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A study on cross-flow ultrafiltration with various membrane orientations
Separation and Purification Technology 39 (2004) 13–22
A study on cross-flow ultrafiltration with various membrane orientations Tung-Wen Cheng∗ , Chih-Ta Lin Department of Chemical Engineering, Tamkang University, Tamsui, Taipei 251, Taiwan, ROC
Abstract In this study, the flux behavior of an-inclined flat-membrane cross-flow ultrafiltration was investigated. The experiment was conducted using a cellulose ester membrane (10,000 MWCO) and dextran T500 as the test solution. The membrane inclination included 0◦ (flow above membrane), 90◦ (vertical membrane), and 180◦ (flow below membrane). Experimental results showed that raising the cross-flow velocity increased the permeate flux and the time required to reach a steady state was also shortened. The effect of inclination on the flux enhancement is significant when the conventional ultrafiltration is operated at a low liquid flow rate. In two-phase flow ultrafiltration, the introduction of gas enhances the permeate flux even at a high liquid flow rate and the influence of gas sparging is particularly prominent as the membrane is installed at 180◦ orientation. Under the present operating conditions, the filtration flux is maximum at 180◦ and minimum at 0◦ inclination. © 2004 Elsevier B.V. All rights reserved. Keywords: Ultrafiltration; Membrane orientation; Gas slug; Flux enhancement
1. Introduction Ultrafiltration of macromolecular solutions has become an increasingly important separation process, and its applications include the treatments of industrial effluents, oil emulsion wastewater, biological macromolecules, colloidal paint suspensions and medical therapeutics. Ultrafiltration is a pressure-driven membrane separation process. The working pressure, usually applied to the solution in the range of 100–1000 kPa, provides the driving potential to force the solvent and the smaller molecules to flow through the membrane while the larger molecules are rejected ∗ Corresponding author. Tel.: +886-2-26219554; fax: +886-2-26209887. E-mail address: [email protected] (T.-W. Cheng).
by the membrane. The concentration of rejected solute on the membrane surface is always higher than that in the bulk solution. This is the so-called concentration polarization phenomenon, which results in fouling and solute adsorption on the membrane as well as a flux decline. In the search of ways to decrease concentration polarization in order to increase the permeate flux, the method of gas–liquid two-phase flow in the membrane filtration has been proven as a simple and economical technique to enhance the permeate flux effectively. The addition of air to the liquid stream increases turbulence on the membrane surface and suppresses the formation of the concentration boundary layer, leading to enhancement in the flux of the filtration process. Ultrafiltration of bacterial cell suspensions has been improved up to 100% using air slugs [1]. Cui and Wright [2] investigated the effect
1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2003.12.008
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of air sparging on ultrafiltrating macromolecular solutions in a tubular membrane mounted vertically or horizontally. The maximum flux enhancement was 60% for undyed 162 kDa dextran, 113% for 162 kDa dyed dextran, and 91% for 69 kDa BSA solutions; and the permeate flux for the upward flow in the vertically mounted membrane was 10–20% higher than that in the horizontally installed membrane. Bellara et al. [3] employed a pilot-plant scale hollow-fiber module, and investigated the use of gas–liquid two-phase cross-flow to overcome concentration polarization in the ultrafiltration of macromolecular solutions. Their work showed that the flux enhancements were 20–50% for dextran and 10–60% for albumin, and the sieving coefficient of albumin, i.e. concentration in permeate/concentration in feed, was considerably reduced when gas-sparging technique was used. The flow pattern of gas–liquid two-phase flow is important in determining the performance of air-sparging ultrafiltration system. Mercier et al. [4] consider that the slug-flow pattern is the best regime for enhancing the permeate flux. It was also noted that the effect of gas slugs on flux enhancement depends on the resistance of the concentration polarization layer in the conventional liquid-phase ultrafiltration system [5].
