EXCESS PROPERTIES AND PHASE-EQUILIBRIA CALCULATED FROM A GENERALIZED VAN-DER-WAALS-EQUATION OF STATE FOR THE EXAMPLE OF THE MIXTURE METHANE PLUS ETHANE

A generalized van der Waals equation of state, formulated recently for pure non-polar substances, is now adapted for the treatment of mixtures. The main contribution to the van der Waals attractive term, the critical temperature T-c, is now considered as a mixture parameter for the equivalent substance (the one-fluid mixture model). It is determined from the orthobaric liquid density of the mixture, i.e. from the excess volume. All other parameters are supposed to be additive in mole fraction. With these mixing rules, the equation gives good agreement for the excess Gibbs energy of methane + ethane over a larger temperature range. The phase equilibria are calculated according to the algorithm of Deiters (1985) and cover the whole range of temperatures and composition up to the critical curve. Apart from the immediate neighborhood of the critical points of the mixtures, the agreement is very satisfactory. The densities in the homogeneous region and the interaction second virial coefficients are also well reproduced.