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Benzene and toluene are virtually impossible to separate from close boiling nonaromatic hydrocarbons by rectification. Benzene and toluene can be readily separated from similar boiling nonaromatics by using extractive distillation in which the extractive distillation agent is a proper mixture of organic compounds boiling higher than benzene or toluene. A typical extractive distillation agent for benzene is a mixture of phthalic anhydride, maleic anhydride, and adiponitrile; for toluene, phthalic anhydride, maleic anhydride, and glycerol triacetate.Extractive distillation is the name applied to the method of altering the relative volatility of two or more compounds by distilling them in the presence of a quantity of a higher boiling liquid. Typically the extractive distillation agent boils at least 20°C higher than the compounds being separated, forms no azeotrope with them, and is miscible with them at distilling temperature. It was the objective of this research to find extractive distillation agents which will increase the apparent relative volatility of benzene or toluene to similar boiling nonaromatic hydrocarbons to a value high enough to permit separation in a rectification column having a moderate number of theoretical plates thus making rectification an economically

The use of a single compound as the agent in modifying the apparent relative volatility in extractive distillation is well known. In the separation of ethylbenzene from para-and meta-xylenes by extractive distillation, it was found that mixtures containing from two to four components were more effective than any known single compound. Hundreds of combinations of extractive agents were investigated in vapor-liquid equilibrium stills, and from these, twenty-seven different combinations of chlorinated organic compounds showed merit. The relative volatility of ethylbenzene to pxylene is 1.06 and to m-xylene 1.08. The promising combinations were investigated in three glass perforated plate extractive distillation columns containing twelve, fifteen and nineteen theoretical plates, respectively. Of the twenty-seven different extractive agents tested in the columns, sixteen yielded a relative volatility greater than 1.20. The best one, a mixture of pentachlorophenol, benzene hexachloride and 1,2,4-trichlorobenzene, gave a relative volatility of 1.27. Other combinations approaching this relative volatility are pentachlorophenol, benzene hexachloride and 2,4dichlorotoluene giving 1.25 and polychlorobenzene, pentachlorophenol, benzene hexachloride and 1,2,4-trichlorobenzene giving 1.26, when used in the ratio of two parts of extractive agent to one part of ethylbenzenexylene mixture. All the extractive agents boil sufficiently above xylene to make their recovery by distillation easy and complete. Life studies showed no excessive decomposition of any ofthem while in use.The separation of ethylbenzene (boiling point 136.15"C.) from p-xylene (boiling point 138.4"C.) and mxylene (boiling point 139.2"C.) presents considerable difficulty by reason of the proximity of their boiling points and the accompanying similarity of their vapor pressures as to require the use of fractioning columns containing a great number of plates and high reflux ratios in order to assure effective separation thereof. A major end use for these compounds is in plastics, where high purity is essential; for example, ethylbenzene is converted to styrene and much of p-xylene is made into dimethyl terephthalate. As a result, the cost of accomplishing such a separation by conventional rectification is relatively uneconomical. Since ethylbenzene, p-xylene and m-xylene are all isomers of C, aromatic hydrocarbons, their chemical prop-

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