# Heavy inorganics: salt, sulphur, nitrogen and phosphorus: Sulphur and sulphuric acid: the workhorse acid Source page: https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p1/ [Understanding Chemicals](https://www.materialinsights.org/learn/chemicals/) / [Module 4: Heavy inorganics: salt, sulphur, nitrogen and phosphorus](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/) Part 1 of 5 ## Sulphur and sulphuric acid: the workhorse acid Sulphuric acid is made in larger tonnage than any other chemical, yet almost none stays in a final product. Its economics turn on three questions: where the sulphur comes from, how completely the sulphur dioxide is converted, and how much of the plant's heat can be sold as steam and power. ### 1.1 Most sulphur is now a by-product of cleaning oil and gas, so supply ignores demand Sulphur reaches an acid plant by three routes. Elemental sulphur is the cleanest feed. Pyrites (iron sulphide) can be roasted in fluid beds at 850-940 C to give a gas of 7-14% sulphur dioxide (SO2). Copper, zinc and lead smelters also give off SO2, which they must capture anyway. Kent (2012) shows the shift. Mined sulphur fell from 3.2 million tonnes in 1999 to 0.3 million in 2009. Sulphur recovered from oil and gas rose from 37.5 to 46.0 million tonnes over the same decade. Recovered sulphur comes from the Claus process. Hydrogen sulphide (H2S) stripped from sour gas or refinery streams is partly burned to SO2 in a furnace at 950-1,250 C. The rest of the H2S then reacts with that SO2 over a catalyst: 2H2S + SO2 → 3S + 2H2O. Modern three-stage units recover over 98% of the sulphur, against 85-90% in the late 1980s. This changes the economics of the raw material. A refinery must get rid of its sulphur at almost any price, because it has little storage and cannot stop desulphurising fuel, so small surpluses cause price collapses (Kent). Spot sulphur fell to the tens of dollars a tonne at Tampa and Vancouver in mid-2001, against $140 and $108 in 1990. The Tampa price reached zero in the first quarter of 2009. For an acid maker, the main raw material is often cheap but always volatile. India has almost no elemental sulphur of its own. Dryden (1997) notes that nearly all Indian acid plants burn imported sulphur. The exceptions are smelter-gas plants at Khetri, Ghatsila, Udaipur and Alwaye, and one pyrites plant at Amjhore in Bihar. Data World sulphur production in all forms (million tonnes) | Source | 1999 | 2009 | |---|---|---| | Mined (Frasch and other) | 3.2 | 0.3 | | Recovered from oil | 15.6 | 22.7 | | Recovered from gas | 21.9 | 23.3 | | Recovered, other | 1.1 | 2.4 | | Other forms (pyrites, smelter gas and others) | 15.3 | 19.2 | | Total | 57.1 | 67.9 | Recovered sulphur is supplied whatever the price, which is why prices swing so widely. Source: Kent (2012), Ch 25 Sulfur and Sulfuric Acid, Table 25.1, pdf p. 1002 Case #### The end of Frasch mining Herman Frasch showed in 1894 how to melt underground sulphur with water at about 160 C, pumped down concentric pipes, and lift it with compressed air. Cheap Frasch sulphur ended the Italian monopoly and let acid makers abandon pyrites. A century later it lost to its customers' by-product: refiners sold recovered sulphur cheaply while gas for heating mine water grew dear, and US Frasch mining stopped in 2001. - Kent (2012), Ch 25 Sulfur and Sulfuric Acid · pdf pp. 1001-1009, 1013 - Dryden (1997), Ch IIA Sulfur and Sulfuric Acid · pdf pp. 65-66 ### 1.2 The contact process burns, converts and absorbs, and double absorption removes the last SO2 The contact process has five steps: make SO2, clean and cool the gas, convert SO2 to sulphur trioxide (SO3) on a catalyst, cool the SO3 gas, and absorb it (Kent). In a sulphur-burning plant, molten sulphur is sprayed into a furnace with dried air, giving gas at 1,000-1,200 C with 10-12% SO2. The air is dried first, because