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Liquefaction of gases is physical conversion of agas into aliquid state (condensation). The liquefaction of gases is a complicated process that uses various compressions and expansions to achieve high pressures and very low temperatures, using, for example,turboexpanders.
Liquefaction processes are used for scientific, industrial and commercial purposes. Many gases can be put into a liquid state at normalatmospheric pressure by simple cooling; a few, such ascarbon dioxide, require pressurization as well. Liquefaction is used for analyzing the fundamental properties of gas molecules (intermolecular forces), or for the storage of gases, for example:LPG, and inrefrigeration andair conditioning. There the gas is liquefied in thecondenser, where theheat of vaporization is released, and evaporated in theevaporator, where the heat of vaporization is absorbed.Ammonia was the first suchrefrigerant, and is still in widespread use in industrial refrigeration, but it has largely been replaced by compounds derived frompetroleum andhalogens in residential and commercial applications.
Liquid oxygen is provided to hospitals for conversion to gas for patients with breathing problems, andliquid nitrogen is used in the medical field forcryosurgery, by inseminators to freezesemen, and by field and lab scientists to preserve samples. Liquefiedchlorine is transported for eventual solution in water, after which it is used for water purification, sanitation ofindustrial waste, sewage and swimming pools, bleaching of pulp and textiles and manufacture ofcarbon tetrachloride,glycol and numerous other organic compounds as well asphosgene gas.
Liquefaction ofhelium (4He) with the precooledHampson–Linde cycle led to aNobel Prize forHeike Kamerlingh Onnes in 1913. At ambient pressure the boiling point ofliquefied helium is 4.22 K (−268.93 °C). Below 2.17 K liquid4He becomes asuperfluid (Nobel Prize 1978,Pyotr Kapitsa) and shows characteristic properties such as heat conduction throughsecond sound, zeroviscosity and thefountain effect among others.
The liquefaction of air is used to obtainnitrogen,oxygen, andargon and other atmosphericnoble gases by separating the air components byfractional distillation in acryogenic air separation unit.
Air is liquefied by theLinde process, in which air is alternately compressed, cooled, and expanded, each expansion results in a considerable reduction in temperature. With the lower temperature the molecules move more slowly and occupy less space, so the air changes phase to become liquid.
Air can also be liquefied byClaude's process in which the gas is allowed to expandisentropically twice in two chambers. While expanding, the gas has to do work as it is led through anexpansion turbine. The gas is not yet liquid, since that would destroy the turbine.[citation needed] Commercial air liquefication plants bypass this problem by expanding the air atsupercritical pressures.[1] Final liquefaction takes place byisenthalpic expansion in athermal expansion valve.