Mixtures, azeotropic

Mixtures with low relative volatility or which exhibit azeotropic behavior. The most common means of dealing with the separation of low-relative-volatility and azeotropic mixtures is to use extractive or azeotropic distillation. These processes are considered in detail later. Crystallization and liquid-liquid extraction also can be used. 

If an azeotropic mixture is to be separated by distillation, then use of pressure change to alter the azeotropic composition should be considered before use of an extraneous mass-separating agent. Avoiding the use of extraneous materials often can prevent environmental problems later in the design. 

At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

Examples of azeotropic mixtures of minimum boiling point are collected in Table I, 4, A. 

Examples of azeotropic mixtures of maximum boiling point are tabulated below these are not as numerous as those of minimum boiling point. 

Table I, 4, B. Azeotropic Mixtures op Maximum Boilino Point...

The breaking up of azeotropic mixtures. The behaviour of constant boiling point mixtures simulates that of a pure compound, because the composition of the liquid phase is identical with that of the vapour phase. The composition, however, depends upon the pressure at which the distillation is conducted and also rarely corresponds to stoichiometric proportions. The methods adopted in practice will of necessity depend upon the nature of the components of the binary azeotropic mixture, and include —... 

Other mixtures which may be employed are carbon tetrachloride (b.p. 77°) and toluene (b.p. 110-111°) chloroform (b.p. 61°) and toluene methyl alcohol (b.p. 65°) and water (b.p. 100°). The last example is of interest because almost pure methyl alcohol may be isolated no constant boiling point mixture (or azeotropic mixture) is formed (compare ethyl alcohol and water, Sections 1,4 and 1,5). Attention is directed to the poisonous character of methyl alcohol the vapour should therefore not be inhaled. 

Pure acrylonitrile boils at 78°. Acrylonitrile vapour is highly toxic it should therefore be handled with due caution and all operations with it should be conducted in a fume cupboard provided with an efficient draught. Acrylonitrile forms an azeotropic mixture with water, b.p. 70-5° (12-5 per cent, water). The commercial product may contain tte polymer it should be redistilled before use and the fraction b.p. 76 -5-78° collected separately as a colourless liquid. 

The first binary mixture quantitatively studied was the water-thiazole system, thiazole being a very hygroscopic compound (104), Determining the purity of thiazole sample obtained by distillation, Metzger and Distel-dorf (287) observed the existence of a stable azeotropic mixture, the characteristics of which are the following ... 

An azeotropic mixture con tains two or more substances that distill together at a con stant boiling point The benzene-water azeotrope contains 9% water and boils at 69 C... 

Another way to shift the position of equilibrium to favor the formation of ester is by removing water from the reaction mixture This can be accomplished by adding benzene as a cosolvent and distilling the azeotropic mixture of benzene and water... 

Metal chlorides which are not readily salted out by hydrochloric acid can require high concentrations of HCl for precipitation. This property is used to recover hydrogen chloride from azeotropic mixtures. A typical example is the calcium chloride [10043-52-4] addition used to breakup the HCl—H2O azeotrope and permit recovery of HCl gas by distillation (see Distillation, azeotropic and extractive). 

The cmde wax is refined by extracting at 90—100°C with an azeotropic mixture of benzene and a mixture of alcohols, typically 85% benzene and 15% methanol (see Distillation, azeotropic and extractive). Distilling the solvent leaves a wax too daddy colored to be used without added refining. 

The principle of azeotropic distillation depends on the abiHty of a chemically dissimilar compound to cause one or both components of a mixture to boil at a temperature other than the one expected. Thus, the addition of a nonindigenous component forms an azeotropic mixture with one of the components of the mixture, thereby lowering the boiling point and faciHtating separation by distillation. The separation of components of similar volatiHty may become economical if an entrainer can be found that effectively changes the relative volatiHty. It is also desirable that the entrainer be reasonably cheap, stable, nontoxic, and readily recoverable from the components. In practice, it is probably the ready recoverabiHty that limits the appHcation of extractive and azeotropic distillation. 

Vinyl acetate is a colorless, flammable Hquid having an initially pleasant odor which quickly becomes sharp and irritating. Table 1 Hsts the physical properties of the monomer. Information on properties, safety, and handling of vinyl acetate has been pubUshed (5—9). The vapor pressure, heat of vaporization, vapor heat capacity, Hquid heat capacity, Hquid density, vapor viscosity, Hquid viscosity, surface tension, vapor thermal conductivity, and Hquid thermal conductivity profile over temperature ranges have also been pubHshed (10). Table 2 (11) Hsts the solubiHty information for vinyl acetate. Unlike monomers such as styrene, vinyl acetate has a significant level of solubiHty in water which contributes to unique polymerization behavior. Vinyl acetate forms azeotropic mixtures (Table 3) (12). 

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