When the resistance of the polarization layer is large, i.e. the system is operated with low liquid velocity, high transmembrane pressure or high feed concentration, a low gas flow rate cannot disturb effectively the concentration polarization layer, and a minimum gas velocity is required to disturb this layer. Beyond the critical gas velocity, gas slugs can enhance the permeate flux significantly. The permeate flux of ultrafiltration in a vertical tubular membrane is larger than that in a horizontal one [2]. Experimental data of single liquid-phase ultrafiltration obtained with a flat membrane channel cell at various orientations also show that natural convection instability enhances the flux at the unstable gravitational orientation (membrane above the flow channel) [6]. The previous works [7,8] indicated that the flux of gas–liquid two-phase ultrafiltration in tubular membrane is significantly influenced by the membrane inclination. The objective of this work was to study the act of gas slug under various membrane inclination on influencing the permeate flux of ultrafiltration. The experiments were carried out with a flat-plate membrane module connected to a rotating panel. The result will be also discussed under various liquid velocities, gas velocities, and membrane inclinations.
Fig. 1. Flow diagram of ultrafiltration experiment: (1) thermostat, (2) feed tank, (3) pump, (4) flow meter, (5) pressure gauge, (6) valve, (7) compressed air, (8) rotating module, (9) membrane, (10) collector, (11) balance, (12) data processor.
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2. Experiment
3. Results and discussion
The experimental apparatus used in this work is shown in Fig. 1. This ultrafiltration experiment was carried out in an inclined flat-membrane module. The membrane medium used was a 10,000 Da MWCO cellulose ester membrane (Spectrum Co.) with 784 mm2 effective membrane area (49 mm length and 16 mm width). The cross-sectional area of the flow channel is 160 mm2 (10 mm height and 16 mm width). The tested solute was dextran T500 (Pharmacia Co.) which was more than 99% retained by the membrane used. The solvent was distilled water. The feed solution was circulated by a diaphragm pump (CDP 6800, Aquatec), and the liquid flow rate was measured by a flowmeter (60648, Cole Parmer). The compressed air supply was directed into the liquid stream. The gas flow rate was controlled by an adjustable valve and measured by a flowmeter (F150-AV1-B-125-30-SAP, Porter). The feed pressure was controlled by using an adjustable valve at the outlet of the membrane module. The pressures at the module inlet (Pi ) and outlet (Po ) were measured with pressure transmitter (E713, Bourden Sedme Co.). The inclination angles, θ, of the membrane module in the experiment were 0◦ , 90◦ and 180◦ , respectively, where θ = 0◦ stands for fluid flow above the membrane and θ = 180◦ for below the membrane. The superficial liquid velocities, uL , were 0, 0.02 and 0.05 m/s, where uL = 0 means a dead-end operation; and the superficial gas velocities, uG , were 0, 0.002, 0.019 and 0.053 m/s, where uG = 0 represents a conventional ultrafiltration. The transmembrane pressure was adjusted to 200 kPa. The feed solution temperature in all experiments was kept at 30 ◦ C by a thermostat. The amount of permeate is small as compared to the feed solution, so, the concentration of solute in the feed tank is assumed to be constant. After each experiment, the membrane was cleaned by using sonic cleaner and distilled water. The cleaning procedure was repeated until the original water flux had been restored. Experimental data shows that the membrane used can restore to its original water flux after cleaning and the flux measurement of ultrafiltration system conducted in this work has a good reproducibility.
