Unsteady-state shear strategies to enhance mass-transfer for the implementation of ultrapermeable membranes in reverse osmosis

Advances in material science promise the development of a new generation of ultrapermeable membranes (UPMs) for reverse osmosis (RO) desalination and water reclamation, which will lead to reduced footprint and lower capital costs. However, due to the attendant increased concentration-polarization (CP) and membrane fouling effects, the higher fl uxes are not possible unless the boundary-layer mass-transfer is enhanced to match the fl ux increase. In a conventional module, a two-fold increase in fl ux via UPM would require a four-fold increase in cross fl ow, generating a 12-fold increase in channel pressure drop. To overcome this, the application of unsteady-state shear to the membrane surface has the potential to be more energy-ef fi cient than a steady-state high shear approach. Hence, this paper reviews a range of unsteady-state shear strategies, including gas sparging, vibrations, particle fl uidization, and fl ow pulsations. Analysis shows that unsteady-state shear could allow for an enhancement of two- to fi ve-fold at an incremental power cost of about 10% compared to the conventional RO desalination process. Some of the practical constraints to implementation are discussed and the promising options identi fi ed for further development. Novel modules and modes of operation could provide a challenge for material science and membrane preparation.

In this review, we conclude that exploitation of the high flux potential of the next generation ultrapermeable membranes (UPMs) for RO requires a radical change of the module hydrodynamics. The higher fluxes must be matched by an increased boundary-layer mass-transfer coefficient, k. Increased shear in conventional modules will lead to unreasonably high pressure drops and energy usage. The development of unsteady-state shear techniques to increase k should be considered. The techniques assessed in this review are gas-sparging, membrane vibrations, particle fluidization and flow pulsation. A useful approach is to compare the potential flux enhancements, the estimated shear rates and the specific power for the various techniques. Using the specific power (with units of W/m2) for a typical seawater RO process as a benchmark, we conclude that the majority of the techniques could achieve useful enhancements for b10% of the benchmark. Gas-sparging, vibrations and particle fluidization are judged to be the most promising with enhancements of at least two-fold and possibly up to five-fold. Each technique brings specific challenges. Overall, there is a need to develop modified modules and to have the option to use UPMs in the hollow fiber configuration. The development of hollow fiber UPMs is another challenge for material scientists and membrane fabricators.