Importance of the Relative Static Permittivity in electrolyte SAFT-VR Mie Equations of State

Georgios M Kontogeorgis,Xiaodong Liang,Pierre J Walker

doi:10.1016/j.fluid.2021.113256

Georgios M Kontogeorgis, Xiaodong Liang + Show 1 more

Open Access

https://doi.org/10.1016/j.fluid.2021.113256

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Abstract

The influence and importance of the relative static permittivity (RSP) in electrolyte equations of state is examined for the case of aqueous sodium chloride. Using the SAFT-VR Mie model, the Debye-Hückel (DH) or Mean-Spherical Approximation (MSA) terms, as well as the Born-solvation term, are used to formulate an electrolyte equation of state. The RSP is obtained from a variety of models, each differing in their dependencies; we consider constant, temperature-, density- and composition-dependent models. For a fair comparison between different combinations of electrostatic and RSP models, all ion-related parameters are obtained a priori. A novel combining rule is proposed to obtain the unlike parameters between solvents and ions; its reliability is examined for a variety of electrolyte systems. We also compare its performance relative to parameterised electrolyte models. Both the DH and MSA terms yield similar results for almost all properties and conditions. The RSP models used have the more-significant impact. Liquid densities and solvent saturation pressures showed limited changes between RSP models whereas osmotic coefficients, mean ionic activity coefficients and carbon dioxide solubilities observed drastically different behaviour. Analysing the contributions of the various terms to the activities of each species in an electrolyte mixture reveals an important balance between the Born-solvation and the DH or MSA terms where the RSP models have a significant influence over this balance, particularly when these carry a solvent- or ion-composition dependence.

Highlights

Electrolyte systems are of growing importance in many areas of research and industry; examples include carbon capture [1], acid gas scrubbing [2], carriers of active pharmaceutical ingredients and formulation additives [3], and PUREX processes [4]
As we aim to compare a great many combinations of relative static permittivity (RSP) and electrostatic interaction models, as well as the existing Statistical Associating Fluid Theory (SAFT)-VR Mie electrolyte models [31,32], we shall focus most of our attention on a single electrolyte system: aqueous sodium chloride
The properties discussed in the previous section only examined a single phase; here, we examine the influence of electrolytes on a system at equilibrium with two phases, involving derivatives in both volume and composition

Summary

Introduction

Electrolyte systems are of growing importance in many areas of research and industry; examples include carbon capture [1], acid gas scrubbing [2], carriers of active pharmaceutical ingredients and formulation additives [3], and PUREX processes [4]. To model the thermodynamic properties of such systems, one needs to account for a wide variety of interactions: short-range london-dispersion, highly directional dipolar, quadrupolar and hydrogen bonding interactions, and long-range electrostatic interactions. Short-range dispersive interactions can be modelled accurately using cubic equations of state, such as Peng– Robinson (PR) [5] or Soave–Redlich–Kwong (SRK) [6]. Whilst polar interactions and hydrogen-bonding interactions can be modelled as associative type interactions, for which the various Statistical Associating Fluid Theory (SAFT) equations of state [7,8,9,10] have proven to give excellent descriptions of, the former can be modelled as its own distinct interaction [11]. For the long-range electrostatic interactions, the two most-common approaches are the Debye– Hückel [12] (DH) and the primitive Mean-spherical approximation [13] (MSA) equations

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