Azeotropes locus

Figure 7. Vapor-liquid equilibria for the system Xe-HCl from theory (lines) and experiment (27 (points). The dash-dot line is the azeotropic locus.

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.

The mathematical solution of Eq. (A.15) is tedious. An elegant graphical solution has been proposed by Stichlmair and Fair [1]. The occurrence of a reactive azeotrope is expressed geometrically by the necessary condition that the tangent to the residue (distillation) curve be collinear with the stoichiometric line. Such points form the locus of potential reactive azeotropes. In order to become a true reactive azeotrope the intersection point must also belong to the chemical equilibrium... 

This is a remarkable result, for it is merely the Clapeyron equation (8.2.27) extended from pure substances to azeotropic mixtures. The derivative on the Ihs represents the slope of the locus of azeotropes on a PT diagram. We can use (9.3.21) as a basis for correlating azeotropic temperatures and pressures, just as we used it in 8.2.6 for correlating pure-component vapor pressures. We obtain the same generalized form of the Clausius-Clapeyron equation. 

P3/P2 is indeterminate. This special case is known to occur when a binary mixture exhibits azeotropic behavior in the critical region that is, when the locus of azeotropic conditions intersects the locus of critical conditions ( 5) ... 

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