Abstract
SIR FRANK SMITH in his James Forrest Lecture to the Institution of Civil Engineers on May 3 pointed out that in recent years very low temperatures (that is, great reductions of molecular motion) have been attained, and the properties of materials at very low temperatures have been studied. The corresponding advance in engineering technique has resulted in the development of refrigerating engineering. The Joule -Thomson effect, whereby the work done in separating the molecules of a gas allowed to expand from a high pressure results in the gas being lowered in temperature, is used in many commercial processes for cooling gases, and in some cases for liquefying them. In order to obtain pure gases from air, the process of rectification is employed. The process depends on the fact that when equilibrium exists between a mixture of two liquids and the vapour it gives off, the composition of the vapour is always different from the composition of the liquid if the two liquids in the mixture have different boiling-points. For example, oxygen boils at 90° K. and nitrogen at 77° K., and when equilibrium exists between liquid air and the vapour above, the vapour contains about 7 per cent of oxygen and 93 per cent of nitrogen. The separation of oxygen and nitrogen by rectification is therefore carried out by allowing liquid air to drop slowly into liquid oxygen ; the vapour given off by the latter condenses as it comes into contact with the drops, while part of the nitrogen in the drops simultaneously evaporates, and if the drops are sufficiently small and fall very slowly, all the nitrogen evaporates before the drops reach the vessel containing liquid oxygen. The process is continued, with the result that the gas becomes steadily richer in nitrogen. Other gases can also be extracted from the air by low-temperature separation.
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Disorderly Molecules and Refrigerating Engineering. Nature 141, 864 (1938). https://doi.org/10.1038/141864c0
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DOI: https://doi.org/10.1038/141864c0