Third virial coefficients of nonpolar gases and their mixtures

Abstract

While much attention has been given to second virial coefficients of nonpolar gases, experimental and theoretical studies on third virial coefficients are scarce. This work presents a correlation of third virial coefficients within the framework of the corresponding states principle. The correlation is useful for estimating third virial coefficients of pure and mixed nonpolar gases, including the quantum gases helium, hydrogen, and neon. The importance of third virial cross coefficients in phase equilibrium predictions is illustrated with calculations for the solid-gas, methane-hydrogen system a t 76°K.Brief attention is given to the pressure series form of the virial equation. Because of fortuitous cancellations, it is shown that for reduced temperatures above 1.4, the pressure series, truncated after the second term, is applicable to a wider range of density than the density series truncated after the second term. However, when both series are truncated after the third term, the density series appears to be superior regardless of reduced temperature.To describe the volumetric properties of gases, many For practical work, the advantages and disadvantages of the virial e uation have often been discussed (16, 46, 47, 48) ; briefy, th e advantages follow from the direct relationship between virial coefficients and intermolecular forces and the disadvantages follow from our inadequate quantitative knowled e of virial coefficients higher than at moderate densities, below the critical, but has little practical utility at high densities beyond the critical.The technical literature abounds with studies of the second virial coefficient and, as a result of much theoretical and experimental work, it is now possible to make good estimates of the second virial coefficient of a large number of gases from a minimum of experimental data (29, 30, 58). For typical applied calculations, the correlation of Pitzer and Curl (27, 45) is probably the most useful. However, much less attention has been given to the third virial coefficient, primarily for two reasons: first, because of experimental difficulties, good data for the third virial coefficient are scarce, and second, theoretical calculations with potential energy functions used are tedious and, for accurate results, require corrections to the assumption of pairwise additivity which are at best known only approximately (55 to 57). In this work we have collected and examined the limited amount of experimental third virial coefficients now available and we have correlated them as best as we can. Our correlation is limited to nonpolar gases but holds also for the quantum fluids helium, hydrogen, and neon. Since the purpose of our correlation is application oriented, we have given the second. As a resu 7 t, the virial equation is most useful brief attention to the third virial coefficient of the pressure series virial equation which, while theoretically less significant, is sometimes more convenient. More important, we have considered how our correlation may be used to estimate th...

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