Influence of the Non‐Perfect Step Input Concentration at the Feed Side of the Membrane Surface on the Diffusion Coefficient Evaluation

Abstract

Summary: Vapor diffusion coefficients in polymeric membranes were evaluated from dynamic permeation experiments. A membrane separated the diffusion cell into two parts – upstream and downstream. At the start of the experiment the concentration change in the upstream part (feed side) was made by substituting the input stream of pure nitrogen by the stream of permeant vapors. The solution of the Fick's second law with the step input concentration function is used for the evaluation of diffusion coefficients. The realization of the step input function can be difficult and its imperfection can negatively influence the evaluation process. This contribution deals with the description of the experimentally obtained input function and the study of its influence on evaluated values of diffusion coefficients. The mathematical model, which includes the non‐perfect step input concentration function and the transport through a polymer was developed. The results of this study enable the estimation of diffusion coefficient evaluation errors as dependence on the experimental arrangements and on the membrane transport properties.Diffusion apparatus for measurement of the steady‐state permeation process.magnified imageDiffusion apparatus for measurement of the steady‐state permeation process.

New apparatus for gas permeability, diffusivity and solubility assessing in dense polymeric membranes

“…4(a)). Both cell volumes V o 1 and V o 2 are in the opened position sufficiently small for to ensure a fast response to the feed concentration changes, which diminishes errors of diffusion coefficients [21] and sorption determination. The cell construction is designed so that volumes V o 1 and V o 2 are independent of the membrane thickness.…”