Single-Component Permeation Maximum with Respect to Temperature: A Lattice Density Functional Theory Study

Membrane permeability and flux of pure gases can exhibit maxima with respect to temperature. For zeolites, this has been explained as a competition between surface and nonsurface diffusion within pores and as a process that depends on the diffusive activation energy and the heat of adsorption. This behavior is reproduced for nanoscale pores by using the lattice density functional theory approach for modeling diffusion. The approach can give expressions for the permeability of noncondensable fluids through nanoscale pores in terms of bulk densities and intermolecular interactions; it also gives the following molecular explanation for the permeation maximum: with decreasing temperature, (i) attractions at the poreʹs entrance increase permeation because molecules in the pore experience difficulty in back-diffusing to the feed, and (ii) at even lower temperatures, attractions at the poreʹs exit reduce permeation because molecules in the pore experience difficulty in escaping from the walls at the end of the pore. The approach shows that permeation maxima can occur without competition between surface and nonsurface diffusion. However, when this competition occurs, the maximum in the surface flux leads to the overall permeation maximum with respect to temperature.