MODIFIED SPHCT EQUATION OF STATE FOR IMPROVED PREDICTIONS OF EQUILIBRIUM AND VOLUMETRIC PROPERTIES OF PURE FLUIDS

Segment-segment interaction models, such as the simplified-perturbed-h ard-chain theory (SPHCT) equation of state (EOS), offer the potential of improved volumetric and equilibrium property predictions. Our previ ous evaluations of the SPHCT EOS and similar evaluations in the litera ture lead us to believe that proper modifications can make the SPHCT m ore accurate. In the present study, we report our results for pure-flu id property predictions. The SPHCT EOS is reduced from a three-paramet er to a one-parameter equation of state by using classical critical po int constraints. The temperature dependence of the attractive portion of the equation is modified to provide improved vapor pressure predict ions. In addition, volumetric predictions are enhanced by incorporatin g a phenomenological volume translation strategy. The modified SPHCT e quation of state is compared with the original version and with the Pe ng-Robinson (PR) equation using experimental data for a variety of che mical species. The modified SPHCT equation yields better vapor pressur e predictions than either the original SPHCT or PR equations (1.2% ave rage absolute percent deviation for the modified SPHCT, 3.4% for the o riginal SPHCT, and 4.2% for PR). Improved results are also obtained fo r liquid and vapor phase densities (2.7% and 1.9% for the modified SPH CT, 6.8% and 6.2% far the original SPHCT, and 6.6% and 3.1% for PR for liquid and vapor phases respectively).