A NONCUBIC EQUATION OF STATE FOR THE TREATMENT OF HYDROCARBON FLUIDS AT RESERVOIR CONDITIONS

A modified version of the Benedict-Webb-Rubin equation of state (EOS) has been developed. Attention was focused on near-critical conditions, where good behavior was obtained by forcing a tight fit to the experimental critical isotherms, including the critical density. The new model requires component critical constants T-c and P-c and acentric factor omega. Generalized expressions have been developed for the calculation of the EOS parameters from the reduced temperature and the acentric factor. Pure-component vapor-pressure and density data were reproduced accurately in a wide range of conditions (reduced temperatures down to 0.4 and pressures up to 1000 bar, both for liquids and gases, including the critical region) and omega values up to 0.6. Mixtures are treated like pure compounds. Their T-c, P-c, and omega are calculated by new mixing rules derived from those of cubic EOS's. With these rules, experimental VLE data were reproduced accurately, with no need of binary interaction constants for alkane-alkane pairs and one constant for the other pairs. Procedures were developed for a fast and safe solution of the equation of state.