Vapor-liquid equilibrium maximum boiling azeotropes

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.

Many liquid mixtures exhibit azeotropes at intermedrate concentrations such that the liquid and its equilibrium vapor have the same composition. No separation of (his concentration is possible by partial vaporization. A binary mixture may have a minimum bailing azeotrope, where the boiling temperature of the azeotrope is less than that of the pare components, or a maximum boiling azeotrope, where (he boiling temperature is higher (han that of the pare components. About 90% of die known azeotropes are of the minimum variety. 

Figure 8-7 Minimum boiling azeotrope (top), and maximum boiling azeotrope (bottom). Data from Vapor-Liquid Equilibrium Data Collection, J. Gmeliling, U. Onken, and W. Arlt, DECHEMA. 

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. 

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... 

For the mixture isopropyl ether/isopropyl alcohol, positive deviations from Raoult s law occur. They are so strong that the total pressure reaches a maximum for a certain concentration. The vapor pressure maximum corresponds to a boiling temperature minimum as it is depicted in the middle row. Boiling curve and dewpoint curve contact each other in the so-called azeotropic points. Here the mole fractions of vapor y and liquid have the same value which also follows from the equilibrium diagram below. In the azeotropic point A the equilibrium curve intersects the diagonal Pa = To the left of point H the mole fraction of compo-... 

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