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Ultrasound effect on cross-flow filtration of polyacrylonitrile ultrafiltration membranes
Journal of Membrane Science 148 (1998) 129±135
Short Communication
Ultrasound effect on cross-¯ow ®ltration of polyacrylonitrile ultra®ltration membranes Xijun Chai, Takaomi Kobayashi*, Nobuyuki Fujii Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka 940-2188, Japan Received 9 October 1997; received in revised form 23 April 1998; accepted 5 June 1998
Abstract Ultrasound effects on permeate ¯ux and rejection of solute were studied by using two types of polyacrylonitrile (PAN) ultra®ltration (UF) membranes reinforced with non-woven cloth. The membrane was set in cross-¯ow UF cell and the ®ltration was carried out by permeating 1 wt% dextran solutions under 30 kPa until steady-state ¯ux was obtained. Then, ultrasound with 45 kHz of frequency was irradiated on the UF cell in water. It was found that the ultrasound irradiation signi®cantly increased the permeate ¯ux of different molecular weight (MW) dextran solutions. Evidence was presented that ultrasound irradiation was effective to enhance the membrane permeate ¯ux for dextran solution. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Ultrasound; Ultra®ltration membrane; Polyacrylonitrile; Dextran; Mass transfer enhancement
1. Introduction The most important disadvantage of membrane ®ltration is the declination of permeate ¯ux due to membrane fouling, resulting from gel layer formation on the membrane surface, adsorption of solute on the membrane pores and blocking of pores by rejected solutes [1±4]. Serious problems in performing ef®cient membrane operation are encountered in both micro®ltration and ultra®ltration (UF). For two decades, this problem has attracted much attention and a number of ways have been available for reducing the extent of fouling. To control fouling and concentration polarization, improvement methods of membrane performance are classi®ed into four categories: pretreat*Corresponding author. Fax: +81-258-47-9300.
ment of feed solution, adjustment of membrane material properties [1], membrane cleaning and improvement by operation condition. Practically, what is most widely used in membrane separation process is the cleaning technique by hydraulic, mechanical, chemical [3,4] and electrical methods [5]. The hydraulic cleaning including back-washing [2] is the most important method for reducing membrane fouling. However, this technique interrupts the continuous ®ltration process and cannot maintain the membrane ®ltration operating at high permeate ¯ux. More recently, electric cleaning technique [5] was developed, which was applied by using pulsed electric ®eld to membrane ®ltration. As a result, the charged particles fouled on membrane were removed away from the membrane. Moreover, other techniques using unsteady jet [6], pulsation ¯owing [7,8] and gas±liquid
0376-7388/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00145-8
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X. Chai et al. / Journal of Membrane Science 148 (1998) 129±135
two-phase ¯ow formation [9] were developed to overcome ¯ux decline of membrane. Ultrasound effect has been studied for the enhancement of permeate ¯ux in the capsule membrane process [10]. It was found that the transport rate of NaCl across nylon capsule membrane coated with amphiphiles was affected by ultrasound irradiation. Ultrasound effect has also been studied for the transport of organic compounds across the polymer membrane. The experiment was carried out in aqueous solution by using 20 kHz sound wave and ultrasound led to increase in the permeability of hydrocortizone and benzoic acid in a cellulose membrane and polydimethylsiloxane ®lm, respectively [11]. Ultrasound effect on membrane ®ltration was found to increase the electrolyte diffusion through dialysis or ionexchange membranes [12±15]. It was reported that the permeability enhancement was resulted from the acoustic pressure, agitation and microcurrent of feed ¯uid by ultrasound irradiation. Effect of ultrasound was further studied on the micro®ltration of anatase and china clay [16]. It was found that ultrasound can promote fouling prevention and facilitate improved separation rates. The rate of fouling was affected by ultrasound strength, suspension concentration and cross-¯ow velocity. The present study aims to understand the effect of ultrasound irradiation on enhancement of membrane permeate ¯ux. Ultrasound effect was studied for cross-¯ow ®ltration process having ¯at sheet polyacrylonitrile UF membrane reinforced with non-woven cloth.
casting solution was immediately immersed in a water bath at 308C and kept overnight. The solidi®ed PAN membrane on the non-woven cloth was washed with a large quantity of water to remove the solvent DMSO. In order to control the membrane permeability and rejection property, casting solutions having 8 and 15 wt% PAN concentrations were used. The corresponding membranes were abbreviated as PAN8 and PAN15 membrane, respectively. The resultant membranes have thickness of 80±90 mm and dimension size of 200150 mm2. Scanning electronic microscope observation indicated that both membranes have asymmetric structure with ®nger-like pore in supporting layer. For characterization of ®ltration properties, 0.1 wt% aqueous solutions of dextran having different MW were applied to determine the molecular size exclusion properties of resultant membranes. The experiments were carried out as reported previously [17]. The concentration of dextran in feed and permeate solutions was analyzed by GPC instrument (Type CCPD UV8000 of Toyo Soda with a 30 cm column of TSK gel G5000 PWX1) equipped with a refractometer (RI8000). The membrane rejection was calculated from the areas of GPC curves for permeate and feed solutions. The rejection data for different MW dextran are shown in Fig. 1. The results for the molecular weight cutoff curves (MWCO) mean that the resultant
2. Experimental 2.1. Membranes For UF experiments, polyacrylonitrile (PAN) membranes reinforced with polyester non-woven cloth (Barren, MDF-60K, 0.07 mm of thickness, 1 mm average pore size) were used. The membranes were prepared as follows: PAN having molecular weight (MW) of about 7104 was dissolved in dimethyl sulfoxide (DMSO) overnight at 508C for preparing casting solution. The solution was spread on the non-woven cloth on which a pair of 100 mm thickness spacers were placed to keep the membrane thickness constant. After casting, the non-woven cloth involving PAN
Fig. 1. MWCO curves for PAN8 (*) and PAN15 (&) membranes.
