Pii: 0009-2509(72)80096-4

A modified Redlich-Kwong equation of state is proposed. Vapor pressures of pure compounds can be closely reproduced by assuming the parameter a in the original equation to be temperature-dependent. With the introduction of the acentric factor as a third parameter, a generalized correlation for the modified parameter can be derived. It applies to all nonpolar compounds. With the application of the original generalized mixing rules, the proposed equation can be extended successfully to multicomponent-VLE calculations, for mixtures of nonpolar substances, with the exclusion of carbon dioxide. Less accurate results are obtained for hydrogen-containing mixtures. INTRODUCTION THE REDLICH-KWONG equation [ l] is commonly considered the best of two-parameter equations of state proposed until now. While it can be used to calculate, with a good degree of accuracy, volumetric and thermal properties of pure compounds and of mixtures, its application to multicomponent -VLE calculations often gives poor results. This fact cannot be attributed solely to the imperfection of the mixing rules (although several authors have tried to improve the equation by introducing empirical binary interaction constants [2]), but it must be ascribed partly to the equation’s lack of accuracy in expressing the influence of temperature. As a matter of fact, the accuracy is not better when calculating the vapor pressures of pure substances, which are not influenced by any mixing rule. This conclusion was attained by several authors[3] who tried to improve the original equation by assuming temperature-dependent parameters. This work arises from the assumption that an improvement in reproducing saturation conditions of pure substances also leads to an improvement for mixtures. Good agreement between calculated and experimental vapor pressures of pure compounds is of course not a sufficient condition for a good fitting for mixtures also, but is a necessary one. The original Redlich-Kwong equation of state is: RT al To.= p=-v-b v(v+ 6) ’ (1) In this work, it has been modified by replacing the term a/T0.5 with a more general temperaturedependent term a(T) : RT a(T) -p=v-b v(v+b)’ Letting: RT v=zP