275727 Molecular Simulation of Gas Transport and Entrance Effects in Micro and Mesoporous Carbons

Wednesday, October 31, 2012: 3:15 PM
405 (Convention Center )
Mahnaz Firouzi, Energy Resources Engineering, Stanford University, Stanford, CA, Abby Kirchofer, Earth, Energy, and Environmental Sciences, Stanford University, Stanford, CA and Jennifer Wilcox, Department of Energy Resources Engineering, Stanford University, Stanford, CA

Molecular dynamics simulations have been carried out to study gas slippage and the Klinkenberg effects of methane and carbon dioxide, in addition to their mixtures, confined in carbon slit micro- and mesopores. We used non-equilibrium molecular dynamics (NEMD) simulations with an external driving force imposed on the system. Simulations were conducted to determine the effect of pore size and exposure to an external potential on the velocity profile and slip-stick boundary conditions.

The results indicate that for small channels the particle-wall collisions may influence the velocity profile, which deviates significantly from the Navier-Stokes hydrodynamic prediction. In small pores, unlike in large pores where continuum flow occurs, the fluid velocity at the walls is nonzero. It is shown that the velocity profile is uniform for pore sizes less than 2 nm (micropores) where the transport is mainly due to molecular streaming and, to a lesser extent, molecular diffusion.  As pore sizes increase to 10 nm parabolic velocity profiles are observed due to reduced interaction of gas molecules with carbon atoms of the pore walls. Also, the shape of the velocity profile is found to be independent of the applied pressure gradient in micropores. The simulation results are compared with available experimental Klinkenberg data, in order to understand both the merit and limitations of the simulation approach.

Extensive NEMD simulations have also been carried out to investigate entrance effects of gas diffusion in micropores. The dependence of pore filling and transport of carbon dioxide on the pore size, external pressure gradient, and temperature, have been investigated in carbon slit pores. The results indicate the significance of the pore structure and the fluids’ state to their transport through a porous material. The goal of this work is to determine how physical characteristics (e.g., pore diameter) influence mass transfer, and to determine the dominant mass-transfer mechanisms in pores of varying diameter.

These fundamental studies indicate the significance of molecular-scale phenomena, which lead to an understanding of the mechanism associated with methane and carbon dioxide transport in the confined spaces for carbon capture and sequestration applications.

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