Transport through composite membrane, part 1: Is there an optimal support membrane?

The effects of the support membrane pore size and porosity on diffusive transport through composite membranes have been investigated theoretically. Both 3D and 2D models were developed to mechanistically describe the relationship between support membrane pore structure, support material permeability, coating film thickness and the resulting composite membrane permeability; in addition, an analytical model was developed as an approximate, but more convenient approach for assessing trends of composite membrane transport. Model results suggest the choice of support is increasingly important as thin film permeability increases (i.e., less dense or thinner). For fixed coating film permeability, changes to support membrane pore structure create practically important changes to observed water flux and salt rejection by nanofiltration, brackish water reverse osmosis (RO), and seawater RO membranes. The diffusivity of the permeating species in the microporous phase of the support material may also contribute to the overall permeability of the composite membrane. Finally, a systematic numerical study suggests, for the first time, that the local permeate water flux through composite membranes is dictated by support membrane pore morphology, creating localized high flux “hot spots” with potentially high fouling and scaling propensity. The variation in the flux scaled against the average flux through the membrane, for various scaled membrane thicknesses, H, which is the ratio of the thin-film thickness and the support pore radius. Localized regions of high flux and no flux (i.e., stagnant zones) correspond to the location of pores in the underlying film–support interface. [Display omitted] ► Support membrane pore structure influences composite membrane flux and rejection. ► Small pores and high porosity maximize flux, while the opposite maximize rejection. ► Looser membranes are more strongly affected by support membrane properties. ► Localized permeate flux is variable; maximum over pores and minimum in between. ► Localized permeate flux maxima may increase local fouling and scaling propensity.