Ethanol-water mixtures ternary azeotropes

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. 

By far, the most common prejudice (sometimes completely overlooking the other alternative) is to propose the most volatile composition (low-boiling node) as distillate (Fig. 28). In this region containing the decant aqueous layer, the lightest composition is the ternary azeotrope. With enough stages, all of the azeotropic composition in the decant aqueous layer may be recovered, and the underflow will contain only a binary ethanol-water mixture. If the distillate of... 

Azeotropic mixtures require special methods for their separation, which usually consist of adding a third component that has the ability to "break" the azeotrope. Perhaps the most famous case is that of ethanol-water, which has an azeotropic mole fraction in ethanol of 0.8943 at atmospheric pressure (Table 6.8). Here the added component is benzene and results, on distillation, in the recovery of pure ethanol and a ternary azeotrope containing benzene. That mixture, on condensation, results in two immiscible aqueous and organic layers, which are separated and further processed by distillation. 

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. 

Absolute (100%) ethanol is often made by adding benzene to the ethanol -water binary azeotrope (two components), to make a ternary azeotrope (three components). This ternary alcohol-water-benzene (18.5 7.4 74.1) azeotrope comes over until all the water is gone, followed by a benzene-ethanol mixture. Finally, absolute ethanol gets its chance to appear, marred only slightly by traces of benzene. 

A mixture of 681 g. (5.0 moles) of levulinic acid, 583 ml. (10.0 moles) of 96% ethanol, 500 ml. of benzene, and 5 ml. of concentrated sulfuric acid is refluxed in an apparatus which provides for taking off the benzene-water-ethanol ternary azeotrope, condensing it, and returning the upper benzene layer continuously to the reaction vessel. A 15-hour reflux period is required to remove all the water in this... 

Water is probably the easiest product to separate, because it is immiscible with many organic solvents and may often be removed from the reaction mixture as an azeotrope. As a typical example, consider the formation of an ethyl ester. The acid, somewhat more than an equivalent amount of ethyl alcohol, and about three times the volume of benzene are placed in a flask equipped with a Dean-Stark water trap (Fig. 5-21). The solution is heated to reflux, and the ternary azeotrope (7.4 per cent water, 18.5 per cent alcohol, and 74.1 per cent benzene, bp 65°) is vaporized and transferred to the trap. The liquid upon cooling separates into two layers— the lower, a solution of ethanol in water, and the upper, a solution of ethanol in benzene. The upper layer returns to the flask to be recycled, and the amount of water may be noted in the graduated lower part of the trap. As this fills up, the water may be withdrawn. The completion of the reaction is taken as the point at which water no longer collects in the trap. In order to ensure completion of the reaction, a small additional amount of ethanol may be added to the reaction flask and the heating continued. If water still does not collect in the trap, the reaction is complete, and an essentially quantitative yield of product should be obtained. 

Azeotropic distillation involves either an embedded azeotrope, present in the feed mixture, or a contrived azeotrope, formed by the addition of an extraneous component called an entrainer. Benzene-water may be separated into high-purity benzene and the benzene-water azeotrope this is frequently practiced to remove water from benzene when very dry benzene is needed for chemical processing. More commonly encountered are distillation separations that are enhanced through the addition of an entrainer to form an azeotrope. Perhaps the best known separation of this type is the production of anhydrous ethanol from the ethanol-water azeotrope. Here, benzene is added as the entrainer, with the result that a low-boiling ternary azeotrope is formed between benzene, ethanol, and water. This permits the higher-boiling ethanol to be taken from the bottom of the column. The distillate condenses to a heterogeneous mixture of benzene and alcohol-water phases. 

