An enthalpy-based cubic equation of state mixing rule for cross-prediction of excess thermodynamic properties of hydrocarbon and halogenated refrigerant mixtures

Abstract

Mixing rules based on excess enthalpy of a binary fluid mixture were derived for Soave's equation. Specifically, the mixture SRK equation was cast in a form with complete separation of equation of state constants from thermodynamic variables P, V, and T yielding an equation of state with two volume parameters and one energy-volume parameter. An expression for the binary excess enthalpy was obtained, and the infinite-pressure limit was determined to provide a composition and temperature explicit expression for the SRK parameter a M; a conventional linear mixing rule was used for b M. This set of mixing rules was used with published excess enthalpy data to predict VLE for eleven binary systems comprised of simple aliphatic and halogenated hydrocarbons. A systematic analysis of the results by type of system shows the new mixing rule to be marginally superior to the original Soave polynomial mixing rules with k 12 = 0 or to an athermal form of the new mixing rule for a M ( H E = 0) for simple hydrocarbon systems. However, a marked improvement was observed for nonideal mixtures containing halogenated compounds for which conventional mixing rules with k 12 = 0 for the liquid phase generally fail. Moreover, when only a few excess enthalpy data for the single liquid phase (or supercritical fluid region) which could be fit to a simple polynomial expansion were available, excellent predictions were usually observed. Finally, the new mixing rules produce an approximately quadratic composition dependence of the vapor mixture second virial coefficient at low pressure as is required by statistical mechanics.