Extension of the Elliott−Suresh−Donohue Equation of State to Dipolar Systems

The Elliott−Suresh−Donohue (ESD) equation of state (EOS) is extended to dipolar systems by adding a contribution as perturbation to the Helmholtz energy of the system. The polar contribution is based on the Padé approximation for square-well (SW) reference potential. Pure compound parameters of the new equation were regressed by vapor pressure and saturated density analyses. The parameters were then generalized for nonassociating compounds based on the data for 38 slightly polar substances, mostly including hydrocarbons. Using the polar perturbation term, the accuracy of the ESD EOS to predict saturation properties of pure components is improved in both generalized (nonassociating) and nongeneralized (associating) forms. The proposed model is applied to vapor−liquid equilibrium (VLE) and liquid−liquid equilibrium (LLE) in binary polar systems including associating compounds. Using the polar contribution, the required binary interaction parameter was decreased in comparison with the ESD EOS. By some illustrations, it is demonstrated that the polar perturbation term improves the accuracy and applicability of the ESD EOS.