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of membrane ¯ux. For the cross-¯ow ®ltration of dextran, the 1 wt% solution was passed through the membrane surface with a peristaltic pump. The ¯ow rate of feed solution was controlled with 325 ml/min ¯ow-rate. Operating pressure was ®xed at 30 kPa and temperature was 258C. The feed and retentate ¯ow rates were measured by ¯ow meters (Model RK400, Ko¯oc, Japan) and converted to volume ¯ux, m3/(m2/ s). The permeate solution through PAN membrane was collected for GPC measurements of dextran solute. 3. Results and discussion 3.1. Change of membrane permeate flux with time in the absence and presence of ultrasound irradiation
Fig. 2. Schematic diagram of the cross-flow UF experiments for ultrasound effect. PG is pressure gauge. The sonicator emits 45 kHz ultrasound with 248 W power. Feed tank contains 1 wt% dextran solution with 500 ml. The applied pressure was fixed at 30 kPa through the UF experiments.
In order to investigate the ultrasound effect on permeability of PAN UF membranes for 1 wt% solutions of different MWs, two types of PAN8 and PAN15 membranes were used for cross-¯ow ®ltration. Fig. 3 shows the change of permeate ¯ux of dextran solution through PAN8 membrane with and without irradiation of ultrasound. The ultrasound effects on permeate ¯ux
PAN8 membrane has lower rejection than PAN15 membrane. 2.2. Cross-flow ultrafiltration experiments As schematically illustrated in Fig. 2, sonication effect on UF was studied by using experimental setup of cross-¯ow ®ltration unit that was made of stainless steel (Type Minitan S, Millipore). The prepared PAN UF membrane was cut into 80120 mm2 size and was set in the UF cell. Then the ®ltration cell was immersed in water bath of sonicator (Branson ultrasonic cleaner of Type Bransonic B-52 with ultrasonic wave frequency of 45 kHz and input power of 248 W) as the ®ltration was carried out. The size of the water bath of sonicator was 300240160 mm3. The membrane sheet was 50 mm away from the transducers of sonicator and the dense skin layer of the membrane faced the transmission direction of ultrasound wave. During the ®ltration experiments, the temperature increase of water bath (3 l) was checked as less than 18C. Hence, this temperature increase through the ®ltration experiment is negligible in measurement
Fig. 3. Change of permeate flux with filtration time in the absence and the presence of ultrasound irradiation for PAN8 membrane. Water (*); 1 wt% dextran solution of MW 1104 (*); 7104 (~); 5105 (&) and 2 106 (&).
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were examined for water and 1 wt% solution of dextran having MW of 1104, 7104, 5105 and 2106, respectively. At ®rst, all of the ®ltration experiments for each solution were conducted without sonication (as indicated with sonication OFF). The permeate ¯ux observed for water (shown with ``*'' in Fig. 3) is 310ÿ5 m3/(m2/s). After 80 min ®ltration, then, the ultrasound irradiation was carried out between 80 and 110 min for an interval of 30 min (as indicated with sonication ON). However, the permeate ¯ux of water was kept unchanged during the following 30 min ®ltration, although the sonication was applied. This phenomenon suggests that no pore size change may occur in the PAN membrane under ultrasound irradiation. On the other hand, in the cases of dextran solution with 1104 (*) and 7104 (~) MWs, the permeate ¯ux gradually decreases during the ®ltration period between 0 and 80 min. For 5105 (&) and 2106 (&) MW dextran, the values of the permeate ¯ux are much lower than those for the above two kinds of dextran solutions. The results for solutions of 5105 and 2106 MW dextrans are due to the concentration polarization because of high rejection by membrane. The high concentration of solute rejected on the membrane surface results in high osmotic pressure, which in turn decreases membrane permeate ¯ux [18]. As shown in Fig. 1, the results for the MWCO curves indicate that PAN8 membrane completely permeates dextran molecules having 1104 MW, while the membrane rejects 5105 and 2106 MW dextran with 45% and 70% rejection, respectively. After 80 min ®ltration for solution of 1104 and 7104 MW dextrans, the values of the permeate ¯ux increase by the ultrasound irradiation and become similar to that of water, 310ÿ5 m3/ (m2/s). For 1 wt% solutions of 5105 and 2106 MW dextran, the values of permeate ¯ux increase to 610ÿ6 m3/(m2/s) by the 30 min sonication. As the ®ltration was operated again without sonication after the sonication, the values of permeate ¯ux for dextran solutions gradually decreased in the following 30 min ®ltration period. Then, when sonication was irradiated again to the UF cell, the value of the permeate ¯ux for each dextran solution increased in same way as observed in the ®rst circulation. As well as the ultrasound experiments for PAN8 membrane, the permeate ¯ux of water and aqueous solutions of dextrans for PAN15 membrane were also
Fig. 4. Permeate flux without and with ultrasound irradiation for PAN15 membrane. Water (*); 1 wt% dextran solution of MW 1104 (*) and 2106 (&).
