Air Liquefaction Distillation

R. Agrawal and D. M. Herron, Air Products and Chemicals, Hamilton Boulevard, Allentown, PA, USA

Copyright © 2000 Air Products and Chemicals, Inc

Oxygen, nitrogen and argon, the major components of air, have been separated by distillation at cryogenic temperatures for nearly a century. Air was commercially liquefied as early as 1895 by Carl von Linde and also by William Hampson. Linde separated oxygen from air by distillation in a single column in 1902. A commercial plant producing pure nitrogen was already in operation by 1904. The first double-col umn distillation system, the predecessor to current double-column processes, was commissioned in 1910 by Linde. Argon was produced on an industrial scale by 1913. Today the major industrial companies supplying products from air distillation and liquefaction and also the equipment for this purpose are: AGA, Air Liquide, Air Products and Chemicals, the BOC Group, Linde, Messer Group, Nippon Sanso and Praxair.

The composition of dry and impurities-free air is given in Table 1. The critical temperature and normal boiling point (at 101.3 kPa) for each component is also listed. In this table, and in the rest of this chapter, concentration in p.p.m. refers to parts per million on a volume basis. The gases listed in Table 1 are used in

Table 1 Composition of air and thermodynamic properties of its constituent gases

Constituentgas Concentration Boilingtemperature Criticaltemperature (mol%) (°Cf (°C)

a Boiling temperature at 101.3 kPa.

a wide range of industrial and medical applications. Typical industries using these gases include: ferrous and nonferrous metals, chemicals, petroleum, food, paper, glass, textile and electronics. Oxygen is generally used as an oxidant while nitrogen and argon are used to provide inert atmospheres. Krypton is used in light bulbs, lasers, sputtering of electronic components and high energy physics. Neon is used in fluorescent lighting, infrared detection equipment and experimental physics at cryogenic temperatures. Xenon is used in electronic flashlights, as an anaesthetic and in a new application where an on-board xenon ion propulsion system is used for positioning a satellite. Helium is not generally recovered from air due to its low concentration.

The history of air distillation started with oxygen production followed by recovery of other constituents. Therefore, the distillation processes to produce oxygen are described first here followed by argon and then nitrogen. These topics are followed by liquefaction processes and finally a brief description is given of the major equipment used in cryogenic air separation and liquefaction processes.

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