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The application of ultrasound to dairy ultrafiltration: The influence of operating conditions
Journal of Food Engineering 81 (2007) 364–373 www.elsevier.com/locate/jfoodeng
The application of ultrasound to dairy ultrafiltration: The influence of operating conditions Shobha Muthukumaran a, Sandra E. Kentish a,*, Geoffrey W. Stevens a, Muthupandian Ashokkumar b, Raymond Mawson c a
Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, University of Melbourne, Vic. 3010, Australia b Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, Vic. 3010, Australia c Food Science Australia, Private Bag 16, Werribee, Vic. 3030, Australia Received 7 November 2005; received in revised form 9 November 2006; accepted 9 November 2006 Available online 2 January 2007
Abstract Work previously presented has shown that ultrasound can be effective in enhancing both the production and cleaning cycles of dairy membrane processes. In this present work we extend these previous results to consider the effect of ultrasonic frequency and the use of intermittent ultrasound. These results show that the use of continuous low frequency (50 kHz) ultrasound is most effective in both the fouling and cleaning cycles. The application of intermittent high frequency (1 MHz) ultrasound is less effective. At higher transmembrane pressure, high frequency pulsed sonication can indeed lead to a reduction in steady state membrane flux. The benefits of ultrasound arise from a reduction in both concentration polarization and in the resistance provided by the more labile protein deposits that are removed during a water wash. Conversely, the loss of membrane flux when high frequency pulsed sonication is used arises from a significant increase in the more tenacious ‘irreversible’ fouling deposit. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ultrasound; Whey solutions; Ultrafiltration; Flux; Fouling; Cleaning
1. Introduction One of the critical issues in the development of effective whey ultrafiltration processes is the decline in system performance due to protein fouling, which limits the economic efficiency of the processing operation. Membrane fouling is generally characterized as a reduction of permeate flux through the membrane as a result of increased flow resistance due to pore blocking and cake formation. Several approaches have been proposed to reduce such membrane fouling and to improve the membrane cleaning efficiency. Such methods include intermittent backflushing, flow pulsation and electrical field inducement. Of particular interest to the present work is the use of ultrasound for this purpose. *
Corresponding author. Tel.: +61 3 8344 6682; fax: +61 3 8344 4153. E-mail address: [email protected] (S.E. Kentish).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.11.008
Ultrasound has been studied in both cross-flow systems (Chai, Kobayashi, & Fujii, 1998, 1999; Kobayashi, Chai, & Fujii, 1999; Kobayashi, Kobayashi, Hosaka, & Fujii, 2003; Li, Sanderson, & Jacobs, 2002; Matsumoto, Miwa, Nakao, & Kimura, 1996; Wakeman & Tarleton, 1991) as well as in dead end filtration (Simon, Gondrexon, Taha, Cabon, & Dorange, 2000a; Simon, Penpenic, Gondrexon, Taha, & Dorange, 2000), often in combination with chemical and or water cleaning (Kobayashi et al., 2003). In our earlier studies (Muthukumaran, Kentish, Ashokkumar, & Stevens, 2005; Muthukumaran, Kentish, Lalchandani, et al., 2005; Muthukumaran et al., 2004), we have shown that the use of low frequency (50 kHz) ultrasound at low power densities enhances whey ultrafiltration and the cleaning of whey fouled membranes. Ultrasound has been found to increase the flux by both increasing the mass transfer coefficient within the concentration polarization layer and by providing a less compressible or ‘looser’ fouling cake.
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Nomenclature Jf Jwi Jwr Jwc DP Rtotal Rm Rb Rf f0 R0
permeate flux (l/m2 h) pure water flux (l/m2 h) water flux after fouling (l/m2 h) water flux after cleaning (l/m2 h) transmembrane pressure difference (kPa) total resistance (m1) clean membrane resistance (m1) reversible resistance (m1) irreversible resistance (m1) cake growth factor (m/kg)
Our work to date has considered only the use of continuous ultrasound at a single ultrasonic frequency (50 kHz). In this report, our aim is to consider a broader range of operating parameters. Firstly, we consider the use of intermittent rather than continuous ultrasonic application as this is likely to be less energy intensive. Pulsed ultrasound has previously been shown to be very effective in maintaining high fluxes and high permeability of bovine serum albumin (BSA) during cross-flow ultrafiltration (Yuk & Youm, 2003). Similarly, Matsumoto et al. (1996) found that alternate operation of an ultrasonic generator and the feed pump was effective in removing BSA fouling layers from a membrane during cross-flow microfiltration. Conversely, Simon et al. (2000a) indicated that the use of intermittent ultrasonic application was less effective than continuous application when applied to polymer solutions. In this paper, we also consider the use of ultrasound at a higher frequency (1 MHz). It is well established that at low ultrasonic frequencies, bubbles produced through acoustic cavitation are relatively large and their violent collapse leads to microjetting and localized turbulence. At higher frequencies, smaller bubbles are produced leading to less intensive cavitational collapse events. Conversely, the acoustic energy radiated from a higher frequency ultrasound source is more readily absorbed by the sonicating fluid, leading to greater acoustic streaming flow rates than at lower frequencies for the same power intensity (Suslick, 1988). Both Kobayashi et al. (1999) and Lamminen et al. (2004) found that the ultrasonic enhancement of membrane processes was maximized at low frequency. Kobayashi et al. (1999) found that the permeate flux of dextran solution could be maximized by using concurrent continuous ultrasound at 28 kHz in a cross-flow system. There was no effect on permeate flux at 100 kHz. Lamminen, Walker, and Weavers (2004) noted that the effectiveness of membrane cleaning was maximized at 70 kHz but that ultrasound still had some effect at the maximum frequency of 1062 kHz. Finally, the experimental apparatus used in our prior work was restricted in the range of cross-flow velocities that could be studied. In this earlier work (Muthukumaran, Kentish, Ashokkumar, et al., 2005), the ultrasonic flux
Rpo Rc Eus TMP CFV
initial resistance of the protein deposit (m1) cleaned resistance (m1) ultrasonic enhancement factor applied transmembrane pressure difference (kPa) cross-flow velocity (m/s)
Greek symbols a pore blockage parameter (m2/kg) l solution viscosity (kg/m s)
enhancement was consistent across a range of cross-flow velocities. However, the range of velocities that could be tested was limited by equipment constraints. In this study, we also consider an alternate membrane module design in an attempt to increase the cross-flow velocity and hence the shear stresses over the membrane. The intent here is to consider membrane performance over shear stress ranges that are more consistent with the commercial application of spiral wound polymeric membrane systems. 2. Materials and methods 2.1. Experimental set-up The same experimental set-up as described in our previous reports (Muthukumaran, Kentish, Ashokkumar, et al., 2005; Muthukumaran, Kentish, Lalchandani, et al., 2005; Muthukumaran et al., 2004) has been used for this study. As shown in Fig. 1, a gear pump operating at 300– 1000 ml/min is used to pump a feed solution through a cross-flow ultrafiltration unit. Permeate mass is measured at 1 min intervals by an electronic balance connected to a PC. During the experiments the retentate is recycled to the feed tank.
permeate
retentate
Membrane unit
Transducer array pump Feed tank
balance
Fig. 1. Experimental set-up. The cross-flow membrane module is immersed in a water bath equipped with ultrasonic transducers. Dairy whey solution is pumped through the membrane unit and the retentate recycled to the feed tank, while permeate mass is recorded on an automated balance.
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Fig. 2. Images of the membrane modules: (a) Minitan unit and (b) Vivaflow unit.
