# Sulfuric Acid from Sulfur

Laplace’s Mini World · Plate 68 · Matter, Light & Sound · Matter

> Burnt sulfur gains an oxygen on four cooled beds of vanadium catalyst; the trioxide goes into strong acid, not water.

- Interactive model: https://world.lapla.si/en/contact/
- Embed: https://world.lapla.si/embed/en/contact/
- 中文: https://world.lapla.si/zh/contact/
- Updated: 2026-10-31

## 01 Sulfur First

The contact process has three steps: burn sulfur to sulfur dioxide, turn the dioxide into sulfur trioxide on a catalyst, and absorb the trioxide in strong sulfuric acid (US EPA). At the front left are a few lumps of sulfur.

Most sulfur today is not mined but recovered when refineries and gas plants take it out of oil and natural gas. Liquid sulfur burns to sulfur dioxide in a furnace at 900⁠–⁠1,500 °C, giving off about 297 kJ per mole. The gas leaves with up to about 18 % sulfur dioxide and is diluted with air to 10⁠–⁠11 % before conversion.

The air for burning is first dried with strong sulfuric acid in a drying tower (US EPA). Water in the gas would later make an acid mist that is hard to remove.

*S + O₂ → SO₂ · about 297 kJ per mole · burner at 900⁠–⁠1,500 °C · into the converter: 10⁠–⁠11 % SO₂*

## 02 One More Oxygen

The second step is SO₂ + ½ O₂ ⇌ SO₃, which gives off about 99 kJ for every mole of trioxide (EU BREF). It can run backwards too: the trioxide keeps breaking down again.

For a reaction that gives off heat, the cooler it is, the further the equilibrium lies towards the trioxide. Computed here from thermodynamic data: a gas with 11 % sulfur dioxide at atmospheric pressure can be about 99 % converted at 400 °C, only about 72 % at 600 °C.

On its own the reaction is slow. BASF’s Rudolf Knietsch said in 1901 that, though it gives off heat, it runs “with extraordinary slowness”. Without a catalyst it needs a high temperature to go fast, and by then the equilibrium has swung back towards sulfur dioxide (EFMA 2000). A catalyst does not move the equilibrium; it makes the reaction fast enough at a little over 400 °C.

*SO₂ + ½ O₂ ⇌ SO₃ · about 99 kJ per mole · at equilibrium (11 % SO₂): about 99 % at 400 °C, about 72 % at 600 °C (computed)*

## 03 The Heat Stays in the Gas

The heat a bed gives off has nowhere to go and stays in the gas: as it goes down, the more it converts, the hotter it gets. With 11 % sulfur dioxide, each percentage point converted warms the gas by about 3 °C (computed here; about 3.1 °C in EFMA’s worked example).

The hotter it gets, the worse the equilibrium. However deep the bed, its conversion can’t pass the point where the warming line meets the equilibrium curve: for gas in at 430 °C, about 63 % at 629 °C (computed here). Much hotter, and the catalyst reaches its upper limit of 600⁠–⁠650 °C (BREF).

In EFMA’s example the first bed takes the gas in at 430 °C and out at 582 °C, about 49 % converted; in a large sulfur-burning plant, the first bed was measured converting about 56 % (MECS). The streaks in the section are a schematic: the more orange, the hotter the gas.

*About 3 °C per percentage point (computed) · first bed: in at 430 °C, out at 582 °C, about 49 % (EFMA example) · one bed at most about 63 % (computed)*

## 04 Four Beds

The answer is to split the catalyst into beds and cool the gas back to a little over 400 °C between them. Cooled, the equilibrium swings back towards the trioxide and the next bed carries on. A converter usually holds four beds (BREF).

In EFMA’s example (11 % sulfur dioxide), the second bed takes the gas from 430 °C to 520 °C, about 78 % converted in all; the third from 430 °C to 478 °C, about 93 %. Less and less sulfur dioxide is left, and each bed warms the gas less.

