Azeotrope with maximum boiling point

Mixtures forming an azeotrope with maximum, boiling point distillate the comj)o-nent in excess, pure bottom product azeotropic mixture of the two components. 

Fig. 231 shows the equilibrium curves of the azeotropic nii.xture acetone-chloroform to which have been added various amounts of the extracting agent, ineth>l-isobntylketone. (The quantities are mole fractions.) The binary S3 stem gives an azeotrope with maximum boiling point at 34.5 mol%. This disappears when 30 mol , of the additive is present further additions cau.se a still larger increase in the relative volatility [50]. 

An important system in distillation is an azeotropic mixture. An azeotrope is a liquid mixture which when vaporized, produces the same composition as the liquid. The VLE plots illustrated in Figure 11 show two different azeotropic systems one with a minimum boiling point and one with a maximum boiling point. In both plots, the equilibrium curves cross the diagonal lines. 

For a solution or mixture of two or more distinct liquid components, an azeotrope is that composition (typically measured in mole fractions or percent weight and referred to as the azeotropic solution) with which there is either a maximum point (a negative azeotrope) or a minimum point (a positive azeotrope) in a boiling point versus composition diagram at constant pressure. 

In systems with negative deviation from ideal behavior, maximum-boiling-point azeotropes can occur. This is illustrated in Fig. 8 for the chloroform-acetone system, treated in Example 1. This system shows negative deviation from ideal behavior due to the possibility of hydrogen bonds between chloroform and acetone, which cannot occur with the pure components. 

The mixture with the maximum boiling point is called maximum bailing azeotrope and behaves as if it is a pure chemical compound of two components, because it boils at a constant temperature and the composition of the liquid and vapour is the same. But the azeotrope is not a chemical compound, because its composition is not constant under conditions and rarely corresponds to stoichiometric proportions. 

Azeotropes with a minimum boiling point (for examples, see Fig. 43, column 3/III) are far more numerous than those with a maximum boiling point (Fig. 43, column o/III). According to the tables of Lecat [20], who li.sts 6287 azeotro]ies and 700 > iion-azeotropes, the ratio is about 9 to I. 

The appearance of azeotropic points has important consequences for the distillation of the mixtures concerned. First let us consider a system with a boiling point maximum (Fig. 14.23). A liquid mixture having the composition x boils at temperature Ti and its corresponding vapor is enriched by the more volatile component B (xf). If the vapor is removed continuously from equilibrium by simple distillation, meaning by condensation in a receiver, the composition of the... 

Azeotropes are of great importance to distillation and rectification. At the azeotrope gas and liquid have the same concentration y = x) and, in turn, no driving force for interfacial mass transfer exists. Azeotropic mixtures behave in some respects like pure substances. They cannot be fractionated by simple distillation. Azeotropes can exhibit a boiling point minimum (minimum azeotropes) or a boiling point maximum (maximum azeotropes). In multicomponent mixtures saddle point azeotropes with intermediate boiling temperature can also exist. 

Figure 9.16 Different types of liquid-vapor phase diagrams for a binary liquid mixture of component A and B as functions of the mole fraction of the component with the higher boiling temperature, (a) The phase diagram for a system with a low-boiling azeotrope (minimum boiling point) and (b) the phase diagram for a system with a high-boiling azeotrope (maximum boiling point). The arrows show how the paths for various distillation processes depend upon the position of the initial composition relative to the azeotrope.

III also crosses the 45 line but curve II cuts the 45 line with the slope less than 1 while curve III crosses the 45 line with the slope greater than 1. Curve III is of the maximum boiling-point type, and the particular composition at which the curve crosses the 45 line is called a maximum constant boiling mixture or a maximum boiling azeotrope. 

Figure 8.9d is practically the vertical mirror image of Figure 8.8d. It shows that this system has a maximum boiling point, and two regions with different behaviors on either side of the liquid composition at which that maximum occurs. This type of azeotrope is less common than minimum boiling azeotropes, but it also makes separation by distillation difficult. If we start with a mixture to the left of the azeotrope in...

Figure A2.5.5. Phase diagrams for two-eomponent systems with deviations from ideal behaviour (temperature T versus mole fraetion v at eonstant pressure). Liquid-gas phase diagrams with maximum (a) and minimum (b) boiling mixtures (azeotropes), (e) Liquid-liquid phase separation, with a eoexistenee eurve and a eritieal point.

Figure 14.3 Boiling temperature against composition phase diagram for (xi or yi) C6F6 + (jt2 or y ) Cgf at a pressure of 0.664 MPa. Evident in the diagram is a minimum boiling azeotrope at point A and a maximum boiling azeotrope at point B. Reprinted with permission from W. J. Gaw and F. L. Swinton, Occurrence of a Double Azeotrope in the Binary System Hexafluorobenzene + Benzene Nature (London), 212, 284 (1966). Copyright MacMillan Magazines Ltd.

Figure 14.11 The critical locus for (xiQHp + x2C6F6), a system with a maximum boiling azeotrope. In (A), the circles represent the critical points (a and b) of pure components (1) and (2) the solid lines represent (vapor + liquid) equilibrium for the pure substances the dashed line is the critical locus, and the short-dashed line represents the azeotrope composition, which intersects the critical locus at point c. (B) shows the intersection of the (vapor + liquid) equilibrium lines with the critical locus.

We predicted their behavior earlier using infinite-dilution /f-values, with the results at 1 atm shown in Table VIII. Only the acetone and chloroform appear to display azeotropic behavior. With this information and that for pure species boiling points at the pressure of interest, we can sketch the ternary diagram for this mixture. We can also use a computer code to generate it, which was done for Fig. 25. We see that there is one maximum-boiling azeotrope between acetone and chloroform. 

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