Vapor-Liquid Equilibrium Based on Activity Coefficient Models

5 VAPOR-LIQUID EQUILIBRIUM BASED ON ACTIVITY COEFFICIENT MODELS 

In order to model liquid-phase nonideality at moderate pressures, the liquid activity coefficient y, must be known  

Yi varies with composition and temperature. There are three popular activity coefficient models3, 

These models are semiempirical and are based on the concept that intermolecular forces will cause nonrandom arrangement of molecules in the mixture. The models account for the arrangement of molecules of different sizes and the preferred orientation of molecules. In each case, the models are fitted to experimental binary vapor-liquid equilibrium data. This gives binary interaction parameters that can be used to predict multicomponent vapor-liquid equilibrium. In the case of the UNIQUAC equation, if experimentally determined vapor-liquid equilibrium data are not available, the Universal Quasi-chemical Functional Group Activity Coefficients (UNIFAC) method can be used to estimate UNIQUAC parameters from the molecular structures of the components in the mixture3. 

For each binary pair, there are two adjustable parameters 

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