Unit III: Fluid Phase Equilibria in Mixtures

We have already encountered the phase equilibrium problem in our discussions of pure fluids. In Unit I, we were concerned with the quality of the steam. In Unit II, we developed generalized relations for the vapor pressure. These analyses enable us to estimate both the conditions when a liquid/vapor phase transition occurs and the ratio of vapor to liquid. In Unit III, we are not only concerned with the ratio of vapor to liquid, but also with the ratio of each component in the liquid to that in the vapor. These ratios may not be the same because all components are not equally soluble in all phases. These issues arise in a number of applications (e.g., distillation or extraction) that are extremely common. Unfortunately, prediction of the desired properties to the required accuracy is challenging. In fact, no currently available method is entirely satisfactory, even for the limited types of phase equilibria commonly encountered in the chemical processing industries. Nevertheless, the available methods do provide an adequate basis for correlating the available data and for making modest extrapolations, and the methods can be successfully applied to process design. Understanding of the difference between modest extrapolations and radical predictions is facilitated by a careful appreciation of the underlying theory as developed from the molecular level. Developing this understanding is strongly encouraged as a means of avoiding extrapolations that are unreasonable. This unit relies on straightforward extensions of the concepts of energy, entropy, and equilibrium to provide a solid background in the molecular thermodynamics of non-reactive mixtures. The final unit of the text, Unit IV, treats reactive systems.

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