Adsorption and diffusion of pure components and binary mixtures containing methane, ethane, propane, n-butane, isobutane and hydrogen at 300K were investigated in various configurations of carbon nanotubes using configurational-bias Monte Carlo simulations and molecular dynamics simulations. Both the self-diffusivities and the Maxwell-Stefan diffusivities were determined for a variety of molecular loadings approaching saturation limits. For comparison, self-diffusivities were also determined in pure fluids of varying densities using molecular dynamics. At low loadings, the self-diffusivities corresponded to the value for low-density gases. With increasing loadings, the self-diffusivities in carbon nanotubes were slightly higher than the values in fluids when compared at the same molecular density. In carbon nanotubes, the self-diffusivity was significantly smaller in magnitude than the Maxwell-Stefan diffusivity; indicating strong correlations between molecular jumps along the tube. Thus for mixture diffusion the component self-diffusivities were close together. Molecular dynamics simulations of binary-mixture diffusion demonstrated that the mixture-diffusion characteristics could be estimated with good accuracy from the pure-component diffusion parameters by using the Maxwell-Stefan diffusion formulation. In the estimation procedure, the binary-exchange parameter D12 was estimated from the pure-component self-exchange coefficients D11 and D22 by using an interpolation scheme previously suggested for transport in zeolites.

Describing Binary Mixture Diffusion in Carbon Nanotubes with the Maxwell-Stefan Equations. an Investigation using Molecular Dynamics Simulations. Krishna, R., Van Baten, J.M.: Industrial and Engineering Chemistry Research, 2006, 45[6], 2084-93