An equation of state for polar and non-polar substances and mixtures

Abstract

Simple cubic equations of state, such as the Redlich–Kwong–Soave (RKS) equation, have been used successfully to describe the behaviour of weakly polar substances for both single- and two-phase properties and equilibria.

In this paper we point out how important it is for an equation of state for a substance to fit the entire vapour-pressure curve from the triple point to the critical point. We show how even a simple equation that fits the entire vapour-pressure curve for a substance can quite accurately model a very wide range of compounds and their mixtures. Our new equation is a simple modification of the RKS equation. For each substance it represents accurately the vapour-pressure curve using two constants which are obtained by regression from vapour-pressure data covering the whole range of temperatures from the triple point to the critical point.

We show here that the new equation can represent accurately the vapour pressures, enthalpies, heats of vaporisation and vapour densities of the individual substances, but like most cubic equations it is inaccurate for liquid densities. With binary interaction parameters the equation also describes two-phase equilibria for a wide range of mixtures of both strongly and weakly polar substances with an accuracy similar to that of the RKS equation for mixtures of weakly polar substances.

We give constants for 12 substances: H₂, CH₄, HCl, CS₂, C₂H₆, n-C₅H₁₂, n-C₈H₁₈, H₂O, CH₃OH, CO₂, C₆H₆ and anthracene (C₁₄H₁₀), chosen to illustrate the range of substances to which the new equation applies. We compare calculated results with experimental data for six binary systems (CH₄+ C₂H₆, CH₄+ n-C₈H₁₈, CH₃OH + H₂O, CO₂+ H₂O and n-C₅H₁₂+ H₂O), selected to cover the widest range of types of mixtures. The results demonstrate that the new equation of state describes the behaviour of all these binary systems with an accuracy similar to that of the RKS equation for weakly polar systems. For several of these mixtures this is the first equation of state able to describe their behaviour.