(222bg) Modeling of Thermodynamic, Volumetric, and Electrical Properties With the Electrolyte CPA Equation of State

Maribo-Mogensen, B., Technical University of Denmark
Kontogeorgis, G., Center for Energy Resources Engineering (CERE), Technical University of Denmark
Thomsen, K., Technical University of Denmark

Complex mixtures of associating/polar components and electrolytes are often encountered in the oil- and gas and chemical industry. It is well-known in the oil- and gas industry that electrolytes have a substantial effect on e.g. solubilities of gases in water-hydrocarbon mixtures (salting-out effect), and furthermore, the presence of electrolytes may enhance the inhibitory effect of methanol and glycol on the formation of gas hydrates in natural gas pipelines, thus allowing for problem-free flow [1].

The osmotic coefficients and mean ionic activity coefficients of aqueous and mixed solvent electrolytes have been successfully modeled through activity coefficient models such as electrolyte NRTL [2] or Extended UNIQUAC [3]. These properties have also been represented by electrolyte equations of state (EoS) [1], but the e-EoS are still in their infancy as they require a more accurate physical representation to simultaneously account for volumetric and thermodynamic properties.

Molecular simulations have been used to assess our physical understanding of the complex interactions arising from the strong and long-range Columbic forces [4]. While the basic ion-ion interactions may be modeled through the Debye-Hückel or mean spherical approximation (MSA) models [4,5], these terms become less valid at high concentrations or in low-permittivity solvents where the electrostatic interactions lead to the formation solvation shells and short-lived ion pair complexes that may either be charged or neutral with dipole or multipole moments. While the thermodynamic properties represent a combination of all of the physical effects, the individual processes have also been studied indirectly through dielectric spectroscopy [6], density [7] and conductivity measurements [8].

CPA has successfully been applied in modeling of the phase equilibrium of natural gas components in mixtures of water, glycols or alcohols. CPA was combined with a simple geometrical model and the Onsager / Kirkwood / Fröhlich framework for calculation of the static permittivity [9] . We have studied how the effect of ion association can be used to extend the Cubic Plus Association (CPA) EoS to predict and correlate the thermodynamic, volumetric, dielectric, and electric properties of electrolytes in mixed solvents for a selection of ions; (Li+, Na+, K+, Cs+, Mg2+, Ca2+, Cl-, Br-, I-, NO3-, SO42-). Finally, we show how the electrolyte CPA can be used to predict important flow assurance properties including gas solubility and gas hydrate inhibition in mixed solvents.  

[1] G. M. Kontogeorgis; G. Folas Thermodynamic Models for Industrial Applications, Wiley, Chichester, 2010, ISBN: 978-0-470-69726-9

[2] Chen, C. C., & Evans, L. B. (1986). A local composition model for the excess Gibbs energy of aqueous electrolyte systems. AIChE Journal, 32(3), 444-454.

[3] Thomsen, K., & Rasmussen, P. Modeling of vapor–liquid–solid equilibrium in gas–aqueous electrolyte systems. Chemical Engineering Science (1999), 54(12), 1787-1802.

[4] Fisher, M. E., & Levin, Y.. Criticality in ionic fluids: Debye-Hückel theory, Bjerrum, and beyond. Physical review letters (1993), 71(23), 3826-3829.

[5] B. Maribo-Mogensen, G. M. Kontogeorgis, K. Thomsen, Comparison of the Debye–Hückel and the Mean Spherical Approximation Theories for Electrolyte Solutions, Ind. Eng. Chem. Res. (2012), 51, 5353

[6] Buchner, R., & Barthel, J. (2001). Dielectric relaxation in solutions. Annual Reports Section" C"(Physical Chemistry), 97, 349-382.

[7] Marcus, Y. (2010). On the intrinsic volumes of ions in aqueous solutions.Journal of solution chemistry39(7), 1031-1038.

[8] Safonova, L. P., & Kolker, A. M.. Conductometry of electrolyte solutions. Russian Chemical Reviews, (1992), 61(9), 959-973.

[9] B. Maribo-Mogensen, G. M. Kontogeorgis, K. Thomsen, Modeling of Dielectric Properties of Complex Fluids with an Equation of State, J. Phys. Chem. B (2013), 117, 3389