Liquefaction processes are used for scientific, industrial, and commercial purposes. Many gases can be put into a liquid state at normal
atmospheric pressure by simple cooling; a few, such as
carbon dioxide, require pressurization as well. Liquefaction is used to analyze the fundamental properties of gas molecules (intermolecular forces), to store gases (e.g.,
LPG), and in
refrigeration and
air conditioning. There, the gas is liquefied in the
condenser, where the
heat of vaporization is released, and evaporated in the
evaporator, where the heat of vaporization is absorbed.
Ammonia was the first such
refrigerant, and is still in widespread use in industrial refrigeration. In residential and commercial applications, it has largely been replaced by compounds derived from
petroleum and
halogens.
Liquid oxygen is provided to hospitals for conversion to gas for patients with breathing problems, and
liquid nitrogen is used in the medical field for
cryosurgery, by inseminators to freeze
semen, and by field and lab scientists to preserve samples. Liquefied
chlorine is transported for eventual solution in water, after which it is used for water purification, sanitation of
industrial waste, sewage and swimming pools, bleaching of pulp and textiles and manufacture of
carbon tetrachloride,
glycol and numerous other organic compounds as well as
phosgene gas. Liquefaction of
helium (
4He) with the precooled
Hampson–Linde cycle led to a
Nobel Prize for
Heike Kamerlingh Onnes in 1913. At ambient pressure the boiling point of
liquefied helium is . Below 2.17 K liquid 4He becomes a
superfluid (
Nobel Prize 1978,
Pyotr Kapitsa) and shows characteristic properties such as heat conduction through
second sound, zero
viscosity and the
fountain effect among others. The liquefaction of air is used to obtain
nitrogen,
oxygen, and
argon and other atmospheric
noble gases by separating the air components by
fractional distillation in a
cryogenic air separation unit. ==History==