The contact process makes concentrated sulfuric acid in three steps: burn sulfur or roast pyrite to sulfur dioxide, oxidise the SO2 to SO3 over a vanadium pentoxide catalyst at 400-450 °C, and absorb the SO3 in existing sulfuric acid, never in water. SO3 meeting water reacts violently and exothermically and throws off a fine acid mist that is carried deep into the lungs and does far more damage there than liquid acid does to skin. The absorption step is the whole engineering problem.
Part of Industrial Chemistry
Catalytic manufacture of concentrated sulfuric acid, with the mechanism by which SO3 absorption produces a hazardous mist.
Three Steps From The Carrier
Step 1: make SO2. Burn sulfur (S + O2 → SO2) or roast sulfide ore such as pyrite (4FeS2 + 11O2 → 2Fe2O3 + 8SO2). See Pyrite Roasting.
Step 2: oxidise SO2 to SO3. Pass SO2 with excess air over a catalyst at 400-450 °C: 2SO2 + O2 → 2SO3. The carrier names vanadium pentoxide (V2O5) as ideal, platinum as scarce but workable, and finely divided iron oxide as less efficient but available. The bed design is in Catalyst Beds.
Step 3: absorb SO3. The carrier says not to dissolve SO3 directly in water. Dissolve it in existing sulfuric acid to form oleum, then dilute the oleum with water to the desired concentration.
The simpler, older route that avoids a catalyst bed is the Lead Chamber process, which gives only dilute acid; the acid itself is covered in Sulfuric Acid.
Why The Absorption Step Is The Whole Problem
SO3 + H2O → H2SO4 releases a great deal of heat. When SO3 gas meets water, the heat boils the water and the acid at the surface, and the vapour recondenses in the cool gas above as a cloud of sub-micron droplets. Three consequences, all general chemistry and engineering and not corpus figures:
- The mist is the product, escaping. Droplets this small do not settle or coalesce in the gas stream, so most of the acid passes through the absorber and out of the stack.
- It reaches deep in the lung. Acid aerosol small enough to inhale reaches the airways and air sacs, where it burns wet tissue; skin has a dead outer layer and lungs do not. Lung damage can appear hours after the exposure.
- Strong acid absorbs SO3 quietly. In acid of the right strength the SO3 dissolves without a mist because there is little free water to react at the gas-liquid boundary. A plant keeps the absorbing acid at a set strength by adding water to a recirculating stream and drawing off product.
The carrier states the rule and the reason in a single sentence: violently exothermic, dangerous acid mist. The mechanism is why the rule has no exceptions.
What You Need
- A sulfur or pyrite burner with controlled air.
- A cooled gas train, dust removal (dust and arsenic poison the catalyst) and a heated catalyst converter.
- An absorber: a tower in which gas rises against a circulating flow of strong acid, in glass, glazed ceramic or acid-resistant steel (Chemical Vessels).
- A way to dilute the circulating acid under control, with water added slowly into acid in a cooled vessel.
- A stack that vents outdoors, well away from people, and a scrubber if possible.
- Acid-proof gloves, face shield and apron.
Method: Order Of Work (No Quantities)
Step 1: Burn the sulfur with controlled air and cool the gas.
Step 2: Clean the gas of dust before the catalyst.
Step 3: Pass the gas over the preheated catalyst, holding temperature in the carrier’s range.
Step 4: Pass the SO3-bearing gas up the absorber against strong acid, never water.
Step 5: Dilute product acid only by pouring acid into water in a cooled vessel, slowly.
Worked Example: Mass And Air For One Kilogram Of Sulfur
Rounded atomic masses: S 32, O 16, H 1.
- S = 32 g per mole; SO2 = 64; SO3 = 80; H2SO4 = 98. One kilogram of sulfur is 1,000 ÷ 32 = 31.25 mol.
- SO2: 31.25 × 64 = 2,000 g (2 kg). SO3: 31.25 × 80 = 2,500 g (2.5 kg). H2SO4: 31.25 × 98 = 3,063 g (3.06 kg) at 100% yield.
- Oxygen: burning needs 1 mol O2 per mole of S, and SO2 to SO3 needs 0.5 mol, so 1.5 × 31.25 = 46.9 mol O2.
- Air is about 21% oxygen by volume (recalled), so 46.9 ÷ 0.21 = 223 mol of air. At about 24 L per mole, 223 × 24 = 5,350 litres, 5.4 cubic metres of air per kilogram of sulfur, before the excess air the carrier calls for.
The carrier gives the concentrated acid at density 1.84 g/cm³ for a quick hydrometer check (see Industrial Chemistry).
Common Mistakes
- Sending SO3 into water to “save a step”.
- Letting dust or sulfur-bearing contaminants reach the catalyst. The carrier names sulfur compounds, phosphorus and arsenic as poisons.
- Running the absorber without a stack.
- Adding water to concentrated acid.
Safety
SO3 and its mist, SO2, and hot concentrated acid
SO3 reacts violently with water and forms an acid mist that burns the airways and lungs; the injury can appear hours later. SO2 is itself a toxic irritant gas. Concentrated sulfuric acid destroys tissue and boils if water is added to it: add acid to water, slowly. Work outdoors, stay upwind, never put your face over an absorber, and keep clean water and a way to flush skin and eyes at hand. See Toxic Gases and Chemical Burns.
What’s Next
- Learn next: Catalyst Beds — housing the vanadium catalyst
- Combine with: Toxic Gases — SO2 and mist exposure