3.1. Conventional ultrafiltration under various liquid velocities and inclinations Fig. 2 shows the permeate flux over time under various liquid velocities and the membrane module was installed at 0◦ . The flux decreases sharply at the beginning of filtration, and then approaches a steady state flux. At uL = 0, the dead-end filtration, the steady state flux is near zero. This implies that the filtration resistance increases with the operating time, and then reaches a maximum. The time required to reach a steady state decreases with the increase in liquid flow velocity, while the steady state flux increases with increasing the liquid flow velocity. The increase in cross-flow velocity can enhance the back diffusion of solute accumulated on the membrane surface, and hence, improve the permeate flux. Fig. 3 is plot of the permeate flux when membrane was installed at 90◦ . These fluxes are close to the fluxes in the case of 0◦ except there is a slightly larger flux at uL = 0 in this case. Fig. 4 is the result when membrane orientation was 180◦ . The steady state fluxes under various operating conditions were listed in Table 1. The flux of uL = 0 at 180◦ inclination is obviously larger than that at 90 or 0◦ . For a dead-end ultrafiltration, the effect of membrane inclination is significant. Membrane inclination will create a natural convection caused by density gradient between the concentration polarization layer and the bulk fluid. However, at a high cross-flow velocity, for instance uL = 0.05 m/s, the effect of forced convection caused by the flow velocity on disturbing the concentration polarization layer is obviously larger than the effect of natural convection induced by inclining the membrane. Therefore, the permeate fluxes with different membrane inclinations are almost equal at a high liquid velocity. 3.2. Effect of gas velocity on flux The influence of gas slugs on the ultrafiltratuin flux was investigated with high liquid velocity. Fig. 5 shows the permeate flux under various gas velocities when ultrafiltration operated at 0.05 m/s liquid velocity and θ = 0◦ . It is noted that the addition of gas slug into the liquid stream is not an effective way on enhancing the flux in this case. When the gas velocity is
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low, the gas slug passing the membrane module is near to the upper plate or away from the membrane surface. Thus, the gas slug cannot disturb the concentration polarization layer effectively but increases the pressure drop in the module leading to decreasing the transmembrane pressure. Therefore, the flux of two-phase ultrafiltration is lower than the conventional ultrafiltration until the gas velocity is high enough, e.g. 0.053 m/s, that can disturb concentration boundary layer in compensation for the loss of transmembrane pressure. Fig. 6 shows the flux under effect of gas slugs when membrane was installed at θ = 90◦ . The introduction of gas slugs enhances flux in this case. The gas slug passing the membrane module should locate symmetrically to the centerline of a vertical flow channel when the wall is impermeable. However, in this membrane module the wall suction is asymmetric, the passing gas slug will deviate from the centerline and more close to the membrane wall. Gas slugs have a slight effect on disturbing the concentration polarization layer when the gas velocity is low. At a high gas velocity, gas slugs move closer to the membrane wall because the size of gas slug increases, thus gas slugs affect effectively on the concentration boundary layer and the flux enhancement is relatively significant. Fig. 7 shows the permeate flux under various gas velocities when membrane was installed at θ = 180◦ . It is noted that the addition of gas slugs into the liquid stream is very effective in enhancing the flux. At this membrane orientation, the gas slug passing the membrane module is near to the membrane surface. The gas slug can disturb the concentration polarization layer and improve the flux effectively. The flux enhancement increases with the increase of gas veloc-
ity owing to the increase of cross-flow velocity and frequency of the gas slugs. 3.3. Membrane orientation at various gas velocities Fig. 8 shows the permeate flux under different membrane orientation when the gas velocity was 0.002 m/s. The steady fluxes of θ = 90◦ and θ = 180◦ are almost equal and higher than that of θ = 0◦ . The effect of membrane inclination is significant as the membrane orientation changed from θ = 0 to 90◦ or 180◦ even when a small amount of gas slugs were introduced. The flux result also implies that the gas slug position located in the flow channel is similar between the cases of θ = 90◦ and 180◦ . As the gas velocity increased to 0.019 m/s, the effect of membrane inclination on the permeate flux is enhanced and the difference between the effects under θ = 90◦ and 180◦ is significant, as shown in Fig. 9. The gas slug position will deviate more from the centerline towards the membrane surface as the gas velocity increases, particularly for membrane installed at 180◦ position. Fig. 10 shows the flux variations at a higher gas velocity, uG = 0.053 m/s. The permeate flux of θ = 180◦ is still significantly larger than that of θ = 90◦ . The flux data also shows that the flux decline over time is slight in the case of 180◦ inclination because the gas slugs reduce the filtration resistance effectively. Therefore, membrane inclination not only enhances but also sustains the flux of the gas-sparging ultrafiltration. Experimental result of this work indicated that membrane installed at the upper position in an asymmetric permeable module is an optimal orientation in enhancing the ultrafiltration flux. The hydrodynamic characteristics of gas–liquid two-phase flow in an
Table 1 Steady state fluxesa under various experimental conditions uG (m/s)
uL (m/s) 0
0 0.002 0.019 0.053 a b
0.02
0.05
0◦b
90◦b
180◦b
0◦b
90◦b
180◦b
0◦b
90◦b
180◦b
0.64
1.07
3.63
4.06
4.48
4.91
5.98 5.59 5.55 5.98
6.04 6.66 6.55 7.90
6.09 6.68 7.05 9.18
The flux in unit of 10−6 m3 /m2 s. Angle θ.
