liquid air

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For the automobile, see Liquid Air.

Liquid air is air that has been cooled to very low temperatures so that it has condensed to a liquid. At room temperature, it must be kept in a vacuum flask. Liquid air can absorb heat rapidly and revert to its gaseous state. It is often used for condensing other substances into liquid and/or solidifying them, and as a source of nitrogen, oxygen, argon, and other inert gases.

Properties

Liquified air has a density of approximately 870 kg/m³, though the density may vary depending on the elemental composition of the air. Since gaseous air has 78% nitrogen and 21% oxygen, the density of liquid air at standard composition is calculated by the decimal percentage of the components by their respective liquid densities. See liquid nitrogen and liquid oxygen. Its freezing point is -216.7°C and its boiling point is -194.35°C.

Preparation

Principle of production

The constituents of air were once known as "permanent gases" as they could not be liquified solely by compression at room temperature. A compression process will raise the temperature of the gas. This heat is removed by cooling to the ambient temperature in a heat exchanger and then the gas is expanded by venting into a chamber. This expansion causes a lowering of the temperature and by counter-flow heat exchange of the expanded air the pressurized air entering the expander is further cooled. With sufficient compression, flow, and heat removal eventually droplets of liquid air will form which may then be employed directly for low temperature demonstrations. (A device for such production is simple enough to be fabricated by the experimenter using commonly available materials.)

Process of manufacture

The most common process for the preparation of liquid air is the two-column Hampson-Linde cycle using the Joule-Thomson effect. Air is fed at high pressure >60 psig (520 kPa) into the lower column, in which it is separated into pure nitrogen and oxygen-rich liquid. The rich liquid and some of the nitrogen are fed as reflux into the upper column, which operates at low pressure <10 psig (170 kPa), where the final separation into pure nitrogen and oxygen occurs. A raw argon product can be removed from the middle of the upper column for further purification.^[1]

Application

In manufacturing processes the liquid air product is fractionated into its constituent gasses in liquid or gaseous form, as the oxygen is especially useful for use in fuel gas welding and cutting, and the argon is useful as an oxygen-excluding shielding gas in some forms of shielded metal arc welding, while Liquid nitrogen is useful in various low-temperature applications, being nonreactive at normal temperatures (unlike the oxygen) and boiling at −196 °C (−321 °F). During World War Two, Nazi Germany reportedly experimented with a bomb made from liquid air and coal dust. ^[2]

See also

• Liquefaction of gases

References