Vapor-Liquid Equilibria

What we know about vapor-liquid equilibria is mainly based on experimental data. There are of course predictive methods, but they are based on the results of measurements for the determination of the parameters they involve and the control of their validity. Accordingly, this aspect must be emphasized. However, we will not develop here the experimental techniques that must often be adapted to the case under study taking into account the nature of the mixture, and the domains of pressure and temperature. The reviews by Abbott (1986), Marsh (1989), Deiters et al. (1986) and Holste et al. (1986) can be con­sulted in this connection.

Likewise, the bibliographical or numerical data bases that cover the results of these measurements are also important. The largest was initiated by the University of Dortmund (Gmehling et al. 1978-1984), but mention should also be made of the one by Kehiaian (1973-1993), which covers all deviations from ideality and provides carefully controlled quality data. Lastly, the biblio­graphic base of Wichterle et al. (1973-1985, 1993) is virtually exhaustive.

A good understanding of the vaporization and condensation phenomena of mixtures is required to use them.

It should be pointed out that the way in which vapor-liquid equilibrium data are acquired and therefore presented has evolved. The oldest data are generally isobaric, they reflect better the operation of a distillation column, where normally small variations in pressure correspond to pressure drop between the trays, while the variations in temperature may be significant. This type of data could be directly utilized by graphic methods of calculating columns, such as the McCabe and Thiele method for example.

More recently, the trend has been to acquire isothermal data that are eas­ier to model. The influence of pressure on liquid phase deviations from ideal­ity is in fact generally negligible, contrary to the influence of temperature.

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