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Fig. 8. Effect of membrane inclination on ultrafiltration flux: uG = 0.002 m/s.
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Dextran T500 solutes C = 3 g/l P = 2 bar uL = 0.05 m/s uG = 0.053 m/s MWCO = 10K
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asymmetric permeable module need further study in order to quantitatively clarify the influence of membrane inclination on the two-phase ultrafiltration.
4. Conclusion The influence of the membrane inclination on the permeate flux of ultrafiltration was investigated experimentally. The experiments were carried out in an asymmetric flat-plate membrane module by using aqueous dextran T500 solution as tested solution. The permeate fluxes were measured under various liquid flow rates, gas flow rates and membrane inclinations. For dead-end ultrafiltration, membrane inclination will create natural convection that disturbs the concentration boundary layer affecting the permeate flux significantly. In cross-flow conventional ultrafiltration, the enhancement in permeate flux is mainly dominated by forced convection or cross-flow velocity. The natural convection induced by varying membrane inclination is relatively weak as the flow velocity increases.
Membrane inclination is important in enhancing and sustaining the flux of gas–liquid two-phase ultrafiltration system. The position of gas slug is close to the membrane surface when the membrane orientation is installed at 180◦ inclination. Thus, gas slugs disturb the concentration boundary layer effectively resulting in a high flux enhancement.
Acknowledgements The authors wish to express their thanks to the National Science Council of Taiwan for financial aid.
References [1] C.K. Lee, W.G. Chang, Y.H. Ju, Air slugs entrapped crossflow filtration of bacterial suspensions, Biotechnol. Bioeng. 41 (1993) 525. [2] Z.F. Cui, K.I.T. Wright, Gas–liquid two-phase cross-flow utrafiltration of BSA and dextran solution, J. Membr. Sci. 90 (1994) 183.
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[3] S.R. Bellara, Z.F. Cui, D.S. Pepper, Gas-sparging to enhance permeate flux in ultrafiltration using hollow fiber membranes, J. Membr. Sci. 121 (1996) 175. [4] M. Mercier, C. Fonade, C. Lafforgue-Delorme, Influence of the flow regime on the efficiency of a gas–liquid two-phase medium filtration, Biotechnol. Tech. 9 (1995) 853. [5] T.W. Cheng, H.M. Yeh, C.T. Gau, Enhancement of permeate flux by gas slugs for cross-flow ultrafiltration in tubular membrane module, Sep. Sci. Technol. 33 (1998) 2295.
[6] K.H. Youm, A.G. Fane, D.E. Wiley, Effects of natural convection instability on membrane performance in dead-end and cross-flow ultrafiltration, J. Membr. Sci. 116 (1996) 229. [7] T.W. Cheng, H.M. Yeh, J.H. Wu, Effects of gas slugs and inclination angle on the ultrafiltration flux in tubular membrane module, J. Membr. Sci. 158 (1999) 223. [8] T.W. Cheng, Influence of inclination on gas-sparged cross-flow ultrafiltration through an inorganic tubular membrane, J. Membr. Sci. 196 (2002) 103.
