Phase equilibrium by equation of state: a short-cut method allowing for association

Abstract

A continuous association model is used to calculate phase equilibrium by equation of state. Following a suggestion by Scott as well as by Heidemann and Prausnitz, association is taken into account only by the properties of the monomers and by the extent of association. The equation of state is divided into a physical and a chemical part. Alternatively, three different cubic equations of state are used for the physical part. The contribution of the present paper consists of modifications applied to the chemical part, leading to accurate results including cases of strongly associated compounds. Test calculations at atmospheric and subatmospheric pressure were done in five binary systems involving a self-associated compound and a hydrocarbon. The self-associated compounds are methanol, ethanol, phenol and methylamine. The pure-component models are fitted to vapor pressure, vapor and liquid density, and to the critical temperature and pressure. For binary vapor—liquid equilibrium, a single binary fit parameter is used. For liquid-liquid equilibrium, in the case of the mixture methanol-hexane, two binary fit parameters are required. To achieve high accuracy, these parameters are made linear functions of temperature. Computation time is essentially that of classical methods.