SEPARATION OF GASES FROM AIR

SEPARATION OF GASES FROM AIR

In order to separate the major components of air, it is first purified, then liquified and finally fractionally distilled. The steps involved in the process are as follows -

Purification of Air:

• Air generally contains carbon dioxide gas, hydrogen sulphide gas and sulphur dioxide gas as impurities. In addition to it there are dust particles also.
• First of all air is washed by passing it through water, where the dust particles are removed.
• The washed air is passed through dilute caustic soda solution, where the gases like carbon dioxide, sulphur dioxide and hydrogen sulphide are removed.
• The purified air, however, contains moisture. The moist air is passed through pipes, maintained at a temperature below – 20o C, where vapour present in it freezes and hence, air becomes dry.
• The air leaving the cooling pipes is free from all impurities.

Liquification of Air:

• The cool air, free from all impurities is compressed to a pressure 200 times more than the atmospheric pressure. The compression raises the temperature of the air.
• The hot compressed air is then passed through cooling tank in which cold water enters from one end and warm water leaves from the other end.
• The compressed and cooled air is passed through a spiral pipe, placed in a vacuum flask. The end of spiral pipe is provided with a fine jet.
•  When compressed air suddenly escapes from the jet, its pressure suddenly falls. Thus, its molecules move wide apart. When the molecules move wide apart, they need energy. This energy is taken by the molecules from themselves and hence their temperature drops.
• The air so cooled, is now at a pressure equal to that of atmosphere. This cooled air rises up and in the process further cools the incoming compressed air in spiral tube. The air is then sucked again by the compression pump and the cycle is repeated. With every cycle, the temperature of air drops, till it liquefies.

Fractional Distillation of Air :

•  The liquid air mainly of nitrogen and oxygen, and is at a temperature of – 200oC.

•  The boiling point of liquid nitrogen is – 195oC and that of liquid oxygen is – 183oC.
•  The liquid is gradually warmed to -1950 C, when nitrogen starts boiling off from the liquid air. The nitrogen gas so formed, is compressed and filled is steel cylinders.
• The liquified oxygen left behind, is also changed to gas and then filled in compressed state in steel cylinders.