Industrial Preparation of Sulphuric Acid by Contact Process

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Industrially, the main method of manufacture of sulphuric acid is the Contact Process. In this process, sulphur dioxide is first catalytically oxidized to sulphur trioxide. The trioxide is then absorbed by concentrated sulphuric acid. The product is then diluted to the desired concentration of the acid required.

The main steps are as follows:

1. Production of Sulphur Dioxide

Sulphur dioxide is prepared by burning sulphur or iron pyrites.

2. Purification of Sulphur Dioxide

Before catalytic oxidation of sulphur dioxide, impurities like dust, arsenic etc. have to be removed otherwise they spoil the catalyst.

3. Catalytic Oxidation of Sulphur Dioxide to Sulphur Trioxide with Oxygen of the air

The catalysts used in this process are platinum or vanadium pentoxide (V₂O₅) that are heated to a high temperature. The optimum temperature required is 450 - 500^oC.

Remember:-

Low temperature favours the oxidation of sulphur dioxide. If the temperature is too low, the rate of reaction is slowed down. At high temperature sulphur trioxide may decompose back to form sulphur dioxide and oxygen. So an optimum temperature of 450-500^oC is generally used.

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4. Absorption of Sulphur Trioxide in 98% Sulphuric Acid

The sulphur trioxide formed is not dissolved in water directly to yield sulphuric acid. This is because this reaction is highly exothermic. It forms clouds of sulphur trioxide and water vapour, which corrode the plant and machinery and make intolerant working conditions. Instead, sulphur trioxide is absorbed by 98% sulphuric acid to produce pyrosulphuric acid or oleum.

Remember:-

Oleum is also called pyrosulphuric acid or orthosulphuric acid. Its usual formula is H₂S₂O₇. It can also be represented as H₂SO₄SO₃.

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5. Dilution of Oleum or Pyrosulphuric Acid with Water

 

Requisite quantity of water is then added to Oleum, so as to obtain sulphuric acid of the desired concentration.

Summary of Contact Process

Step 1

Production of SO₂ from pyrites or sulphur.

Step 2

Purification of SO₂ from air impurities like dust, arsenic etc. are removed as otherwise these will lead to poisoning of catalyst.

Step 3

Catalytic oxidation of SO₂ to SO₃.

Catalyst - V₂O₅ (Vanadium pentoxide) or platinum

Temperature: 450^oC - 500^oC.

Pressure: 760 - 1520 mm of Hg

Step 4

Absorption of SO₃ in 98% sulphuric acid to form oleum.

Step 5

Dilution of oleum to obtain sulphuric acid of desired concentration.