How Sulphuric Acid Is Manufactured By Contact Process?

The contact process produces sulphuric acid in three stages.

First sulphur dioxide is produced, either by burning sulphur in air or by heating sulphide ores like pyrites in excess oxygen.

Second the sulphur dioxide is converted into sulphur trioxide. This is done at a temperature of four hundred to four hundred and fifty degrees Celsius, at one to two atmospheres of pressure and in the presence of a Vanadium Oxide catalyst. This reaction is a reversible reaction and the conditions used are a compromise between equilibrium and rate considerations.

The reaction is an exothermic reaction, so the the equilibrium would produce more sulphur trioxide at lower temperature. The reaction would be far too slow at lower temperatures, however, so a higher temperature is used.

The reaction would produce more sulphur trioxide at higher pressure, but economic considerations mean that the pressure is not much over standard pressure.

The catalyst speeds the reaction up considerably and so is important.

The third step is the production of sulphuric acid from sulphur trioxide. This is done by dissolving it in concentrated sulphuric acid to make oleum and then adding water which makes sulphuric acid. It is done this way because the addition of sulphur dioxide straight to water is uncontrollable and produces a fog of sulphuric acid fumes.

Contact Process was developed by knietsch in Germany. Basically it involves the catalytic combination of sulphur and oxygen to form sulphur monooxide (SO), which is then dissolved in water to form sulphuric acid.
Various steps in the process are given below:-

1. Production of sulphur dioxide :

Sulphur dioxide is produced by burning sulphur or roasting of iron sulphide in air in pyrite burners.
S + O2 -> SO2
4FeS2 + 1102 -> 2Fe2O3 + 8SO2

2. Purification of sulphur dioxide :

Purifying unit consists of dust remover,cooling pipes,scrubbers,drying tower and testing box.

Dust particles are removed from the mixture of SO2 and O2 with steam.These gases are passed through lead pipes to cool them to 100 C. Cooled gases are washed by spray of water.The gases are then dried with concentrated sulphuric acid trickling through coke filled in the tower.The purified gases are dried and passed through a beam of light to confirm the absence of solid particles.

3. Oxidation of SO2 to SO3 in Contact Tower:

Purified and heated gases are passed through contact tower where SO2 is oxidized to SO3 in the presence of the catalyst V2O5 at 400-500 C.
2SO2 + O2 ------> 2SO3

4. Production of sulphuric acid :

Sulphur trioxide from the contact tower is passed onto absorption tower where it is absorbed by descending stream of 98% sulphuric acid to form oleum. It is diluted with water to get sulphuric acid of desired concentration.

SO3 + H2SO4 -----> H2S2O7
H2S2O7 + H20 -----> 2H2SO4

A molecule of Sulphuric Acid, H2SO4, consists of two atoms of hydrogen, one atom of sulphur and four atoms of oxygen. Sulphuric acid is oily and colorless liquid. Its melting point is 10.3 degrees and 338 degree centigrade. Sulphuric acid is produced by dissolving Sulphur-trioxide in water. It is a strong acid in lead-chamber process. It was used for manufacturing Sulphuric acid from the mid-1930s, but these days this process has now been inter changed by the contact process, including the catalytic oxidation of sulphur dioxide.

The contact method which is made by burning sulphur or by roasting sulphide ores is used for manufacturing Sulphuric acid and fuming Sulphuric acid from sulphur dioxide,
For making sulphur trioxide in the presence of a catalyst, sulphur dioxide is combined with oxygen in the form of air. Valadium oxide (V2O5) is used as a catalyst. To keep the temperature at 450 degrees C, the conditions are controlled. The fuming sulphuric acid is called Oleum. Sulphur trioxide dissolves in Sulphuric acid and makes Oleum.

S(s) + O2(g) ==> SO2(g)
Sulphur
Dioxide
2SO2(g) + O2(g) ==> 2SO3(g)
Sulphur
Trioxide

SO3(g) + H2SO4(l)==> H2S2O7(l)
Oleum

This Oleum, H2S2O7, is then mixed with Water, H2O, to produce concentrated Sulphuric Acid, H2SO4.

H2S2O7(l)+ H2O (l) ==> 2 H2SO4(l)
Oleum Sulphuric
Acid

Sulphuric acid was first prepared in the laboratory by a Muslim scientist Jabir-Bin-Hayan. It is one of the most important chemical compounds known. In Pakistan and other countries sulphuric acid is manufactured by the contact process. The six main steps involved in the contact process. First of all sulphur dioxide is produced by burning sulphur or roasting sulphide ore in the air. Sulphur dioxide is then purified by removing impurities like dust and arsenic compounds which would otherwise poison the catalyst. It is then passed through purifying chamber and sprayed by steam. The solid particles of impurities stick to the droplets of water and settle down.

Then the humid gases (sulphur dioxide and air) are dried by passing through a drying tower in which concentrated sulphuric acid is sprayed. After the complete removal of dust and other impurities, sulphur dioxide gas is passed over vanadiumpentaoxide or platinum catalyst at 450°c and 2-3 atmospheric pressure, though a contact chamber. The reaction is reversible but under these conditions of temperature and pressure 98% conversion to sulphur trioxide is achieved. It is an exothermic reaction and lot of energy is released.
Sulphur trioxide gas formed in the contact chamber is then absorbed into 98% sulphuric acid to produce oleum or pryosulphuric acid which is then diluted with appropriate amount to produce 98-100% sulphuric acid.

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