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Oxygen : Fractionation Of Liquid Air |
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In order to obtain liquid oxygen from liquid air, it would appear simplest to allow the liquid to evaporate slowly, when the nitrogen would pass off and liquid oxygen might be left. This would, however, lead to serious loss of oxygen, as is seen from the table below, giving the results of Linde's experiments:
The gas coming from fresh liquid air contains only 7.5 per cent, of oxygen; when the evaporation has proceeded until the liquid contains 50 per cent, of oxygen, or about two-thirds of the liquid has evaporated, the gas is of the same composition as ordinary air. It is only when 95 per cent, of the liquid has disappeared that the gas contains 90 per cent, of oxygen, and if the remaining liquid is evaporated to produce this rich gas, we recover only 19 per cent, of the oxygen originally present in the liquid air. Linde (1902) avoided this loss by making use of a rectifying column, in which the escaping gas is scrubbed by liquid air or oxygen passing down in the opposite direction. Claude in 1906 introduced two new principles: (1) he liquefied the air in stages, obtaining two liquids, one rich in oxygen and the other in nitrogen; (2) the expanding gas was allowed to do work in an engine, and the heat equivalent of this work was taken from it. (This had been previously suggested by Rayleigh.) A tall rectifying column is used, the liquid rich in nitrogen being discharged into the top, whilst the liquid rich in oxygen is introduced at a point lower down, where the descending liquid has become enriched to the same composition. Claude's apparatus is shown in
Compressed air at about 20-35 atm., cooled by an interchanger as in the Linde process, enters A, already partly liquefied, into a vessel containing two sets of vertical pipes. The first drain into A, and the second form a ring round the first and drain into C. Both sets are immersed in the bath, S, which, when the machine is operating, contains nearly pure liquid oxygen. The condensation of the compressed air evaporates a portion of this oxygen, part of the vapour going up the rectifying column, D, where it is practically completely condensed, displacing nitrogen from the liquid, and returns to S. The oxygen gas from S goes off by the pipe G to the heat-inter-changer where its cold is utilised, after which it is pumped at 120 atm. pressure into steel cylinders for use. The compressed air passes up the central group of pipes in 5, and a liquid condenses there which is relatively rich in oxygen, which drains back into A. The gaseous residue passes through the outer ring of pipes, liquefies in them, and falls into C, the liquid being richer in nitrogen. This liquid in C is then taken to the top of the column, that in A to a lower compartment L, containing scrubbed liquid of the same composition. Gas rich in nitrogen escapes at the top of the column and its cold is utilised in the heat inter changers. The liquid condensed in the inner pipes is scrubbed by air passing on. Liquid air as obtained from the liquefier is richer in oxygen than gaseous air. Usually it contains from 50 to 60 per cent, by weight of oxygen as compared with 23 in gaseous air. Liquid air or oxygen is stored in spherical metallic vacuum vessels, holding 5-30 galls., the inner vessel being suspended by a thin metallic neck, and the annular space exhausted. A high vacuum is produced by means of a tube of absorbent charcoal, open at the end exposed to the vacuous space, and with the other (closed) end immersed in the liquid air itself. The daily rate of loss does not exceed 5 per cent. When used in connection with aviation at high altitudes, a smaller metallic vacuum vessel is provided with a mechanism for controlling the rate of evaporation of the liquid oxygen, and a tube leading to the inhaling mouthpiece. The controlling mechanism consists of a siphon dropping liquid oxygen at a controlled rate into an evaporation chamber. This control is necessary, since great fluctuations in the rate of evaporation are caused by movements from higher to lower altitudes where the atmospheric pressure is higher. Solid oxygen (blue) is obtained by cooling the liquid in a tube immersed in liquid hydrogen. It cannot be obtained by the rapid evaporation of the liquid, as is the case with solid nitrogen. Liquid oxygen is strongly magnetic; the gas is weakly magnetic. A second, denser form of the solid probably exists at temperatures much below the melting-point, which is -219° at 12 mm. pressure. About 85 per cent, of the commercial oxygen is used, in about equal proportions, for cutting and welding metals by the oxy-acetylene or oxy-coal gas blowpipe. The rest is used in medicine for treating cases of pneumonia, gas-poisoning, etc., for oxidising linseed oil, for maturing spirits, for the conversion of acetaldehyde into acetic acid, absorbed as liquid in charcoal as an explosive, and in the synthesis of nitric acid. Oxygen is used mixed with nitrous oxide, ether vapour, or other anaesthetics. Some carbon dioxide is often mixed with the oxygen, since this stimulates breathing, and this gas is also used in cases of poisoning and collapse for restoring respiration. |
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