what is the contact process for the manufacture of sulphuric acid.

Explanation

Sulphuric acid is prepared in the following steps :
i) Formation of sulphur dioxide
`SO_2` is formed by heating sulphur or iron pyrites in the excess of air.
`S + O_ 2 to SO_2 , 4FeS_2 + 11O_2 to2 Fe_2O_3 + 8SO_2`
Catalytic oxidation of `SO_2` into `SO_3`
`2SO_2+O_2overset(V_2O_5)to2SO_3`
Following are the favourable conditions to get maximum yield of `SO_3`.
a) High concentration of reactants. b) Low temperature.
c) Presence of catalyst. d) High pressure.
iii) Absorption of `SO_3` in 98% `H_2SO_4`
`SO_3+H_2SO_4tounderset("Oleum")(H_2S_2O_7)`
iv) Dilution of oleum with water
`H_2S_2O_7+H_2Oto2H_2SO_4`
Plant and its working
a) Pyrite or sulphur burners : In this unit `SO_2` is formed.
b) Purifying unit
1) Dust chamber : Steam is introduced from the top and dust particles are settled down.
ii) Coolers: The temperature of the gas is lowered to 373 K.

iii)Scrubber : Mist and water soluble impurities are removed.
iv) Drying tower: Gaseous mixture is dehydrated by spraying concentrated `H_2SO_4`.
v) Arsenic purifier : Freshly prepared ferric hydroxide `[Fe(OH)_3]` absorbs the impurities of arsenic.
c) Testing box : Purity of a gas is checked in the testing box. Impure gas is again passed to the purifying unit.
d) Contact tower : In contact tower, `SO_3` is formed from `SO_2`.
`2SO_2+O_2overset(V_2O_5)to2SO_3, Delta_r H^(Theta) = - 196.6 kJ`.
e) Absorption tower
`SO_3` is absorbed in fuming sulphuric acid.
`H_2SO_4+SO_3underset("oleum")toH_2S_2O_7,H_2S_2O_7+H_2Oto2H_2SO_4`