle bituminous and sub-bituminous coal but not lignite. A newer statistic estimates a figure of 19 % for 2001 [1387]. [518-525]. resulting in a total energy consumption of 50 – 56 GJ per tonne NH3. where the gas pressure is lower than the steam pressure. Shift conversion. A second Rectisol stage. 4. and Lurgi processes have been used commercially for ammonia production [493]. described in Section Feedstock Pretreatment and Raw Gas Production. Steinmüller's waste-heat boiler product line is now owned by Borsig. The Texaco Coal Gasification Process [495]. the inlet tubesheet and the tubesheet welds are exposed to the extreme temperature of the reformed gas. Water-tube boilers. 527. the degree of integration is considerably lower. and the product gas contains up to 15 % CH4. Several failures in this application have page 48 of 77 . The tubesheet of Babcock-Borsig (today Borsig) [837]. Winkler/HTW. The moving-bed generator. The traditional leading ammonia contractors have to rely on proprietary processes licensed from different companies. equivalent to 90 % of the reaction heat can be generated. For example there may be fewer vendors or special fabrication procedures might become necessary. with which more than 90 plants have been revamped so far. not entailing excessive cost. Many papers on this subject were presented at the “AIChE Annual Symposia for Safety In Ammonia Plants and Related Facilities” [1350].4. and Borsig [1347]. Other measures. and installing a more effective CO2 removal are other options.7. 1318]. Drying of the make-up gas. [1330]. Another method is to perform a part of the primary reforming in a pre-reformer [431-438] that uses low-level heat. Topsøe [1011]. addition of hydrogen recovery from purge gas. numerous modernization possibilities exist for partial oxidation plants. reduced synthesis pressure. and this also represents an overview of the revamp options for existing steam reforming plants. Figure 76. The Shell process uses propretary designs (Fasel – Lentjes) [1349]. But the simple formula should be handled with care because it is only valid as long as a given process configuration is scaled up and the dimensions of the equipment for very large capacities do not lead to disproportionate price increases. which cool the exit gas of the generator from 1400 °C to around 350 °C. [813-816] its proprietary ACAR technology. A horizontal synthesis waste-heat boiler was developed by Balcke – Dürr with straight tubes and thick tubesheets on both ends [835]. 76) are available from Uhde [1342]. With the aMDEA system. Single-train Capacity Limitations – Mega-Ammonia Plants The principle of the economy of scale is well known in production processes. [1254]. On the other hand certain process developments like autothermal reforming and exchanger reforming could reduce the investment for the higher capacities. page 49 of 77 . [1345]. Proven U-type designs (Fig. are introduction of combustion air preheating and reducing the primary reformer load. [1084]. or a combination thereof. such as labor. The thick-tubesheet concepts in various configurations are more generally accepted now. but reviews on this subject and useful information are given in [1005-1017]. Balcke – Dürr [833]. Elegant variants of this principle are the Jacobs BYAS process [1034]. less efficient plants so that they can stay competitive. According to most of these studies a capacity of 5000 t/d seems to be possible.6. [1028]. (see also Section Feedstock Pretreatment and Raw Gas Production) could be considered. It has a capacity of 3300 t/d and came on-stream in 2006. So the scale factor could increase for very large single-train capacities. if possible. which can be flexibly tailored to fit into existing process configurations. [1398]. For ammonia plants a scale exponent 0. Several studies [1317]. adjusting the activator concentration accordingly to achieve zero or only minor equipment modification. 4. Also hot potash processes have been converted in this way to aMDEA. [1098-1101]. The largest plant so far was built by Uhde in Saudi-Arabia. Forced water circulation around the entrance nozzles helps to stand the high heat flux (700 kJ m–2s–1) at this location. involving more additional hardware and engineering work. As the market possibilities for a company do not increase in steps of 1000 or 1500 t/d but slowly and continuously. Just a few of the frequently used revamp possibilities should be mentioned here. higher conversion. If oxygen is available. and they may even outnumber those for steam reforming plants. possible to simply replace the MEA solvent with the aMDEA solution. A limitation could be the synthesis compressor which can probably be designed for 4500 t/d only. Application of indirect cooling and smaller catalyst particles are frequently chosen to reduce energy consumption through lower pressure drop. In steam reforming plants it is often possible to lower the steam/carbon ratio by using improved reforming catalysts and copper-promoted HT shift catalysts. Most revamp projects have been combined with a moderate capacity increase because some of the original equipment was oversized and only specific bottlenecks had to be eliminated. [1088]. Uhde U-tube synthesis loop boiler A special design is Borsig's hot/cold tubesheet. [1087]. Borsig syngas waste-heat boiler for Texaco gasification Additional information on waste-heat boilers is found in [762]. and overhead expenses. [404]. The hot and cold end of the tube are arranged alternately. where just the loop which produces only two thirds of the ammonia is at high pressure whereas the once-through converter is at lower pressure. face additional difficulties. Another capacity limiting factor is pipe diameter in the synthesis section. For a revamp project first an updated base-line flow sheet of the existing plant should be prepared from which the proposed improvement can be measured [10021004]. simple modifications of existing equipment are preferable to replacement. shifting duties from overtaxed units to oversized ones. [1353]. Section Further Developments describes modifications that lower the energy consumption in newer plant generations compared to the first generation of single-train ammonia plants. Modernization of Older Plants (Revamping) With rising feedstock prices and hard competition in the market. which will reduce H2 loss (methanation) and inert content in the make-up gas. such a moderate capacity addition will involve less risk and will be more economical than building a new plant. The gas contains soot and probably some fly ash particles. [1403]. Apart from replacing existing ammonia converters. so that a hot shank is always to a cold shank and vice versa. Depending on the objective (energy saving and/or capacity increase) the following guidelines should be kept in mind: maximum use of capacity reserves in existing equipment. The advantage is that the tubesheet can kept below 380°C [836. Common to both plant types is the potential for improvement of the synthesis loop and converter. 77) by Borsig [1348]. Similarly. [1026] mostly uses its Series 200 configuration. More active LT shift catalysts lower the residual CO content. This also holds true for the Dual Pressure Process of Uhde. [406]. The KBR KAAP process has there an advantage because of its low synthesis pressure. [1355]. it is. [767].Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience been been reported [842]. Steam reforming furnaces of the required size have already been built for other applications (methanol) and neccessary vessel diameters seem also to be fabricable. many producers have looked for possibilities to “revamp” or modernize their older. [1269] and the Foster Wheeler AM2 process [1033]. KBR [838]. One possibility is to increase the duty of the secondary reformer and use air in excess of the stoichiometric demand. [1391]. Very high gas velocities and appropriate design are necessary to prevent any deposition on the heat-exchange surfaces and to reduce the danger of attrition as well. for example. Alternative methods to enlarge the reforming capacity make use of the process heat of the secondary reformer in an exchanger reformer such as ICI's GHR [1031] or Kellogg's KRES [469].7 is widely used as a rule of thumb: The advantage of larger plants is not only the investment but also the reduction of fixed costs. 4. [1346]. With a conventional process concept for the front end there is no restriction for producing syngas for 5000 t/d of ammonia. A special design for Texaco gasifications (orginally by Schmidt'sche Heißdampf GmbH and Steinmüller GmbH) is offered now in an improved version (Fig. in situ modification of the internals of installed converters is a very economic approach. Description of executed revamp projects are given in [1270]. Figure 77. Waste-heat boilers in partial oxidation plants. the amount of additional equipment should be kept to a minimum [1003]. To give an exhaustive list or description of the individual modification options is beyond the scope of this article. Nevertheless the specific investment will decrease with the increase of capacity. [1381-1386] have been made by various contractors to investigate the feasibility and the size limitations for steam reforming plants. This latter option is used when the revamp objective is capacity increase and the primary reformer is identified as a bottleneck. maintenance. installation of a parallel autothermal reformer or a parallel Uhde CAR unit [472-476]. For export-orientated plants at locations with cheap natural gas there is growing interest in very large capacities. [834]. [1019-1024]. [1341]. [1027]. [1344]. and Ammonia Casale [767-769]. Vessels for which temper embrittlement is anticipated should not be pressurized below a certain temperature. [879]. 