measured at various ®ltration times. Fig. 4 shows permeate ¯ux of water and dextran solutions without and with ultrasound operation. For water, the permeate ¯ux of PAN15 membrane is 310ÿ6 m3/(m2/s) and is about one order of magnitude smaller than that of PAN8 membrane. In the case of PAN15 membrane, water permeate ¯ux has no change with ®ltration time even if ultrasound is irradiated to the UF cell. For 1 wt% solution of 1104 MW dextran, little was observed for the decrease of the permeate ¯ux during the 80 min ®ltration. In addition, the value of permeate ¯ux did not increase signi®cantly by the 30 min sonication. However, the permeate ¯ux for solution of 2106 MW dextran signi®cantly decreases during the 80 min ®ltration. This means that the highly rejected 2106 MW dextran results in more serious concentration polarization. Then, by the ultrasound irradiation, the permeate ¯ux increases through the following 30 min sonication. As shown in Table 1, the 2106 MW dextran is almost totally rejected by PAN15 membrane, while the 1104 MW dextran is slightly rejected by PAN15 membrane. We note that, for the 1104 MW dextran, both data of Figs. 3 and 4 have similar tendency when sonicator was turned on or off. Namely, as the dextran rejection by membrane is
X. Chai et al. / Journal of Membrane Science 148 (1998) 129±135
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Table 1 Rejection of dextrans by both PAN membranes for cross-flow filtration of 1 wt% dextran solution Time (min)
Sonication
PAN8 membrane 4
0 80 110 190 220
OFF ON OFF ON OFF
PAN15 membrane
110 MW
210 MW
1104 MW
2106 MW
1.4 1.1 0 1 1
59 63 57 62 58
3 3.5 3 4 2
93 94 93 94 93
very low, the ultrasound effect on the permeate ¯ux enhancement is also low. In such cases, probably, concentration polarization is slight. On the other hand, for the 2106 MW dextran solution, which results in serious concentration polarization, the permeate ¯ux decreases signi®cantly during ®ltration process without sonication. When subjected to sonication, the enhancement of the permeate ¯ux for the 2106 MW dextran solution is observed. This may be due to the sonication enhancing bulk mass transfer in concentration polarization layer on the membrane surface. For estimation of ultrasound effect on the rejection properties of the membranes, Table 1 summarizes rejection data of both membranes used for dextran solution at different ®ltration time. It can be seen that, although the permeate ¯ux is recovered by ultrasound irradiation, there are no signi®cant changes of rejection for 1104 and 2106 MW dextrans with and without ultrasound irradiation. This suggests that the PAN membranes show no change in pore sizes by ultrasound irradiation during the ®ltration process. Consequently, we are sure of practical applicability of the ultrasound effect on ¯ux enhancement in cross¯ow ®ltration. Particularly, for the 2106 MW dextran solution, serious concentration polarization occurs, the ultrasound irradiation is effective to enhance membrane permeate ¯ux. 3.2. Continuous ultrasound irradiation to cross-flow filtration To verify the ultrasound effect on enhancement of the permeate ¯ux for both PAN membranes, sonication was operated from the time when the ®ltration was started. The permeate ¯ux for 1 wt% solution of 2106 MW dextran was measured under 30 kPa
6
Fig. 5. Permeate flux observed with and without sonication for cross-flow filtration of 1 wt% dextran of 2106 MW. PAN8 (*, *) and PAN15 (&, &). Open symbols are for data of permeate flux without sonication and closed symbols are for data with sonication. The sonication was operated through the filtration experiments for the 80 min period.
