Analysis of transport through thin film composite membranes using an improved Wheatstone bridge resistance model

Abstract

The two-dimensional Wheatstone bridge resistance analog model for permeation through thin film composite (TFC) membranes proposed by Karode et al. [5] has been extended to also include the cases where the coating layer thickness is of similar magnitude as the pore radius in the support matrix. The effect of the constriction resistance, i.e. the resistance encountered by the permeating species in traveling in a radial direction to find a pore to diffuse through is highlighted by considering three generic types of TFC membranes: (i) TFC membranes where the support has very low surface porosity; (ii) TFC membranes with moderate support layer surface porosity; and (iii) TFC membranes incorporating an intermediate gutter layer between the top coating and the bottom support. The model predictions are compared with experimental data reported in literature and various effects are highlighted by considering a few hypothetical cases. The calculations indicate that PRISM type membranes do not require pore filling in order to achieve a composite selectivity close to that of the support material as the high constriction resistance due to low surface porosity effectively prevents transport along the less permselective pathway. In case of less permeable but highly selective top layers, the constriction resistance can be significantly decreased by the gutter layer concept resulting in higher permeabilities and selectivities controlled by the coating layer. In general, the constriction resistance becomes dominant when the permeability of the top layer is low or when the support surface porosity is low.