reformer CH4 2

Preprints of the 8th IFAC Symposium on Advanced Control of Chemical ProcessesThe International Federation of Automatic Control Furama Riverfront, Singapore, July 10-13, 2012 Optimization of a methane autothermal reforming-based hydrogen production system with low CO2 emissions Wei Wu*,a and Chutima Tungpanututhb a b Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, R.O.C. Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin 64002, Taiwan, R.O.C. Abstract: The aim of this work is to develop a hydrogen production system with respect to saving energy and reducing carbon dioxide. Methane and carbon dioxide are major feedstocks of an autothermal reformer (ATR) and a CO2 reformer, respectively. The waste heat recovery technique is applied to build a stand-alone ideal heat-integrated system. The proposed system configuration and optimal operating conditions are verified with the Aspen HYSYS simulator. Keywords: Autothermal reforming; CO2 reformer; Hydrogen production; Heat recovery; Optimization Ersoz et al. [13] provided a simulation study for a fuel cell system combined with a methane ATR-based hydrogen production process. Furthermore, the optimization of the methane ATR process with regard to the conflict between the increase in hydrogen yield and the reduction in carbon monoxide has also been investigated [14, 15]. Using a similar approach, Silva et al. [16] undertook the simulation and optimization of a glycerol ATR-based hydrogen production process with the assistance of Aspen HYSYS. 1. Introduction About 95% of the hydrogen that was produced in the U.S. used a reforming process with natural gas as the feedstock. The conventional process primarily consists of two steps. First, the reformation of the feedstock with high temperature steam to obtain a syngas and, second, a water gas shift (WGS) reaction that produces hydrogen, carbon dioxide, and a few residual materials [1]. As for fuel cell applications, the pressure swing adsorption (PSA) process is connected to the WGS reactor such that the purity of hydrogen can reach almost 99.99+%, and the waste gas of PSA may flow into a preferential oxidation (PROX) reactor to produce the high temperature flue gas [2]. Moreover, the modeling, simulation and optimization of a class of hydrogen production processes using the Aspen HYSYS process simulator have been studied [3, 4]. 2. Description of Major Process Units There are five major process units in our methane autothermal reforming-based hydrogen production process. Three two-phase streams of methane, water and air (oxygen) are well mixed at room temperature with two molar ratios of water-to-methane (H 2 O/CH 4 ) and oxygen-to-methane (O 2 /CH 4 ) . The mixed stream is pre-heated by a heater equipment and flows into the autothermal reforming (ATR) reactor at the prescribed inlet temperature, TATR,in . Assuming In general, methane reforming processes constantly generate a large amount of carbon dioxide from the steam methane reformer (SMR), WGS reactor and PROX reactor. The CO2 capture technologies including MEA absorption and membrane processes can be devoted to reducing greenhouse gas emissions [5, 6], but the capture and storage of CO2 need additional electricity. Nord et al. [7] used the pre-combustion CO2 capture technique to effectively reduce greenhouse gas emissions. Recently, Fan et al. [8] utilized greenhouse gases through the CO2 reforming process with specific catalysts to improve hourly space velocity and hydrogen production, and Fan et al. [9] studied the optimization of hydrogen production from CO2 reforming of methane via experiments. With regard to methane reforming-based hydrogen production processes, a combination of partial oxidation and steam reforming of methane in an autothermal reforming (ATR) reactor has recently become more popular. The modeling, kinetic analysis, and simulation of ATR of methane for prescribed reactors have been studied [10-12]. © IFAC, 2012. All rights reserved. that the system is an adiabatic reactor with packed catalysts, the major reactions are described as follows. CH + 2O ⟶ CO + 2H O ( ), ∆H10 =-802.3kJ/mol (1) CH + H O ⟷ CO + 3H ( ), ∆H 0 = 206.2 kJ/mol 2 (2) CO + H O ⟷ CO + H ( ), (3) ∆H30= - 41.2 kJ/mol CH + 2H O ⟷ CO + 4H ( ), ∆H 40 =165.0 kJ/mol (4) and the kinetic rate equations for 661 - are expressed as [10], 8th IFAC Symposium on Advanced Control of Chemical Processes Furama Riverfront, Singapore, July 10-13, 2012 where = . . ( = . = & !"