Sulphuric Acid (Contact Process)
2nd Year Chemistry Notes
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
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