Analysis of Transient Permeation through a Membrane with Immobilizing Chemical Reaction

摘要

Membranes are drawing increasing attention as feasible means of gas separation. The quest for selective and permeable substrates has focused on glassy polymers although chemically reactive liquid films have also attracted considerable interest. The source of permselectivity is invariably a secondary phenomenon-reaction, sorption in glassy microvoids, polymer chain motions-that affords the diffusion process 'non-Fickian' characteristics such as thickness-dependent permeability. The transient phase of membrane permeation with immobilizing, reversible reaction is explored, with emphasis upon the permeation 'time lag'. When reaction is slow there is a two-stage approach to steady-state permeation, from which the reaction kinetics can be determined directly. With fast reaction the data from equilibrium uptake, plus transient and steady-state permeation measurements, may mislead the observer to conclude that permeability is affected by reaction. Previously published measurements of carbon dioxide transport in an ion-exchange membrane are reexamined in this light. Earlier investigators appear to have erroneously concluded that an immobilizing reaction that delays the approach to steady state also affects steady-state transport.