For the present investigation two types of membrane modules were used. The first membrane module was a standard Millipore Minitan module as used previously (Fig. 2(a)). In this unit, a polysulfone (PS) or polyethersulfone (PES) membrane (15 cm 8 cm) was placed between two acrylic manifolds of thickness 2.3 cm, which were in turn held in place by metal plates of 1.1 cm thickness. The original membrane holder design was modified by replacing the lower stainless steel plate with a perforated aluminium plate to increase the penetration of ultrasonic waves. Standard silicone separators of approximately 0.5 mm thickness were used on both the feed and permeate side of the membrane, to create nine linear flow channels each of approximately 7 mm width. The second membrane module was a Sartorius Vivaflow 50 modular cross-flow system (Fig. 2(b)). This unit contains an inbuilt polyethersulfone ultrafiltration membrane with 10000 MWCO and an effective area of 50 cm2. The acrylic manifold encasing the membrane in this case is much thinner, of the order of 2 mm, thus allowing for greater penetration of ultrasonic irradiation to the mem-
brane itself. The technical specifications of both units are summarised in Table 1. Further, in the present work, two alternate ultrasonic units were used to generate low and high frequency ultrasonic irradiation respectively. As in our previous work, an ultrasonic bath (Ultrasonics Australia, Model FXP14DH) of internal dimensions 29.5 cm 24 cm 20 cm provided continuous low frequency ultrasound (50 kHz) using 4 disc transducers with a total nominal power of 300 W. However we also employed a megasonic system (SONOSYSÒ Ultraschallsysteme GmbH, Germany) to provide high frequency (1 MHz) ultrasound with a total nominal power of 300 W. This unit was comprised of 4 linear arrays of transducer elements, arranged inside a rectangular plate (16 cm 16 cm 3.5 cm). The plate was immersed in a stainless steel bath of internal dimensions 29.5 cm 24 cm 23.5 cm so that the total depth of water above the transducers matched that in the ultrasonic bath above. This megasonic unit could not be operated continuously but rather in either an intermittent sequential mode, where the four linear transducer arrays
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Table 1 Technical specifications of the Minitan and Vivaflow membrane units Technical specifications
Minitan S unit
Vivaflow 50 unit
Membrane used (MWCO)
Polysulfone 8000 and 30000 Polyethersulfone 10000 Hydrophobic 30 550 0.5 63 0.28 0.0022 290
Polyethersulfone 10000 (inbuilt)
Membrane surface properties Active membrane area (cm2) Feed flowrate (ml/min) Flow channel depth and width (mm) Cross-flow velocity (m/s) Wall shear stress (Pa) Reynolds numbers
Neutral 50 400 0.3 15 1.5 0.02 900
Wall shear stress for laminar flow = 4ul/w where u = cross-flow velocity, l = viscosity and w = channel depth.
in the plate switched sequentially or in an intermittent pulsed mode, where all four transducers operated simultaneously for a given period. In either case the pulse ‘on’ time was 8 s and the ‘off’ time was 2 s. This implies that the power delivery to any particular area of the membrane is of greater intensity but lower pulsing frequency in the sequential mode when all power is delivered through one linear array at a time. Both the membrane units were completely immersed in 5000 ml of water and kept 3 cm above the transducer arrays throughout the entire experimental period. The bath water was replaced as necessary to maintain the temperature in the range, 20–22 °C. Whey solutions were used as the foulant in all experiments. These solutions were reconstituted to 6 wt% total solids from spray dried non-hygroscopic sweet whey powder (Bonlac Foods Ltd. Australia) of approximately 12 wt% total protein. The powder was mixed with deionised water and stirred at 50 °C for at least 30 min or until completely clear. The solution was then cooled to room temperature. The pH of the prepared whey solution was 6.4 and the average molecular weight of the constituents of the whey powder was approximately 24,000 Da. Deionised or distilled water was used throughout for membrane flushing, cleaning and measurement of water fluxes. 2.2. Experimental procedure 2.2.1. Ultrasonic power determination The power transferred from the transducers into the water bath was measured calorimetrically by observing the temperature change with time (Kimura et al., 1996; Ratoarinoro, Wilhelm, Berlan, & Delmas, 1995) using a digital thermometer. Power calibrations were carried out without the membrane unit in place, but with the bath filled to the same vertical height as during experiments. While both ultrasonic units had a nominal power input of 300 W, the average power dissipation into the bath was found to be only 100 W (20 W/l or 0.14 W/cm2) for the low frequency unit. For the megasonic unit in pulsed mode, the average dissipation was 75 W but this equates to 94 W during the ‘on’ times of the pulses. Similarly, the
average dissipation in sequential mode was measured as 60 W equating to 75 W during the ultrasonically active periods. The actual power penetrating to the membrane surface would be less than this again, due to dissipation and reflection from the membrane holder. Kobayashi et al. (2003) indicated that the power intensity reaching the membrane in a Minitan unit similar to the present system would be reduced by a factor of 10 by these effects. However, in the present work, the use of a perforated aluminium lower plate (acoustic impedance of aluminium = 17 106 kg/ m2 s) in place of a solid stainless steel plate (acoustic impedance of steel = 45 106 kg/m2 s) would increase transmission of the ultrasonic waves. Similarly, the Vivaflow unit has a thin plastic casing (acoustic impedance of plastic = 3 106 kg/m2 s) so there should be relatively little reduction in energy transmission to the membrane surface. 2.2.2. Ultrasound assisted cross-flow experiments In all experiments, the permeate water flux (Jwi) was initially measured and this value was used to obtain the clean membrane resistance (Rm) using the well known equation: J¼
DP lR
ð1Þ
where DP is the transmembrane pressure, l is the viscosity of the permeate solution and R is the resistance to solvent permeation. Subsequently, the membrane was fouled for 4 h with a freshly prepared 6% w/w whey solution. In selected experiments, ultrasound of a given frequency and power level was applied during this fouling cycle. In all cases, the majority of the flux reduction occurred in the first 30 min of operation (see Fig. 3 for an example) with flow stabilizing beyond this point. The steady state permeate flux (Jf) was determined by averaging the last 10 recorded values of permeate mass (i.e. last 10 min of flow in a 4 h run). Selected experiments were performed in duplicate and an average experimental error in this steady state whey flux was found to be ±6% or ±1 l/m2 h. The total fouling resistance Rtotal was calculated from this steady state value, again using Eq. (1).
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The model contains three adjustable parameters i.e., the pore blockage factor a which describes the extent of blocked pores, the initial resistance of the cake deposit (Rpo) and the cake growth factor (f0 R0 ) which is the product of the fractional amount of protein present that contributes to deposit growth and the specific protein layer resistance. The best-fit values of these parameters were determined by minimizing the sum of the squared residuals between the experimental filtrate flux data and the model calculations.
2 Permeate Flux (l/m.hr)
50
40 Continuous Ultrasound Intermittent Ultrasound No Ultrasound
30
20
10 0
10
20
30 40 Time (min)
50
60
70
Fig. 3. Effect of intermittent sonication on whey permeation using the Minitan unit and a PS30000 MWCO membrane (TMP = 55 kPa, CFV = 0.28 m/s).
After this fouling cycle, the membrane surface was rinsed with water for 10 min. This rinsing is intended to remove the reversible fouling resistance that results from both concentration polarization and labile surface deposits, leaving the more tenacious deposits. A water flux value (Jwr) was recorded over the last 5 min of this 10 min flush. Results from these experiments were analysed using the well known resistance in series model. In this case, the total resistance Rtotal can be considered as the sum of three terms i.e., the membrane resistance, Rm, the reversible resistance arising from both concentration polarization and loose protein deposits, Rb and the resistance due to the irreversible, tenacious deposits, Rf: Rtotal ¼ Rm þ Rb þ Rf
ð2Þ
As given above, Rm was determined from the initial pure water flux, Rtotal from the steady state flux after fouling, and the sum Rm + Rf from the water rinsed flux. By difference, Rf and Rb could then be individually determined. The effect of ultrasound was also determined via an ultrasonic enhancement factor (Muthukumaran, Kentish, Lalchandani, et al., 2005), Eus ¼
J f; ultrasound J f;no ultrasound
ð3Þ
where Jf, ultrasound and Jf, no ultrasound are the average steady state permeate fluxes achieved with and without ultrasound. Finally, we also utilized in our analysis the model developed by Ho and Zydney (2000) that describes the flux decay as a function of time when both pore blockage and cake filtration mechanisms are active. This model requires the simultaneous solution of two equations: J aDPC b Rm aDPC b ¼ exp t þ 1 exp t ð4Þ J0 lRm Rm þ Rp lRm sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 2f 0 R DPC b t Rp ¼ ðRm þ Rpo Þ 1 þ ð5Þ Rm 2 lðRm þ Rpo Þ
2.2.3. Ultrasound assisted cleaning experiments Cleaning experiments followed a protocol similar to the one discussed above. After recording the clean water flux, the membrane was fouled for 30 min with freshly prepared 6% w/w whey solution. The membrane was then rinsed with water for 10 min to remove the reversible fouling load and the water flux again recorded as above Jwr. This was used to calculate Rf, the resistance of the irreversible fouling deposit (Eq. (1)). The fouled membrane was next cleaned for a further 10 min under different levels of both ultrasonic power and frequency. After this cleaning cycle, the membrane surface was rinsed with water again for 10 min to remove cleaning solutions and provide a consistent regime for flux calculation. The cleaned membrane flux (Jwc) was determined over the final 5 min of this water rinse and the cleaned resistance Rc calculated. Results from these cleaning experiments are presented here as a cleaning efficiency (CE) defined by Matzinos and Alvarez (2002): CE ¼
Rf R c 100 Rf Rm
ð6Þ
In both the fouling and cleaning experiments, the membrane was finally cleaned by circulating 0.1 M sodium hydroxide and 15 mM of sodium dodecyl sulfate for 5 min. The membrane was then left to soak in this solution. Milli-Q water/distilled water was fed into the unit to flush out the cleaning solution. A further permeate water flux was recorded and compared to the initial water flux. If necessary, further membrane cleaning was conducted until the original clean membrane flux was restored. The experimental variability in the final cleaned membrane flux measurement was ±9 l/m2 h or ±5% of the total value. 3. Results and discussion 3.1. Influence of intermittent ultrasound on the permeate flux As discussed above, the high frequency ultrasonic unit could only be operated in an intermittent format. Hence, to determine the effects of intermittent operation, the low frequency ultrasonic bath was first utilized in an intermittent mode for 1 h (5 min on/5 min off) and results compared to continuous operation. As shown in Fig. 3, while such intermittent ultrasound appeared effective at short operation times, there was little flux improvement over
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longer time frames. This is consistent with the results of Simon et al. (2000a). The ultrasonic enhancement factor for intermittent ultrasound after 1 h was calculated as 1.1 compared to a factor of 1.4 for continuous ultrasound. 3.2. Frequency effects Fig. 4 compares the long term permeate flux achieved when using the Minitan membrane unit and a 30000 MWCO membrane. In the absence of ultrasound, the permeate flux declines as the transmembrane pressure increases. This trend is somewhat unusual and can be related to both the low cross-flow velocities in the Minitan unit, and the deformable nature of colloidal solids such as whey proteins. At higher transmembrane pressures, the compressive forces exerted on the cake layer favor a thicker and more densely packed cake layer which provides more resistance to permeate flow. With the addition of low frequency ultrasound, the flux increases across the full range of transmembrane pressures. The ultrasonic enhancement factor of around 1.5 (see Table 2) is consistent with our previous results across a range of transmembrane pressures. The high frequency pulsed unit provides a flux enhancement of 1.2 at low transmembrane pressures. While lower than the enhancement provided by the continuous low frequency ultrasound, this result is consistent with the results for intermittent application described above. Of greater concern is the reduction in flux observed under these con-
2 Permeate Flux (l/m.hr)
20
50 kHz continuous 1 MHz Pulsed No Ultrasound
15
10
5
0 0
50
100
150
200
250
300
350
Transmembrane Pressure (kPa)
Fig. 4. Effect of ultrasonic frequency on whey permeation using the Minitan unit and a PS30000 MWCO membrane (CFV = 0.28 m/s).