An example of azeotropic distillation is the use of benzene to permit the complete separation of ethanol and water, which forms a minimum-boiling azeotrope with 95.6 weight percent alcohol. The alcohol-water mixture with about 95 percent alcohol is fed to the azeotropic distillation column with a benzene-rich stream added at the top. The bottom product is nearly pure alcohol, and the overhead vapor is a ternary azeotrope. The overhead vapor is condensed and separated into two phases. The organic layer is refluxed, and the water layer is sent to a benzene recovery column. All the benzene and some alcohol is taken overhead and sent back to the first column, and the bottoms stream is distilled in a third column to give pure water and some of the binary azeotrope. 

It may be observed that the ternary azeotrope m falls Inside the heterogeneous region. Thus, an overhead vapour of this composition splits by decantation in two phases, one rich in entrainer o other in water w the ethanol being distributed in both. Moreover, o, and w, are in different distillation regions. By clever mixing with other streams, these streams can produce feasible feeds for ethanol and water recovery columns, by overcoming the constraints of the distillation boundaries. Hence, liquid-liquid decantation creates opportunities for the separation of an azeotropic mixture. 

In a fortunate case, distillation aimed at purification may also result in the drying of the solvent. Distillation may be particularly effective in the removal of moisture if azeotropic mixtures with low boiling points are formed. For instance, the first step in the dehydration of ethanol is distillation after the addition of benzene, when water is removed in the volatile ternary azeotrope. Other solvents may also be dehydrated by distillation, e.g., benzene, chloroform, carbon tetrachloride, ethylene dichloride, heptane, hexane, toluene and xylene. In distillations with the aim of dehydration, the apparatus must be fitted with moisture traps (containing calcium chloride, silica gel or some other drying agent). It must be borne in mind that many anhydrous organic solvents are hygroscopic. 

Most investigations of azeotropes have dealt with binary or ternary mixtures. The binary pairs of a multicomponent mixture may separately form azeotropes, but these are submerged by die possible azeotropes involving the Aril mixture. It is sometimes possible to utilize binary pair azeotrope information in estimating Are role that a multicomponent azeotn will play in the distillation separation. As an exariqile, the ethanol-water system exhibits a minimum boiling azeotrope. At atmospheric pressure, the boiling points are... 

In the main esterification process for acetic acid ethyl ester (ethyl acetate) a mixture of acetic acid and ethanol with a small amount of sulphuric acid is preheated and fed to an esterifying column where it is refluxed. The mixture removed goes to a second refluxing column where a ternary azeotrope containing 85% ethyl acetate is removed. Water is mixed with the distillate after which it separates into two layers. The top layer is fed to a refluxing column from which the residue containing 95% ethyl acetate is distilled to remove any impurities. Ethyl acetate and ethyl lactate are used as solvents. Some ethyl esters of carboxylic acids are used as flavor and aroma substances. 

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. 

Acetonitrile. CH3CN, b.p. 81.6° [1, 7, before a-Acetoxyacrylonitrile]. Aprotic. water-miscible Diels-Alder solvent (1, 239). Supplier of pure solvent. 1, 1110. For polara-graphic use. Moe1 notes that the main impurity is acrylonitrile, which differs from it in b.p. by only 4.2°, and recommends separation by distillation of the ternary and secondary azeotropes which the two liquids form with ethanol and with water. Ethanol (95%) is added to practical acetonitrile and the mixture is distilled through an H. Stedman column of 60-65 theoretical plates. The purified material is suitable also for U V spectroscopy. 

The mode of operation and the dimensioning of a heteroazeotropic distillation as exemplified by the separation of the system water-acetic acid has been described by Wolf et al. [61b]. Morozova and Platonov [61c] analyzed the structure of phase diagrams of multicomponent mixtures using a digital computer. They studied the requirements for the separation of azeotropic mixtures. In order to achieve optimum column combinations Serafimov et al. [58 c] studied the ternary mi.xture isopropanol/ benzene/water on the basis of a mathematical treatment of the separation of heteroazeotropic mixtures. In another paper [58 d] a procedure was presented for the separation into its components of the water-containing mixture with acetone, ethanol, benzene and butyl acetate by means of the thermodynamic and topological analysis of the phase diagram structure. 

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