SULFURIC ACID AND SULFUR TRIOXIDE1. Introduction Sulfuric acid [7664-93-9], H2SO4, is a colorless, viscous liquid with a speciﬁc gravity of 1.8357 and a normal boiling point of $ 2748C. Its anhydride, SO3 [7446-11-9], is also a liquid with a speciﬁc gravity of 1.857 and a normal boiling point of 44.88C. Sulfuric acid is by far the largest volume chemical commodity produced and is sold or used commercially in a number of different concentra´ ´ tions including 78 wt% (608 Be), 93 wt% (668 Be), 96 wt%, 98–99 wt%, 100%, and as various oleums (fuming sulfuric acid, H2SO4 þ SO3) [8014-95-7]. Stabilized and unstabilized liquid SO3 are items of commerce. Sulfuric acid has many desirable properties that lead to its use in a wide variety of applications including production of basic chemicals, steel (qv), copper (qv), fertilizer (qv), ﬁbers (qv), plastics, gasoline (see GASOLINE AND OTHER MOTOR FUELS), explosives (see EXPLOSIVES; PROPELLANTS), electronic chips, batteries (qv), and pharmaceuticals (qv). It typically is less costly than other acids; it can be readily handled in steel or common alloys at normal commercial concentrations. It is available and readily handled at concentrations >100 wt% (oleum). Sulfuric acid is a strong acid. It reacts readily with many organic compounds to produce useful products. Sulfuric acid forms a slightly soluble salt or precipitate with calcium oxide or hydroxide, the least expensive and most readily available base. This is a useful property when it comes to disposing of sulfuric acid. Concentrated sulfuric acid is also a good dehydrating agent and under some circumstances it functions as an oxidizing agent. 2. History Sulfuric acid has been an important item of commerce since the early-to-mid1700s. It has been known and used since the Middle Ages. In the eighteenth and nineteenth centuries, it was produced almost entirely by the chamber process, in which oxides of nitrogen (as nitrosyl compounds) are used as homogeneous catalysts for the oxidation of sulfur dioxide. The product made by this ´ process is of rather low concentration (typically 608 Baume, or 77–78 wt% H2SO4). This is not high enough for many of the commercial uses developed since the early 1900s. The chamber process is therefore considered obsolete for primary sulfuric acid production. However, more recently, modiﬁcations to the chamber process have been used to produce sulfuric acid from metallurgical off-gases in several European plants (1–3). During the ﬁrst part of the twentieth century, the chamber process was gradually replaced by the contact process. The primary impetus for development of the contact process came from a need for high strength acid and oleum to make synthetic dyes and organic chemicals. The contact process employing platinum catalysts began to be used on a large scale for this purpose late in the nineteenth century. Its development accelerated during World War I to provide concentrated mixtures of sulfuric and nitric acid for explosives production. 1 Kirk-Othmer Encyclopedia of Chemical Technology. Copyright John Wiley & Sons, Inc. All rights reserved. 2 SULFURIC ACID AND SULFUR TRIOXIDE Vol. 23 In 1875, a paper by Winkler awakened interest in the contact process, ﬁrst patented in 1831. Winkler claimed that successful conversion of SO2 to SO3 could only be achieved with stoichiometric, undiluted ratios of SO2 and O2. Although erroneous, this belief was widely accepted for > 20 years and was employed by a number of ﬁrms. Meanwhile, other German ﬁrms expended a tremendous amount of time and money on research. This culminated in 1901 with Knietsch’s lecture before the German Chemical Society (4) revealing some of the investigations carried out by the Badische Anilin-und-Soda-Fabrik. This revealed the abandonment of Winkler’s theory and further described principles necessary for successful application of the contact process. In 1915, an effective vanadium catalyst for the contact process was developed and used by Badische in Germany. This type of catalyst was employed in the United States starting in 1926 and gradually replaced platinum catalysts over the next few decades. Vanadium catalysts have the advantages of exhibiting superior resistance to poisoning and of being relatively abundant and inexpensive compared to platinum. After World War II, the typical size of individual contact plants increased dramatically in the United States and around the world to supply the rapidly increasing demands of the phosphate fertilizer industry. The largest sulfur burning plant as of 2005 is $ 4500 metric tons of acid/day. Plants using sulfur in other forms, especially SO2 from smelting operations (metallurgical plants), have also increased in size. One metallurgical plant has been built to produce 3700 metric tons of acid/day (5). Another signiﬁcant change in the contact process occurred in 1963, when Bayer AG announced the ﬁrst large-scale use of the double-contact (doubleabsorption) process and was granted several patents (6–9). In this process, SO2 gas that has been partially converted to SO3 by catalysis is cooled, passed through sulfuric acid to remove SO3, reheated, and then passed through another one or two catalyst beds. Through these means, overall conversions can be increased from $ 98 to >99.7%, thereby reducing emissions of unconverted SO2 to the atmosphere. Because of worldwide pressures to reduce SO2 emissions, most plants as of 2005 utilize double-absorption. An early U.S. patent (10) disclosed the general concept, but apparently was not reduced to practice at that time. 3. Physical Properties 3.1. Sulfur Trioxide. Pure sulfur trioxide [7446-11-9] at room temperature and atmospheric pressure is a colorless liquid that fumes in air. Sulfur trioxide can exist in both monomeric and polymeric forms. In the gaseous and liquid state, pure SO3 is an equilibrium mixture of monomeric SO3 and trimeric S3O9 (11), also called g-SO3. 3 SO3 $ S3 O9 In the gaseous state, the equilibrium lies far to the left (12). In the liquid state, the amount of S3O9 is reported by some investigators to be $25% (258C) Vol. 23 SULFURIC ACID AND SULFUR TRIOXIDE 3 (13). Others report that the liquid is primarily S3O9 (14). For both gas (12,15) and liquid (16–18), the degree of association increases with decreasing temperature. If the SO3 is pure, it freezes to g-SO3, also called ice-like SO3, at 16.868C (19). It is possible that some monomeric SO3 may also be present in the crystal structure of the ice-like form (17,20–22). Traces of moisture, ie, of H2SO4, as low as 10À3 mol% (17,23) cause liquid SO3 to polymerize ﬁrst to a low melting, asbestos-like form, b-SO3. The b-SO3 forms crystals with a silky luster. Additional reaction involves cross-linking of b-SO3 to form a high melting asbestos-like form, a-SO3. The a-SO3 crystals resemble ice needles. An additional form has been mentioned in the literature as vaselinartiges (24), ie, vaseline-like, or gelatinous (25–27). This form, it has been speculated, is partially cross-linked b-SO3. Both the alpha and beta forms melt to give g-SO3. The early literature used reversed nomenclature for a-, b-, and g-SO3; when a- was the ice-like form; b- was the low melting, asbestos-like form; and g- was the high melting, asbestos-like form. b-SO3 consists of helical chain molecules (28) of unknown length (23). The aSO3 form is also a polymer similar to b-SO3, but probably in a layered crosslinked structure (22,23). Melting points, or more precisely triple points, of 32.5 and 62.28C have been given to beta and alpha polymers, respectively (22,29). The presence of these values persists in product literature, but they are rarely observed in industrial practice. The b-polymer melts only slowly a few degrees above its reported melting point (30). The a-polymer can best be melted by heating under pressure to 808C (25,30). Without pressure, the polymer sublimes. Initial melting of the b-polymer often leaves behind a residue that is much more difﬁcult to melt. This residue is probably a cross-linked b-polymer that may be a precursor to a pure a-polymer (26). Because of the slowness of melting and the lack of a distinct melting point, the melting process is believed to be a slow depolymerization rather than a general disintegration of the entire polymer molecule (31). Even in the presence of considerable moisture, solid polymer never forms > 308C (25). Below 308C, liquid stability decreases with increasing moisture and decreasing temperature (25). The actual formation of solid polymer has been hypothesized to involve the formation in the liquid of high molecular weight polysulfuric acids, followed by precipitation. In perfectly dry SO3 no sulfuric acid molecules would exist. But in the presence of even a trace of water, it seems likely that high molecular weight polysulfuric acids exist. As the higher polyacids form, the increasing molecular weight would be expected to decrease their solubility, leading to precipitation. Owing to kinetic considerations, lower temperatures also favor the formation, and hence precipitation of higher polyacids (26). The formation of polyacids in solution may be relatively rapid and the rate of formation of solid b-polymer controlled by the rate of nucleation (26). Temperatures < 08C do not appear to accelerate polymer formation as is sometimes believed. Instead, samples quickly cooled to À30 and À788C, held, and then quickly rewarmed showed less polymer than SO3 slowly cooled to 08C, and then slowly rewarmed even though the cycle times for all samples were the same (26). A study on the thermodynamic properties of the three SO3 phases is given in Ref. 32. Table 1 presents a summary of thermodynamic properties of pure 4 SULFURIC ACID AND SULFUR TRIOXIDE Vol. 23 sulfur trioxide. A signiﬁcantly lower value has been reported for the heat of fusion of g-SO3, 24.05 kJ/kg (5.75 kcal/kg) (33), as have slightly different critical temperature, pressure, and density values (34). Figure 1 shows the density of sulfur trioxide as a function of temperature. This curve is a composite of data taken from the literature (44–48). The vapor pressures of sulfur trioxide’s a-, b-, and g- phases are presented in Fig. 2 (49). Different values of SO3 vapor pressure for a-, b-, and g- phases have been reported in Refs. 32 and 34 (see Table 2). Additional data is available (50). The thermodynamic properties of sulfur trioxide, and of the oxidation reaction of sulfur dioxide are summarized in Tables 3 and 4, respectively. Thermodynamic data from Ref. 52 are believed to be more accurate than those of Ref. 51 at temperatures below $ 4358C. 3.2. Sulfuric Acid. Sulfuric acid is a dense, colorless liquid at room temperature, having a speciﬁc gravity as shown in Fig. 3 (53). Historically, the concentration of sulfuric acid has been reported as speciﬁc gravity (sp gr) in degrees ´ ´ Baume. In the United States, the Baume scale is calculated by the following formula: B´ ¼ 145 À e 145 sp gr ´ In Germany and France, the Baume scale is calculated using 144.3 as the ´ constant. The Baume scale only includes the sulfuric acid concentration range of ´ 0–93.19% H2SO4. Higher concentrations are not included in the Baume scale because density is not a unique function of concentration between 93 and 100% acid. The density of sulfuric acid versus temperature and concentration is shown in Fig. 4 (53). Figures 5 and 6 present the electrical conductivity of sulfuric acid solutions (54,55). For sulfuric acid solutions in the 90–100% H2SO4 concentration range, the electrical conductivity measurements reported by Ref. 55 are believed to be the best values; other conductivity data are also available (56,57). The viscosity of sulfuric acid solutions is plotted in Fig. 7 (58); other viscosity data may be found in Refs. 57–63. Surface tension of sulfuric acid solutions is presented in Fig. 8 (64). Surface tension of selected concentrations of sulfuric acid as a function of temperature up to the boiling point is given in Ref. 65; other data are also available (61,62,66–68). The index of refraction of sulfuric acid solutions (65) and additional related data (69) along with solubility data for oxygen in sulfuric acid solutions (70), are available in the literature. The solubility of sulfur dioxide in concentrated sulfuric acid is shown in Fig. 9 (71); additional data is also available (72). Data on chemical properties, such as self-dissociation constants for sulfuric and dideuterosulfuric acid (63,68,73,74), as well as an excellent graphical representation of physical property data of 100% H2SO4 (75) are available in the literature. Critical temperatures of sulfuric acid solutions are presented in Fig. 10 (76). Boiling points of sulfuric acid are given in Fig. 11. There is some uncertainty in the data close to 100% H2SO4 (77). Freezing points also are not well Vol. 23 SULFURIC ACID AND SULFUR TRIOXIDE 5 established, in part because acid purity and cooling rates signiﬁcantly affect the observed freezing points. Acid impurities lower the freezing point, and cooling rates may be such that subcooled liquid sulfuric acid is produced. Figure 12 shows the freezing points (78). Additional freezing point data (79), and discussions (62,80) are available. At atmospheric pressure, sulfuric acid has a maximum boiling azeotrope at $ 98.48% (81,82). At 258C, the minimum vapor pressure occurs at 99.4% (81). Data and a discussion on the azeotropic composition of sulfuric acid as a function of pressure can also be found in these two references. The vapor pressure exerted by sulfuric acid solutions below the azeotrope is primarily from water vapor; above the azeotropic concentration SO3 is the primary component of the vapor phase. The vapor of sulfuric acid solutions between 85% H2SO4 and 35% free SO3 is a mixture of sulfuric acid, water, and sulfur trioxide vapors. At the boiling point, sulfuric acid solutions containing <85% H2SO4 evaporate water exclusively; those containing >35% free SO3 (oleum) evaporate exclusively sulfur trioxide. A tabulation of the partial pressures of sulfuric acid, water, and sulfur trioxide for sulfuric acid solutions can be found in Ref. 83 from data reported in Ref. 84. Figure 13 is a plot of total vapor pressures for 0–100% H2SO4 versus temperature. References 84–86 present thermodynamic modeling studies for the sulfuric acid–water system. Vapor pressure, enthalpy and dew point data are available (82). Figure 14 shows the heat of mixing of sulfuric acid and water (87). Additional data are in Ref. 88. 