Static turbulence promoters in cross-flow membrane filtration: a review

The main problems of membrane-based separation methods especially pressure driven ones are fouling and concentration polarization. After increasing cross-flow velocity, use of promoters for inducing turbulence in filtration zone is one of the easiest ways of mitigating concentration polarization in membrane separation processes. Turbulence promoters have the ability to significantly increase the permeate flux in membrane filtration systems by generating alterations in the flow path of the feed. The complexities involved in fluid dynamics which lead to flux enhancement are still not been clearly conceptualized. Various researchers have investigated the effectiveness of promoters on the overall improvement of the filtration efficiency in membrane filtration processes. The flow field generated by these promoters creates turbulence and secondary flows which lead to high shear rates in the vicinity of the filtration surface resulting in scouring of foulant materials and reduction of cake thickness. This ultimately helps in attaining higher levels of flux and mass transfer. Turbulence promoters such as static mixers, Kenics mixers, helical elements, cylindrical rods, thin wires, spacers are widely used in filtration systems to induce turbulence in order to control polarization. This paper reviews the utilization of promoters in membrane-based separation systems, dependence of flux on operational parameters, and effects of different type of promoters on membranes. The effects of cross-flow velocity, Reynolds number, feed properties, membrane properties, pressure, and temperature are reviewed.