Heteroazeotropy and Analytical Models for Phase Equilibria

Heteroazeotropy in binary systems is of theoretical interest because it provides a physical relation between vapor-liquid (VLE) and liquid-liquid (LLE) equilibria. Criteria for localization of the vapor phase composition in heteroazeotropic points for nonassociating and low-pressure binary systems are established. It is shown that the localization of heteroazeotropic compositions cannot be made without assumption of a model for the liquid phase, although it can be roughly approximated from pure compound vapor pressure information. The developed criteria are illustrated using well-known activity coefficient models such as the Wohl expansion and the NRTL and Gothard models. It is also shown that solution of the problem must be accompanied by a global phase stability analysis.