On the stand, a model of the principle of an older converter: a boiler-steel shell lined with firebrick, each bed of catalyst on a cast-iron grid with a layer of quartz under and over it (MECS); the gas leaves through a nozzle in the side, is cooled in an exchanger and comes back into the next pass. Newer converters have stainless-steel shells (BREF).

*Usually four beds (BREF) · EFMA example: about 49 % after bed 1, 78 % after bed 2, 93 % after bed 3 · cooled to about 430 °C between*

## 05 A Catalyst That Melts

Today the catalyst is almost always vanadium: 4⁠–⁠9 % vanadium pentoxide with potassium sulfate, on silica (BREF). At working temperature the vanadium and alkali sulfates melt into a liquid, and the reaction is thought to take place in that melt.

An ordinary catalyst works down to 410⁠–⁠440 °C, one with caesium down to 380⁠–⁠410 °C; above 600⁠–⁠650 °C its inner surface shrinks and it loses activity for good (BREF). So each bed’s inlet must stay above the lower limit and its outlet below the upper.

The catalyst is made as rings or star-shaped rings, so the gas passes easily and dust doesn’t block it (BREF). The dish holds rings at real size, about 1 cm (Topsoe); the rings in the section are drawn many times larger. The first bed catches the last of the dust and is screened every 2⁠–⁠3 years, the others about every 10 (BREF).

*4⁠–⁠9 % V₂O₅ with potassium sulfate, on silica · molten at work · lower limit 410⁠–⁠440 °C, upper 600⁠–⁠650 °C (BREF)*

## 06 From Platinum to Vanadium

In 1831 Peregrine Phillips, a vinegar maker in Bristol, received a British patent for turning sulfur dioxide into trioxide on platinum. For long after, most sulfuric acid was still made in lead chambers, where oxides of nitrogen oxidised the sulfur dioxide and the acid came out at only about 60⁠–⁠78 % (GDCh, ATSDR).

In 1875 Clemens Winkler in Freiberg made fuming acid on platinised asbestos. BASF’s Rudolf Knietsch turned the contact process into a large-scale method: BASF made almost all its acid that way by 1899, and he made it public in Berlin on 19 October 1901. Platinum feared arsenic: arsenic at only 1⁠–⁠2 % of the platinum, Knietsch said, made the catalyst completely inert.

BASF patented a vanadium catalyst in 1913; from the 1920s vanadium, cheaper and untroubled by arsenic, took over from platinum. In North America and western Europe the lead chambers’ share fell from 80 % in 1910 to 15 % in 1960, then to almost nothing (ATSDR).

*1831: Phillips’s platinum patent · 1901: Knietsch makes BASF’s contact process public · 1913: the vanadium catalyst patent*

## 07 Absorb, Then Convert Again

Absorbing only once, conversion rarely passes 98 %; most single-absorption plants in the US ran at 95⁠–⁠98 % (US EPA). The rest of the sulfur dioxide goes out with the tail gas: at 98 % conversion, about 13 kg for every tonne of acid.

Double contact, double absorption: after three beds 85⁠–⁠95 % is converted; the gas is cooled to about 190 °C, the trioxide is absorbed in an interpass absorber, and the gas is heated back to a little over 400 °C for the last bed (BREF). With the product taken away, the equilibrium moves well towards the trioxide. In one large plant the last bed converted about 92 % of the sulfur dioxide that reached it (MECS). The total reaches 99.7⁠–⁠99.9 %, about 2 kg per tonne.

The interpass absorber stands at the front left, joined to the third exchanger; the gas coming back from it is reheated from about 80 °C to about 430 °C (EFMA example) with heat from the conversion (BREF). The idea was tried in Freiberg in 1888 and patented by Hoechst in 1900 (GDCh); the first industrial plant dates from 1964, under a Bayer licence.

*Single absorption: about 98 %, about 13 kg SO₂ per tonne of acid · double: 99.7⁠–⁠99.9 %, about 2 kg (US EPA, BREF)*

## 08 Into Strong Acid

Sulfur trioxide and water make sulfuric acid, but plants don’t absorb it in water. Below its dew point, sulfur trioxide meeting water vapour forms an acid mist so stable that the absorber removes only a little of it (US EPA).

