Calculation of Fluid‐Fluid and Solid‐Fluid Phase Equilibria in Binary Mixtures at High Pressures

Abstract

Abstract By integrating an equation of state, an expression for the Gibbs energy of binary fluid mixtures is derived and used to define the thermodynamic conditions of phase equilibrium. These conditions are solved numerically for the equilibrium concentrations. The same equation of state is used for liquid and vapour phases. From additional solid density data and the sublimation/melting pressure, solid‐liquid and solid‐gas equilibria can be calculated, provided that no miscibility occurs in the solid state. ‐ Calculations of phase equilibria have been carried out for several binary mixtures of nonpolar substances (noble gases, CO 2 hydrocarbons) for pressures up to 200 MPa, using the Redlich‐Kwong equation and our own equation of state, which has been published earlier [1]. ‐ The Redlich‐Kwong equation represents the experimental phase equilibrium data at low pressures only, whereas the other equation achieves good agreement over the whole pressure range. I f the molecules differ very much in size, deviations from one‐fluid theory can be accounted for by using the Leland‐Mansoori‐Carnahan‐Starling function for rigid sphere mixtures in the repulsion term of our equation of state.