Ind. Eng. Chem. Res. 1992,31,515-523 Science; Buckingham, A. D., Ed.; University Park Press: Baltimore, 1973;Vol. 6,pp 41-113. Crundwell, F. K. The Influence of the Electronic Structure of Solids on the Anodic Dissolution and Leaching of Semiconducting Sulphide Minerals. Hydrometallurgy 1988,21,155-190. Felker, D. L.; Bautista, R. G. Electrochemical Processes in Recovering Metals from Ores. JOM 1990,42(4),60-63. Gerischer, H. Semiconductor Electrochemistry. In Physical Chemistry; Eyring, H., Ed.; Academic Press: New York, 1970;Vol. E A , pp 463-542. Goodisman,J. Electrochemistry: Theoretical Foundations; WileyInterscience: New York, 1987;pp 275-286, 294-299, 357-364. Hamnett, A. Semiconductor Electrochemistry. In Comprehensive Chemical Kinetics; Compton, R. G., Ed.; Elsevier: New York, 1987;Vol. 27,Electrode Kinetics: Reactions, pp 61-246. Kimura, R. T.; Haunschild, P. A.; Liddell, K. C. A Mathematical Model for Calculation of the Equilibrium Solution Speciations for the FeC13-FeC12-CuC12-CuCl-HC1-NaCl-Hz0 System at 25 "C. Metall. Trans. B. 1984,15B,213-219. King, J. A.; Burkin, A. R.; Ferreira, R. C. H. Leaching of Chalcocite by Acidic Ferric Chloride Solution. In Leaching and Reduction in Hydrometallurgy; Burkin, A. R.; Ed.; Institution of Mining and Metallurgy: London, 1975; pp 36-45. MacKinnon, D. J. Fluidised-bed anodic dissolution of chalcocite. Hydrometallurgy 1976,1,241-257.
515
Memming, R. Processes at Semiconductor Electrodes. In Comprehensive Treatise of Electrochemistry; Conway, B. E., Bockris,J. O'M., Yeager, E., Khan, S. U. M., White, R. Ed., Eds.; Plenum Press: New York, 1983;pp 529-592. Paramguru, R. K.; Sircar, S. C.; Bose,S. K. Electrode kinetics studies on compacted CuzS electrodes in perchlorate baths. Trans. Indian Znst. Met. 1983,36,114-120. Parikh, R. S. Orthogonal Collocation Simulation of the Rotating Ring Disk Electrode and Ita Application in the Anodic Dissolution of Chalcocite. Ph.D. Dissertation, Washineton State Universitv. _. Pullman, 1988. Parikh, R. S.;Liddell, K. C. Mechanism of Anodic Dissolution of Chalcocite in Hydrochloric Acid Solution. Znd. Ena. - Chem. Res. 1990,29,187-193. Peters, E. Electrochemistry of Sulphide Minerals. In Trends in Electrochemistry; Bockris, J. OM., Rand, D. A. J., Welch, B. J., Eds.; Plenum Press: New York, 1977;pp 267-290. Shuey, R. T. Semiconducting Ore Minerals; Elsevier: New York, 1975. Srinivasan, V. Mechanism for Anodic Dissolution of Chalcocite in Cupric Chloride Solution. M.S. Thesis, Washington State University, Pullman, 1990. 1
Received for reuiew September 29, 1991 Accepted October 27,1991
MATERIALS AND INTERFACES Pulsed Electrokinetic Cleaning of Cellulose Nitrate Microfiltration Membranes W. Richard Bowen* and Hoze A. M. Sabuni Biochemical Engineering Group, Department of Chemical Engineering, University College of Swansea, University of Wales, Swansea SA2 8PP,U.K.
An experimental study has shown that the use of pulsed electric fields can be an effective means of removing particulate materials from polymeric membrane filters and hence of improving the rate of cross-flow filtration. Pulsed electrophoretic membrane cleaning can improve filtration rates by factors of up to 4.8 under the conditions studied, and pulsed electroosmotic membrane cleaning is moderately effective, if membrane and particles have the same sign of {-potential. The likely success of pulsed electrophoretic membrane cleaning is in qualitative agreement with a force balance model under such conditions. For particles and membranes of differing sign of {-potential, such particles being partly aggregated, neither process was very successful a t improving filtration rates, the effecb being generally less favorable than predicted by a force balance model. Pulsed electrophoretic membrane cleaning is a promising process meriting further investigation. 1. Introduction The most important disadvantage of conventional cake filtration is the declining rate due to the growth of the cake on the filter medium. The flow rate of the liquid through the medium can be kept high only if little or no cake is allowed to form on the medium. A number of ways are available for reducing the amount of cake forming, but for fine particles and colloids the most important is cross-flow filtration (Svarvosky, 1990). Here the stationary membrane surface is swept by flowing the process fluid tangentially across it, normally at a velocity in the range of 2-8 m s-l.
* To whom correspondence should be addressed.
