Contact process
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In British English, Sulphur is spelt with a ph, in American English sulfur is spelt with an f.
The contact process is the current method of producing sulphuric acid in the high concentrations needed for industrial processes. Vanadium(V) oxide is the catalyst employed.
This process was patented in 1831 by the British vinegar merchant Peregrine Phillips, as a far more economical process for producing sulphur trioxide and concentrated sulphuric acid than the previous lead chamber process method used for producing sulphuric acid.
Manufacturing sulphuric acid by the contact process
The process can be divided into three stages:
- Preparation and purification of sulphur dioxide
- Catalytic oxidation (using vanadium oxide catalyst) of sulphur dioxide to sulphur trioxide
- Conversion of sulphur trioxide to sulphuric acid
To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.
sulphur 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 (200 kPa or 2 atm), and vanadium(V) oxide (V2O5) are used to ensure a 95% conversion. Platinum would be a more suitable catalyst, but it is very costly and easily poisoned.
Hot sulphur 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. 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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