7.5 Gases in the Atmosphere

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7.5 Gases in the Atmosphere

You will recall that about 78 percent of the air is nitrogen and 21 percent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Liquefying the air

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

·         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.

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7.5 Gases in the Atmosphere

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. 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.

Uses of nitrogen and oxygen

·         liquid nitrogen is used to freeze food

·         food is packaged in gaseous nitrogen to increase its shelf life

·         oil tankers are flushed with gaseous nitrogen to reduce the chance of explosion

·         oxygen is used in the manufacture of steel and in medicine.

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