Correlation effects in diffusion of CH4/CF4 mixtures in MFI zeolite. A study linking MD simulations with the Maxwell-Stefan formulation

Correlation effects in diffusion of CH<SUB>4 </SUB>and CF<SUB>4</SUB> in MFI zeolite have been investigated with the help of molecular dynamics (MD) simulations and the Maxwell-Stefan (M-S) formulation. For single-component diffusion, the correlations are captured by the self-exchange coefficient &#208; <SUP>corr</SUP><SUB> ii</SUB>; in the published literature this coefficient has been assumed to be equal to the single-component M-S diffusivity, &#208;<SUB>i</SUB>. A detailed analysis of single-component diffusivity data from MD, along with published kinetic Monte Carlo (KMC) simulations, reveals that &#208; <SUP>corr</SUP><SUB> ii</SUB>;/&#208; <SUB>i </SUB>is a decreasing function of the molecular loading, depends on the guest-host combination, and is affected by intermolecular repulsion (attraction) forces. A comparison of published KMC simulations for diffusion of various molecules in MFI, with those of primitive square and cubic lattices, shows that the self-exchange coefficient increases with increasing connectivity. Correlations in CH<SUB>4</SUB>/CF<SUB>4</SUB> binary mixtures are described by the binary exchange coefficient &#208; <SUP>corr</SUP><SUB>12</SUB>; this exchange coefficient has been examined using Onsager transport coefficients computed from MD simulations. Analysis of the MD data leads to the development of a logarithmic interpolation formula to relate &#208; <SUP>corr</SUP><SUB>12</SUB>; with the self-exchange coefficient &#208; <SUP>corr</SUP><SUB>12</SUB>; of the constituents. The suggested procedure for estimation of &#208; <SUP>corr</SUP><SUB>12</SUB>; is validated by comparison with MD simulations of the Onsager and Fick transport coefficients for a variety of loadings and compositions. Our studies show that a combination of the M-S formulation and the ideal adsorbed solution theory allows good predictions of binary mixture transport on the basis of only pure component diffusion and sorption data

Packing configurations for methane storage in carbon nanotubes

In this paper we investigate methane packing in single-walled carbon nanotubes. We employ classical applied mathematical modelling using the basic principles of mechanics to exploit the Lennard- Jones potential function and the continuous approximation, which assumes that intermolecular interactions can be approximated by average atomic surface densities.We consider both zigzag and spiral configurations formed by packing methane molecules into (9, 5), (8, 8) and (10, 10) carbon nanotubes, and we derive analytical expressions for the interaction potential energy of these configurations. Our findings indicate that for the zigzag configuration for a (9, 5) tube, the potential energy of the system is minimized when the methane molecules simply form a linear chain along the tube axis, but genuine zigzag patterns are found as the tube size increases such as for the (8, 8) and (10, 10) tubes. For the spiral configuration, the potential energy of the system is minimized when the angular spacing is approximately equal to π for the (9, 5) and (8, 8) tubes, and π/2 for the (10, 10) tube. Overall, our results are in good agreement with molecular dynamics simulations in the literature and show that the most energetically efficient packing configuration of the three tubes studied, occurs for a (10, 10) tube with a zigzag packing, while a (10, 10) tube with a spiral packing configuration has the largest free-cavity volume for methane adsorption at higher temperatures.O. O. Adisa, B. J. Cox, and J. M. Hil