A study on cross-flow ultrafiltration with various membrane orientations Tung-Wen Cheng∗ , Chih-Ta Lin Department of Chemical Engineering, Tamkang University, Tamsui, Taipei 251, Taiwan, ROC
Abstract In this study, the flux behavior of an-inclined flat-membrane cross-flow ultrafiltration was investigated. The experiment was conducted using a cellulose ester membrane (10,000 MWCO) and dextran T500 as the test solution. The membrane inclination included 0◦ (flow above membrane), 90◦ (vertical membrane), and 180◦ (flow below membrane). Experimental results showed that raising the cross-flow velocity increased the permeate flux and the time required to reach a steady state was also shortened. The effect of inclination on the flux enhancement is significant when the conventional ultrafiltration is operated at a low liquid flow rate. In two-phase flow ultrafiltration, the introduction of gas enhances the permeate flux even at a high liquid flow rate and the influence of gas sparging is particularly prominent as the membrane is installed at 180◦ orientation. Under the present operating conditions, the filtration flux is maximum at 180◦ and minimum at 0◦ inclination. © 2004 Elsevier B.V. All rights reserved. Keywords: Ultrafiltration; Membrane orientation; Gas slug; Flux enhancement
1. Introduction Ultrafiltration of macromolecular solutions has become an increasingly important separation process, and its applications include the treatments of industrial effluents, oil emulsion wastewater, biological macromolecules, colloidal paint suspensions and medical therapeutics. Ultrafiltration is a pressure-driven membrane separation process. The working pressure, usually applied to the solution in the range of 100–1000 kPa, provides the driving potential to force the solvent and the smaller molecules to flow through the membrane while the larger molecules are rejected ∗ Corresponding author. Tel.: +886-2-26219554; fax: +886-2-26209887. E-mail address: [email protected] (T.-W. Cheng).
by the membrane. The concentration of rejected solute on the membrane surface is always higher than that in the bulk solution. This is the so-called concentration polarization phenomenon, which results in fouling and solute adsorption on the membrane as well as a flux decline. In the search of ways to decrease concentration polarization in order to increase the permeate flux, the method of gas–liquid two-phase flow in the membrane filtration has been proven as a simple and economical technique to enhance the permeate flux effectively. The addition of air to the liquid stream increases turbulence on the membrane surface and suppresses the formation of the concentration boundary layer, leading to enhancement in the flux of the filtration process. Ultrafiltration of bacterial cell suspensions has been improved up to 100% using air slugs [1]. Cui and Wright [2] investigated the effect
1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2003.12.008
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T.-W. Cheng, C.-T. Lin / Separation and Purification Technology 39 (2004) 13–22
of air sparging on ultrafiltrating macromolecular solutions in a tubular membrane mounted vertically or horizontally. The maximum flux enhancement was 60% for undyed 162 kDa dextran, 113% for 162 kDa dyed dextran, and 91% for 69 kDa BSA solutions; and the permeate flux for the upward flow in the vertically mounted membrane was 10–20% higher than that in the horizontally installed membrane. Bellara et al. [3] employed a pilot-plant scale hollow-fiber module, and investigated the use of gas–liquid two-phase cross-flow to overcome concentration polarization in the ultrafiltration of macromolecular solutions. Their work showed that the flux enhancements were 20–50% for dextran and 10–60% for albumin, and the sieving coefficient of albumin, i.e. concentration in permeate/concentration in feed, was considerably reduced when gas-sparging technique was used. The flow pattern of gas–liquid two-phase flow is important in determining the performance of air-sparging ultrafiltration system. Mercier et al. [4] consider that the slug-flow pattern is the best regime for enhancing the permeate flux. It was also noted that the effect of gas slugs on flux enhancement depends on the resistance of the concentration polarization layer in the conventional liquid-phase ultrafiltration system [5].
When the resistance of the polarization layer is large, i.e. the system is operated with low liquid velocity, high transmembrane pressure or high feed concentration, a low gas flow rate cannot disturb effectively the concentration polarization layer, and a minimum gas velocity is required to disturb this layer. Beyond the critical gas velocity, gas slugs can enhance the permeate flux significantly. The permeate flux of ultrafiltration in a vertical tubular membrane is larger than that in a horizontal one [2]. Experimental data of single liquid-phase ultrafiltration obtained with a flat membrane channel cell at various orientations also show that natural convection instability enhances the flux at the unstable gravitational orientation (membrane above the flow channel) [6]. The previous works [7,8] indicated that the flux of gas–liquid two-phase ultrafiltration in tubular membrane is significantly influenced by the membrane inclination. The objective of this work was to study the act of gas slug under various membrane inclination on influencing the permeate flux of ultrafiltration. The experiments were carried out with a flat-plate membrane module connected to a rotating panel. The result will be also discussed under various liquid velocities, gas velocities, and membrane inclinations.
