Atomistic molecular dynamics simulations for CO2 - H2O - NaCl at conditions relevant to CCS processes
Authors: Moultos Othon 
Published: November 2013

Emissions of long-lived greenhouse gases are believed to be a major driver of climate change. Carbon dioxide is the most important greenhouse gas[1] and one of the most prominent strategies to lower its emissions is carbon capture and sequestration (CCS)[2]. CO2 can be stored in geological repositories, in which sodium chloride (NaCl) is the most common dissolved salt. For the optimum design of any CCS process, accurate experimental data and computational models are necessary. This study focuses on generating and validating molecular-based models and methodologies to allow for reliable prediction of the thermodynamic and transport properties of CO2-brine mixtures over a broad range of temperatures and pressures relevant for geological storage. We employ Atomistic Molecular Dynamics (MD) simulations, which is widely used to predict phase equilibria, transport, and other properties of gases, liquids etc. The current study aims at: (a)Determining the accuracy of current molecular models over a broad range of temperatures, pressures and salt concentration relevant to CCS processes with respect to phase behavior and transport properties of CO2 - H2O - NaCl mixture, (b)Developing efficient computational methods and improved potential models for these properties, (c) Assessing the accuracy of SAFT/PC-SAFT based models for the phase behavior of CO2-H2O-NaCl mixture and improving the models using also data generated through molecular simulations and (d)Developing appropriate engineering models for the correlation of viscosity and self-diffusion coefficient experimental and molecular simulation data for the CO2-H2O-NaCl mixture. More specifically, in this work, we study the mutual solubility and interfacial tension in the CO2-H2O-NaCl system over a broad range of temperatures, pressures and NaCl concentrations, using direct interfacial MD simulations. Also we calculate transport properties such as viscosity, self-diffusion coefficient, thermal conductivity. Additionally, we assess the predictive abilities of several combinations of existing H2O, CO2 and NaCl models. To describe water, CO2 and NaCl we use various intermolecular potential models[4,5,6,7,8] that provide property predictions for the CO2-H2O-NaCl mixture over a broad range of temperature, pressure and composition. In addition, the estimated accuracy of the model for each property and for specific conditions will be reported. Our group takes advantage of recent developments of efficiently parallelized codes such as LAMMPS[9] and GROMACS[10]. [1] Metz B, Intergovernmental Panel on Climate Change. Working Group III. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the IInter-governmental Panel on Climate Change. Cambridge; New York: Cambridge University Press (2007). [2] International Energy Agency, A Policy Strategy for Carbon Capture and Storage (2012). [3] Jorgensen, W.L. J. Am. Chem. Soc., 103(2), 335 - 340 (1981). [4] Berendsen, H.J.C., Grigera, J.R. and Straatsma, T.P., J. Chem. Phys., 91(24), 6269 - 6271 (1987). [5] Harris, J.G. and Yung, K.H. J. Phys. Chem., 99(31), 12021 - 12024 (1995). [6] Potoff, J.J. and Siepmann, J.I. AlChE J., 47(7), 1676 - 1682 (2001). [7] Smith, D.E. and Dang, L.X. J. Chem. Phys., 100(5), 3757 - 3766 (1994). [8] Deublein, S., Reiser, S., Vrabec, J. and Hasse H. to appear (2012). [9] See: http://lammps.sandia.gov/. [10] See: http://www.gromacs.org/.