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