Rates of Reaction and the Equilibrium Conditions
Equilibrium Considerations
To maximise the yield (force equilibrium to the right) we would have to introduce:
* excess oxygen
* high pressure (due to 1.5:1 ratio)
* low temperatures (as reaction is exothermic)
Kinetic Considerations
* a catalyst
* high temperature
* high pressure (increasing frequency of collisions as concentrations increases)
Energy Considerations
To minimise energy use (thus cost):
* atmospheric pressure (as high-pressure containers are expensive)
* no catalysts, as cost of researching, producing and using them are high
* heating from exothermic reactions (combustion of sulfur)
Compromise Conditions
A gas pressure between 100 and 200 kPa increases the collision frequency between the reacting gases. They increase the yield by driving the reaction to the right (1.5 moles to 1 mole of gas). These pressures are also sufficiently low to avoid expensive apparatuses.
An excess of oxygen will drive the reaction to the product side, increasing the yield. Thus, a 5:1 air:SO2 ratio is used (essentially 1:1 O2:SO2) creating an excess of oxygen.
A catalyst is used to increase the rate of reaction, compensating for the lower temperature. Its identity is V2O5 supported on a silica bed.
Sulfur trioxide is removed from the reaction mix, just before the last bed, by passing it through an interpass absorption tower (producing oleum). It helps shift the reaction to the right and increases yield. As such the gases removed to the atmosphere contain no more than 0.3% SO2.