200 °C) and a hydrogen partial pressure as low as 7 bar. As the name implies. the pressure shells of reaction vessels as well as the connecting pipes are in contact with hydrogen at elevated pressure and temperature with a potential risk of material deterioration [872-874]. A special revamping option is the integration of other processes into an ammonia plant [1318]. For this reason it is advisable not to cool vessels too rapidly when taking them out of service. but at present the following situation must be accepted: virtually all high-temperature alloys are vulnerable to metal dusting. used for the converter basket. such as those between ferritic and austenitic steels [886]. h) Catchpot. It is assumed that thermodynamically favored sites exist for these elements at the surface and enhance the carbon deposition if the gas composition corresponds to a carbon activity >1 [898]. In the nitrided areas. higher steam/carbon ratios reduces this sort of corrosion. leading to less stress on account of thicker walls. For example a CO production from a side stream upstream of the HT shift is possible [1356]. Mn. tungsten. Numerous studies. P. j) Let-down vessel In the less integrated partial oxidation plants coproduction schemes are easier to be incorporated and there are several large installations which were directly designed to produce ammonia. aluminum. which may happen simultaneously with chemical attack.. typically above 800 °C. improvements may be expected by additional surface coating (for example with aluminum). Sn) in steels [892]. The risk of attack may exist at quite moderate temperatures (ca. on this subject. With materials such as Inconel 601. an induction period may be observed until metal dusting manifests itself as pitting or general attack. The result is a progressive deterioration of the material that lowers its toughness until the affected piece of equipment cracks and ultimately ruptures. Areas highly susceptible to attack are those which have the greatest probability of containing unstable carbides. strongly catalyzed by iron. In [880]. [1388] is a corrosion phenomenon which has come into focus again in the last few years with the introduction of exchanger reformer technology and the operation of steam superheaters in the hot process gas downstream of the secondary reformer waste-heat boiler. The rate of deterioration of the material depends on the pressure of the trapped methane. An example of coproduction of methanol is shown in Figure 78. [1408] can occur. [1029]. [878]. such as welding seams [877]. These Nelson diagrams give the stability limits for various steels as a function of temperature and hydrogen partial pressure. transforming the steel from a ductile to a brittle state. A promising loop modernization option for the future will be the Kellogg KAAP process [470]. metal dusting occurs at 500 – 800 °C on iron – nickel and iron – cobalt based alloys with gases containing carbon monoxide. f) Steam drum. Holding a weld and the heataffected zone for a prolonged period at elevated temperature (an operation known as soaking) allows the majority of included hydrogen to diffuse out of the material. c) HT shift.5 – 3.. Normally. [881]. This phenomenon was already recognized and principally understood by BOSCH et al. but this can increase to 60 °C and more (temper embrittlement). [878]. I) Syngas compressor. Austenitic steel. and others). In several steps of the ammonia production process. Hydrogen can also be produced from a side stream by using PSA. [1294]. Hydrogen diffuses into the steel and reacts with the carbon that is responsible for the strength of the material to form methane. Newer experience gained in industrial applications required several revisions of the original Nelson diagram. [1398]. for a longer time no problems were experienced for the same converter design in other cases where non-hygroscopic flux was used for welding and a more conservative vessel code was adopted. which on account of its higher molecular volume cannot escape. especially in the synthesis section. [894-900] [1318]. In contrast to the hydrogen attack described above this phenomenon is reversible. d) LT shift. hydrogen is always soluble to a minor extent in construction steels. With unalloyed and low-alloy steels.8. The active sites at the page 50 of 77 . This was surprising because the operating conditions for the material (2. which tend to flake off. Chemical Hydrogen Attack. 601H. molybdenum. Metal dusting [404]. 0. and the carbon originates from cracking of hydrocarbons. methanol and hydrogen [1357]. even in plants which have already made modifications. It occurs in the presence of ammonia on the surface of steel at temperatures above 300 °C [888]. [893]. high-pressure steam pipes) can lead to a decline in impact strength [1405]. [93] when they developed the first ammonia process. Conventional carburization is a familiar problem with high-temperature alloys in steam reforming furnaces caused by inward migration of carbon leading to formation of carbides in the metal matrix. However. which is sensitive to hydrogen attack. But this may not be sufficient if moisture was present during the original welding operation (for example if wet electrodes or hygroscopic fluxes were used). [1358]. The type of carbides and their activity are strongly influenced by the quality of post-weld heat treatment (PWHT).Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience With a “split-flow” configuration Kellogg proposed an in situ revamp option to change its four-bed quench converter into a two-stage intercooled converter (with two parallel beds for the second stage) using smaller catalyst particles of 1. It happens at high temperatures. because traces of atomic hydrogen are formed by thermal decomposition of water under the intense heat of the welding procedure. A possible mechanism was proposed by GRABKE [902] and HOCHMANN [903]. It occurs when adsorbed molecular hydrogen dissociates at higher temperatures into atomic hydrogen. Under certain conditions chemical hydrogen attack [875]. the risk of formation of brittle surface cracks exists. Methanol coproduction (side-stream loop) a) Secondary reformer. A rival theory [883]. It has been reported [882]. still valid today. Metal Dusting. Revamp histories are a constant topic at the “AIChE Annual Symposium Ammonia Plant and Related Facilities” [1350] and a wealth of information and practical experience can be found there. Material Considerations for Equipment Fabrication Hydrogen Attack. CLASS gives an extensive survey. [876]. Figure 78. The phenomenon is also referred to as hydrogen embrittlement. Wherever hydrogen atoms recombine to molecules in the material structure (at second-phase particles or material defects such as dislocations) internal stress becomes established within the material. transition temperatures (precipitous decline of notched bar impact values) of below 0 °C are encountered. may increase the risk of brittle fracture. develops very thin but hard and brittle nitride layers. e) Purification. Depending on the defects in a protective oxide film on the metal surface and the ability of the material to sustain this film. is generally regarded as the source of the carbon in this case. [885] attributes the damage to physical hydrogen attack resulting from the use of agglomerated hygroscopic flux in combination with insufficient soaking. g) Methanol converter.5 mm and 3 – 6 mm [1293]. Of great importance in this field was the work of NELSON [871]. [883] that hot-wall converters in which the pressure shell is in contact with ammonia-containing synthesis gas at 400 °C have developed cracks in circumferential welding seams to a depth nearly approaching the wall thickness in places. At higher temperature and partial pressure. chromium. A related phenomenon is physical hydrogen attack. Temper Embrittlement. which is hydrogenated to methane.g. So there are still questions left regarding the two theories. The resulting pressure causes cavity growth along the grain boundaries. The fact that more than 45 % of world ammonia plants are older than 30 – 35 years suggests that there is a major potential for revamp projects. At least from a theoretical point of view. But in the meantime also these converters developed cracks. High residual welding stress and internal pressure are considered to be essential for the propagation of the cracks. [899].25 and 0. For heat-resistant steels long-term service at temperatures above 400 °C (e. Highly critical in this respect are dissimilar welds [1406]. in most cases without any significant prior deformation. b) Waste-heat boiler.25 Cr/1 Mo) were well below the Nelson curve. the creep rate of the material. which can diffuse through the material structure. This may finally reach a point where the affected vessel or pipe ruptures.5 Mo steels are now regarded as ordinary nonalloyed steels with respect to their hydrogen resistance [875]. The resistance of steel against this sort of attack can be enhanced by alloy components which react with the carbon to form stable carbides (e. Nitriding is a problem specific to the ammonia converter. 4. The Boudouard reaction. [889-891]. or silicon oxide films should exhibit an increased resistance. Physical Hydrogen Attack. and cobalt. alloys formulated to form chromium. Efforts to find solutions for this problem are continuing. where the formation of martensite. 625 and similar alloys it is at least possible to reduce the attack to a level which is tolerable in practical operation. experiments and careful investigations of failures have made it possible to largely prevent hydrogen attack in modern ammonia plants by proper selection of hydrogen-tolerant alloys with the appropriate content of metals that form stable alloys. the nitride layer grows with time to a thickness of several millimeters. and its grain structure. and to hold them at atmospheric pressure for some hours at 300 °C so that the hydrogen can largely diffuse out (soaking). Nitriding proceeding along microcracks could transform carbides normally stable against hydrogen into nitrides and carbonitrides to give free active carbon. nickel. One investigation [884] concluded that hydrogen attack had occurred by a special mechanism at temperatures lower than predicted by the Nelson diagram. For example.g. the affected material disintegrates into fine metal and metal oxide particles mixed with carbon. The susceptibility to temper embrittlement can be reduced by controlling the level of trace elements (Si. who summarized available experimental and operational experience in graphical form. In contrast. It is most likely to occur in welds that not received proper PWHT. A coat of reflecting paint or. Extensive studies [887]. A review is given in [923]. or by pipeline. has page 51 of 77 . 79). Figure 79. Atmospheric storage at – 33 °C in insulated cylindrical tanks for amounts to about 50 000 t per vessel 3. Storage Three methods exist for storing liquid ammonia [1155]. Reduced pressure storage at about 0 °C in insulated. then appropriate transport must be arranged [1154]. Ammonia is usually handled in liquid form. but the mechanism is so far not fully understood. However. handling. in which case the ratio tonnes ammonia per tonne of steel will become about 6.6 MPa to avoid wall thicknesses above 30 mm. and transportation of ammonia. Reference [402. however. [914]. [923]. centrifugal pumps for discharging into liquid ammonia supply piping and for liquid ammonia loading. rail or tank cars. Storage and Shipping Producing and processing ammonia requires storage facilities to smooth out fluctuations in production and usage or shipments. [Top of Page] 5. Preventive measures include maintaining a certain water content in the ammonia and excluding even traces of oxygen and carbon dioxide. the combination of atmospheric and pressure storage may be the most economic concept (Fig.1.2 % in transport vessels [915]. 