Intensification of cross-flow membrane filtration using dielectrophoresis with a novel electrode configuration

Journal of Membrane Science 448 (2013) 256–261

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Intensification of cross-flow membrane filtration using dielectrophoresis with a novel electrode configuration F. Du, P. Ciaciuch, S. Bohlen, Y. Wang, M. Baune, J. Thöming n Center for Environmental Research and Sustainable Technology (UFT), Leobener Str. UFT, 28359 Bremen, Germany

art ic l e i nf o

a b s t r a c t

Article history: Received 21 December 2012 Received in revised form 13 June 2013 Accepted 9 August 2013 Available online 20 August 2013

Fouling can be a serious problem in membrane filtration processes. Here we present a solution that avoids addition of chemicals and allows for uninterrupted operation of cross-flow micro-filtration process. A novel electrode configuration is suggested for generating an inhomogeneous electric (ac) field for repelling particles from the membrane by means of dielectrophoresis (DEP). Undesired electrochemical reactions are avoided even in case of aqueous suspensions due to a sufficiently high frequency of the applied field (200 kHz). The inhomogeneous electric field generated by interdigitatedly installed cylindrical electrodes (IDE) could move clay particles (size 100–3000 nm) away from membrane (pore size 0.2 mm), thereby reducing fouling in membrane filtration process. A specific cosine-wave like DEP force distribution induced by the IDE along the feed flow was observed to allow for enhancing the DEP fouling suppression by removing the particles agglomerate adhered to the membrane at lower electric field regions. The fouling suppression performance of this new DEP filtration process was revealed comparing experimentally the permeate flux using same membrane with and without electrical field. With continuous DEP operation period with more than 69% of initial permeate flux was 9 times higher. In this lab-scale process with feed flow rate of 12 mL/min, Joule heating was observed elevating permeate temperature with rate of 3 K/h indicating a demand for electrical power (dissipated as heat) of about 2.5 W. & 2013 Elsevier B.V. All rights reserved.

Keywords: Suppression of fouling Dielectrophoresis Interdigitated electrode configuration Intensification of cross-flow micro-filtration

1. Introduction Membrane filtration process is being applied in separation industries as one of the most important technologies. However, a significant decrease of permeate flux, mainly caused by two phenomena: concentration polarization and fouling, always occurs in membrane filtration process [1]. Membrane fouling can be minimized by the pretreatment, the membrane stacking structure and geometry, mechanical washing, and chemical cleaning [2–6]. However, the mechanical washing, such as backwashing and backpulsing, has to interrupt operation of membrane filtration [7]. In addition, the chemicals used in the chemical cleaning often result in damage of membrane, and potential secondary pollution [8]. Therefore, the application of additional forces for suppressing fouling has been attracting more and more attentions recently [4]. These additional forces are generated by fields spatially superimposed over membrane, such as ultrasonic field and electric field. Both fields could suppress fouling without interrupting process. However, the ultrasonic intensity must be controlled carefully to avoid

n Corresponding author. Tel.: þ 49 421 218 63300, þ 49 421 218 63301, þ49 421 218 63371; fax: þ 49 421 218 8297. E-mail address: [email protected] (J. Thöming).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.08.016

damage of membrane, and its bulky system together with the difficulty in integrating it into a membrane filtration system is an obstacle for this method to be realized in industry [4]. In the application of electric field for reducing fouling in membrane filtration, two mechanisms were proposed [6]. One, termed as electrofiltration, is theoretically based on electrophoresis (EP), an effect of charged particle movement produced by Coulomb force in a homogeneous electric field [9]. The application of direct current (dc) electric field in filtration module will repel negatively charged particles from the membrane, under which a cathode is installed, so as to reduce the contact chances of particles with membrane [9–13]. Another electrokinetic effect, dc electro-osmosis, may contribute large contribution to the total permeate flux due to the transport of liquid through membrane and cake layer due to the electric potential through both porous media in some systems [13–16]. Besides suppressing fouling, electrofiltration can be applied in enhancing rejection of some specific charged substances such as total organic content (TOC) [12], benzophenone-3 (BP3) [17], and microbial poly-(3-hydroxybutyrate) [18]. The major problems of this technique are the electrodes corrosion caused by potential electrochemical reaction [12,19] and the high risk of electric circuit and human electric shock [20]. In order to avoid electrodes corrosion, inert metals are often

