Multiscale modeling of transport and residence times in nanostructured membranes

Modeling and simulation at different scales were used to study mass transport and residence times of particles in nanostructured membranes with uniform cylindrical pores of 10-150 nm diameter and up to 5 μm long. 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 present only at temperatures < 700 K. Knudsen diffusion was identified as the dominant mechanism. Simulations based on its principles were performed using an ensemble of particles in a boundary-driven simulation cell, providing the average number of hits between a particle and the pore wall and the dependency of the residence time on the pore dimensions. The differences between operating a nanostructured membrane reactor in sweep-gas and pass-through modes were also investigated.