## Ge RT ln y[14

where G,E = (dnGE/dnt) T,p,nj. Some of these expressions, such as the two-parameter equations of van Laar, those of Margules, and the multiparameter equations of Redlich and Kister, are listed in Table 3. Additional expressions involving higher-order terms, such as those based on the local composition concept proposed by Wilson in 1964, the nonrandom two-liquid (NRTL) model by Renon and Prausnitz in 1968, and the universal quasi-chemical theory (UNIQUAC) equation by Abrams and Pransnitz in 1975, are frequently applied to activity coefficient calculations for binary and multicomponent systems.

According to eqn [8], the relation between ln y1 and ln y2 for a binary system at constant temperature and at low to moderate pressure, is given by:

which indicates that the two activity coefficients are not independent of each other. These activity coefficients may be evaluated by means of the equations listed in Table 4. If the vapour phase can be considered ideal at low pressures, the calculation of y is much simplified. Pure-component fugacities may be substituted directly by the pure-component vapour pressures:

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