SUMMARY

SINCE cracked gasolines were first produced in marketable quantities, the refining of this type of motor spirit has been an important problem. These gasolines, owing to the comparatively high content of unsaturated compounds, are unstable and readily form gum and darken in colour on storage. Refining has been difficult owing to the necessity of rendering them stable on storage and otherwise satisfactory as a motor fuel, without incurring heavy refining losses. Recent advances in vapour-phase cracking processes for producing gasoline of high anti-knock value has still further emphasised the importance of this problem. The high anti-knock value of this type of cracked gasoline is due both to the particular character of the unsaturated hydrocarbons and the large amount present. Moreover, it has been found that many of the least stable unsaturated compounds have the highest anti-knock value, so that refining them sufficiently to render them stable on storage usually results in an appreciable decrease in antiknock value. Similar difficulties have also arisen with regard to the refining of spirits obtained by the high- and low-temperature carbonisation of coal, and to a less extent those obtained by the hydrogenation of coal and tars. Clearly, any refining treatment applied- to such products should be reduced to. the minimum to achieve the desired results of rendering the refined material stable on storage and of removing sulphur, malodorous, and other objectionable constituents with the least destruction of the unsaturated and other valuable hydrocarbons. When it is necessary to market a "water-white "product, the refining treatment must also remove colour and ensure that the product remains colourless on storage. At the present time there is an increasing tendency for the complete refining treatment to consist of a number of processes, each applied for a specific purpose. In this way, the refining losses can be kept lower than when only one process is employed capable of achieving simultaneously all the desired objects of the separate processes. The choice of any particular method of treatment obviously depends very largely on the character of the material to be refined. In order to obtain a stable product, the refining treatment must remove the unstable gum-forming compounds, unless inhibitors are used. The addition of these substances delays for considerable periods the formation of gum from the unstable unsaturated compounds, thus enabling both the stable and unstable unsaturated hydrocarbons to be retained for use as motor fuel, and allowing the refining treatment to be cut down still further. From th

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