369
ditions at higher transmembrane pressures. Under these conditions, the long term flux falls below the level provided in the absence of ultrasound. Similar results are obtained when the membrane pore size is reduced from 30000 to 8000 MWCO (Table 2). These results are consistent with the work of Kobayashi et al. (1999) who found that while operating at 30 kPa transmembrane pressure, 20 kHz ultrasound was effective, but operation at 100 kHz resulted in no ultrasonic flux enhancement. Similarly Duriyabunleng and Petmunee (2001) find that there is an optimum transmembrane pressure beyond which ultrasound is less effective. The distinction in the present case is that the ultrasound appears to be positively detrimental when both high transmembrane pressures and ultrasonic frequencies are applied in combination. In order to elucidate the cause of this loss in flux, the behaviour of the individual resistances to flow were considered. As expected, the hydraulic resistance of the membrane Rm is insensitive to sonication. At all pressures, the ‘reversible’ fouling resistance, Rb is lower when either 50 kHz or 1 MHz ultrasound is applied (Fig. 5(a)), relative to the no-ultrasound case. This is consistent with the results of our previous work (Muthukumaran, Kentish, Ashokkumar, et al., 2005), which indicate that ultrasound increases the mass transfer coefficient within the polarization boundary layer. The irreversible fouling resistance Rf (Fig. 5(b)), provided by the no-ultrasound case and the 50 kHz ultrasound are comparable, indicating that ultrasound is relatively ineffective in reducing the extent of more tightly bound deposits within the fouling cake and any pore blockage. Again, this is consistent with our previous work which showed that pore blockage was little affected by low frequency ultrasonic application. Of greater note is the dramatic increase in irreversible fouling that occurs with the combination of high frequency ultrasonic pulsed application and high transmembrane pressures. The fouling load in this case is clearly greater than in the absence of ultrasound. This increase in fouling could result either from compaction of the protein deposits into a more densely packed cake layer or from such deposits being forced into the membrane pores under the influence of this ultrasonic field. To gain further insight into these phenomena, the above results were also analysed using the theoretical model
Table 2 Whey permeate flux and ultrasonic enhancement factor at variable frequencies using the Minitan unit (CFV = 0.28 m/s) Average permeate flux (l/m2 h) Membrane pore size (MWCO)
30000
Transmembrane pressure (kPa)
55
150
Mode of operation 50 kHz continuous 1 MHz pulsed No ultrasound
15 12 11
13 9.5 8.7
Ultrasonic enhancement factor 8000
30000
8000
300
300
55
150
300
300
12 4.3 7.2
7.6 3.8 4.8
1.4 1.2
1.5 1.1
1.7 0.6
1.6 0.8
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developed by Ho and Zydney (2000). The best-fit values of a, Rpo and f0 R0 across a range of pressures are shown in Fig. 6 for the 30000 MWCO membrane. Results for the no-ultrasound and 50 kHz ultrasound are comparable to our prior results. The use of ultrasound reduces the pore blockage and initial deposit resistance a little, but has most effect on the cake growth factor, reducing this value by an order of magnitude. This implies that the fouling cake is either thinner (lower f0 ) and/or less compressed (lower R0 ) and thus offers less resistance. There is no evidence of the high frequency ultrasound providing increased pore blockage, as the pore blockage parameter (a) is within the range of data for both low frequency and no ultrasound, and if anything decreases as transmembrane pressure increases. The initial deposit resistance matches that of the no-ultrasound case. However, the cake growth factor, while matching that for no ultrasound at low transmembrane pressures, is significantly larger at higher TMP values. This indicates that either the fraction of the total protein contributing to cake formation (f0 ) is increasing or that this cake layer is offering more resistance (R0 ) at high TMP. Sonication has been reported to cause both particle agglomeration (Thompson & Doraiswamy, 1990; Vimini, Kemp, & Fox, 1983) and possibly whey protein denaturation (Villamiel & de Jong, 2000) and either of these effects could contribute to a thicker and/or more compacted fouling cake.
14 No Ultrasound 30000MWCO 1 MHz Pulsed 30000MWCO 50 kHz Continuous 30000MWCO No Ultrasound 8000 MWCO 1 MHz Pulsed 8000MWCO 50 kHz Continuous 8000MWCO
8 6 4 2 0 0
5 50
1 100
150
20 200
250
300
350
T Transmembrane Pressure (kPa)
-1
Irreversible Resistance (Rf) x10 (m )
25 13
No Ultrasound 30000MWCO 1 MHz Pulsed 30000MWCO 50 kHz Continuous 30000MWCO No Ultrasound 8000 MWCO 1 MHz Plused 8000MWCO 50 kHz Continuous 8000MWCO
20
15
10
5
0 0
50
100
150
200
250
300
350
Transmembrane Pressure (kPa)
3.3. Results with a high cross-flow velocity membrane module Experiments were also carried out using the Viva flow module. This unit was operated at a cross-flow rate around
Initial Deposit Resistance
7 6 5 (m 2 /kg)
Pore Blockage Parameter
Fig. 5. Permeation resistance values as a function of transmembrane pressure in the Minitan unit with a PS30000 MWCO membrane (CFV = 0.28 m/s): (a) reversible resistance and (b) irreversible resistance.
4 3 2 1 0 0
100
200
300
-1
10
13
12
x10 ( m )
13
-1
Reversible Resistance (Rb) x10 (m )
370
9 8 7 6 5 4 3 2 1 0 0
400
Transmembrane Pressure (kPa)
100
200
300
400
Transmembrane Pressure (kPa)
Cake Growth Factor 13 x10 (m/kg)
100 10
50 kHz Continous 1 MHz Pulsed
1
No Ultrasound 0.1 0.01 0
100
200
300
400
Transmembrane Pressure (kPa)
Fig. 6. Best-fit values of the fouling parameters as a function of the transmembrane pressure using the Minitan unit and a PS30000 MWCO membrane (CFV = 0.28 m/s).
