High-Pressure Phase Behavior of Mixtures

Phase Diagrams of Binary Mixtures Liquids and Vapors 

The bubble and dew points terminate at the mixture critical point (for temperatures above the critical temperature of the SCF). The mixture critical point is the greatest pressure and composition at a given temperature at which both liquid and vapor phases can coexist Above the mixture critical point, a single fluid phase exists that is completely miscible in all proportions of the two components. As seen from the diagram, this pressure is always greater than the critical pressure of the pure SCF, e.g., CO2. 

The phase behavior and solubility of soHds in supercritical fluids is quite different than that in liquids. First, it is assumed that the solubility of the fluid-phase component, such as supercritical CO2, in the solid phase is negligible. This is unlike the equihbrirmi with liquids, where one must consider the mutual solubilities of both the liquid solute and the fluid phase. From thermodynamics, the mole fraction solubility of a solute in a supercritical fluid, y2, is given by Eq. (4), where, P is the sublimation pressure of the solid which is a function of temperature alone, the exponential term is called the Poynting correction (usual values are 1 to 4) to account for hydrostatic pressure, and the fugacity coefficient, ] , which accounts for the non-idealities of the fluid phase at a certain temperature, pressure, and concentration. At low pressures, the behavior is ideal and the solubility y2 is equal to P /P. 

Global Fluid Phase Behavior of Liquids and Cases 

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Hasch, B. M., M. A. Meilchen, S.-H. Lee, and M. A. McHugh. 1992. High pressure phase behavior of mixtures of poly(ethylene-co-methyl acrylate) with low molecular weight hydrocarbons. J. Polym. Sci. Polym. Phys. Ed., 30 1365-1373. 

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