Production of SO2
If the process is started with elemental sulfur, it is first melted then sprayed into an excess of dry air at atmospheric pressure. The air is dried by passage through sulfuric acid, a dehydrating agent. This is done to avoid acid mist and corrosion in downstream pipes.
Relevant Equations: S(l) + O2(g) -> SO2(g)
An excess of air (oxygen) ensures the sulfur reacts completely. The combustion of sulfur generates a lot of heat. It is necessary to cool the gas stream from 1000oC to around 400oC, he optimum temperature for the next step. Heat exchangers are used to remove the excess heat and recycle this heat energy so it can be used to remelt more sulfur or power turbines for electrical energy.
Alternatively, SO2 can be obtained from the smelting of metal oxide ores.
2CuFeS2(s) + 5O2(g) -> 2Cu(l) + 2FeO(s) + 4SO2(g)
Catalytic Oxidation of SO2 and SO3
Clean, dry sulfur dioxide is the feedstock for this stage. Electrostatic precipitators are used to remove any ash from the stream.
Relevant Equations: SO2(g) + 1/2O2(g) <-> SO3(g) $\Delta$ = -99 kJ/mol
The sulfur dioxide is mixed with air at pressure slightly higher than atmospheric, and is passed through a catalyst tower, called a converter. The tower contains 3 or 4 layers of vanadium oxide catalyst. Unreacted gases are recycled back into the gas stream.