Manufacture of sulphuric acid by contact process, properties and uses

Sulphuric acid

Sulphuric acid, H₂SO₄ is a colorless, odorless, extremely corrosive, oily liquid. It was initially called oil of vitriol.

• It occurs in the free state and combined state.

• It is also formed near sulphide beds.

In the combined form, it occurs as salts in minerals as metallic sulphates.

Manufacture of Sulphuric Acid

On industrial scale, sulphuric acid can be prepared by the following two methods.

• Lead chamber process
• Contact process

Sulphuric acid is prepared by contact process all over the world.

Preparation of sulphuric acid by contact process is based upon the catalytic oxidation of SO₂ to SO₃.

There are five steps in the manufacture of sulphuric acid by the contact process:

• Production of sulphur dioxide

• Purification of gases

• Oxidation of sulphur dioxide to sulphur trioxide

• Absorption of sulphur trioxide in sulphuric acid

• Dilution of oleum

Pure sulphur is burnt in pure and excess of oxygen, in the sulphur burners to obtain a mixture of sulphur dioxide gas and oxygen.

Burning of sulphur or iron pyrites in oxygen is preferred to purified air since heat energy is wasted in heating the unreactive nitrogen component of the air.

The mixture of sulphur dioxide and air obtained contains various impurities which must be removed; or else the catalyst loses its efficiency.

There are dust particles, pyrite dust and arsenious oxide. The mixture is passed through a purifier called electric precipitator consisting of a chamber with high electric potential wires. The electric charge attracts solid particles which are removed. The gas is then led to a water scrubber where it is completely freed from dust particles. It then dried by a spray of concentrated sulphuric acid in another chamber through arsenic purifier where every trace of arsenic oxide is removed.

The clean, dried gaseous mixture of sulphur dioxide and air is passed through a tower loosely packed with vanadium pentoxide or platinum on perforated shelves. The catalyst is placed in vertical iron pipes inside a cylindrical iron tower called the converter. Here the preheated mixture of sulphur dioxide and air form sulphur trioxide.

As the reaction is exothermic, the catalyst is heated in the beginning and the temperature is maintained by the heat evolved during the reaction. Initially it is heated to about 450^oC.

This is the main chemical reaction occurring in the Contact process. It is a reversible, exothermic reaction accompanied by a decrease in volume.

Applying Le Chatelier's principle, to get a good yield of sulphur trioxide,

the following points are to be borne in mind:

A low temperature is necessary. An optimum temperature of

410-450^oC is found suitable.

High pressure would favour the forward reaction. It is not possible to build acid resistant towers which can withstand high pressures. A pressure of 760 - 1520 mm of Hg is used.

Excess of oxygen is necessary as it would favour the forward reaction.

A suitable catalyst is required. Platinum is a more efficient catalyst than vanadium pentoxide. But it is more expensive. It also gets poisoned by impurities like arsenic (III) oxide, Vanadium pentoxide though less efficient is cheaper and not affected by impurities. K₂O may be used as a promoter to enhance the activity of the catalyst.

Absorption of SO

₃ in H₂SO₄

Sulphur trioxide is not dissolved directly in water as it emits a lot of heat. It forms misty droplets of sulphuric acid, which do not condense easily and is not absorbed by water directly. Sulphur trioxide is cooled in a heat exchanger and is then absorbed in concentric sulphuric acid in another tower to give oleum(H₂S₂O₇).

Sulphuric acid is soluble in water in all proportions. Dilution of oleum or pyrosulphuric acid is done only by the addition of acid to water. If water is added, there is a sudden increase in temperature and the acid being in bulk tends to spurt out.

If acid is added to water, the water is in a larger amount and the acid being heavier settles down. The heat evolved is dissipated in the water and hence spurting of acid is reduced.

Sulphuric acid forms a constant boiling mixture at 338^oC containing 98.5% of the acid.

A constant boiling mixture is one, which boils without change in composition. Hence on boiling the above mixture, vapours of both acid and water evolve in the same proportion as in the liquid mixture.

Uses of sulphuric acid

• In the manufacture of fertilizers, ammonium phosphate and calcium super phosphate.

• In the manufacture of rayon and nylon and also in the preparation of dyes and drugs from coal tar derivatives.

• In the manufacture of the explosives such as Tri-nitro toluene , Tri-nitro glycerine and picric acid.

• In the manufacture of nitric acid, hydrochloric acid and phosphoric acid.

• In the manufacture of sodium sulphate for glass industry and ferrous sulphate for ink industry.

• In the purification of petrol, kerosene, and lubricants.

• It is used in metallurgy for extraction of metals. Leaching of metallic compounds gives sulphates which on electrolysis gives the metal in pure form .It is used for pickling of metals.

• It is used in storage of batteries.

• It is used as a laboratory reagent for the preparation of iodine, carbon monoxide and hydrogen.