3.3. Oleum. Oleum strengths are usually reported as weight percent free SO3 or percent equivalent sulfuric acid. The formula for converting percent oleum to equivalent sulfuric acid is %H2 SO4 ¼ 100 þ % oleum=4:444 Thus, 20% oleum is equivalent to 104.5% H2SO4. Oleum is generally thought of as a mixture of sulfuric acid and free sulfur trioxide. In various strength, oleums the free SO3 actually forms disulfuric acid, H2S2O7, and trisulfuric acid, H2S3O10 (89). Researchers have also argued for the existence of higher molecular weight polyacids although their presence in significant amount has been the subject of considerable controversy. An excellent review and discussion of the compositions of both the liquid and vapor phases of oleum, including a thermodynamic description of oleum with regard to the formation of H2S2O7, is available (89). The density of oleum at 20 (79) and at 258C (42) has been reported. The boiling points of oleum are presented in Fig. 15 (90). Freezing points are shown in Fig. 16 (78,91). An excellent discussion on the crystallization points of oleum is available (72). The solubility of sulfur dioxide in oleum has been reported (71,72). Viscosity of oleum is summarized in Fig. 17 (58); additional viscosity data are available (79). A composite curve of heat of inﬁnite dilution of oleum from reported data (4,92–94) is presented in a compiled form in the literature (95), where heats of 5 to 110 Torr) is given in the literature (96). . typical plant designs use sulfuric acid from the process as a drying agent. ie. Some applications of Haldor Topsøe WSA-2 wet gas catalysis process are described in the literature (101). 72. The principal direct raw materials used to make sulfuric acid are elemental sulfur. depending largely on the raw material used. In the past. the air used for sulfur burning is dried. 19. Elemental sulfur is by far the most widely used. Oleum heat capacity data are presented in Fig. When acid is produced from elemental sulfur. The gas stream containing sulfur dioxide is either dried before passing to the catalytic oxidation step. Sulfuric acid may be produced by the contact process from a wide range of sulfur-bearing raw materials by several different process variants. and production of sulfuric acid from a variety of waste products. or (in a wet gas process) is oxidized in the presence of water vapor with subsequent acid condensation and removal.45 to 1247 psi) over the entire concentration range of oleum is available (97). Vapor pressure curves for oleum calculated from these equations are shown in Fig. primarily as an economical or convenient means of minimizing air pollution (qv) or disposing of unwanted byproducts. Heat of vaporization data are also available (96). ‘‘Wet’’ catalytic oxidation is becoming more common. including off-gases and spent sulfuric acid. contaminant-free gas stream containing appreciable sulfur dioxide and some oxygen. A review of existing vapor pressure data plus additional data from 10 to 8600 kPa (1.06 to 14 kPa (0. roasting sulﬁde ores of copper. 23 formation of oleums from liquid or gaseous SO3 are also reported (Tables 5 and 6). lead. In some cases. If the initial oxygen concentration of the process gas is low. spent (contaminated and diluted) sulfuric acid.4. however. additional air or oxygen must be added prior to or during catalytic oxidation to ensure that there is an excess over stoichiometric needs for conversion of SO2 to SO3. substantially change the nature of the process or the process equipment. This requires careful design of equipment to minimize mist formation in the condensation–absorption portion of the plant. the ﬁrst steps in the process have the objective of producing a reasonably continuous. There are signiﬁcant differences in various sets of published data for oleum vapor pressure. In almost all cases.99). 18 (79). Pyrites plants are still found around the world. Additional vapor pressure data from 0. In all types of contact plants. A large amount of sulfuric acid is also produced as a by-product of nonferrous metal smelting. especially for treating weak sulfurous gas streams. Double absorption did not. The gas stream is preferably dry. eg. Manufacture. but plants can be designed to handle wet gas directly. solubility data of SO2 in oleum can be found in Ref. but as of 2005 are not common except in China (100). 3. led to a number of new process and equipment modiﬁcations (98.6 SULFURIC ACID AND SULFUR TRIOXIDE Vol. In the 1970s and 1980s the increased value of energy. from H2S combustion. The contact process remained virtually unchanged from its introduction in the late 1800s until the 1960s when the double absorption process was introduced to reduce atmospheric SO2 emissions. molybdenum. sulfuric acid is made as a by-product of other operations. including equations for vapor pressure versus temperature. zinc. and hydrogen sulﬁde. or others. iron pyrites or related compounds were often used. and sulfuric acid is added to maintain desired oleum concentrations.5% H2SO4. or where special circumstances exist. particularly the use of cesium promoted catalyst. this typically resulted in SO2 stack emissions of 1500–2000 parts per million (ppm) by volume. The catalytic oxidation of SO2 to SO3 is highly exothermic and. sulfuric acid of $ 98. which can be utilized either for heating requirements in other processes or to generate power . this is about the minimum temperature level required for typical commercial catalysts to function. the sum of SO3. SO3. four pass converters were commonly used to obtain conversions of from 97 to slightly > 98%. Oleums containing SO3 >40 wt% are usually produced by mixing SO3 with low concentration oleum. Partial absorption of SO3 occurs in these towers. oleum up to $ 35 wt% free SO3 content can be made in a single tower. Generally. 102.5% concentration is used because it is near the concentration with minimum total vapor pressure. the partial pressure of SO3 becomes signiﬁcant. air pollution requirements led to the adoption of the double contact or double absorption process. With catalyst improvements. Plants producing oleum or liquid SO3 typically have one or two additional packed towers irrigated with oleum ahead of the normal SO3 absorption towers. Both factors lead to unacceptable. and H2SO4 partial pressures.Vol. Consequently. Sulfur dioxide concentrations in the gas stream range from 4 to 14 vol%. The double absorption process employs the principle of intermediate removal of the reaction product. Where elemental sulfur or hydrogen sulﬁde is used as raw material. Liquid SO3 is produced by heating oleum in a boiler to generate SO3 gas. In early years. For sulfur burning plants. A few single absorption plants are still being built in some areas of the world. control measures and emissions calculations can be found in Ref. plant catalytic reactors (converters) are typically designed as multistage adiabatic units with gas cooling between each stage. much of the heat is typically used to produce steam. the contact process frequently employed only two or three catalyst stages (passes) to obtain overall SO2 conversions of $ 95–96%. Unfortunately. In the early 1970s. ie. At much higher acid concentrations. Normally. conversions near 99% have been achieved. 23 SULFURIC ACID AND SULFUR TRIOXIDE 7 The sulfur trioxide produced by catalytic oxidation is absorbed in a circulating stream of 98–99% H2SO4 that is cooled to $ 70–808C. At acid concentrations much < 98. which provides overall conversions of > 99. lower or higher concentrations occasionally being handled by special process or plant modiﬁcations. as expected. H2O. Later. Water or weaker acid is added as needed to maintain acid concentration. to obtain favorable equilibria and kinetics in later stages of the reaction. In such plants. relatively intractable aerosols of sulfuric acid mist particles are formed by vapor-phase reaction of SO3 and H2O.7%. which is then condensed. but most industrialized nations now have emission standards that cannot be achieved without double absorption or tail-gas scrubbers. equilibrium becomes increasingly unfavorable for SO3 formation as temperature increases > 410–4308C. visible atmospheric emissions of sulfuric acid mist. A discussion of sulfuric acid plant air emissions. ie. considerable heat is evolved during the initial combustion. two towers are used for 40 wt% SO3. Additional heat is generated by catalytic oxidation to SO3 and by the reaction of SO3 and H2O to form H2SO4. but typically is 80–1708C. In areas of the world where solid sulfur is still handled. oil. most sulfur used in the United States and Europe has been shipped and handled as a liquid containing very low ash concentrations. however. Since the early 1970s. Various equipment combinations including humidiﬁcation towers. it usually is decomposed in furnaces ﬁred by gas. Labsorb. spent acids. large plants of this type are essentially coproducers of steam or power and sulfuric acid. Where spent acid is used as raw material. sodium hydroxide. All three processes have been commercialized. These include CANSOLV. and are rarely used. Louis) and is marketed under the trade name ClausMaster. or other fuels (sometimes H2S or sulfur). In many cases. ie. most frequently as an add-on to an existing plant. reverse jet scrubbers. both products have signiﬁcant economic value. 23 via turbines. These processes have also been proposed as alternatives to dual absorption for new plants. A number of other tail gas scrubbing processes have been used. Hollriegelskreuth.8 SULFURIC ACID AND SULFUR TRIOXIDE Vol. Tail gas scrubbers are sometimes used on single absorption plants to meet SO2 emission requirements. Before the advent of relatively pure Frasch or recovered sulfur. Ammonia (qv) scrubbing has been popular. Sulfur shipped as a solid frequently becomes contaminated with dirt and scale during shipping and handling. lime and soda ash. or in combination with. molten sulfur is frequently ﬁltered prior to use as an alternative to. Such mists are objectionable because of both corrosion in the process and stack emissions. rather than on a new plant. Germany) is now licensed to (formerly Monsanto Enviro-Chem Systems) MECS. including use of hydrogen peroxide. [The Solinox ¨ process (Linde AG. and Solinox/ClausMaster (103–107). some tail gas scrubbers can reduce emissions beyond that achievable by dual absorption alone. typically <0.] These processes absorb SO2 from gas stream and regenerate it as concentrated SO2. Higher and lower dew point temperatures are possible depending on the SO3 concentration and moisture content of the gas. Using this type of raw material. Plants that burn good quality elemental sulfur or H2S gas generally have no facilities for purifying SO2. Though not economically justiﬁed. (St. The dew point varies with gas composition and pressure. In general. Small amounts of sulfuric acid mist or aerosol are always formed in sulfuric acid plants whenever gas streams are cooled. and the absorption and subsequent release of SO2 from a sodium bisulﬁte solution (the Wellman-Lord process). plants using SO2 gas derived from metallic sulﬁdes. typically ammonium sulfate for fertilizer use. hot gas puriﬁcation was frequently used in which the SO2 gas stream was passed through beds of granular solids to ﬁlter out ﬁne dust particles just prior to entering the converter. Other regenerative scrubbing processes have also been introduced. neither sulfur ﬁltration nor hot gas puriﬁcation are essential. . hot gas puriﬁcation. but to achieve good economics the ammonia value must be recovered as a usable product. and the high temperature gas from such furnaces can also generate steam or power (see POWER GENERATION). or SO3 and H2O react. impingement tray columns and electrostatic precipitators are used to clean the gas. gas puriﬁcation. wet.005%. Inc. or gypsum anhydrite purify the gas stream before drying it by cold. below the sulfuric acid dew point. packed gas cooling towers. The SO2 can be recycled back to the front end of the acid plant or used elsewhere. which ends up either as trace amounts in product acids or. in the range of 135. Sulfur burners are normally operated at moderate pressures. the lines and spray nozzles must be steam jacketed and steam pressure must hold the molten sulfur within the range of 135–1558C. with or without atomization air.76 MPa (150 psi) or higher. small amounts of nitrogen will react with oxygen to form nitrogen oxides. some form nitrosylsulfuric acid. as condensed acid collected in mist eliminators.8–170. Atomization typically is accomplished either by pressure spray nozzles. At high ﬂame temperatures. At a combustion air temperature of 558C.5. in considerably higher concentrations. 