SAFT-Based Models for Polar Systems

Eirini Karakatsani, Dr. Ioannis G. Economou

In this work, SAFT and PC-SAFT equations of state are modified to account explicitly for dipole-dipole, quadrupole-quadrupole, dipole-quadrupole interactions, as well as for the polarizability effect. The modifications are based on recent work by Nezbeda and Pavlicek [1] for dipolar fluids and on earlier work of Larsen, Rasaiah and Stell [2]. The new models, PSAFT (Polar SAFT) and PC-PSAFT (Polar PC-SAFT) are applied to several pure polar fluids, and used for the correlation of the vapor-liquid and liquid-liquid equilibria of binary mixtures. Finally, model predictions for representative polar ternary mixtures and a quaternary one are calculated. In all cases, very good agreement with experimental data is obtained.

A polar interactions term and a dipole-induced dipole interactions term are added as perturbations to the reference short-range part of the potential. The model views a polarizable polar molecule as a spherical one of effective diameter D, used for the definition of the transition range of polar interactions. The free einergy contribution due to polar interactions and due to the polarizabilities have the form of simple Pade approximants: and , respectively. Both contributions are expressed as a function of reduced temperature, density, dipole moment, quadrupole moment, polarizability and the aforementioned effective diameter.

Figure 1 shows some representative results of the PSAFT model. More details about the model are given by Karakatsani et al. [3].


Figure 1: VLE of carbon dioxide(1)/methanolol(2)/ethanol(3) (k12 = 0.066, k13 = 0.030, k23 = 0.056) at 313.2 K (left)
and LLE of methanol(1)/n-decane(2) at atmospheric pressure (k12 = 0.048)(right).

References
[1] Nezbeda, Pavlicek J., Fluid Phase Equilibria, 1996, 116, 530.
[2] Larsen B., Rasaiah J.C., Stell G., Molec. Phys., 1977, 33, 987.
[3] Karakatsani E.K., Spyriouni T., Economou I.G.,AIChE J., 2005, 51, 2328 .