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3.4. Effect of ultrasonic frequency on the cleaning efficiency Experiments were also carried out to determine the effect of ultrasound on cleaning efficiency. It should be noted that these cleaning experiments were conducted on a fouled membrane that had already been subjected to a 10 min water flush, so the results are indicative of the ability of ultrasound to remove the irreversible fouling layer. Fig. 8 shows the effect of sonication time on the membrane permeate flux for a fixed fouling time of 30 min using the Vivaflow unit. It can be seen from this figure, that the cleaning efficiency is relatively low for all forms of ultrasound, consistent with the discussion above that suggests ultrasound is less effective on this irreversible fouling. Cleaning is most effective when the low frequency continuous ultrasound is used relative to the 1 MHz pulsed source. Both modes of operation provide higher cleaning efficiency 35
60
Cleaning Efficiency (%)
13
membrane pressure utilized in this experiments (200 kPa) is lower than the highest value used in the Minitan unit (300 kPa). The ‘reversible’ fouling resistance (Rb) is much greater than in the Minitan module, but the ultrasonic flux enhancement again occurs predominantly through a reduction in this ‘reversible’ resistance. These effects may again be attributed to an increase in the mass transfer coefficient within the flow boundary layer. Conversely, the irreversible fouling resistance (Rf) is unaffected by sonication at any frequency.
Vivaflow Module Minitan Module
-1
Total Resistance (Rtot) x10 (m )
five times that used in the Minitan unit (1.5 versus 0.28 m/ s) resulting in wall stresses a factor of 10 greater in magnitude. However, experimental results with this unit displayed a significant reduction in the steady state permeate flux after 4 h of operation. The surface potential of the PES membrane used in the Vivaflow unit is described as neutral whereas hydrophobic membranes were employed in the Minitan unit. This difference in membrane surface potential results in a significantly different pattern in the total flow resistance over time. As shown in Fig. 7, while the resistance is initially lower for the Vivaflow unit, this resistance increases almost linearly with time, whereas the resistance in the Minitan unit levels off rapidly. Table 3 shows the steady state whey permeate flux and the permeation resistance values in the Vivaflow unit under variable sonication modes after 4 h of permeation. The ultrasonic enhancement factor with the low frequency unit is comparable to that in the Minitan unit. This indicates that even with the increased wall shear stresses present in this unit, ultrasound is still effective. This is consistent with our prior work (Muthukumaran, Kentish, Ashokkumar, et al., 2005) which showed that the addition of membrane spacers also did not dramatically effect the ultrasonic enhancement. However, contrary to the Minitan module, ultrasound remains effective even with the use of high frequency ultrasound. The greater effectiveness of the high frequency unit in this case can be related to both the higher cross-flow velocities that will assist in preventing the deposition of a thicker fouling cake and the fact that the trans-
371
50 40 30 20 10
30 25 20 15 50 kHz Continuous 1 MHz Sequential 1 MHz Pulsed No Ultrasound
10 5
0 0
0.5
1
1.5
2
2.5
3
3.5
4
0 0
5
10
15
20
25
30
35
Water Flush Time (min)
Time (Hours)
Fig. 7. Total resistance as a function of time (Minitan unit, PES10000 MWCO membrane, TMP = 300 kPa, CFV = 0.28 m/s: Viva flow unit, PES10000 MWCO membrane, TMP = 200 kPa, CFV = 1.5 m/s).
Fig. 8. Effect of ultrasonic frequency on the cleaning efficiency using the Vivaflow unit for 30 min fouling at 200 kPa, 1.5 m/s and cleaning at 60 kPa, 1.5 m/s.
Table 3 Whey permeate flux and permeation resistances at variable frequencies using the Vivaflow unit (TMP = 200 kPa, CFV = 1.5 m/s) Mode of operation
Average permeate flux (l/m2 h)
Ultrasonic enhancement factor
Resistance 1013 (m1) Rm
Rf
Rb
50 kHz continuous 1 MHz pulsed 1 MHz sequential No ultrasound
1.6 1.6 1.6 1.2
1.4 1.4 1.4
0.40 0.44 0.51 0.41
12 11 12 11
32 32 33 51
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Table 4 Effect of ultrasonic frequency on cleaning efficiency using the Minitan unit and a PS30000 MWCO membrane (30 min fouling at 0.28 m/s, 55 kPa and 10 min cleaning at 0.5 m/s, 55 kPa) Mode
(CE) (%)
50 kHz continuous 1 MHz pulsed 1 MHz sequential No ultrasound
57 ± 3 50 ± 3 48 ± 3 48 ± 3
been provided through a University of Melbourne Research and Development Grants Scheme award and a University of Melbourne-CSIRO Collaborative Research Support Scheme award. Infrastucture and equipment is also provided through the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council. This support is gratefully acknowledged. References
than when no ultrasound is applied (i.e. when a 10 min water flush is used in place of a sonication sequence). As with our prior work, there is little gain in sonicating for longer than 10 min. Similar experiments were carried out using the Minitan unit. Results again show that the maximum cleaning efficiency occurs with the continuous low frequency ultrasound (Table 4). As in the fouling experiments, the high frequency pulsed ultrasound in this unit is less effective. These results are consistent with the work of Lamminen et al. (2004) who also show that cleaning is optimized at low frequencies. In this published work, the ultrasound is used to remove both the reversible and irreversible fouling layer, so the overall cleaning effectiveness is higher than in the present work. 4. Conclusion These experiments have shown that while the use of ultrasound in membrane ultrafiltration is generally positive, there are conditions under which it can be less effective or even have a negative effect on filtration performance. We find that the use of intermittent ultrasound does little to enhance flux rates at any frequency. Further, the use of intermittent (pulsed) ultrasound at high frequency can cause a net reduction in flux rates when high transmembrane pressures and low cross-flow velocities are employed. Pulsed high frequency ultrasound is also less effective in cleaning membrane surfaces after the fouling cycle. These results are consistent with other workers who also find that high frequency ultrasound is less effective. Our results show that continuous low frequency sonication generally reduces the components of the total flow resistance that are readily reversed during water flush. This includes both the mass transfer resistance arising from concentration polarization and from the more labile protein deposits that are readily removed. This is consistent with our previous results. Conversely, the high frequency pulsed ultrasound can cause a significant increase in the ‘irreversible’ flow resistance. This increase in resistance is associated with a thicker and/or more compacted cake layer, rather than the blockage of pores. Acknowledgements S. Muthukumaran is the recipient of an Australian Postgraduate Award. Financial support for this project has
Chai, X., Kobayashi, T., & Fujii, N. (1998). Ultrasound effect on crossflow filtration of polyacrylonitrile ultrafiltration membranes. Journal of Membrane Science, 148(1), 129–135. Chai, X., Kobayashi, T., & Fujii, N. (1999). Ultrasound-associated cleaning of polymeric membranes for water treatment. Separation and Purification Technology, 15(2), 139–146. Duriyabunleng, H., & Petmunee, J. Mu (2001). Effects of the ultrasonic waves on microfiltration in plate and frame module. Journal of Chemical Engineering of Japan, 34(8), 985–989. Ho, C., & Zydney, A. L. (2000). A combined pore blockage and cake filtration model for protein fouling during microfiltration. Journal of Colloid and Interface Science, 232, 389–399. Kimura, T., Sakamoto, T., Leveque, J.-M., Sohmiya, H., Fujita, M., Ikeda, S., et al. (1996). Standardization of ultrasonic power for sonochemical reaction. Ultrasonics Sonochemistry, 3(3), S157–S161. Kobayashi, T., Chai, X., & Fujii, N. (1999). Ultrasound enhanced crossflow membrane filtration. Separation and Purification Technology, 17(1), 31–40. Kobayashi, T., Kobayashi, T., Hosaka, Y., & Fujii, N. (2003). Ultrasound-enhanced membrane-cleaning processes applied water treatments: influence of sonic frequency on filtration treatments. Ultrasonics, 41(3), 185–190. Lamminen, M. O., Walker, H. W., & Weavers, L. K. (2004). Mechanisms and factors influencing the ultrasonic cleaning of particle-fouled ceramic membranes. Journal of Membrane Science, 237(1–2), 213–223. Li, J., Sanderson, R. D., & Jacobs, E. P. (2002). Ultrasonic cleaning of nylon microfiltration membranes fouled by Kraft paper mill effluent. Journal of Membrane Science, 205(1–2), 247–257. Matsumoto, Y., Miwa, T., Nakao, S., & Kimura, S. (1996). Improvement of membrane permeation performance by ultrasonic microfiltration. Journal of Chemical Engineering of Japan, 29(4), 561–567. Matzinos, P., & Alvarez, R. (2002). Effect of ionic strength on rinsing and alkaline cleaning of ultrafiltration inorganic membranes fouled with whey proteins. Journal of Membrane Science, 208(1–2), 23–30. Muthukumaran, S., Kentish, S., Ashokkumar, M., & Stevens, G. W. (2005). Mechanisms for the ultrasonic enhancement of dairy whey ultrafiltration. Journal of Membrane Science, 258(1–2), 106–114. Muthukumaran, S., Kentish, S., Lalchandani, S., Ashokkumar, M., Mawson, R., Stevens, G. W., et al. (2005). The optimisation of ultrasonic cleaning procedures for dairy fouled ultrafiltration membranes. Ultrasonic Sonochemistry, 12(1–2), 29–35. Muthukumaran, S., Yang, K., Seuren, A., Kentish, S., Ashokkumar, M., Stevens, G. W., et al. (2004). The use of ultrasonic cleaning for ultrafiltration membranes in the dairy industry. Separation and Purification Technology, 39(1–2), 99–107. Ratoarinoro, C. F., Wilhelm, A. M., Berlan, J., & Delmas, H. (1995). Power measurement in sonochemistry. Ultrasonics Sonochemistry, 2(1), S43–S47. Simon, A., Gondrexon, N., Taha, S., Cabon, J., & Dorange, G. (2000a). Low frequency ultrasound to improve dead-end ultrafiltration performance. Separation Science and Technology, 35(16), 2619–2637. Simon, A., Penpenic, L., Gondrexon, N., Taha, S., & Dorange, G. (2000). A comparative study between classical stirred and ultrasonicallyassisted dead-end ultrafiltration. Ultrasonic Sonochemistry, 7(4), 183–186.
