Contact Process (Sulfuric Acid)

The three stages

Industrial sulfuric acid is made in three serial stages, each in a different vessel, but the gas stream flows continuously from one to the next.

1. Burn sulfur to SO₂. Molten elemental sulfur (or roasted pyrite ore, FeS₂) is sprayed into a furnace with dry air. S(l) + O₂(g) → SO₂(g), ΔH = −297 kJ mol⁻¹. The combustion gas leaves at ~1 000 °C, then is cooled to ~430 °C through a heat-recovery boiler that raises high-pressure steam.
2. Catalytically oxidize SO₂ to SO₃. The cooled SO₂/air mixture is passed through a converter containing four (sometimes five) shallow beds of vanadium(V) oxide catalyst. 2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g), ΔH = −197 kJ mol⁻¹. Each pass through a bed releases enough heat to need inter-bed cooling.
3. Absorb SO₃ in concentrated acid. SO₃ does not absorb cleanly in water (see the FAQ). Instead, the SO₃ stream meets a falling film of 98 % H₂SO₄ in a packed absorber. SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l) — the product is oleum, also called fuming sulfuric acid. Water is added downstream in a controlled mixing tank to dilute oleum to the desired final concentration.

Total water of hydration in the final acid is the only water in the loop — the converter feed and absorber gas are kept rigorously dry, otherwise sulfuric mist forms and corrodes everything.

Why 450 °C, why V₂O₅

The middle stage is the bottleneck. The reaction 2 SO₂ + O₂ ⇌ 2 SO₃ is fast and complete at low temperature by thermodynamics, but agonizingly slow until you heat it. V₂O₅ on a porous silica support changes that: it dissolves SO₂ and O₂ as a thin molten potassium-vanadate layer that runs the redox cycle on its surface.

Reduction:  V₂O₅ + SO₂ → V₂O₄ + SO₃
Reoxidation: 2 V₂O₄ + O₂ → 2 V₂O₅

Below ~400 °C the molten salt freezes and activity dies. Above ~620 °C the equilibrium constant K_eq drops below 10, conversion sinks under 80 %, and the catalyst slowly sinters. 440-470 °C is the sweet spot for the first bed; later beds run progressively cooler (430 °C, 440 °C, 430 °C is a typical four-bed profile) because the gas already contains significant SO₃ and you want to push the equilibrium harder.

Single absorption vs double absorption

Single-absorption plants from the 1950s ran the gas through all four beds in series, then to one final absorber. Equilibrium across the last bed is the limit — at 99 % conversion you've stripped most of the driving force. Double-absorption plants, the world standard since the 1970s, intercept the gas after bed 3, scrub out the SO₃, and return SO₂-rich gas to bed 4. With product gone, equilibrium pushes the last fraction of SO₂ over the line.

	Lead-chamber (1746-1970s)	Single absorption	Double absorption (modern)
Catalyst / mediator	NO/NO₂ in lead-lined tanks	V₂O₅	V₂O₅
Operating temperature	~80 °C	~450 °C	~450 °C
Final acid concentration	~78 % (chamber acid)	~98 %	~98-99 % + oleum
SO₂ conversion	~65 %	~98 %	99.7-99.9 %
Stack SO₂ (g per kg acid)	~50	~10-15	~0.5
Plant footprint per Mt yr⁻¹	large (lead chambers)	medium	medium
Heat recovery	low	partial (steam)	up to 1.3 t HP steam per t acid

The Contact Process beats the lead-chamber route on every axis except capital cost per beginning-of-life tonne, which is why the older method survived in small plants until air-quality regulation made its SO₂ emissions untenable.

Worked example: a 2 000 t day⁻¹ plant

A typical sulfur-burning plant delivers 2 000 t day⁻¹ of 98 % H₂SO₄ — about 730 kt yr⁻¹.

• Sulfur in: 2 000 t × 0.98 × (32 / 98) = 640 t S day⁻¹.
• Air in: stoichiometry needs 1.5 mol O₂ per mol S, but plants run at 1.7-1.8 to keep equilibrium favorable. About 4 500 t day⁻¹ of dry air enters the burner.
• Converter inlet: ~10-11 % SO₂, ~11 % O₂, balance N₂ at 430-440 °C.
• Bed-by-bed conversion (typical):
  • Bed 1: 0 → 65 %, exit ~600 °C (cooled before bed 2).
  • Bed 2: 65 → 88 %, exit ~510 °C.
  • Bed 3: 88 → 95 %, exit ~470 °C.
  • Inter-pass absorber strips SO₃ → gas re-enters bed 4 at ~430 °C.
  • Bed 4: 95 → 99.8 %, exit ~430 °C.
• Final absorber: ~660 t day⁻¹ of SO₃ produces 800-900 t day⁻¹ of fresh H₂SO₄ via reaction with water that is metered in to keep the absorber acid at 98.0 ± 0.2 %.
• Steam co-product: ~1.1 t high-pressure (40 bar) steam per t acid — sold to the grid or used to drive the main air blower.