Fig. 1. Flow diagram of ultrafiltration experiment: (1) thermostat, (2) feed tank, (3) pump, (4) flow meter, (5) pressure gauge, (6) valve, (7) compressed air, (8) rotating module, (9) membrane, (10) collector, (11) balance, (12) data processor.
T.-W. Cheng, C.-T. Lin / Separation and Purification Technology 39 (2004) 13–22
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2. Experiment
3. Results and discussion
The experimental apparatus used in this work is shown in Fig. 1. This ultrafiltration experiment was carried out in an inclined flat-membrane module. The membrane medium used was a 10,000 Da MWCO cellulose ester membrane (Spectrum Co.) with 784 mm2 effective membrane area (49 mm length and 16 mm width). The cross-sectional area of the flow channel is 160 mm2 (10 mm height and 16 mm width). The tested solute was dextran T500 (Pharmacia Co.) which was more than 99% retained by the membrane used. The solvent was distilled water. The feed solution was circulated by a diaphragm pump (CDP 6800, Aquatec), and the liquid flow rate was measured by a flowmeter (60648, Cole Parmer). The compressed air supply was directed into the liquid stream. The gas flow rate was controlled by an adjustable valve and measured by a flowmeter (F150-AV1-B-125-30-SAP, Porter). The feed pressure was controlled by using an adjustable valve at the outlet of the membrane module. The pressures at the module inlet (Pi ) and outlet (Po ) were measured with pressure transmitter (E713, Bourden Sedme Co.). The inclination angles, θ, of the membrane module in the experiment were 0◦ , 90◦ and 180◦ , respectively, where θ = 0◦ stands for fluid flow above the membrane and θ = 180◦ for below the membrane. The superficial liquid velocities, uL , were 0, 0.02 and 0.05 m/s, where uL = 0 means a dead-end operation; and the superficial gas velocities, uG , were 0, 0.002, 0.019 and 0.053 m/s, where uG = 0 represents a conventional ultrafiltration. The transmembrane pressure was adjusted to 200 kPa. The feed solution temperature in all experiments was kept at 30 ◦ C by a thermostat. The amount of permeate is small as compared to the feed solution, so, the concentration of solute in the feed tank is assumed to be constant. After each experiment, the membrane was cleaned by using sonic cleaner and distilled water. The cleaning procedure was repeated until the original water flux had been restored. Experimental data shows that the membrane used can restore to its original water flux after cleaning and the flux measurement of ultrafiltration system conducted in this work has a good reproducibility.
3.1. Conventional ultrafiltration under various liquid velocities and inclinations Fig. 2 shows the permeate flux over time under various liquid velocities and the membrane module was installed at 0◦ . The flux decreases sharply at the beginning of filtration, and then approaches a steady state flux. At uL = 0, the dead-end filtration, the steady state flux is near zero. This implies that the filtration resistance increases with the operating time, and then reaches a maximum. The time required to reach a steady state decreases with the increase in liquid flow velocity, while the steady state flux increases with increasing the liquid flow velocity. The increase in cross-flow velocity can enhance the back diffusion of solute accumulated on the membrane surface, and hence, improve the permeate flux. Fig. 3 is plot of the permeate flux when membrane was installed at 90◦ . These fluxes are close to the fluxes in the case of 0◦ except there is a slightly larger flux at uL = 0 in this case. Fig. 4 is the result when membrane orientation was 180◦ . The steady state fluxes under various operating conditions were listed in Table 1. The flux of uL = 0 at 180◦ inclination is obviously larger than that at 90 or 0◦ . For a dead-end ultrafiltration, the effect of membrane inclination is significant. Membrane inclination will create a natural convection caused by density gradient between the concentration polarization layer and the bulk fluid. However, at a high cross-flow velocity, for instance uL = 0.05 m/s, the effect of forced convection caused by the flow velocity on disturbing the concentration polarization layer is obviously larger than the effect of natural convection induced by inclining the membrane. Therefore, the permeate fluxes with different membrane inclinations are almost equal at a high liquid velocity. 3.2. Effect of gas velocity on flux The influence of gas slugs on the ultrafiltratuin flux was investigated with high liquid velocity. Fig. 5 shows the permeate flux under various gas velocities when ultrafiltration operated at 0.05 m/s liquid velocity and θ = 0◦ . It is noted that the addition of gas slug into the liquid stream is not an effective way on enhancing the flux in this case. When the gas velocity is
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6000
9000
Time (sec) Fig. 4. Flux profile of conventional ultrafiltration (uG = 0) installed at θ = 180◦ .