5.5 MPa (25 bar): meters and flow controls for pressurized ammonia feed and effluent streams. for static and safety reasons. Its storage and distribution technologies therefore have much in common with other liquefied gases. h) Booster. 79). river barges. [901]. b) Tank at ambient pressure (refrigerated). More recent research [919]. storage and handling facilities for this product will also be needed. t ammonia per t bar °C steel 16 – 18 3–5 1. frequently in hot climates. tank cars. [904-914] have defined the conditions under which SSC in liquid ammonia is likely to occur and how it may largely be prevented. IV] summarizes the literature on storage. e) Air-cooled condenser. obtained as a byproduct of purge-gas scrubbing or. Sometimes high-strength or fine-grained steels are used in making pressure vessels. [1156]: 1. and the role of oxygen are not yet completely understood.1. The determining factors for the type of storage — apart from the required size — are temperature and the quantity of ammonia flowing into and out of storage [1159]. Stress corrosion cracking in pressurized ammonia vessels and tanks is a problem which has been discussed in many papers [901]. Liquid ammonia is a liquefied gas. usually spherical pressure vessels for quantities up to about 2500 t per sphere The first two methods are preferred. 0 – 33 2. It occurs at ambient temperature under pressure as well as in atmospheric storage tanks at –33 °C. vol. into a water wash system (Fig. usually horizontal for up to about 150 t Spherical vessels resting on tangentially arranged support columns but also more recently.1 – 1. and in atmospheric tanks it is important to select the appropriate welding electrodes.1.8 10 41 – 45 Capacity. in a suitably shaped shallow depression for about 250 – 1500 t. but compared to liquid ammonia the demand is negligible. but in some cases delivery of ammonia vapor to downstream consumers on site may have some advantage due to savings of refrigeration energy in the ammonia plant. [915] it has become a widely used practice to maintain a water content of 0. As a rule. and marine vessels carrying pressurized ammonia Entrance to or exit from pipeline systems Usually. for example. Safely avoiding this hazard requires careful thermal stress relief after completing all welding on the vessel. pressure control is provided for the storage drum by controlled bleeding of the inert gases through a pressure-reducing valve. i) Rail car or truck. If there is an opportunity for on-site usage or marketing of aqueous ammonia. Corrosion by hydrogen sulfide in partial oxidation plants can be controlled by the use of austenitic steels. For pressurized storage. thus inhibiting the initiation of metal dusting [899]. Protection may also be achieved by aluminum or zinc metal spray coating [921]. Pressure Storage This system is especially suitable for: – – – – Storing small quantities of ammonia Balancing production variations with downstream units processing pressurized ammonia Loading and unloading trucks. As it is generally accepted that addition of water may inhibit stress corrosion [402]. in many cases. k) Heater Characteristic features of the three types of storage are summarized in Table 30[404]. Characteristic features of ammonia storage tanks Type Typical pressure. cylindrical pressure vessels are designed for about 2. Hydrogen Sulfide Corrosion. in spite of extensive research. g) Barge for ammonia at ambient pressure. These may be susceptible to stress corrosion cracking by ammonia.5 MPa. Spraying the vessel with water is very effective against intense solar radiation but does stain and damage the paint. but special care to ensure proper stress relief of welds is advisable to avoid stress corrosion cracking in these plants caused by traces of chlorine sometimes present in the feed oil. This may be by ocean-going ships. t ammonia < 270* 450 – 2700 4500 – 45 000 Refrigeration compressor none single stage two-stage Pressure storage* Semi-refrigerated storage Low-temperature storage * Refers to cylindrical tanks (“bullets”). avoid excessive differences of the thickness of plates welded together and choose the correct geometry of welding seams.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience metal surface which catalyze the Boudourd reaction can be poisoned by H2S. 79 and [1156]).2 ambient ca. Design temperature. spherical vessels see Section Pressure Storage. deliberately produced. spheres with a capacity up to 1500 t have been constructed. c) Pumps. equipment for safe pressure relief for ammonia vapor and inerts (see Fig. Welds in pressurized vessels must be properly stress-relieved. [923]. less frequently.5. liquid ammonia fed to storage from a synthesis loop carries inert gases with it. f) Barge with pressure tank. Ammonia terminal with loading and unloading facilities a) Sphere at ambient temperature. If manufacture and use occur in separate locations. Besides the prescribed safety relief valves. an outer covering of insulation may be used on the vessels to avoid solar radiation heating. Stress Corrosion Cracking. Pressure storage at ambient temperature in spherical or cylindrical pressure vessels having capacities up to about 1500 t 2. Table 30. Stress corrosion cracking (SCC) of many steels in liquid ammonia is a peculiar phenomenon. d) Compressors. 5. The larger spherical vessels are designed only for about 1. Design of pressure storage tanks and the related safety aspects are discussed in [1158]. the influence of water. The mechanism of this phenomenon. and there is growing opinion that reduced-pressure storage is less attractive [402]. Ancillary equipment. The shape of the vessel depends above all on its capacity: Cylindrical. designed for at least 2. [904-922]. The prevailing opinion was that stress corrosion should not occur in atmospheric storage tanks [1162]. i) Loose Perlite or lightweight concrete and rock wool. [1161]. Underground Storage For years. heating is unnecessary. a jetty and equipment for ship loading and facilities for loading rail and/or tank cars. and the vapors from the flash vessel are also fed to the refrigeration unit. Refrigeration units have at least one stand-by unit. which contains an atmosphere of ammonia vapor. the tank bottom is on piles above ground level. Two principle “double integrity” designs are used as shown in Figure 80. g) Inner steel base. which could lift it. from steels which retain their notch ductility strength at low temperature. 5. correspondingly large storage facilities are necessary at ship and pipeline loading and unloading points. If it is necessary to avoid contaminating the aqueous ammonia by iron hydroxide. Further safety provisions include surrounding the tanks by dikes or placing them in concrete basins to contain the liquid ammonia in the event of a total failure. The inner tank has a suspended roof and the insulation fills the annular gap. Single-Wall Tanks. j) Outer steel shell and base.2. 5.. In addition. b) Steel roof. The insulation for the inner shell may fill the total annular space [e. n) Steel roof and shell or prestressed concrete lined for vapor containment.4. e. for example. 5.5 × 106. The single-wall tank has a single-shell designed for the full operating pressure. o) Loose Perlite or insulation on liner or in space The tanks are fabricated by welding on site. lightweight concrete [1165] or Perlite (B)] or may consist of an organic foam attached to the outside of the inner shell (A). The outside insulation must be completely vapor tight to avoid icing and requires the highest standards of construction and maintenance to avoid hazardous deterioration by meteorological influences.3. [1163]. The most widely used methods are: – With pressure vessels for anhydrous ammonia (maximum allowable operating pressure about 2. Arrangements of refrigerated ammonia tanks a) Waterproof roofing. at least 40 m3 storage volume for 30 m3 tank cars. Double-wall tanks are known in various designs. cellular glass or lightweight concrete [1165] is used for the floor insulation in all cold tank systems. This applies especially to storage of ammonia coming from the synthesis loop at low temperature and loading and unloading of refrigerated ships. metal casks. and repair procedures can be found in [922]. [1168]. Transportation in Trucks and Rail Cars with Capacities Normally up to 100 m3 . DuPont has operated a rock cavern in the United States with a capacity of 20 000 t. [928-935]. tanks are used. A foundation as shown in Figure 80 A and B requires underground heating to prevent formation of a continuous ice sheet under the tank. no further storage facilities of this kind have been built for a long time. m) Pile foundation. it was somewhat surprising when cases of stress corrosion in atmospheric ammonia tanks were reported [925-927]. to about 80 kWh/t ammonia throughput. For comparable large storage volumes. Energy consumption would also increase considerably. Influence of climatic conditions on design and operation is considered in [1244]. for example. in-situ applied polyurethane foam) are used for insulation of single wall tanks. The annular space between the two walls is filled with insulation material.. f) Stay bolts. l) Suspended deck with insulation. Perlite. The most common containers are: – Cylindrical steel bottles and pressurized flasks for about 20 – 200 kg anhydrous ammonia to meet the requirements of laboratories. mostly powered by a diesel engine. A metal sheet covering is normally applied. A rough budget figure of the capital investment in 2005 for a double-integrity tank with a capacity of 45 000 t including cooling compressors is around $ 10 × 106.. Because of the static loading. Transportation Transportation in Small Containers. A modern rail car loading station for anhydrous and aqueous ammonia is described in [1167]. transporting large amounts of ammonia by ship or pipeline and stockpiling to balance fluctuations in the ammonia market have great importance. is only covered by the water-tight insulation. the capital investment. The cylindrical tanks have a flat bottom and a domed roof and are completely insulated. Design pressure is usually 1. Descriptions of further incidents. For stress corrosion occurred in atmospheric tanks see Section Pressure Storage. In Figure 80 A and B the inner tank has a solid steel roof and is pressure-tight. Discussions of such secondary containment are found in [924].5 MPa) – With atmospheric pressure vessels