Ind. Eng. Chem. Res. 1991,30, 1573-1579
1573
SEPARATIONS Electrically Enhanced Membrane Filtration at Low Cross-Flow Velocities W. Richard Bowen* and Hoze A. M. Sabuni Colloid and Interface Group, Department of Chemical Engineering, University of Wales, Swamea SA2 8PP, U.K.
In situ electrochemical membrane cleaning (IEMC) utilizes solvent electrolysis produced by brief, intermittent current pulses to remove deposited material from electrically conducting microfiiters. An important potential advantage of this technique is the elimination of the need for the use of high cross-flow velocities. Data from a systematic study of the application of IEMC at low cross-flow velocities (0.27m 8-l) in the filtration of titanium oxide are presented. The data has been analyzed by using classical cake filtration theory modified to allow for operation with a complete recycle of permeate and retentate. It is shown that the use of low cross-flow velocities in conjunction with IEMC can result in filtration without a continuous buildup of particles on the filter surface. A further significant feature of the technique is that the application of current pulses can change the subsequent adhesion of filtered particles to the filter surface, a phenomenon that is termed "electrochemical filter conditioning". 1. Introduction The large-scale use of cross-flow microfiltration shows considerable growth potential (Bowen, 1987). The realization of this potential will require the development of means of maintaining high membrane permeation rates for extended periods of time. This means that effective means of combatting residual filter-cake formation and membrane fouling will have to be utilized. Membrane fouling and filter-cake formation may be limited in a number of ways. Suitable methods include feed pretreatment (pH or ionic strength adjustment), choice of membrane materials, selection of membrane configuration, and control of module hydrodynamics. This latter normally involves the use of high cross-flow velocities, in the range of 2-8 m s-l, possibly coupled with the use of turbulence promoters. The use of electric fields in the control of membrane fouling and filter-cake formation has also been studied by a number of workers. Most studies have been directed toward the use of continuous electric fields (Henry et al., 1977;Yukawa et al., 1983; Bowen and Turner, 1984). This approach, termed electrofiltration, makes use of the inherent surface charge of dispersed materials and their consequent electrophoretic transport away from the membrane under the influence of the applied field. This can be an effective means of reducing both concentration polarization and membrane depoeition, but it has several disadvantages. These include the limitation to process streams of relatively low conductivity, a high energy requirement, substantial heat production, and changes in the process feed due to reactions at the electrodes. The latter may only be avoided by the use of modules of relatively complex construction in which the process feed is protected from the electrodes by additional membranes. The possible establishment of electrically enhanced membrane processes as acceptable
* To whom correspondence should be addressed.
unit operations will also require the minimization of energy use and heat production. The latter is especially important in the processing of biological materials. 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 and ultrafiltration. For these reasons, attention has been directed recently to the use of pulsed electric fields (Wakeman and Tarleton, 1987). If relatively infrequent pulses are effective, then the main drawbacks of continuous field application (as described in the preceding paragraph) can be substantially diminished. It may then be acceptable to incorporate the electrodes required for electrically enhanced membrane processes into membrane modules with minimum modification. We have recently described the basis of an innovative type of electrically enhanced membrane process applicable to electrically conducting membranes, such as stainless steel microfilters (Bowen et al., 1989). This process makes use of pulsed electric fields, and we have termed it "in situ electrolytic membrane cleaning" (IEMC). Unlike electrofiltration, this process does not depend on the surface charge of the dispersed materials for its primary action. It is, therefore, applicable to process streams of a wide range of conductivity. Preliminary results obtained with a simple manually operated test rig at normal cross-flow velocities were described. A computer-controlled test rig that enables the collection of reliable and reproducible data over a wide range of conditions has new been developed. The present paper describes a systematic study of the operation of IEMC as applied to the filtration of titanium dioxide at cross-flow velocities (0.27m 8-l) too low to have a significant effect in preventing the deposition of particles at the filter surface. That is, the cross-flow is essentially a means of introducing the feed into the filtration module. A quantitative analysis of the data is presented. It is shown that the use of low cross-flow velocities in conjunction with IEMC is an effective way of operating a filtration process.
