A generalized method for predicting vapor‐liquid equilibrium

Abstract

Vapor-liquid equilibrium distribution ratios for hydrocarbons can be computed by the new equations presented in this work with higher accuracy than i s possible by previous generalized methods. In this proposed new method, the three coefficients of the combination Yiyi/+i are newly developed and generalized functions.The vapor-phase fugacity coefficient @i was formulated via a new generalized equation of state, and the liquid-phase activity coefficient yi is predicted via a proposed new equation that contains three generalized binary interaction coefficients for each binary set in a mixture. These interaction coefficients were developed from experimental K value data. Two expressions are employed in predicting the liquid fugacity coefficient vi, one for the real liquid state and one for the hypothetical liquid state, the latter being developed empirically from experimental K value data also. These two vi equations are also generalized.This new K value prediction method gave an average absolute deviation of 5.25% on a total of 3,504 data points on 19 hydrocarbons in binary and ternary mixtures, these data points being selected so that they cover the range of pure component T r = 0.5 and up and pressures up to 0.9 of the true mixture critical pressure. inclusion of nonhydrocarbon-hydrocarbon systems in the evaluation gave an overall absolute deviation of 6.33% for 4,290 K value data points on 23 components.The phase distribution ratio, Ki = yi/xi, for the components of coexisting equilibrium vapor and liquid mixtures is a complex function of the conditions and compositions of the coexisting phases. Analytical prediction methods for the K value have been based on the equal-fugacity criterion of equilibria, that is, the fugacity of each component is the same in both vapor and liquid phases. Two approaches have been used in predicting equilibrium K values by this equal-fugacity criterion.In one approach, an equation of state, describing PVT behavior, is used to compute the fugacities for the components of both phases. In the other approach, liquidphase fugacities are found via an activity coefficient plus pure component reference state fugacity relationship and combined with equation of state vapor-phase fugacities to obtain the K values. MATHEMATICAL MODELIn terms of fugacity and activity coefficients, the equalfugacity criterion of equilibria may be written asThe left-hand side terms in Equation (1) ship for correlating and predicting K values of hydrocarbons. Chao and Seader ( 2 ) used Equation ( 2 ) as the basis for their generalized correlation. Equation ( 2 ) is the model for the present generalized K value prediction method.In the present work, as in the previous Chao-Seader ( 2 ) work, analytical expressions were derived for the three right-hand terms of Equation ( 2 ) . These semitheoretical expressions were based upon a vapor-phase equation of state and a large quantity of experimental vapor-liquid equilibria data, which were used to determine constants in the theoretical and empirical expressions derived ...

Acentric Factor. A Valuable Correlating Parameter for the Properties of Hydrocarbons

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