The absorber is a packed tower: 98.5⁠–⁠99.5 % acid trickles down from the top while the gas comes in at the bottom (BREF, US EPA); the trioxide joins the little water in the acid to make more acid. The acid runs at 70⁠–⁠120 °C. It grows stronger as it absorbs, and water or dilute acid is added to hold it at about 99 %.

Candle filters at the top catch the last of the mist. The acid leaves the plant at 94 %, 96 % or 98.5⁠–⁠99.5 % and is diluted for its use (BREF). To dilute it, pour the acid into water, never the other way: a little water added to strong acid can flash to steam and spatter the acid (ATSDR).

*SO₃ + H₂O → H₂SO₄ · absorber acid 98.5⁠–⁠99.5 % at 70⁠–⁠120 °C · acid mist, once formed, is very stable · to dilute: acid into water*

## 09 Heat to Spare

All three steps give off heat: burning, converting and absorbing (BASF’s brochure gives about 132 kJ per mole for the last). The boiler after the burner and the exchangers between beds turn it into high-pressure steam; on the stand, the thin pipes from the first two exchangers stand for steam lines.

In the EU BREF’s energy balance for a sulfur-burning plant making 1,000 t of acid a day, burning and the waste-heat boiler take 37 % of the heat, the catalyst beds and gas cooling 30 %: together 67 % as high-pressure steam. Cooling the acid takes 31 %, at only 85⁠–⁠120 °C, good for drying or low-pressure steam.

A modern sulfur-burning double-contact plant can export about 6 GJ of energy for every tonne of acid, while using 35⁠–⁠50 kWh of electricity (BREF).

*About 6 GJ exported per tonne of acid · 67 % of the waste heat as high-pressure steam · 35⁠–⁠50 kWh of electricity (BREF)*

## 10 More Than Any Other Chemical

The EU BREF says more sulfuric acid is made than any other chemical in the world. The world total it gives, about 150 million tonnes, is for 1997.

Most of it goes into phosphate fertiliser: sulfuric acid breaks down phosphate rock into phosphoric acid and gypsum (BREF). In the US almost 70 % of the acid went to fertiliser (US EPA, 1990s); the rest to oil refining, ore processing, pigments, paper, plastics and more.

The world produced about 84 million tonnes of sulfur in 2025, most of it recovered when oil and natural gas are cleaned (USGS). In the US about 90 % of the sulfur used is used as sulfuric acid.

*The most produced chemical (BREF; about 150 Mt in 1997) · Ca₃(PO₄)₂ + 3 H₂SO₄ → 2 H₃PO₄ + 3 CaSO₄ · almost 70 % for fertiliser in the US · world sulfur 2025: about 84 Mt (USGS)*

## 11 Pass by Pass

The converter comes apart into its head and four passes: each a ring of shell and brick, a bed of catalyst on its grid, and the plate beneath. Beside it stand three exchangers and the interpass absorber.

The part under the pointer lifts alone and is named; left alone, the passes stand apart together.

*Head · pass 1 · pass 2 · pass 3 · pass 4 · exchangers 1 and 3 · interpass absorber*

## 12 Gas Temperature and Strength

Computed here: the gas is sulfur burnt in dry air (with 11 % sulfur dioxide it holds about 10 % oxygen), at atmospheric pressure; every bed is fed at the same temperature and runs all the way to where its warming line meets the equilibrium. Real beds stop short of that, so the totals here are a little higher than a plant’s.

Readings: the conversion after four beds, and the sulfur dioxide left per tonne of acid by the US EPA’s factor. On the chart the slopes are the warming in a bed, the flat lines the cooling between beds and the curves the equilibrium; when the first bed’s outlet passes 630 °C, the catalyst’s upper limit is marked.

*Readings: total conversion · SO₂ per tonne of acid (682 − 6.82 × conversion in %) · every bed to equilibrium (computed)*

**Try this**: Raise the gas into each bed from 430 °C to 460 °C: how do the first bed’s outlet conversion and temperature change? Then switch the interpass absorption off: how much sulfur dioxide is left after four beds?