OSSS-SSS5/91/26S0-1S73$02.50~0 Q 1991 American Chemical Society
1574 Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991
2. Experimental Section Experiments were carried out in a flat-sheet cross-flow microfiltration module constructed in our department. The module was constructed in Perspex and had a process feed chamber of dimensions 40.0 cm X 2.5 cm X 0.8 cm. The membrane filters were supported on a stainless steel mesh, which also provided electrical contact. The effective working area of the filters was nominally 80 cm2. In all experiments the membrane was the cathode. The electrochemical cell was completed by means of a platinized titanium mesh counter electrode mounted flush with the other side of the process feed chamber. Experiments were controlled by an IBM-compatible microcomputer ( h t r a d PC1512) fitted with an IEEE448 interface (CIL Microsystems Ltd.). The power supply used for applying the pulses of electric field (Farnell instruments Ltd. AP60-50) was linked directly to this interface. This power supply uses switching techniques that 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 (CIL PCI 6380). These were a gear pump (Micropump Model 122 with 5003U drive unit), pressure transducers (RS Ltd. type 303-337),temperature control unit, pH probe, and flow measurement devices. The retentate flow was measured by using a Pelton wheel turbine (Titan Enterprises Ltd. type 265). The permeate flow, which for laboratory-scale studies of this type may be to low for reliable measurement by Pelton wheel devices, was measured by using a specially developed level sensing unit. This recorded the time for collection of a fixed volume of permeate, normally 50 mL. Experiments were carried out in a simple pump recirculation loop, both retentate and permeate being returned to the feed reservoir. The cross-flow velocity was 0.27 m s-l, the inlet pressure was 152 kPa, and the outlet pressure was 124 kPa. Most experiments have been carried out with stainless steel filters with a nominal pore size of 3 pm (Sheepbridge Sintered Products Ltd.), the finest grade available at the start of the project. Results are also reported for stainless steel filters of nominal pore sizes 2 and 1 pm (Fairey Filtration Development Centre), which became available during the course of the work. Samples of 5-L dispersions of 5 g L-' Ti02in 0.01 M KN03at pH 9.75 were prepared in the feed reservoir. The conductivity of the dispersions was 0.125 S m-l. Titanium dioxide was technical grade and potassium nitrate was Analar grade (BDH Ltd.). Reverse-osmosis water was used in feed preparation. The size distribution of the particles in these dispersions was determined by laser diffraction (Malvern Mastersizer), The particles showed a broad distribution, 90% being under 1.69 pm, 50% under 0.48 pm, and 10% under 0.19 pm. The electrophoretic mobility of the particles was determined by microelectrophoresis (Malvern Zetasizer). The mean mobility was found to be 1.60 pm s-l/(V cm-'). Neither the size distribution nor the electrophoretic mobility varied significantly during the experimental runs. All filtration experiments were of 4-h duration. Samples required for carrying out mass balancea were obtained from the feed tank as 20-mL aliquots, which were dried in an oven at 95 OC for 48 h before weighing. After each experiment the stainless steel filters were cleaned electrolytically, as previously described (Bowen et al., 1989). 3. Results 3.1. Basic Data: Variation of Limiting Collection Times. Typical data showing the time for collection of
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PERMEATE VOLUME/mL
Figure 1. Typical data for the microfiltration of titanium dioxide dispersions: ( 0 )conventional filtration: (0)IEMC. Conditions an in text. Table 1. Effect of the Variation of Minimum Permeation Rate in the Filtration of Titanium Dioxide Diewreione minimum permeation average permeation rateo/(L m-2 h-l) rate/(L m-* h-l) 110 205 216 325 386 430 ~
Cf.: Average permeation rate for conventional filtration 73 L m-2 h-1
a given volume of permeate as a function of the total permeate volume are presented in Figure 1, for conventional microfiltration and for microfiltration with the application of IEMC. This form of presentation is used as it shows the data exactly as it was collected. Under the chosen conditions, the time for collection of a given volume of permeate increases continuously during the course of the experiment. In the case of electrically enhanced filtration, the collection time was kept at nominally less than 200 s by the application of electrical pulses at the peaks of the saw teeth. The pulses were of 8.0 A (1.016 kA m-2) for 9.5 s. After the application of each pulse, the collection time was substantially reduced, that is, the permeation rate was increased, due to removal of deposited material from the microfilter. After cessation of the pulse, deposition of material again takes place, giving gradually increasing collection times. Data of this type has been collected for three different limiting collection times, for nine different current densities, and for five different pulse durations. Variation of the collection time at which the pulses are applied allows the attainment of substantial improvementa in the membrane permeation rate. Data is shown in Table I for maximum allowed times to collect 50 mL (minimum allowed permeation rates) of 200, 100, and 50 s, respectively. (Cleaning pulses were applied immediately after recording a collection time exceeding these values). 