Cross-flow filtration processes are classified according to the size of particles separated, but particle surface electrochemical properties also need consideration. Most substancea acquire a surface eledrical charge when brought into contact with a polar (e.g. aqueous) medium. This may arise by ion dissociation, ion adsorption, or ion dissolution. In aqueous solutions, proton equilibria are especially important. The charge properties of membranes and process streams can have a significant effect on their separation characteristics (McDonough et al., 1989; Bowen and Hughes, 1990). The utilization of such properties by the application of external electric fields can potentially give substantial improvements in the performance of membrane filtration. Such processes make use of two electrokinetic phenomena: firstly, electrophoresis, the trans-
0888-5885/92/2631-0515$03.00/00 1992 American Chemical Society
516 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992
port of a charged surface relative to a liquid in an electric field, for example, the movement of particles; secondly, electroosmosis, the transport of a liquid relative to an immobile charged surface by an electric field, for example, the movement of an electrolyte solution through a membrane pore or filter cake (Hunter, 1981). The magnitudes of both phenomena depend on the applied electric field gradient and the potential at the surface of shear between the surface and the solution, the &potential. The filtration processes utilizing these phenomena are known collectively as “electrofiltration” or “electrically enhanced membrane processes”. Investigations of electrically enhanced membrane processes have usually been directed toward the use of continuously applied electric fields (Henry et al., 1977; Bowen and Turner, 1984, Visvanathan and Ben Aim, 1989). This approach, “conventional electrofiltration”, makes use mostly of the electrophoretic transport of the dispersed materials away from the membrane under the influence of the applied field. This can be an effective way of reducing both concentration polarization and membrane deposition. However, it has several disadvantages, including a high energy requirement, substantial heat production, and changes in the process stream due to reactions at the electrodes. The latter may be avoided by the use of modules in which the process feed is protected from the electrodes by ion-permeable membranes, though this complicates module design. The possible establishment of electrically enhanced membrane processes as acceptable unit operations will require the minimization of energy use and heat production. The use of such processes will also be facilitated if they can be carried out in modules closely comparable to those used conventionally for cross-flow microfiltration, especially if the electrodes may be placed directly in the process feed and permeate chambers. For these reasons, attention has been directed to the use of pulsed fields. If relatively infrequent pulses are effective, then the main drawbacks of continuous field application can be substantially diminished. We have described the development of an electrically enhanced membrane process using intermittent electric fields which could be applied to electrically conducting membranes, for example, stainless steel microfilters (Bowen et al., 1989). This could substantially improve filtration rates. An important advantage of such processes is that they can eliminate the need for high cross-flow velocities (Bowen and Sabuni, 1991). There have also been preliminary reports that the use of pulsed electric fields across nonconducting polymeric or inorganic membranes can lead to substantial improvements in filtration rates (Wakeman and Tarleton, 1987; Bowen and Goenaga, 1989). In microfiltration, materials deposited on the membrane surface often retain a surface charge and hence an electrophoretic mobility. It should therefore be possible to remove such materials by the periodic application of electric field pulses. This is the probable mechanism of the improvements in filtration rate described in the cited preliminary reports. Such processes may be described by the term “pulsed electrophoretic membrane cleaning”. One of the aims of the present paper is to present a more detailed experimental investigation and analysis of such a process. It is well-known that substantial improvements in membrane performance can result from temporary reversal of mass flow through the membrane, backwashing. Mechanical reversal of flow can be complex, so the use of electric fields to produce such reversal is a potentially attractive alternative. This type of process has been applied to reverse osmosis membranes (Spiegler and Ma-
cleish, 1981). Such “electroosmotic backwashing” was found to be an effective process with the advantage that only periodic application of the electric field was required. The only published report of the application of electroosmotic backwashing to cross-flow microfiltration used continuous alternating electric fields in a multiple stack filtration cell (Visvanathan and Ben Aim, 1990). Hence, the further aims of the present paper are to present an experimental investigation and analysis of “pulsed electroosmotic membrane cleaning” as applied to microfiltration and to compare the potential of this process with “pulsed electrophoretic membrane cleaning”. 2. Experimental Section Cross-flow filtration experiments were carried out in a module constructed in our department. The module was constructed in perspex and had a process feed chamber of dimensions 9.8 cm X 2.5 cm X 0.246 cm with an effective membrane working area of 23.0 cm2. The membranes, which formed one side of the process feed chamber, were supported on a porous polyethylene sheet of pore size 95 pm. This polyethylene rested on a platinized titanium mesh which acted as an electrode. The electrochemical circuit was completed by a platinized titanium sheet electrode mounted flush with the other side of the process feed chamber. Experiments were controlled by an IBM-compatible microcomputer fitted with an IEEE-448 interface. The power supply used for applying the electric field (Farnell Instruments Ltd. AP100-30) was linked directly to this interface. This power supply uses switching techniques which result in a fast rise time, 0.6 s under the experimental conditions, and a fall time of 1.0 s. Other components were linked via an A/D converter. These were a temperature control unit, pH probe, and flow measurement devices. The permeate flow, which for laboratoryscale studies of this type may be too low for reliable measurement by Pelton wheel devices, was measured using a level-sensing unit which recorded the time for the collection of a fixed volume of permeate, normally 20 mL. Experiments were carried out in a simple pump recirculation loop, using a gear pump (Micropump Model 221), both retentate and permeate being returned to the feed reservoir. The cross-flow velocity was 1.5 m s-l and the transmembrane pressure 130 kN me2. Cellulose nitrate membranes with a rated pore size of 0.2 pm were obtained from Sartorius GmbH. Titanium dioxide (technicalgrade) dispersions at 5 g L-l were made up in KN03 solutions at ionic strengths in the range of 10-1-10-4 M. The pH was adjusted to 8.0 or 3.5 by the addition of small amounts of KOH or HNOBsolutions. Reverse osmosis water was used in feed preparation. Experiments were normally of 3-h duration. The electrokinetic properties of the membranes were determined using an electroosmotic technique (Bowen and Clarke, 1984). These characterization measurements were carried out on 25-mm-membrane discs. Such measurementa require the application of a constant current across the membrane and the measurement of the resulting electroosmotic flow by means of an electronic balance connected to a microcomputer. The membrane electrolyte permeation rates were measured using 25-mm discs held in a 10-mL filtration cell connected to a solution reservoir with a maximum capacity of 1.0 L. The system was press& with nitrogen gas, and rates of permeation were determined by continuously weighing the permeate on an electronic balance connected to a microcomputer. The electrokinetic properties of the dispersed particles were determined using single-particle electrophoresis
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 517 2500
mk 2000 E
I
I
I
2500
I
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IC 2000
7 I?
i
2
\
1500
f 1500
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800
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2
400
4-
1
p 300 C
2 1 0
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I
I
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Figure 1. Data for the cross-flow microfitration of titanium dioxide dispersionsfor (a) EL and (b) OS: (0) pulsed electrokineticcleaning; (0) conventional filtration. lo-* M KNOB,pH 8.0.
equipment (Rank Brothers Mark JI). The size distribution of the particles was determined by laser diffraction (Malvern Mastersizer).