10.0
Dextran T500 solute C = 3 g/l P = 2 bar uL = 0.05 m/s MWCO = 10K θ = 0o
0 m/s 0.002 m/s 0.019 m/s
Jv X106(m3/m2-s)
8.0
uG
0.053 m/s
6.0
0
3000
6000
9000
Time (sec) Fig. 5. Effect of gas velocity on ultrafiltration flux as membrane installed at θ = 0◦ .
17
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T.-W. Cheng, C.-T. Lin / Separation and Purification Technology 39 (2004) 13–22
low, the gas slug passing the membrane module is near to the upper plate or away from the membrane surface. Thus, the gas slug cannot disturb the concentration polarization layer effectively but increases the pressure drop in the module leading to decreasing the transmembrane pressure. Therefore, the flux of two-phase ultrafiltration is lower than the conventional ultrafiltration until the gas velocity is high enough, e.g. 0.053 m/s, that can disturb concentration boundary layer in compensation for the loss of transmembrane pressure. Fig. 6 shows the flux under effect of gas slugs when membrane was installed at θ = 90◦ . The introduction of gas slugs enhances flux in this case. The gas slug passing the membrane module should locate symmetrically to the centerline of a vertical flow channel when the wall is impermeable. However, in this membrane module the wall suction is asymmetric, the passing gas slug will deviate from the centerline and more close to the membrane wall. Gas slugs have a slight effect on disturbing the concentration polarization layer when the gas velocity is low. At a high gas velocity, gas slugs move closer to the membrane wall because the size of gas slug increases, thus gas slugs affect effectively on the concentration boundary layer and the flux enhancement is relatively significant. Fig. 7 shows the permeate flux under various gas velocities when membrane was installed at θ = 180◦ . It is noted that the addition of gas slugs into the liquid stream is very effective in enhancing the flux. At this membrane orientation, the gas slug passing the membrane module is near to the membrane surface. The gas slug can disturb the concentration polarization layer and improve the flux effectively. The flux enhancement increases with the increase of gas veloc-
ity owing to the increase of cross-flow velocity and frequency of the gas slugs. 3.3. Membrane orientation at various gas velocities Fig. 8 shows the permeate flux under different membrane orientation when the gas velocity was 0.002 m/s. The steady fluxes of θ = 90◦ and θ = 180◦ are almost equal and higher than that of θ = 0◦ . The effect of membrane inclination is significant as the membrane orientation changed from θ = 0 to 90◦ or 180◦ even when a small amount of gas slugs were introduced. The flux result also implies that the gas slug position located in the flow channel is similar between the cases of θ = 90◦ and 180◦ . As the gas velocity increased to 0.019 m/s, the effect of membrane inclination on the permeate flux is enhanced and the difference between the effects under θ = 90◦ and 180◦ is significant, as shown in Fig. 9. The gas slug position will deviate more from the centerline towards the membrane surface as the gas velocity increases, particularly for membrane installed at 180◦ position. Fig. 10 shows the flux variations at a higher gas velocity, uG = 0.053 m/s. The permeate flux of θ = 180◦ is still significantly larger than that of θ = 90◦ . The flux data also shows that the flux decline over time is slight in the case of 180◦ inclination because the gas slugs reduce the filtration resistance effectively. Therefore, membrane inclination not only enhances but also sustains the flux of the gas-sparging ultrafiltration. Experimental result of this work indicated that membrane installed at the upper position in an asymmetric permeable module is an optimal orientation in enhancing the ultrafiltration flux. The hydrodynamic characteristics of gas–liquid two-phase flow in an
Table 1 Steady state fluxesa under various experimental conditions uG (m/s)
uL (m/s) 0
0 0.002 0.019 0.053 a b
0.02
0.05
0◦b
90◦b
180◦b
0◦b
90◦b
180◦b
0◦b
90◦b
180◦b
0.64
1.07
3.63
4.06
4.48
4.91
5.98 5.59 5.55 5.98
6.04 6.66 6.55 7.90
6.09 6.68 7.05 9.18
The flux in unit of 10−6 m3 /m2 s. Angle θ.
