Preface This book is the culmination of about ten years of studying sulfuric acid plants. Its objectives are to introduce readers to sulfuric acid manufacture and to show how acid production may be controlled and optimized. One of the authors (MJK) operated an acid plant while writing this book. His Ph.D. work also centered on analyzing sulfuric acid manufacture. He is now a sulfuric acid and smelter specialist with Hatch. The other author (WGD) has been interested in sulfuric acid plants since his 1957 student internship at Cominco's lead/zinc smelter in Trail, British Columbia. Cominco was making sulfuric acid from lead and zinc roaster offgases at that time. It was also making ammonium sulfate fertilizer. In the book, we consider SO2(g) to be the raw material for sulfuric acid manufacture. Industrially it comes from: (a) burning elemental sulfur with air (b) smelting and roasting metal sulfide minerals (c) decomposing spent acid from organic catalysis. These sources are detailed in the book, but our main subject is production of sulfuric acid from SO2(g). Readers interested in smelting and roasting offgases might enjoy our other books evitcartxE ygrullateM of reppoC (2002) and hsalF gnitlemS (2003). The book begins with a 9 chapter description of sulfuric acid manufacture. These chapters introduce the reader to industrial acidmaking and give reasons for each process step. They also present considerable industrial acid plant operating data. We thank our industrial colleagues profusely for so graciously providing this information. The book follows with a mathematical analysis of sulfuric acid manufacture. It concentrates on catalytic SO2(g) + (cid:1)89 )-- 3OS oxidation. It also examines temperature control and production of )g(4OS2H from SO3(g). We have tried to make our analysis completely transparent so that readers can adapt it to their own purposes. We have used this approach quite successfully in our examinations of several metallurgical processes. We hope that we have also succeeded here. vi We have used Microsoft Excel for all our calculations. We have found it especially useful for matrix calculations. We also like its Goal Seek, Visual Basic and Chart Wizard features. All the Excel techniques used in this book are detailed in our forthcoming book Excel for Freshmen. Please note that, consistent with Excel, we use (cid:12)9 for multiply throughout the book. A note on units- we have used SI-based units throughout. The only controversial choice is the use of K for temperature. We use it because it greatly simplifies thermodynamic calculations. We use bar as our pressure unit for the same reason. Lastly we use Nm 3 as our gas volume unit. It is 1 m 3 of gas at 273 K and 1 atmosphere (1.01325 bar) pressure. 22.4 Nm 3 contain 1 kg-mole of ideal gas. We were helped enormously by our industrial colleagues during preparation of this book. We thank them all most deeply. As with all our publications, Margaret Davenport read every word of our typescript. While she may not be an expert on sulfuric acid, she is an expert on logic and the English language. We know that if she gives her approval to a typescript, it is ready for the publisher. We also wish to thank George Davenport for his technical assistance and Vijala Kiruvanayagam of Elsevier Science Ltd. for her unflagging support during our preparation of this and other books. Lastly, we hope that our book Sulfuric Acid Manufacture brings us as much joy and insight as Professor Dr von Igelfeld's masterpiece Portuguese Irregular Verbs # has brought him. William G. Davenport Matthew J. King Tucson, Arizona Perth, Western Australia # See, for example, At eht alliV of Reduced ,secnatsmucriC Anchor Books, a Division of Random House, Inc., New York (2005), p63. CHAPTER 1 Overview Sulfuric acid is a dense clear liquid. It is used for making fertilizers, leaching metallic ores, refining petroleum and for manufacturing a myriad of chemicals and materials. Worldwide, about 180 million tonnes of sulfuric acid are consumed per year (Kitto, 2004). The raw material for sulfuric acid is 2OS gas. It is obtained by: (a) burning elemental sulfur with air (b) smelting and roasting metal sulfide minerals (c) decomposing contaminated (spent) sulfuric acid catalyst. Elemental sulfur is far and away the largest source. Table 1.1 describes three sulfuric acid plant feed gases. It shows that acid plant 2OS feed is always mixed with other gases. Table 1.1. Compositions of acid plant feed gases entering 2OS oxidation 'converters', 2005. ehT gases may also contain small amounts of 2OC ro SO3. The data era from the industrial