The air is filtered to remove any dust
Any carbon dioxide or water vapour in the air must be removed, otherwise when the air is cooled they would solidify and block up the pipes. Carbon dioxide is removed by passing the air through an alkali like sodium hydroxide.
Sodium hydroxide + carbon dioxide -> Sodium Carbonate + water.
Water is removed in a drying tower by using a drying agent such as silica gel
The air is then compressed to 200 atmospheres. On compression, the air gets hotter. This compressed air is then suddenly allowed to expand. This causes the temperature of the air to drop. The cold air is then returned to the compressor. Here is mixes with more incoming air and helps to lower its temperature. This process of expansion and compression continues till the temperature of the air falls to -200 degrees Celsius. By this time most of the all the gases in the air have been liquefied, except neon and helium.
The liquid air is pale blue in colour due to the presence of liquid oxygen. It is passed through the fractionating column where it is slowly heated. The first gas to boil off is nitrogen at -196 degrees Celsius. When all the nitrogen has been converted to gas, the temperature begins to rise again. Argon boils at -186 degrees Celsius. Oxygen boils at -183 degrees Celsius. Each gas is collected and stored separately in cylinders under pressure.