S. Muthukumaran et al. / Journal of Food Engineering 81 (2007) 364–373 Suslick, K. S. (1988). Ultrasound: Its chemical physical and biological effects. New York: VCH Publishers. Thompson, L. H., & Doraiswamy, L. K. (1990). Sonochemistry: science and engineering. Industrial & Engineering Chemistry Research, 38, 1215–1249. Villamiel, M., & de Jong, P. (2000). Influence of high-intensity ultrasound and heat treatment in continuous flow on fat, proteins and native enzymes of milk. Journal of Agricultural and Food Chemistry, 48(2), 472–478.
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Vimini, R. J., Kemp, J. D., & Fox, J. D. (1983). Effects of low-frequency ultrasound on properties of restructured beef rolls. Journal of Food Science, 48, 1572. Wakeman, R. J., & Tarleton, E. S. (1991). An experimental study of electroacoustic crossflow microfiltration. Chemical Engineering Research and Design, 69(A5), 386–397. Yuk, Y. J., & Youm, K. H. (2003). Enhancement of ultrafiltration performance using ultrasound. Memburein, 13(4), 283–290.
The application of ultrasound to dairy ultrafiltration: The influence of operating conditions Shobha Muthukumaran a, Sandra E. Kentish a,*, Geoffrey W. Stevens a, Muthupandian Ashokkumar b, Raymond Mawson c a
Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, University of Melbourne, Vic. 3010, Australia b Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, Vic. 3010, Australia c Food Science Australia, Private Bag 16, Werribee, Vic. 3030, Australia Received 7 November 2005; received in revised form 9 November 2006; accepted 9 November 2006 Available online 2 January 2007
Abstract Work previously presented has shown that ultrasound can be effective in enhancing both the production and cleaning cycles of dairy membrane processes. In this present work we extend these previous results to consider the effect of ultrasonic frequency and the use of intermittent ultrasound. These results show that the use of continuous low frequency (50 kHz) ultrasound is most effective in both the fouling and cleaning cycles. The application of intermittent high frequency (1 MHz) ultrasound is less effective. At higher transmembrane pressure, high frequency pulsed sonication can indeed lead to a reduction in steady state membrane flux. The benefits of ultrasound arise from a reduction in both concentration polarization and in the resistance provided by the more labile protein deposits that are removed during a water wash. Conversely, the loss of membrane flux when high frequency pulsed sonication is used arises from a significant increase in the more tenacious ‘irreversible’ fouling deposit. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ultrasound; Whey solutions; Ultrafiltration; Flux; Fouling; Cleaning
1. Introduction One of the critical issues in the development of effective whey ultrafiltration processes is the decline in system performance due to protein fouling, which limits the economic efficiency of the processing operation. Membrane fouling is generally characterized as a reduction of permeate flux through the membrane as a result of increased flow resistance due to pore blocking and cake formation. Several approaches have been proposed to reduce such membrane fouling and to improve the membrane cleaning efficiency. Such methods include intermittent backflushing, flow pulsation and electrical field inducement. Of particular interest to the present work is the use of ultrasound for this purpose. *
Corresponding author. Tel.: +61 3 8344 6682; fax: +61 3 8344 4153. E-mail address: [email protected] (S.E. Kentish).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.11.008
Ultrasound has been studied in both cross-flow systems (Chai, Kobayashi, & Fujii, 1998, 1999; Kobayashi, Chai, & Fujii, 1999; Kobayashi, Kobayashi, Hosaka, & Fujii, 2003; Li, Sanderson, & Jacobs, 2002; Matsumoto, Miwa, Nakao, & Kimura, 1996; Wakeman & Tarleton, 1991) as well as in dead end filtration (Simon, Gondrexon, Taha, Cabon, & Dorange, 2000a; Simon, Penpenic, Gondrexon, Taha, & Dorange, 2000), often in combination with chemical and or water cleaning (Kobayashi et al., 2003). In our earlier studies (Muthukumaran, Kentish, Ashokkumar, & Stevens, 2005; Muthukumaran, Kentish, Lalchandani, et al., 2005; Muthukumaran et al., 2004), we have shown that the use of low frequency (50 kHz) ultrasound at low power densities enhances whey ultrafiltration and the cleaning of whey fouled membranes. Ultrasound has been found to increase the flux by both increasing the mass transfer coefficient within the concentration polarization layer and by providing a less compressible or ‘looser’ fouling cake.
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Nomenclature Jf Jwi Jwr Jwc DP Rtotal Rm Rb Rf f0 R0
permeate flux (l/m2 h) pure water flux (l/m2 h) water flux after fouling (l/m2 h) water flux after cleaning (l/m2 h) transmembrane pressure difference (kPa) total resistance (m1) clean membrane resistance (m1) reversible resistance (m1) irreversible resistance (m1) cake growth factor (m/kg)
Our work to date has considered only the use of continuous ultrasound at a single ultrasonic frequency (50 kHz). In this report, our aim is to consider a broader range of operating parameters. Firstly, we consider the use of intermittent rather than continuous ultrasonic application as this is likely to be less energy intensive. Pulsed ultrasound has previously been shown to be very effective in maintaining high fluxes and high permeability of bovine serum albumin (BSA) during cross-flow ultrafiltration (Yuk & Youm, 2003). Similarly, Matsumoto et al. (1996) found that alternate operation of an ultrasonic generator and the feed pump was effective in removing BSA fouling layers from a membrane during cross-flow microfiltration. Conversely, Simon et al. (2000a) indicated that the use of intermittent ultrasonic application was less effective than continuous application when applied to polymer solutions. In this paper, we also consider the use of ultrasound at a higher frequency (1 MHz). It is well established that at low ultrasonic frequencies, bubbles produced through acoustic cavitation are relatively large and their violent collapse leads to microjetting and localized turbulence. At higher frequencies, smaller bubbles are produced leading to less intensive cavitational collapse events. Conversely, the acoustic energy radiated from a higher frequency ultrasound source is more readily absorbed by the sonicating fluid, leading to greater acoustic streaming flow rates than at lower frequencies for the same power intensity (Suslick, 1988). Both Kobayashi et al. (1999) and Lamminen et al. (2004) found that the ultrasonic enhancement of membrane processes was maximized at low frequency. Kobayashi et al. (1999) found that the permeate flux of dextran solution could be maximized by using concurrent continuous ultrasound at 28 kHz in a cross-flow system. There was no effect on permeate flux at 100 kHz. Lamminen, Walker, and Weavers (2004) noted that the effectiveness of membrane cleaning was maximized at 70 kHz but that ultrasound still had some effect at the maximum frequency of 1062 kHz. Finally, the experimental apparatus used in our prior work was restricted in the range of cross-flow velocities that could be studied. In this earlier work (Muthukumaran, Kentish, Ashokkumar, et al., 2005), the ultrasonic flux
Rpo Rc Eus TMP CFV
initial resistance of the protein deposit (m1) cleaned resistance (m1) ultrasonic enhancement factor applied transmembrane pressure difference (kPa) cross-flow velocity (m/s)
Greek symbols a pore blockage parameter (m2/kg) l solution viscosity (kg/m s)
enhancement was consistent across a range of cross-flow velocities. However, the range of velocities that could be tested was limited by equipment constraints. In this study, we also consider an alternate membrane module design in an attempt to increase the cross-flow velocity and hence the shear stresses over the membrane. The intent here is to consider membrane performance over shear stress ranges that are more consistent with the commercial application of spiral wound polymeric membrane systems. 2. Materials and methods 2.1. Experimental set-up The same experimental set-up as described in our previous reports (Muthukumaran, Kentish, Ashokkumar, et al., 2005; Muthukumaran, Kentish, Lalchandani, et al., 2005; Muthukumaran et al., 2004) has been used for this study. As shown in Fig. 1, a gear pump operating at 300– 1000 ml/min is used to pump a feed solution through a cross-flow ultrafiltration unit. Permeate mass is measured at 1 min intervals by an electronic balance connected to a PC. During the experiments the retentate is recycled to the feed tank.
permeate
retentate
Membrane unit
Transducer array pump Feed tank
balance
Fig. 1. Experimental set-up. The cross-flow membrane module is immersed in a water bath equipped with ultrasonic transducers. Dairy whey solution is pumped through the membrane unit and the retentate recycled to the feed tank, while permeate mass is recorded on an automated balance.