3. Results and Discussion 3.1. Electrically Enhanced Cross-Flow Microfiltration. Operating Patterns. Experiments have been carried out with 5 g L-’ titanium dioxide dispersions at four ionic strengths: lO-l, and lo4 M. At each ionic strength, experiments were carried out at pH 8.0 and pH 3.5. This gives a total of eight solution conditions which were chosen for the variation in solution and particle electrochemical properties which they produced. For each solution condition, three types of cross-flow microfiltration experiment were carried out: (i) cross-flow filtration with no in situ membrane cleaning-“conventional microfiltration”; (ii) electrically enhanced filtration with a cathode on the permeate side of the membrane and an anode on the feed side of the membrane (such an arrangement will be designated with the letters “EL” as particles in industrial process feeds most commonly have a negative {-potential so that this is the electrode arrangement most likely to be used for electrophoretic membrane cleaning; however, experiments with particles of positive {-potential will also be described); and (iii) electrically enhanced filtration with an anode on the permeate side of the membrane and a cathode on the feed side (such an arrangement will be designated with the letters “OS”as polymeric membranes most commonly have a negative {-potential so that this is the arrangement most likely to be used for electroosmotic membrane backwashing). In real situations the effects on application of an electric field are more complex than such a classification indicates. This will be discussed in more detail in a later section.
-2
200
=J If 100
0 0
40
80
120
160
200
Time/min
Figure 2. Data for the cross-flow microfitration of titanium dioxide dispersions for (a) EL and (b) O S (0) pulsed electrokineticcleaning; (0)conventional filtration. lo4 M KNOB,pH 8.0.
The measured rates of filtration for these three different types of operation varied widely as the solution conditions were changed. Unless otherwise indicated, the electric field was applied for 60 s every 10 min with an applied voltage of 100 V, the maximum obtainable with the power supply. Some typical data are shown in Figure la,b for dispersions in M KNOBat pH 8.0. In this case the time dependence for conventional microfiitration is compared to that for electrically enhanced filtration in the EL and OS modes. Each time that the electric field is applied in the EL mode (Figure la), there is a substantial increase in the filtration rate, followed by a decline. This demonstrates removal of deposited particles on application of the electric field followed by redeposition until the field is again applied. Under these solution conditions, there is also an improvement in the case of application of the electric field in the OS mode (Figure lb), though this is smaller in magnitude. Another pattern of behavior is shown in Figure 2a,b for dispersions in lo4 M KNOBat pH 8.0. Here the effect of electrically enhanced filtration in the EL mode is a very substantial increase in the filtration rate after the application of the electrical field (Figure 2a). However, in this case the application of an electric field in the OS mode at identical time intervals and for identical duration resulted in a lowering of the filtration rate (Figure 2b). Under some solution conditions, no improvement in filtration rate was measured in either the EL or OS mode. This is shown in Figure 3a,b for dispersions in M KN03at pH 3.5. In both cases there is an increase in the rate of filtration on application of the electric field, but this is followed by a rapid decrease in the rate to levels below that occurring in the conventional filtration experiment, which serves as a control.
518 Ind. Eng. Chem. Res.,Vol. 31, No. 2, 1992 1.5
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Figure 4. Electroosmoticcharacterization of cellulose nitrate membranes: ( 0 ) M KN03, pH 8.0,15.3 mA cm-2;(0) lo4 M KNOB, pH 8.0, 1.02 mA cm-2.
1600
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80 120 TI rne / rnIn
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Figure 3. Data for the croes-flow microfitration of titanium dioxide dispersions for (a) EL and (b) O S (-) pulsed electrokinetic cleaning; (- -) conventional filtration. M KNOB, pH 3.5. Discrete points are not shown due to substantial overlap at relatively high, constant filtration rates.
3.2. Electrically Enhanced Cross-Flow Microfiltration. Data Compilation. Data for experiments under all operating conditions are given in Table I. Solution conditions and cleaning type are specified. A voltage of 100 V was applied in each case, and the current reported is the average current at the start of the electric field pulses. This is determined by the electrical resistances in the cell due to the solution, filter cake, membrane, and membrane support. In most experiments 18 pulses were applied. The “average minimum filtration rate” is the average rate of filtration immediately before application of the electric field pulses. The “average maximum filtration rate” is the average rate of filtration immediately after application of the electric field pulses. The “average rate of filtration” is the time average for the whole experiment, including the time for which the electric field was applied. The “performance ratio” is the average rate of filtration for an electrically enhanced filtration experiment divided by the average rate of filtration for the conventional filtration experiment carried out under the same conditions. A performance ratio greater than 1indicates that the use of the electric field has been beneficial; a ratio less than 1indicates that the use of the electric field has been detrimental. The data at pH 8.0 are considered first. For the case of the EL mode of operation there were substantial improvements in the average rate of filtration in and lo4 M solutions with performance ratios in the range of 2.3-4.8. In the case of experiments in M solution, the effect of pulse duration was also investigated. For the EL mode, increasing the pulse duration from 15 to 60 s gave an increase in performance ratio from 2.6 to 4.8.
