Development of novel crossflow ultrafiltration module integrated with ultrasonic transducers for herbal extracts purification

A major obstacle of applying membrane filtration for purification and separation of herbal extracts is the rapid decline of permeate flux resulting from membrane fouling. With ultrasound irradiation, the combination effects of acoustic cavitation, microstreaming and micro-jets are capable of keeping the particles away from membrane surface to elevate filtration efficiency. However, with conventional configuration, only 10% of ultrasonic intensity can transmit into filtration channel after propagating though housing material. Rather than immersing crossflow module into ultrasonic bath, a customized CF module integrated with ultrasonic transducers was fabricated and resistance in series model was adapted to evaluate the fouling profile of polysaccharides suspension derived from Radix Astragalus water extracts. The filtration enhancement increased significantly from 1.31 to 2.95 with 28 kHz at 200W input comparing to previous setup and DI-H2O flux was recovered to 100% after water flushing. US irradiation dramatically decreased not only the reversible fouling contributed by cake layer and concentration polarization but also the irreversible fouling caused by pore blocking. Evidence was presented by the detected sonic acoustic pressure distribution inside the crossflow channel as well as membrane morphology patterns provided by SEM.;In order to further improve the filtration efficiency, polypropylene permeate spacer was replaced by stainless steel (S.S.) permeate spacers with various porosities. Six sets of S.S. permeate spacers were inserted into CF module to evaluate the effect of porosity on permeate flux as well as on RAE filtration enhancement in both US enhanced and integrated system. The different mechanisms of US effect in these two CF systems played important role. Response Surface Methodology (RSM) coupled with Box–Behnken design (BBD) of experiments were employed to investigate the effects of control variables (ultrasound output power, porosity of permeate spacer and transmembrane pressure) on the absolute permeate collection for RAE Ultrafiltration in US integrated CF system. The optimal operational conditions established by desirability function approach were as follow: a US output power of 120W, a spacer porosity of 60.8% and a TMP of 20 psi. By applying these process parameter values, the permeation efficiency can be improved up to 211%.