Ideal and non-ideal diffusion through polymers - Application to pervaporation

Using the approach developed in a previous paper, we integrate the generali sed Stefan-Maxwell (SM) diffusion equations for a unique species in both La grangian and Cartesian coordinates. The dusty gas membrane model is conside red. Two activity-concentration relationships are used: the Flory-Huggins e quation and the Freundlich relationship. The ethyl acetate (EA)/PDMS system follows the Flory-Huggins equation and the computed interaction parameter allows to predict quantitatively the non-dimensional EA flux through a PDMS membrane when the Lagrangian coordinates are utilised. The prediction is l ess satisfactory when integrating the SM equation in the Cartesian coordina tes. The system EA/PDMS can be qualified as an ideal system in diffusion. T he water/ethanol/PVA-based membrane at 60 degreesC system follows Freundlic h's equation at equilibrium. The bad fitting of the experimental water flux versus volume fraction of water at the feed side of the membrane suggests that this system cannot be described by the dusty gas/Stefan-Maxwell theory even when the Lagrangian coordinates are used. This type of system, define d here as non-ideal, obeys theories of the free volume type, as quoted in a recent paper. (C) 2001 Elsevier Science B.V. All rights reserved.