for 25 % aqueous ammonia – With vessels designed for elevated pressure for high-concentration aqueous ammonia (maximum allowable operating pressure in accordance with the ammonia content up to 1. The capacity of the transporting equipment determines the storage volume. In the design according to Figure 80 C the outer tank is pressure-tight. Therefore.2. A small facility uses mostly cylindrical vessels. In the simplest version an inner tank designed for storage temperature and pressure is surrounded by a second tank. Jumbo Rail Cars Holding up to 150 m3. and the like – Polyethylene canisters. For larger capacities. Storage of Aqueous Ammonia To allow storage and transportation at temperatures up to 35 °C.1 – 1. This so-called double-integrity tank concept provides an additional safety measure as the outer tank can hold the full content if the inner shell fails. [1242]. Corresponding to the decreasing static pressure. it is still more economical than pressure storage. inspection techniques. as in Figure 80 C. Specific cases and foundation problems are discussed in [1239-1242]. Technical details and design questions are treated in [402 Vol IV]. This technique has been applied to ammonia also. because of its vapor pressure. the concentration of aqueous ammonia should not exceed 25 %. Low-Temperature Storage Modern single-train plants need to have large-volume storage facilities available to compensate for ammonia production or consumption outages.g. Storage equivalent to about 20 days is customary for this. k) Cellular glass or lightweight concrete.g.1. Incoming ammonia of ambient temperature is flashed to –33 °C before entering the tank.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience shown that water may not give complete protection. and sometimes a bond wall of reinforced concrete or steel is added [1163].1. [1160]. small refrigeration systems. Therefore. Refrigeration is provided by recompression of the boil-off. [1164]. For atmospheric storage at – 33 °C single tanks with a capacity up to 50 000 t of ammonia are available [1160]. the capital investment costs for atmospheric pressure storage are substantially lower than for pressure storage. Mats or panels of rock wool or foamed organics (e. substantial investment will be necessary for a complete terminal including. austenitic (stainless) steels may be used instead of the usual carbon steels. Inclusion of facilities to receive or deliver pressurized ammonia at ambient temperature would add a few millions more. In spite of higher energy costs for maintaining the pressure and for feed into and out of atmospheric storage. usually with two-stage reciprocating compressors. [1237]. If. [1237].1. In 1995. page 52 of 77 . Moreover. [1164]. the liquefied petroleum gas (LPG) industry has used pressurized underground liquefied gas storage. h) Concrete ring wall. [1243]. c) Inner steel shell.6 MPa). Figure 80. for a 1000-t pressurized ammonia storage facility amounted to about $ 3. Maintenance and inspection procedures are covered in [1247-1253]. The main purpose of the outer shell is to contain and hold the insulation.5 bar (plus the static pressure of the ammonia). Retrofitting of existing storage tanks is described in [1245]. Norsk Hydro has one in Norway at 50 000 t. including ancillary equipment. 5. steel plate thickness in the cylindrical shell is reduced stepwise from bottom to top. d) Foamed organic materials or rock wool. and the like for 25 % aqueous ammonia. Because of the contaminants occurring in liquid ammonia stored this way and the lack of suitable construction sites. e) Heating coils. which can withstand the full hydrostatic pressure at operating temperature. whereas the gap between inner and outer shell. Today's usual practice is to design the outer shell to the same standard as the inner shell.g. Single-wall and double-wall tanks are used in the industry. the containers should be made of seawater-resistant aluminum (magnesium alloyed) or austenitic steels. the concentration is 25 – 30 % NH3 and the iron content less than 10 ppm. max not spec'd not spec'd Oil ppm by wt 5. The world's longest ammonia pipeline has been in operation since 1983 in Russia.98 99. so that the volume which can be released between two valves is limited to 400 t. min 99. Analysis.0 Free of H2S. can detect ammonia in a concentration of 1 ppm. and Missouri. Oil may be introduced by the seal oil of the synthesis compressor.1 – 0. For the quantitative determination of ammonia in air. For example. The relevant standards are US Specification OA-445a. with low emissions. calcium (ammonia catalyst) and other impurities can then be determined in the ether-insoluble residue. In 2004. water and soil pollution. but a few are pressure vessels.98 * 0. To inhibit stress corrosion cracking a water content of at least 0. in the agricultural practice of direct fertilization. low energy consumption due to high-efficiency process design.9 °C ** The noncondensable gases dissolved in ammonia are H . but on account of its low solubility in liquid ammonia it usually settles out on storage. see [1198] ). shipping liquid ammonia by truck is used only where other means of transport are not available. there is no significant pipeline transport in Europe. [Top of Page] 7. Overseas shipping gained great momentum through exports from producers in countries with low natural gas prices or low-price policies [1181]. The inert gas content is analyzed volumetrically after the vaporized ammonia has been absorbed in water. such as Nessler's reagent (HgI in KOH). individual (manual) and continuous (recorded) analyses can be made (for a measurement station for automatic determination of ammonium/ammonia. and Ar. Exact knowledge of the p – V – T properties is important [1172]. Normally. whereas plants which receive the feedgas after a methanation without further drying may give a water content of 0. 0. Shipping in pressure vessels is necessary for ammonia contents above 25 % because of its elevated vapor pressure. Various concentrations and purities of aqueous ammonia are on the market. Their amounts depend on the methods of synthesis and storage. Quality Specifications and Analysis The quality of the ammonia product depends to some extent on the operating conditions of the production plant and storage. Environmental Aspects of Ammonia Production and Handling Measured by its overall environmental impact — air. As ammonia is transported at a temperature of at least 2 °C it has to be warmed up at the supplier terminal and cooled down again to – 33 °C at the receiver terminal. for example. Shipping in Ocean-Going Vessels and River Barges. shipping of anhydrous ammonia is far more important than transport by rail. [Top of Page] 6. Pipelines. so that only a minor concentration will remain in commercial deliveries. Ocean-going vessels may transport as much as 50 000 t of fully refrigerated ammonia.015 0. Illinois. the hold up is about 20 000 t. Mostly. The Gulf Central Pipeline connects the major producers along the Texas and Louisiana Gulf coast with terminals in Iowa. The oil content of liquid ammonia can be tested gravimetrically by first evaporating the ammonia liquid and then concentrating the ether extract of the residue. Nebraska. N . rail transport of liquid ammonia may to some extent supplement large marine and pipeline shipments. [1189]The MidAmerica Pipeline. among others: Acidimetry and volumetric analysis by absorption Gas chromatography Infrared absorption Thermal conductivity measurement Electrical conductivity measurement Measurement of heat of neutralization Density measurement (for aqueous ammonia) Normally. Minimum quality requirements for ammonia Quality Commercial grade USA Purity Water wt %. synthesis gas. Environmental. which regularly reports shipping prices. page 53 of 77 . which are intensive agricultural areas. max 0. For more stringent purity requirements for aqueous ammonia. the water content of liquid ammonia is determined volumetrically as the ammonia-containing residue on evaporation or gravimetrically by fully vaporizing the ammonia sample and absorbing the water on KOH. The inerts 2 2 4 amount to about 50 mL/kg for atmospheric storage. where ammonia is predominantly further processed in downstream facilities on site and where no widespread direct application to agricultural crops exists.5 0. DIN 8960 (1972) for refrigeration-grade ammonia. However. and Iowa.2 Inerts ** mL/g.. today Nitrogen and Syngas).1.0 not spec'd * Allowable boiling point change on vaporization of 5 – 97 % of the test sample. aluminum. a total of 13 × 106 t of anhydrous ammonia was transported by ocean-going vessels (IFA statistics).2 0. and aqueous solutions. published by British Sulphur. [1157] over great distances is far more economical than by river barge or rail. most of them shorter than 50 km. In the USA the MidAmerica Pipeline System transports ammonia to terminals in Kansas. materials and energy consumption — ammonia production is a rather clean technology. in which improving one environmental effect worsens another.08 3. Iron.1 0. has a peak delivery capacity of 8000 t/d for a number of destinations [1170]. for example. and no severe cross-media dilemmas.0 5. pyridine.g. Then the inert gas composition is analyzed chromatographically. Table 31.2 % for shipped and pipelined liquid ammonia is generally recommended and is mandatory in the USA. CH . e. Nebraska. Most river barges have loading capacities of 400 to 2500 t and mostly have refrigerated load. and naphthene 99.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience The distribution in rail cars and trucks primarily serves to supply smaller processing operations and wholesale merchants.5 Refrigeration grade Germany Germany USA 99. for more up-todate information consult the journal Nitrogen (later Nitrogen & Methanol. Table 31 lists the commercial specifications. Supplement 5 (1963) and. Transport of large volumes of ammonia by pipeline [1173]. Typical emissions are given in EFMA's publication Production of Ammonia in the series “Best Available Techniques and Control in the European Fertilizer Industry” [1174]. Reference [1169] examines rail freight cost. Safety. Ammonia is readily detectable in air in the range of a few parts per million by its characteristic odor and alkaline reaction. in Germany. Apart from some short pipeline connections. A comparison of shipping costs is given in [1173].2 %. Maintenance is described in [1171]. Regarding the transport volume. Specific indicators. connecting the large production facilities Togliatti/Gordlovka with the terminals Grigorowski/Odessa at the Black Sea over a distance of 2424 km [1166]. water content from a synthesis loop receiving a dry make-up gas is about zero. The methods used include. Automatic lock valves are installed at intervals of 10 miles. and Health Aspects 7.5 wt %. There are two commercial qualities of anhydrous ammonia: commercial (technical) grade (ammonia as received from production and storage) and refrigeration grade (technical product purified by distillation [1197] or molecular sieve adsorption). In the Netherlands and Germany limits for emissions from boilers also apply for ammonia plants. This directive requests an exchange between national environmental authorities.2 300 0. as well as an investigation of the causes are found in [1400]. The total NOx emission per tonne of product may be somewhat lower than for steam reforming plants. or synthesis section (75 % hydrogen). the EU commission has proposed a Council Directive Concerning Integrated Pollution Prevention Control (IPPC Directive). Apart from proper design. 