The adsorption and diffusion of pure components and binary mixtures containing methane, ethane, propane, n-butane, isobutane, and hydrogen at 300K in various carbon nanotube configurations were investigated using configurational-bias Monte Carlo and molecular dynamics simulations. Both the self-diffusivities, Ds, and the Maxwell-Stefan diffusivities, Di, were determined for various molecular loadings which approached the saturation limits. For comparison, self-diffusivities were also determined for pure fluids of various densities by using molecular dynamics. At low loadings, Ds corresponded to the value for low-density gases. With increasing loading, the Ds for nanotubes were slightly higher than the values for fluids when compared at the same molecular density. In nanotubes, the Ds was significantly lower in magnitude than the Maxwell-Stefan diffusivity, signifying a strong correlation between molecular jumps along the tube. 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 molecular dynamics 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 that had been suggested previously for treating transport in zeolites.

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