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Fig. 2. Images of the membrane modules: (a) Minitan unit and (b) Vivaflow unit.
For the present investigation two types of membrane modules were used. The first membrane module was a standard Millipore Minitan module as used previously (Fig. 2(a)). In this unit, a polysulfone (PS) or polyethersulfone (PES) membrane (15 cm 8 cm) was placed between two acrylic manifolds of thickness 2.3 cm, which were in turn held in place by metal plates of 1.1 cm thickness. The original membrane holder design was modified by replacing the lower stainless steel plate with a perforated aluminium plate to increase the penetration of ultrasonic waves. Standard silicone separators of approximately 0.5 mm thickness were used on both the feed and permeate side of the membrane, to create nine linear flow channels each of approximately 7 mm width. The second membrane module was a Sartorius Vivaflow 50 modular cross-flow system (Fig. 2(b)). This unit contains an inbuilt polyethersulfone ultrafiltration membrane with 10000 MWCO and an effective area of 50 cm2. The acrylic manifold encasing the membrane in this case is much thinner, of the order of 2 mm, thus allowing for greater penetration of ultrasonic irradiation to the mem-
brane itself. The technical specifications of both units are summarised in Table 1. Further, in the present work, two alternate ultrasonic units were used to generate low and high frequency ultrasonic irradiation respectively. As in our previous work, an ultrasonic bath (Ultrasonics Australia, Model FXP14DH) of internal dimensions 29.5 cm 24 cm 20 cm provided continuous low frequency ultrasound (50 kHz) using 4 disc transducers with a total nominal power of 300 W. However we also employed a megasonic system (SONOSYSÒ Ultraschallsysteme GmbH, Germany) to provide high frequency (1 MHz) ultrasound with a total nominal power of 300 W. This unit was comprised of 4 linear arrays of transducer elements, arranged inside a rectangular plate (16 cm 16 cm 3.5 cm). The plate was immersed in a stainless steel bath of internal dimensions 29.5 cm 24 cm 23.5 cm so that the total depth of water above the transducers matched that in the ultrasonic bath above. This megasonic unit could not be operated continuously but rather in either an intermittent sequential mode, where the four linear transducer arrays
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Table 1 Technical specifications of the Minitan and Vivaflow membrane units Technical specifications
Minitan S unit
Vivaflow 50 unit
Membrane used (MWCO)
Polysulfone 8000 and 30000 Polyethersulfone 10000 Hydrophobic 30 550 0.5 63 0.28 0.0022 290
Polyethersulfone 10000 (inbuilt)
Membrane surface properties Active membrane area (cm2) Feed flowrate (ml/min) Flow channel depth and width (mm) Cross-flow velocity (m/s) Wall shear stress (Pa) Reynolds numbers
Neutral 50 400 0.3 15 1.5 0.02 900
Wall shear stress for laminar flow = 4ul/w where u = cross-flow velocity, l = viscosity and w = channel depth.
in the plate switched sequentially or in an intermittent pulsed mode, where all four transducers operated simultaneously for a given period. In either case the pulse ‘on’ time was 8 s and the ‘off’ time was 2 s. This implies that the power delivery to any particular area of the membrane is of greater intensity but lower pulsing frequency in the sequential mode when all power is delivered through one linear array at a time. Both the membrane units were completely immersed in 5000 ml of water and kept 3 cm above the transducer arrays throughout the entire experimental period. The bath water was replaced as necessary to maintain the temperature in the range, 20–22 °C. Whey solutions were used as the foulant in all experiments. These solutions were reconstituted to 6 wt% total solids from spray dried non-hygroscopic sweet whey powder (Bonlac Foods Ltd. Australia) of approximately 12 wt% total protein. The powder was mixed with deionised water and stirred at 50 °C for at least 30 min or until completely clear. The solution was then cooled to room temperature. The pH of the prepared whey solution was 6.4 and the average molecular weight of the constituents of the whey powder was approximately 24,000 Da. Deionised or distilled water was used throughout for membrane flushing, cleaning and measurement of water fluxes. 2.2. Experimental procedure 2.2.1. Ultrasonic power determination The power transferred from the transducers into the water bath was measured calorimetrically by observing the temperature change with time (Kimura et al., 1996; Ratoarinoro, Wilhelm, Berlan, & Delmas, 1995) using a digital thermometer. Power calibrations were carried out without the membrane unit in place, but with the bath filled to the same vertical height as during experiments. While both ultrasonic units had a nominal power input of 300 W, the average power dissipation into the bath was found to be only 100 W (20 W/l or 0.14 W/cm2) for the low frequency unit. For the megasonic unit in pulsed mode, the average dissipation was 75 W but this equates to 94 W during the ‘on’ times of the pulses. Similarly, the
average dissipation in sequential mode was measured as 60 W equating to 75 W during the ultrasonically active periods. The actual power penetrating to the membrane surface would be less than this again, due to dissipation and reflection from the membrane holder. Kobayashi et al. (2003) indicated that the power intensity reaching the membrane in a Minitan unit similar to the present system would be reduced by a factor of 10 by these effects. However, in the present work, the use of a perforated aluminium lower plate (acoustic impedance of aluminium = 17 106 kg/ m2 s) in place of a solid stainless steel plate (acoustic impedance of steel = 45 106 kg/m2 s) would increase transmission of the ultrasonic waves. Similarly, the Vivaflow unit has a thin plastic casing (acoustic impedance of plastic = 3 106 kg/m2 s) so there should be relatively little reduction in energy transmission to the membrane surface. 2.2.2. Ultrasound assisted cross-flow experiments In all experiments, the permeate water flux (Jwi) was initially measured and this value was used to obtain the clean membrane resistance (Rm) using the well known equation: J¼
DP lR
ð1Þ
where DP is the transmembrane pressure, l is the viscosity of the permeate solution and R is the resistance to solvent permeation. Subsequently, the membrane was fouled for 4 h with a freshly prepared 6% w/w whey solution. In selected experiments, ultrasound of a given frequency and power level was applied during this fouling cycle. In all cases, the majority of the flux reduction occurred in the first 30 min of operation (see Fig. 3 for an example) with flow stabilizing beyond this point. The steady state permeate flux (Jf) was determined by averaging the last 10 recorded values of permeate mass (i.e. last 10 min of flow in a 4 h run). Selected experiments were performed in duplicate and an average experimental error in this steady state whey flux was found to be ±6% or ±1 l/m2 h. The total fouling resistance Rtotal was calculated from this steady state value, again using Eq. (1).
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The model contains three adjustable parameters i.e., the pore blockage factor a which describes the extent of blocked pores, the initial resistance of the cake deposit (Rpo) and the cake growth factor (f0 R0 ) which is the product of the fractional amount of protein present that contributes to deposit growth and the specific protein layer resistance. The best-fit values of these parameters were determined by minimizing the sum of the squared residuals between the experimental filtrate flux data and the model calculations.
2 Permeate Flux (l/m.hr)
50
40 Continuous Ultrasound Intermittent Ultrasound No Ultrasound
30
20
10 0
10
20
30 40 Time (min)
50
60
70
Fig. 3. Effect of intermittent sonication on whey permeation using the Minitan unit and a PS30000 MWCO membrane (TMP = 55 kPa, CFV = 0.28 m/s).
