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.
The process can be divided into three stages:
- Preparation and purification of sulfur dioxide
- Catalytic oxidation (using vanadium oxide catalyst) of sulfur dioxide to sulfur trioxide
- Conversion of sulfur trioxide to sulfuric acid
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)