Oxidation of Sulphur Dioxide to Sulphur Trioxide
Cleaned and dried sulphur dioxide is passed with air over catalyst pellet beds in four cylindrical vessels called converters. The pressure is normally no more than 2 atmospheres.
The catalyst used is predominantly vanadium (V) oxide, V2O5, with alkali metal sulphate promoters (mainly potassium or caesium sulphate) on a silica ceramic base.
The catalyst components are mixed into a paste and extruded as small cylindrical pellets, rings or star-ring shapes, and baked at high temperatures. The catalyst bed may be some 8m in diameter and some 60 cm deep.
This reaction is exothermic, and the heat must be removed to prevent the equilibrium moving to the left. The heat can be used elsewhere in the plant.