3.2. Variation of the Magnitude of the Applied Current. 3.2.1. Effect on the Average Permeation Rate. The dependence of the average permeation rata on the applied current density is shown in Figure 2 for applied current pulses in the range 0.2-1.6 kA m-2 (-2-16 A in the test rig) of 9.5-5 duration at a nominal minimum permeation rate of 110 L m-2 h-l. The average permeation rate is based on time including the duration of the pulse. Increasing the applied current density up to 0.5 kA m-2 gave substantial increases in the average permeation rate, but the use of higher currents gave relatively small improvements. T h e limiting value of the average permeation
Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991 1676 2 00
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Figure 2. Variation of average permeation rate with applied current density in application of JEMC. Nominal minimum permeation rate 110 L m3 h-l. 50
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Figure 3. Variation of average pulse interval with applied current density in the application of JEMC. Nominal minimum permeation rate 110 L m-*h-l.
Figure 5. Variation of average permeation rate with pulse duration in the application of IEMC: (0)1.106 kA mJ; (0)0.314 kA m-*. Nominal minimum permeation rate 110 L m-* h-l.
rate is a system-dependent parameter, depending on the minimum permeation rate at which cleaning was applied, the rate of deposition of particles after cleaning, and the other module operating conditions. The permeation rate immediately after cleaning does not show such a limiting value (see section 3.4). 3.2.2. Effect on t h e Interval between Pulses. The variation of the average interval between pulses with the applied current density is shown in Figure 3, again for a nominal minimum permeation rate of 110 L m-2 h-l. Higher density pulses required less frequent application for the maintenance of a given nominal minimum permeation rate. The time between pulses was between 4 and 44 min. There was a substantial increase in this interval for current densities of up to -0.5 kA m-2, above which there was a leveling off. This leveling off is a system-dependent operational parameter. Although higher current densities give very high permeation rates immediately after cleaning (see section 3.41, the time-averaged effect of these rates is small due to rapid particle deposition. 32.3. Effect on Power Consumption. There was also a significant variation with applied current of the power consumption required for the maintenance of a given nominal minimum permeation rate, shown in Figure 4 for a limiting value of 110 L m-2 h-l. Power consumption was in the range of 0.3-1.8 kW h m-8 permeate, with a minimum in the region of currents of 0.5 kA m-2.
3.3. Variation of the Duration of the Applied Current. The experiments so far reported were carried out with pulses of 9.6 s duration. The effect of pulse duration on the average permeation rate is shown in Figure 5 for relatively high (1.016 kA m-2) and relatively low (0.314 kA m-2) currents with the same nominal minimum permeation rate of 110 L m-2 h-l. Applied pulse durations were in the range of 2.6-13.6 s. The lower limit was determined by the response time of the power supply and interface. Over this range the average permeation rate was essentially independent of the pulse duration. However, more frequent application of the shorter duration pulses was required. 3.4. Maximum Permeation Rates. From the point of view of understanding the process, it is useful to determine the maximum permeation rate immediately after each pulse. It is difficult in practice to measure this instantaneous rate, so the "maximum permeation ratesn presented in Figure 6 as a function of the applied current density are in fact the average rates for the second 50-mL aliquot of permeate collected. Unlike the process-averaged data of Figure 2, there is no leveling off of these "maximum" permeation rates at the higher current densities. There does, however, appear to be a minimum current density below which there was effectively no improvement in permeation rate on the application of the electric pulses, occurring in the region of 0.2 kA m-2.
