(375c) Prediction of Thermodynamic and Transport Properties of H2O – CO2, H2O – NaCl and H2O – CO2 – NaCl Mixtures: Towards a Predictive Molecular Model for CCS Processes

Accurate knowledge of thermodynamic and transport properties and phase equilibria of CO₂mixtures over a wide range of temperature and pressure is critical for the optimum design of Carbon Capture and Sequestration (CCS) processes.  These properties can be either measured experimentally or calculated using appropriate models.  Experimental measurements are time consuming and costly.  Thermodynamic models in the form of equations of state, activity coefficient models or simple empirical correlations are used widely for process design calculations. In this case, some experimental data are needed to tune interaction parameters of the models. A powerful approach evolved in recent years refers to molecular simulation.  Thanks to the unprecedented increase of computing power and the development of accurate atomistic force fields, molecular simulation can be used to generate reliable predictions for various physical properties of complex chemical systems, in the absence of experimental data.

In this work, a wide range of equilibrium thermodynamic and transport properties for the ternary mixture H₂O – CO₂ – NaCl and its constituting binaries are predicted using Monte Carlo (MC) and Molecular Dynamics (MD) simulations.  Two different classes of atomistic force fields were used to calculate intermolecular interactions. Initially, two-body potential models were examined. The TIP4P/2005 – EPM2 and the SPC/E – TraPPE combinations were the more accurate for the prediction of CO₂ diffusion coefficient in H₂O below 323 K and above 323 K, respectively [1].  These force fields were used subsequently to predict the diffusion coefficient of H₂O in CO­₂ in liquid and supercritical conditions [2]. Monte Carlo simulations in the Gibbs Ensemble revealed that none of the existing Lennard-Jones or exponential – 6 (exp-6) models reproduce accurately the mutual H₂O – CO₂ solubilities.  By adjusting the oxygen – oxygen unlike interaction energy parameter in the exp-6 model, GEMC predictions are much closer to experimental data in the range 423 – 523 K [3].  For the case of H₂O – NaCl mixture, no combination of force fields is able to predict accurately all properties examined [4].  Specifically, vapor pressures are well represented using the SPC - Joung-Cheathem (JC) model combination.  For viscosities, the combination of SPC/E - Smith-Dang (SD) is the best accurate one. For interfacial tensions, the combination of the semiflexible SPC/E with SD or JC gives the best results.  Inclusion of water flexibility resulted did not improve predictions for all properties uniformly.

A more powerful, but significantly more computing time consuming class of force fields refer to polarizable ones.  In this work, the Drude oscillator based polarizable water (BK3) and ion (AH/BK3) force fields developed by Kiss and Baranyai are examined [6,7]. In the Drude oscillator (or charge-on-spring) approach, a partial charge is attached to a particular interaction site. In particular, liquid densities, electrolyte and crystal chemical potentials of NaCl, salt solubilities, mean ionic activity coefficients, vapor pressures, vapor-liquid interfacial tensions, and viscosities were obtained as functions of temperature, pressure and salt concentration. In all cases, predictions from these models are better or comparable to predictions from the best two-body force-field.  The model is extended to ternary H₂O – CO₂– NaCl mixture predictions.

Acknowledgment

This publication was made possible by NPRP grant number 6-1157-2-471 form the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. We are grateful to the High Performance Computing Center of Texas A&M University at Qatar for generous resource allocation.

References

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