Contact process

The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Vanadium(V) oxide is the catalyst employed.

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This process was patented in 1831 by the British vinegar merchant Peregrine Phillips, as a far more economical process for producing sulfur trioxide and concentrated sulfuric acid than the previous lead chamber process method used for producing sulfuric acid.

Process

The process can be divided into three stages:

1. Preparation and purification of sulfur dioxide
2. Catalytic oxidation (using vanadium oxide catalyst) of sulfur dioxide to sulfur trioxide
3. Conversion of sulfur trioxide to sulfuric acid

Purification of air and SO₂ is necessary to avoid catalyst poisoning (ie. removing catalytic activities). The gas is then washed with water and dried by H₂SO₄.

To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.

Sulfur dioxide and oxygen then react in the manner as follows:

2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g) : ΔH = −197 kJ mol⁻¹

To increase the reaction rate, high temperatures (450 °C), high pressures (10)atm), and vanadium(V) oxide (V₂O₅) are used to ensure a 99.5% conversion. Platinum would be a more suitable catalyst, but it is very costly and easily poisoned. The catalyst only serves to increase the rate of reaction - it has no effect on how much SO₃ is produced.

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated H₂SO₄ in the absorption tower to form oleum:

H₂SO₄(l) + SO₃ → H₂S₂O₇(l)

Note that directly dissolving SO₃ in water is impractical due to the highly exothermic nature of the reaction. Acidic vapour or mists are formed instead of a liquid.

Oleum is reacted with water to form concentrated H₂SO₄.

The average percentage yield of this reaction is around 30%.

H₂S₂O₇(l) + H₂O(l) → 2 H₂SO₄(l)

References

• Jim Clark (2002). The Contact Process. Chemguide.