Page 8 - Curriculum Visions Dynamic Book

“Refining” nitrogen
Nitrogen at normal air pressure and temperature contains so much energy
that it is a gas. Nitrogen makes up about three-quarters of the mixture of gases in the atmosphere. However, nitrogen is never found in nature as a liquid or a solid.
Liquid nitrogen
To make nitrogen into a liquid the gas
has to be deprived of energy by cooling. However, this is not very easy because the temperature at which nitrogen liquifies
is -196°C.
To make liquid nitrogen easier to obtain, air is cooled while under pressure. You can think of the process as being rather like
an air “refinery”. Each of the gases in the air has its own condensation point (where it changes from gas to liquid). As this
is reached, the element liquifies. Liquid oxygen forms at -183°C. Nitrogen has the lowest boiling point of any of the gases in the air and it is the last to be collected.
Distilling nitrogen
Often the easiest way to obtain nitrogen is to liquify air in bulk and then allow the oxygen to boil off. This process is called distillation, and it is a very low temperature version of, for example, how oil and other mixtures are obtained from hydrocarbons. The process
takes place in a tall column (called a fractionation column) made of a set of “leaky trays”. Liquid air is poured onto the trays. There is a boiler at the bottom and a condenser (a trap for the gas given off) at the top.
As the air begins to evaporate, nitrogen is given off as a gas, while
the oxygen (which goes into a gas less easily than nitrogen – it is less volatile) drips to the bottom. In this way carefully controlled conditions can separate out the nitrogen from the oxygen.
The liquid air is kept under a pressure of five atmospheres in the tower, at which pressure the boiling point of the nitrogen and oxygen are much higher (and so easier to maintain) than at normal atmospheric pressure.
 The composition of clean air.
Nitrogen 78%
Carbon dioxide 0.03%
Noble gases 1% (helium, neon, argon, krypton and xenon)
Oxygen 21%
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