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Contact Process



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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