Computer simulation of isothermal mass transport in graphite slit pores

Results are presented from a simulation study of the mass transport of oxyg en and nitrogen through graphite slit pores. The work is motivated by an at tempt to understand the molecular origins of the kinetic selectivity displa yed when air is separated into its major components using pressure swing ad sorption. A combination of non-equilibrium molecular dynamics (NEMD), equil ibrium molecular dynamics (EMD) and grand canonical Monte Carlo methods has been employed in our study to extract the maximum information. Transport d iffusivities, self-diffusivities, permeabilities and Darken thermodynamic f actors have been calculated as a function of pore width and temperature for pure component oxygen and nitrogen. In addition, new EMD simulation data f or an 80:20 mixture of nitrogen and oxygen is reported, including a direct calculation of the Stefan-Maxwell coefficients. The results are discussed i n terms of the oxygen selectivity and the possible mechanisms, which increa se or decrease this quantity. We find that the pore width behaviour of the diffusion coefficients consist s of three distinct regimes: a regime at larger pore widths in which single component diffusion coefficients are largely independent of pore width, an optimum pore width at which both diffusivities increase substantially but the slit pore is selective towards nitrogen, and a regime at very low pore widths at which the diffusivities decrease sharply, but the slits are selec tive towards oxygen. The mechanism behind each of these regimes is discusse d in terms of ''entropic'' effects and potential barrier heights. We have also found that permeability selectivity is substantially reduced i n a mixture of the two gases with a composition similar to that of air. Cro ss diffusion coefficients in the mixture have been calculated and shown to be non-negligible.