# !# $ − !"$ !# $ − & ' (× *+ (× '& *+ and K e5 = exp(−30782/ T + 42.97) . The CO2 reformer produces syngas which is similar to the steam reforming of methane, i.e. the large amount of CO in the outlet stream of the CO2 reformer is reproduced. Moreover, another high temperature water gas shift (HTWGS) reactor with packed catalyst is connected in order to convert components of CO and H2O in the stream into CO2 and H2. The 600K operating temperature of the HTWGS reactor is lower than that of the CO2 reformer, so another heat exchanger for heat removal (i.e. a cooler) is added to adjust the inlet temperature of HTWGS reactor, (5) ) k5 = 8.71 × 10−2 exp(23.7/ RT ) (6) TWGS ,in . Moreover, the kinetic model of HTWGS is (7) expressed by, PCO2 PH2   −95  1.1 0.53  rWGS = 7.79 × 1014 exp  PCO PH2O  1 −    K e2 PCO PH2O   RT   = where /# $ &. !"# !# $ − (× ' *+ (8) (11) ,- = 1 + /"$ !"$ + /# !# + /"# !"# + Afterward the outlet stream of the HTWGS reactor flows into the pressure swing adsorption (PSA) unit to produce high purity hydrogen (99.95+%) by using a solid adsorbent, e.g., activated carbon can be separated. Assuming that the PSA process is an isothermal one, the outlet composition of the PSA is expressed as follows . The reaction rate constant 01 = 021 exp(−61 /89), j=1, …,4, the adsorption constant for the combustion of " methane /:" = /2: exp(−∆<:" /89) , i=CH4, O2, and the adsorption constant of species i /: = /2: exp(−∆<: /89), i=CO, H2, CH4, H2O. Notably, the rate equation is based on the temperature dependence of the Langmuir-Hinshelwood-Hougen-Watson (LHHW) model. All kinetic data including kinetic parameters ( 021 , 61 ) , equilibrium constants ( /> , /> & , /> ) and adsorption constants (/21 , ∆<: ) are taken from the reference cited [10]. Since the outlet temperature of the ATR reactor could be over 850K and the stream contains a high percentage of carbon dioxide, a CO2 reformer aims to reduce the greenhouse gas emission and increase hydrogen production accordingly. In our approach, the outlet stream of the ATR reactor needs to be mixed with the second feed flow of methane ?CH |ABBC at room temperature, and the well-mixed stream flows into the CO2 reformer at the prescribed inlet temperature, TCO R,in . Assuming that the system is a yH2 ,PSA ,out = S PSA y i ,PSA ,out = (1 − S PSA ) i = H2, CH4, CO, CO2, H2O 1− G H G G G ( (13) , (14) where S PSA (= 99.95%) represents the hydrogen purity from the PSA. Moreover, the mass flow rates of H2 and CO2 at two exits of the PSA unit are, (9) with the corresponding kinetic model = 0D F"# F"$ , i ,PSA ,in and the outlet composition of PSA waste gas is shown by y i ,PSA ,in yɶ i ,PSA ,out = ∑ yi ,PSA ,in + (1 − S PSA ) × y H2 ,PSA ,in 2 D ∑y i = CH4, CO, CO2, H2O nonisothermal tubular reactor with packed catalysts, the reversible endothermic reaction proceeds in a CO2 reformer [9], CH + CO ⟷ 2CO + 2H ( D ), ∆HDE = 205 kJ/mol y i ,PSA ,in (12) (10) 662 8th IFAC Symposium on Advanced Control of Chemical Processes Furama Riverfront, Singapore, July 10-13, 2012 CH4 |LIJJK 298K, 1atm CH4 |MIJJK 298K, 1atm Figure 1 Methane autothermal reforming-based hydrogen production system FH2 = y H2 ,PSA ,out × FPSA ,in methane ?CH |ABBC and inlet temperature of the CO2 (15) reformer , TCO R ,in , are adjustable variables of the CO2 2 FCO2 = yɶ CO2 ,PSA ,out × FPSA ,in (16) reformer, and inlet temperature of the HTWGS reactor , TWGS ,in , is an adjustable variable of the HTWGS reactor. When the waste gas of the PSA flows into a preferential oxidation (PROX) reactor, the high temperature flue gas is produced. The kinetics of the PROX reactor are simplified as 1 Cat C (17) CO+ O2  → CO2 , ∆HCO = −282.99 kJ/mol 2 Moreover, the operating conditions are set by the ATR with H2O/CH4 =1.5 , O2 /CH 4 = 0.55 and TATR ,in = 773K , the CO2 reformer with ?CH |ABBC = 9.92 kg/h at 25°C, and the HTWGS reactor with TWGS ,in = 664K . Moreover, Fig. 2(a) and the corresponding reaction rate is shown by 0.5 −0.1 O2 CO rCO = 3.528 × 10 exp( −33092 / RT )P P 2 (18) demonstrates that the proposed hydrogen production system with the aid of the CO2 reformer can ensure a higher Notably, the built-in property database in the Aspen HYSYS hydrogen flow rate and lower carbon dioxide flow rate than provides accurate thermodynamic data, which are calculated with the conventional design in Fig. 2(b). Obviously, the CO2 with the Peng-Robinson equation of