3. The temperature of gas leaving the sulfur furnace is a good indication of SO2 concentration.Vol. Special burners capable of producing higher SO2 concentrations are also available (see SULFUR COMPOUNDS). NO. Burners for spent acid or hydrogen sulﬁde are generally similar to those used for sulfur with a few critical differences. The self-sustaining ignition temperature of pure sulfur is $ 2608C. Because the degree of atomization is a key factor in producing efﬁcient combustion. or by mechanically driven spinning cups. Use of this type of equipment has been a signiﬁcant factor in reducing stack emissions of acid mist to acceptable levels. SO2 concentrations in the range of $ 3–14 vol% can be produced by the burners described. a source of ignition is not required if the combustion chamber is preheated to $ 400–4258C before sulfur is admitted. using air supplied by the main blower for the plant.000 times that at 1508C. a temperature of 9558C corresponds to $ 10. 23 SULFURIC ACID AND SULFUR TRIOXIDE 9 Since the 1960s. In a few cases. where its viscosity is at a minimum. Consequently. Sulfur Burning. 10348C–11. Some designs contain bafﬂes or secondary air inlets to promote mixing and effective combustion. 11128C–12. sulfur nozzle pressures are typically 2. In any process involving the handling of molten sulfur. NOx. Special types of nozzles are required both for H2S. ie. Generation of Sulfur Dioxide Gas. spent acids may be so viscous that . When burning sulfur in air. sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small diameter glass or Teﬂon ﬁbers. however. Ultimately. Spent Acid or H2S Burning. Above 1608C the viscosity rises sharply and at 1908C its viscosity is 13. Such units are called mist eliminators (see AIR POLLUTION CONTROL METHODS). sulfur-containing organic impurities.0 vol% SO2. but may be slightly higher for dark sulfur. and for the corrosive and viscous spent acids. primarily nitric oxide.0 vol% SO2. current practice is to use horizontal. a gaseous fuel.3 kPa (5–10 psig).0 vol% SO2. brick-lined combustion chambers with dried air and atomized molten sulfur introduced at one end. The chemistry of these nitrogen oxides is complex. Other temperatures and concentrations are in similar proportion. Sulfur furnaces are typically designed as proprietary items by companies specializing in acid plant design and construction. even though thermocouples employed for temperature measurement (qv) frequently read somewhat lower than the true temperatures because of radiation and convection errors. Packed ﬁber mist eliminators are considered BACT for mist removal. With the trend to very large single train plants. but costs escalate rapidly when the sulfuric acid concentration in spent acid (impurity free basis) falls below $ 75%. A wide variety of spent acids can be processed in this way. Relatively high (typically 980–12008C) temperatures are required to decompose spent acids at reasonable burner retention times. These smelting processes are often followed by traditional batch converting and the combined gas ﬂow to the acid plant is more uniform and higher strength than when treating converting gases alone. To supply the additional heat required. The resulting gases are high (25–75%) in SO2. Because combustion of H2S is highly exothermic. Traditionally. nickel. In some cases. auxiliary fuels. in other cases. intensive smelting processes have been developed that use highly oxygen-enriched air or even technically pure oxygen to minimize fuel consumption. This allows efﬁcient acid plant design including high levels of energy recovery that was formerly only possible in sulfur-burning acid plants. smelters has been limited to treatment of gases from roasters. Spent acid burning is actually a misnomer. eg. but low concentration off-gas. Moreover. there is considerable time when the furnaces are off-line for charging and transferring metals. sulfur recovery from copper (qv). lead (qv). Generation of SO2 at nonferrous metal smelters is determined primarily by the needs of the various metallurgical processes and only incidentally by requirements of the sulfuric acid process. Ore Roasting. sintering machines. Traditional converters. sulfur and H2S are excellent auxiliary fuels. must be burned. Weak spent acids are frequently concentrated by evaporation prior to decomposition. for such acids are decomposed to SO2 and H2O at high temperatures in an endothermic reaction. not dried air. or by mixing in fresh sulfuric acid of high concentration. Treatment of these cyclical and low grade gases is costly and involves technical problems that make efﬁcient acid production difﬁcult. The most modern smelters ($ 2005) now use continuous smelting and converting processes that utilize high levels of oxygen-enriched air to produce a uniform ﬂow of high strength process gas. More recently. is supplied to the burners. produce gases having varying concentrations of SO2. When available. 23 only a spinning cup can satisfactorily atomize them. oil or gas. Sintering. but generally low in oxygen. Excess water in the acid is also vaporized. Temperatures depend on the type of spent acid. Because large amounts of water vapor are produced by combustion of H2S or spent acids. and converters.10 SULFURIC ACID AND SULFUR TRIOXIDE Vol. preheated combustion air at low pressure may be supplied by ducts. A few relatively uncontaminated spent acids can be reused without decomposition by evaporating the excess water in concentrators. Any organic compounds present in the spent acid will oxidize to produce some of the required heat. ambient. Roasters and sintering machines operate continuously and produce a fairly uniform. or Smelting. burners are operated at pressures slightly below atmospheric pressure to pull in outside air. Acid decomposition and water vaporization require considerable heat. . careful design is necessary to avoid excessive temperatures. which operate as batch reactors. and zinc (qv). . increasing concentration of O2 in the process gas stream by air dilution or oxygen enrichment. to cool the process gas entering the last one or two converter passes. and increasing the catalytic converter operating pressure (pressure plants). This is equivalent to emissions of 12–27 lbm SO2/ton of acid. Sulfur Burning Plants. permitting agencies may require limits considerably below 4 lbm/ton for plants in areas with poor air quality. reversible. known as the double absorption process. Gas leaving the converter passed through a single SO3 absorption tower (hence the term ‘‘single absorption’’) then passed to the atmosphere. the sulfuric acid industry has attempted one or a combination of the following process design modiﬁcations: increasing concentration of SO2 in the process gas stream. or air dilution designs and higher ($ 10 vol%) inlet gas strength. To achieve this high conversion efﬁciency producers normally use the double absorption process. removing the SO3 product by interpass absorption. (In practice. those associated with smelters. This requires very aggressive use of catalyst.7%. ie. 4. The additional air improved conversion at the ﬁnal converter pass by increasing oxygen concentration and reducing equivalent sulfur dioxide concentration of the process gas. lowering catalytic converter inlet operating temperatures. In the United States. increasing the number of catalyst beds. Single absorption sulfuric acid plants were standard in the industry for many years. the third converter pass (3 þ 1 conﬁguration). in the direction of the desired product.1. tail gas scrubbers or both. These used either relatively low strength ($ 8 vol%) SO2 gas without air dilution. SO2 was oxidized to SO3 in a multipass converter. ie. may have different regulations. It is exothermic. In the single absorption plant. eg. New Source Performance Standards (NSPS) limit SO2 stack emissions for new sulfuric acid plants to limit to 2 kg of SO2 per metric ton of 100% acid produced (4 lbm/short ton). To improve equilibrium or driving force for the reaction. Plants with ﬁve passes in a 3 þ 2 conﬁguration have also been built. more commonly.Vol. Even with current improved catalysts. instead of heat exchangers. which is equivalent to a sulfur dioxide conversion efﬁciency of 99. Process Details and Flow Sheets The stoichiometric relation between reactants and products for the contact process may be represented as follows: SO2 þ 0:5 O2 $ SO3 SO3 þ H2 O ! H2 SO4 ð1Þ ð2Þ There are three important characteristics of the ﬁrst of these equations. and shows a decrease in molar volume on the righthand side. equilibrium considerations in a single absorption plant limit overall SO2 conversion to 98–99%. Typical double absorption plant design uses intermediate SO3 absorption after the second or. Its chief advantage was reduced investment over designs with heat exchangers for interpass cooling. using better catalysts. Air dilution was a common design option using additional dry air. 23 SULFURIC ACID AND SULFUR TRIOXIDE 11 4.) Acid plants used as pollution control devices. Process air in sulfur burning plants is dried by contacting it with 93– 98 wt% sulfuric acid in a countercurrent packed tower. catalyst bed depth. (The catalyst ignition temperature is the temperature below which substantial catalytic conversion (approaching equilibrium) cannot be sustained in any given bed or pass. The typical acid inlet temperature for 98. a common pump tank is usually used for both the drying and absorbing towers. called the interpass absorption tower.5% acid. If 668 Be acid product is not needed. have a considerably lower sustainable ignition temperature ($ 3908C) and have proved useful in special situations (108). vanadium catalysts do not have sustained catalytic activity at temperatures < 400–4108C. Most of the heat of combustion from the sulfur furnace is removed in a boiler. This increases overall plant energy recovery and improves SO3 absorption by lowering the process gas temperature entering the absorption tower. Typical converter operating conditions are shown in Table 7. although fresh catalyst may have an initial reaction ignition temperature as low as 3858C. where SO3 is absorbed. exit temperatures are limited by thermodynamic equilibrium to $ 5008C or less. In some cases. The temperature of hot gas exiting the ﬁrst pass is then lowered to the desired second pass inlet temperature (430–4508C) by removing the heat of reaction in a steam superheater or second boiler. Sulfur dioxide gas is catalytically oxidized to SO3 in a ﬁxed-bed reactor (converter) that operates adiabatically in each catalyst pass. double absorption sulfur burning plant. Gas leaving the converter is normally cooled to 165–2308C using boiler feedwater in an ‘‘economizer’’. which reduces the process gas temperature to the desired converter inlet temperature. a gas cooler is used instead of an economizer. thus reducing corrosion problems and stack mist emissions. To obtain optimum conversion. Plants of this design burn sulfur to generate a process gas stream ´ of about 10. sodium.12 SULFURIC ACID AND SULFUR TRIOXIDE Vol. The scheme shown produces 668 Be and 98. even though SO2 and O2 concentrations can vary widely. Gas leaving the economizer ﬂows to a packed tower.or potassium-promoted. The process gas is not cooled to a lower temperature to avoid the possibility of corrosion from condensing sulfuric acid originating from trace water in the gas stream. The temperature rise of the process gas is almost directly proportional to the SO2 converted in each pass. 23 Figure 20 shows a typical ﬂow sheet for a 3 þ 1.0–12. ie. The heat of reaction raises the process gas temperature in the ﬁrst pass to $ 6008C (see Table 7). If oleum is desired. the inlet temperature (Table 7) to the ﬁrst converter pass is dictated by catalyst performance. Concentrated sulfuric acid circulates in the tower and cools the gas to within a few degrees of the acid inlet temperature. an oleum tower and associated pump and cooler would be ´ installed ahead of the interpass absorption tower. Such low ignition temperatures cannot be sustained by conventional catalysts.5% sulfuric acid absorption towers . In converter passes downstream of the ﬁrst pass. and process gas strength.) Catalysts promoted with cesium. the heat of reaction from succeeding converter passes is removed by superheaters or gas to gas heat exchangers.0% SO2. Dry process air is used to minimize sulfuric acid mist formation in downstream equipment. Typically. Standard. 