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.
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.
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