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1576 Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991
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4. Discussion
In order to provide an understanding of the nature and scope of IEMC, this section will have two main objectives: first, to provide an analysis of the time dependence of the membrane permeation rate; second, to provide evidence concerning the mechanism of permeation rate restoration by the electrical pulses. 4.1. Analysis of the Time Dependence of the Permeation Rate. The theoretical description of cross-flow microfiltration is a subject of considerable complexity. The process has been widely treated by applications of the film model initially developed to describe ultrafiltration (Bertera et al., 1984). However, in rigorously quantitative terms, this approach is unsatisfactory due to the low diffusion coefficienta of fine particles. Experimental fluxes for colloidal suspensions are often 1 or 2 orders of magnitude higher than those predicted by the film model (Porter, 1988). This "flux paradox" has been treated in a number of ways (Davis and Leighton, 1987). An alternative approach is to consider the various resistances to flow using classical cake filtration theory as a starting point (Schneider and Klein, 1982). The formation of filter cakes is undoubtedly of importance in cross-flow microfiltration, and they are sometimes given the more sophisticated title of "dynamically formed membranew(Tanny, 1978). It has been shown that classical cake filtration theory may be used to describe the time dependence of membrane permeation rates under cross-flow conditions until the growth of the cake becomes limited by the applied fluid flow (Suki et al., 1984;Nakao, 1990). The present study has been carried out at a cross-flow velocity substantially lower than those that are conventionally used. It is therefore expected that cake filtration theory should be applicable over a longer period of filtration than is normally observed. Observation of particles in the permeate for only the first few seconds of filtration suggests that cake formation may be preceded by a brief period of pore blocking. Further, filtration theories have the advantage of relative mathematical simplicity. For these reasons, and as the objective of the present work is to develop electrically enhanced processes rather than to develop models for cross-flow microfiltration, we have based data analysis on these theories. The basis of the classical theories is the description of the growth of a filter cake due to the deposition of particles from the process feed. Classical cake filtration with a mass balance gives rise to the expression
(1) where J ( t ) is the permeation rate at a given time t , AP is the pressure difference across the filter, R is the hydraulic resistance of the filer, V is the volume of filtrate, A is the effective area of the filter, Cb is the bulk concentration of the dispersed material, and p is the filtrate viscosity. This expression assumes that all particles filtered adhere to the filter and form a cake. This is not necessarily the case when the feed is flowing across the face of the membrane. A possible modification is then to introduce a term to decribe the back-diffusion of particles into the bulk of the feed. This gives rise to an expression of the form (Fane, 1986) J ( t ) = l/A(dV/dt) = AP/(R + (V/A - Jss)CYCb)p (2)
