Phase Equilibrium and Thermodynamic Properties - Summary

The integral in Equation 4.84 can be evaluated from an equation of state3. However, before this entropy departure function can be applied to calculate entropy, the reference state must be defined. Unlike enthalpy, the reference state cannot be defined at zero pressure, as the entropy of a gas is infinite at zero pressure. To avoid this difficulty, the standard state can be defined as a reference state at low pressure P0 (usually chosen to be 1 bar or 1 atm) and at the temperature under consideration. Thus, 

Prediction of liquid-liquid equilibrium also requires an activity coefficient model. The choice of models of liquid-liquid equilibrium is more restricted than that for vapor-liquid equilibrium, and predictions are particularly sensitive to the model parameters used. 

Both enthalpy and entropy can be calculated from an equation of state to predict the deviation from ideal gas behavior. Having calculated the ideal gas enthalpy or entropy from experimentally correlated data, the enthalpy or entropy departure function from the reference state can then be calculated from an equation of state. 

Finally, it should be noted that the methods outlined in this chapter are not appropriate for systems in which the components exhibit chemical association in the vapor phase (e.g. acetic acid) or to electrolytic systems. 

For the following mixtures, suggest suitable models for both the liquid and vapor phases to predict vapor-liquid equilibrium, a. H2S and water at 20° C and 1.013 bar. 

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