3 lbm/short ton). The double absorption process removes this limitation by removing the SO3 from the gas stream. 23 SULFURIC ACID AND SULFUR TRIOXIDE 13 is $ 75–808C. 20). which is reasonably constant for any degree of conversion. Reheating the process gas is accomplished in gas-to-gas heat exchangers (see Fig. Reheating can be done by burning additional sulfur and adding the hot gas from the sulfur furnace to the main gas stream. The smaller amount of sulfur trioxide absorbed in the ﬁnal absorption tower of double absorption plants typically raises its acid temperature to only 1058C or less. Each initial SO2 gas concentration has its own characteristic equilibrium curve. the gas is reheated to 390– 4308C.Vol. thereby altering the equilibrium curve. In a dual absorption plant without additional sulfur . the adiabatic temperature rise lines can approach the equilibrium curve.65 kg SO2/metric ton of 100% acid produced (1. For a given gas composition. or in drying towers operated at 93– 96% H2SO4. The hot product acid leaving the tower is cooled in heat exchangers before being recirculated or pumped into storage tanks. With appropriate loadings of newer catalysts. eg. but never cross it. and sent to the ﬁnal converter pass where nearly all the remaining SO2 is converted to SO3. Approximately 93–97% of total sulfur dioxide is converted to sulfur trioxide in the ﬁrst three converter passes.2. Acid plants having a SO2 source other than sulfur burning. If oleum is desired. The slopes of the adiabatic temperature rise lines are directly proportional to the speciﬁc heat capacity of the process gas. The conversion efﬁciency of any sulfuric acid plant can be presented as an equilibrium-stage process. the sulfur trioxide produced in ﬁnal converter pass is absorbed in the ﬁnal absorbing tower. Acid temperature rise within the tower comes from the heat of hydration of sulfur trioxide and sensible heat of the process gas. 4. new dual absorption plants can achieve an overall conversion of $ 99. The equilibrium curve limits conversion in a single absorption plant to $ 99%. Non-Sulfur Burning Plants. The curve in Fig. The irrigation rate in the tower is designed so that the sulfuric acid exits the absorption tower at 100–1258C. even with aggressive catalyst use. 21 represents SO2 equilibrium conversions versus temperature for the initial SO2 and O2 gas concentrations.5% to minimize its vapor pressure. Where lower concentration product acid is desired.9%. With the SO3 removed from the process gas. it is made either in separate dilution facilities. This is equivalent to an emission rate of 0. using some of the heat from the initial converter passes. using heat from the converter as the heat source. SO3 is absorbed in an oleum tower ahead of the interpass absorption tower. or using gas-to-gas heat exchangers. A typical equilibrium curve after the SO3 has been absorbed and the operating line for the ﬁnal catalyst pass are shown in Fig. and is absorbed in the interpass absorption tower. not sulfur trioxide concentration. 22. Figure 21 presents the equilibrium-stage diagram of a single absorption sulfur-burning plant using 8% SO2 burner gas. usually receive cold process gas that must be heated to the reaction temperature of $ 4258C before entering the converter. Interpass and ﬁnal absorbing towers are very similar in size since tower diameter is dependent on total gas throughput. from metallurgical plants or spent acid decomposition. depending on the catalyst used. Acid circulated over SO3 absorbing towers is maintained at $ 98. 3. Liquid SO3 is usually produced by distilling SO3 vapor from oleum and condensing it. Absorption and Drying Towers. At some plants.1.14 SULFURIC ACID AND SULFUR TRIOXIDE Vol. plus high reactivity with almost all organic substances and water. gaseous 100% SO3 is utilized directly instead of producing liquid. SO3 is absorbed in one or more special absorption towers irrigated by recirculated oleum. 4. The anhydride of sulfuric acid. When polymerization occurs. and instructions for its storage and use are available from suppliers. For some oleum uses. gas leaving the oleum tower must be processed in a nonfuming absorption tower. Sulfur Trioxide.88C) boiling point.4. Equipment 5. The absorption of SO3 for oleum production is carried out over a relatively narrow temperature range. To produce fuming sulfuric acid (oleum). 4. 5. In addition. Liquid SO3 is a difﬁcult material to handle because of its relatively low (44. a high percentage of the heat of reaction in the converter is needed for heating and reheating the process gas. trace amounts of water (sulfuric acid) act as polymerization catalysts for liquid SO3 and produce a series of high molecular weight polymers with elevated melting points. it is practical to add small amounts of HNO3 as an antifreeze (109). Its principal applications are in production of detergents and as a raw material for chlorosulfuric acid and 65% oleum. The upper temperature is set to provide a reasonable partial pressure driving force for the oleum concentration used. 23 burning. Polymerization is promoted by cooling the liquid < 308C. Stabilized liquid SO3 is an item of commerce. Because of oleum vapor pressure limitations the amount of SO3 absorbed from the process gas is typically limited to < 70%. More recently. is a strong organic sulfonating and dehydrating agent that has some specialized uses (see SULFONATION AND SULFATION).005% boric oxide (30). its tendency to form solid polymers < 308C. When this is done. It reacts explosively with water because of a very high heat of reaction. it is frequently possible to utilize unstabilized liquid SO3 if precautions are taken to prevent its freezing before use. SO3 gas has been added to cooled combustion gases at many coal burning power plants to improve dust removal in electrostatic precipitators (see AIR POLLUTION CONTROL METHODS). which is high enough to be a problem in shipping and handling as well. This operation is normally carried out at a sulfuric acid plant where the stripped oleum can be readily refortiﬁed or reused. such as 0. Towers are typically carbon steel vessels lined with acid proof brick and mortar and packed with ceramic saddles . SO3. Oleum Manufacture.3% dimethyl sulfate with 0. It can be inhibited by adding small amounts of various patented stabilizers to the liquid. Because absorption of SO3 is incomplete. Eliminating all traces of sulfuric acid from the SO3 vapor stream is important to minimize polymerization of the liquid condensate. attempts to melt the solids can cause piping or equipment failures because high (exceeding atmospheric pressure) vapor pressures develop before melting occurs. The lower practical temperature limit is the freezing point of oleums. Saramet. and especially when the main blower is downstream from the drying tower. Though it predates a few of the newer alloys. Beginning in the early 1990s. More recently. Ref.Vol. They are not recommended for acid services > 93% concentration. Structured packing has also been used. A detailed discussion of oxidation catalysts for sulfuric acid production is available (115). Although graphite is relatively inert. Catalysts. Various acid distributors have been used. The main disadvantage of graphite coolers is brittleness. Anodic passivation and its application to sulfuric acid equipment. or ZeCor have been installed in several acid plants. Distributors were formerly made out of cast iron and were limited in the number of distribution points per square foot that could be achieved. 23 SULFURIC ACID AND SULFUR TRIOXIDE 15 (see ABSORPTION). typically 99. Glass or Teﬂon packed ﬁber bed mist eliminators are standard in interpass and ﬁnal absorption towers. anodically passivated stainless steel shell and tube acid coolers became commercially available. graphite coolers provide excellent service in weak acids. They are subject to damage by mechanical or hydraulic shocks. such as shell and tube coolers and carbon steel storage tanks.2. Drying towers are typically designed using special glass ﬁber mesh pads at the gas exit to remove acid spray and large mist particles. A brief discussion of the evolution of acid coolers is included. Acid Coolers. Saramet.9%. as well as plate coolers with plates welded together to minimize gasketing. 114 is a good discussion of acid plant coolers. or ZeCor. 5.5%. Commercial sulfuric acid catalysts typically consist of vanadium and potassium salts supported on silica.8 to >99. 5. all metal towers (with no brick lining) have been built from high silicon stainless steel alloys such as SX. Plate and frame coolers using Hastelloy C-276. In 1970. where high efﬁciency collection of acid mist is more critical. . Sometimes. Well-designed absorption or drying towers operate at absorption efﬁciencies >99. usually diatomaceous earth (see DIATOMITE). Some designs are prone to plugging from packing chips and sulfates. Anodic passivation uses an impressed voltage from an external electrical power source to reduce metal corrosion. once the industry standard. shapes. Because of the close clearance between plates cooling water for plate coolers must be relatively clean. this type of cooler became widely used. are considered obsolete.3. Because these proved to have signiﬁcant maintenance savings and other advantages. is discussed in the literature (111–113). and sizes depending on the manufacturer and the particular converter pass in which they are to be used. Impervious (impregnated) graphite coolers are utilized in acid service in which the sulfuric acid concentration is 90–93% or less. When operated properly. including perforated pipes and trough and downcomer designs. These materials do not require anodic protection. more efﬁcient packed bed ﬁber mist eliminators are installed. Anodically protected plate coolers are available. Catalyst is available in various formulations. and thus strainers are frequently used in acid lines. Hastelloy D205. Designs using stainless steel alloys can provide up to four distribution points per square foot (43/m2). the graphite impregnating agent is attacked by concentrated sulfuric acid. Cast iron trombone coolers (110). and SMO254 plates have been used successfully. shell and tube coolers made from SX. which is very difﬁcult to reoxidize under converter conditions. ﬁrst converter pass catalyst is screened at every major turnaround. vanadium catalysts in ﬁrst and second pass service slowly lose catalytic activity over time. Catalytic Converters.4. Under normal operating conditions. Converters vary in design with different engineering ﬁrms. including rings with longitudinal ribs. It is well documented that at operating temperatures the active catalytic ingredient is a molten salt that migrates from catalyst into adjacent dust. sometimes partly or wholly brick lined. carbon steel is protected from hot gases by brick lining or a sprayed aluminum coating. The various ring-shaped catalysts also have greater resistance to dust fouling. Exposure to moisture above the dew point is not detrimental to sulfuric acid catalyst. although carbon steel converters are also still used. Moisture also damages the binders that hold together the silica support. Screening losses depend on screen mesh size and catalyst hardness as well as on screening rate. are used almost exclusively as of 2005. Stainless steel converters are generally preferred. signiﬁcant scaling has been observed in stainless steel converters. Prolonged exposure to moisture reduces the vanadium to the þ3 oxidation state.16 SULFURIC ACID AND SULFUR TRIOXIDE Vol. Traditional carbon steel converter designs use steel shells. One innovation in stainless steel converter design is to use ‘‘structurally shaped’’ support grids and division plates (116). has not been sufﬁciently serious to result in abandonment of stainless steel designs. with cast iron and alloy internals. Both carbon and stainless steel converters are insulated to reduce heat losses. Stainless steel offers better corrosion resistance and signiﬁcantly higher strength at operating temperatures than carbon steel. exposure to moisture at temperatures below the dew point produces irreversible damage. This reduces catalyst hardness resulting in higher than normal screening losses. However. Ring catalysts also have somewhat higher activity per unit volume than pellet catalysts. 5. so far. Catalyst aging is accelerated by increasing temperatures and temperature cycling. typically every 12–36 months. although signiﬁcant metal loss has not been found. Ring-shaped catalysts or variations. Stainless steel designs generally use all welded interior construction. Essentially all designs use horizontal catalyst beds arranged one over another with gas ﬂowing down through the catalyst. Increased catalyst-bed pressure drop caused by dust fouling reduces production of acid and signiﬁcantly increases energy consumption by the plant’s blower. In plants having high temperatures and high NOx content. This problem appears to be more closely linked to high NOx content than high temperature and. 23 Formerly. Two advantages are claimed for the structural shape of the support grids and division plates: improved . Typical screening losses range from 10 to 15% of the catalyst bed per screening. Catalyst aging is a combination of a loss of catalytically active material from the catalyst pellets and irreversible changes within the catalyst. primarily as a means of saving energy via reduced gas pressure drop. Second pass catalyst is screened less frequently because the ﬁrst converter pass catalyst bed acts as a ﬁlter for the rest of the converter. To avoid these problems. In the high temperature converter passes. most catalysts were supplied as solid cylindrical extrudates or pellets ranging from 4 to 10 mm diameter. with stainless screens for catalyst supports. 