(222ag) Modelling Electrolyte Solutions Using the SAFT-VRE Mie Equation of State

The use of an intermolecular potential of variable form, such as the Mie (generalized Lennard-Jones), within SAFT type equations of state [1] leads to an excellent description not only of thermodynamic fluid phase behaviour, as has become expected of the SAFT family of equations, with a much-improved description in the region of the critical point, but also of a remarkably good representation of second-derivative properties, such as heat capacities and Joule-Thomson coefficients [2,3]. It is highly desirable to be able to treat all fluids that one is likely to encounter in modern chemical engineering practice using a single platform. However, to this point, no version of the SAFT approach for Mie potentials has been available for the treatment of electrolytes. In this paper we report on the extension of SAFT-VR Mie to treat electrolyte systems. In the resulting SAFT-VRE Mie approach, as in its parent EOS, the Mie pair potential [3] is used to represent the short-range repulsive and dispersive interactions. As usual in SAFT implementations, hydrogen bond formation is treated using the first-order thermodynamic perturbation theory (TPT1) of Wertheim [4–7]. In our implementation, however, the association integral, Δ, is evaluated using an essentially exact radial distribution function of the reference fluid, obtained from integral-equation theory; this leads to  much more accurate models for hydrogen bonding systems than can be obtained using previously used approximations of Δ,  and provides for an excellent thermodynamic description of water. Coulombic interactions between ions are incorporated using an unrestricted primitive model, such as augmented Debye-Hückel theory [8,9] or the mean spherical approximation (MSA) [10,11]; the dielectric constant is represented using a new empirical temperature-dependent correlation with the density of the solvent (water) and a classic Born term is included in order to calculate the solvation energies of the ions. The ionic models proposed will be discussed in some detail, especially in terms of their relationships with measurable physical quantities. Results will be presented for single and mixed salt solutions, and for single and mixed solvent solutions including mixtures of brines with CO₂, which are of great importance in the topical area of sequestration of CO₂ in saline aquifers.

References

[1]   T. Lafitte, A. Apostolakou, C. Avendaño, A. Galindo, C.S. Adjiman, E.A. Müller, G. Jackson

       (2013), in preparation.

[2]   T. Lafitte, D. Bessieres, M. M. Piñeiro, J.-L. Daridon, J. Chem. Phys., 124, 024509 (2006).

[3]   G. Mie., Annalen der Physik, 316, 8, 657–697 (1903).

[4]   M. S. Wertheim, J. Stat. Phys., 35, 1–2, 19–34 (1984).