tables ni Chapters 3 through .9 Sulfur burning Sulfide mineral Spent acid decom- furnace smelters and roasters ,position furnace Gas volume % SO 2 11 01 9 0 2 01 11 11 2N 97 97 67 Sulfuric acid is made from these gases by: (a) catalytically reacting their SOz and 2O to form SO3(g) (b) reacting (a)'s product )g(3OS with the )g(02H in 98.5 mass% ,4OS2H 5.1 mass% 02H sulfuric acid. Industrially, both processes are carried out rapidly and continuously, Fig. 1.1. Fig. 1.1. Schematic of sulfur burning sulfuric acid plant, courtesy Outokumpu OYJ www.outokumpu.com The main components are the catalytic 2OS + (cid:1)89 ~-- 3OS 'converter' (tall, back), twin 4082H making ('absorption') towers (middle distance) and large molten sulfur storage tank (front). The combustion air filter dna air dehydration ('drying') tower are on the right. The sulfur burning furnace is hidden behind. Catalytic converters are typically 21 m diameter. 1.1 Catalytic Oxidation of S02 to S03 0 2 does not oxidize SO2 to 3OS without a catalyst. All industrial SO2 oxidation is done by sending 2OS bearing gas down through 'beds' of catalyst, Fig. 1.2. The reaction "si 700-900 K 1 SO2(g) + -- O2(g) ~ SO3(g) 2 in dry ,2OS ,2O ni feed gas catalyst in ,3OS 2OS (1.1). 2N feed gas ,2O 2N gas It is strongly exothermic (AH ~ ~ -100 MJ per kg-mole of SO3). Its heat of reaction provides considerable energy for operating the acid plant. Fig. 1.2. Catalyst pieces ni a catalytic 2OS oxidation 'converter'. Converters m are --15 high dna 21 m ni diameter. They typically contain four, 98(cid:1) m thick catalyst beds. SO2-bearing gas descends the bed at--3000 Nm 3 per minute. Individual pieces of catalyst are shown in Fig. 8.1. They are-~0.01 m ni diameter dna length. 1.1.1 Catalyst At its operating temperature, 700-900 K, SO 2 oxidation catalyst consists of a molten film of V, K, Na, (Cs) pyrosulfate salt on a solid porous SiO2 substrate. The molten film rapidly absorbs )g(2OS and Oz(g) - and rapidly produces and desorbs SO3(g), Chapters 7 and 8. 1.1.2 Feed gas drying Eqn. (1.1) indicates that catalytic oxidation feed gas is always dry .# This dryness avoids: (a) accidental formation of 4OS2H by reaction of )g(02H with the )g(3OS product of catalytic SOz oxidation (b) condensation of the 4OS2H in cool flues and heat exchangers (c) corrosion. The HzO(g) is removed by cooling/condensation (Chapter 4) and by dehydration with HzSO4(g), Chapter 6. A # llams tnuoma of ciruflus dica si edam yb tew .sisylatac This si dessucsid ni noitceS 9.1 dna retpahC .52 1.2 H2SO 4 Production Catalytic oxidation's SO3(g) product is made into H2SO4 by contacting catalytic oxidation's exit gas with strong sulfuric acid, Fig. 1.3. The reaction is: 350-380 K SO3(g) H20(g) >-- )~(4OS2H ni ,3OS ,2OS ni 98.5% ,4OS2H ni strengthened (1.2) ,2O 2 N sag %5.1 H20 sulfuric acid sulfuric acid AH ~ ~- 130 MJ per kg mole of SO .3 Reaction (1.2) produces strengthened sulfuric acid because it consumes H20(Q and makes HzSO4(g). )g(4OS2H is not made by reacting )g(3OS with water. This is because Reaction (1.2) is so exothermic that the product of the SO3(g) + HzO(g) ~-- H2SO4 reaction would be hot OSzH 4 vapor- which is difficult and expensive to condense. The small amount of H20(t) and the massive amount of )t(4OS2H in Reaction (1.2)'s input acid avoids this problem. The small amount of H20(g) limits the extent of the reaction. The large amount of )g(4OSzH warms only 25 K while it absorbs Eqn. (1.2)'s heat of reaction. Fig. 1.3. Top of gnikam-4OS2H ('absorption') tower, courtesy Monsanto Enviro-Chem Systems, Inc. www.enviro-chem.com The tower si packed with ceramic saddles. 98.5 mass% ,4OS2H 5.1 mass% 02H sulfuric acid si distributed uniformly across this packed bed. Distributor headers and 'downcomer' pipes era shown. ehT acid flows through slots ni the downcomers down across the bed (see buried downcomers below the right distributor). It descends around the saddles while SO3-rich sag ascends, giving excellent gas-liquid contact. The result is efficient 4OS2H production by Reaction (1.2). A tower si 7~- m diameter. stI packed bed si -4 m deep. About 52 3 m of acid descends per minute while 3000 Nm 3 of gas ascends per minute. 1.3 Industrial Flowsheet Fig. 1.4 is a sulfuric acid manufacture flowsheet. It shows: (a) the three sources of SO 2 for acid manufacture (metallurgical, sulfur burning and spent acid decomposition gas) (b) acid manufacture from SO 2 by Reactions (1.1) and (1.2). (b) is the same for all three sources of SO .2 The next three sections describe (a)'s three 2OS sources. 