F. Du et al. / Journal of Membrane Science 448 (2013) 256–261

used as electrode material [12], which in turn increases cost. Even so, the water hydrolysis will occur in aqueous suspension with high conductivity as 1 mS/cm, and thereby increasing temperature and changing properties of feed [12]. Molla and Bhattacharjee presented another method for suppression of fouling using inhomogeneous alternating current (ac) electric field and experimentally validated its possibility in a labscaled experimental setup without membrane involved [21,22]. This method is theoretically based on dielectrophoresis (DEP), a motion of neutral particle caused by dielectric polarization in inhomogeneous electric field [23]. With the polarization of particle in electric field, a dipole moment is induced on the particle, which can be represented as two equal but opposite charges at the particle boundary [24]. The nonuniformly distributed charges on the particle surface hence generate a macroscopic dipole. If the electric field is inhomogeneous, the local electric field and resulting force on both sides of particle are different, and thereby a net force arises, termed as dielectrophoretic force FDEP. For a spherical particle with a radius of a suspending in a medium with dielectric constant of εm in an electric field E, FDEP is given as, F DEP ¼ 4π a3 ε0 εM re½K~ ðE∇ÞE where a is particle radius, the value of 8.85410

–12

ð1Þ

ε0 is the permittivity of free space with

h i F m  1, re K~ is the real part of Clausius–

Mossotti factor K~ , a parameter with a magnitude between 0.5 and 1 defining the effective dielectric polarizability of the particle, and E is electric field intensity [23]. The Clausius–Mossotti factor is a function of frequency of the electric field, depending upon the particle and medium's dielectric properties as,

ε~ P ε~ M K~ ¼ ε~ P þ 2ε~ M

ð2Þ

where the complex permittivities of particle and medium ε~ P and ε~ M can be given as, js

ε~ ¼ ε ϖ

ð3Þ

In which pffiffiffiffiffiffiffis is conductivity of medium or particle respectively with j ¼ 1 and ω ¼ 2π f . The (geometric) gradient of the square of the field intensity  ðE∇ÞE ¼ 12∇Ej2 , which solely determines the dielectrophoretic force for a given suspension at a certain frequency, as an example of cylindrical electrode configuration can be given [23], ∇jEj2 ¼

2U 2M

  2 r 3 ln rr12

ð4Þ

where UM is the voltage across medium, r is the distance between particle and electrode, r1 is the radius of central electrode, and r2 is the characteristic length of electrode configuration. Depending on the difference of permittivities between particle and medium at a certain frequency of electric field, the dielectrophoretic force moves particle either toward lower electric field region (larger permittivity of medium than that of particle), presenting negative DEP effect, or toward higher electric field region (lower permittivity of medium than that of particle), presenting positive DEP effect. Due to the lower polarizability of most of particles compared to that of aqueous medium, the particle could be moved away from membrane in an inhomogeneous electric field. Du et al. experimentally validated the feasibility of DEP application using alternating current (ac) in intensifying micro-membrane filtration in inhomogeneous electric field generated by two oppositely installed electrodes [6].

