Ethanol ternary azeotropes with

Ethanol forms an azeotrope containing 5 wt % water. In older installations, dissolved salts were employed to break the azeotrope. Typical data are in Figure 13.28(c). Several substances form ternary azeotropes with ethanol and water, including benzene, gasoline, and trichlorethylene. The first is not satisfactory because of slight decomposition under distillation conditions. A flowsketeh of a... 

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... 

Using benzene as the entrainer to the ethanol/water solution forms a ternary azeotrope at 100 kPa, 64.86°C, containing 22.81 mole% ethanol, 23.32 mole% water, and 53.87 mole% benzene. By adding benzene to the distillation column, the ternary azeotrope, with its lower boiling point than that of ethanol at 78.4°C, leaves the column in the distillate, and purified ethanol is recovered in the bottoms. 

An early application of azeotropic distillation was proposed by Guinot and Clark11 for the separation of ethanol and water by the use of benzene as the solvent. This process is based on the fact that benzene forms a ternary azeotrope with ethanol and water, which has a higher ratio of water to ethanol than does the ethanol-water azeotrope. In the first column, shown in Fig. 6-3, an azeotropic distillation is carried out. A two-phase liquid separation at 20°C in the decanter is used to concentrate the benzene in the reflux to the first column. The solvent benzene is recovered in the second column and water is removed in the third column. 

Table 7.4 Settling characteristics of water entrainers in ternary azeotropes with ethanol Density of...

Benzene forms a ternary azeotrope with water and ethanol (18.5% ethanol, 7.4 % water, and 74.1% benzene), which distills first. 

By redistillation with an additional substance which can form a ternary azeotropic mixture (as in ethanol-... 

Eresh aqueous ethanol feed is first preconcentrated to nearly the azeotropic composition in column C3, while producing a water bottoms product. The distillate from C3 is sent to column Cl, which is refluxed with the entire organic (entrainer-rich) layer, recycled from a decanter. Mixing of these two streams is the key to this sequence as it allows the overall feed composition to cross the distillation boundary into Region II. column Cl is operated to recover pure high-boihng node ethanol as a bottoms product and to produce a distillate close to the ternary azeotrope. If the ternary azeotrope is heterogeneous (as it... 

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. 

A distillative separation task with this net feed may then be specified to simultaneously produce a pure ethanol underflow and the ternary azeotrope overhead. This is feasible because the net feed, distillate, and bottoms compositions are collinear by mass balance, and because the distillate and bottoms are on the same residue curve (since all the curves in that region originate at the ternary... 

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. 

Ethanol is commonly obtained in a 95 per cent and an absolute grade. The former has the composition of the azeotrope with water (bp 78.2°) and except for the water is quite pure. If absolute ethanol (bp 78.3°) is required, it may be prepared by heating the 95 per cent ethanol to reflux with calcium oxide for several hours and then distilling. However, absolute ethanol is available at a reasonable cost and is rarely prepared in a laboratory. The commercial absolute ethanol often contains a small amount of benzene, since it is prepared from 95 per cent ethanol by removing the water through the ternary azeotrope of benzene-ethanol-water (bp 65°). The absolute ethanol is therefore not suitable for use as a solvent for ultraviolet spectroscopy, and the 95 per cent ethanol is usually used. 

For example, during the dehydration of ethanol with toluene a new ternary azeotrope is formed (Fig. 2.42). Due to the phase separation of the added toluene from water, toluene can be continuously recycled using rectification equipment with two columns. 

Condensation of the top product from column A, which is the ternary azeotrope, then allows two phases to separate. The upper phase is rich in benzene (84.5%) with only a trace of water present (1.0%). The lower phase of the separator is rich in ethanol and water. The upper phase is returned to an upper plate of the first column to recycle most of the benzene added to this column. 

Assuming perfect distillation, with pure ethanol in the bottoms product and only the ternary azeotrope in the distillate, find the entrainer to feed flow rate ratio if the ethanol solution feed stream contains 90 mole% ethanol and 10 mole% water. 

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. 

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