5. In drying towers of sulfur-burning plants. High efﬁciency mist eliminators are usually used ahead of drying towers of spent acid plants. Process gas leaving the spent acid regeneration furnace and waste heat boiler must be cleaned prior to entering the drying tower. ﬁber diameter. Most modern mist eliminators use ﬁber beds enclosed or supported by stainless steel or alloy-20 wire mesh or the like.8. Acid mist eliminators use three aerosol collection mechanisms: inertial impaction. liquid distributors. and boilers. The corrosiveness of oleum decreases with increasing SO3 concentration. except for special services.. One notable exception is process iron. Relatively low oleum velocities must be used in steel piping to prevent excessive corrosion. but their higher price may make these materials less economical.118). 5.6. The traditional material of construction for oleum is carbon steel. It is well documented that gray cast iron fails catastrophically by cracking in oleum service. and the tubes run vertically in a typical shell and tube arrangement.Vol. bed depth. S. Mist eliminator design. Steel is borderline from 5% to $ 15% SO3. which is in turn determined largely by the application. Gas–gas heat exchangers in double absorption plants are built of carbon steel or stainless steel. Alonizing signiﬁcantly increases the life of carbon steel with the added beneﬁt of no loss of heat transfer from accumulation of corrosion scale. followed by a mesh pad in the drying tower exit. Process iron has been used successfully in oleum service. oleum. etc. Standard stainless steels have signiﬁcantly greater corrosion resistance to oleum than carbon steel. control. 5. To reduce corrosion and scaling. Packed ﬁber-bed mist eliminators can be designed to operate at various particle collection efﬁciencies. is determined by particle size and loading. interception. Spent Acid Regeneration Gas Cleaning Equipment. Typical tube diameters range from 37. Mist Eliminators. (St. Metallurgical plants may require wet electrostatic precipitators ahead of drying towers. mesh pads. A good discussion of sulfuric acid mist generation. or inertial impactiontype mist eliminators are usually adequate. which is a proprietary aluminum alloy coating vapor diffused into the metal surface. Fibers are generally glass or in some cases Teﬂon. Oleum Equipment and Piping. 5. depending on temperature. packing density.7. Lewis Co. The mechanism or the cause of cracking is not well understood. such as valves. Missouri). but it is agreed that the failures are due to a buildup of internal stresses within the cast iron. ie. Brownian motion and interception work well with aerosols having smaller particle diameters. carbon steel is not recommended because of excessive corrosion. and Brownian motion. a proprietary cast iron of Chas. depending on allowable pressure drop and cost. Inertial impaction works well for aerosols with particle diameters > 3 mm. carbon steel tubes in high temperature service are Alonized. Except where spent gases . Louis.5. For oleum concentrations < 5% SO3. 23 SULFURIC ACID AND SULFUR TRIOXIDE 17 resistance to temperature differentials during startup and a higher strength design requiring less metal.5 to 100 mm. Gas–Gas Heat Exchangers. and mist eliminator design is available (117. Gray cast iron is not a recommended construction material for oleum. Saramet. or from catastrophic. Sulfuric Acid Piping. the U. plus tolerable corrosion. 5. Venturi scrubbers and cyclone separators were sometimes used to remove acid mist and spray instead of electrostatic precipitators. was successfully introduced by MECS (St. For many years. Zecor.S.9. Mondi ductile iron has increased Si and Cu for reported lower corrosion rates in acid service. These options offer potentially higher reliability and lower iron content in product acid than ductile iron. Ductile iron used in acid service should have thicker walls for added corrosion allowance. Missouri) (119). but may be as high as 8008C. spent gas puriﬁcation equipment consisted of brick and lead-lined spray humidiﬁcation towers. the ultimate failure of gray iron pipe may be catastrophic if thinning is not properly monitored. The economics . The expected service life of gray cast iron pipe is $ 7–15 years. This produces a highly turbulent froth zone that efﬁciently cools and scrubs gas leaving the spent boiler. the main application of this technology for sulfuric acid plants is the Reverse Jet Scrubber in which a jet of scrubbing liquid is sprayed into the gas stream so as to balance the liquid and gas momentums.5 m/s) or other special services. it is also necessary to remove as much water as possible from the spent gas to avoid water balance problems in the absorption portion of the plant. Owing to its brittle nature. 23 make up only a small portion of the total SO2 stream. Karbate cooler/condensers. Gray iron pipe and ﬁttings within 3–4 m of anodically protected acid coolers will fail catastrophically in 3 months to 1 year by spontaneous cracking similar to that observed in oleum service.18 SULFURIC ACID AND SULFUR TRIOXIDE Vol. Temperatures entering the primary reverse jet scrubber are typically $ 3258C. Tests have shown that the expected life of ductile iron pipe in concentrated sulfuric acid service is about the same as gray cast iron pipe. In the late 1980s. with the possible exception of high velocity (>2. Gray cast iron pipe and ﬁttings should not be utilized immediately adjacent to anodically protected coolers. packed cooling towers. a new type of gas cleaning technology. Small (<75 mm) diameter pipe is typically stainless steel or alloy 20. Mondi is sold as a special piping speciﬁcation and material speciﬁcally for acid service. and velocity of the acid. and impingement tray towers. depending on the temperature. In 1977. depending on concentration and temperature. or anodically protected 304 or 316 stainless steel piping. based on patented DuPont froth scrubbing technology. water supply industry and the centrifugally cast iron pipe foundries made a change from gray cast iron to ductile cast iron pipe as the preferred material. electrostatic precipitators. DynaWave equipment is typically smaller than conventional equipment and is usually made of ﬁberglass reinforced plastic (FRP) at considerable capital savings over conventional materials. In contrast to gray cast iron. DynaWave is now in use at a number of plants worldwide. The traditional material of construction for handling concentrated sulfuric acid (>70 wt%) for most of the past century has been gray cast iron. concentration. ductile iron does not suffer from spontaneous catastrophic cracking in acid service near anodically protected equipment. Ductile iron pipe is available only in diameters of !75 mm. Trademarked DynaWave. A few plants have been built using SX. Gray cast iron was the preferred material because of its low price and availability. brittle failure at the end of its useful life. Louis. molybdenum. acid velocity. and inspection is required for safe operation. High nickel/chrome–moly alloys.Vol. The contaminants in weak acid usually vary too greatly to allow use of an economical alloy that will give adequate service. Materials of Construction Resistance of alloys to concentrated sulfuric acid corrosion increases with increasing chromium. Alloy 20 type alloys have suffered inconsistent and variable corrosion performance at moderate temperatures. 6. For nickel containing alloys in particular. eg. including oleum. An excellent compilation of the relatively scarce literature data on corrosion of alloys in liquid sulfur trioxide and oleum may be found in Ref. Rigorous design. Resistance of these alloys is extended to higher temperatures than the alloy 20 type alloys for certain concentrations. the weak acid scrubbing circuit is typically handled by plastic or glass ﬁber reinforced plastic (FRP) pipe. 23 SULFURIC ACID AND SULFUR TRIOXIDE 19 of these options versus ductile iron or other alloys are highly dependent on piping diameter and expected piping life. Sulfuric Acid and Oleum Storage Tanks. Carbon steel is not suitable for handling sulfuric acid in concentrations between 80 and 90% or < 68% even under quiescent conditions. and silicon content. An excellent summary is available (122). eg. can handle sulfuric acid at ambient and slightly elevated temperatures throughout the concentration range. and acid impurities. The corrosiveness of sulfuric acid solutions is highly dependent on concentration. More detailed discussions are also available (125– 130). Impurities and acid velocities also are important in choosing the optimum piping material. temperature. heavy wall steel pipe is sometimes used for transferring product acid. where high concentrations of acid (>98%) or oleum service is assured. versatile. which is designated as CN-7M or Durimet 20 in cast form. fabrication. 131. At the high temperatures and concentrations . 123 and 124. especially <93% (132). Where product contamination or low corrosion rates are important. 5. In the gas cleaning sections of spent acid or metallurgical sulfuric acid plants. Alloy 20 and similar alloys. Carbon steel is not normally a suitable piping material for concentrated sulfuric acid because of high corrosion rates in ﬂowing acid. alloy C-22. Failures of sulfuric acid storage tanks have occurred owing to the corrosive nature of the acid and the peculiarities of the corrosion phenomenon (120). Reference 131 is appropriate. Excellent guidelines exist (121). unless passivating agents are present. where temperatures and ﬂow rates are low. This is probably due the allowable variations in metal chemistry and sensitivity to oxidizing or reducing agents impurities. will also handle sulfuric acid at all concentrations. However. Alloy 20 offers no advantage over Type 304 and 316 stainless alloys. Carbon steel is used in concentrated sulfuric acid storage tanks because in quiescent and low temperature conditions its corrosion rate is acceptable. alloy C-276. One of the more resistant. and economical alloys available in wrought form is alloy 20.10. either linings or the use of anodic protection are both well proven. and alloy 59. Good general discussions of materials of construction used in modern sulfuric acid plants may be found in Refs. alloy 20-Cb3. Many plastics do not resist acid above 50–60 wt% H2SO4. Tantalum’s high price prevents it from becoming a common material of construction in concentrated sulfuric acid service. should be used as the criterion of selection because metal skin temperatures may be signiﬁcantly higher or lower than bulk acid temperatures. nozzles. nitric acid. Hastelloy B-2 performs extremely well in concentrated sulfuric acid. nitric acid and ferric ions. easily formable high (5–6%) silicon nickel base alloy designated alloy D-205. which is being used for plate and frame heat exchanger plates to cool 93 and 99% acid in contact plants. Consequently.135). Haynes has more recently developed a wrought.133). Zirconium has excellent corrosion resistance to sulfuric acid solutions up to the boiling point in the concentration range of 0–65 wt% H2SO4. modern practice uses various acid-resisting polymeric materials instead of lead whenever practical. Hastelloy B-2 is not recommended for oleum service. extensive external supports are normally required. and ZeCor. Since the mid-1980s the high (5–6% Si) silicon stainless steels have made inroads as materials of construction for pipe and equipment handling contact plant acid in the 93–99% concentration ranges at operating temperatures. MECS discovered an operating window at very high ($ 170–2008C) temperatures and 99% concentration where high chromium stainless steels. Because of lead’s low physical strength and loss of strength at high temperatures. an unnecessarily more expensive higher alloy may be chosen on the basis of bulk acid temperature. This is particularly important when designing heattransfer equipment: Heating coils have failed catastrophically because alloy selection was based on acid bulk temperatures instead of the higher metal skin temperature of the heating coil. 23 found in contact plant absorbing towers the Lewmet 55 (cast) and Lewmet 66 (wrought) alloys (Chas. eg. or free SO3 in sulfuric acid. In the past. generally unacceptable in acid service below $ 90% or in areas where localized oleum may be present. These materials are. St. and screens (131. but corrodes rapidly in the presence of ferric ions. but very poor corrosion resistance to oleum. For this reason. such as oriﬁce plates pump parts. The