MULTICOMPONENT DIFFUSION IN DIALYSIS MEMBRANES

Multicomponent diffusion through porous media is usually described by an effective diffusivity for each component. In such a diffusivity many different effects are lumped together, which makes its behaviour very difficult to understand. In this study we use a different approach in which each species has a driving force which is counteracted by friction due to its motion relative to the surroundings. The resulting equation is a difference form of what is known as the generalized Maxwell Stefan equation (GMS). We apply this to describe transport through cellulose dialysis membranes. Friction between water and the membrane matrix is determined by isobaric dialysis experiments in mixtures with methanol, ethanol and 2-propanol. The water-membrane friction strongly depends on the water content. The friction of methanol or ethanol with the membrane is almost constant, while that of 2-propanol decreases with an increase in the 2-propanol concentration. The resulting friction coefficients give a quantitative description of transport of a ternary liquid mixture through the membrane. Using similar mixtures with a hollow fibre device shows that only the external area of the fibre bundle is effectively used. Apparently there is insufficient flow of the external phase between the fibres. In a second set of experiments a multicomponent system is studied. At the feed side of the membrane a solution of water, 2-propanol and sodium oleate is applied; on the permeate side a NaCl solution, A small pressure gradient from feed to permeate is applied. Initially a mass flux against the pressure gradient is observed. After some time the flux changes direction and becomes two to ten times larger than the permeation rate would be for the feed solution alone with the same applied pressure. These effects cannot be explained using effective diffusivities, but they can be understood qualitatively from the GMS equations.