257

In contrast to electrofiltration, insulated electrodes can be used in ac-DEP system for protecting electrodes and avoiding electrochemical reactions as well as the risk of short circuit and human electric shock. The electrochemical reactions on the bare electrodes used in electrofiltration result in high electric current. Hence, the current in electrofiltration is much higher than that in DEP system, in which electrodes are insulated. However, due to a high-pass-filter effect resulted from the application of insulated electrode in an aqueous DEP system [6], the voltage required by DEP system is often very high. It results in an approximately 100 times higher voltage required by the DEP system than that often applied in electrofiltration process. Consequently electrofiltration requires lower voltage than DEP system but higher currents. Although the work from Du et al. demonstrated a 2-fold and 3.3-fold increased permeate flux with lower energy consumption by using continuously and pulsed applied DEP respectively [6], the concept reported is limited. This is because of a very small distance between electrodes, which is required for a sufficiently high DEP force (Eqs. (1) and (4)) and limits both the filtration throughput, and membrane modules design. This limitation of such systems with electrodes that are oppositely installed with the membrane in-between does not allow for an operation in industrial scale. An alternative electrode concept is required. A DEP system with low electric potential requires very small electrodes distance to generate high electric field, which could not be realized yet in case of high flow rates. On the other hand, a DEP system, which is based on a high electric potential and a large distance between electrodes, requires an energy input that is accordingly high. A novel electrode configuration is required for trade off the two opposing parameters, voltage and distance between electrodes, so as to realize scaling up DEP application in suppression of fouling. In this study, we present a novel designed electrode configuration, which could realize high DEP force using small electrode distance so as to increase the intensification function of DEP on the cross-flow membrane filtration process (DEP-MF) by suppressing the fouling, while being able to be scaled up for industrial application.

2. Materials and methods 2.1. Electrode configuration Interdigitated electrode (IDE) configuration (Fig. 1) is applied in this work. Two wires (0.6 mm diameter cylindrical with insulation film of 0.03 mm thickness) insulated by polyurethane (PU) with a relative dielectric constant of about 3, as shown in Fig. 1b, were installed interdigitatedly on a circuit board with a distance of 1 mm between two electrodes. The simulation performed using OpenFoam demonstrates that the area around electrodes presents higher electric field strength, while both regions between electrodes (in the direction of x in Fig. 1b) and far away (in the direction of H in Fig. 1b) from electrodes show weaker electric field, as presented in Fig. 1b. Due to the negative DEP of particles in aqueous medium, the repulsive DEP force on particles is only required to be functional on the surface of membrane. Therefore, the application of IDE could keep small distance between electrodes and similar voltage input while providing sufficient DEP force to move particles away from membrane and the opportunity to scale up simply by multiplying the number of interdigitatedly installed wires. In order to avoid electrode corrosion from electrochemical reaction and high risks of short circuit and human electric shocking with bare electrodes, insulated electrodes were applied. However, the both insulated electrodes present a high-pass-filter

258

F. Du et al. / Journal of Membrane Science 448 (2013) 256–261 2 E / [V 2m--3]

4.18E15 3

H / [mm]

1E14 2

1E12 1

0

1

2

1E10 5.14E09

3

X / [mm]

Fig. 1. The top view (a) and the front view (b) of the interdigitated electrode configuration, and the simulation of electric field gradient distribution generated by two wires (white circles) in the IDE configuration with a voltage input of 200 Veff at frequency of 200 kHz using OpenFoam (c). In a DEP intensified cross-flow filtration process, membrane (indicated as in (b) and (c)) is mounted on electrodes.

1 Suspension 0.9

2

0.8 Retentate

0.7

4 3

1

0.6 0.5 0.4 Permeate

200 Veff, 200 kHz

0.3

0.1 0 102

Fig. 3. Sketch of DEP intensified cross-flow membrane filtration process, which is composed of membrane filtration module (1), pump (2), membrane (3), and two electrodes (4), which are interdigitatedly installed below membrane and connected to ac power supply (200 Veff, 200 kHz).

0.03 mm 0.125 mm 0.25 mm 0.5 mm

0.2

103

104

105

106

107

Fig. 2. Comparison of influence of high pass filter effect between IDE configuration and electrode configuration applied in the work of Du et al. due to different insulation thicknesses 30 mm in IDE configuration and 250 mm in the work of Du et al. [6]. Dashed line indicates the working frequency (200 kHz) used in this work and in the work of Du et al.

