Azeotropic composition, definition

Butadiene and acrylonitrile have different copolymerisation parameters. The azeotropic composition of the monomer mixture is about 38% acrylonitrile (at 25 °C). Where the mixture of monomers contains less than 38 % acrylonitrile the copolymers have acrylonitrile contents that are larger than those which would correspond to the acrylonitrile inputs. This results in continuous alteration of the monomer composition and hence in a lack of chemical uniformity. In industrial production additional acrylonitrile is therefore incorporated in a series of stages when definite degrees of polymerisation have been reached. 

Bajoras and Makuska investigated the effect of hydrogen bonding complexes on the reactivities of (meth)acrylic and isotonic acids in a binary mixture of dimethyl sulfoxide and water using IR spectroscopy (Bajoras and Makuska, 1986). They demonstrated that by altering the solvent composition it was possible to carry out copolymerization in the azeotropic which resulted in the production of homogeneous copolymers of definite compositions at high conversions. Furthermore, it was shown that water solvent fraction determines the rate of copolymerization and the reactivity ratios of the comonomers. This in turn determines the copolymer composition. 

The anhydrous acetates of the rare earths have recently been prepared. Moeller et al. [355] obtained them for La, Dy, Ho, Er, Yb and Y by the azeotropic distillation of a mixture of hydrated acetates with N,N -dimethylformamide (DMF) and benzene. In the case of Ce, Pr, Nd, Sm, Eu and Gd the same method gave a monosolvated acetate, M(C2Hs02)3 DMF. However, the anhydrous acetates of Ce, Pr, Nd, Sm, Eu and Gd can be prepared [355] by vacuum desolvation of the monosolvated compounds. A direct desolvation of the acetates in vacuum at —150° C was attempted by Witt and Onstott [389] after dissolution of the rare earth oxides in 50 per cent acetic acid, and anhydrous acetates of definite composition were obtained for La, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y. 

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

The instantaneous copolymer composition X generally doesn t coincide with the monomer feed composition x from which the copolymer was produced. Such a coincidence X = x can occur only under some special values of monomer feed composition x, called azeotropic . According to definition these values can be calculated in the case of the terminal model (2.8) from a system of non-linear algebraic equations ... 

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. 

Working with azeotropes - not all liquid mixtures can be separated by distillation. In some cases an azeotrope, a mixture of the liquids of definite composition, which boils at a constant temperature, is formed. For example, an azeotrope containing 95.5% ethanol and 4.5% water boils at 78.15 C, which is below the boiling point of pure ethanol (78.3 C). Therefore 100% ethanol cannot be obtained by distillation of ethanol-water mixtures, even though their boiling points are about 22 C apart. 

Binary systems are known that form solid solutions over the entire range of composition and which exhibit either a maximum or a minimum in the melting point. The Uquidus-solidus curves have an appearance similar to that of the liquid-vapor curves in systems which f orm azeotropes. The mixture having the composition at the maximum or minimum of the curve melts sharply and simulates a pure substance in this respect just as an azeotrope boils at a definite temperature and distills unchanged. Mixtures having a maximum in the melting-point curve are comparatively rare. 

These definitions present convenient and simplifying properties ( ) the dimensions of the system are reduced, simplifying the depiction of equilibrium (figure 3.3) (Frey and Stichlmair, 19996 Barbosa and Doherty, 1987a) (ii) they have the same numerical values before and after reaction (m) they sum up to unity (iv) they clearly indicate the presence of reactive azeotropes when Xi =Yf, (v) the nonreactive hmits are well defined (vi) the number of linear independent transformed composition variables coincides with the number of independent variables that describe the chemical equihbrium problem and vii) the lever rule is valid as the chemical reaction no longer impacts the material balance. 

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