operation pressure. Fig. 5 shows plots of the permeate ¯uxes versus ®ltration time for PAN8 and PAN15 membranes. At the beginning of the operation, PAN8 and PAN15 membranes have the permeate ¯ux of 1.410ÿ6 and 2.510ÿ6 m3/(m2/s), respectively. By operating sonication, the values of the permeate ¯ux immediately increase for both PAN8 and PAN15 membranes. Keeping on operating the sonicator for the following 80 min ®ltration, it is shown that there is no decline of the permeate ¯ux during the sonication operation. However, in the membrane ®ltration
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without sonication (indicated by dashed lines) the permeate ¯ux decreases with operating time and approaches a steady value. In this case, the permeate ¯ux is lower than the original permeate ¯ux. The comparison of experimental data suggests that the ultrasound irradiation process is very effective on enhancement of membrane permeability in the ®ltration. The enhancement of membrane permeate ¯ux is resulted by the increase in bulk mass transfer due to vibration of the membrane caused by the ultrasound. 3.3. Degradation of dextran solute under ultrasound irradiation It is known that macromolecular compounds decompose by ultrasound irradiation [19]. Namely, the MW of macromolecule solute decreases by breakage of the molecular chain in the presence of sonication. Therefore, we examined the degradation of dextran solute after sonication in this work, since the decomposed solute may in¯uence the ®ltration behavior. At ®rst, the degradation experiment was carried out by directly immersing the glass ¯ask containing 500 ml of 1 wt% solution of the 2106 MW dextran in the water bath of sonicator and then the ultrasound was irradiated for 30 min to the dextran solution through the thin glass with 1 mm thickness. The GPC chromatograms of dextran before and after sonication were measured (Fig. 6(a)) to check dextran molecular size. Some change can be seen in the chromatograms before and after sonication. That is, the peak intensity appeared near 10 min retention time increases by the sonication treatment. This means that the dextran MW is decreased by the ultrasound irradiation. On the other hand, the ultrasound was irradiated to cross-¯ow UF cell without membrane. Here, the 1 wt% solution of 2106 MW dextran was circulated for 30 min in the presence of ultrasound and then the dextran solution was sampled from feed tank. The GPC chromatograms of these samples before and after sonication are shown in Fig. 6(b). A little GPC trace change can be seen in the chromatograms. We note that the change of latter case is smaller than the former one. Hence, this suggests that the degradation of dextran solute is very low in the ®ltration experiments. This may be due to shielding of the ultrasound by the UF cell made of stainless steel and plastics boards. Although the exact mechanism of degradation of
Fig. 6. GPC chromatograms for 1 wt% solution of 2106 dextran MW before (solid line) and after (dashed line) ultrasound irradiation. (a) irradiated directly to dextran solution, (b) irradiated through the UF cell operated without membrane.
macromolecules by ultrasound is not clear, PAN membrane it is generally agreed that the hydrodynamic forces are of primary importance. There has been report on ultrasonic degradation of dextran [20]. It was known that the shear forces generated by cavitation collapse are responsible for the breakage of chemical bonds of the polymer. 4. Conclusions Ultrasound effect on permeate ¯ux and dextran rejection by the PAN membranes was studied using various MWs of dextran. We found that the ultrasound irradiation to the cross-¯ow UF cell having PAN membrane was very effective to enhance the ®ltration. Under the experimental conditions, it was found that the sonication treatment increases permeate ¯ux for dextran solute which is highly rejected by the PAN membrane. However, there was no ultrasound effect for water and small effect for dextran with low rejection by the membrane. Evidence was shown that the increase in the permeate ¯ux is attributed to the enhancement of bulk mass transfer in concentration polarization layer near membrane.
X. Chai et al. / Journal of Membrane Science 148 (1998) 129±135
Acknowledgements This study was partially supported by Grants-in-Aid for Scienti®c Research of Ministry of Health and Welfare, Japan. References [1] A. Kulkarni, D. Mukherjee, W.N. Gill, Flux enhancement by hydrophilization of thin film composite RO membranes, J. Membr. Sci. 114 (1996) 39. [2] H. Li, C.D. Bertram, D.E. Wiley, Buck flushing effect of pulsatile flow on cross-flow microfiltration, Proceeding of the 1996 International Congress on Membrane and Membrane Processes, Yokohama, Japan, 1996, p. 240. [3] D.E. Wiley, M.J. Munoz-Aguado, A.G. Fane, The action of enzymes and surfactants during the cleaning of ultrafiltration membranes, Proceedings of the 1996 international congress on membranes and membrane processes, Yokohama, Japan, 1996, p. 220. [4] M. Pontie, X. Chasseray, D. Lemordant, J.M. Laine, The streaming potential method for the characterization of ultrafiltration organic membranes and the control of cleaning treatments, J. Membr. Sci. 129 (1997) 125. [5] W.R. Bowen, H.A.M. Sabuni, Pulsed electrokinetic cleaning of cellulose nitrate microfiltration membranes, Ind. Eng. Chem. Res. 31 (1992) 515. [6] C. Maranges, C. Fonade, Flux enhancement in cross-flow filtration using an unsteady jet, J. Membr. Sci. 123 (1997) 1. [7] S. Najarian, B.J. Bellhouse, Effect of liquid pulsation on protein fraction using UF processes, J. Membr. Sci. 114 (1996) 245. [8] S.M. Finnigam, J.A. Howell, The effect of pulsed flow on ultrafiltration fluxes in a baffled tubular membrane systems, Chem. Eng. Res. Des. 67 (1989) 278.
