Calculation of equilibria between fluid and solid phases in binary mixtures at high pressures from equations of state

Abstract

The Redlich-Kwong equation and a three-parameter equation of state of our own (Deiters, 1981, 1982), in connection with appropriate mixing rules, have been used to derive expressions for the Gibbs energy and to evaluate the thermodynamic conditions of fluid-fluid phase equilibrium in binary mixtures. With little additional information it is furthermore possible to extend our theory to equilibria between a fluid mixture and a pure solid phase, so that melting diagrams, solid-vapour equilibria, and solid-liquid-gas three-phase lines can be computed. We have calculated isothermal fluid-fluid (vapour-liquid and gas-gas) and solid-fluid phase diagrams for several binary mixtures of nonpolar substances for pressures up to 200 MPa. The Redlich-Kwong equation is found to represent the experimental data well at low pressures only whereas the results of the other equation of state agree well with the experimental data even at very high pressures. Furthermore it is possible to predict solid-fluid equilibria from fluid-fluid equilibrium data successfully.