Question

What are the significances of the three stages respectively of Fractional distillation of liquefied air?

Answer 1

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say that the air has been liquefied.

Here's what happens as the air liquefies (note that you do not need to recall the boiling points of the different gases):

water vapour condenses, and is removed using absorbent filters

carbon dioxide freezes at –79ºC, and is removed

oxygen liquefies at –183ºC

nitrogen liquefies at –196ºC.

The liquid nitrogen and oxygen are then separated by fractional distillation.

Fractional distillation

The liquefied air is passed into the bottom of a fractionating column. Just as in the columns used to separate oil fractions, the column is warmer at the bottom than it is at the top.

Fractional distillation

The liquid nitrogen boils at the bottom of the column. Gaseous nitrogen rises to the top, where it is piped off and stored. Liquid oxygen collects at the bottom of the column. The boiling point of argon - the noble gas that forms 0.9 percent of the air - is close to the boiling point of oxygen, so a second fractionating column is often used to separate the argon from the oxygen.