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Liquefaction of gases
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Gases must be cooled below the critical temperature.
A gas can be liquefied only below its critical temperature; techniques include compression, cooling and Joule–Thomson expansion. Nitrogen boils at 77 K, helium at 4.2 K.
Linde process for air.
Above T_c no pressure suffices.
In more detail
Gases are liquefied by cooling below the critical temperature and compressing them. Above the critical temperature, a gas cannot be liquefied by pressure alone.
Real-world uses
- Liquid nitrogen and oxygen are produced by liquefaction of air.
- Natural gas is liquefied for transport as LNG.
- Industrial gas plants liquefy oxygen and nitrogen by cooling them below their critical temperatures and then compressing them.
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Frequently asked questions
What is the critical temperature?
The temperature above which a gas cannot be liquefied, however much pressure is applied.
Why is liquefaction energy-intensive?
It requires removing heat and doing work against molecular attractions.
Why must a gas be cooled before it can liquefy?
Above the critical temperature, no amount of pressure can make the gas liquid, so cooling is necessary.
Related topics in Kinetic Theory & Statistical Physics
Real gases · Van der Waals equation · Compressibility factor · Microstates and macrostates · Boltzmann distribution · Partition function