Molecular Dynamics Simulations of Transport and Separation of Supercritical Fluid Mixtures through Nanoporous Membranes

Mahnaz Firouzi, Theodore T. Tsotsis, and Muhammad Sahimi
University of Southern California, US

Keywords: Molecular dynamics , carbon nanopore , separation, supercritical, non-equilibrium, alkane

Abstract:
Molecular dynamics simulations have been carried out to study the transport and separation properties of binary mixtures consisting of carbon dioxide and alkanes through a carbon nanopore and also in carbon molecular-sieve membranes with interconnected pores, under supercritical conditions which represent a high pressure and relatively low temperature system. We use non-equilibrium molecular dynamics simulation with an external chemical potential or pressure gradient imposed on the system. The alkanes are generated by the RATTLE algorithm. The membrane is represented by a three dimensional pore space, generated, atomistically, by Voronoi tessellation of the space, using tens of thousands of atoms. Extensive simulations are carried out to study the effect of the pore structure and the imposed external potential on the quantities of interest, such as the fluids' distributions in the system, the separation factor, and the adsorption isotherms, for broad ranges of conditions. It is shown that the fluids form dynamic molecular clusters that travel the pore space. Their sizes varies with the time in a seemingly oscillatory manner. Compared with the subcritical conditions, the supercritical fluids enhances the separation of a fluid mixtures into its constituent components. The results are compared with the relevant experimental data collected in our laboratory, in order to understand the merit and weakness of molecular modeling, and supercritical fluid separation processes.

NSTI Nanotech 2003 Conference Technical Program Abstract