7.21 Describe the manufacture of H2SO4 by contact process?

Sulphuric acid is manufactured by the contact process. It involves the following steps:

Step (i):

Sulphur or sulphide ores are burnt in air to form SO2.

Step (ii):

By a reaction with oxygen, SO2 is converted into SO3 in the presence of V2O5 as a catalyst.

2SO(g) + O2(g) $\stackrel{{V}_{2}{O}_{5}}{\to }$ 2SO3(g)

Step (iii):

SO3 produced is absorbed on H2SO4 to give H2S2O7 (oleum).

SO+ H2SO→ H2S2O7

This oleum is then diluted to obtain H2SOof the desired concentration. In practice, the plant is operated at 2 bar (pressure) and 720 K (temperature). The sulphuric acid thus obtained is 96-98% pure.