An isomorphic Peng–Robinson equation for phase-equilibria properties of hydrocarbon mixtures in the critical region

The principle of isomorphism of critical phenomena asserts that the equation of state of fluid mixtures in the critical region has the same form as that of the pure components, provided that the mixture is not considered at a fixed composition but at a fixed value of a constant field variable related to the chemical potentials of the components of the mixtures. This principle has been successfully applied by various investigators to formulate nonclassical equations of state for fluid mixtures in the critical region. In this paper we show how the same principle can be applied to simple classical equations of state using the Peng–Robinson equation as a representative example. The isomorphic Peng–Robinson equation thus obtained is applied to represent phase equilibria properties of some binary mixtures of methane, butane and decane.