water would form corrosive acid mist downstream. In the converter, SO2 + 1/2 O2 → SO3 releases heat and is reversible. Equilibrium favours low temperature, while the rate favours high temperature. Shreve (1984) explains the compromise. Gas enters the first bed of vanadium pentoxide catalyst at about 410-440 C and heats itself as it reacts. In a Monsanto four-pass converter, the first pass took the gas from 410 to 602 C and did 74% of the conversion. After cooling between beds, the next three passes added 18.4, 4.3 and 1.3 points, for 98% overall. Single absorption stops at 97.5-98% conversion. Double absorption takes the SO3 out after the second or third bed in an intermediate absorber, reheats the gas and sends it through the last bed or beds. With SO3 removed, the reaction can move forward again, and overall conversion reaches 99.5-99.8%, or 99.9% with caesium catalyst. Kent calls this the industry standard. From the early 1970s, US rules limited new sulphur-burning plants to 2 kg of SO2 per tonne of acid. Absorption is done in acid, not water. SO3 meeting water makes a fog that is almost impossible to catch. It is absorbed in acid near 98.3% H2SO4, where vapour pressures are lowest, with over 99.9% efficiency. Water is added to hold the acid's strength, and surplus acid is drawn off as product. Process flowchart Double absorption contact process (sulphur burning), per tonne of 100% acid Inputs **Sulphur**about 0.33 t (from Dryden's 0.67 t SO2) **Air**1,450-2,200 Nm3 **Process water**makes up the acid strength **Boiler feedwater**for steam export Process steps 1 **Air drying tower** air dried with 93-98% acid at 50-60 C 2 **Sulphur burner** S + O2 → SO2; gas at 1,000-1,200 C with 10-12% SO2 3 **Waste heat boiler** cools gas to 420-440 C, raises 40-60 bar steam 4 **Converter beds 1-3 (V2O5)** enter 410-440 C; first bed exits near 600 C with 60-75% conversion; cooled between beds 5 **Intermediate absorber** SO3 absorbed in 98-99% acid; gas reheated 6 **Final bed(s)** conversion rises to 99.5-99.8% 7 **Final absorber and acid coolers** 98.3% acid at 60-80 C; mist eliminators Outputs **Sulphuric acid**1 t (sold as 93-98% acid or oleum) **High-pressure steam**over 1.3 t **Warm cooling water**about 40% of the heat **Tail gas**under 2 kg SO2 (US limit, 1970s) Plant sizes in the sources range from 50 to 2,800 t/day. Source: Kent (2012), Ch 25 Sulfur and Sulfuric Acid; Shreve (1984), Ch 19 Sulfur and Sulfuric Acid; Dryden (1997), Ch IIA Sulfur and Sulfuric Acid, Kent pdf pp. 1013-1017; Shreve pdf pp. 347-356; Dryden pdf p. 75 Common misreading #### A better catalyst does not buy the last 2% It is tempting to think a better catalyst explains the jump from 98% to 99.7% conversion. Shreve points out that it does not. A catalyst only speeds the approach to equilibrium. The gain comes from removing SO3 between beds, which shifts the equilibrium itself. The same idea, removing product so a reversible reaction can keep going, reappears in the ammonia loop and the urea plant. - Kent (2012), Ch 25 Sulfur and Sulfuric Acid · pdf pp. 1013-1016 - Shreve (1984), Ch 19 Sulfur and Sulfuric Acid · pdf pp. 347-356 - Dryden (1997), Ch IIA Sulfur and Sulfuric Acid · pdf pp. 73-76 ### 1.3 A modern acid plant is also a power station, and much of its margin lies there Three reactions in the plant release heat: burning sulphur, oxidising SO2 and forming H2SO4. Kent puts the thermal output of a 1,000 t/day sulphur-burning plant at about 63 MW. About 97% of the energy input comes from burning sulphur. Only 3% is electricity, mostly for the main blower. The arithmetic per tonne is simple. 