Discuss the favourble conditions for the manufacture of sulphur trioxide from Sulphur dioxide ?

Explanation

Sulphur trioxide is manufactured by the direct oxidation of sulphur dioxide with atmospheric oxygen.
`2SO_2 (g) +O_2 (g) underset("Endothermic")overset("Exothermic")hArr 2SO_3 (g)`
According to Le-Chatelier.s principle the favourable condition for the manufacture of `SO_3` are : 1. High Concentration : High concentration of So, and O, favour the formation of `SO_3` . 2. Low Temperature : Since forward reaction is exothermic hence low temperature favour the formation of `SO_3`. But at low temperature the rate of reaction is very slow. Thus an optimum temperature 623-723 K is maintained to get the maximum yield of `SO_3` 3. High Pressure : Since the formation `SO_3` takes place with the decrease in volume, therefore high pressure favour the formation of maximum yield of `SO_3`. A high pressure of about 2 atmosphere is applied. 4. Catalyst : To increase the reaction rate vanadium pentaoxide (`V_2 O_5`) is used as a catalyst. 5. Purity of Cases : The impurities can act as the poison for the gases and thus decrease the efficiency. So, `SO_2` and `O_2` gases must be purified before they made to react scrubbing tower in which is sprayed from the top. The gases can washed. 6. Drying tower : The moist gases from the scrubbing tower are now passed through drying tower from which conc. `H_2 SO_4` is sprayed. Moisture is absorbed by conc. `H_2 SO_4` and gases are dried.
7. Arsenic Purifier : The dry gaseous mix from the tower is passed through arsenic purifier. The purifier contains gelatinous ferric hydroxide resin on slalow. It absorbs many arsenic impurity from the gases. 8. Testing Box : The gases are passed through the testing box to remove any solid particles. Gases if found solid particle and passed on to the contact tower, so these are to be purified again. 9. Catalyst Chamber : Pure gases are passed through a preheater heated to about 920 K. These gases are passed into catalyst chamber and oleum is obtained which is diluted with water to get sulphuric acid.