High Pressure Thermodynamics of Mixtures

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 GE, the excess enthalpy HE, the excess entropy SE and the excess heat capacity CE is discussed. It can be obtained from the knowledge of the excess volume yE as a function of pressure, temperature and composition. Experimental results demonstrate that the variations of yE as a function of pressure and temperature can be important and comparable to those of the molar volumes themselves. A detailed discussion shows that further progress in this field depends on the development of an accurate equation of state for mixtures. Since furthermore direct calorimetric measurements are practically completely lacking at high pressures most thermodynamic informations have to be deduced up to now 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 gas—gas equilibria will be shortly reviewed. Mainly binary systems will be treated but phase separation phenomena in some ternary and multicomponent systems will be equally considered. Recent results concerning the rate of phase separation will be additionally presented. 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 for binary mixtures of hydrocarbons with carbon dioxide and methane and for some inert gas systems. The significance of high pressure phase equilibria in fluid mixtures for practical applications is shortly discussed, e.g. for high pressure extractions, supercritical fluid chromatography and for some other high pressure techniques and processes. Methods for the calculation of fluid phase equilibria in mixtures under high pressure are reviewed. They start from equations of state or from theories of mixtures using sometimes sophisticated mixing rules for the parameters. Some results are presented and compared with experimental data. Finally a characteristic example for the pressure dependence of chemical equilibria in liquid solutions is considered and the standard value of the reaction volume i V° is determined from measurements of the equilibrium constant K as a function of pressure at constant temperature. It is shown how standard values of the reaction enthalpy iH° can be obtained from temperature jump experiments in such solutions at high pressure.