A new approach for incorporation of the excess free energy from an activity coefficient model (ACM) into an equation of state (EOS) has been proposed. The approach is based on a concept that any ACM is valid at a low but finite value of compressibility factor. This condition allows us to analyze the “infinite pressure” approximation in a general way. It is shown that the “infinite pressure” approach neglects the contribution of the repulsive term of the EOS to the excess free energy of a mixture. This, in turn, introduces inconsistency which results in the difference in the calculated excess free energy of the EOS and the base ACM. The new approach defines parameter a of the EOS as an implicit function of compositions and temperature, thus requiring an iterative procedure using the “infinite pressure limit” as a starting point. The new approach can be modified to provide the correct composition dependence of a mixture second virial coefficient. Applicability of the new method has been tested for binary systems under vapor–liquid and liquid–liquid equilibrium conditions.
[1]
Syed S. H. Rizvi,et al.
Vapor-liquid equilibria in the ammonia-water system
,
1987
.
[2]
Michael L. Michelsen,et al.
A modified Huron-Vidal mixing rule for cubic equations of state
,
1990
.
[3]
H. J. Van Der Kooi,et al.
Vapor-liquid equilibria for the system ammonia + water up to the critical region.
,
1990
.
[4]
David Shan-Hill Wong,et al.
A theoretically correct mixing rule for cubic equations of state
,
1992
.
[5]
Combined excess free energy models and equations of state
,
1990
.
[6]
M. Michelsen,et al.
High‐pressure vapor‐liquid equilibrium with a UNIFAC‐based equation of state
,
1990
.