These durations correspond to electric field application for between 2.5 and 10% of the overall process time. Pulses of 60-sduration were used in comparing effects at different solution conditions, longer pulse times not being studied as one of the aims of the work was to reduce the overall time for electric field application by working with intermittent fields. In the lo-’ M solution the EL mode had a detrimental effect on the average filtration rate. The data at pH 3.5 showed a very different pattern. The only substantial improvement in the average filtration rate with the electrically enhanced process was observed for the M solution. A small improvement was EL mode in observed for the EL mode in lo-* M solution and a marginal improvement for the OS mode in lo4 M solution. In all other cases the effect of the electric field in both the EL and OS modes was detrimental. 3.3. Membrane and Particle Characterization. The electrokinetic properties of the membranes used can have an important effect on electrically enhanced filtration pruceaws. In the present work these have been determined by measuring the rate of electroosmotic flow at membrane discs. Typical data are shown in Figure 4, which shows the amount of electrolyte electroosmotically transported and through a cellulose nitrate membrane at pH 8.0 in loa M solutions as a function of time. The specific electroosmotic flow rate (ueOm)is calculated from the slopes of such plots. Data for all of the solution conditions studied are given in Table IIa. These data are in good agreement with our previous more-detailed studies of the electrokinetic properties of polymeric microfiltration membranes (Bowen and Cooke, 1990,1991). The other important membrane characteristic for the present work is the electrolyte permeation rate. This has been measured for all of the electrolyte conditions studied. The data are reported as resistance values (R,) in Table 111, where the resistance has been calculated using the relationship uom
= */ccR
om
(1)
where uom is the electrolyte permeation rate, AP is the applied pressure, p is the electrolyte viscosity, and R, is the resistance. When an electrolyte flows in a charged porous medium, a streaming potential is established that produces a back-flow of liquid by an electroosmotic effect. The net effect is a reduced flow in the forward direction, an example of an electroviscous effect (Hunter, 1981). Hence, electrolyte permeation rates are expected to vary
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 519 Table I. Application of Pulsed Electrokinetic Membrane Cleaning in the Cross-Flow Filtration of Titanium Dioxide Dispersions at Cellulose Nitrate Microfiltration Membranes ionic cleaning pulse filtration rate/(L m-2 h-l) performance strength/M type time/s current/(kA m+) av min av mas av ratio Data at pH 8.0 10-1 595 10-1 EL 60 1.18 490 1278 509 0.85 10-1 os 60 0.96 374 836 404 0.68 10-2 98 10-2 EL 15 0.49 196 373 254 2.58 os 15 0.68 149 162 159 1.61 10-2 10-2 EL 30 0.61 233 1045 365 3.71 10-2 os 30 0.66 142 167 157 1.60 10-2 EL 60 0.70 272 1711 474 4.82 10-2 os 60 0.83 153 212 178 1.81 10-3 170 10-3 EL 60 0.15 243 1912 480 2.82 10-3 os 60 0.091 150 151 155 0.91 10-4 221 10-4 EL 60 0.091 273 1867 508 2.30 10-4 os 60 0.040 170 174 176 0.80 Data at DH 3.5 10-1 10-1 10-1 10-2 10-2 10-2 10-3 10-3 10-3 10-4 10-4 10-4
EL
os
60 60
3.29 1.18
658 565
2808 1415
EL
os
60 60
0.59 1.13
471 600
1175 1056
EL
os
60 60
0.10 0.22
677 448
1045 458
EL
60 60
0.067 0.123
436 510
700 514
os
915 964 658 731 547 679 492 721 460 508 476 517
1.05 0.72 0.75 0.93 1.47 0.93 0.94 1.02
Table I1 (a) Membrane-SDecific Electroosmotic Flow Rates membrane-specific ionic electroosmotic flow RH strength/M rate/(pm d / ( A m-2)) -1.55 X lo-' 10-1 8 -0.141 10-2 8 10-3 -1.32 8 -5.14 10-4 8 -1.70 X lo-' 10-1 3.5 10-2 -0.123 3.5 -1.60 10-3 3.5 -10.66 10-4 3.5
-
1
0.1
1.0
0.1
1.0
10.0 Particle size/um
100.0
(b) Particle Electrophoretic Mobilities PH 8 8 8 8 3.5 3.5 3.5 3.5
ionic strength/M 10-1 10-2
10-3 10-4 10-1 10-2 10-3 10-4
electrophoretic mobility X lo2/ (pm s-'/(V rn-l)) -2.94 -2.68 -3.41 -3.68 1.16 1.46 1.59 1.71
with electrolyte concentration and pH. However, the variation of the values of R, with electrolyte concentration at pH 8.0 shown in Table I11 is much greater than such an effect would produce, and the variation at pH 3.5 is somewhat irregular. This indicates that variation of solution conditions is producing changes in pore structure of the membrane, possibly due to swelling of the polymer. Mean particle electrophoretic mobilities (up) are reported in Table IIb, and particle size distributions are presented in Figures 5a,b. The mobility is that corresponding to a negative {-potential at pH 8.0 and a positive {-potential at pH 3.5. At pH 8.0 the particles had a rel-
w
-s 0
50
10.0 Particle size/vm
100.0
Figure 5. Particle size distributions of titanium dioxide dispersions M, (3) 10" M, in KNOBsolutions. (a) At pH 8.0; (1)lo-' M, (2) (4) lo4 M. (b) At pH 3.5 (1) lo-' M, (2) M; (3) 10" M; (4) lo-' M.