63 MW for one day is 63 x 86,400 = 5.4 million MJ, or about 5.4 GJ per tonne of acid. Most plants recover 55-60% of this as high-pressure steam at 40-60 bar and 400-480 C, which is roughly 3 GJ per tonne. About 40% is lost as warm water from the acid coolers. Shreve put steam output from large plants at over 1.3 t per tonne of acid in 1984. The lost 40% sits in acid at 110-120 C, too cool to raise useful steam. Monsanto's heat recovery system, first run at Namhae in South Korea in 1987, runs the absorber at about 200 C in a special stainless steel. It raises 10-bar steam and lifts total recovery to about 90%. Twenty-one were built by 2005 (Kent). When energy was cheap, plants were simple. Once energy prices rose, the heat became a product, plants were redesigned for cogeneration, and acid costs fell (Shreve). A plant beside a phosphate complex can use its steam; a stand-alone plant may struggle to sell it. Capital is modest by chemical-industry standards. Shreve gives roughly $25 million for an 1,800 t/day double absorption, sulphur-burning plant in the early 1980s. A plant on smelter or pyrites gas cost three to four times as much, because it needs roasters, dust removal, scrubbing and gas reheating, and it exports much less steam. Data Where the energy goes in a 1,000 t/day sulphur-burning double absorption plant | Item | Value | |---|---| | Thermal capacity | about 63 MW (about 5.4 GJ per tonne of acid) | | Share of energy input from burning sulphur | about 97% | | Share from electricity (blowers) | about 3% | | Recovered as HP steam, standard design | 55-60% (40-60 bar, 400-480 C) | | Lost in acid cooling water | about 40% | | Recovered with economisers and low pressure-drop catalyst | up to 70% | | Recovered with hot absorption (HRS) | about 90% | Source: Kent (2012), Ch 25 Sulfur and Sulfuric Acid, pdf pp. 1017-1018 Worked example #### Steam credit against sulphur cost Each tonne of acid needs about 0.33 t of sulphur. At the 1990 Tampa price of $140 a tonne, that is about $46 of sulphur per tonne of acid. The same tonne exports over 1.3 t of high-pressure steam (Shreve). Brennan's indicative 2018 price for 40-bar steam is $10-16 a tonne, a credit of about $13-21. The years differ, but the point holds: at 1990 prices sulphur outweighs steam, and when sulphur fell to zero in 2009 the steam was worth more. - Kent (2012), Ch 25 Sulfur and Sulfuric Acid · pdf pp. 1017-1018 - Shreve (1984), Ch 19 Sulfur and Sulfuric Acid · pdf pp. 346-349, 358 - Brennan (2020), Ch 4 Operating cost estimation, Table 4.4 · pdf p. 110 ### 1.4 Acid follows fertiliser, and India built many small plants beside its customers World output of new sulphuric acid was 158.7 million tonnes in 1999 and 191.3 million tonnes in 2008 (Kent). China accounted for most of the growth, from 21.6 to 53.8 million tonnes. India rose from 6.1 to 7.9 million tonnes. One customer dominates. The United States made about 27 million tonnes in 2008, and some 74% went into fertilisers, mainly phosphates. Petroleum refining, which uses the acid as a catalyst for alkylation (joining small hydrocarbons into petrol), is the largest of the other uses. Copper leaching, titanium dioxide, pulp and steel pickling follow. The acid seldom ends up in the product: a phosphate plant turns it into gypsum. Acid is low in value per tonne and awkward to move, so plants sit near their customers. Dryden explains that in India it proved cheaper to carry sulphur inland and make acid beside the fertiliser plant than to ship acid from a coastal plant. The result in 1993-94 was about 6 million tonnes a year of capacity spread over many dozens of sites. Many were built to a standard design of about 33,000 t a year, which is about 100 t/day. Dryden gives typical capacities of 50-1,000 t/day, while Shreve describes a 2,800 t/day plant started in North Carolina in 1982. Small plants cost more per tonne to build and run, and they export less steam. But they avoid freight and fit a scattered fertiliser industry. Dryden blamed high Indian chemical costs on small plants running below capacity, costly imported equipment, raw material imports and dear