where the term J$&b describes the convective transport of the solute at steady state expressed by the film model or by experimentally determined values of Jw A disadvantage of this approach is that only exceptionally is a true steady state achieved, so there is always a degree of arbitrariness in the chosen value of Jw Steady-state behavior was not observed in the present work. Hence, in the present analysis a model-independent assumption will be made. This is, that, for a given dispersion, filter, and operating conditions, only a certain fraction of particles (j3)from the volume of feed filtered will actually adhere to the filter suface to form a cake. The remainder will return to the bulk of the process feed. Possible values of j3 are in the range from 1.0 (when all filtered particles are retained in a filter cake) to zero (when no filter cake is formed). The extent of adherence is likely to be a function of the morphology of the filter surface, the surface properties, and shape of the particles and the available transport mechanisms. Our experiments also differ in an important way from the conditions described by eq 1,which assumes a constant particle concentration in the feed. In the present case both retentate and permeate are returned to the feed tank, so the experiments are operating at a constant feed volume but with a decreasing particle concentration, as some particles are forming the filter cake. The change in particle concentration depends on operating conditions but could be as high as 20% for some experiments under the low cross-flow conditions of the present work. With these changes, the expression describing the filtration process becomes 4 t ) = l/A(dV/dt) = U / ( R + (1 - eXP(-BV/Vo)(crVoCo/A)))r(3) where Vo is the total feed volume and Co is the initial particle concentration. Equation 3 contains three variables that may not be directly determined during filtration, CY, R, and 8. A common procedure in cross-flow filtration studies is to carry out unstirred dead-end filtration experiments to determine the values of a and R and to use these values in the interpretation of cross-flow data. In the present work, this procedure was not sufficient for two reasons. First, it is difficult to carry out reliable dead-end filtration studies with the titanium dioxide used without sedimentation of the larger particles becoming appreciable on the time scale of the measurements. Second, it is necessary to interpret the time dependence of the permeation rate after each electrical cleaning pulse. The value of R for each segment of curves such as those shown in Figure 1 will depend on the state of the filter immediately after each pulse. These values give information on the degree of cleaning effected, due to both removal of the filter cake and possible cleaning of blocked pores.
Ind. Eng. Chem. Res., Vol. 30,No. 7, 1991 1677 Table 11. Analysis of Data for the Filtration of Titanium Dioxide Dispersions with the Application of IEMC at a Nominally 3 - p Stainlesr Steel FilteP Dulse number current/(kA m-l) 0 1 2 3 4 5 6 7 8 9 0.314 R 0.19 R 3.52 3.42 3.59 3.31 3.59 3.45 3.46 3.46 3.47 a 9.38 1.04 0.99 0.93 1.01 1.03 1.00 0.97 /3 1.03 1.03 0.15 R 2.60 2.68 2.66 2.63 2.63 2.58 2.59 2.55 2.49 0.376 R 12.3 a j3 0.75 0.78 0.82 0.81 0.79 0.80 0.79 0.78 0.79 -0,063 0.502 R R 1.49 1.77 1.94 1.96 2.04 1.96 1.86 a B 0.87 0.90 0.85 0.84 0.78 0.84 0.80 9.61 0.753 R R 1.37 1.57 1.49 1.54 1.54 1.57 0.086 a 11.7 /3 0.61 0.57 0.57 0.56 0.57 0.58 1.016 R 1.27 1.12 1.06 0.20 R 0.80 0.80 a @ 0.87 0.87 0.71 0.66 0.61 8.97 1.267 R 0.091 R 0.77 0.90 0.96 0.96 0.88 a /3 0.97 0.87 0.83 0.76 0.75 7.58 1.594 R R 0.79 0.95 0.91 0.99 0.85 0.78 0.28 a B 0.90 0.88 0.83 0.82 0.80 0.82 8.33 a Minimum
permeation rate 108 L m-l h-l. Units: R, 10l2m-l; a,10l2m kg-l; 8, dimensionless.
Table 111. Analysis of Data for the Filtration of Titanium Dioxide Dispersions with the Application of IEMC at Nominally 2and I-" Stainless Steel Filters" pulse number 0 1 2 3 4 2-pm filter R 4042 R 1.61 1.02 1.06 B 0.83 0.52 0.32 a 5.7 R 1.22 1.18 1.13 1.06 1-pm filter R 0.035 B 0.82 0.73 0.67 0.59 a 6.9 Applied current 0.987 kA m-l. Minimum permeation rate 108 L m-l h-l. Units As for Table 11.