state. reformer can contribute to increased hydrogen production by Demonstration: In Fig. 1, the water-to-methane molar ratio consuming carbon dioxide. However, the CO2 reformer in (H2O/CH4 ) , oxygen-to-methane molar ratio (O 2 /CH 4 ) , inlet this process design needs an additional methane feed flow due to a very low composition of methane in the outlet of temperature of the ATR reactor, TATR ,in , are denoted as ATR. adjustable variables of the ATR unit, the second feed flow of (a) 663 8th IFAC Symposium on Advanced Control of Chemical Processes Furama Riverfront, Singapore, July 10-13, 2012 (b) Figure 2 Methane ATR-based hydrogen production system: (a) CO2 reformer with the second feed of methane (b) without CO2 reformer To achieve reduce the energy needed and the carbon dioxide produced, a heat recovery method, shown in Fig. 3, is developed. The flue gas produced from the PROX reactor goes through two heat exchangers to rapidly heat the inlet flow of the ATR reactor, and the recirculating streams go Figure 3 through another two heat exchangers to cool the outlet flow of the CO2 reformer. Finally, the outlet stream of the fourth heat exchanger (HX4) reaches a temperature of 774 K and this can be treated as the heat source for the CO2 reformer. Methane ATR-based hydrogen production system with waste heat recovery I, the optimization algorithm for maximizing hydrogen selectivity is described as follows. (20) max S H Notably, the original devices of heater and cooler shown in Fig. 2(a) are replaced by four heat exchangers. Using the Aspen HYSYS simulator, although the waste heat recovery design may reduce the hydrogen yield by about 2.5%, but the total benefits with regard to saving energy make this process worthwhile. ui subject to 573 K 3. Process Optimization Maximizing the hydrogen yield and minimizing carbon dioxide emissions are the optimization goals for the hydrogen production system; presented in this work. Two optimization strategies are introduced. First, the hydrogen selectivity in terms of the flow rates leaving the reactor is defined as T2UV- WU2X # 2YZ RS = ∑(T2UV- WU2X 2W 1) \]^ 2 , j = CH4, H2O, CO, CO2, O2 (19) ≤ TATR ,in ≤ 823 K 0.3 ≤ O2 /CH4 1 ≤ H2O/CH4 ≤ ≤ 0.7 2 803 K ≤ TCO2R ,in ≤ 983 K 573 K ≤ TWGS ,in ≤ 773 K (21) and the corresponding model equations have been built in the Aspen HYSYS. Notably, This shows that if the hydrogen product increases or the carbon dioxide decreases in the outlet flow of the HTWGS reactor, then the value of RS definitely increases. For Case { 2 ui ∈ TATR ,in ,O2 /CH4 ,H2O/CH4 ,TCO2R ,in ,CH4 Feed ,TWGS ,in 664 } 8th IFAC Symposium on Advanced Control of Chemical Processes Furama Riverfront, Singapore, July 10-13, 2012 represents one of decision variables. The upper and lower limits of physical constraints are based on the previous study of operating conditions for each reactor. 4. Conclusions The configuration proposed in this work is an energy-saving process, since the ATR reactor reduces energy consumption by up to 57.4%, and the waste heat recovery design can completely take over functions of heater and cooler. The proposed system can also reduce carbon dioxide emissions, because the CO2 reformer can reduce these by 42.54% and increase hydrogen yield by over 13.60%. It has been verified that the ATR reactor plus CO2 reformer can provide greater hydrogen produce than the conventional SMR process. However, both HTWGS and PROX reactors may generate a large amount of CO2 if the waste heat is not enough and the concentration of CO is too high. Remark 2: The hydrogen selectivity is denoted as a sole objective function, but the maximization of hydrogen selectivity could identify the operating scenarios for maximizing the hydrogen yield and minimizing carbon dioxide emissions simultaneously. Seven decision variables are determined by solving this constrained optimization problem. The optimal operating conditions are verified with the Aspen HYSYS simulator. Second, the carbon dioxide selectivity for the evaluation of the effect of carbon dioxide reduction is shown by T2UV- WU2X "$ 2YZ R_` = ∑(T2UV- WU2X 2W )\]^ , k = CH4, H2, H2O, CO, O2 (22) Acknowledgment Notably, the total carbon dioxide reduction is relevant to decrease the value of R_` . Thereby, Case II for the optimization algorithm in regard to the minimization of R_` is described as (23) min S CO ui The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 100-2211-E-006-264. 