On individual resistances of selective skin, porous support and diffusion boundary layer in water vapor permeation

Abstract

The efficiency of gas permeation processes is typically determined by two main parameters: selectivity and permeance. While selectivity depends to a large extent on chemical nature of the selective polymer layer, permeance is often dominated by fluid dynamics next to the membrane interface. A well-known phenomenon in terms of diffusive transport limitation is concentration polarization. It occurs in both the stagnant boundary layer and the porous support. The latter is called internal concentration polarization. The more selective and permeable a material is, the more severe the impact of concentration polarization can be. As a result process optimization is an issue far beyond the mere optimization of the selective membrane layer. The accurate identification and quantification of the limiting resistances other than the selective layer is a major challenge. Hence we introduce a systematic characterization approach to deconvolute the total mass transfer resistance. By combining single and mixed gas measurements, we quantify contributions of the selective skin, the porous support and the stagnant boundary layer. All tests were performed with a system comprising water vapor and air, with water vapor as the preferentially permeating component. Depending on process parameters, boundary layer resistance was found to be larger or in the same order of magnitude as the one of the selective layer. The influence of the porous substrate varied with the materials used. The methodology developed is important for humidification as well as dehydration processes, in particular for enthalpy exchangers in building ventilation systems.