Fluid mixtures at high pressures

Abstract

In the present survey some important trends in the high pressure thermodynamics of fluid mixtures of non-electrolytes are reviewed. First the pressure dependence of excess functions such as the excess Gibbs energy G, the excess enthalpy 5E, the excess en— tropy 5E, and the excess heat capacity CE is discussed. They can be obtained from a knowledge of the xcess volume VE as a function of pressure, temperature, and composition. Experimental results demonstrate that the variations of E as a function of pressure can be important and that even a change of the sign of VE with increasing pressure has been found in some cases. The pressure dependence of HE values thus obtained agree well with literature values determined from direct f lowcalorimetric measurements. Until now most thermodynamic information has had to be deduced from high-pressure phase equilibria and critical phenomena where our knowledge is much better. The pressure dependence and critical phenomena of liquid-gas, liquid-liquid, and gasgas equilibria will be shortly reviewed. Mainly binary systems will be treated, but phase-separation phenomena in some ternary systems will also be considered. New developments during recent years have shown that the limits between liquid-gas, liquid-liquid, and gas-gas equilibria are not well defined and that continuous transitions occur. This continuity will be demonstrated on recent results for binary mixtures of hydrocarbons with tetrafluoromethane and nitrogen. Methods for the calculation and correlation of high-pressure phase equilibria in fluid mixtures under pressure are reviewed. They start from equations of state or from theories of mixtures using sometimes rather complicated mixing rules for the mixture parameters. Some results are presented and compared with experimental data. The significance of high-pressure phase equilibria in fluid mixtures for practical applications is briefly discussed e.g. f or fluid extraction, supercritical fluid chromatography (SFC), and for some high—pressure techniques and processes.