T.-W. Cheng, C.-T. Lin / Separation and Purification Technology 39 (2004) 13–22 14.0
Dextran T500 solute C = 3 g/l P = 2 bar uL = 0.05 m/s MWCO = 10K θ = 90o
Jv X106(m3/m2-s)
12.0
uG 0 m/s 0.002 m/s 0.019 m/s
10.0
0.053 m/s
8.0
6.0
0
3000
6000
9000
Time (sec) Fig. 6. Effect of gas velocity on ultrafiltration flux as membrane installed at θ = 90◦ .
14.0
Dextran T500 solute C = 3 g/l P = 2 bar uL = 0.05 m/s MWCO = 10K θ = 180o
Jv X106(m3/m2-s)
12.0
uG 0 m/s 0.002 m/s 0.019 m/s 0.053 m/s
10.0
8.0
6.0
0
3000
6000
9000
Time (sec) Fig. 7. Effect of gas velocity on ultrafiltration flux as membrane installed at θ = 180◦ .
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T.-W. Cheng, C.-T. Lin / Separation and Purification Technology 39 (2004) 13–22 10.0
Dextran T500 solutes C = 3 g/l P = 2 bar uL = 0.05 m/s uG = 0.002 m/s MWCO = 10K
Jv X106(m3/m2-s)
8.0
θ 0o 90 o 180 o
6.0
0
3000
6000
9000
Time (sec)
Fig. 8. Effect of membrane inclination on ultrafiltration flux: uG = 0.002 m/s.
10.0
Jv X106(m3/m2-s)
20
Dextran T500 solutes C = 3 g/l P = 2 bar uL = 0.05 m/s uG = 0.019 m/s MWCO = 10K
8.0
θ 0o 90 o 180 o
6.0
0
3000
6000
Time (sec)
9000
Fig. 9. Effect of membrane inclination on ultrafiltration flux: uG = 0.019 m/s.
T.-W. Cheng, C.-T. Lin / Separation and Purification Technology 39 (2004) 13–22
Dextran T500 solutes C = 3 g/l P = 2 bar uL = 0.05 m/s uG = 0.053 m/s MWCO = 10K
14.0
Jv X106(m3/m2-s)
12.0
21
θ 0o 90 o 180 o
10.0
8.0
6.0
0
3000
6000
9000
Time (sec) Fig. 10. Effect of membrane inclination on ultrafiltration flux: uG = 0.053 m/s.
asymmetric permeable module need further study in order to quantitatively clarify the influence of membrane inclination on the two-phase ultrafiltration.
4. Conclusion The influence of the membrane inclination on the permeate flux of ultrafiltration was investigated experimentally. The experiments were carried out in an asymmetric flat-plate membrane module by using aqueous dextran T500 solution as tested solution. The permeate fluxes were measured under various liquid flow rates, gas flow rates and membrane inclinations. For dead-end ultrafiltration, membrane inclination will create natural convection that disturbs the concentration boundary layer affecting the permeate flux significantly. In cross-flow conventional ultrafiltration, the enhancement in permeate flux is mainly dominated by forced convection or cross-flow velocity. The natural convection induced by varying membrane inclination is relatively weak as the flow velocity increases.
Membrane inclination is important in enhancing and sustaining the flux of gas–liquid two-phase ultrafiltration system. The position of gas slug is close to the membrane surface when the membrane orientation is installed at 180◦ inclination. Thus, gas slugs disturb the concentration boundary layer effectively resulting in a high flux enhancement.
Acknowledgements The authors wish to express their thanks to the National Science Council of Taiwan for financial aid.
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