Adsorption and diffusion of pure components and binary mixtures containing methane, ethane, propane, n-butane, isobutane, and hydrogen at 300K in a variety of configurations of carbon nanotubes were investigated using configurational-bias Monte Carlo simulations and molecular dynamics simulations. Both self-diffusivities, Di,self, and the Maxwell-Stefan diffusivities, Di, were determined for a variety of molecular loadings, approaching saturation limits. For comparison purposes, self-diffusivities were also determined in pure fluids of varying densities using molecular dynamics. At low molecular loadings, the Di,self corresponded to the value for low-density gases. With increasing loadings, however, the Di,self carbon nanotubes were slightly higher than the values in fluids when compared at the same molecular density. In carbon nanotubes, the Di,self was significantly smaller in magnitude than the Maxwell-Stefan diffusivity Di, signifying strong correlations between molecular jumps along the tube. Consequently, 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 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 using an interpolation scheme suggested earlier for transport in zeolites (Skoulidas et al. 2003).

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