Binary Mixtures of Fluids at High Pressures

Binary mixtures can exhibit so many different kinds of fiuid-phase behavior [23, 24] that we need a way to organize and classify them. The classification presented here is based on a scheme first suggested by Scott and van Kon)menbm g [25, 26]. In their original work Scott and van Konynenbmg used the van der Waals equation to identify different forms for mixture critical lines. Since that work, more complicated PvT equations of state have been used to identify additional shapes for critical lines however, many of those have not been observed experimentally. 

A three-phase line will terminate when two of the three phases become identical such states mark the intersection of the three-phase line with a critical line. Consider the three-phase vapor-hquid-hquid situation. If the two liquid phases become identical, then the VLLE line has intersected a locus of liquid-liquid critical points. Similarly, if the vapor phase becomes identical to one of the liquid phases, then the VLLE line has intersected a gas-hquid critical line. These intersections are called upper critical end points (UCEP) if they occur at a maximum temperature on the VLLE locus they are called lower critical end points (LCEP) if they occur at a minimum temperature. The number and kinds of critical end points help distinguish the classes in the Scott-van Konynenburg scheme. 

2 Binaries with Continuous VapoivLiquid Critical Lines 

Mixtures in this first group all have gas-liquid critical points that form a continuous line between the critical points of the pure components. The three classes in this group can be distinguished by the number of critical end points (CEPs). 

Class A. These binaries never exhibit LLE and therefore have no critical end points, although many form azeotropes. However, most mixtures in class A would exhibit LLE with UCEPs, except that solidification occurs at temperatures above that at which a liquid-liquid split would occur. Although the mixture critical line is continuous, the mixture critical T and P may not be bounded by the pure-component critical points. In a few class A mixtures phase splits occur at temperatures above the critical points of both pures. This could hardly be called liquid-liquid equilibrium instead, such an immisdbility is called gas-gas equilibrium (GGE). Three kinds of gas-gas equilibria have been identified that in class A is called GGE of the third kind. 

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