Contact Process - Process

Process

The process can be divided into five stages:

1. combining of sulphur and oxygen;
2. purifying sulphur dioxide in the purification unit;
3. adding excess of oxygen to sulphur dioxide in presence of catalyst vanadium oxide;
4. sulphur trioxide formed is added to sulphuric acid which gives rise to oleum (disulphuric acid);
5. the oleum then is added to water to form sulphuric acid which is very concentrated.

Purification of air and SO₂ is necessary to avoid catalyst poisoning (i.e. removing catalytic activities). The gas is then washed with water and dried by sulphuric acid.

To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.

Sulphur dioxide and oxygen then react as follows:

2 SO₂(g) + O₂(g) ⇌ 2 SO₃(g) : ΔH = −197 kJ mol−1

According to the Le Chatelier's principle, a lower temperature should be used to shift the chemical equilibrium towards the right, hence increasing the percentage yield. However 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 a 96% conversion. Platinum would be a more effective catalyst, but it is very costly and easily poisoned. The catalyst only serves to increase the rate of reaction as it does not change the position of the thermodynamic equilibrium. The mechanism for the action of the catalyst comprises two steps:

1. Oxidation of SO₂ into SO₃ by V5+:

2 SO₂ + 4V5+ + 2 O2- → 2 SO₃ + 4V4+

2. Oxidation of V4+ back into V5+ by oxygen (catalyst regeneration):

4 V4+ + O₂ → 4 V5+ + 2 O2-

Hot sulphur trioxide passes through the heat exchanger and is dissolved in concentrated H₂SO₄ in the absorption tower to form oleum:

H₂SO₄(l) + SO₃(g) → H₂S₂O₇(l)

Note that directly dissolving SO₃ in water is impractical due to the highly exothermic nature of the reaction. Acidic vapor or mists are formed instead of a liquid.

Oleum is reacted with water to form concentrated H₂SO₄.

The average percentage yield of this reaction is around 30%.

H₂S₂O₇(l) + H₂O(l) → 2 H₂SO₄(l)