power and transport. Data World production of new sulphuric acid (million tonnes, 100% H2SO4) | Region | 1999 | 2008 | |---|---|---| | Western Europe | 17.5 | 14.2 | | Central Europe and former USSR | 15.6 | 17.9 | | North America | 46.2 | 30.8 | | Latin America | 10.2 | 18.5 | | Africa (Morocco 8.9 in 2008) | 18.1 | 18.5 | | Asia | 42.0 | 68.9 | | of which China | 21.6 | 53.8 | | of which India | 6.1 | 7.9 | | World | 158.7 | 191.3 | Kent's table labels the later column 2008. Source: Kent (2012), Ch 25 Sulfur and Sulfuric Acid, Table 25.7, pdf p. 1018 In the data #### Acid as a barometer Dryden observed that Indian acid output in the mid-1990s matched the US level of about 1900, and that consumption per person was about 4% of the American figure. Sulphuric acid use was long treated as a barometer of industrial progress, an idea Kent also repeats. Dryden adds that it no longer holds in advanced economies, where industry keeps growing while acid output stays flat. - Kent (2012), Ch 25 Sulfur and Sulfuric Acid · pdf pp. 1010-1011, 1018-1019 - Dryden (1997), Ch IIA Sulfur and Sulfuric Acid · pdf pp. 64, 77-82 - Dryden (1997), Ch II introduction · pdf p. 64 - Shreve (1984), Ch 19 Sulfur and Sulfuric Acid · pdf p. 347 ### See it in the table [H₂O Water](https://www.materialinsights.org/commodities/water/)[Cu Copper](https://www.materialinsights.org/commodities/copper/)[Zn Zinc](https://www.materialinsights.org/commodities/zinc/)[St Steel](https://www.materialinsights.org/commodities/steel/)[V Vanadium](https://www.materialinsights.org/commodities/vanadium/)[Ti Titanium](https://www.materialinsights.org/commodities/titanium/)[Cs Caesium](https://www.materialinsights.org/commodities/cesium/)[Gyp Gypsum](https://www.materialinsights.org/commodities/gypsum/)[H2SO4 Sulphuric acid](https://www.materialinsights.org/chemicals/sulphuric_acid/)[TiO2 Titanium dioxide](https://www.materialinsights.org/chemicals/titanium_dioxide/)[H2 Hydrogen](https://www.materialinsights.org/chemicals/hydrogen/)[NH3 Ammonia](https://www.materialinsights.org/chemicals/ammonia/)[Ur Urea](https://www.materialinsights.org/chemicals/urea/) ### Check your understanding Why does a double absorption plant reach about 99.7% conversion when a single absorption plant stops near 98%? The catalyst only sets the speed. Taking SO3 out in an intermediate absorber shifts the equilibrium, so the remaining SO2 can react in the final bed. Roughly how much heat does a sulphur-burning acid plant release per tonne of acid, using Kent's 63 MW for a 1,000 t/day plant? 63 MW x 86,400 seconds is about 5.4 million MJ a day, divided by 1,000 t gives about 5.4 GJ per tonne. About 55-60% of it is recovered as HP steam. Why did Indian acid plants tend to be small and spread out? Dryden explains that acid is costly to transport relative to its value, so plants were built next to fertiliser and other users, many to a standard 33,000 t a year design. [← Module contents](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/)[Mark read · Part 2: Salt: chlor-alkali, hydrochloric acid and soda ash →](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p2/) ## Other sections - [Module overview](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/index.md) - [Salt: chlor-alkali, hydrochloric acid and soda ash](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p2/index.md) - [Ammonia: fixing nitrogen with natural gas](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p3/index.md) - [From ammonia and rock to fertiliser: nitric acid, urea, ammonium nitrate and phosphates](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p4/index.md) - [Industrial gases, and the heavy inorganics side by side](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/p5/index.md) - [Key terms, quiz and sources](https://www.materialinsights.org/learn/chemicals/heavy-inorganic-chemicals/review/index.md)