Azeotropic distillation absolute alcohol

To conclude this section an example will be given of an azeotropic distillation in which a ternary azeotrope is formed — the preparation of absolute alcohol by distillation with benzene as auxiliary substance. If 95% wt. alcohol is distilled in batch with the calculated quantity of benzene, there first passes over at 64.9 °C a ternary azeotroj)e, separating at 28°C into two layers the compositions of the azeotrope and these layers are as follows ... 

The first successful appHcation of heterogeneous azeotropic distillation was in 1902 (87) and involved using benzene to produce absolute alcohol from a binary mixture of ethanol and water. This batch process was patented in 1903 (88) and later converted to a continuous process (89). Good reviews of the early development and widespread appHcation of continuous azeotropic distillation in the prewar chemical industry are available (90). 

Most alcohol is sold as 95% ethanol-5 % water since it forms an azeotrope at that temperature. To obtain absolute alcohol, a third component, such as benzene, must be added during the distillation. This tertiary azeotrope carries over the water and leaves the pure alcohol behind. Common industrial alcohol is denatured, and additives are purposely included to make it nondrinkable and therefore not subject to the high taxes of the alcoholic beverage industry. 

The esterification step occurs with 85% sulfuric acid at 24 to 27°C, and dilution to 20% concentration is done in a separate tank. The iso-propyl alcohol is distilled from the dilute acid that is concentrated and returned to the esterification reactor. The /so-propyl alcohol is originally distilled as a 91% azeotrope with water. Absolute iso-propyl alcohol, boiling point 82.5°C, is obtained by distilling a tertiary azeotrope with isopropyl ether. A 95% yield is realized. 

For both minimum- and maximum-boiling-point azeotropes, fractional distillation can only separate a solution into a pure component and the azeotrope, not into the two pure components. It is not possible to distill past an azeotropic concentration. Absolute alcohol cannot be prepared by simply distilling aqueous solutions obtained by fermenting grains. 

The 95% alcohol produced by distillation is well suited for use as a solvent and a reagent when traces of water do not affect the reaction. When absolute alcohol (100% ethanol) is required, the 95% azeotrope is passed through a dehydrating agent such as anhydrous calcium oxide (CaO), which removes the final 5% of water. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

Keyes process. A distillation process involving the addition of benzene to a constant-boiling 95% alcohol-water solution to obtain absolute (100%) alcohol. On distillation, a ternary azeotropic mixture containing all three components leaves the top of the column while anhydrous alcohol leaves the bottom. The azeotrope (which separates into two layers) is redistilled separately for recovery and reuse of the benzene and alcohol. 

As obtained synthetically, ethanol contains 4 wt.-% of water, as well as acetaldehyde and acetone. Alcohol obtained by fermentation contains also higher alcohols. Further impurities are the denaturing agents that are usually added (methanol, pyridine bases, petroleum). Absolute alcohol obtained commercially by azeotropic distillation may contain also traces of benzene or trichloroethylene. 

An azeotrope frequently becomes richer in the low-boiling component as the ])ressure is reduced. By continuing the reduction, a j)ressure is reached at which the azeotropic point vanishes. As an example, the mixture ethanol-water may be quoted at 70 mm Hg this system no longer has an azeotropic point (c/. section 6.2.1). Hence it is possible to prepare absolute alcohol by the distillation of dilute spirits at a pressure below 70 mm, without the addition of auxiliar substances. The boiling point, however, is then rather low (28°C). In this connection it is obvious that the choice of distillation pressure may also depend on the temperature of the cooling medium. 

According to Gibbs phase rule a completely soluble binary mixture is enriched in both phases, whilst an immiscible binary mixture, with its three phases, cannot be enriched (see Fig. 29, a—d). It wiU be recognized, on the other hand, that three-component systems having a miscibility gap, f.e. showing two liquid phases and one vapour phase, are separable by countercurrent distillation [1]. A typical example is the preparation of absolute alcohol by azeotropic distillation with benzene. 

The result of a diminution in pressure is frequently that the azeotropic composition becomes richer in the low-boiling constituent. A reduced pressure may thus be reached at which the special point vanishes. As examples, the mixtures ethanol-water and water-phenol may be taken (see Fig. 226). By distilling dilute alcohol at 70 mm, absolute alcohol may be prepared without the use of anentrainer. The azeotropic point water-phenol is eliminated at 32 ram. The shift in the azeotropic point has been determined by Sheinker and Peresleni [45] for two other systems ... 

Absolute Alcohol is obtained from 95% Alcohol by using a ternary azeotrope (i.e. by distillation using a three component Azeotrope). 

Thus, when a mixture of 95% ethanol and benzene Is distilled, the above ternary azeotrope distills first, followed by the binary azeotrope, and the final Action (b.p. 78.30 C) is absolute alcohol. 

Absolute alcohol n. Ethyl alcohol that has been refined by azeotropic distillation to 99.9% purity (200 proof). Other commercial ethanols contain about 5% water and may contain denaturants that make the alcohol undrinkable. Pure anhydrous ethyl alcohol (ethanol). The term is used to distinguish it from the several varieties of alcohol which are available, and which contain varying amounts of water and/or other impurities. 

Production of Absolute Alcohol by Azeotropic Distillation with Benzene. 

Anhydrous or absolute alcohol is produced by several methods. A third component such as benzene may be added, and the mixture distilled. The ternary azeotrope thus formed carries over the water, leaving behind anhydrous alcohol. Another method uses counter-current extraction with a third component such as glycerine or ethylene glycol. The added component depresses the vapor pressure of the water and allows anhydrous alcohol to be distilled from the top of the extraction column. Both these methods are run using continuous columns. 

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