Explain the manufacture of sulphuric acid by contact process.

Explanation

Theory: Manufacture of `H_2 SO_4` by contact process involves following steps:
(i) Production of sulphur dioxide : By buring powdered sulphur or by roasting sulphur rich ores.
`S_3 + 8O_2 to 8SO_2`
`4FeS_2 + 11O_2 to 2Fe_2 O_3 + 8SO_2`
(ii) Oxidation of sulphur dioxide t sulphur troxide.
`2SO_2 (g) + O_2 (g) hArr 2SO_3 (g)`
`DeltaH = 196.6 KJ`
According to Le-chatelier.s principles the conditions for maximum yield of `SO_3` are as follows:
(a) Temperature : As the reaction is exothermic, a low temperature favours the forward reaction. In optimum temperature of 673–723 K is used.
Pressure : As the forward reaction reaction proceeds with the decrease in number of gascous moles, pressure favours the formation of `SO_3`. Thus a pressure of 1.5 -2 atm, is maintained.
(c) Catalyst : As such at low temperature the reaction is slow. Therefore, a catalyst is used. The catalyst used now-a-days is vanadium pentaoxide `(V_2 O_5)`.
(d) Purity of gases : To prevent the poisoning of catalyst, the gases must be free from impurities of `As_2 O_3`, dust particles and moisture.
(e) Excess of oxygen : To get a better yiled of `SO_3` an excess of oxygen is used.
(iii) Conversion of `SO_3` in to `H_2 SO_4`: Sulphur trioxide is not directly dissovled in water as it leads to the formation of dense fog which is difficult to settle. Hence `SO_3` is dissolved in conc. `H_2 SO_4` to get which can be diluted to get desired concentration of `H_2 SO_4` .
`SO_3 + H_2 SO_4 to H_2 S_2 O_7`
`H_2 S_2 O_7 + H_2 O to 2H_2 SO_4`
Plant and its working : The plant used in the contact process is as shown in the fig. It consists of following parts:
(i) Sulphur burner : Here `SO_2` is produced.
(ii) Purification unit: The different plants of purification unit are :
(a) Dust precipitator: It removes dust from gases.
(b) Scrubber : It removes soluble impurities. In this tower gases are washed with water.
(c) Drying tower : In this tower gases are dried with conc. `H_2 SO_4`.
(d) Arsenic purifier : In this tower impurity of `As_2 O_3` is removed by using gelatinous `Fe(OH)_3`. The testing in this box is based on Tyndal effect.
The gases coming out of this unit are tested in a testing box. If impurities are still present these are recirculated again through the purification unit.
(iii) Catalytic convertor : The pure gases coming from the testing box are preheated to 673 of 723 K in preheater. Hot gases are then passed through the catalyst chamber where `SO_2` is oxidised to `SO_3`.
(iv) Absorption tower : This tower is packed with quartz pieces or acid proof flint over which 98% `H_2 SO_4` trickles down. Here `SO_3` is dissovled in `H_2 SO_4` to get oleum. Oleum is then diluted with water to get desired concentration of `H_2 SO_4`.