1.4 Sulfur Burning About 70% of sulfuric acid is made from elemental sulfur. All the sulfur is obtained as a byproduct from refining natural gas and petroleum. The sulfur is made into SO 2 acid plant feed by: melting the sulfur spraying it into a hot furnace burning the droplets with dried air. The reaction :si 0041 K S(g) + 02(g) ~-- SO2(g) (1.3) ni ria ni ,2OS ,2O 2N sag HA ~ ~ -300 MJ per kg-mole of S(g). Very little SO3(g) forms at the 1400 K flame temperature of this reaction, Fig. 7.4. This explains Fig. 1.4's two-step oxidation, i.e.: (a) burning of sulfur to SO 2 then: (b) catalytic oxidation of SO 2 to SO3, 700 K. The product of sulfur burning is hot, dry 802, 02, 2N gas. After cooling to -700 K, it is ready for catalytic 2OS oxidation and subsequent H2SO4-making. 1.5 Metallurgical Offgas SO2 in smelting and roasting gas accounts for about 20% of sulfuric acid production. The 2OS is ready for sulfuric acid manufacture, but the gas is dusty. If left in the gas, o E b---- (cid:12)9 -- ~0 -~~ ,-.,.,.,....,~ E .o _ ~- ,J ._ ".~ x:E ~ + . L,, "2 o ~'~ .~ ~ _~C~ ~ O ~,~ n _~__~ N Z ._ o ~ c- Q; N ._ ~ ~' '- ' ? ~ "- i.,.. .,,,..,,,: t~l i.~ o -6 .~- t~ 0 v "~ .... I~.'._.;I. ,. "f:':-,: __ 0 o "6 o ~. o ~ " ..-.. z ~ o: Q) i_ . ~ ~+~t '~ ~. :~ (cid:12)9 ~ I o 2 + ._. O0 .-. ~ ~ / ~ z ~-~ e-- ~:~ {~ ~ . ID ~.- o~ ~ tO "~ ,.~ ~o ~ O ~ ~ --,~ Eo m~ ~e m o m O z~ I... ~'~ o ~ r ~ ,-~ o m ~ ~ 7_ the dust would plug the downstream catalyst layers and block gas flow. It must be removed before the gas goes to catalytic zOS oxidation. It is removed by combinations :fo (a) settling in waste heat boilers (b) electrostatic precipitation (c) scrubbing with water (which also removes impurity vapors). After treatment, the gas contains -1 milligram of dust per Nm 3 of gas. It is ready for drying, catalytic 2OS oxidation and 4OS2H making. 1.6 Spent Acid Regeneration A major use of sulfuric acid is as catalyst for petroleum refining and polymer manufacture, Chapter .5 The acid becomes contaminated with water, hydrocarbons and other compounds during this use. It is regenerated by: (a) spraying the acid into a hot (-1300 K) furnace- where the acid decomposes to SO2, 0 2 and H20(g) (b) cleaning and drying the furnace offgas (c) catalytically oxidizing the offgas's 2OS to 3OS (d) making the resulting )g(3OS into wen )g(4OS2H by contact with strong sulfuric acid, Fig. 1.4. About 10% of sulfuric acid is made this way. Virtually all is re-used for petroleum refining and polymer manufacture. 1.7 Sulfuric Acid Product Most industrial acid plants have three flows of sulfuric acid - one gas-dehydration flow and two H2SO4-making flows. These flows are connected through automatic control valves to: (a) maintain proper flows and 4OS2H concentrations in the three acid circuits (b) draw off newly made acid. Water is added where necessary to give prescribed acid strengths. Sulfuric acid si sold in grades of 39 to 99 mass% 4OS2H according to market demand. The main product in cold climates is-94% 4OS2H because of its low (238 K) freezing point (Gable et al., 1950). A small amount of oleum (H2804 with dissolved )3OS is also made and sold (BASF, 2005). Sulfuric acid is mainly shipped in stainless steel trucks, steel rail tank cars (DuPont, 2003) and double-hulled steel barges and ships (Barge, 1998; Bulk, 2003). Great care si taken to avoid spillage. 1.8 Recent Developments The three main recent developments in sulfuric acidmaking have been: (a) improved materials of construction, specifically more corrosion resistant materials (Salehi and Hopp, 2001, 2004; Sulphur, 2004) (b) improved SO2(g) + (cid:1)89 ~ SO3(g) catalyst, specifically V, Cs, K, Na, ,S O, 2OiS catalyst with low activation temperatures (Hansen, 2004) (c) improved techniques for recovering the heat from Reactions (1.1), (1.2) and (1.3) (Puricelli et al., 1998). All of these improve OS2H 4 and energy recovery. 9.1 Alternative Process An alternative to the conventional acidmaking described here si Wet Sulfuric Acidmaking (Laursen, 2005; Topsoe, 2005; WSA, 2005). This process: (a) catalytically oxidizes the 208 in H20(g), 802, ,20 2N gas (b) condenses )g(4OS2H directly from the gas. It is described in Chapter 25. In 2005, it is mainly used for low flow, low% 2OS gases. It accounts for 1 or 2% of world 4OS2H production. Development of a large, rapid-heat-removal condenser will likely widen its use. I.I0 Summary About 081 million tonnes of sulfuric acid are produced/consumed per year. The acid is used for making fertilizer, leaching metal ores, refining petroleum and for manufac- turing a myriad of products. Sulfuric acid is made from dry ,2OS ,20 2N gas. The gas comes from: