Vapor‐liquid equilibrium constants of binary methane systems for the sub‐critical, critical, and retrograde regions

Abstract

Several methods are presented in the literature for the prediction of vapor-liquid equilibrium constants of mixtures ( 1 , 5,11,12, 13, 15, 19, 20) from thermodynamic relationships. Although these methods are valuable, they can be used only for the determination of vapor-liquid equilibrium constants for moderate pressure conditions. Significant deviations result between calculated and experimental values at elevated pressures, particularly for conditions near the critical point.For a mixture in which the vapor phase behaves as an ideal gas and the liquid phase as an ideal solution, the vapor-liquid equilibrium constant can be expressed as K = y/x = p 0 / g . To account for the effect of pressure on K in a real mixture, Mehra and Thodos (14) have introduced the pseudo-vapor pressure of a component, Kr, and have normalized this quantity with the actual critical pressure of the mixture to obtain the reduced pseudo-vapor pressure, 8 =: K?r/Pc. For a component in its pure state, this ratio is 8" = Po/Poc, the reduced vapor pressure of the substance at the reduced temperature of the mixture. Mehra and Thodos (14) utilized the binary systems, ethane-n-butane, n-butane-n-heptane, and ethane-nheptane, to develop a series of charts which relate the ratio p//P to the reduced temperature of the system with composition as a parameter for different values of the ratio of the normal boiling points of the two components I -TR 1 Fig. 1. Relationships between p andfor the methane-ethane system which exhibits norrnaf behavior.