3. Gas transport in membrane materials

The essential starting point for designing a gas permeation process is to have quantitative data on the rate at which compounds pass through a given membrane material. To achieve this objective, we need to study and model the mechanisms of gas transport in materials (dense or porous), a vast field of solid-state physics. From a general point of view, the transport rate of a species can be expressed as a phenomenological flux law, like the laws of flow in porous media (Darcy's law), electrical conduction (Ohm's law) or thermal conduction (Fourier's law). The entity's steady-state flow (material flow of the compound in the medium, denoted J) then involves two terms: the driving force, derived from the generalized electrochemical potential gradient of the transferred compound, and an intrinsic characteristic of the system, the phenomenological transport coefficientL, which depends on the structure...