atively narrow particle size distribution in lo-*, and lo4 M solutions with a mean diameter of 0.45 pm. Under other solution conditions the particles were flocculated to varying extents. This has a bearing on the mean mobilities as the electrophoretic equipment requires the observation of single particles, which introduces an element of sub-
520 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 Table 111. Resistances during Filtration and Electrokineticallv Enhanced Filtration
8 8 8 8 8 8 8 8 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5
10-1 10-1 10-2 10-2 10-3 10-3 10-4 10-4 10-1 10-1 10-2 10-2 10-3 10-3 10-4 10-4
EL
os EL os EL os EL os EL os EL os EL os EL os
0.89 0.89 1.11 1.11
1.53 1.53 1.70 1.70 0.88 0.88 1.05 1.05 0.97 0.97 0.97 0.97
9.88 13.38 13.34 29.15 13.36 27.49 13.35 25.59 6.39 7.77 8.33 5.80 4.85 9.04 9.54 7.69
jectivity and possibly a bias toward the observation of larger particles. In particular, at pH 3.5 a few particles were observed to have negative mobilities. 3.4. Force Balance Model. 3.4.1. Basic Model and Calculation of Fb and F,. The theoretical treatment of cross-flow microfiltration is a subject of considerable complexity, and there is no comprehensively acceptable approach. The early stages of cross-flow filtration can often be quantified in terms of conventionalcake filtration theory (Schneiderand Klein, 1982). The film model is also used, though experimental fluxes for colloidal suspensions can be 1or 2 orders of magnitude higher than those predicted (Porter, 1988). This "flux paradox" has been treated in a number of ways (Davis and Leighton, 1987; Belfort, 1987; McDonough et al., 1989), recognizing that cake mobility and nondiffusive back-transport can be important. Cross-flow electrofiltration can be treated theoretically as cross-flow filtration with superimposed electric fields. An analysis of the modification to the various resistances to flow occurring on the application of a continuous electric field can provide a good understanding of the importance of the role of the various electrical effects (Henry et al., 1977; Radovich and Chao, 1982). An analysis of particle trajectories allowing the prediction of the reduction in particle deposition due to electrophoresis on the application of continuous electric fields has been presented (Wakeman and Tarleton, 1987). Such an analysis is valuable but does not consider all of the effects occurring on application of an electric field. Indeed, cross-flow electrofiltration is such a complex process that a complete fundamental analysis of the phenomena occurring is not at present possible. Further, previous analyses have been concerned with the application of continuous electric fields. In the present work only intermittent electric field application is considered. In the case of intermittent electric field application the improvement in filtration rate is due to periodic removal of particles in the filter cake forming on the membrane surface. In analyzing the data the forces acting on a particle on the surface of such a cake will be considered. If such a particle is to be removed from the cake, then the resultant force on such a particle should be in a direction away from the membrane surface. For conventional cross-flow filtration, at steady state the balance of forces perpendicular to the membrane is given by Fy + Fb = 0 (2) where F,, is the viscous drag on the particles due to the flow of permeate and Fb is the reentraining force due to the particle concentration gradient or other effects resulting
0.53 0.53 5.95 5.95 7.93 7.93 5.22 5.22 1.16 1.16 2.39 2.39 2.37 2.37 2.21 2.21
0.79 1.04 0.47 -0.59 0.39 -0.81 0.24 -0.50 0.67 1.01 0.50 6.14 0.52 -0.52 0.14 -0.12
7.96 17.96 4.53 -9.33 2.32 -7.32 0.72 -2.21 7.93 7.19 16.59 3.32 8.03 3.88 -23.04 2.38
0.52 0.54 3.80 18.47 2.99 -370 0.94 -5.13 1.19 1.15 2.66 2.00 2.68 1.89 2.93 1.52
9.27 19.53 8.80 8.54 5.70 -378 1.89 -7.85 9.80 9.34 19.74 11.46 11.24 5.25 -19.97 3.78
in back-transport (McDonough et al., 1989). The viscous force may be expressed as Fy = f u , (3) where f is a friction factor (=67rpu, for a spherical particle of radius a), and u is the filtration rate. For very small particles it is possihe to write a simple expression for Fb based on the concentration gradient over the boundary film at the membrane surface. However, for the particles in the present work, diffusion will not be the only backtransport mechanism, and a fundamental theoretical description is complex. It is therefore best to express Fb in terms of an experimentally determinable parameter Fb = -fuss (4) where u, is the filtration rate at steady state, where the filtration cake is in equilibrium with the cross-flowing solution. On application of an electric field, a particle at the outer surface of the filter cake will experience an electrophoretic force, which may be written F, = -fu$ (5) where upis the particle electrophoretic mobility and E is the electric field gradient. The direction of this force depends on the sign of the electrophoretic mobility and the direction of the electric field gradient. Hence, on this analysis the criterion for removing a particle from the surface of the cake on the initial application of an electric field is Fy + Fb + F, < 0 (6) where forces are defined as positive if they act toward the membrane. In the application of this model, Fb can be calculated from the experimentallymeasureable steady-state flux, u,, using eq 4. F, can be calculated using eq 5. In this case the electrophoretic mobility, up,is directly determinable experimentally. A knowledge of the electric field gradient across the solution, E, is also required. This may not be calculated from the overall applied voltage, for the voltage drops at the electrode-solution interfaces (overpotential) are unknown. Hence, E is calculated from the unambiguous value of the current (I),the cell dimensions, and the known conductivity of the bulk solution (X,) using Ohm's law. The use of current in calculations considerably simplifies the application of an analysis such as the present of electrofiltration data, for the same current passes through all parts of the "equivalent circuit" which comprise the filtration cell (feed chamber, membrane and permeate chamber).
