ask mattrab Visit www.askmattrab.com for more academic resources.

Manufacture of Sulphuric Acid by Contact Process

 sanjiv %289%29

↪ The contact process derives its name from the fact that the oxidation of SO2 to SO3 takes place only when the reactants come in contact with the surface of a catalyst.
↪ It is the most modern method.


↪ Sulphur dioxide is obtained by burning sulphur or iron pyrites.

S + O2 → SO2
4FeS2 + 11O2 → 2Fe2O3 + 8SO2

↪ The contact process for the manufacture of sulphuric acid is based on the catalytic oxidation of SO2 to SO3 by atmospheric oxygen.
↪ SO2 and air are passed over a catalyst (V2O5) heated to 450-500°C.
↪ About 98% of the possible yield of sulphur trioxide is obtained.

2SO2(g) + O2(g) ⇌ 2SO3(g) + 45.0 kcal

↪ The mechanism of oxidation of SO2 to SO3 by using V2O5 catalyst consists of two steps.

Step 1: Oxidation of SO2 into SO3 by V5+

2SO2 + 4V5+ + 10O2- → 2SO3 + 4V4- + 8O2- 

Step 2: Regeneration of catalyst  by oxidation of V4+ to V5+ by Oxygen (O2).

4V4+ + 8O2- + O2 → 4V5+ + 10O2-

↪ The sulphur trioxide can't be satisfactorily absorbed by water.
↪ A mist of fine drops of dilute sulphuric acid fill the factory if direct absorption is tried.

↪ Therefore, SO3 produced is absorbed in conc.
↪ H2SO4 and the fuming liquid called Oleum or pyro sulphuric acid, liquid is formed and it is diluted water to get sulphuric acid of required concentration.

H2SO4 + SO3(g) → H2S2O7(l) {Oleum}

H2S2O7(l) + H2O(l) → 2H2SO4(aq.)

↪ Thus, the manufacture of H2SO4 is nothing but getting SO3 in excess from air by the oxidation of SO2.

Suitable Conditions for maximum yield 

↪ The reaction 2SO2 + O2 ⇌ 2SO3 + 45 kcal is reversible, exothermic and proceeds with decrease in volume.

⁕ Low temperature

↪ Low temperature will favor the forward reaction because of its exothermic nature.

⁕ High Pressure

↪ High pressure favors the forward reaction.
↪ A pressure of 2-3 atms is sufficient to make steady flow of gases.

⁕ Excess of Oxygen

↪ A slight excess of oxygen in the reacting mixture helps to carry the reaction in the direction of the formation of sulphur trioxide.

⁕ Use of Catalyst (V2O5)

↪ At the low temperature maintained, the velocity of reaction is very slow and in order to accelerate the formation of SO3, the use of a catalyst is extremely necessary.

⁕ Purity of gases

↪ As the catalyst is open to poisoning, the mixture of SO2 and air of oxygen should be pure and dry and free from all types of impurities, particularly the dust.

⁕ Flowsheet diagram of Manufacture of Sulphuric Acid by Contact Process