## What the Model Leaves Out

The stand is a model of the principle: the look, size and place of the converter, exchangers and absorber are drawn, not taken from any plant. The burner, waste-heat boiler, drying tower, final absorber and blower are left out. The catalyst rings in the section are drawn many times larger than real, and the streaks and their colour are a schematic. The lab computes a gas of sulfur burnt in dry air at atmospheric pressure, every bed running to equilibrium, with no rates and no diffusion inside the catalyst; the interpass absorber is taken to remove all the trioxide.

## Easily Misread

A catalyst doesn’t move the equilibrium; it makes the reaction fast enough at a lower temperature. Cooling favours the trioxide, but below about 410 °C an ordinary catalyst stops working well. Sulfur trioxide is absorbed in 98⁠–⁠99 % sulfuric acid, not in water. A contact converter runs just above atmospheric pressure; it is not a high-pressure reactor. “More than any other chemical” comes from an EU document of 2007 whose world total is for 1997; no authoritative figure for today’s world output was found, so none is given. Today’s contact process uses vanadium, not platinum.

## Common questions

### Does the catalyst change the equilibrium?

A catalyst does not move the equilibrium; it makes the reaction fast enough at a little over 400 °C.

See: [02 One More Oxygen](https://world.lapla.si/en/contact/#oxygen)

### Why can’t one bed of catalyst convert all the sulfur dioxide?

However deep the bed, its conversion can’t pass the point where the warming line meets the equilibrium curve: for gas in at 430 °C, about 63 % at 629 °C (computed here).

See: [03 The Heat Stays in the Gas](https://world.lapla.si/en/contact/#heat)

### Why isn’t sulfur trioxide absorbed in water?

Below its dew point, sulfur trioxide meeting water vapour forms an acid mist so stable that the absorber removes only a little of it (US EPA).

See: [08 Into Strong Acid](https://world.lapla.si/en/contact/#absorb)

### What is double contact, double absorption?

Double contact, double absorption: after three beds 85⁠–⁠95 % is converted; the gas is cooled to about 190 °C, the trioxide is absorbed in an interpass absorber, and the gas is heated back to a little over 400 °C for the last bed (BREF).

See: [07 Absorb, Then Convert Again](https://world.lapla.si/en/contact/#double)

### What is most sulfuric acid used for?

In the US almost 70 % of the acid went to fertiliser (US EPA, 1990s); the rest to oil refining, ore processing, pigments, paper, plastics and more.

See: [10 More Than Any Other Chemical](https://world.lapla.si/en/contact/#scale)

