Evaluation of the simplified perturbed hard chain theory (SPHCT) for prediction of phase behavior of n-paraffins and mixtures of n-paraffins with ethane

Abstract

The ability of the simplified perturbed hard chain theory (SPHCT) equation of state (EOS) to represent the phase behavior of both pure n-paraffins and mixtures of ethane + n-paraffins has been evaluated. The SPHCT EOS predicts reasonably well the vapor pressures and saturated liquid densities of n-paraffins extending from methane to n-C 64 (average absolute deviations, AAD, of 4% and 7%, respectively). Less accurate predictions, however, are obtained from the SPHCT at temperatures near the triple and the critical points. In general, comparable predictions of phase compositions are obtained from the SPHCT and the Soave (SRK) EOS for ethane + n-paraffin binary systems. For the heavier paraffins (n-C 20 and heavier), however, the SPHCT EOS produces better results. Accurate representation of the experimental data (root mean square error (RMSE within 1 bar for bubble point pressures) requires the use of an interaction parameter for ethane with each n-paraffin. Optimum binary interaction parameters for ethane with n-paraffin solvents extending from C 3 to n-C 44 are presented for the two equations. Simple generalized correlations have been developed for the SPHCT and SRK EOS input variables ( T *, v * and c for SPHCT; and T c, P c, and w for SRK). Using such correlations, a signle parameter ( C ij ) produces useful predictions for ethane in the complete n-paraffin series (RMSE of 1.5 bar for bubble point pressures) including the heaviest members of the series.