Ind. Eng. Chem. Res., Vol. 31, No. 2,1992 521 The calculation of Fy is more complex and is considered in the next section. 3.4.2. Calculation of F The calculation of Fr must include the change in the fiiration rate, u ,on apphcation of the electric field due to the effects of eiectroosmosis in the membrane and in the filter cake (Henry et al., 1977). Hence, in the general case, on application of the electric field u y = */PR, (7)
.
where R, is the total resistance to flow and is given by Rt = R , + Rc + R f (8) where R, is the membrane resistance, R, is the resistance of the filter cake, and R f is the resistance due to the boundary film and any other resistances that are not explicitly defined. The present problem is the calculation of these resistances when the electric field is applied. Following previous analysis for the continuous application of electric fields (Henry et al., 1977), in the case of electrofiltration the membrane resistance includes the effects of both membrane permeability and electroosmosis. Two membrane resistances may be defined: the resistance in the absence of an electric field, R,, as given by eq 1,and urn = Al'/firn (9) where u, is the flux in the presence of the field. If the contributions due to permeability and electroosmotic effects are assumed to be additive, (10) u, = u,, + u,,E where u,, is the electroosmoticmobility of the membrane. Then, from combination of ( 9 ) , (lo), and ( l ) , Rm R o m / ( l + (UemE/Uorn)) (11)
It is also possible to write an equivalent expression for the filter cake (12) Rc = R , / ( 1 + (Ue8/uoc)) where R , is the cake resistance in the absence of an electric field, uecis the electroosmotic mobility of the cake, and u, is the flux through the cake in the absence of an electric field. In applying eqs 7, 8, 11, and 12 to a practical case of electric field pulse application,the following procedure may then be adopted. (i) R,, is known from the electrolyte permeation rate. (ii) Analysis of the initial part of each filtration curve in the absence of an electric field according to constant pressure cake filtration theory yields a straight line plot of t / V against V (where t is the time and V the total volume of filtrate), from the intercept of which the sum R,, + Ref, and hence Rof,can be evaluated. This is assumed to be constant for each filtration run of identical feed concentration, applied pressure, and cross-flow velocity. (iii) Application of eqs 7 and 8 to the filtration rate (u,) immediately before application of the electric field allows the evaluation of R,. (iv) Equations 11and 12 can then be used to calculate R , and R,. In these expressions at this time, u,,,, = u,, = u,. It is not necessary to know the voltage gradient across the membrane as for defined solution conditions ueJ3 = ueoJ where the membranespecific electroosmotic flow rate is independently characterized (Table IIa) and I (the current density) is easily measured. Such a direct electroosmotic determination is not easily carried out for a cake of such fine particles. Instead, the expression u J 3 =
uJ/Xo is used, where A, is the bulk electrolyte conductivity. This expression will hold well at the higher electrolyte strengths where KU > 1, where K is the Debye length (Hunter, 1981) but is an approximation at the lower ionic strengths where the conductivity in the pores of the cake will be greater than the bulk solution conductivity. (v) Finally, the value of R f allowing for the effect of electrophoretictransport on the resistance of the boundary film is calculated using the expression R f = R,t/(l + ( p R , f / W ( u & ) ) (13) The results of this analysis applied to the changes in resistance occurring on application of the electric field are given in Table 111. The changes become increasingly signifkant as the ionic strength is reduced. The main features of this analysis may be summarized as follows. EL at pH 8.0. The membrane resistance is reduced; the cake resistance is very substantially reduced. The film layer resistance is decreased at all ionic strengths. OS at pH 8.0. In lo-' M solution the cake and membrane resistances are increased. At all other ionic strengths these resistances become negative as electroosmosisdominates. The film layer resistance is increased at the higher ionic strengths and becomes negative at the lower ionic strengths (reversal of the concentration gradient). EL at pH 3.5. In lO-l, and M solutions, the membrane resistance is decreased and the cake resistance is increased. In lo4 M solution, the membrane resistance is decreased and the cake resistance become negative. The film layer resistance increases slightly at all ionic strengths. OS at pH 3.5. The membrane resistance is increased M solutions and becomes negative in in lo-' and M solutions; the cake resistance is decreased at and all ionic strengths. The film layer resistance decreases slightly at all ionic strengths. This analysis shows the importance of considering changes in the membrane properties and, in particular, cake properties even in the case of electrode polarity EL, a factor often ignored in studies of conventional electrofiltration. Having calculated R,, the values of which are reported in Table ID, it is possible to calculate uy and hence Fy using eq 4. 3.4.3. Application of Force Balance. The force balance model requires a knowledge of Fy,Fb, and F,. The latter two may be calculated relatively simply, as described in section 3.4.1. Fy may be calculated as described in section 3.4.2. The directions in which the forces act on application of the electric field are shown in Figure 6. The force Fy is not shown, but rather the direction of the viscous drag immediately prior to application of the electric field (Fay) and the forces due to electroosmosis in the cake (Fa) and in the membrane (Feom). The calculated average forces acting a t the start of the electric field pulses are reported in Table IV. For conditions where some particles were flocculated, the forces are reported for discrete particles (0.45 pm), assuming them to be spherical. The forces acting on the larger particles (aggregates) will be proportional to the reported values (proportional to the effective particle radius). The experimental performance ratios (PR) are also reported alongside the values of F,for convenience. The overall effects may be summarized as follows. EL at pH 8.0. The qualitative nature of the effect of application of the electric field is in agreement with the force balance model. The application of the electric field gives an improvement in the filtration rate (PR > 1)if F, < 0 and has a deleterious effect (PR < 1) if F, > 0, in agreement with eq 6. The correlation between the mag-
522 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 Table IV. Forces Acting at the Start of Electrical Cleaning Pulses pH ionic strength/M cleaning type FyX 1012/N Fb X 1012/N 0.53 -0.41 10-1 EL 8 0.25 -0.41 8 os 10-1 -0.12 0.66 8 10-2 EL 0.68 -0.12 8 10-2 os 1.05 -0.17 8 10-3 EL -0.17 -0.016 8 10-3 os -0.21 3.09 10-4 EL 8 -0.21 -0.75 8 os 10-4 -0.72 0.50 3.5 10-1 EL -0.72 0.53 3.5 os 10-1 -0.76 0.30 10-2 EL 3.5 0.51 -0.76 3.5 10-2 os 0.29 -0.27 10-3 EL 3.5 0.62 -0.27 3.5 10-3 os -0.26 -0.36 3.5 10-4 EL 1.37 -0.36 3.5 10-4 os Membrane and parttcies with negative electrokinetic mobility
Membrane with negative and particles with positive electrokinetic mobility
EL
I I I
os
I I I
I
0 ; I I
I I
M
!I
I
M
Figure 6. Schematic representation of the forces acting during pulsed electrokineticcleaning. Explanation in text. Key: (e)anode; (e)cathode; (M) membrane.