7. Legally binding emission values for certain pollutants associated with ammonia production 2. For ammonia in air the authorities in Germany require that the achievable minimum level should correspond to the state of the art. The European Fertilizer Manufacture's Associatiom (EFMA) has published a series: “Best Available Techniques for Pollution Prevention and Control in the European Fertilizer Industry” Booklet No. NOx emission level values are categorized in the Clean Air Act.1 < 0. and periodic technical inspections of the equipment used. Further information is given in [1350]. page 54 of 77 . but aqueous ammonia is classified in the Water Endangering Group 1 as a weakly endangering substance. Statistics of plant on-stream factors.25 mg NH3 per liter. The long industrial experience is summarized in a number of national codes and standards which have to be applied for design. Thus the industry as a whole has the opportunity to learn from the specific experiences of its individual members. Safety Features In ammonia production three potential hazard events can be identified: fire/explosion hazard from the hydrocarbon feed system. size and partially severe operating conditions of vessels and piping remarkably few failures occur.3 mL/m3). Noise. and the stripped condensate can be recycled to the boiler feed water after polishing with ion exchangers. is in fact a marginal quantity. following a very meticulous procedure apparatus after apparatus is checked for potential failure and risk possibilities together with a proposal for the appropriate remedies. but in considerably smaller quantities. This is especially important for equipment operating at high temperatures and/or pressures. In Germany there is no maximum ammonia concentration laid down for wastewater.2 kg/t NH3 The source of the NOx emissions is the flue gas of the fired primary reformer. process engineers (also from contractors). A previous BAT document on ammonia was published in 1990 [1175]. and in plants without a fired tubular reformer NOx is emitted from fired heaters and auxiliary boilers.9 0. Table 32 summarizes the emission values for steam reforming ammonia plants. Many investigations have dealt with noise generation and noise abatement in ammonia plants [1217-1221]. operation. too. individual operation permits are negotiated.45 0. steam blowing. In the other European countries no national emission limits or guidelines exist. The time frame of unscheduled shut downs because of problems which need urgent repairs can be used for routine maintenance operations. could serve as a guideline. fabrication. shut-down frequencies and time. Table 32. and accidents are reported and discussed including the manner how the problems were handled and dealt with. Availability.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience Steam Reforming Ammonia Plants. The long history of ammonia production since 1913 has demonstrated this production technology is a very safe operation. Only about 0. A similar approach. A short desciptions of a number of accidents is found in [1371]. 1 is “Production of Ammonia” [1174]. The present limit in Germany is 200 mg NOx/m3 for furnaces up to 300 MWth. 2 ** Spent catalysts. because in this case the pollutants would be transferred from water to air. In addition there is also a potential toxic hazard in handling and storing of liquid ammonia. In this context the question of adequate maintenance is very important and to which extent preventive maintenance is provided. To harmonize European regulations concerning emission value permits for industrial production. sound-reducing enclosures for compressors or housing them in closed buildings. also smaller revamps. Guideline values not legally binding but providing the background for requirements laid down in individual operating permits In Europe such legally binding emission levels relevant to ammonia production exist only in Germany. However. blowers. which was usually defined by economic criteria [1335]. for example. which defines a significance level for the source according to the total emission (10 to 100 t/a) on the one hand. 1974). usually orientated on other cases and countries. For the limitation of sound levels at working places within the plant occupational safety regulations are valid which also require ear protection above a certain level. resonance vibrations in the flue gas ducts. Partial oxidation ammonia plants have the same emission sources except for the primary reformer flue gas. Measures for noise reduction include installing low-noise let-down valves. [1407]. It is largely thanks to such forums as the annual AIChE Safety in Ammonia Plants and Related Facilities Symposium that failures. originally projected values in a draft of TA Luft [1215]. A special point is the assessment of remaining service life of equipment because many ammonia plants operate beyond there originally anticipated lifetime. fire/explosion hazard due to leaks in the synthesis gas purification. HAZOP studies introduced by ICI [1359-1362] for general application in chemical plants proved to be very helpful not only for existing plants but also in the planning phase of new ones. There are two categories of regulations: 1. Specific emission guideline values are laid down in the United Kingdom. if they fit in the time schedule. Usually single-train ammonia plants are shut down every two to five years for some weeks for changing of catalyst. and assigns a threshold limit to the geographical area (100 t/a to 10 t/a) on the other. The NOx content of the flue gas depends on the configuration of the auxiliary boiler and on the extent electric power generation on the site as opposed to outside supply. for example for the reformer furnace. A concentration of 1. For noise emissions usually local permits are negotiated. which could provide a common accepted basis for emission limit value regulations of the individual EU member states. burner noise. Other possible emissions are H2S (< 0. compared to the total amount from human activities. were not subsequently adopted. [1370]. The severe impacts of rare events with explosions seem to be confined to a radius of around 60 m [1174]. called Reliability. It is especially important to define and configure trip systems and trip strategies for safe plant shut downs in case of offset conditions. A key element is the concept of Best Available Techniques (BAT). Emissions from steam reforming ammonia plants Existing plants New plants Emission to air NOx (as NO2)* ppmv (mL/m3) 150 mg/Nm3 kg/t NH3 Emission to water NH3/NH4 (as N) kg/t NH3 Waste material** * At 3 % O . NOx emissions from ammonia production. equipment inspections. A BREF for ammonia is being prepared to serve as pilot project. use of silencers. Another possibility is use of the process condensate for feed gas saturation [458]. The following major sources can be identified: depressurizing of large gas quantities for control or venting. can largely be avoided. 75 150 0. [1397]. maintenance and. Maintainability (RAM) was developed by KBR [1399]. An increasing awareness has developed for noise generation and emission from ammonia plants especially in the neighborhood of residential areas [1216]. Emission Limits and Guideline Values for Ammonia Production. some MIK values (maximum imission concentrations) are given in VDI Richtlinie 2310 (Sept. skilled and well-trained operators and effective and timely maintenance are essential for safe plant operation. industry and nongovernmental organizations with the aim of preparing IPPC BAT Reference Documents (BREFs). and noise from compressors. CO (30 mL/m3) and traces of dust. and toxic hazard from release of liquid ammonia from the synthesis loop. The inspections are widely carried out with nondestructive testing [1402]. Despite the complexity. incidents. it is only referred to the general interdiction to introduce poisons or harmful substances into rivers and lakes. which on account of the sulfur content of the fuel oil release a flue gas containing SO2 (< 1500 mg/m3).2.1 < 0. General practice today is to make so-called HAZOP (hazards and operability) studies with an experienced team consisting of operating personal. generally originating from the condensate from the condensation of surplus process steam ahead of the CO2 removal system. material selection. Emissions to water. Steam stripping without recycling to the reformer would produce a cross media effect. For most up-to-date immission regulations contact the relevant authorities in the different countries.16 % of the anthropogenic NOx emissions come from ammonia production. compression. and pumps. stated to be harmful to fish. Minor concentrations of methanol and amines can be removed and recycled to the reformer feed by stripping with process steam. In the United States. The plants have an auxiliary boiler to generate steam for power production and fired heaters. experts in process control and safety experts (often independent consultats from outside the company) in which. whereby in most instances the contribution to the sound level in the surrounding residential areas will be limited. In the “Catalog of Water Endangering Substances” (Germany) anhydrous ammonia is not listed. In contrast to some other liquefied gases (e. Training of operating personal for ammonia handling with special regard to ammonia spills is important [1208]. In the environmental sector ammonia is used in various processes for removing SO2 from flue gases of fossil-fuel power plants. increased salivation. [1214]. The production of smaller volumes of hydrogen/nitrogen mixtures used as protective gases for chemical products [5] and for metal-working processes [6] by page 55 of 77 . ammonia is toxic and even a short exposure to concentrations of 2500 ppm may be fatal. such as polyamides. laryngospasm. On