The Contact process is used to manufacture sulfuric acid. The main steps involved in the process are:
1. Production of sulfur dioxide (SO2) by burning sulfur or iron pyrite (FeS2) in air:
S + O2 → SO2
2FeS2 + 11O2 → 2Fe2O3 + 8SO2
2. Conversion of SO2 to sulfur trioxide (SO3) using a catalyst, usually vanadium pentoxide (V2O5):
2SO2 + O2 ⇌ 2SO3 (ΔH = -196 kJ/mol)
3. Absorption of SO3 into concentrated sulfuric acid to form oleum (H2S2O7):
SO3 + H2SO4 → H2S2O7
4. Dilution of oleum with water to form concentrated sulfuric acid (H2SO4):
H2S2O7 + H2O → 2H2SO4
The physical conditions required to achieve maximum yield in the Contact process are:
1. High pressure: The reaction between SO2 and O2 to form SO3 is exothermic, meaning that it releases heat. Higher pressure shifts the equilibrium towards the product side, resulting in a higher yield of SO3. The typical operating pressure is around 1-2 atmospheres.
2. Low temperature: The reaction between SO2 and O2 to form SO3 is also exothermic, meaning that it releases heat. Lowering the temperature of the reaction helps to reduce the amount of heat released and shift the equilibrium towards the product side. The typical operating temperature is around 400-450°C.
3. Catalyst: A catalyst, usually vanadium pentoxide, is used to increase the rate of the reaction between SO2 and O2 to form SO3. The catalyst is typically supported on a porous material, such as silica, to increase its surface area and enhance its activity.
Here is a flow sheet diagram for the manufacture of sulfuric acid using the Contact process:
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Sulfur or Iron Pyrite + Air → SO2 → Catalyst → SO3 → H2SO4
(Concentration and Dilution with Water)
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