resistance and properties of polymers can vary widely depending on exact composition. very brittle and use is limited because of its propensity to fracture when subjected to thermal or mechanical shock. Duriron. Louis. can be used (134. may signiﬁcantly alter the performance of alloys in sulfuric acid. Tantalum has excellent corrosion resistance to concentrated sulfuric acid at high temperatures. eg. Special precautions should be observed when utilizing isocorrosion charts as guidelines for alloy selection. It is. type 310. S. Saramet. Reference 127 is an excellent compilation of isocorrosion diagrams for metals in sulfuric acid service. has excellent corrosion resistance to sulfuric acid at all concentrations up to the boiling point. Oxidizing contaminants. etc. a cast high silicon iron. Missouri) are used for velocitysensitive components. . These include SX. For example. at temperatures <458C. Hence. Lewis Co. In the case of acid cooling. lead was widely used as a material of construction for sulfuric acid concentrations less than $ 80 wt% and occasionally for concentrations up to 93 wt%. not bulk acid temperature.. In the mid-1980s. degree of polymerization. however. however. Metal skin temperature.20 SULFURIC ACID AND SULFUR TRIOXIDE Vol. Vol. capable of processing weak. eg.2. and various proprietary baked phenolic resins are reasonably effective up to 94–95 wt% H2SO4 at temperatures near ambient. wet gas streams directly (136). Denmark) developed the Wet gas Sulfuric Acid (WSA) process. the WSA process can treat SO2 and H2S gases with concentrations down to 500 ppm volume (136). The concentration of SO2 (or other sulfur compounds) is frequently too weak to sustain ‘‘auto-thermal’’ conditions in the converter. Use of sulfur compounds in ﬂue gases and sulfurous off-gases for sulfuric acid production is problematic for conventional acid plants. Speciﬁc impurities or operating conditions may have major impact on resistance. During the 1970s and 1980s a dramatic increase in energy cost. Even if ﬂue gases come to the acid plant hot. Butyl rubber. gaskets and tank linings. Tetraﬂuoroethylene (TFE). By using supplemental heat. Wet Gas Process. In the early 1980s. plus governmental regulation regarding cogeneration of electric power. they often have a high moisture content and may be very dirty. gas duct and piping. Many of these changes became standard by the 1990s. metallurgical. This eliminates traditional absorption towers. 7. More recently. led to signiﬁcant modiﬁcations and variations in plant design. and of FRP for vessels. References 137–139 describe some applications of the WSA process. > 40 WSA plants have been build worldwide treating process gases from a wide range of sources including the petroleum. The uniqueness of the WSA process is the direct production of sulfuric acid by vapor-phase reaction of SO3 and water. ﬂuorinated ethylene propylene (FEP). Energy Efﬁcient Plants. To date. Poly(vinylidene ﬂuoride) (PVDF). the need for additional electric generating capacity in many regions of the country has become a force in the development of energy recovery (see ENERGY MANAGEMENT. extensive use is made of polyproplyene for tower packings and lined pipe. poly(ethylene-chlorotriﬂuorethylene) (ECTFE). Using available process heat (including hydrolysis of SO3. and power industries (140). In the gas cleaning sections of metallurgical and spent acid regeneration plants. Teﬂon. and perﬂuoroalkoxy polymer (PFA) materials. eg. Cleaning and drying operations produce a cold gas exacerbating the auto-thermal issue. 23 SULFURIC ACID AND SULFUR TRIOXIDE 21 speciﬁc data or information should be obtained from the manufacturer or a materials consultant. the WSA process can treat very low gas compositions without supplemental heat (see Table 8). Special Plant Designs 7. are the only common plastics that resist all acid concentrations (with temperature limitations). . PROCESS ENERGY CONSERVATION). heat of condensation and sensible heat in the process exit gas) to heat the incoming gas.1. but limits product acid concentration to the sulfuric acid azeotrope. 7. Haldor Topsøe A/S (Lyngby. Hypalon and ethylene–propylene diene monomer (EPDM) have useful resistance in these systems where elastomers are needed. and the use of a glass-tube falling ﬁlm condenser to condense the product acid from the process gas. reduced plant pressure drop via new catalyst shapes. including at least two in excess of 4500 metric tons/day.142). 900 psi. This achieves thermal efﬁciencies of 90–95% compared with $ 55% for plants in the early 1970s. Although proposed processes for oxygen enrichment have been published. 7. 30 vol% O2.144). Oxygen or oxygen-enriched air is sometimes used in spent acid decomposition furnaces to increase furnace capacity. Increased use of oxygen in smelters is becoming very common and leads to much more concentrated and reactive gases being fed to metallurgical acid plants with reduced sizes. operating conditions and operating costs. The HRS process requires extremely careful control of acid concentration. This reduces the amount of inerts in the gas stream in the furnace and gas puriﬁcation equipment. increased (up to 6. A number of HRS plants have been built or retroﬁtted. low pressure equipment design. 7. recovering the sensible heat of the gas stream and the heat of reaction as steam with pressures up to 1140 kPa (150 psig). or by-product gypsum from phosphate fertilizers is occasionally used to produce sulfuric acid and cement (qv). a number of alloys have very low corrosion rates at temperatures as high as 2008C. the cost of obtaining oxygen (qv) at high concentrations makes its use uneconomical at sulfuric acid plants unless special circumstances exist. Approximately 1 kg of Portland cement is produced for each kilogram of sulfuric acid. In the HRS process. Concentrators for increasing the strength of dilute sulfuric acid by removing water have been used since the . These changes allowed sulfur-burning plants to recover $ 70% of the available energy. Cement Plants. That plant. and 4808C) steam pressure and superheat. nitrogen (145. balance. Because the capital requirement for such installations is approximately six to eight times the cost of an elemental sulfur burning plant. 23 Design changes have included increased gas strength (up to 12% for sulfur plants and 18% SO2 for metallurgical plants). Catastrophic corrosion rates will result if tower concentrations are signiﬁcantly outside of the prescribed range.22 SULFURIC ACID AND SULFUR TRIOXIDE Vol. gypsum. 7. perhaps only one plant of this type operated commercially for a sustained period. This permits higher SO2 throughput and helps both the heat and water balance in the conversion and absorption sections of the plant (147). (Cominco) to use 25 vol% SO2.3. CaSO4Á2H2O. remodeled by Consolidated Mining and Smelting Co. they are uneconomical except under special circumstances. Sulfuric Acid Concentrators. Oxygen-Enriched Processes. and installation of turbogenerators to convert steam to electricity. suction side dry towers.146) is no longer in existence. absorbing towers operate at temperatures as high as 2208C.2 MPa. low temperature economizers. A signiﬁcant development occurred in the mid-1980s with the introduction of the heat recovery system (HRS) developed by MECS (141.4. ie. This requires rapid and proper response to boiler or other leaks that can introduce water to the process. and improved tower packing. dry towers placed on the suction side of the main compressor. Typically.5. Calcium sulfate in the form of anhydrite. The HRS process uses conventional stainless alloys and was made possible by the discovery that > 99% acid. Good general discussions on the sources of heat and the energy balance within a sulfuric acid plant are available (143. but may purify it as well. 23 SULFURIC ACID AND SULFUR TRIOXIDE 23 early days of the industry. is growing. Flash evaporation is sometimes employed as an initial puriﬁcation step (153. and other chemicals. Vacuum evaporation is widely used. In the United States. Alkylate producers have a choice of either a hydroﬂuoric acid or sulfuric acid process. Demand for production of fertilizer in the United States is expected to decline as capacity is added in China and elsewhere. the demand for sulfuric acid for fertilizer and for leaching ores. while all other major uses are growing only slightly or declining. In the United States. thus reducing corrosion. because of heavy sulfuric acid usage by the phosphate fertilizer industry.5 Â 106 metric tons. in water treatment. A two-volume text on this subject was published in 1924 (148). the acid is diluted and partially degraded or contaminated. In the past. Examples of some options available are discussed in Ref. notably copper and nickel. or recycling. Economic Aspects Historically. Modern concentrators frequently not only concentrate the acid. Increased cost and regulation has all but removed the second. Sulfur trioxide. either as liquid or from oleum. Its main advantage is the ability to produce relatively high product acid concentrations at low operating temperatures. > 74% went into phosphate fertilizers (157) as compared to 45% in 1970 and 64% in 1980. more attractive and in many cases even a necessity. small amounts of organic contaminants are frequently partially oxidized and sometimes can be largely removed by treating with small amounts of hydrogen peroxide or nitric acid to accelerate oxidation (156) (see EVAPORATION). use in the production of alkylate for gasoline has grown. concerns over the safety or potential regulation of hydroﬂuoric acid are likely to result in most of the growth being for the sulfuric acid process. pulp and paper. the elimination of MTBE (methyl tertiary-butyl ether) and the increased use of ethanol for gasoline production are increasing the demand for petroleum alkylate. removing both organic and inorganic materials.149–154). plastics. In addition. detergents. This statement is far less valid in 2005. Worldwide. Other uses of sulfuric acid are as a pickling agent for iron and steel. 153. more recent discussions of the subject are also available (34. The need for acid concentrators exists because many uses of sulfuric acid do not lead to its consumption. as a component of lead storage batteries. Concern over contaminants entering the food chain through fertilizer removed the ﬁrst option. and in the production of textile ﬁbers. sulfuric acid consumption in 2001 of 37.155). large amounts of acid were disposed of either by using it in the phosphate fertilizer industry to dissolve phosphate rock or by neutralization and discharge to waterways. inorganic pigments. This has made concentration.Vol. However. consumption of sulfuric acid was a good measure of a country’s degree of industrialization and also a good barometer of general business conditions. Instead. explosives.S. This trend is expected to continue. ﬁnds significant use as a sulfonating or sulfating agent for surfactants. . and other products. 8. Both processes are widely used today. Of total U. the need and means to export sulfur. Production and consumption trends in the United States are shown in Tables 9 and 10. As of 2005. Louisiana. prices had fallen to $11–55/t ($10–50/short ton). The cost and price of sulfuric acid depend in large part on raw material cost and on freight costs. In many areas. Overall these have resulted in a more than ample supply of sulfur. the primary role of sulfuric acid is to convert phosphate rock to phosphoric acid and solid calcium sulfates. the oil reﬁning industry underwent a divesting phase followed by acquisition so that a few major reﬁning entities are responsible for most of the sulfur produced. Wyoming. Sulfuric acid prices are usually quoted per ton of equivalent 100% H2SO4. sulfuric acid industry. Additional areas for growth in the United States are expected to be in copper leaching. First is the continuing growth of oil reﬁning. phosphate fertilizer exports. which are removed by ﬁltration. fertilizer plants are located in Florida. the delivered cost of sulfur is the most important factor affecting sulfuric acid pricing. and North Carolina. As of 1997–2001. but the remote locations of many . costs at smelters are essentially zero. Prices for oleum are typically higher than for nonfuming acid and increase substantially for concentrations > 30% free SO3. which has created a large demand for sulfur offshore.1 Â 109 for 2005.S.S. methyl methacrylate. eg. ie. and batteries (157). Third is the growth of the fertilizer industry in China and elsewhere. 