Binary azeotropes containing water

MgS04, CaO, K2CO3, Ca or solid NaOH, followed by refluxing with, and distn from, calcium, magnesium activated with iodine, aluminium amalgam or sodium. Can also dry with molecular sieves, or by refluxing with n-butyl phthalate or succinate. (For method, see Ethanol.) n-Butanol can also be dried by efficient fractional distn, water passing over in the first fractn as a binary azeotrope (contains about 37% water). An ultraviolet-... 

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. 

FIG. 13-58 (Continued) Residue curve maps, (h) MEK-MIPK-water system containing two minumum-hoiling binary azeotropes. 

Because a large amount of water is entrained in the side stream, this is removed in the column C-3. Raw acetonitrile, namely a binary azeotrope with 20% water, separates in top. The bottom stream contains water with heavy impurities. Vacuum distillation at 0.5 bar is adequate to limit the bottom temperature. In the next step pure acetonitrile can be obtained by using pressure-swing distillation. 

FIG. 13-78b Residue curve map MEK-MIPK-water system at 1 atm containing two minumum-boiling binary azeotropes. 

Table 4-1. Some binary azeotropic systems containing water...

These layers are separated continuously in an automatic separator the upper layer is returned to the still, while the lower is taken off and measured. As the reaction nears completion, the amount of water that separates diminishes until there is none the temperature in the still, after rising steadily, flattens out at the refluxing temperature of l e butyl acetate. The crude ester is cooled and neutralized with aqueous sodium hydroxide. After separation of the water layer, the ester is ready for refining by distillation. The first fraction is the ester-water binary azeotrope which is caught in an automatic separator, from which the ester layer is returned to the still. The next is a smtdl fraction that contains some water, as shown by turlndity when it is mixed with 10 vol. of benzene. This is added to the next batch. The rest of the distillate is finished ester and goes to storage. 

Consider the separation of 100 kmol/hr of an equimolar stream of tetrahydrofuran (THF) and water using pressure-swing distillation, as shown in Figure 7.37. The tower T1 operates at 1 bar, with the pressure of the tower T2 increased to 10 bar. As shown in the T-x-y diagrams in Figure 7.38, the binary azeotrope shifts from 19 mol% water at 1 bar to 33 mol% water at 10 bar. Assume that the bottoms product from T1 contains pure water and that from D2 contains pure THF. Also, assume that the distillates from T1 and T2 are at their azeotropic compositions. Determine the unknown flow rates of the product and internal streams. Note that data for the calculation of vapor-liquid equilibria are provided in Table 7.5. 

Figure 5.2-5 shows equilibrium diagrams for binary systems containing maximum boiling (acetone-chlorofotm) and nunimum boiling (ethanol-water) azeotropes. It also shows an example of an azeotrope which, when condensed, forms two liquid phases (n-butanol-water) this is called a heterogeneous azeotrope. 

We examine separation of the mixtures, concentration space of which contains region of existence of two hquid phases and points of heteroazeotropes. It is considerably easier to separate such mixtures into pure components because one can use for separation the combination of distillation columns and decanters (i.e., heteroazeotropic and heteroextractive complexes). Such complexes are widely used now for separation of binary azeotropic mixtures (e.g., of ethanol and water) and of mixtures that form a tangential azeotrope (e.g., acetic acid and water), adding an entrainer that forms two liquid phases with one or both components of the separated azeotropic mixture. In a number of cases, the initial mixture itself contains a component that forms two liquid phases with one or several components of this mixture. Such a component is an autoentrainer, and it is the easiest to separate the mixture under consideration with the help of heteroazeotropic or heteroextractive complex. The example can be the mixture of acetone, butanol, and water, where butanol is autoentrainer. First, heteroazeotropic distillation of the mixture of ethanol and water with the help of benzene as an entrainer was offered in the work (Young, 1902) in the form of a periodical process and then in the form of a continuous process in the work (Kubierschky, 1915). 

Of the more than 800 binary azeotropes listed in Ref. 3, Appendix 2, slightly less than half (377) are likely to not raise concern about compatibility with water as they either don t contain water or their Ra values relative to water are more than 16 MPa (Figure 3.36). Of the remainder, 103 do contain water (VOC exempt in the US), and an additional 379 have Ra values relative to water less than 16 MPa and are likely to be miscible to some extent with water. It is these latter 379 which have more (rather than less) potential to interact with water. 

Water can form binary azeotropes with a few halogenated solvents, but none of them containing cleaning solvents are of consequence. 

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. 

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