Theory: This process involves the catalytic oxidation of sulphur dioxide into sulphur trioxide by atmospheric air.

2SO₂ + O₂ → 2 SO_3; A_H = - 196.6 kJ

The reaction is reversible, exothermic and involves a decrease in the number of moles. Therefore, according to the Le-Chatelier’s principle, the favourable conditions for the maximum yield of sulphur trioxide are as follows.

[i]. Low temperature: A decrease in temperature would favour the forward reaction. The optimum temperature is experimentally found to be 670-720 K.

[ii]. High pressure: An increase in pressure should favour the forward reaction because the reaction involves a decrease in a number of moles. In practice, about 2-atmosphere pressure is maintained.

[iii]. Excess of oxygen: An increase in the concentration of reactants favours the forward reaction. Hence, oxygen [air] is used in excess.

[iv]. Use of catalyst:

1] Even at low temperature and high pressure, the yield of sulphur trioxide is not very satisfactory.

2] Hence, a suitable catalyst must be used to increase the rate of formation of sulphur trioxide.

3] Several catalysts such as platinized asbestos, platinized silica gel, vanadium pentoxide, etc., may be used for the purpose.

4] Nowadays, vanadium pentoxide is extensively used as a catalyst in this process. It gives good results at 67-72 K and is less susceptible to poisoning.

The sulphur trioxide obtained as above is dissolved in concentrated sulphuric acid to get oleum. Sulphuric acid of desired concentration can be obtained by diluting it with water.

Plant Used and the Procedure:

The plant used in the process consists of following parts.

[i]. Sulphur burners: In these burners, sulphur is burnt to produce sulphur dioxide.

S + O₂ → SO₂

Iron pyrites may also be used in place of sulphur.

4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

Sulphur dioxide thus produced is mixed with air and is taken to the purification unit.

[ii]. Purification unit: The impurities present in the mixture of sulphur dioxide and air coming from sulphur burners are removed in this unit. The unit consists of following parts.

[a] Dust precipitator: It makes the dust particles to settle down.

[b] Washing and cooling tower: The dust-free gases are washed with water to remove the impurities and to cool the gases.

[c] Drying Tower: In this tower, the incoming gases are dried by a spray of conc. H₂SO₄.

[d] Arsenic purifier: It consists of gelatinous ferric hydroxide and absorbs any impurity of arsenic oxide present in the gases.

[e] Testing box: This box detects the presence of solid particles in the incoming gases. When the gases are found free from solid particles, they are allowed to move onwards to the contact tower.

[iii] Contact tower:

1] It consists of iron pipes packed with vanadium pentoxide.

2] The gases coming out of the testing box are heated to 725 K in a pre-heater and then allowed to enter into this tower.

3] As the mixture of sulphur dioxide and air passed through the pipes containing the catalyst, sulphur dioxide gets oxidized to sulphur trioxide.

4] The conversion of SO₂ into SO₃is exothermic and heat generated in the reaction is sufficient to maintain the temperature of the catalyst.

5] Therefore, once the process has started, the gases are not heated in the pre-heater and are passed directly into the contact tower.

[iv]. Absorption tower:

1] The sulphur trioxide formed in the contact tower is now led to the absorption tower where it is brought in contact with conc. H₂SO₄which is sprayed from the top of the tower.

2] Sulphur trioxide gets dissolved in conc. Sulphuric acid to give oleum. This can be converted into sulphuric acid of desired concentration by diluting it with a calculated amount of water.

3] Sulphur trioxide is not absorbed in water directly because it results in the formation of the dense fog of sulphuric acid particles which makes the operation difficult to handle.

4] This is why it is preferred to dissolve SO₃ into conc. Sulphuric acid to obtain oleum.