effect, which limited voltage output through aqueous medium (Um) at low frequency [25]. The voltage fraction (Um/U0) between voltage across medium Um and applied voltage U0, increases with the increase of frequency, as presented in Fig. 2. This frequencydependent voltage fraction depends on the dielectric constant and thickness of insulation material [6,26]. When dielectric constants of insulation materials are identical, the voltage fraction increases with the decrease of insulation material thickness. As presented in Fig. 2, the voltage fraction in this work is about 0.99 at the frequency of 200 kHz when the thickness of insulation material is 30 mm. As a comparison, one electrode was insulated with 250 mm thick insulation material in the work of Du et al. for reaching the voltage fraction of 0.8 at 200 kHz frequency [6]. Hence, with identical voltage applied, the new IDE configuration could provide a higher voltage across medium. An increment of 25% of voltage results in a DEP force 1.56 times higher than that with thicker insulation material (Eq. (4)). This is the case when

replacing the relative thick insulation of the standard process by a thinner one in connection with the new IDE configuration. The voltage fraction Um/U0 of the standard case is 0.8, and if U0 is equal to 200 V, the voltage across medium results to 160 V. With thinner insulation however, the voltage fraction is 0.99, hence, the voltage across medium reaches about 200 V and thus DEP force is enhanced by 56% at similar energy consumption. However, it is noticed from Fig. 2 that the high frequency as 200 kHz is still required. With the increment of frequency, the energy consumption will be increased due to the reduced impedance and thereby increased current in the system.

2.2. Materials and setup The experimental setup (Fig. 3) was composed of a cross-flow membrane filtration cell (1) made of Plexiglass with a filtration area of 0.0031 m2, 0.116 m long and 0.026 m wide and a pump (2) (Harton DC 5/15, transmembrane pressure 1.2 bar). A flat membrane (3) (FM MV020, 0.2 mm, PVDF, NADIRs), was mounted on the IDE (4), connected to a power amplifier (FM 1290, FM ELECTRONIK BERLIN) signaled by a function generator (VOLTCRAFTs 7202) for sinewave electrical signal up to 2 MHz with an effective voltage range between 0 and 280 Veff.

F. Du et al. / Journal of Membrane Science 448 (2013) 256–261

A suspension of clay in demineralized water was used with a concentration of 5 g/L and particles size ranging from 100 to 3000 nm [6], for reaching a severe fouling problem. The permeate flux was measured in the process with and without electric field for examining the intensification effect from DEP in micro-membrane filtration process. In the process with DEP, the suspension was fed into the filtration cell at a volume flow of 12 mL/ min, with electric field switched on. Due to the negative DEP effect, clay particles were levitated and moved away from membrane and then drifted along the medium flow as retentate, while the pure water was separated out of the filtration system as permeate. Both retentate and permeate were collected and volumetrically measured in a certain process time. The time-averaged permeate and retentate fluxes were calculated by dividing the collected volumes of permeate and retentate by the process time. Two processes, continuous DEP (continuously applied DEP) and pulsed DEP (intervallic applied DEP) with 5 min electric field on and 15 min electric field off (Pulsed DEP 5-15), were examined and compared.

3. Results and discussion Without electric field applied in the cross-flow microfiltration process (MF), the normalized permeate flux, which is obtained by relating the flux data to the initial permeate flux in the first minute, reduced to about 41% after six hours of operation (Fig. 4). This can be assumed to occur due to blockage of membrane pores by particles with diameters similar to those of pores. Though with continuous application of electric field the diminishing rate of permeate flux decreased. During six hours of filtration the normalized flux reduced down to 69% of the initial permeate flux. By assuming the stead-state flux as threshold (69% in this case) to be the minimum relative flux needed for operation we obtained a number which we called service time of filtration process before next backwashing is required. From Fig. 4 it can be depicted that for DEP-MF service time is 360 min of operation. In contrary without DEP, service time diminished to 40 min and the normalized permeate flux decreased further down to 41% after 360 min of operation. This means that the process without DEP can provide normalized permeate flux above 69% only for 40 min. By comparing both processes, MF and DEP-MF, the process with DEP provides nine-fold longer service time. Further, after 360 min of 100

Normalized permeate flow [%]

90 80 69 60 50 40 30

without DEP

20

Results: Trend lines:

10 0

With DEP

0

50

100

150

200

250

300

350

400

Time t / [min] Fig. 4. Comparison of normalized permeate fluxes between with and without continuous DEP intensified cross-flow membrane filtration processes using (IDE) configuration. Dashed line indicates a threshold (69%) as the minimum relative flux needed for operation.

