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The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes.

 

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        

To increase the reaction rate, high temperatures (450 °C), high pressures (200 kPa or 2 atm) are used to ensure a 99.5% conversion.

H₂SO₄(l) + SO₃(g)  → H₂S₂O₇(l)

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

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

Oleum is reacted with water to form concentrated H₂SO₄.

H₂S₂O₇(l) + H₂O(l) → 2 H₂SO₄(l)

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