Sulphuric acid by the contact process

Why it matters

Sulphuric acid is the largest-volume chemical in the world and the backbone of phosphatic fertiliser production, so its output is often used as an index of a country's industrial activity. The contact process is a textbook case of a reversible, exothermic catalytic reaction where the engineer must trade equilibrium conversion against reaction rate, and of absorption with a strong heat effect.

Key ideas

Raw materials. Elemental sulphur (most common today, often recovered from refineries), pyrites (FeS₂), smelter off-gases from copper and zinc roasting, or spent acid.

Process steps (sulphur-burning plant).

1. Melting and filtering sulphur, then burning it in dry air: S + O₂ → SO₂, strongly exothermic. The burner gas typically contains 8–12 % SO₂. A waste-heat boiler recovers the heat as high-pressure steam.
2. Air drying: combustion air is dried in a packed tower irrigated with 93–98 % H₂SO₄. Moisture would form acid mist with SO₃ that is very hard to absorb and corrodes downstream equipment.
3. Catalytic conversion: SO₂ + ½O₂ ⇌ SO₃, ΔH ≈ −99 kJ/mol. The catalyst is vanadium pentoxide (V₂O₅) promoted with potassium sulphate on a silica support; it is active between about 400 °C (ignition) and 600 °C (above which it deactivates). The converter has 3–4 adiabatic beds with cooling between beds. In each bed the temperature rises as conversion rises; inter-bed coolers bring the gas back near 420–440 °C so that the next bed can push conversion closer to the falling equilibrium line.
4. Absorption: SO₃ is absorbed in 98–98.5 % H₂SO₄, not in water. Water would react with SO₃ in the gas phase to form a fine acid mist that passes through the tower. The acid strength is held constant by adding water to the circulating acid. Absorbing SO₃ in 98 % acid without adding enough water gives oleum (fuming sulphuric acid, H₂SO₄ with free SO₃).
5. Double contact double absorption (DCDA): after 2–3 beds the gas goes to an intermediate absorber, which removes SO₃. Removing a product shifts the equilibrium, so the remaining SO₂ converts further in the last bed. Overall conversion reaches 99.7 % or more, compared with about 98 % for single absorption. This is what keeps stack SO₂ within emission limits.

Le Chatelier's view of the converter. The reaction is exothermic and decreases moles (1.5 → 1), so low temperature and high pressure favour equilibrium. In practice the plant runs near atmospheric pressure (excess air and DCDA give enough conversion without compressors) and at the lowest temperature at which the catalyst is still fast enough.

Why V₂O₅ and not platinum. Platinum was used in early plants but is expensive and easily poisoned by arsenic and other impurities. V₂O₅ is cheap and tolerant.

Comparison with the lead chamber process. The older chamber process used nitrogen oxides as homogeneous catalysts and gave only about 62–78 % acid. The contact process gives 98 % acid and oleum directly.

Energy. The plant is a net steam exporter: combustion, conversion and absorption are all exothermic.

Formulas

S + O₂ → SO₂, SO₂ + ½O₂ ⇌ SO₃, SO₃ + H₂O → H₂SO₄

Overall: S + 1.5O₂ + H₂O → H₂SO₄ (32.06 kg S gives 98.08 kg acid)

X = (n_SO₂,in − n_SO₂,out) / n_SO₂,in

• X: fractional conversion of SO₂ (dimensionless), n: molar flow (kmol/h).

Kp = p_SO₃ / (p_SO₂ · p_O₂^0.5) (units bar^−0.5); Kp falls as temperature rises because the reaction is exothermic.

Equivalent H₂SO₄ in oleum (%) = (100 − w) + w × 98.08 / 80.06

• w: mass % free SO₃ in the oleum. Gives the mass of 100 % acid obtained per 100 kg oleum after adding water. Example: 20 % oleum = 104.5 % H₂SO₄.

Molar masses: S 32.06, SO₂ 64.06, SO₃ 80.06, H₂SO₄ 98.08 g/mol.

Worked examples

Example 1 (standard): sulphur demand. A plant makes 100 t/day of H₂SO₄ (100 % basis). Overall sulphur-to-acid efficiency is 99.5 %. Find the sulphur required.

1. 1 mol S gives 1 mol H₂SO₄, so the theoretical sulphur = 100 × 32.06 / 98.08 = 32.69 t/day.
2. Actual = 32.69 / 0.995 = 32.85 t/day.

Answer: about 32.9 t/day of sulphur.

Example 2 (GATE level): converter balance and stack SO₂. Burner gas: 10 mol % SO₂, 11 % O₂, 79 % N₂. Conversion of SO₂ in the converter is 98 %, and all SO₃ is absorbed in a single absorber. Find the SO₂ content of the gas leaving the absorber, in ppm (by volume).

1. Basis 100 mol of feed: SO₂ 10, O₂ 11, N₂ 79.
2. SO₂ converted = 0.98 × 10 = 9.8 mol, giving 9.8 mol SO₃. O₂ used = 9.8 / 2 = 4.9 mol.
3. Converter exit: SO₂ 0.2, O₂ 6.1, SO₃ 9.8, N₂ 79; total 95.1 mol.
4. After the absorber removes all SO₃: 0.2 + 6.1 + 79 = 85.3 mol.
5. SO₂ fraction = 0.2 / 85.3 = 2.345 × 10⁻³.

Answer: about 2340 ppm SO₂. This is far too high for modern limits; DCDA at 99.7 % conversion cuts it to about 350 ppm, which is why DCDA is standard.

Common mistakes

• Saying SO₃ is absorbed in water. It is absorbed in 98–98.5 % acid; water gives acid mist.
• Thinking higher temperature always helps. It speeds the reaction but lowers equilibrium conversion; above about 600 °C the catalyst also deactivates.
• Forgetting the volume change when computing mole fractions after conversion: total moles fall by half the SO₂ converted.
• Treating oleum percentage as acid strength. 20 % oleum means 20 mass % free SO₃, equivalent to 104.5 % H₂SO₄.
• Assuming the catalyst shifts equilibrium. It changes only the rate.

For GATE CH

Typical questions: catalyst identity and its promoter, the function of the drying tower and the absorber, why 98 % acid is the absorbent, why DCDA is used, and the effect of temperature and pressure on conversion. Numericals: sulphur or pyrite requirement, converter material balances with fractional conversion, mole fractions after reaction, oleum equivalents and simple Kp calculations. Practise balances on a 100 mol basis.

Quick check

1. What is the catalyst and its usual promoter?
2. Why is the gas cooled between converter beds?
3. In which liquid is SO₃ absorbed?
4. What does DCDA achieve?

Answers: 1. V₂O₅ promoted with K₂SO₄ on silica; 2. the reaction is exothermic, and cooling restores a higher equilibrium conversion for the next bed; 3. 98–98.5 % sulphuric acid; 4. intermediate SO₃ removal shifts equilibrium so overall conversion exceeds 99.7 %, cutting stack SO₂.