(262az) A SAFT-Based Equation of State for Electrolyte Solutions; Focused on Second Order Thermodynamic Properties

In this study, the electrolyte Perturbed Chain Statistical Associating Fluid Theory (ePC-SAFT) equation of state (EoS) has been utilized to predict the second order thermodynamic properties of electrolyte and non-electrolyte solutions over wide range of pressure and temperature. The main motivation of our work is the need for prediction of derivative properties of electrolyte solutions. The accurate description of second order derivative properties is important for transport properties which are difficult to predict. We need a robust and efficient thermodynamic model to provide good prediction of phase equilibrium calculations and thermo-physical properties over wide range of operational conditions. During last decade, several EoS have been developed based on SAFT theory to account the phase equilibrium calculation of numerous systems. However, the capability of presented model for prediction of second order derivative properties such as heat capacity, speed of sound and etc. is not clear. In the case of electrolyte solutions, several type of electrolyte EoS have focused on phase equilibrium calculations of single or mixed electrolyte (or solvent) systems, while prediction of second order derivative properties of electrolyte solutions have been spared.

In this work, a model has been presented which is capable in phase equilibrium calculations of electrolyte solutions while can predict second order properties efficiently. In the case of electrolyte solutions, calculation of solvent properties play an important role. In this regard and in the first step and in order to provide a comprehensive understanding of the model capability, the second order derivative properties of water over wide range of pressure and temperature has been predicted. The results show that, this model accurately predict the isobaric heat capacity, speed of sound and joule-Thomson coefficient of water when is modelled as the 4C association scheme. Meanwhile vapor pressure and saturated liquid density of pure water up to critical temperature was correlated accurately.

In the case of electrolyte solutions, the solvent permittivity plays a crucial role as it is the main characteristic of the solvent which is used by the model, so having a correct estimation about permittivity will be useful. Considering this fact, the dielectric constant of solvent has been modified using recent methodology proposed by Maribo-Mogensen et al. [J. Phys. Chem. B. 2013;117:10523] to account the pressure, temperature and ion concentration effect on permittivity. This method was based on the framework developed by Onsager, Kirkwood, and Fröhlich. Finally, the modified dielectric constant has been incorporated into the Born and Deby-Hückel term to account the solvation and long range interactions. A new set of parameters are obtained by fitting the mean ionic activity coefficient (MIAC) and apparent molal volume (AMV) experimental data, simultaneously. The ion-specific parameters are utilized to estimate the phase behavior, the infinite dilution heat capacity, enthalpy and Gibbs free energy of electrolyte solutions without any adjustment. On the other hand, the isobaric heat capacity of some 1-1, 1-2 and 2-1 electrolyte solution have been predicted up to 3 molal. The results show that satisfactory results can be obtained in phase equilibrium calculations and second order properties prediction simultaneously if temperature and concentration dependency of permittivity is considered.