The Manufacturing Process of Sulphuric Acid ( Contact Process)

The manufacturing of sulphuric acid is one of the most important chemical inductries at the present time. Sulphuric acid, H₂SO₄ is a non-volatile diprotic acid. Concentrated sulphuric acid is a viscous colourless liquid. Sulphuric acid is an important chemical used to make other manufactures substances. The uses of sulphuric acid are to manufacture fertilizers, detergents, pesticides, synthetic fibre, paint pigments, as an electrolyte in lead acid accumulators, to remove metal oxides from metal surfaces before electroplating and etc.

Sulphuric acid is manufactured by the Contact process in industry. The raw materials used in the Contact process are sulphur or sulphide mineral), air and water. The Contact process involves three steps:

sulphur -> sulphur dioxide -> sulphur trioxide -> sulphuric acid

Step I: Production of sulphur dioxide gas, SO_2.

This is done by two methods.
(a) Burning of sulphur in air.

S + O₂ --> SO₂

(b) Burning of metal sulphide such as zinc sulphide or iron(III) sulphide in air.

2ZnS + 3O₂ --> 2SO₂ + 2ZnO

Step II: Conversion of sulphur dioxide to sulohur trioxide, SO₃

(a) Pure and dry sulphur dioxide with excess dry oxygen(from air) are passed

through a converter.

(b) A high percentage of sulphur dioxide is converted into sulphur trioxide under the following conditions:

(i) The presence of vanadium(V) oxide,V₂O₅ as a catalyst.

(ii) A temperature of 450’C – 550’C.

(iii) A pressure of one atmosphere.

2SO₂ + O₂ <--> 2SO₃

Step III: Production of sulphuric acid

(a) The sulphur trioxide produced is dissolved in concentrated sulphuric acid to produce oleum, H₂S₂O₇, a viscous liquid.

SO₃ + H₂SO₄ --> H₂S₂O₇

(b) Oleum is then diluted with an equal volume of water to produce concentrated sulphuric acid(98%).

H₂S₂O₇ + H₂O --> 2H₂SO₄

Sulphur trioxide is not dissolved directly in water to produce sulphuric acid.

SO₃ + H₂O --> H₂SO₄

This is because:

(a) the solubility of sulphur trioxide in water is low.

(b) The process releases a large amount of heat which will vapourise sulphuric acid to form fumes.

The sulphur dioxide gas is dried and purified before the production of sulphur trioxide gas.This is to remove:

(a) water vapour ( the reaction of water with sulphur trioxide will produce heat that will vapourise the acid)

(b) Contaminants such as arsenic compounds( found in the sulphur or sulphide mineral) which will poison the catalyst and make it ineffective.

The conversion of sulphur dioxide into sulphur trioxide is a reversible process. This means that not all the sulphur dioxide can be concerted into sulphur trioxide. The optimum temperature of 450’C – 550’C, the pressure of 1 atm and the use of V2O5 as catalyst are needed to ensure that a high percentage of sulphur trioxide is formed. Under these conditions, the conversion efficiency is about 98%.