2 [1] subject to the same constraints in Eq. (21). Table 1 Comparisons of process before and after optimization Component/ before Case I for Case II for selectivity optimization maximizing minimizing H2 CH4 H 2O CO CO2 O2 (kg/h) SH2 (kg/h) SCO2 (kg/h) 23.53 0.09 46.58 65.86 87.46 12.81 22.71 0.99 18.46 82.70 58.49 10.92 21.12 5.44 21.87 79.45 51.40 9.15 [2] [3] [4] [5] Remark 3: Similarly, the carbon dioxide selectivity is another objective function. The minimization of carbon dioxide selectivity could identify the operating scenarios for maximizing the hydrogen yield and minimizing carbon dioxide emissions simultaneously. The optimal operating conditions can be found by solving the minimization algorithm according to seven decision variables. Similarly, the system’s performance with regard to the reduction in carbon dioxide emissions can be evaluated by the Aspen HYSYS simulator. Furthermore, a comparison of the system before and after using optimization strategies is shown in Table 1. Obviously, both optimization cases may induce a lower hydrogen yield than the original design with the emissions by 38.4%~45.9%. Moreover, if the waste heat recovery design is employed, then the hydrogen yield for the system using Cases I and II usually decrease by 2.5%. However, it is truly compensated by saving energy and reducing carbon dioxide emissions. he mass flow of CO2 falls from 87.46 kg/h to 58.49 kg/h, and the optimization of Case II can achieve up to a 41.23% reduction in carbon dioxide emissions. [6] [7] [8] [9] 665 References Xu J, Froment GF. Methane steam reforming, methanation and water-gas shift: I. intrinsic kinetics. AIChE J. 1989;35: pp.88-96. Liu K, Song C, Subramani V. Hydrogen and Syngas Production and Purification Technologies, A John Wiley & Sons, Inc., 2010. Posada A, Manousiouthakis V. Heat and power integration of methane reforming based hydrogen production. Industrial & Engineering Chemistry Research 2005;44:9113-19. Zamaniyan A, Behroozsarand A, Ebrahimi H. Modeling and simulation of large scale hydrogen production. Journal of Natural Gas Science and Engineering 2010;2: 293-301. Aresta M. Carbon Dioxide Recovery and Utilization, Kluwer Academic Publishers, Netherlands, 2003. Tarun CB, Croiset E, Douglas PL, Gupta M, Chowdhury MHM. Techno-economic study of CO2 capture from natural gas based hydrogen plants. Journal of Greenhouse Gas Control 2007;1:55-61. Nord LO, Anantharaman R, Bolland O. Design and off-design analyses of a pre-combustion CO2 capture process in a natural gas combined cycle power plant. International Journal of Greenhouse Gas Control 2009;3:385-92. Fan MS, Abdullah AZ, Bhatia S. Utilization of greenhouse gases through carbon dioxide reforming of methane over NieCo/MgOeZrO2: preparation, characterization and activity studies. Appl Catal B 2010;100:365-77. Fan MS, Abdullah AZ, Bhatia S. Hydrogen production from carbon dioxide reforming of methane over NieCo/MgOeZrO2 catalyst: Process optimization. International Journal of Hydrogen Energy 2011;36: 4875-86. 8th IFAC Symposium on Advanced Control of Chemical Processes Furama Riverfront, Singapore, July 10-13, 2012 [10] [11] [12] [13] [14] [15] [16] Hoang DL, Chan SH. Modeling of a catalytic autothermal methane reformer for cell applications. Applied Catalysis A: General 2004; 268:207-16. Zahedi nezhad M, Rowshanzamir S, Eikani MH. Autothermal reforming of methane to synthesis gas: Modeling and simulation. International Journal of Hydrogen Energy 2009;34: 1292-1300. Halabi MH, de Croon MHJM, van der Schaaf J, Cobden PD, Schouten JC. Modeling and analysis of autothermal reforming of methane to hydrogen in a fixed bed reformer. Chemical Engineering Journal 2008; 137: 568–78. Ersoz A, Olgun H, Ozdogan S. Simulation study of a proton exchange membrane (PEM) fuel cell system with autothermal reforming. Energy 2006;36:1490-1500. Behroozsarand A, Ebrahimi H, Zamaniyan A. Multiobjective optimization of industrial autothermal reformer for syngas production using nonsorting genetic algorithm II. Ind. Eng. Chem. Res., 2009;48:7529-39. Akbari MH, Sharafian Ardakani AH, Andisheh Tadbir M. A microreactor modeling, analysis and optimization for methane autothermal reforming in fuel cell applications. Chemical Engineering Journal 2011; 166:1116-25. Silva GF, Fereira ALO, Cartaxo SJM, Fernandes FAN. Simulation and optimization of H2 production by autothermal reforming of glycerol. Computer Aided Chemical Engineering 2009:27:987-92. 666