At the very low cross-flow velocities used in the present work, it has been determined by mass balance that for conventional microfiltration j3 is equal to 1.0. That is, all of the particles filtered take part in the formation of the cake, back-diffusion or radial transport of particles being insignificant. This applies to the first segment of each IEMC experiment (before application of any electrical pulse) of the type shown in Figure 1. The value of a and the initial value of R have then been determined for each experimental run by fitting the data to eq 3 with 6 = 1.0 using the Marquart algorithm. Good fits were achieved, and the values are reported in the first columns of Tables I1 and 111. The segments of data obtained after each electrical pulse were then analyzed by fixing a at the value obtained for the fvst data segment of each experiment and determining R and j3 by the same nonlinear regression. Data are presented in Tables I1 and 111. Here the values of R are a measure of the state of the membrane immediately after each pulse, and the values of j3 are a measure of the tendency of the particles to form a filter cake. This procedure has the advantage of a direct determination of a for each set of experimental data, thus allowing for any changes that might arise in producing batches of feed over an extended period of time. Mass balances showed that immediately after each pulse the particle concentration in the feed was restored to the initial value Cotwithin experimental error. The values in Table I1 have a number of interesting features. The values of R and a in the first columns show a certain variation. In the case of R, this reflects different states of the filter at the start of each experiment due to differences in the effectiveness of the cleaning procedure adopted between experiments. In one case the value of R is negative. This is a phenomenon previously reported in cake filtration and sometimes interpreted as being due to an increasing fine particle content of the filter cake as filtration proceeds (Tiller and Leu, 1984), though this explanation is not well established. In the case of a,the variation reflects differences in the particles forming the
cake, probably produced by a certain irreproducibility in the properties of the batches of titania used. The variation in these values of R and a is comparable to that obtained by other workers. The values of R reported in the subsequent columns of Table I1 show a clear pattern. The higher the value of the current used then the lower is the value of R. This shows that the higher current density pulses clean the membrane more effectively, as already indicated by the maximum permeation rate data shown in Figure 6. There is no systematic variation in the values of R obtained for successive pulses during the same experiment. A notable feature is that the values of j3 after the application of p u h differ significantly from unity. This has been confirmed by mass balance. Further, the values decrease with successive pulses. This variation in the values of j3 is even more pronounced in the data reported in Table 111 for the finer pore size filters. The declining values of j3 indicate that there is a decreased tendency for the particles being filtered to form a cake. This is an unexpected result requiring further consideration. The applied pressure and croes-flow velocity were constant during each experiment. The changes cannot therefore be due to hydrodynamic effects. Further, the size distribution of the particles and their electrophoretic mobility in the bulk of the feed did not change during the course of the experiments. So it is unlikely that the changed values of B are due to changes in the particle properties. (This constancy of particle properties also supports the assumption in the analysis that the value of a is constant for each experiment.) It is therefore p r o p o d that the decreasing values of j3 are a result of changes in the morphology of firmly adhering layers produced by the action of the electric pulses. The local changes in pH which will occur during electrolysis could produce changes in the surface chemistry of deposited particles that 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 stainless steel
1578 Ind. Eng, Chem. Res., Vol. 30,No. 7, 1991
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filters are rough on the scale of many tens of microns. Particles transported to the "valleys" in the surface will be protected from the shearing effect of the cross flow. If the applied current pulses could restructure the filter cake closeat to the surface so as to fill these valleys with a porous layer that presented a smoother surface to the process stream, then such a decrease in /3 would be expected. This postulated effect we have termed "electrochemical filter conditioning". 4%. Mode of Cleaning. The application of the electric field pulses results in the formation of microbubbles at the membrane surface due to solvent electrolysis. It is apparent from direct visual and microscopic observation that these bubbles push deposited material from the surface and out into the feed stream. This material is then carried away by the cross flow. The direction of the electric field is such that electrophoresis may have some role to play in the removal of materials from the surface and their subsequent transport into the bulk. The rate and distribution of gas evolution at the membrane surface will play dominant roles in determining the extent of removal of the deposited materials. The evolution of gas at a rough, porous surface is Micult to describe theoretically. The most quantitative studies of gas evolution at electrodes has been carried out at planar surfaces (Vogt, 1984). Vogt found that only a fraction of the gas generated actually forms bubbles at the surface. The remainder is transported to the bulk in dissolved form. He presented theoretical expressions for calculating the efficiency of gas evolution at an electrode surface. These require the estimation of a number of parameters, in particular, the maximum size of bubbles, their fractional surface coverage, and the supersaturated concentration of dissolved gas at the interface. The latter is probably the most uncertain, through reasonable estimates are available (Shibata, 1963). In the present study, gas evolved at the filter surface will be shielded from the cross flow by the deposited layers. Under these conditions it is possible to use Vogt's expressions to estimate that the efficiency of gas evolution at the surface for the range of current densities studied will only vary over the range of %33%, with the minimum value occurring at intermediate current densities. This variation is not significant for the present experimente. The rate of generation of bubbles at the filter will therefore be directly proportional to the applied current density. The data of Figure 6 and the values of R in Tables I1 and I11 hence show that the effectiveness of removal of deposited materials is directly proportional to the rate of generation of bubbles at the surface.
