Molecular dynamics simulations of transport and separation of supercritical carbon dioxide-alkane mixtures in supported membranes

Slippage and viscosity predictions in carbon micropores and their influence on CO2 and CH4 transport

Non-equilibrium molecular dynamics simulations of pure carbon dioxide and methane and their equimolar mixtures have been carried out with an external driving force imposed on carbon slit pores to investigate gas slippage and Klinkenberg effects. 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 simulations indicate that molecule-wall collisions influence the velocity profile, which deviates significantly from the Navier-Stokes hydrodynamic prediction for micro- and mesopores. Also, the shape of the velocity profile is found to be independent of the applied pressure gradient in micropores. The results indicate that the velocity profile is uniform for pore sizes less than 2 nm (micropores) where the transport is mainly due to molecular streaming or Knudsen diffusion and, to a lesser extent, molecular diffusion. As pore sizes increase to 10 nm, parabolic profiles are observed due to the reduced interaction of gas molecules with the pore walls. A 3D pore network, representative of porous carbon-based materials, has been generated atomistically using the Voronoi tessellation method. Simulations have been carried out to determine the effect of the pore structure and modeled viscosity on permeability and Klinkenberg parameters. The use of the bulk-phase viscosity for estimating the permeability of CO(2) in units of Darcy in a 3D micropore network is not an appropriate assumption as it significantly underestimates the CO(2) permeability. On the other hand, since the transport properties of CH(4) are less influenced by the pore walls compared with CO(2), the use of the bulk-phase CH(4) viscosity estimates are a reasonable assumption.

Monte Carlo study of structural ordering of Lennard-Jones fluids confined in nanochannels

We investigate quantitatively the ordering of Lennard-Jones fluids confined in a thin and infinitely long nanochannel with square cross section. The most probable spatial configurations of the atoms were examined by Monte Carlo simulations, and the order parameter was calculated. The effect of the various parameters, such as the wall-fluid attractive interaction, the size of constriction, and the temperature, was studied. The results indicate that for strong wall-fluid interactions and small constrictions, the ordering of the fluid particles is almost perfect. Geometrical mismatch, as well as increasing the system's temperature, deteriorates the ordering phenomenon, even for very small openings. We observe a nontrivial trend in the dependence of the order parameter on the size of the opening of the channel with a linear size smaller than five atomic layers. We also examined the rearrangements of the fluid's atoms in more symmetrical pores--slitlike pores and cylindrical nanopores--and discuss their similarities and differences with the square channels.