mucous membranes alkaline ammonium hydroxide is formed. LPG. Mutagenicity. The resulting ammonium sulfate is sold as fertilizer. Toxicology. pharynx. and polyacrylonitrile. polyurethanes. loading/unloading of ships and barges. is the production of plastics and fibers. [1206] of 15 – 27 % are rather narrow. Ammonia is absorbed rapidly by the wet membranes of body surfaces as ammonium hydroxide. the resulting cloud can contain a mist of liquid ammonia. Health Aspects and Toxicity of Ammonia In ammonia production. amides. Urea production consumed about 40 % of the ammonia produced in 1995. It is beyond the scope of this article to list them completely and to check for the most up-to-date status. cataract. [1278] and in an excellent review on safety in ammonia storage [1188]. The inhaled ammonia is partly neutralized by carbon dioxide present in the alveoli [1286]. applying to fabrication and operation of equipment. damage of the iris. it tolerates moisture. ammonium nitrate. and ammonium bicarbonate. transfer. Repeated inhalation can cause a higher tolerance because the mucous membranes become increasingly resistant [1300]. such as edema of the glottis. The capacity of detoxification via urea is sufficient to eliminate the ammonium ion when ammonia is inhaled in nonirritating concentrations. can form mixtures which are denser than air. However. leading to keratitis. at 250 – 500 ppm tachypnoe and tachycardia [1306]. Spill experiments on land and on water in various dimensions have been carried out by various companies and organizations. sulfate) or directly applied to arable soil. day or night) into account. and nephritis. For maintaining safety in ammonia storage. tracheobronchitis. and the density of the cloud may be greater than that of air [1176]. 2500 to 4500 ppm may be fatal after short exposure. rail cars. The threshold of perception of ammonia varies from person to person and may also be influenced by atmospheric conditions. [1200]. [1295-1299]. Ammonia may also serve as a solvent in certain processes. Because of its corrosive action constrictions of the esophagus may result. Major inorganic products are nitric acid. and pipeline transport in which major spills occurred is described in [1222-1229]. Another report [1192] gives concentration limits for short time exposures as follows: 100 ppm (10 min). storage. On accidental release. sodium cyanide. urea – formaldehyde – phenol resins. partly after conversion to nitric acid. and glaucoma [1284]. An old and still flourishing business is the use of ammonia as refrigerant. At 1700 ppm coughing with labored breathing. 70 % of that of ambient air.. ammonium chloride. and oil contaminants. In the selective catalytic reduction process (SCR) the NOx in flue gases is reduced in a catalytic reaction of the nitrogen oxides with a stoichiometric amount of ammonia [1233-1236]. amines. it is cheap and there are many suppliers. [1180]. Underwater releases were also studied. Models have been developed to mathematically describe ammonia dispersion in such events [1182-1187]. 100 ppm irritates the nose and throat and causes a burning sensation in the eyes and tachypnoe [1305]. as the flammability limits [1177-1179]. laryngitis. This reference also provides quantitative information on temperature development and evaporation rate of small and large ammonia pools at certain wind velocities and also takes radiation influences (sunny or overcast sky. [1246]. Lifetime ingestion of ammonium hydroxide in drinking water by mice was without any carcinogenic effects [1302]. and interstitial lung edema [1283]. [1309-1315]. An important use of the ammonia nitrogen. 5000 ppm and higher causes death by respiratory arrest [1188]. A discussion of metabolism and of acute and chronic health problems caused by ammonia can be found in [1196]. which dissolves cellular proteins and causes severe necrosis (corrosive effect). Ammonia is a strong local irritant.3 % at an inhalation concentration of 230 ppm) [1287]. converted to urea. Regulations and Guidelines. Figures for the proportion flashed as a function of pressure and temperature of the spilled ammonia are given in [1180]. This instantaneous flash is followed by a period of slow evaporation of the remaining liquid ammonia [1176]. ammonia and air. [1246]. This behavior has to be taken into account in dispersion models. melamine-based resins. immediate onset of burning sensations in the eyes [1307]. 1000 ppm causes immediate coughing [1308]. nausea.5 – 3 ppm) are reported in [1190]. in addition to the above symptoms [1306]. In addition to the high specific refrigeration effect. Carcinogenicity. This recommendation was supplemented by a value for short-time exposure: 35 ppm for 15 min. Actually every nitrogen atom in industrially produced chemicals compounds comes directly or indirectly from ammonia. Ammonia is either converted into solid fertilizers (urea.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience An area which deserves special attention with respect to safety is the storage of liquid ammonia. at 700 ppm. Surveys [1191] found concentrations from 9 – 45 ppm in various plant areas. The primary target organ is the pulmonary system. values as low as 0. Ammonia vapor at the boiling point of – 33 °C has vapor density of ca. Human Exposure. and esophagus and cause the shock syndrome. nitriles and other organic nitrogen compounds. The metabolism seems not to be significantly changed after exposure to 800 ppm [1308]. and handling the main potential health hazard is the toxicity of the product itself. but with increasing environmental concern regarding the application of CFCs ammonia's position is strengthening again. bronchospasm. sometimes with momentary inability to breath (coughing of rescued persons may continue for hours). hydrazine. Also noncatalytic reduction is applied with ammonia or urea solutions. [1177]. The symptomatology of various exposure levels is also described in [1193-1195]. sodium nitrate. Though initially irritated. A drawback is its toxicity. An overview of such regulations and standards is given in [1335]. toxic hepatitis. It is recommanded to consult the relevant governmental authorities of the individual countries and the relevant industrial codes. exposed persons may quickly become accustomed to these concentrations.g. LNG). reflectoric bradycardia. and the following symptoms can be observed: pharyngitis. The explosion hazard from air/ammonia mixtures is rather low. vomiting. 200 ppm will cause headache and nausea. Only a small fraction of the ammonia is exhaled unchanged by the lungs (12. Ammonia is not mutagenic in the Ames Salmonella system and in Saccharomyces cerevisiae [1303]. dirt. The industrial use of ammonia is around 17 % [1238]. 75 ppm (30 min) 50 ppm (60 min). In an ammonia spill a portion of the liquid ammonia released flashes as ammonia vapor. and handling many regulations and guidelines exist in the various countries. and excreted by the kidneys [1285]. the amount of the initial flash is only a few percent of the total. but 50 ppm may easily detected by everybody [1304]. Another application is the nitriding of steel. the ammonia has the following advantages: it is noncorrosive. Exposure to higher ammonia concentration has the following effects: 50 – 72 ppm does not disturb respiration significantly [1305]. Ammonia or ammonium hydroxide can penetrate the cornea rapidly. The MAK value is 50 ppm. The ignition temperature is 651 °C. [Top of Page] 8. which serve as intermediates for dyes and pharmaceuticals. based on its low boiling point and its high heat of evaporation. 7. For this reason this section concentrates on ammonia only. tanks. For some time heavy competition came from chlorofluorocarbons (CFCs). Additional information on the toxicology of ammonia can be found in [1288-1292]. because the mixture is at lower temperature due to evaporation of ammonia. and life-threatening symptoms. whereas in a release from a pressure vessel at about 9 bar and 24 °C approximately 20 % of the spilled ammonia would flash. Other toxic substances such as carbon monoxide or traces of nickel carbonyl (which may be formed during shut down in the methanation stage) may be only a risk in maintenance operations and need appropriate protection provisions as well as blanketing or flushing with nitrogen.3.4 – 2 mg/m3 (0. Another application is the manufacture of explosives. under certain conditions. Oral ingestion of aqueous ammonia can corrode the mucous membranes of the oral cavity. The time-weighted average Threshhold Limit Value (TLV) of the ACGIH is 25 ppm (or 18 mg/m3) [1193]. A number of real incidents with tank cars. Ammonia is applied in a large number of industrial and commercial refrigeration units and air-conditioning installations [1-4]. Uses In 2003 about 83 % [1238] of ammonia production was consumed for fertilizers. In a release from a refrigerated tank operating slightly above atmospheric pressure. Ammonia Spills. Ammonia failed to produce an increase in the incidence of tumors in Sprague Dawley rats even when the protein ratio in the diet was increased or when urea was added [1301]. phosphate. [1363-1365] Year 1985 1990 1991 1992 1993 1994 1995 1998 2000 2001 2003 2004 2005 2006 Capacity 143.4 % of the world consumption of fossil energy (not including combustion of wood) goes into the production of ammonia. Because of the surplus of capacity over demand from the mid-1980s to the mid-1990s the tendency to invest in new plants was rather small and revamping measures aiming at energy saving dominated. In developing countries. Economic growth with the tendency of higher meat consumption and improvement of nutrition in developing countries are driving forces for increased fertilizer production. An indication of the natural gas prices is given in Table 34. Table 33. the nitrogen required to produce the animal feed is in the range of 20 – 30 % of the total nitrogen demand for fertilizers.4 142.8 151. lost their leading position (54 % in 1969) to Asia.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience decomposition of ammonia over iron. Development of ammonia production and world population The political changes in the former Eastern Bloc caused a dramatic slump in 1991 – 1993.8 126. which now together have a 26 % capacity share.5 Capacity utilization.9 148. since the industry is capable to reach plant availabilities of 91 – 93 %.3 142. are dominant in ammonia export. China is the largest producer with 36. Venezuela). The future growth of the nitrogen demand supplied by synthetic ammonia is expected to be larger than the growth of the global population. 