Development of a new empirical non-cubic equation of state

A new empirical non-cubic equation of state (EOS) has been developed for pure substances. The functional form of the equation was devised by fitting the critical isotherms of test substances, with emphasis on exact reproduction of the critical point, and enforcement of van der Waals conditions of criticality. The equation was fit to critical isotherms of methane, propane, and hydrogen, and showed improvement in some areas over both the original Benedict–Webb–Rubin (BWR) EOS and the recent modification of Soave [G. Soave, Ind. Eng. Chem. Res. 34 (1995) 3981–3994], with average absolute percent deviations of less than 1% in both pressure and volume for each substance. Temperature dependence of the parameters was then determined by forcing reproduction of vapour pressures and saturation liquid and vapour densities for methane at numerous subcritical temperatures. Above the critical temperature, parameter values were determined by least-squares fits to supercritical isotherms. The parameter values determined in these ways were then fit as functions of temperature using a least-squares method. The equation was able to fit vapour pressures, saturation liquid and vapour specific volumes, and vaporization enthalpy for methane, with average absolute percent deviations from smoothed data of 0.14%, 0.23%, 0.31%, and 0.66%, respectively, utilizing 16 parameters, 13 of which are adjustable. The maximum deviations for specific volumes and vaporization enthalpy occurred in the immediate vicinity of the critical point.