419b Formulation and Behavior of a Symmetric Electrolyte Nrtl Model for Gibbs Energy of Electrolyte Systems

Chau-Chyun Chen¹, Yuhua Song¹, and George M. Bollas². (1) R&D, Aspen Technology, Inc., 200 Wheeler Road, Burlington, MA 01803, (2) Department of Chemical Engineering - Process Systems Engineering Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Ave., RM 66-363, Cambridge, MA 02139

For decades, the electrolyte NRTL model[1] has been extensively used as a versatile engineering expression for excess Gibbs energy of aqueous and mixed solvent electrolyte systems. Incorporating the “like-ion repulsion” and “local electroneutrality” assumptions for the liquid lattice structure, the model combines the local composition non-random two-liquid (NRTL) model[2] for short-range local interactions and the unsymmetric Pitzer-Debye-Hückel (PDH) expression[3] for long-range ion-ion interactions. While electrolyte thermodynamic data and constants in the literature are mostly reported for aqueous electrolyte systems, the model follows the convention and uses the aqueous phase infinite dilution reference state for ionic species and the pure liquid reference state for water and other solvents. However, the cumbersomeness of using aqueous phase infinite dilution reference state for ionic species is evident when applying the model for electrolytes or ionic liquids in mixed solvents or non-aqueous solvents[4]. In this paper we replace the asymmetric PDH long-range interaction expression with the symmetric PDH expression of Pitzer and Simonson[5] and we use the fused salt reference state for electrolytes in both the local composition NRTL short-range interaction term and the PDH long-range interaction term. The resulting symmetric electrolyte NRTL model offers a simple and consistent thermodynamic framework for all electrolyte solutions. We present model formulation and model behavior of this symmetric electrolyte NRTL equation for various electrolyte systems.

References

1. Chen, C.-C., J.F. Boston, H.I. Britt, L.B. Evans, “Local Composition Model for the Excess Gibbs Energy of Electrolyte Systems, Part I: Single Solvent, Single Completely Dissociated Electrolyte Systems,” AIChE Journal, 1982, 23, 588-596

2. Renon, H., J. M. Prausnitz, “Local Compositions in Thermodynamic Excess Functions for Liquid Mixtures,” AIChE Journal, 1968, 14, 135-144

3. Pitzer, K.S., “Electrolytes. From Dilute Solutions to Fused Salts,” J. Am. Chem. Soc., 1980, 102, 2902-2906

4. Chen, C.-C., Y. Song, “Extension of Non-Random Two-Liquid Segment Activity Coefficient Model for Electrolytes,” Ind. Eng. Chem. Res., 2005, 44, 8909-8921

5. Pitzer, K.S., J.M. Simonson, “Thermodynamics of Multicomponent, Miscible, Ionic Systems: Theory and Equations,” J. Phys. Chem., 1986, 90, 3005-3009