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

Using the approach developed in a previous paper, we integrate the generalised Stefan–Maxwell (SM) diffusion equations for a unique species in both Lagrangian and Cartesian coordinates. The dusty gas membrane model is considered. Two activity–concentration relationships are used: the Flory–Huggins equation 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 less satisfactory when integrating the SM equation in the Cartesian coordinates. The system EA/PDMS can be qualified as an ideal system in diffusion. The water/ethanol/PVA-based membrane at 60°C system follows Freundlich’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, defined here as non-ideal, obeys theories of the free volume type, as quoted in a recent paper.