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Figure 8. Data for (a) conventional microfitration, (b)IEMC with a nominal minimum permeation rata of 110 L m-z h-' and (c) IEMC with a nominal minimum permeation rate of 386 L m-2h-l. Crossflow velocity 0.27 m 8-I in all cases.
It would be interesting to compare directly the effectiveness of electrolytic cleaning with the rate of removal of deposited materials by electrophoresis alone. It is not possible to carry out such a comparison directly. However, Figure 7 compares the velocity of filtration immediately after the application of an electric field with the electrophoretic velocity of the dispersed materials under the influence of the applied voltage gradient. The figure shows that the improvement in fitration velocity achieved by the application of the electric pulse is greater than could have been achieved by electrophoresis alone. This is especially the case at the higher current densities. 4.3. Effectiveness of Cleaning. In order to appreciate the effectiveness of this approach it is useful to return to the classical filtration theory as presented in eq 1. This equation leads to the prediction of a linear relationship between t / V and V for pure cake filtration. For ideal cross-flow filtration, the slope of such a plot should be zero. Data for three experiments is presented in this way in Figure 8, for conventional filtration and for IEMC with minimum permeation rates of 111and 386 L m-2 h-l, respectively. It can be seen that as the minimum-allowed permeation rate is increased, the filtration approaches effectively ideal cross-flow behavior, but due to the effect of the applied electrical pulses rather than the cross-flow velocity. This "ideal" behavior is reached after a short initial period during which the flux declines. In the present case, this initial period is most likely due to some pore blocking in the early stages of filtration (see section 4.1). 5. Conclusions
The use of pulsed electric fields is a very effective way of removing deposited materials from stainless steel mi-
crofilters. This technique requires relatively infrequent electrical pulses and may be applied without interruption of the filtration process. The data may be analyzed in terms of a modified cake filtration theory, which allows a quantitative analysis of the effectiveness of cleaning (in terms of R ) and the tendency of particles to redeposit on the filter after cleaning (0). It is shown that the effectiveness of cleaning increases with the applied current density, but that the pulse duration has little effect in the range studied. After the application of pulses there is a reduced tendency of particles to redeposit on the filter surface. It is postulated that this effect, which is sometimes quite pronounced, is due to changes in the mor-
Znd. Eng. Chem. Res. 1991,30, 1579-1582 phology of the filter produced by particles remaining on the surface.
Acknowledgment We thank the Science and Engineering Research Council for support, including a Research Assistantship for H. A.M.S.
Literature Cited Bertera, R.; Steven, H.; Metcalfe, M. Development Studies of Crossflow Microfiltration. Chem. Eng. 1984 (June), 10-14. Bowen, W. R. The Economics of Membrane Separations in the Processing of Biological Materials. I m t . Chem. Eng. Symp. Ser. 1987,NO. 105,37-46. Bowen, W.R.; Turner, A. D. Electrical Separation Processes. In Solid-liquid separation; Gregory, J., Ed.; Ellis Horwood: ChiChester, England, 19W,pp 9-28. Bowen, W. R.; Kingdon, R. S.; Sabuni, H. A. M. Electrically Enhanced Separation Processes: the Basis of In Situ Intermittent Electrolytic Membrane 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. Fane, A. G. Ultrafiltration: Factors Influencing Flux and Rejection. In Progress in filtration and separation; Wakeman, R. J., Ed.; Elsevier: 1986,Vol. 4, pp 101-179. Gatenholm, P.; Fell C. J. D.; Fane, A. G. Influence of the Membrane Structure on the Composition of the Deposit Layer During Processing of Microbial Suspensions. Eztended Abstracts of the International Membrane Technology Conference, G1, Sydney, Australia; School of Chemical Engineering and Industrial Chemistry, University of New South Wales: 1988.