1207].00 Canada 4.8 71.6 151. Geographical distribution of world ammonia capacity [1364] Acccording to the figures for 2002 [1366]. World ammonia capacity and demand The geographical distribution of world ammonia capacity is shown in Figure 83.1 151. [1199].8 Western Europe 4. Much of this idle capacity is in the United States (because of the extreme increase of the natural gas price) and in Eastern Europe.7 113. whereas.7 77. ammonia is generally one of the first products of industrialization.9 85. on account of their enormous natural gas reserves. Nearly three-quarters of world natural gas reserves (2003) are concentrated in two areas: in the former Soviet Union (32 %) and the Arabian Gulf (41 % including Iran).0 85. Trinidad. which owns 15 % of the world gas reserves.3 175. [1368] Location $/106 Btu* USA > 6.34 81. This was roughly the case in the mid-1980s as can be seen from Figure 81 [406].9 *1 Btu (British thermal unit) = 1055 J.6 126. the FSU countries with 15.9 × 106 t/a. whose producers export predominantly to Asia.7 From 2002/2003 onwards new ammonia projects materialized in increasing capacity numbers (see Fig.7 147. The required plant availabilities to satisfiy the expected demand are between 80 and 85 %. [Top of Page] 9.7 × 106 t) and the United States (2002: 5. which now accounts for 46 % (17 % in 1969).0 81. The nonfertilizer use of nitrogen is governed by the general economic development and by environmental legislation [1365]. Trinidad (3. around 40 000 t/d of capacity is idle now. Natural gas prices at different locations around 2004 [1365]. Table 34. page 56 of 77 .7 148. [1367].6 × 106 t/a NH3. A retarding effect resulted also from the financial crises in Asia in the early 1990s associated with a retreat from goverment subsidizing.0 132. R. Energy-related applications of ammonia are proposed in [1235].4 × 106 t/a. This has been mainly for economic reasons in developing countries and for ecological reasons in industrialized countries.5 168.4 Trinidad 1.6 Venezuela 0.6 × 106 t). as may be seen from Figure 83. However.9 8. Economic Aspects About 1.7 114.2 153.1 74. % 77.7 142. As 83 % of world nitrogen consumption is for fertilizers it might be expected that ammonia production should develop approximately in proportion to the growth of world population.2 165. Table 33 shows the development from 1985 to 2006.0 Demand 111.2 146.6 119. Figure 83. Figure 82. since then the rate of increase in production has been markedly slower than that of world population. followed by North America (USA and Canada) with 16.9 × 106 t/a and Western Europe 11. Europe and North America. the FSU countries (4.1 109.2 113.7 129. Meat consumption is presently increasing with around 3 % per year [1238]. The shift is to plants in areas with low gas cost (Middle East.6 × 106 t/a. Figure 81.1 80.0 84.25 Russia 0. the P.5 × 106 t) and the countries in the Middle East. World ammonia supply/demand balance (106 t/a ammonia) [1201-1203.5 × 106 t). India 11.4 115.or nickel-based catalysts at 800 – 900 °C may be an economic alternative where production or purchase of pure hydrogen is too expensive (see also Hydrogen – Hydrogen from Ammonia).75 Argentina 1. Because of the steadily increasing natural gas prices in the United States and Western Europe ammonia imports from low-gas-price locations are constantly growing and a number of ammonia plants have been temporarily or permanently closed.07 86.9 141.5 83. Iran.3 80. The major importers of ammonia are Western Europe (2002: 3. is so far not a major ammonia producer and not an ammonia exporter. 82).1 163. Consumption figures for older plants can be considerably higher than those for modern ones as used in Table 35.6 – 2. Table 35. DAP = Diammonium phosphate. World reserves and consumption rate of fossil feestocks in 2004 [1368] Coal. MAP = Magnesium – ammonium phosphate Even with inclusion of the shipping costs from the low-gas-price areas Trinidad ($ 25/t) and from Russia/Ukraine ($ 65-70 /t) to the US Gulf Coast these imports can easily compete with the domestic production as to be seen from Table 37. 106 Btub per tonne NH3 27 38. the total investment for a 2000 t/d capacity could easily reach $ 450 for a natural gas steam reforming plant.5 162. So the figures for the capital related costs in the table assuming debt/equity ratio 60:40. With natural gas prices above $ 9/106 Btu in the USA coal-based production would be competitive at full production costs [1411]. $/106 Btub (LHV) Consumption. by Ferticon. $ per tonne NH3 Freight. which compares ammonia production costs in North-West Europe for different feedstocks using today's best technological standards for each process.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience For for comparative production cost evaluations these figures should be taken with some care. In the example of Table 35. This can be seen from Table 35. The total investment for a new ammonia production can vary considerably depending on the location and its specific conditions. China where as a consequence of its large coal reserves and limited gas and oil reserves more than 60 % of the ammonia production uses coal. 1412] as can be seen from world reserves of fossil energy and their present consumption rate (Table 36). For a specific plant the on-stream factor might be lower for market reasons or because of outages due to technical problems. But the relative high investment cost for the coal-based plants create more difficulties for the project financing.0 148. Ammonia production costs from various feedstocks (2004)a Feedstock (Process) Natural gas(steam reforming) 4. Capital related costs depend highly on the financing condition and on ROI (return on investment) objectives of the investor.. Natural gas is by far the most economical feedstock for ammonia production.0 16 – 31 30 25 [1370] to USA 70 [1370] to USA 25 – 70 to Asia 130 71 – 131 309 – 340 105 – 155 page 57 of 77 . $ per tonne NH3 124. $ per tonne NH3 Cash costs. Only for this proportion prices are regularly published in Nitrogen + Syngas.2 96.3 243.6* Vacuum residue (partial oxidation) 2. $ per tonne NH3 Total cash costs. $ 1.5 75.0 50 30 Russia 0.5* 45 112. FINDS.3 250 Feed + energy (fuel). Investment figures for the vacuum residue and coal-based plants are derived from the steam reforming figure by the well-known investment relation factors. Ammonia cash costs 2005* USA 9 – 10 Nat. Nm3 456 × 109 162 × 109 180 × 1012 2778 × 106 3868 × 106 2692 × 109 Years of expected supply 164 42 67 Only about 14 % of the ammonia is traded as such. 8 % interest on debts. For the future coal has prospects [1411. 84). The feedstock question is discussed in [1204]. Figure 84.5 370 Coal(partial oxidation) 2. TFI statistics and other publications.3 625 Feedstock price. 20 % uses natural gas and the rest is oil-based [1370].1 227. are only an example to illustrate this substantial contribution to the total costs. Applications of ammonia UAN = Urea – ammonium nitrate. and also in Europe the $ 5 line was crossed. Green Market. IFA. t Reserves Consumption per year Oil. In 2005 gas prices dramatically increased in the USA. fire fighting. 16 % ROI on equity. gas price $/106 Btu Feedstock costs. $ per tonne NH3 Investmentc. Nevertheles there are already feasibility studies made in the USA for coal-based production using Illinois coal ($ 30/t. EFMA. $ per tonne NH3 30 Trinidad 1.9 30 30 Arabian Gulf 0. cThe investment for steam reforming is a budget figure based on recent estimates for a 2000 t/d plant inside battery limits and excludes any offsites. where they reached in some areas values above $ 9/106 Btu. The price of the vaccuum residue is the heavy fuel oil price minus a deduction of $ 50/t [1369]. an industralized site is assumed where all the necessary off-site installations are already existing. administration.7* 36 97. $ per tonne NH3 310 Other costs. For coal as an alternative to natural gas see also [1374]. a 45 000 t ammonia tank with refrigeration is included. b1 Btu (British thermal unit) = 1055 J. achieving the lowest energy consumption and requiring the lowest investment. t Natural gas. $ per tonne NH3 Capital related costsc. and a few housing buildings and roads have to be included.2 51. so that the world-wide average figure for steam reforming plants is probably between 31 and 33 106 Btu per tonne NH3.0 162.5 – 1. For the annual production 330 days of operation per year were assumed. because they are statistically compiled and might not always be suited for an individual case or a concrete situation. For a remote site with heavy forest vegetation where all neccessary offsite facilities like workshops. which will increase the fixed costs per tonne. A special case is the P. It is assumed that the utilities are supplied at battery limit in required specification.2 65. Table 37.30/106 Btu) [1372]. Apart from some direct uses (3%) all ammonia is converteted into downstream products (Fig.2 Other cash costs. depreciation 6%. R. In the rest of the world 90 % of the ammonia production uses natural gas. 106 $ aThe 2004 Feedstock prices are taken from the BP Statistical Review of World Energy 2005 [1205]. $ per tonne NH3 Total production costs. Table 36.0 187.8 350. ambulance. whereby the host plant indirectly supplies the large amount of energy needed. Technical feasability studies for 5000 t/d plants are made by various contractors (see Section Single-train Capacity Limitations – Mega-Ammonia Plants). Based on the foregoing discussion and the evaluation of present technology (see also [406]) and research efforts one may formulate the scenario for the future development of ammonia production as follows: 1.g. and wide capacity range. For the synthesis of the nitrogenase in the cell the so-called NIF gene is responsible. it is obvious that the future of the ammonia industry is very closely bound up with future fertilizer needs and the pattern of the world supply. Biological Processes. Ammonia synthesis is performed by the enzyme nitrogenase [1147-1149]. Abiotic Processes [1209]. Possibilities of converting molecular nitrogen into ammonia in homogeneous solution by using organometallic complexes have been studied from around 1966 onwards. 3. the existing ammonia plants represent a considerable capital investment and a great number of them may reliably operate for at least another 20 to 30 years from a mere technical point of view. In investigations on Rizobium [1211]. The prospects of this route are not judged to be very promising in terms of energy consumption and with respect to the costs of the rather sophisticated catalyst systems. and corn. and transform it into organic nitrogen compounds via intermediate ammonium ions. [1142]. beans. 1151]: 1. leads to the following questions: Would it be preferable to fix nitrogen in the leaves where energy is available or in the roots. by themselves or in symbiosis with a host plant.. 