A New Correlation of Vapor-Liquid Equilibrium Ratios Internally Consistent With Critical Behavior

Abstract

ABSTRACT Equilibrium ratios (K = Y/X) for calculating vapor-liquid equilibrium states have been correlated as functions of pressure, temperature, and convergence pressure for the normal paraffin hydrocarbons from methane through decane, isobutane, isopentane, nitrogen, and carbon dioxide. The functions were designed to yield K = 1 at the convergence pressure but, in the interest of more general application, were not subjected to the other constraints associated with binary systems. Convergence pressures were determined by the critical composition method, using previously developed critical locus and critical pressure correlations. A linear least-squares routine was used to fit the K-functions to 17,245 published experimental equilibrium states in binary, ternary, multicomponent, and complex systems covering a range of temperatures from − 280°F to 700°F, pressures to 12,000 psia, and convergence pressures to 23,200 psia. For the fourteen components, the average absolute deviation of the predicted K-values from experimental is 5.1%. A model is proposed which relates the derivatives of the K-functions at convergence to the isothermal critical composition locus of the system and is found to give very reasonable results for binary and ternary systems and two multicomponent mixtures. A vapor-liquid phase-equilibrium algorithm incorporating this locus into the convergence pressure calculation was used to compute equilibrium phase compositions for comparison with experimental data from selected binary, ternary and multicomponent systems.