Check of VLE Data for Thermodynamic Consistency

Not in all cases, the quality of the published data is suflScient. The quality of complete data (P, T, x,. y,) can be checked with the help of thermodynamic consistency tests. A large number of consistency tests have been developed. Most of ten the so-called area test is applied. The derivation of the required equations for the area test is started from the following equation (see Section 4.3)  

After integration from xi = 0 to xi = 1 an expression is obtained, which can be applied for the graphical examination of complete VLE data for thermodynamic consistency. 

In the case of isothermal or isobaric data one term in the equation above can be cancelled. Since the pressure dependence can usually be neglected, in the case of isothermal VLE data the following simple relation can be used for checking the thermodynamic consistency of VLE data  

The consistency test (Redlich-Kister test) is performed by plotting the logarithmic value of the ratio of the activity coefficients as a function of the mole fraction xi. If the VLE data are thermodynamically consistent the area above and below the x-axis should be equal. 

In the case of isobaric data the excess enthalpy part has to be taken into account. This can be done if the excess enthalpies for the system investigated are known. Since the excess enthalpies are usually not known, in the area test the contribution is taken into account empirically using the quantity J as suggested by Redlich and Kister [25]  

---