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[9] Z.F. Cui, K.I.T. Wright, Gas±liquid two-phase cross-flow ultrafiltration of BSA and dextran, J. Membr. Sci. 90 (1994) 183. [10] Y. Okahata, H. Noguchi, Ultrasound-responsible permeability control of bilayer-coated capsule membranes, Chem. Lett. (1983) 1517. [11] T.N. Julian, G.M. Zentner, Ultrasonically mediated solute permeation through polymer barriers, J. Pharm. Pharmacol. 38 (1986) 871. [12] H. Li, E. Ohdaria, M. Ide, Effect of ultrasound irradiation on permeability of dialysis membrane, Jpn. J. Appl. Phys. 35(5B) (1996) 3255. [13] M. Band, M. Gutman, V. Faerman, E. Korngold, J. Kost, P.J. Plath, V. Gontar, Influence of specially modulated ultrasound on the water desalination process with ion-exchange hollow fibers, Desalination 109 (1997) 303. [14] I. Lenart, D. Auslander, The effect of ultrasound on diffusion through membranes, Ultrasounics (1980) 216. [15] J. Kost, R. Langer, Ultrasound Enhancement of Membrane Permeability, US Patent No. 4,780,212, 1988. [16] E.S. Tarleton, R.J. Wakeman, Microfiltration Enhancement by Electrical and Ultrasonic force Fields, Filtration and Separation 27 (1990) 192. [17] T. Kobayashi, T. Miyamoto, T. Nagai, N. Fujii, Negatively charged ultrafiltration membranes of polyacrylonitrile having amphiphilic quaternary ammonium counter ions, J. Membr. Sci. 90 (1994) 141. [18] G. Jonsson, Boundary layer phenomena during ultrafiltration of dextran and whey protein solutions, Desalination 51 (1984) 61. [19] J.P. Lorimer, Ultrasound in polymer chemistry, in: T.J. Mason (Ed.), Sonochemistry: the Uses of Ultrasound in Chemistry, Royal Society of Chemistry, England, 1990, p. 118. [20] J.P. Lorimer, T.J. Mason, T.C. Cuthbert, E.A. Brookfiled, Effect of ultrasound on the degradation of aqueous native dextran, Ultrasonics Sonochemistry 2(1) (1995) S55.
Short Communication
Ultrasound effect on cross-¯ow ®ltration of polyacrylonitrile ultra®ltration membranes Xijun Chai, Takaomi Kobayashi*, Nobuyuki Fujii Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka 940-2188, Japan Received 9 October 1997; received in revised form 23 April 1998; accepted 5 June 1998
Abstract Ultrasound effects on permeate ¯ux and rejection of solute were studied by using two types of polyacrylonitrile (PAN) ultra®ltration (UF) membranes reinforced with non-woven cloth. The membrane was set in cross-¯ow UF cell and the ®ltration was carried out by permeating 1 wt% dextran solutions under 30 kPa until steady-state ¯ux was obtained. Then, ultrasound with 45 kHz of frequency was irradiated on the UF cell in water. It was found that the ultrasound irradiation signi®cantly increased the permeate ¯ux of different molecular weight (MW) dextran solutions. Evidence was presented that ultrasound irradiation was effective to enhance the membrane permeate ¯ux for dextran solution. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Ultrasound; Ultra®ltration membrane; Polyacrylonitrile; Dextran; Mass transfer enhancement
1. Introduction The most important disadvantage of membrane ®ltration is the declination of permeate ¯ux due to membrane fouling, resulting from gel layer formation on the membrane surface, adsorption of solute on the membrane pores and blocking of pores by rejected solutes [1±4]. Serious problems in performing ef®cient membrane operation are encountered in both micro®ltration and ultra®ltration (UF). For two decades, this problem has attracted much attention and a number of ways have been available for reducing the extent of fouling. To control fouling and concentration polarization, improvement methods of membrane performance are classi®ed into four categories: pretreat*Corresponding author. Fax: +81-258-47-9300.
ment of feed solution, adjustment of membrane material properties [1], membrane cleaning and improvement by operation condition. Practically, what is most widely used in membrane separation process is the cleaning technique by hydraulic, mechanical, chemical [3,4] and electrical methods [5]. The hydraulic cleaning including back-washing [2] is the most important method for reducing membrane fouling. However, this technique interrupts the continuous ®ltration process and cannot maintain the membrane ®ltration operating at high permeate ¯ux. More recently, electric cleaning technique [5] was developed, which was applied by using pulsed electric ®eld to membrane ®ltration. As a result, the charged particles fouled on membrane were removed away from the membrane. Moreover, other techniques using unsteady jet [6], pulsation ¯owing [7,8] and gas±liquid
0376-7388/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00145-8
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X. Chai et al. / Journal of Membrane Science 148 (1998) 129±135
two-phase ¯ow formation [9] were developed to overcome ¯ux decline of membrane. Ultrasound effect has been studied for the enhancement of permeate ¯ux in the capsule membrane process [10]. It was found that the transport rate of NaCl across nylon capsule membrane coated with amphiphiles was affected by ultrasound irradiation. Ultrasound effect has also been studied for the transport of organic compounds across the polymer membrane. The experiment was carried out in aqueous solution by using 20 kHz sound wave and ultrasound led to increase in the permeability of hydrocortizone and benzoic acid in a cellulose membrane and polydimethylsiloxane ®lm, respectively [11]. Ultrasound effect on membrane ®ltration was found to increase the electrolyte diffusion through dialysis or ionexchange membranes [12±15]. It was reported that the permeability enhancement was resulted from the acoustic pressure, agitation and microcurrent of feed ¯uid by ultrasound irradiation. Effect of ultrasound was further studied on the micro®ltration of anatase and china clay [16]. It was found that ultrasound can promote fouling prevention and facilitate improved separation rates. The rate of fouling was affected by ultrasound strength, suspension concentration and cross-¯ow velocity. The present study aims to understand the effect of ultrasound irradiation on enhancement of membrane permeate ¯ux. Ultrasound effect was studied for cross-¯ow ®ltration process having ¯at sheet polyacrylonitrile UF membrane reinforced with non-woven cloth.
