Author: Wang, San Ju
Title: Development of novel crossflow ultrafiltration module integrated with ultrasonic transducers for herbal extracts purification
Degree: Ph.D.
Year: 2012
Subject: Ultrafiltration.
Membrane separation.
Plant extracts -- Therapeutic use.
Ultrasonic transducers.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Biology and Chemical Technology
Pages: xv, 149 leaves : ill. ; 30 cm.
Language: English
Abstract: 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 the addition of ultrasound (US) irradiation, the combination effects of acoustic cavitation, microstreaming and micro-jets are capable of keeping the particle away from membrane surface to elevate filtration efficiency. However, with conventional configuration, only 10% of ultrasonic intensity can transmit into the membrane filtration channel when propagating though housing material. Rather than immersing crossflow (CF) module into ultrasonic bath, in our study, a customized CF module integrated with ultrasonic transducers was fabricated and operated in stainless steel water bath. A resistance in series model was adapted to evaluate the fouling profile of polysaccharides suspension derived from Radix Astragalus water extracts (RAE). The filtration enhancement factor increased significantly from 1.31 to 2.95 with 28 KHz at 200W input comparing to previous conventional setup. With either continuous or intermitted mode of ultrasonic irradiation, DI-H₂O flux of 30 kDa PES membrane recovered to 100% after water flushing. The fouling profile of polysaccharides extract was dominated by reversible fouling taking up to 80% of total resistance. 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. The morphology of fouled membrane and ultrasonic irradiated fouled membrane were characterized using SEM. Furthermore, polysaccharides MW and MW distribution of tested solutions were investigated with GPC.
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 when irradiating with ultrasound in both US enhanced and integrated system. Without proper support to membrane, higher frictional resistance as well as localized turbulent induced by flow across the membrane surface became reasons for significant DI-H₂O flux loss. In the contrast, under the irradiation of US, these reasons turned into positive driving forces for enhanced permeation. The relative RAE filtration enhancement factors presented completely contrary tendencies for the two CF systems when plotting as the function of spacer porosity. The different mechanisms of US effect in these two CF systems played important role. By simply replacing the PP spacer with S.S spacer in permeate channel, the RAE filtration enhancement increased 28% more in US enhanced system and 45% more in US integrated system. Response Surface Methodology (RSM) coupled with BoxBehnken 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 RSM and 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, maximal responses have been predicted. Under these conditions, the permeation efficiency can be improved up to 211%.
Rights: All rights reserved
Access: open access

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