259

operation, normalized permeate flux in continuous DEP is 1.68 times above the flux without DEP. An intensification factor (IF) is defined as ratio of membrane service time between with and without applying DEP. In our former DEP-MF [6] we made use of oppositely installed electrodes (OE). This led to an IF of about 2. Using this set-up as base case and comparing the IF of our actual DEP-MF-IDE (IF¼9) with the IF of the former DEP-MF, it is 4.5 times higher. Further the IDE process resulted in a much lower decrease of normalized permeate flux: after 360 min of operation 160% of the flux of DEP-MF-OE was obtained. In both studies (IDE and OE) we used identical membranes. By comparing here normalized permeate fluxes the enhanced intensification factor, when using DEP-IDE instead of DEP-OE, presents the real improvement that was achieved by the new electrode configuration. This obvious improvement is mainly resulted from the newly designed IDE configuration. On the one hand, IDE electrode array allows smaller electrode distance with no influence on filtration module and application of thinner insulation film for reducing the influence of high-pass-filter effect. Hence the electric field gradient squared and the DEP force is increased, even if the electrode distances in both electrode configurations are equal. On the other hand, the very low electric field gradient squared at the mid between two electrodes (Fig. 1c) in the IDE turns out to form a regularly changing DEP force field. The force field distributes in a way such like a cosine wave along the direction of feed flow, with highest DEP force on the electrode (peak) and lowest DEP force in the mid between two electrodes (valley), as presented in Fig. 5(e). Due to insufficiently high DEP force on smaller particles, smaller particles move to the membrane caused by the permeate flow. The polarization of membrane and particle in the strong electric field induces an electrostatic force, which attracts small particles to the membrane and hence holds them on the surface of membrane, when particles get in contact with the membrane, as presented in Fig. 5(a) [6]. While more particles getting close to those adhered particles on the membrane, a pearl-chain effect occurs by aligning particles along the electric field and thereby forming particles agglomerate (Fig. 5(b)). DEP force could not move the agglomerate from membrane, because the electrostatic force together with viscous drag force induced by permeate flow is stronger than DEP force. The agglomerated small particles can be released easier from membrane surface by the feed flow and moved along the direction of feed flow (Fig. 5(c)) in the region with lower electric field (at the mid between electrodes) because the induced electrostatic force due to polarization is lower in a low electric field (Fig. 5(c)) [6]. As they are moved close to higher electric field region again, the sufficiently high DEP force on these particles with increased size due to agglomeration will move them away from membrane, and thereby increasing the permeate flux, as shown in Fig. 5(d). The cosine-wave distribution of DEP force (Fig. 5(e)) along the feed flow could be achieved by interval application of electric field. The similar effect was observed and demonstrated for pulsed DEP [6]. In pulsation intervals with 5 min electric field on and 15 min electric field off (DEP 5–15), the membrane working time for reaching a 50% normalized permeate flux was 3.3 times longer than the process without DEP and about 1.6 times longer than the process with continuous DEP, in which oppositely installed electrodes were used and thereby with no cosine-wave DEP force distribution [6]. Using the identical pulsation 5–15, the system with IDE configuration demonstrated an identical fouling suppression performance as in continuous DEP process (Fig. 6). With the electric field off, the measured normalized permeate flow values were larger than those in the process without DEP, but below the values collected in the process with continuously applied DEP. Reversely, the much higher permeate flow compared to those in the continuous DEP process was observed with the electric field on, as shown in Fig. 6. The plotted trend line for the normalized permeate flux in process of pulsed 5–15

260

F. Du et al. / Journal of Membrane Science 448 (2013) 256–261

Fig. 5. Mechanism of particle deposition on membrane–electrode assembly and removal by inhomogeneous electric field generated by interdigitatedly installed electrodes (white circles). The simulated DEP force field is indicated with the electric field gradient squared, ∇jEj2 , which is directly proportional to DEP force. (a) Small particles are moved towards the membrane due to viscous drag of permeate flow, while big particle is removed away from membrane by dominating DEP effect. Black arrows indicate particle motion directions. (b) Particles which adhere to the membrane as such they preferentially attract other particles forming pearl-chain [6]. Due to the viscous drag of feed flow, particles on the membrane are mobile. (c) In regions of lower DEP force, agglomerates are less strongly bound to the surface of the membrane due to the lower electrostatic force induced by the lower electric field, and can be released easier from membrane surface by feed flow. (d) At region with higher electric field, grown agglomerates now experience an increased DEP force, which rises cubically with particle size being elevated. (e) Cosine-wave DEP force distribution along the feed flow direction.