casting solution was immediately immersed in a water bath at 308C and kept overnight. The solidi®ed PAN membrane on the non-woven cloth was washed with a large quantity of water to remove the solvent DMSO. In order to control the membrane permeability and rejection property, casting solutions having 8 and 15 wt% PAN concentrations were used. The corresponding membranes were abbreviated as PAN8 and PAN15 membrane, respectively. The resultant membranes have thickness of 80±90 mm and dimension size of 200150 mm2. Scanning electronic microscope observation indicated that both membranes have asymmetric structure with ®nger-like pore in supporting layer. For characterization of ®ltration properties, 0.1 wt% aqueous solutions of dextran having different MW were applied to determine the molecular size exclusion properties of resultant membranes. The experiments were carried out as reported previously [17]. The concentration of dextran in feed and permeate solutions was analyzed by GPC instrument (Type CCPD UV8000 of Toyo Soda with a 30 cm column of TSK gel G5000 PWX1) equipped with a refractometer (RI8000). The membrane rejection was calculated from the areas of GPC curves for permeate and feed solutions. The rejection data for different MW dextran are shown in Fig. 1. The results for the molecular weight cutoff curves (MWCO) mean that the resultant
2. Experimental 2.1. Membranes For UF experiments, polyacrylonitrile (PAN) membranes reinforced with polyester non-woven cloth (Barren, MDF-60K, 0.07 mm of thickness, 1 mm average pore size) were used. The membranes were prepared as follows: PAN having molecular weight (MW) of about 7104 was dissolved in dimethyl sulfoxide (DMSO) overnight at 508C for preparing casting solution. The solution was spread on the non-woven cloth on which a pair of 100 mm thickness spacers were placed to keep the membrane thickness constant. After casting, the non-woven cloth involving PAN
Fig. 1. MWCO curves for PAN8 (*) and PAN15 (&) membranes.
X. Chai et al. / Journal of Membrane Science 148 (1998) 129±135
131
of membrane ¯ux. For the cross-¯ow ®ltration of dextran, the 1 wt% solution was passed through the membrane surface with a peristaltic pump. The ¯ow rate of feed solution was controlled with 325 ml/min ¯ow-rate. Operating pressure was ®xed at 30 kPa and temperature was 258C. The feed and retentate ¯ow rates were measured by ¯ow meters (Model RK400, Ko¯oc, Japan) and converted to volume ¯ux, m3/(m2/ s). The permeate solution through PAN membrane was collected for GPC measurements of dextran solute. 3. Results and discussion 3.1. Change of membrane permeate flux with time in the absence and presence of ultrasound irradiation
Fig. 2. Schematic diagram of the cross-flow UF experiments for ultrasound effect. PG is pressure gauge. The sonicator emits 45 kHz ultrasound with 248 W power. Feed tank contains 1 wt% dextran solution with 500 ml. The applied pressure was fixed at 30 kPa through the UF experiments.
In order to investigate the ultrasound effect on permeability of PAN UF membranes for 1 wt% solutions of different MWs, two types of PAN8 and PAN15 membranes were used for cross-¯ow ®ltration. Fig. 3 shows the change of permeate ¯ux of dextran solution through PAN8 membrane with and without irradiation of ultrasound. The ultrasound effects on permeate ¯ux
PAN8 membrane has lower rejection than PAN15 membrane. 2.2. Cross-flow ultrafiltration experiments As schematically illustrated in Fig. 2, sonication effect on UF was studied by using experimental setup of cross-¯ow ®ltration unit that was made of stainless steel (Type Minitan S, Millipore). The prepared PAN UF membrane was cut into 80120 mm2 size and was set in the UF cell. Then the ®ltration cell was immersed in water bath of sonicator (Branson ultrasonic cleaner of Type Bransonic B-52 with ultrasonic wave frequency of 45 kHz and input power of 248 W) as the ®ltration was carried out. The size of the water bath of sonicator was 300240160 mm3. The membrane sheet was 50 mm away from the transducers of sonicator and the dense skin layer of the membrane faced the transmission direction of ultrasound wave. During the ®ltration experiments, the temperature increase of water bath (3 l) was checked as less than 18C. Hence, this temperature increase through the ®ltration experiment is negligible in measurement
Fig. 3. Change of permeate flux with filtration time in the absence and the presence of ultrasound irradiation for PAN8 membrane. Water (*); 1 wt% dextran solution of MW 1104 (*); 7104 (~); 5105 (&) and 2 106 (&).
