Development and application of tools for modeling mass transport and catalytic reaction in nanostructured membranes.

摘要

Modeling and simulation at different scales were used to study mass transport, residence times and selective oxidation in nanostructured membranes. First, analytical equations of the possible mass transport mechanisms inside the pores were used to determine that diffusion dominates over convection under the conditions of interest for selective oxidation: 700 K and pressure near atmospheric. Molecular dynamics simulations showed that surface diffusion is not present at these conditions. Knudsen diffusion was then identified as the dominant mechanism. Simulations based on its principles were performed using an ensemble of particles in a boundary driven simulation cell. Cylindrical pores with uniform diameter and with multiple sections of different diameters were studied. The average number of hits between a particle and the pore wall were obtained, as well as the dependence of the residence times on the dimensions of the pores, or of the pore sections, and the ratio of their cross-sectional areas in the case of pores with multiple sections. Both sweep-gas and pass-through modes of operation were examined. Analytical expressions were developed to relate the transmission probability in pores of multiple sections to the transmission probabilities of the constituent sections.; The catalytic reactions in the nanostructured membranes were studied using the oxidative dehydrogenation of ethane as a representative system. A continuum-level model and reactive Knudsen dynamics simulations were employed to investigate different operational modes, including the pass-through and the sweep-gas modes. It was found that, by adjusting the pore dimensions, the pass-through mode is capable of achieving high conversions even for slow reactions. This is not possible in the sweep-gas mode, making it attractive only for faster reactions. Pores partially covered in catalyst were also studied in the pass-through mode. It was found that the location of the catalyst affects how effectively it is used, but it does not improve the selectivity for a given conversion over the value obtained for a pore fully covered in catalyst.