XII Chemistry BIEK Topic - Sulphuric Acid (Contact Process)

Sulphuric Acid (Contact Process)
Introduction
On industrial scale two methods are employed for the manufacture of sulphuric acid, the Lead Chamber Process and the Contact Process.




The Contact Process
When a mixture of pure and dry sulphur dioxide and air is passed over a heated catalyst such as Platinum Asbestos, then Sulphur dioxide is oxidized into sulphur trioxide. This gas gives sulphuric acid when treated with water.

The plant used in Contact in Contact Process is composed of following parts.


1. Sulphur and Pyrite Burner

In this part, SO2 is produced by either burning sulphur in air or roasting ores. Like ZnS, PbS, FeS2 in air.

S + O2 ----> SO2

4FeS2 + 11O2 ----> 2Fe2O3 + 8S2


2. Dust Chamber

The SO2 obtained from pyrite burner contains impurities such as unreacted coal, dust, moisture, arsenic compounds etc. These impurities are removed from the gas in dust chamber. In this chamber hot air is blown as a result the heavy particles are removed and unreacted sulphur convert into SO2. Now the gases are passed through a cooling pipe into water chamber.


3. Water Chamber

In this chamber a stream of water is showed on the gas to remove the dust particles. The SO2 gas is now proceed into another chamber, which is known as drying tower.


4. Drying Tower

In drying tower moist SO2 gas is mixed with concentrated H2SO4. As a result the gas become dry and dilute H2SO4 is sink at the bottom.


5. Oxidation Tower

Before oxidation the dry gas is allowed to pass through arsenic purifier containing ferric hydroxide here the arsenic compounds are absorbed. The purified gases are then passed through a strong beam of light to remove any remained dust particle.

The purified and dry SO2 gas is passed through the oxidation tower. This tower consist of iron pipes packed with plantanized asbestos coated with Vanadium Penta Oxide (V2O5), which act as catalyst. In this tower the temperature is kept constant at about 480ºC, under high pressure and high concentration of sulphur dioxide and oxygen, oxidation of SO2 takes place in a favourable condition to yield maximum amount of SO3.

2SO2 + O2 <----> 2SO3 (45000 cal)


6. Absorption Tower

Sulphur trioxide formed in Oxidation Tower is passed up to Absorption tower, where it meets a stream of concentrated sulphuric acid and hence absorbed to form OLEUM. The maximum absorption of sulphur trioxide is 60% at which oleum has maximum density.

SO3 + H2SO4 ----> H2S2O7

Oleum can be diluted to obtain any desired concentration of sulphuric acid.

H2S2O7 + H2O ----> 2H2SO4

Sulphuric acid is obtained from this process is nearly 99.99% pure.