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were examined for water and 1 wt% solution of dextran having MW of 1104, 7104, 5105 and 2106, respectively. At ®rst, all of the ®ltration experiments for each solution were conducted without sonication (as indicated with sonication OFF). The permeate ¯ux observed for water (shown with ``*'' in Fig. 3) is 310ÿ5 m3/(m2/s). After 80 min ®ltration, then, the ultrasound irradiation was carried out between 80 and 110 min for an interval of 30 min (as indicated with sonication ON). However, the permeate ¯ux of water was kept unchanged during the following 30 min ®ltration, although the sonication was applied. This phenomenon suggests that no pore size change may occur in the PAN membrane under ultrasound irradiation. On the other hand, in the cases of dextran solution with 1104 (*) and 7104 (~) MWs, the permeate ¯ux gradually decreases during the ®ltration period between 0 and 80 min. For 5105 (&) and 2106 (&) MW dextran, the values of the permeate ¯ux are much lower than those for the above two kinds of dextran solutions. The results for solutions of 5105 and 2106 MW dextrans are due to the concentration polarization because of high rejection by membrane. The high concentration of solute rejected on the membrane surface results in high osmotic pressure, which in turn decreases membrane permeate ¯ux [18]. As shown in Fig. 1, the results for the MWCO curves indicate that PAN8 membrane completely permeates dextran molecules having 1104 MW, while the membrane rejects 5105 and 2106 MW dextran with 45% and 70% rejection, respectively. After 80 min ®ltration for solution of 1104 and 7104 MW dextrans, the values of the permeate ¯ux increase by the ultrasound irradiation and become similar to that of water, 310ÿ5 m3/ (m2/s). For 1 wt% solutions of 5105 and 2106 MW dextran, the values of permeate ¯ux increase to 610ÿ6 m3/(m2/s) by the 30 min sonication. As the ®ltration was operated again without sonication after the sonication, the values of permeate ¯ux for dextran solutions gradually decreased in the following 30 min ®ltration period. Then, when sonication was irradiated again to the UF cell, the value of the permeate ¯ux for each dextran solution increased in same way as observed in the ®rst circulation. As well as the ultrasound experiments for PAN8 membrane, the permeate ¯ux of water and aqueous solutions of dextrans for PAN15 membrane were also
Fig. 4. Permeate flux without and with ultrasound irradiation for PAN15 membrane. Water (*); 1 wt% dextran solution of MW 1104 (*) and 2106 (&).
measured at various ®ltration times. Fig. 4 shows permeate ¯ux of water and dextran solutions without and with ultrasound operation. For water, the permeate ¯ux of PAN15 membrane is 310ÿ6 m3/(m2/s) and is about one order of magnitude smaller than that of PAN8 membrane. In the case of PAN15 membrane, water permeate ¯ux has no change with ®ltration time even if ultrasound is irradiated to the UF cell. For 1 wt% solution of 1104 MW dextran, little was observed for the decrease of the permeate ¯ux during the 80 min ®ltration. In addition, the value of permeate ¯ux did not increase signi®cantly by the 30 min sonication. However, the permeate ¯ux for solution of 2106 MW dextran signi®cantly decreases during the 80 min ®ltration. This means that the highly rejected 2106 MW dextran results in more serious concentration polarization. Then, by the ultrasound irradiation, the permeate ¯ux increases through the following 30 min sonication. As shown in Table 1, the 2106 MW dextran is almost totally rejected by PAN15 membrane, while the 1104 MW dextran is slightly rejected by PAN15 membrane. We note that, for the 1104 MW dextran, both data of Figs. 3 and 4 have similar tendency when sonicator was turned on or off. Namely, as the dextran rejection by membrane is
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Table 1 Rejection of dextrans by both PAN membranes for cross-flow filtration of 1 wt% dextran solution Time (min)
Sonication
PAN8 membrane 4
0 80 110 190 220
OFF ON OFF ON OFF
PAN15 membrane
110 MW
210 MW
1104 MW
2106 MW
1.4 1.1 0 1 1
59 63 57 62 58
3 3.5 3 4 2
93 94 93 94 93
very low, the ultrasound effect on the permeate ¯ux enhancement is also low. In such cases, probably, concentration polarization is slight. On the other hand, for the 2106 MW dextran solution, which results in serious concentration polarization, the permeate ¯ux decreases signi®cantly during ®ltration process without sonication. When subjected to sonication, the enhancement of the permeate ¯ux for the 2106 MW dextran solution is observed. This may be due to the sonication enhancing bulk mass transfer in concentration polarization layer on the membrane surface. For estimation of ultrasound effect on the rejection properties of the membranes, Table 1 summarizes rejection data of both membranes used for dextran solution at different ®ltration time. It can be seen that, although the permeate ¯ux is recovered by ultrasound irradiation, there are no signi®cant changes of rejection for 1104 and 2106 MW dextrans with and without ultrasound irradiation. This suggests that the PAN membranes show no change in pore sizes by ultrasound irradiation during the ®ltration process. Consequently, we are sure of practical applicability of the ultrasound effect on ¯ux enhancement in cross¯ow ®ltration. Particularly, for the 2106 MW dextran solution, serious concentration polarization occurs, the ultrasound irradiation is effective to enhance membrane permeate ¯ux. 3.2. Continuous ultrasound irradiation to cross-flow filtration To verify the ultrasound effect on enhancement of the permeate ¯ux for both PAN membranes, sonication was operated from the time when the ®ltration was started. The permeate ¯ux for 1 wt% solution of 2106 MW dextran was measured under 30 kPa
6
Fig. 5. Permeate flux observed with and without sonication for cross-flow filtration of 1 wt% dextran of 2106 MW. PAN8 (*, *) and PAN15 (&, &). Open symbols are for data of permeate flux without sonication and closed symbols are for data with sonication. The sonication was operated through the filtration experiments for the 80 min period.