100

Normalized permeate flow [%]

90 80 70 60 50 40 30 With continuous DEP Pulsed DEP 5-15

20

Results

10 0

Trend lines 0

50

100

150

200

250

300

350

Time t / [min] Fig. 6. Normalized permeate flow comparison between continuous DEP and pulsed DEP 5–15, in which electric field was switched on for 5 min, and then off for 15 min.

is nearly identical to the one for the process with continuous DEP. It hints that the IDE configuration in continuous DEP process can present an identical effect to the one shown in the process with pulsed DEP. This in turn proves the hypothesis that the agglomerated particles adhered on the membrane can be alleviated by the lower electric field area in the mid between electrodes in the IDE configurations, and experience a much stronger DEP force due to the grown particle size.

4. Conclusions In this work, an IDE configuration is presented for intensifying cross-flow membrane filtration based on the DEP. The suppression

effect of DEP using IDE was examined experimentally in a labscaled cross-flow micro-membrane filtration cell. With continuous DEP a 9-fold longer service time for a permeate flux of at least 69% of initial was observed compared to the process without DEP. Compared to systems with electrodes that are oppositely installed with the membrane in-between [6] the intensification factor using IDE was about 4.5 and 2.7 times higher for continuous DEP and pulsed DEP 5–15 respectively. The identical intensification function in the process using IDE was observed, when pulsed DEP was applied with a quarter of energy consumption. The cosine-wave DEP force distribution generated by the IDE alleviated the particles adhesion to the membrane at lower DEP force areas, and thereafter repelled agglomerated particles away from membrane at higher DEP force areas, presenting identical effect as in a pulsed DEP process. The application of IDE in cross-flow membrane filtration could be scaled up by simply integrating electrodes into filtration modules with no obstacle in operability. However, due to the high-pass-filter effect mainly caused by the low dielectric constant of insulation film, a high frequency as 200 kHz was still required in this work, which resulted in high energy consumption and high investment in the high-frequency power amplifier. In this lab-scaled process, Joule heating was observed, which increased temperature at a rate of 3 K/ h. It turns out to be an energy consumption of 2.5 W as the heat dissipated. Therefore, further research is needed for a new insulation material with a higher dielectric constant for reducing the influence of high-pass-filter effect thereby decreasing the energy consumption and process costs. An estimate of capabilities and limitations of the method can be deduced from the model described above (Eqs. (1)– (4)). Even in case of a very small difference of permittivities between particles and aqueous medium, negative DEP motion can theoretically be realized. Than the Clausius–Mossotti factor, which indicates the difference of permittivities between particle and medium, defines direction of particle motion. However the magnitude of DEP motion speed for

F. Du et al. / Journal of Membrane Science 448 (2013) 256–261

certain particle with constant size is mainly determined by electric field gradient squared and size of the particle (Eq. (1)). In case of very small ‘particles’ such as hydrophilic macromolecules their speed can be expected to be quite low. The suppression of fouling by macromolecules using this method can be achieved by increasing electric field gradient and changing medium conductivity. In addition, an appropriate frequency of electric field has to be found for achieving maximum difference of permittivities between particle and medium, and this frequency has to be above a critical value set by high pass filter effect of water. However, this method is not suitable for suppressing fouling by very conductive particles, presenting positive DEP effect, such as pure metal particles. In general this approach shows that fouling of membranes can be reduced or even avoided if all parameters in the system are optimized for providing sufficient DEP force to remove particles.

Acknowledgment The authors wish to acknowledge the financial support from the German Federal Ministry of Economics and Technology (BMWi) through the AiF ZIM-KOOP programme (Grant no. KF2162601) and Dr. Lars Dähne (Surflay Nanotec GmbH Berlin) for fruitful discussion.

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