Binary azeotropes containing alcohols

Except for alcoholic beverages, nearly all the ethyl alcohol used is a mixture of 95% alcohol and 5% water, known simply as P5% alcohol. What is so special about the concentration of 95% Whatever the method of preparation, ethyl alcohol is obtained first mixed with water this mixture is then concentrated by fractional distillation. But it happens that the component of lowest boiling point is not ethyl alcohol (b.p. 78.3°) but a binary azeotrope containing 95% alcohol and 5% water (b.p. 78.15°). As an azeotrope, it of course gives a vapor of the same composition, and hence cannot be further concentrated by distillation no matter how efficient the fractionating column used. 

When the water has been eliminated, there follows at 68.3°C a binary azeotrope containing 32.4% of ethanol and 67.6% of benzene, and finally at 78.4°C alcohol of about 99.8% purity distils over. On an industrial scale the preparation takes place continuously, but in the laboratory it is often more convenient to operate in batch and work up the two layers of the ternary azeotrope and the binary azeotrope in portions. 

Ethanol is a monohydric primary alcohol. It melts at -117.3°C and boils at 78.5°C. It is miscible (i.e., mixes without separation) with water in all proportions and is separated from water only with difficulty ethanol that is completely free of water is called absolute ethanol. Ethanol forms a constant-boiling mixture, or azeotrope, with water that contains 95% ethanol and 5% water and that boils at 78.15°C since the boiling point of this binary azeotrope is below that of pure ethanol, absolute ethanol caimot be obtained by simple distillation. However, if benzene is added to 95% ethanol, a ternary azeotrope of benzene, ethanol, and water, with boiling point 64.9°C, can form since the proportion of water to ethanol in this azeotrope is greater than that in 95% ethanol, the water can be removed from 95% ethanol by adding benzene and distilling off this azeotrope. Because small amounts of benzene may remain, absolute ethanol prepared by this process is poisonous. 

Sometimes the new azeotrope which is formed contains all three constituents. The dehydration of ethanol-water mixture with benzene as added substance is an example. Dilute ethanol-water solutions can be continuously rectified to give at best mixtures containing 89.4 mole percent ethanol at atmospheric pressure, since this is the composition of the minimum-boiling azeotrope in the binary system. By introducing benzene into the top of a column fed with an ethanol-water mixture, the ternary azeotrope containing benzene (53.9 mol %), water (23.3 mol %), ethanol (22.8 mol %), boiling at 64.9°C, is readily separated from the ethanol (bp — 78.4 C), which leaves as a residue product. In this case also the azeotropic overhead product separates into two liquid layers, one rich in benzene which is returned to the top of the column as reflux, the other rich in water which is withdrawn. Since the latter contains appreciable quantities of both benzene and ethanol, it must be rectified separately. The ternary azeotrope contains nearly equal molar proportions of ethanol and water, and consequently dilute ethanol-water solutions must be given a preliminary rectification to produce substantially the alcohol-rich binary azeotrope which is used as a feed. 

They performed binary system experiments for each of the pairs in the ternary system. It was found that glycol can move the azeotrope point and therefore, enhance the separation process. Sheikholeslamzadeh et al. have used both the NRTL-SAC and UNIFAC models to perform phase calculations and assess the capacity of the mentioned models in the prediction of binary and ternary systems containing glycol and alcohols [20],... 

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