nitude of PR and Ft is not good but, in the first instance, is not expected to be so as quantifying the magnitude of the cleaning effect requires a knowledge of the dynamics of the process as well as the initial force balance. OS at pH 8.0. In and M solutions, the qualitative prediction of the analysis is correct. In lo-' M M solution, solution, PR < 1even though Ft C 0. In PR > 1 even though Ft > 0. EL and OS at p H 3.6. There is not a good correlation between the magnitude of PR and the sign of Fv In most cases the experimentally measured effect of application of the electric field pulses is small. Hence, the force balance model successfullyprovides a qualitative prediction of the effect of pulsed electrophoretic membrane cleaning for membranes and particles of the same sign of charge (EL at pH 8.0). A more quantitative prediction might result from a detailed calculation of the effects of local electrical heating or electrolytic bubble formation on particle transport. However, such effects are complex and will have less significance in the present case of pulsed cleaning compared to the more usual continuous
F, X 1012/N -0.105 0.085 -0.56 -0.67 -1.25 0.77 -5.10 2.26 0.114 -0.041 0.25 -0.48 0.157 -0.35 0.58 -1.07
Ft X 1012/N 0.016 -0.074 -0,014 1.23 -0.37 0.59 -2.22 1.30 -0.104 -0.234 -0.22 -0.73 0.176 -0.00
-0.045 -0.053
performance ratio 0.85 0.68 4.82 1.81 2.82 0.91 2.30 0.80 1.05 0.72 0.75 0.93 1.47 0.93 0.94 1.02
field application. The model is less successful in predicting the effect of pulsed electroosmoticmembrane cleaning for such materials or of either process if the membrane and particles have opposite sign of charge (OS a t pH 8.0; EL and OS at pH 3.5). This indicates that other factors need consideration. 3.5. Electrokinetic Filter Conditioning. In considering the analysis summarized in Table IV, the following points should be noted. 1. Firstly, the force balance analysis is generally more successful for cases where the particles are not flocculated. Clearly, for the case of flocculated systems (all at pH 3.5) the representation of electrokinetic properties of the particles by a single value for the mobility is an approximation. For example, the particles remaining unflocculated are likely to have higher mobilities than the flocculated particles (that is why they remain unflocculated). The particle electrokinetic properties have a double influence, on F, and Fy (through R, and R J . 2. Secondly, electroosmotic backwashing has been considered by other workers (Visvanathan and Ben Aim, 1990) to disperse depositing particles by creating a perturbing disorder of the laminar sublayer. This has not been considered in the present work. However, in the present case the experimental effect of electroosmotic backwashing was found to be relatively small, suggesting that such an effect cannot be great. 3. Thirdly, and possibly most importantly, under conditions where the force balance analysis is not successful, the application of the electric field pulses produces an unexpected change in the subsequent steady-state filtration rate. This is best seen by comparing the EL data of Figure l a and Figure 3a. The improvement shown in the rate of filtration in Figure l a was in agreement with the force balance model. After each electric field pulse, the filtration rabgradually returned toward the steady-state value. For the data of Figure 3a, the electric field pulse initially gave an improvement in filtration rate in agreement with the force balance model, but this was followed by a rapid decline in filtration rate to below the steadystate value. As the crw-flow conditions remained constant throughout the experiment, this suggests that the properties of the filter cake have been changed on application of the electric field in such a way as to produce a new position of equilibrium with the cross-flowing dispersion. Such an effect is also apparent in the OS data of Figure 3b. We have previously reported that application of electric fields can result in a subsequently reduced tendency to particle deposition during cross-flow filtration at conducting stainless steel microfilters (Bowen and Sabuni, 1991). The present data suggest an increased tendency
Ind. Eng. Chem. Res., Vol. 31,No. 2, 1992 523 of deposition under some conditions. This may be due to changes in the morphology of firmly adhering layers produced by the action of the electric pulses. Local changes of pH in the membrane module on application of the electric field might lead to such effects. The roughness of membrane filters is being increasingly recognized as an important parameter (Gatenholm et al., 1988). The surfaces of polymeric membranes are rough on a scale of microns. If the applied electric field pulses restructured the particles around the pores 80 as to increase the effective surface roughness, then the experimentally observed effects might be expected. The relative charges of particles and membranes might be significant for such an effect as they could control membrane/particle interactions. Whatever the exact explanation, this tendency is especially deleterious when the particles and membrane have {-potentials of opposite sign, that is at pH 3.5, where very little improvement in filtration rate is obtained experimentally even though the force balancesare favorable. We term this effect “electrokinetic filter conditioning”. The existence of such an effect provides a further complication for any quantitative treatment of electrically enhanced membrane processes. Clearly, pulsed electrokinetic membrane cleaning techniques will only be effective under conditions where such an effect is not important. 4. Conclusions The use of pulsed electric fields can be an effective means of removing particulate materials from membrane filters and hence of improving the rate of filtration at polymeric membranes. The most effective mode of operation is pulsed electrophoretic membrane cleaning. For particles of the same sign of {-potential as the membranes, this could improve filtration rates by factors of up to 4.8 under the conditions studied. The experimental results were in qualitative agreement with a force balance model. For particles of the same sign of {-potential as the membranes, pulsed electroosmotic cleaning was under the conditions studied less effective at improving filtration rates. A force balance model was less successful at predicting whether or not such a process was likely to be effective. For particles of differing sign of {-potential to the membrane, such particles being partly aggregated, neither process was very successful at improving filtration rates. The experimentally measured effects were generally much less favorable than predicted by a force balance model. This was due to a rapid decline to filtration rates below steady-state values measured in control experiments, an effect tentatively attributed to changes in morphology of the filter cake produced by the electric field. Pulsed electrophoretic membrane cleaning is a promising process meriting further investigation.