23 Outside the United States. caprolactam. In addition. the amount of sulfur gas emissions from reﬁneries and other sources. and by-product steam from these sulfur burning plants is generally used to support the phosphate process and often to produce electicity. MTBE is not likely to be eliminated. At almost all large fertilizer plants. The sulfur supply will continue to grow in the next decade with the further development of high sulfur bitumen production from Alberta. prices ranged $61–83/t ($55–75/short ton) depending on location within the United States. production was not sold as such. These together have produced more nondiscretionary sulfur in North America than can be used. excess sulfuric acid capacity and a decreased demand for sulfuric acid. By-product raw material. small acid shipments or spot orders typically command a premium vs quoted contract prices in bulk. particularly in China. > 70% of U. Most of the U. even though actual assays may be less or more than 100%. For all acid grades. smelters. Idaho. production was $2.S. and the trend toward increasingly sour feedstocks (see SULFUR REMOVAL AND RECOVERY). Second is the rapid growth outside the United States. Finally. trending toward the eventual reduction in U. By 2001. sulfuric acid is made on site. but the overall growth in gasoline demand and the elimination of the last uses of tetraethyl lead are fostering growth in alkylate production.24 SULFURIC ACID AND SULFUR TRIOXIDE Vol. In mid-1995. Since $ 1995 (through 2005) several factors have had a signiﬁcant impact on the U. they had recovered to $50–77/t ($45–70/short ton). Projected total commercial value of U. facing strong imports of nondiscretionary smelter acid. the increasing need to remove sulfur from products produced from oil. pulp and paper. resulting in the elimination of all Frasch operations and a growing need to export sulfur.S. but used directly by producers to make other materials. has been reduced through more stringent regulations. SO2.S. In the production of phosphate fertilizers. frequently called food-grade acid (although industry-wide. Because iron sulfate is relatively insoluble in concentrated acids. Typical speciﬁcations for several common types or grades of acid are shown in Table 13. Much of this is for increased fertilizer production that will result in a decrease in U. For Western Europe. Eastern Europe and former Soviet Union. which decomposes to both nitrous and nitric compounds. The difﬁculty occurs because nitrogen oxides are usually present as nitrosylsulfuric acid. Exceptions include Federal speciﬁcations for ‘‘Sulfuric Acid. ‘‘food-grade’’ is nonspeciﬁc). This occurs because oleum is produced at relatively low temperatures in the presence of appreciable SO2 in the gas phase. 9. Strong growth is expected in Chile and Peru with the expansion of smelters as well as implementation of stricter environmental controls on existing smelters. It is not possible to strip SO2 from oleums by air blowing. . and is projected to continue growing to > 50 Â 106 metric tons before 2010. owing to increased emphasis on protecting the environment. even at acceptable total iron concentrations. which has extensive phosphate rock deposits. Measurement and speciﬁcation of nitrates and other nitrogen oxide compounds in sulfuric acid is a complex subject. Analysis and Speciﬁcations Speciﬁcations for sulfuric acid vary rather widely between different producers and consumers and for different grades of acid. Technical’’ and ‘‘Sulfuric Acid. The presence of smelter based acid in a given market can seriously affect the price of acid. Very little has been done to establish industry-wide analytical standards in the United States. Similar limits are generally used for other sulfuric acid concentrations. exports as discussed previously. Worldwide sulfuric acid production ﬁgures are shown in Table 11. Worldwide. Nevertheless. supply has declined.Vol. It continues to grow in the Middle East and in Africa. production of nondiscretionary acid is expected to increase. a technique that is frequently applied to product acids of 99% concentration. particularly North Africa. the nondiscretionary nature of smelter acid means that it must be sold if the smelter is to operate. thus leading to high solubility. 23 SULFURIC ACID AND SULFUR TRIOXIDE 25 smelters make freight costs signiﬁcant. but typical values are considerably higher than in acids of 99 wt% concentrations.S. except for development of the ASTM analytical methods. China has undergone explosive growth from $ 30 Â 106 metric tons in 2002 to > 45 Â 106 metric tons in 2005. the turbidities of 98–99% H2SO4 and oleum may be higher than shown. A procedure to measure both types of nitrogen oxide compounds at the same time involves development of a pink color by mixing FeSO4 with sulfuric acid. as third world economies improve. Electrolyte (for storage batteries)’’ and the Food Chemicals Codex speciﬁcation for sulfuric acid. designated as E223-88 and summarized in Table 12. Sulfur dioxide concentrations in oleum are rarely speciﬁed or measured. with the exception of turbidity values for high strength acids (and oleum) and SO2 and niter values in oleums. Hence analytical procedures speciﬁc for nitrates only do not give a complete analysis. Other potential areas for growth include the Middle East and North Africa. organic compounds. particularly with water. there is little standardization in such measurements. and reducing agents. 162–164.17. In addition.166).26 SULFURIC ACID AND SULFUR TRIOXIDE Vol. Descriptions of sulfuric acid analytical procedures not speciﬁed by ASTM are available (34. Federal speciﬁcations also describe the required method of analysis. Sulfuric acid at high concentrations reacts vigorously with water.1.15-171. In some cases. A frequently used procedure is to determine color and turbidity by comparison with standards originally developed by the APHA for examination of water (165). mucosa. Concentrations of 78 and 93 wt% H2SO4 are commonly measured indirectly by determining speciﬁc gravity. 10. 23 followed by measurement or comparison of color intensity. Sulfuric acid is injurious to the skin. or by sonic velocity or other physical property. eg. reagent grade. mercury. Although color and turbidity of acid products are important properties. A number of different grades of acid are produced for specialized uses. Oleums and liquid SO3 frequently react with explosive violence. or in some cases a maximum iron concentration of 10 ppm. Higher acid concentrations are normally determined by titration with a base. Material Safety Data Sheets (MSDS) are available from the U. Health and Safety 10. dangerous amounts of hydrogen may develop in reactions between weakened acid and metals. Shipping and Handling. sufﬁcient amounts of organic ﬂotation agents are volatilized from sulﬁde ores to form brown or black acid. for plant control. Such acid can be used in many applications. hazardous . The Code of Federal Regulations (CFR) includes detailed rules for packaging and shipping. Selenium is not usually present except at a few metallurgical-type plants or at plants using volcanic sulfur as raw material. to minimize possible problems. particularly for fertilizer production. Metallurgical (smelter) plants and spent acid decomposition plants usually produce acid of good (low) color because their SO2 feed gases are extensively puriﬁed prior to use. however. and electronic grade. Sonic velocity has been found to be quite accurate for strength analysis of both fuming and nonfuming acid. Moreover. without signiﬁcant problems arising. Pertinent sections are 49 CFR 171.S. lead. some producers offer special premium priced grades that contain little or no turbidity and color. and European manufactures. but some producers attempt to hold each of these at <1 or <2–5 ppm in the case of lead. food grade. This general type of procedure and possible alternatives are discussed in Refs. and particularly at lead smelters. Certain objectionable elements such as arsenic. Those engaged in handling sulfuric acid should obtain detailed information on safe handling practices. and eyes. and selenium are not commonly speciﬁed for technical grade acid. give artiﬁcial respiration. 49 CFR 172. The acid should be handled only in areas having sufﬁcient ventilation to prevent irritation. Bottom outlets or valves are not allowed. Call a physician. Iron or other solid sulfates may plug lines or retain pockets of acid. preferably mouth to mouth. Tightening ﬂange bolts on pipes ﬁlled with acid is dangerous because of the possibility of mechanical failures. In case of physical contact with sulfuric acid. Sulfuric acid must not come in contact with eyes. .202 and 173. Call a physician. In case of inhalation. immediately ﬂush eyes or skin with plenty of water for at least 15 min while removing contaminated clothing and shoes. Using air pressure to unload is not recommended for safety reasons. nor are internal steam coils. Alternatively. give oxygen. but if air pressure is used. any vessel that has contained sulfuric acid must be thoroughly purged and tested for explosive conditions before welding commences. Acid containers must be kept closed. When diluting. and boots. remove the individual to fresh air. Tank contents must be unloaded via standpipe. and suitable precautions taken. Water must not enter containers.227 and 173. To avoid hydrogen explosions when welding. Wash clothing before reuse and destroy contaminated shoes. hood. Personnel must avoid breathing mist or vapors. When handling containers or operating equipment containing sulfuric acid.227 and 173. rubber acid proof gloves. Drums. face shield and chemical splash goggle combination (not face shield alone). An emptied container retains vapor and product residue. Thus. If breathing is labored.Zone B Corrosive label Corrosive label Corrosive and Poison labels Corrosive and Poison labels Requirements for transport in ships or barges are outlined in Section 46 of the CFR. hazardous material tables. it should always be assumed that a spray of acid may occur.244 Poison Inhalation Hazard . and the following references for speciﬁc materials: Sulfuric acid Oleum (<30%) Oleum (!30%) Sulfur trioxide 49 CFR 173.21 MPa (30 psi).Vol. skin. an appropriate NIOSH/MSHA approved respirator should be worn. gauge pressures should be held at <0. If necessary. 23 SULFURIC ACID AND SULFUR TRIOXIDE 27 material incidents and discharges.101-172. should be periodically vented to prevent accumulations of hydrogen. all labeled safeguards must be observed until the container is cleaned. Handling containers and pipelines also requires that special precautions be observed. are usually employed for high concentrations of sulfuric acid. Steel tank cars. always add the acid slowly with agitation to the surface of the aqueous solution to avoid violent spattering. reconditioned or destroyed. This may include chemical splash goggles. or clothing. often lined to minimize iron contamination.201 and 173. Never add the solution to the concentrated acid.102. boiling and eruption.243 49 CFR 173. if not self-venting.Zone B 49 CFR 173. The following general handling precautions should be observed: General handling precautions should be observed. equipment appropriate for exposure conditions should be worn. In dismantling lines and equipment.242 49 CFR 173.244 Poison Inhalation Hazard . and a full acid proof suit. and previous exposure. residual bronchiectasis. or irritation. The International Agency for Research on Cancer (IARC) has classiﬁed ‘‘occupational exposure to strong inorganic acid mists. ‘‘Thus. Sulfuric acid aerosol or mist is a signiﬁcantly more powerful pulmonary irritant than sulfur dioxide on a sulfur-equivalent basis (167–169). The American Conference of Governmental Hygienists (ACGIH) has classiﬁed ‘‘sulfuric acid containing strong inorganic mists’’ as a Suspected Human Carcinogen. the current epidemioligic data alone do not warrant classifying MSA (mists containing sulfuric acid) as a deﬁnite human carcinogen. This study concluded. a substance that is carcinogenic to humans (177).’’ The overall weight of evidence from animal studies has been evaluated (180). For a constant sulfuric acid aerosol concentration.’’ as a Category 1 carcinogen.182). This study concluded. Studies of prolonged exposure to sulfuric acid fumes have been performed on workers in plants manufacturing lead acid batteries. It rapidly combines with moisture in air to form sulfuric acid mist. protected against freezing. the acid removed by vacuum truck. A TLV of 0.2. Physiological responses to sulfuric acid mist inhalation are highly dependent on the particle size of the aerosol (167. and the spill area ﬂushed with water. Safety showers with deluge heads. Washings should be neutralize with lime or soda ash and pollution control authorities notiﬁed of any runoff into streams or sewers and of any air pollution incidents.’’ The OSHA Permissible Exposure Level (PEL) and the NIOSH Recommended Exposure Limit (REL) for human exposure to sulfuric acid mist are 1 mg/m3 of air (181. keep people away and upwind of the spill. taste. If it is necessary to enter the spill area. should be readily available at appropriate locations in any plant producing or using sulfuric acid. The area should be diked using sand or earth to contain the spill. wear self-contained breathing apparatus and full protective clothing.176). Other factors affecting the physiological response of inhaling sulfuric acid mist are humidity. chronic pulmonary ﬁbrosis. 