The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Platinum used to be the catalyst for this reaction; however, as it is susceptible to reacting with arsenic impurities in the sulphur feedstock, vanadium(V) oxide (V2O5) is now preferred.
The process can be divided into five stages:
1) Combining of sulfur and oxygen (O2) to form sulfur dioxide
2) Purifying the sulfur dioxide in a purification unit
3) Adding an excess of oxygen to sulfur dioxide in the presence of the catalyst vanadium pentoxide at 450 °C and 1-2 atm
4) The sulfur trioxide formed is added to sulfuric acid which gives rise to oleum (disulfuric acid)
5) The oleum is then added to water to form sulfuric acid which is very concentrated.
Purification of the air and sulfur dioxide (SO2) is necessary to avoid catalyst poisoning (i.e. removing catalytic activities). The gas is then washed with water and dried with sulfuric acid.
Too low of a temperature will lower the formation rate to an uneconomical level. Hence to increase the reaction rate, high temperatures (450 °C), medium pressures (1-2 atm), and vanadium(V) oxide (V2O5) are used to ensure an adequate (>95%) conversion. The catalyst only serves to increase the rate of reaction as it does not change the position of the thermodynamic equilibrium.
The industrial production of sulfuric acid involves proper control of temperatures and flow rates of the gases as both the conversion efficiency and absorption are dependent on these.