Transport in nanoporous carbon membranes: Experiments and analysis

Molecular sieving using single wall carbon nanotubes

By using molecular dynamics and grand canonical Monte Carlo simulations, we find that a nanotube with a constriction results in high transport resistance to nitrogen while allowing oxygen to pass at a much higher rate even though these gases have very similar sizes and energetics. This provides an understanding of the reported high permeation rates of oxygen relative to nitrogen in nanoporous carbon membranes and a basis for designing nanotubes with constrictions using available technologies for membrane-based separations.

Mass transport of O2 and N2 in nanoporous carbon (C168 schwarzite) using a quantum mechanical force field and molecular dynamics simulations

A hierarchical approach is used to calculate the single-component fluxes of N2 and O2 in nanoporous carbon molecular sieves (represented by C168 schwarzite) over a wide range of pressures and pressure drops. The self- and corrected diffusivities are calculated using equilibrium molecular dynamics simulations with force fields for the gas-carbon interactions obtained from quantum mechanical calculations. These results are combined with previously reported adsorption isotherms of N2 and O2 in C168 to obtain transport diffusivities and, by use of the Fick's equation of mass transport, to obtain single-component fluxes across the membrane. The diffusion coefficients and fluxes are also calculated using an empirical potential, which has been obtained by fitting low coverage adsorption data of N2 and O2 on a planar graphite sheet. By analyzing the diffusivities calculated with the ab initio potential in the limit of infinite dilution over the temperature range from 80 to 450 K, it is observed that the N2/O2 separation is energetically driven and a high selectivity of O2 over N2 can be obtained at low temperatures. However, with the empirical potential both the energetic and entropic contributions to selectivity were found to be close to unity. Similarly, by calculating single-component fluxes and ideal selectivities at 300 K and finite pressures it is found that the ab initio potential better explains the large O2/N2 selectivities of similarly sized molecules that have been observed experimentally. An interesting reversal in ideal selectivity is observed by adjusting the pressure at the two ends of the membrane. As a consequence, we predict that a highly selective kinetic separation in favor of either nitrogen or oxygen could be obtained with the same membrane depending on the operating conditions.

Nonequilibrium molecular dynamics simulation of gas-mixtures transport in carbon-nanopore membranes

The numerical simulation of nonequilibrium cotransport of H2 and alkane gas mixtures with very different molecular sizes through a porous carbon membrane structure was implemented. Simulated permeabilities and selectivities in binary diffusive systems (and in one ternary system), at pressure about tens of atmospheres and at operating condition of room temperature or higher, can be predicted from single-gas permeabilities. This suggests that the effect is geometrical and an approximate model of the transport is proposed. It can be used for an estimation of the separation factor of a membrane. Simulations are compared with experimental results of two- and three-component codiffusion and counterdiffusion in a carbon membrane. It is shown that diffusion in a porous molecular network and in a carbon nanotube are completely different. The uniqueness of this work lies in the comparison of simulated, approximate, and experimental results, which enables us to identify the important parameters.