## Sources

1. [European Commission, Best Available Techniques reference document: Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers (2007), chapter 4: the reactions and heats, the vanadium catalyst, its temperature range, four beds, single and double contact, absorption, the energy balance, raw materials and uses](https://bureau-industrial-transformation.jrc.ec.europa.eu/sites/default/files/2019-11/lvic_aaf.pdf)
2. [EFMA/ESA (2000): Production of Sulphuric Acid, BAT booklet 3 (Table 15: a worked example of each bed’s inlet and outlet temperatures and conversion)](https://www.fertilizerseurope.com/wp-content/uploads/2019/08/Booklet_3_final.pdf)
3. [US EPA AP-⁠42 §8.10 (1993): Sulfuric Acid (the three steps, drying, single and double absorption, SO₂ per tonne of acid, acid mist, uses)](https://www.epa.gov/sites/default/files/2020-09/documents/8.10_sulfuric_acid.pdf)
4. [NIST Chemistry WebBook: thermochemical data (Shomate equations) for sulfur dioxide](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7446095&Units=SI&Mask=1)
5. [NIST Chemistry WebBook: thermochemical data (Shomate equations) for sulfur trioxide; oxygen and nitrogen likewise](https://webbook.nist.gov/cgi/cbook.cgi?ID=C7446119&Units=SI&Mask=1)
6. [M. W. Chase (1998): NIST-JANAF Thermochemical Tables, tables O-⁠058 sulfur trioxide and O-⁠034 sulfur dioxide (the check)](https://janaf.nist.gov/tables/O-058.txt)
7. [U.S. Geological Survey (2026): Mineral Commodity Summaries, Sulfur (world production, where it comes from)](https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-sulfur.pdf)
8. [ATSDR (1998): Toxicological Profile for Sulfur Trioxide and Sulfuric Acid (the lead chambers’ share, uses, dilution)](https://www.atsdr.cdc.gov/toxprofiles/tp117.pdf)
9. [M. Haustein (2017): Das Kontaktverfahren, Mitteilungen of the GDCh history of chemistry group 25 (Phillips, Winkler, Knietsch, vanadium, early double contact)](https://Gdch.de/fileadmin/downloads/Netzwerk_und_Strukturen/Fachgruppen/Geschichte_der_Chemie/Mitteilungen_Band_25/2017-25-08_neu.pdf)
10. [R. Knietsch (1901): Sulfuric Acid and its Manufacture by the Contact-Process, transl. in Popular Science Monthly 61 (1902) (“extraordinary slowness”, arsenic)](https://en.wikisource.org/wiki/Page:Popular_Science_Monthly_Volume_61.djvu/30)
11. [H. M. Cardwell, T. R. Felthouse (2025): MECS sulfuric acid catalyst: celebrating 100 years, Sulfuric Acid Today (the molten salt, a plant’s measured conversion bed by bed)](https://elessentct.com/wp-content/uploads/2025/03/2025-03-MECS-Catalyst-100-Years-of-Innovation.pdf)
12. [Elessent/MECS (2022): Instructions for Manual Screening (the quartz layers on the grids, where the thermocouples go)](https://elessentct.com/wp-content/uploads/2023/03/Catalyst-Manual-Screening_2022.pdf)
13. [Topsoe: VK38 product page (10 mm rings, the 12 mm Daisy)](https://www.topsoe.com/products/catalysts/vk38)
14. [BASF (2023): Sulfuric Acid Catalysts brochure (the vanadium catalyst patent of 1913; the heat of absorption)](https://chemical-catalysts-and-adsorbents.basf.com/dam/jcr:0c1aaf3c-b9bb-42e5-96aa-c7730254ea1e/basf/chemical-catalysts-and-adsorbents/Sulfuric-Acid/BASF_SulfuricAcid_brochure_screen.pdf)
15. [Hugo Petersen: Milestones (the first double-contact plant, 1964)](https://hugo-petersen.de/en/company/milestones)

## Data

The equilibrium, the heat of reaction and the beds are computed from the NIST Chemistry WebBook’s Shomate coefficients for SO₂, SO₃, O₂ and N₂ (M. W. Chase, NIST-JANAF Thermochemical Tables, 1998), checked against the JANAF tables’ formation constants; the SO₂ left per tonne of acid uses the US EPA’s AP-⁠42 factor. Plant conditions, conversion and emissions are from the EU’s Best Available Techniques reference document for ammonia, acids and fertilisers (2007), EFMA/ESA’s BAT booklet on sulphuric acid (2000) and the US EPA’s AP-⁠42 §8.10; catalysts from the same and from Topsoe, BASF and Elessent (MECS); the history from R. Knietsch’s lecture of 1901 and M. Haustein (GDCh, 2017); sulfur from the USGS (2026); uses and safety from the ATSDR (1998). The converter, exchangers and absorber are a model of the principle, not any one plant or maker’s design. Material presets, parts and the softbox studio of the site’s model kit are ported from AirsupHQ/machine-skill (MIT, © 2026 Air Supply Technologies Inc.).

## Licence CC BY-NC 4.0

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

https://creativecommons.org/licenses/by-nc/4.0/deed.en

## Cite this plate

Laplace’s Mini World, Plate 68, “Sulfuric Acid from Sulfur”, https://world.lapla.si/en/contact/, CC BY-NC 4.0, updated 2026-10-31.