After this fouling cycle, the membrane surface was rinsed with water for 10 min. This rinsing is intended to remove the reversible fouling resistance that results from both concentration polarization and labile surface deposits, leaving the more tenacious deposits. A water flux value (Jwr) was recorded over the last 5 min of this 10 min flush. Results from these experiments were analysed using the well known resistance in series model. In this case, the total resistance Rtotal can be considered as the sum of three terms i.e., the membrane resistance, Rm, the reversible resistance arising from both concentration polarization and loose protein deposits, Rb and the resistance due to the irreversible, tenacious deposits, Rf: Rtotal ¼ Rm þ Rb þ Rf
ð2Þ
As given above, Rm was determined from the initial pure water flux, Rtotal from the steady state flux after fouling, and the sum Rm + Rf from the water rinsed flux. By difference, Rf and Rb could then be individually determined. The effect of ultrasound was also determined via an ultrasonic enhancement factor (Muthukumaran, Kentish, Lalchandani, et al., 2005), Eus ¼
J f; ultrasound J f;no ultrasound
ð3Þ
where Jf, ultrasound and Jf, no ultrasound are the average steady state permeate fluxes achieved with and without ultrasound. Finally, we also utilized in our analysis the model developed by Ho and Zydney (2000) that describes the flux decay as a function of time when both pore blockage and cake filtration mechanisms are active. This model requires the simultaneous solution of two equations: J aDPC b Rm aDPC b ¼ exp t þ 1 exp t ð4Þ J0 lRm Rm þ Rp lRm sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 2f 0 R DPC b t Rp ¼ ðRm þ Rpo Þ 1 þ ð5Þ Rm 2 lðRm þ Rpo Þ
2.2.3. Ultrasound assisted cleaning experiments Cleaning experiments followed a protocol similar to the one discussed above. After recording the clean water flux, the membrane was fouled for 30 min with freshly prepared 6% w/w whey solution. The membrane was then rinsed with water for 10 min to remove the reversible fouling load and the water flux again recorded as above Jwr. This was used to calculate Rf, the resistance of the irreversible fouling deposit (Eq. (1)). The fouled membrane was next cleaned for a further 10 min under different levels of both ultrasonic power and frequency. After this cleaning cycle, the membrane surface was rinsed with water again for 10 min to remove cleaning solutions and provide a consistent regime for flux calculation. The cleaned membrane flux (Jwc) was determined over the final 5 min of this water rinse and the cleaned resistance Rc calculated. Results from these cleaning experiments are presented here as a cleaning efficiency (CE) defined by Matzinos and Alvarez (2002): CE ¼
Rf R c 100 Rf Rm
ð6Þ
In both the fouling and cleaning experiments, the membrane was finally cleaned by circulating 0.1 M sodium hydroxide and 15 mM of sodium dodecyl sulfate for 5 min. The membrane was then left to soak in this solution. Milli-Q water/distilled water was fed into the unit to flush out the cleaning solution. A further permeate water flux was recorded and compared to the initial water flux. If necessary, further membrane cleaning was conducted until the original clean membrane flux was restored. The experimental variability in the final cleaned membrane flux measurement was ±9 l/m2 h or ±5% of the total value. 3. Results and discussion 3.1. Influence of intermittent ultrasound on the permeate flux As discussed above, the high frequency ultrasonic unit could only be operated in an intermittent format. Hence, to determine the effects of intermittent operation, the low frequency ultrasonic bath was first utilized in an intermittent mode for 1 h (5 min on/5 min off) and results compared to continuous operation. As shown in Fig. 3, while such intermittent ultrasound appeared effective at short operation times, there was little flux improvement over
S. Muthukumaran et al. / Journal of Food Engineering 81 (2007) 364–373
longer time frames. This is consistent with the results of Simon et al. (2000a). The ultrasonic enhancement factor for intermittent ultrasound after 1 h was calculated as 1.1 compared to a factor of 1.4 for continuous ultrasound. 3.2. Frequency effects Fig. 4 compares the long term permeate flux achieved when using the Minitan membrane unit and a 30000 MWCO membrane. In the absence of ultrasound, the permeate flux declines as the transmembrane pressure increases. This trend is somewhat unusual and can be related to both the low cross-flow velocities in the Minitan unit, and the deformable nature of colloidal solids such as whey proteins. At higher transmembrane pressures, the compressive forces exerted on the cake layer favor a thicker and more densely packed cake layer which provides more resistance to permeate flow. With the addition of low frequency ultrasound, the flux increases across the full range of transmembrane pressures. The ultrasonic enhancement factor of around 1.5 (see Table 2) is consistent with our previous results across a range of transmembrane pressures. The high frequency pulsed unit provides a flux enhancement of 1.2 at low transmembrane pressures. While lower than the enhancement provided by the continuous low frequency ultrasound, this result is consistent with the results for intermittent application described above. Of greater concern is the reduction in flux observed under these con-
2 Permeate Flux (l/m.hr)
20
50 kHz continuous 1 MHz Pulsed No Ultrasound
15
10
5
0 0
50
100
150
200
250
300
350
Transmembrane Pressure (kPa)
Fig. 4. Effect of ultrasonic frequency on whey permeation using the Minitan unit and a PS30000 MWCO membrane (CFV = 0.28 m/s).
369
ditions at higher transmembrane pressures. Under these conditions, the long term flux falls below the level provided in the absence of ultrasound. Similar results are obtained when the membrane pore size is reduced from 30000 to 8000 MWCO (Table 2). These results are consistent with the work of Kobayashi et al. (1999) who found that while operating at 30 kPa transmembrane pressure, 20 kHz ultrasound was effective, but operation at 100 kHz resulted in no ultrasonic flux enhancement. Similarly Duriyabunleng and Petmunee (2001) find that there is an optimum transmembrane pressure beyond which ultrasound is less effective. The distinction in the present case is that the ultrasound appears to be positively detrimental when both high transmembrane pressures and ultrasonic frequencies are applied in combination. In order to elucidate the cause of this loss in flux, the behaviour of the individual resistances to flow were considered. As expected, the hydraulic resistance of the membrane Rm is insensitive to sonication. At all pressures, the ‘reversible’ fouling resistance, Rb is lower when either 50 kHz or 1 MHz ultrasound is applied (Fig. 5(a)), relative to the no-ultrasound case. This is consistent with the results of our previous work (Muthukumaran, Kentish, Ashokkumar, et al., 2005), which indicate that ultrasound increases the mass transfer coefficient within the polarization boundary layer. The irreversible fouling resistance Rf (Fig. 5(b)), provided by the no-ultrasound case and the 50 kHz ultrasound are comparable, indicating that ultrasound is relatively ineffective in reducing the extent of more tightly bound deposits within the fouling cake and any pore blockage. Again, this is consistent with our previous work which showed that pore blockage was little affected by low frequency ultrasonic application. Of greater note is the dramatic increase in irreversible fouling that occurs with the combination of high frequency ultrasonic pulsed application and high transmembrane pressures. The fouling load in this case is clearly greater than in the absence of ultrasound. This increase in fouling could result either from compaction of the protein deposits into a more densely packed cake layer or from such deposits being forced into the membrane pores under the influence of this ultrasonic field. To gain further insight into these phenomena, the above results were also analysed using the theoretical model
Table 2 Whey permeate flux and ultrasonic enhancement factor at variable frequencies using the Minitan unit (CFV = 0.28 m/s) Average permeate flux (l/m2 h) Membrane pore size (MWCO)
30000
Transmembrane pressure (kPa)
55
150
Mode of operation 50 kHz continuous 1 MHz pulsed No ultrasound
15 12 11
13 9.5 8.7
Ultrasonic enhancement factor 8000
30000
8000
300
300
55
150
300
300
12 4.3 7.2
7.6 3.8 4.8
1.4 1.2
1.5 1.1
1.7 0.6
1.6 0.8
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developed by Ho and Zydney (2000). The best-fit values of a, Rpo and f0 R0 across a range of pressures are shown in Fig. 6 for the 30000 MWCO membrane. Results for the no-ultrasound and 50 kHz ultrasound are comparable to our prior results. The use of ultrasound reduces the pore blockage and initial deposit resistance a little, but has most effect on the cake growth factor, reducing this value by an order of magnitude. This implies that the fouling cake is either thinner (lower f0 ) and/or less compressed (lower R0 ) and thus offers less resistance. There is no evidence of the high frequency ultrasound providing increased pore blockage, as the pore blockage parameter (a) is within the range of data for both low frequency and no ultrasound, and if anything decreases as transmembrane pressure increases. The initial deposit resistance matches that of the no-ultrasound case. However, the cake growth factor, while matching that for no ultrasound at low transmembrane pressures, is significantly larger at higher TMP values. This indicates that either the fraction of the total protein contributing to cake formation (f0 ) is increasing or that this cake layer is offering more resistance (R0 ) at high TMP. Sonication has been reported to cause both particle agglomeration (Thompson & Doraiswamy, 1990; Vimini, Kemp, & Fox, 1983) and possibly whey protein denaturation (Villamiel & de Jong, 2000) and either of these effects could contribute to a thicker and/or more compacted fouling cake.
