To enable air to be separated into its constituents by means of rectification – the actual separation process – a large part of the air volume used must be liquefied. A gas can only be transformed into a liquid state at temperature and pressure conditions below those of its critical point. The critical point of air is Tcrit = –140.7ºC (132.5 K) and Pcrit = 37.7 bar. In other words, air can be liquefied only at temperatures below –140.7ºC (132.5 K).
The vapour pressure curve illustrates the temperatures and pressures at which a gas condenses or a liquid evaporates.
1) Air below atmospheric pressure (1 bar) must be chilled to –192ºC (81.5 K) before it starts to condense
2) Air below a pressure of 6 bar must be chilled to –172ºC (101 K) before it starts to condense
The boiling point and condensation conditions of gas mixtures such as air are not identical. A condensation line and a boiling point line delineate the boiling point range.
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