10. Sulfuric Acid Toxicity. Overexposure to sulfuric acid aerosols results in pulmonary edema. Sulfur trioxide does not exist in the atmosphere except in trace amounts because of its afﬁnity for water.2 mg/m3 is recommended by the ACGIH to prevent pulmonary irritation at particle sizes likely to occur in .169–171). temperature. containing sulfuric acid. although investigations were compromised due to inadequate design and reporting.28 SULFURIC ACID AND SULFUR TRIOXIDE Vol. Additional toxicological information may be found in the literature (175. the irritant action of the aerosol increases with aerosol particle size. The overall weight of evidence from all human studies has been evaluated (179). and pulmonary emphysema. Sulfuric acid aerosols below the TLV are commonly not detected by odor. Prolonged exposure to mineral acid fumes causes the teeth of the exposed subject to deteriorate (172–174). including boots. These classiﬁcations are highly controversial. ie. The ACGIH Threshold Limit Value (TLV) for sulfuric acid mist is 0. ‘‘No evidence of carcinogenic potential was found in these studies.2 mg/m3. (category A2) (178). These classiﬁcations are for mists only and do not apply to sulfuric acid or sulfuric acid solutions. 23 In case of spill or leak. A Two-Stage Process Combination for the Production of Sulfuric Acid or Oleum with Next to Zero SO2 Emissions. Chem. Eng. 10 mg/m3. posting date: December 4. Inc. 1931).. 1998). L. Berechte 34. Allied Chemical Corp. Harrer. Salamone. L. Moller (to Bayer A. U. in ECT 2nd ed. in ECT 4th ed. ERPG-3. 30 (Oct. 2. 23. 53 (May–June 2000).5–2. I. Pat.0–4. C. 1981). pp. W. in ECT 3rd ed. Inc. General Chemical Division. 19.. 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Fairfax. THOMAS L. I. MULLER E. 183. 43 To convert kPa to psi.07 0.25666 2. g/cm3 liquid coefﬁcient of thermal expansion (at 188C). multiply by 0. 8C liquid density (g-phase at 208C).0 1. divide by 4. 8C melting point (g-phase).5843 0.11 negligible References 35 35 35 32 29 29 29 32 34 36 37 34. kJ/(kg-8C)b heat of formation of gas (at 258C).145.6 94. m/s liquid dielectric constant (at 188C) electric conductivity a b Value 217. (MJ-kg)/(mol-8C)b heat of dilution.36 SULFURIC ACID AND SULFUR TRIOXIDE Vol.8 16. g/cm3 solid density (g-phase at À108C).109 324.000013 3. MJ/kgb (a) (b) (g) heat of vaporization (g liquid).8 21. Properties of Sulfur Trioxide Property critical temperature.76 À371. 8C critical pressure.07 0. per 8C liquid heat capacity (at 308C).9224 2.8 8208 0.184. kPaa normal boiling point temperature.38 39 39 39 34 34 34 34 32 32 32 32 40 41 42.002005 3.8518 0. (MJ-kg)/molb free energy of formation of gas (at 258C). MJ/kg diffusion in air (at 808C). g/cm3 triple point temperature (g-phase).630 16. MJ/kgb heat of fusion. kJ/kgb (a) (b) (g) heat of sublimation.0 151. kPaa critical density. 23 Table 1.13 44.8 À183. To convert J to cal.222 À395.7029 0.29 0. 8C transition temperature. (MJ-kg)/molb entropy of gas (at 258C). .7269 0. 8C triple point pressure (g-phase). 005686 0. 34. Vapor Pressure of SO3.660 400.148.30 À23.2 À293. PaÀ1/2 c 13.9 À277. b-.5 À454.Vol. Thermodynamic Properties of Sulfur Trioxidea Temperature. 23 SULFURIC ACID AND SULFUR TRIOXIDE Table 2.000 Ref. Thermodynamic Properties of SO2 þ 0.13 0.000 b 4. divide by 4.860 126.9 À456. Table 4.000 g 5.730 126. c To convert PaÀ1/2 to atmÀ1/2. 51.7445 0. kJ/molb À460. a-.05 À13.3 À326.7 À455.5 Ref. kJ/molb À41.7 À261.148.700 400. K 600 700 800 900 1000 1100 1200 a b DHT.69 À94.999 57.57 À95.002025 0. divide by 6895.02 12.99 À97.94 4.1006 0.51 À92. Pab Temperature. To convert J to cal. divide by 4. multiply by 318.5 O2 ! SO3a Temperature. To convert J to cal. and c-Phasesa Vapor pressure.700 400.59 À32.32.97 À4.1 DFf 8. kJ/molb À97.36 À96.30 DFT. kJ/molb À359. 8C 0 25 50 75 a b 37 a 773 9. 51.2 À459. K 600 700 800 900 1000 1100 1200 a b DHf 8.60 À93.02033 0.266 45.6 À458.000867 Ref. To convert Pa to psi.9 À343.84 Kp.732 86. .9 À457. Table 3.7 À310. 106 Ref.129 1.56 20.83 Ref.009 0. To convert J to cal. MJ/kgb 7.384 9.547 1.17 12. To convert J to cal.6716 0.340 1.632 5.434 5. MJ/kgb 1.086 5.107 1. MJ/kgb 4.919 5. . x ¼ mol total SO3/mol H2O. % 0 10 20 30 40 50 60 70 80 90 100 a b SO3.236 1.311 7.654 7.317 6.841 6.9067 0. x ¼ mol total SO3/mol H2O.6978 0. Table 6. Heat of Formation of Oleums by the Reaction:H2O(liq) þ x SO3(gas) ! H2SO4 Á (1Àx )SO3(liq)a Liquid phase Free SO3. 95.986 11.645 1.803 8.5905 0.445 1. 23 SO3.1333 0 H2O. multiply by 4.260 5.2535 0.79 35.3684 0.5383 H2O. 95.38 SULFURIC ACID AND SULFUR TRIOXIDE Table 5.184.7918 0. multiply by 4.184.84 15.4808 0. MJ/kgb 1.8022 0.061 6.9067 0. Heat of Formation of Oleums by the Reaction:H2O(liq) þ xSO3 (liq) ! H2 SO4 Á (1Àx )SO3 (liq)a Liquid phase Free SO3. % 0 10 20 30 40 50 60 70 80 90 100 a b Vol.019 0.080 9. 23 SULFURIC ACID AND SULFUR TRIOXIDE Table 7. 157. 136. 8C 1 415–420 625 185 2 430–445 530 90 3 430–445 470 30 4 390–430 450 20 39 Table 8. 8C 220 25 25 1–1.423 4.331 742 45.073 1. Sulfuric Acid (100% H2SO4) Production by Sulfur Source. vol% SO2 SO2 H2 S a Required inlet temp. Typical Converter Conditions for a 3 þ 1 Double Absorption Plant with 11.255 1999 32. 8C DT.559 384 36. Typical Gas Compositions Treated by WSA Without Supplemental Heata Range. Table 9.670 3. U.195 361 202 4.5% SO2 Inlet Gas Converter pass inlet.Vol.602 2001 30.457 379 214 3.5 2–2. 8C outlet.332 Ref.a 1000 metric tons Sulfur source elemental sulfur copper smelter gas zinc smelter gas lead and molybdenum smelter gas recycled acid other total a 1997 35.S. .415 467 40.5 !1 Ref.835 337 144 3. Table 11. 158.536 1.S.439 976 933 547 455 1. 106 metric tons Location North America Central and South America Western Europe Eastern Europe Asia Africa Oceania total world a b 1993a 41.047 811 528 454 420 345 115 2.3 157.444 998 918 580 484 422 350 131 1.1 16.a 1000 metric tons Use phosphoric acid other fertilizers copper leaching uranium and vanadium ore processing other ore processing petroleum alkylation methyl methacrylate caprolactam aluminum sulfate hydroﬂuoric acid pulp and paper titanium dioxide cellulosic ﬁbers and plastics all other total a 1997 32. .7 67. Ref.9 164. 23 Table 10.536 Ref.6 33.372 478 1.374 37.023 333 175 5.093 1999 32. World Production of Sulfuric Acid (100% H2SO4).0 19.216 497 2.336 15 334 1.815 2001 27.4 1. 157.5 16.3 135.2 17.6 15.3 1997b 49. Consumption of Sulfuric Acid (100% H2SO4).4 18.6 2001b 40.40 SULFURIC ACID AND SULFUR TRIOXIDE Vol.3 6.0 24.7 4.3 18.6 2.910 6 140 1. U.1 3.1 3. 157.333 537 2.144 41.814 47.221 24 229 1.0 17.8 Ref.5 13. add excess iodine. absorb in pyridine and diethylthiocarbamate. measure color Ref. titrate evolve as arsine. 23 SULFURIC ACID AND SULFUR TRIOXIDE 41 Table 12. reagents total acidity ´ Baume gravity nonvolatile matter iron sulfur dioxide arsenic Principle NaOH titration using phenophthalein hydrometer measurement evaporation and weighing reduce. (see references) 1–7 8–16 17–26 27–33 34–43 44–51 52–61 a Properties general. . ASTM Analytical Methods for Sulfuric Acida Section No.Vol. 159. measure colorimetrically as o-phenanthroline complex remove by air sweep. absorb in alkali. Spec.835–1. zinc.03 1. 23 Table 13.58C nonvolatiles (max). copper. g APHA ¼ American Public Health Association. Limits are also speciﬁed for platinum. % As (max).8347 0.2 1.2 1.03 3 40f 200 20 10 40–80 50–100 5–20 100–200 APHAg Ref. 15.5/15. ppm iron (max). and chloride. O-S-801E) Property % H2SO4 sp gr. O-S-809E) Class 1d 93.42 SULFURIC ACID AND SULFUR TRIOXIDE Vol. d One other class of lower strength acid is included. ppm heavy metals (max) ppm nitrate (max).0 per test Food Chemicals Codexe Technical (industry typical) ´ 668Be (93%) 93.837 0. 161. .025 2.0 1.02–0. nickel. ppm color (max) a b b Technical (Fed. ppm SO2 (max). e Ref. 160. organics. ammonium.5 Class 1c 93. f Reducing substances (as SO2). Typical Sulfuric Acid Speciﬁcationsa Acid type Electrolyte (Fed.8354 0. c Three other classes of lower strength acids are included. selenium. antimony. Spec.0 40 50 5. manganese. (– – –). Speciﬁc gravity of sulfuric acid. 2. g-SO3.2 1. wt % 90 100 Fig.000 100 10 1 0.0 0 25 SULFURIC ACID AND SULFUR TRIOXIDE 43 Liquid Critical point Saturated vapor 50 75 100 125 150 175 200 225 Temperature. 3.4 0. multiply by 0. .4 1.8 1.6 0. (To convert kPa to psi. Density of sulfur trioxide.) 1.000 1.9 1.0 1. 10.1 –50 Vapor pressure. a-SO3 (49).145.0 0.1 1 0 10 20 30 40 50 60 70 80 Sulfuric acid in water.7 Specific gravity 1. °C Fig.5 1.8 1.4 1. °C Fig.3 1. Vapor pressure of sulfur trioxide: (—). kPa 0 50 100 150 200 Temperature. b-SO3. (:::::::::).Vol. 1. g/cm3 1.8 0. where * represents the critical point (44–48).6 1.2 0. 15/48C (53).6 Density. 23 2.2 1. .5 1.4 1. 4.3 1.8 1. Speciﬁc conductance of sulfuric acid (54). °C 80 90 100 Fig.2 1. 180 160 Specific conductance.6 1. S/m 140 120 100 80 60 40 20 0 0 10 20 50 60 70 30 40 Sulfuric acid in water. where the numbers represent wt% of H2SO4 in H2O (53). g/cm3 60 50 40 30 20 0 10 20 30 40 50 60 70 Temperature.7 1.44 SULFURIC ACID AND SULFUR TRIOXIDE 1. 5. Density of sulfuric acid. wt % 80 90 100 104°C 93°C 82°C 71°C 60°C 49°C 38°C 27°C 16°C 4°C –7°C –18°C Fig.9 100 1.9 H 2O 90 80 Vol.1 10 1. 23 70 Density.0 0. Vol. 7. S/m 40 35 30 25 20 15 10 5 0 90 91 92 SULFURIC ACID AND SULFUR TRIOXIDE 45 90°C 80°C 70°C 60°C 50°C 40°C 30°C 20°C 93 94 95 96 97 Sulfuric acid in water. wt % 98 99 100 Fig. 6. wt % 90 95 100 Fig. 25 25°C 20 Viscosity. Viscosity of sulfuric acid (58). mPa⋅s (= cP) 15 45°C 10 60°C 5 80°C 0 55 60 65 70 75 80 85 Sulfuric acid in water. Speciﬁc conductance of sulfuric acid (55). 23 50 45 Specific conductance. . 46 SULFURIC ACID AND SULFUR TRIOXIDE 80 0°C Surface tension. 9. 23 0 10 20 30 40 50 60 70 Sulfuric acid in water. . wt % 80 90 100 10°C Fig. % 10 8 20°C 6 4 2 0 0 10 20 30°C 40°C 50°C 80°C 110°C 30 40 50 60 70 Sulfuric acid in water. 8. Solubility of SO2 in sulfuric acid at SO2 pressure of 101. mN/m (=dyn/cm) 75 30°C 70 65 60 55 50 45 50°C Vol. Surface tension of sulfuric acid (64).3 kPa (1 atm) (71). wt % 80 90 100 Fig. 16 14 12 Solubility of SO2. °C 600 550 500 450 400 350 300 0 10 SULFURIC ACID AND SULFUR TRIOXIDE 47 20 30 40 50 60 70 80 Sulfuric acid in water. wt % 90 100 Fig. 20 10 0 Temperature. 10. 12. Normal boiling point of sulfuric acid (77). wt % 90 100 Fig. . °C 250 200 150 100 50 0 10 20 30 40 50 60 70 80 Sulfuric acid in water.Vol. 350 300 Temperature. Melting points of sulfuric acid (78). wt % 90 100 Fig. °C –10 –20 –30 –40 –50 –60 –70 –80 0 10 20 30 40 50 60 70 80 Sulfuric acid in water. Critical temperatures of sulfuric acid solutions (76). 23 700 650 Critical temperature. 11. To convert J to cal. kJ/mol of H2SO4 Fig.01 0.48 SULFURIC ACID AND SULFUR TRIOXIDE 1. 14.000. 13. . wt % (% oleum) Heat of dilution. Heat of mixing of sulfuric acid from H2O and H2SO4 at 258C (87).000 10. wt % Fig. °C 200 150 100 50 0 0 10 20 30 40 50 60 70 80 90 100 Free SO3. 300 250 Boiling point.000 100.1 0. 15. To convert Pa to mmHg. 23 100°C Fig. 100 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 Sulfuric acid in water. wt % 90 100 20°C 40°C 60°C 80°C Vol.0075.001 0 10 20 30 40 50 60 70 80 Sulfuric acid in water. divide by 4.000 100 10 1 0.184. Normal boiling points of oleum (90). Pa 1. multiply by 0.000 Vapor pressure. Vapor pressure of sulfuric acid (84). 16. 23 40 30 Temperature. 60 50 Viscosity. Viscosity of oleum (58). wt % (% oleum) Fig.6 Heat capacity. 17. wt % (% oleum) Fig.184.4 0 10 20 30 40 50 60 70 80 90 100 Free SO3.4 2.8 1. . Heat capacity of oleum at 208C (79).2 2 1. wt % (% oleum) 80 90 Fig.Vol. Melting points of oleum. To convert J to cal. References 78 (<65%) and 91 (>65%). mPa⋅s (= cP) 40 30 45°C 20 60°C 10 80°C 0 0 10 20 30 40 50 Gas phase 60 70 80 90 100 25 °C Solid phase 25°C Solid phase Free SO3.8 2. divide by 4. 18. °C 20 10 0 –10 –20 SULFURIC ACID AND SULFUR TRIOXIDE 49 0 10 20 30 40 50 60 70 Free SO3. 2. kJ/(kg⋅K) 2.6 1. kPa 10 1 0 10 20 30 40 50 60 70 Free SO3.50 SULFURIC ACID AND SULFUR TRIOXIDE 10. To convert kPa to psi. wt % (% oleum) 80 90 Fig. multiply by 0.000 280°C 1. . 19. 23 Vapor pressure.145.000 240°C 200°C 160°C 120°C 80°C 100 40°C Vol. Vapor pressure of oleum (97). Flow sheet for a 3 þ 1 double absorption sulfur burning plant. 2 waste heat boiler 51 Drying tower Sulfur Main blower Air Sulfur burner Dilution water Economizer superheater Converter Product cooler Hot interpass exchanger To atmosphere No.Vol. 1 waste heat boiler Cold interpass exchanger Dry tower pump tank Interpass absorbing tower Final absorbing tower 93% Acid to storage Economizer Dilution water Interpass tower pump tank Product cooler 99% Acid to storage Dilution water Final tower pump tank Fig. 20. 23 SULFURIC ACID AND SULFUR TRIOXIDE No. . Single absorption equilibrium-stage diagram.52 SULFURIC ACID AND SULFUR TRIOXIDE 100 90 80 70 Conversion.5 99.9% O2. where the equilibrium curve is for 8% SO2. 21.0 380 390 400 410 Temperature.5 97. % 98.0 98. 22. where no appreciable conversion occurs.0 99.5 95.0 97. the diagonal lines represent the adiabatic temperature rise of the process gas within each converter pass. °C 420 430 Fig. .5 Conversion.0 96. Equilibrium stage diagram the ﬁnal pass of a 3 þ 1 dual absorption acid plant. 100. the horizontal lines represent gas cooling between passes. % 60 50 40 30 20 10 0 400 1st pass 2nd pass 4th Vol. °C 600 650 Fig.0 3rd pass 4th pass Equilibrium curve before interpass absorption Equilibrium curve after interpass absorption 95. 23 3rd pass 450 500 550 Temperature.5 96. 12.
Report "30122794 Sulfuric Acid and Sulfur Trioxide"