14 No Ultrasound 30000MWCO 1 MHz Pulsed 30000MWCO 50 kHz Continuous 30000MWCO No Ultrasound 8000 MWCO 1 MHz Pulsed 8000MWCO 50 kHz Continuous 8000MWCO
8 6 4 2 0 0
5 50
1 100
150
20 200
250
300
350
T Transmembrane Pressure (kPa)
-1
Irreversible Resistance (Rf) x10 (m )
25 13
No Ultrasound 30000MWCO 1 MHz Pulsed 30000MWCO 50 kHz Continuous 30000MWCO No Ultrasound 8000 MWCO 1 MHz Plused 8000MWCO 50 kHz Continuous 8000MWCO
20
15
10
5
0 0
50
100
150
200
250
300
350
Transmembrane Pressure (kPa)
3.3. Results with a high cross-flow velocity membrane module Experiments were also carried out using the Viva flow module. This unit was operated at a cross-flow rate around
Initial Deposit Resistance
7 6 5 (m 2 /kg)
Pore Blockage Parameter
Fig. 5. Permeation resistance values as a function of transmembrane pressure in the Minitan unit with a PS30000 MWCO membrane (CFV = 0.28 m/s): (a) reversible resistance and (b) irreversible resistance.
4 3 2 1 0 0
100
200
300
-1
10
13
12
x10 ( m )
13
-1
Reversible Resistance (Rb) x10 (m )
370
9 8 7 6 5 4 3 2 1 0 0
400
Transmembrane Pressure (kPa)
100
200
300
400
Transmembrane Pressure (kPa)
Cake Growth Factor 13 x10 (m/kg)
100 10
50 kHz Continous 1 MHz Pulsed
1
No Ultrasound 0.1 0.01 0
100
200
300
400
Transmembrane Pressure (kPa)
Fig. 6. Best-fit values of the fouling parameters as a function of the transmembrane pressure using the Minitan unit and a PS30000 MWCO membrane (CFV = 0.28 m/s).
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3.4. Effect of ultrasonic frequency on the cleaning efficiency Experiments were also carried out to determine the effect of ultrasound on cleaning efficiency. It should be noted that these cleaning experiments were conducted on a fouled membrane that had already been subjected to a 10 min water flush, so the results are indicative of the ability of ultrasound to remove the irreversible fouling layer. Fig. 8 shows the effect of sonication time on the membrane permeate flux for a fixed fouling time of 30 min using the Vivaflow unit. It can be seen from this figure, that the cleaning efficiency is relatively low for all forms of ultrasound, consistent with the discussion above that suggests ultrasound is less effective on this irreversible fouling. Cleaning is most effective when the low frequency continuous ultrasound is used relative to the 1 MHz pulsed source. Both modes of operation provide higher cleaning efficiency 35
60
Cleaning Efficiency (%)
13
membrane pressure utilized in this experiments (200 kPa) is lower than the highest value used in the Minitan unit (300 kPa). The ‘reversible’ fouling resistance (Rb) is much greater than in the Minitan module, but the ultrasonic flux enhancement again occurs predominantly through a reduction in this ‘reversible’ resistance. These effects may again be attributed to an increase in the mass transfer coefficient within the flow boundary layer. Conversely, the irreversible fouling resistance (Rf) is unaffected by sonication at any frequency.
Vivaflow Module Minitan Module
-1
Total Resistance (Rtot) x10 (m )
five times that used in the Minitan unit (1.5 versus 0.28 m/ s) resulting in wall stresses a factor of 10 greater in magnitude. However, experimental results with this unit displayed a significant reduction in the steady state permeate flux after 4 h of operation. The surface potential of the PES membrane used in the Vivaflow unit is described as neutral whereas hydrophobic membranes were employed in the Minitan unit. This difference in membrane surface potential results in a significantly different pattern in the total flow resistance over time. As shown in Fig. 7, while the resistance is initially lower for the Vivaflow unit, this resistance increases almost linearly with time, whereas the resistance in the Minitan unit levels off rapidly. Table 3 shows the steady state whey permeate flux and the permeation resistance values in the Vivaflow unit under variable sonication modes after 4 h of permeation. The ultrasonic enhancement factor with the low frequency unit is comparable to that in the Minitan unit. This indicates that even with the increased wall shear stresses present in this unit, ultrasound is still effective. This is consistent with our prior work (Muthukumaran, Kentish, Ashokkumar, et al., 2005) which showed that the addition of membrane spacers also did not dramatically effect the ultrasonic enhancement. However, contrary to the Minitan module, ultrasound remains effective even with the use of high frequency ultrasound. The greater effectiveness of the high frequency unit in this case can be related to both the higher cross-flow velocities that will assist in preventing the deposition of a thicker fouling cake and the fact that the trans-
371
50 40 30 20 10
30 25 20 15 50 kHz Continuous 1 MHz Sequential 1 MHz Pulsed No Ultrasound
10 5
0 0
0.5
1
1.5
2
2.5
3
3.5
4
0 0
5
10
15
20
25
30
35
Water Flush Time (min)
Time (Hours)
Fig. 7. Total resistance as a function of time (Minitan unit, PES10000 MWCO membrane, TMP = 300 kPa, CFV = 0.28 m/s: Viva flow unit, PES10000 MWCO membrane, TMP = 200 kPa, CFV = 1.5 m/s).
Fig. 8. Effect of ultrasonic frequency on the cleaning efficiency using the Vivaflow unit for 30 min fouling at 200 kPa, 1.5 m/s and cleaning at 60 kPa, 1.5 m/s.
Table 3 Whey permeate flux and permeation resistances at variable frequencies using the Vivaflow unit (TMP = 200 kPa, CFV = 1.5 m/s) Mode of operation
Average permeate flux (l/m2 h)
Ultrasonic enhancement factor
Resistance 1013 (m1) Rm
Rf
Rb
50 kHz continuous 1 MHz pulsed 1 MHz sequential No ultrasound
1.6 1.6 1.6 1.2
1.4 1.4 1.4
0.40 0.44 0.51 0.41
12 11 12 11
32 32 33 51
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Table 4 Effect of ultrasonic frequency on cleaning efficiency using the Minitan unit and a PS30000 MWCO membrane (30 min fouling at 0.28 m/s, 55 kPa and 10 min cleaning at 0.5 m/s, 55 kPa) Mode
(CE) (%)
50 kHz continuous 1 MHz pulsed 1 MHz sequential No ultrasound
57 ± 3 50 ± 3 48 ± 3 48 ± 3
been provided through a University of Melbourne Research and Development Grants Scheme award and a University of Melbourne-CSIRO Collaborative Research Support Scheme award. Infrastucture and equipment is also provided through the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council. This support is gratefully acknowledged. References
than when no ultrasound is applied (i.e. when a 10 min water flush is used in place of a sonication sequence). As with our prior work, there is little gain in sonicating for longer than 10 min. Similar experiments were carried out using the Minitan unit. Results again show that the maximum cleaning efficiency occurs with the continuous low frequency ultrasound (Table 4). As in the fouling experiments, the high frequency pulsed ultrasound in this unit is less effective. These results are consistent with the work of Lamminen et al. (2004) who also show that cleaning is optimized at low frequencies. In this published work, the ultrasound is used to remove both the reversible and irreversible fouling layer, so the overall cleaning effectiveness is higher than in the present work. 4. Conclusion These experiments have shown that while the use of ultrasound in membrane ultrafiltration is generally positive, there are conditions under which it can be less effective or even have a negative effect on filtration performance. We find that the use of intermittent ultrasound does little to enhance flux rates at any frequency. Further, the use of intermittent (pulsed) ultrasound at high frequency can cause a net reduction in flux rates when high transmembrane pressures and low cross-flow velocities are employed. Pulsed high frequency ultrasound is also less effective in cleaning membrane surfaces after the fouling cycle. These results are consistent with other workers who also find that high frequency ultrasound is less effective. Our results show that continuous low frequency sonication generally reduces the components of the total flow resistance that are readily reversed during water flush. This includes both the mass transfer resistance arising from concentration polarization and from the more labile protein deposits that are readily removed. This is consistent with our previous results. Conversely, the high frequency pulsed ultrasound can cause a significant increase in the ‘irreversible’ flow resistance. This increase in resistance is associated with a thicker and/or more compacted cake layer, rather than the blockage of pores. Acknowledgements S. Muthukumaran is the recipient of an Australian Postgraduate Award. Financial support for this project has
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