operation pressure. Fig. 5 shows plots of the permeate ¯uxes versus ®ltration time for PAN8 and PAN15 membranes. At the beginning of the operation, PAN8 and PAN15 membranes have the permeate ¯ux of 1.410ÿ6 and 2.510ÿ6 m3/(m2/s), respectively. By operating sonication, the values of the permeate ¯ux immediately increase for both PAN8 and PAN15 membranes. Keeping on operating the sonicator for the following 80 min ®ltration, it is shown that there is no decline of the permeate ¯ux during the sonication operation. However, in the membrane ®ltration
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without sonication (indicated by dashed lines) the permeate ¯ux decreases with operating time and approaches a steady value. In this case, the permeate ¯ux is lower than the original permeate ¯ux. The comparison of experimental data suggests that the ultrasound irradiation process is very effective on enhancement of membrane permeability in the ®ltration. The enhancement of membrane permeate ¯ux is resulted by the increase in bulk mass transfer due to vibration of the membrane caused by the ultrasound. 3.3. Degradation of dextran solute under ultrasound irradiation It is known that macromolecular compounds decompose by ultrasound irradiation [19]. Namely, the MW of macromolecule solute decreases by breakage of the molecular chain in the presence of sonication. Therefore, we examined the degradation of dextran solute after sonication in this work, since the decomposed solute may in¯uence the ®ltration behavior. At ®rst, the degradation experiment was carried out by directly immersing the glass ¯ask containing 500 ml of 1 wt% solution of the 2106 MW dextran in the water bath of sonicator and then the ultrasound was irradiated for 30 min to the dextran solution through the thin glass with 1 mm thickness. The GPC chromatograms of dextran before and after sonication were measured (Fig. 6(a)) to check dextran molecular size. Some change can be seen in the chromatograms before and after sonication. That is, the peak intensity appeared near 10 min retention time increases by the sonication treatment. This means that the dextran MW is decreased by the ultrasound irradiation. On the other hand, the ultrasound was irradiated to cross-¯ow UF cell without membrane. Here, the 1 wt% solution of 2106 MW dextran was circulated for 30 min in the presence of ultrasound and then the dextran solution was sampled from feed tank. The GPC chromatograms of these samples before and after sonication are shown in Fig. 6(b). A little GPC trace change can be seen in the chromatograms. We note that the change of latter case is smaller than the former one. Hence, this suggests that the degradation of dextran solute is very low in the ®ltration experiments. This may be due to shielding of the ultrasound by the UF cell made of stainless steel and plastics boards. Although the exact mechanism of degradation of
Fig. 6. GPC chromatograms for 1 wt% solution of 2106 dextran MW before (solid line) and after (dashed line) ultrasound irradiation. (a) irradiated directly to dextran solution, (b) irradiated through the UF cell operated without membrane.
macromolecules by ultrasound is not clear, PAN membrane it is generally agreed that the hydrodynamic forces are of primary importance. There has been report on ultrasonic degradation of dextran [20]. It was known that the shear forces generated by cavitation collapse are responsible for the breakage of chemical bonds of the polymer. 4. Conclusions Ultrasound effect on permeate ¯ux and dextran rejection by the PAN membranes was studied using various MWs of dextran. We found that the ultrasound irradiation to the cross-¯ow UF cell having PAN membrane was very effective to enhance the ®ltration. Under the experimental conditions, it was found that the sonication treatment increases permeate ¯ux for dextran solute which is highly rejected by the PAN membrane. However, there was no ultrasound effect for water and small effect for dextran with low rejection by the membrane. Evidence was shown that the increase in the permeate ¯ux is attributed to the enhancement of bulk mass transfer in concentration polarization layer near membrane.
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