The acid balance is closed: SO₃ generated × (98/80) = acid produced, minus the small mass of SO₂ that slips through the stack. At 99.7 % conversion that slip is roughly 0.5 kg SO₂ per t acid, well below modern 2 kg/t emission limits.

Mechanism arrows on the V₂O₅ surface

     O                     O
     ‖                     ‖
S=O ··· V — O — V       S — O — V — O — V
     ↘ O ↘                  ↗ O   ↗
       O₂                    O   O
                              ↑
                         (lattice O picked up later from O₂)

The arrow-pushing is: a Lewis-basic SO₂ donates a lone pair to a vacant V(V) site, an adjacent lattice oxygen migrates onto the sulfur, and SO₃ desorbs leaving V(IV). Two such cycles consume one O₂, regenerating two V(V) centers. This is a classic Mars-van Krevelen mechanism — the catalyst's own oxygen is consumed and replenished, rather than a Langmuir-Hinshelwood surface reaction.

Oleum and dilution

SO₃ added to 98 % H₂SO₄ first forms oleum (also called fuming sulfuric acid):

SO₃ + H₂SO₄ → H₂S₂O₇  (disulfuric acid, "20 % oleum" if 20 % free SO₃ by mass)

Oleum is shipped at 20 %, 25 %, 40 % and 65 % strengths — the higher numbers are dense, fuming, deliberately water-free, and used in nitration (TNT, dyes) and sulfonation (detergents). To make plain 98 % acid, oleum is diluted in a stirred tank with measured water:

H₂S₂O₇ + H₂O → 2 H₂SO₄

The reaction is wildly exothermic — about 75 kJ per mol of water added. Dilution tanks are cooled with circulating-water jackets, and water is always added to acid (never the reverse) to avoid local boiling and a steam-driven acid spray.

Plant variants in the wild

• Sulfur-burning plants — the cleanest feed, used wherever elemental sulfur is cheap (Frasch, Claus-recovery from natural gas). About 70 % of world capacity.
• Pyrite-roaster plants — burn FeS₂ instead of sulfur, common in copper-, zinc- and lead-smelter regions. The off-gas needs an electrostatic precipitator and a wet scrubber to remove arsenic, selenium, and dust before the catalyst sees it.
• Spent-acid regeneration — refinery alkylation produces dilute, contaminated acid. Plants thermally crack it back to SO₂ at 1 000 °C, then run the SO₂ through a normal Contact converter. About 8 % of world capacity.
• Heat-recovery system (HRS) plants — late-stage absorbers run hot (~200 °C) so the absorber heat itself raises medium-pressure steam, raising overall energy export from ~1.0 to ~1.3 t steam per t acid.

Pitfalls and engineering lessons

• Wet feed gas — even 100 ppm of water in the converter inlet forms an acid aerosol that fouls the heat exchangers and dissolves V₂O₅. Drying towers using 96 % H₂SO₄ are the largest single piece of equipment in many plants.
• Catalyst poisoning — arsenic and chloride from impure pyrite or recycled gas blanket the catalyst. Pre-converter mist precipitators recover 99.9 % of dust before it reaches the V₂O₅.
• Acid-mist emissions — fine H₂SO₄ droplets escape the absorber if the gas/liquid contact is poor. Brink-style fiber-bed mist eliminators reduce stack mist below 0.1 mg m⁻³.
• Carbon-steel attack — concentrated sulfuric is benign on steel below 60 °C and above ~70 % strength, but oleum and hot 80 % acid eat through plain steel within hours. Plant design uses 304 stainless for absorber towers, alloy 20 for hot 95 % acid lines, and PFA-lined steel for dilute streams.
• Heat-exchanger corrosion — the gas-gas heat exchanger between converter beds operates close to the H₂SO₄ dew point. A 5 °C drop too cold and you've got liquid acid eating tube walls. Modern plants use anodically protected steel or alloy 825.

Environmental and economic context

Pre-1970, single-absorption plants were a major source of urban SO₂. The shift to double absorption — driven first by the 1970 U.S. Clean Air Act, then by EU directives in the 1980s and Chinese national standards in 2010 — dropped global SO₂ emissions per tonne of acid by more than 20×. The industry now sells about 280 Mt yr⁻¹ of H₂SO₄ at $50-150 t⁻¹ ex-plant, with a fertilizer-driven price floor: at any global price below ~$30 t⁻¹, marginal pyrite-roaster plants idle, because the Contact Process makes acid as a co-product of metal smelting whether the market wants it or not.