3. see Nitrogen Fixation and [1209]. Natural gas will remain the preferred feedstock of present ammonia production technology in the medium term (15 – 20 years) as may be assumed from the world energy balance shown in Table 26. Genetic engineering to extend biological nitrogen fixation to other plant groups pursues various routes [1150. Future Perspectives It is difficult to venture a prognosis for the future development of ammonia production technology. for example from a fertilizer. peas. to produce ammonia at ambient temperature and atmospheric pressure conditions in presence of a catalysts have up to now attained ammonia yields which are far too low to be economically attractive. It is very likely that genetic engineering will succeed in modifying some classical crops for biological nitrogen fixation and that large-scale application will occur predominantly in areas with strongly growing populations to secure the increasing food demand. which also seems to be unavoidable in vivo. e. simple application. There is no doubt that some day in the future genetic engineering will succeed in designing nitrogen-fixing plants for agricultural crops. Certain bacteria and blue-green algae are able to absorb atmospheric nitrogen. page 58 of 77 . lower investment. A definite answer to this question will only be possible when genetically engineered nitrogen-fixing plants are available in sufficient amounts for field tests. But until this will become a reality. The solution to the run-off problem therefore can also not consist in cutting back fertilizer application. Modifying Rhizobium to broaden its spectrum of host plants Transferring the NIF gene to other bacteria which have a broader spectrum of host plants but have no natural nitrogen-fixing capability Engineering soil bacteria to absorb and convert nitrogen to ammonia and release it to the soil [1143] Inserting the NIF gene directly into plants Especially the last option. ammonia production in the next 15 to 20 years will rely on the classic ammonia synthesis reaction combining nitrogen and hydrogen using a catalyst at elevated temperature and pressure in a recycle process. which settle in their root nodules. is sometimes used as an argument against the use of industrially produced “inorganic” or “synthetic” fertilizers and in favor of more “organic” or “natural” fertilizers. which would be disastrous in developing countries. but is likely to lie in a better management of the timing of fertilizer application. Unlike most diazotropic bacteria. to design nitrogen-fixing species of traditional crops such as wheat. [1148]. even in remote developing areas. 1153]. 4. [1147]. This stoichiometry suggests an efficiency of 75 %. whether there are other options for the future than the present ammonia technology. lupins) and Rhizobium bacteria. biological processes for in vivo conversion of molecular nitrogen into fixed nitrogen would probably be the first choice. but with rather low yields. admittedly rather ambitious and idealized objectives: ambient reaction conditions. solar. This is a somewhat surprising result. requiring a better understanding of the biological nitrogen cycle. rice. [1146]. Nitrate pollution. soybeans. This development may be pushed by the fact that compared to the classical fertilizer route less capital and less energy would be needed. Because of the economy of scale there is the tendency to choose a capacity of 2000 – 2200 t/d for new projects at low-gas-price locations and some investors are looking for even larger sizes. additional literature references can be found in [1140]. Technical ammonia production based on non-fossil resources. This ideal can. (Anabaena azolla) and water fern (Azolla pinata) in tropical rice fields is also very promising [1152. Using this efficiency for the case where all the fixed nitrogen in the plant is supplied only via this route. which is comparable to the free energy change for converting molecular nitrogen to ammonia. But this is a misconception. which suggests that nitrogen supply with nitrate has an energetic advantage over the biological fixation process [1213]. a maximum overall efficiency of 10 – 15 % is estimated for the enzyme. 2. and even less for the bacteria as a whole [1210]. of course. absorbed by the plant is transformed to ammonia involving a free energy change . A well known example is the symbiotic relationship between legumes (e. to which carbohydrates have to be transported? If fixation is performed in the leaves. The largest single stream plant with 3300 t/d was built by Uhde in Saudi Arabia and started up in 2006. For a detailed review of biological and abiotic nitrogen fixation.g. Careful directed application of blue-green algae. In addition. 2. As about 85-87 % of the ammonia consumption goes into the manufacture of fertilizers. the amount of this biological material is far to small to supply the increasing demand of the agriculture necessary to feed the growing world population. water or geothermic energy) or gasification of biomass in this time frame will play no role. [1145]. But even with the introduction of this new approach. Estimates of fixed nitrogen from natural sources and antropogenic activities on global basis are found in [1138-1142]. but if additional functions associated with fixation are taken into account. [1212] it was shown for the nodulated roots of peas that 12 g of glucose are consumed to produce 1 g of fixed nitrogen. which involves severe temperatures and pressures. Partial oxidation of heavy hydrocarbon residues will be limited to special cases. Numerous investigations resulted in the laboratory-scale synthesis of ammonia . The research for new approaches should strive for the following. then how can the problem of the proximity of oxygen generated in the photosynthesis and oxygen-sensitive nitrogenase be dealt with? To what extent will the energy required by a plant to fix nitrogen affect overall productivity? The last question has been discussed in some detail [1209]. but coal gasification could have a renaissance within this period. But besides technical feasability there are questions about the economic risk and financing. For the conversion of molecular nitrogen into ammonia the following stoichiometric equation may be formulated: The oxidation of glucose generates 3140 kJ/mol and with 100 % efficiency only about 0. A fundamental question is. Conclusions. living in symbiosis or providing the required energy from their own metabolism. electrolysis (power from nuclear. as shown in the stoichiometric equation above. [1141]. In the United States field tests with Azospirillium applied to corn could demonstrate a considerable increase in harvest yield [1151]. Nitrate. In addition.11 mol of glucose would be required to produce one mole of ammonia. a yield loss of 18 % would result. This might happen in the future but an estimate regarding extent and time horizon is difficult. because it might be necessary to supplement the biological nitrogen fixation with classical fertilizers. For the major part. blue-green algae are self-sufficient in that they themselves produce the energy necessary for the nitrogen fixation by photosynthesis. which may become a problem in some countries. 4. traditional ammonia synthesis will continue to operate in parallel. Photochemical methods [1144]. less or no consumption of fossil energy. As the major demand is in agriculture. Alternatively abiotic processes (in vitro) with homogeneous catalysis under mild reaction conditions could be discussed for ammonia production. never be achieved. because degradation of biomass and manure also leads to nitrate run-off. [Top of Page] 10. Apart from Rhizobium bacteria a number of other organisms have nitrogen fixation potential. In the molybdenum-containing nitrogenase a transfer of eight electrons is involved in one catalytic cycle: As seen from this equation two electrons are “wasted” in forming molecular hydrogen. application of selected natural nitrogen-fixing microorganisms to crops would be an intermediate solution to reduce industrial fertilizer consumption.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience *For simplicity reasons equal gas consumption and other costs are assumed. L. J. Res. C. H. R. P. Kesler. Chem. 253. Black. 4. Z. J. 1103 – 1107. The bulk of ammonia production in the near future will still be produced in world-scale plants of capacity 1000 – 2000 t NH3 per day. W. p. Tremearne. M. AIChE J. G. 2. Physica (Utrecht) 25 (1959) 840. Ammonia technology will not change fundamentally. C. 18. E. 1960. Haar. Soc. Budapest 1976. 53. G. Dietrich. Gas Wärme 9 (1960) 267. R. Eng. H. F. 56. A. 292. Lee. Miller. Chem. Karlsruhe 1978. Chem. Chem. 33. NSRDS NBS-37. 171 – 175. Links 43. Kholod. I. J. Phys. S. Ithaca. vol. L. Hund. Berlin – Heidelberg – New York 1956. they will need some time to mature to commercial applicability. A. 10 (1977) no. Bartlett. Links 51.. Yu. Reamer. Alesandrini et al. 3. Chem. Chem. 2. A. AIChE J. A. 17. 88. B. G. Eng. Transl. Phys.. 303.) 36 (1962) no. 11 (1972) no. Data 24 (1979) 1. Ing. R. London 1956. Soc. G. Can. K. Larson. Kälte (Hamburg) 23 (1970) no. 5. 293. Springer Verlag. no. v. A. Haldor Topsøe. Phys. Copenhagen 1982. E. 216. 1. 76. 1934. 5. 14. Bartlett et al. 43 (1965) 137 – 142. D. Kopenhagen 1968. Kältetechnik 12 (1960) 282 – 283. Jpn. Michels et al. 5. Phys. Sage. Müller. Data 7 (1978) no. Data 16 (1971) no. Am. Borrmann. Links 26. 47 (1925) 1015. 49. Zeininger. H. Stand. 34 – 37. 7. J. Pauling: The Nature of the Chemical Bond. 40. 23. Jossi et al. Ing.Y. H. F. Barile. 15 (1976) no. part 1. B. pp. H. Husain. T.. 3. W.. Small capacities will be limited to locations where special logistical. Eng. 50 (1928) 1275.. J. for example. 15. Dev. 33 (1961) no. 36. 29. Chem. A. 44. 76 (1972) 938. H. A. Links 47. Eng. Links 58. A. Perelsatein. Kjaer: Enthalpy Tables of Ideal Gases. 2. Klim. N. Tech. Physica (Utrecht) 16 (1950) 831 – 838.): The Nitrogen Industry. 40 (1968) 1192. 16. Am. J. vol. Haar. J. I. Eng. 19 (1980) 580 – 586. 6. F. K. E. probably also mega-plants. JANAF Thermodynamical Tables 2nd ed. Chem. Thus further progress may be confined to minor improvements of individual process steps. Res. Eng. Kälte (Hamburg) 20 (1967) no. Jpn.Ammonia : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience 5.vol.2 GJ per tonne NH3. Pauling: Die Natur der chemischen Bindung. 7 (1974) no. 31 (1925) 95. 45. G. Heide. 34. Verlag C. H. J. Jpn. 41. 152. V. Appl. N. 12. J. Links 38. Ing. H. at least not in the next 10 to 15 years. 12. Appl. Michels et al. no. 6. Chem. Saga. 21. part 1 – 10. 32 (1977) no. 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