Acknowledgment We thank the Science and Engineering Research Council for support, including a Research Assistantship for H.A.M.S.
Literature Cited Belfort, G. Membrane Separation Technology: an Overview. In Advanced Biochemical Engineering; Bungay, H. R., Belfort, G., Eds.; Wiley: New York, 1987;Chapter 10.
Bowen, W. R.; Clarke, R. A. Electro-osmosis at Microporous Membrane and the Determination of Zeta-potential. J. Colloid Interface Sci. 1984,97,401-409. Bowen, W. R.; Turner, A. D. Electrical Separation Processes. In Solid-liquid Separation; Gregory, J., Ed.; Ellis Horwood ChiChester, U.K., 1984; pp 9-28. Bowen, W. R.; Goenaga, X. Electrically Enhanced Membrane Filtration. Cross-flow Microfiltration and Electrofiltration at Aluminium Oxide Membranes. Proceedings of the International Congress on Inorganic Membranes, Montpellier, France; Ecole Nationale Sup6rieure de Chimie: Montpellier, 1989;pp 411414. Bowen, W. R.; Cooke, R. J. Properties of MicrofiltrationMembranes. Computer Automated Determination of the Zeta-potential of Cellulose Nitrate Membranes. Proceedings of the Vth World Filtration Congress, Nice, France; Socibti Franpise de Filtration: Cachan, 1990;pp 231-239. Bowen, W. R.; Hughes, D. T. Properties of Microfiltration Membranes. Adsorption of Bovine Serum Albumin at Aluminium Oxide Membranes. J. Membr. Sci. 1990,51,189-200. Bowen, W. R.; Cooke, R. J. Properties of MicrofiltrationMembranes. Computer Automated Determination of the Electrokinetic P r o p erties of Polycarbonate Membranes. J. Colloid Interface Sci. 1991,141,280-287. Bowen, W. R.; Sabuni, H. A. M. Electrically Enhanced Filtration at Low Cross-Flow Velocities. Znd. Eng. Chem. Res. 1991, 30, 1573-1579. Bowen, W. R;Kingdon, R. S.; Sabuni, H. A. M. Electrically Enhanced Separation Processes: the Basis of In Situ Intermittent ElectrolyticMembrane Cleaning (IIEMC) and In Situ Electrolytic Membrane Restoration (IEMR). J. Membr. Sci. 1989, 40, 219-229. Davis, R. H.; Leighton, D. T. Shear Induced Transport of a Particle Layer Along a Porous Wall. Chem. Eng. Sci. 1987,42,275-281. Gatenholm, P.; Fell, C. J. D.; Fane, A. G. Influence of the Membrane Structure on the Composition of the Deposit Layer During the Processing of Microbial Suspensions. Extended abstracts of the International Membrane Technology Conference, Sydney, Australia; School of Chemical Engineering and Industrial Chemistry, University of New South Wales: Sydney, 1988; pp G 1 4 4 . Henry, J. D.; Lawler, L. F.; Kuo, K. H. A. A Solid/liquid Separation Process Based on Cross-flow and Electrofiltration. AZChE J. 1977,23,851-859. Hunter, R. J. Zeta-potential in Colloid Science; Academic Press: London, 1981;pp 179-216. McDonough, R. M.; Fane, A. G.; Fell, C. J. Charge Effects in the Cross-flow Filtration of Colloids and Particulates. J. Membr. Sci. 1989,21,285-294. Porter, M. C. Membrane Filtration. In Handbook of Separation Processes for Chemical Engineers; Schweitzer, P. A., Ed.; McGraw-Hilk New York, 1988;pp 2-3-2-103. Radovich, J. M.; Chao, 1.-M. Electroultrafiltration of a Cationic Electrodeposition Paint. J. Coat. Technol. 1982,54,33-40. Schneider, K.;Klein, W. The Concentration of Suspensions by Means of Cross-flow Microfiltration. Desalination 1982, 41, 263-275. Spiegler, K. S.; Macleish, J. H. Molecular (Osmotic and Electro-osmotic) Backwash of Cellulose Acetate Hypefitration Membranes. J. Membr. Sci. 1981,8,173-191. Svarovsky,L. Introduction to Solid-liquid Separation. In Solid-liquid Separation, 3rd ed.;Svarowky, L., Ed.; Butterworth: London, 1990;pp 1-10. Visvanathan, C.; Ben Aim, R. Applicationof an Electric Field for the Reduction of Particle and Colloidal Membrane Fouling in Crossflow Microfiltration. Sep. Sci. Technol. 1989,24,383-398. Visvanathan, C.; Ben Aim, R. Enhancing Electrofiltration with the Aid of Electro-osmotic Backwashing Arrangement. Filtr. Sep. 1990,27,42-44. Wakeman, R. J.; Tarleton, E. S. Membrane Fouling Prevention in Crossflow Microfiltration by the Use of Electric Fields. Chem. Eng. Sci. 1987,42,829-842. Received for review April 4, 1991 Revised manuscript received October 2, 1991 Accepted October 13,1991