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Henry, J. D.; Lawler L. F.; Luo, C. H. A. A Solid/liquid Separation Process Based on Cross-flow and Electrofiltration. AIChE J. 1977,23,851-859. Nakao, S.; Nomura, S.; Kimura, S. Transport Phenomena of the Cross-flow Microfiltration Process. Proceedings of the Fifth World Filtration Congress, Nice, France: S w i M Francaise de Filtration: 1990; Vol. 1, pp 564-570. Porter, M. C. Membrane Filtration. In Handbook of Separation Processes for Chemical Engineers; Schweitzer, P. A., Ed.; McGraw-Hill: New York, 1988,pp 2-3-2-103. Schneider, K.; Klein, W. The Concentration of Suspensions by Means of Cross-flow Microfiltration. Desalination 1982, 41, 263-275. Shibata, S. The Concentration of Molecular Hydrogen on the Platinum Cathode. Bull. Chem. SOC.Jpn. 1963,36,53-57. Suki, A,; Fane, A. G.; Fell, C. J. D. Flux Decline in Protein Ultrafiltration. J. Membr. Sci. 1984,21,269-283. Tanny, G. B. Dynamic Membranes in Ultrafiltration and Reverse Osmosis. Sep. Purif. Methods 1978,7, 183-220. Tiller, F. M.; Leu, W. Filtration. In Handbook of Chemical Engineering Calculations; Chopey, N. P., Hicks T. G., Eds.; McGrawHill: New York, 1984;p 11-6. Vogt, H. The Rate of Gas Evolution at Electrodes-1. An Estimate of the Efficiency of Gas Evolution from the Supersaturation of Electrolyte Adjacent to a Gas-evolving Electrode. Electrochim. Acta 1984,29,167-173. 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. Yukawa, H.; Shimura, K.; Suda, S.; Maniwa, A. Cross-flow Electroultrafiltration for Colloidal Separation of Protein. J . Chem. Eng. Jpn. 1983,16,305-311.
Received for review September 10,1990 Revised manuscript received January 26, 1991 Accepted January 29,1991
Study of Relationships between Solvent Effectiveness in Coal Tar Pitch Extractions and Solvent Solubility Parameters Carlos G. Blanco and Maria D. GuillBn* Instituto Nacional del Carbdn, CSIC, A p . 73, 33080 Oviedo, Spain
Relationships between solvent extractive ability with coal tar pitches and Hildebrand’s solubility parameter have been studied. In addition, taking into account Hansen’s solubility parameter components, relationships between these and solvent effectiveness in coal tar pitch extractions have been studied. It is proved that organic solvents with a high solubility parameter dispersive component are the best coal tar pitch solvents, providing their polarity and their ability to give rise to hydrogen bonding interactions do not exceed certain limits.
Introduction The study of coal solubility in organic solvents has received much attention in the literature (Dryden, 1951; Van Krevelen, 1965; Angelovich et al., 1970) and still remains a subject of widespread interest (Chawla and Davis, 1989). In addition, the extraction of pitches using organic solvents is a very common procedure in fractionating schemes (Bartle, 19721, in order to characterize pitches by different techniques (Fischer et al., 1978; Borwitzky and &homburg, 1979), or for the obtention of pitch fractions (free of certain components) suitable for the manufacture of special materials (Weishauptova and Medek, 1985; Riggs, 1984). However, in spite of the evidently wide interest in the solubility of pitches, to the best of our knowledge, there is no research work on relationships between the solubility of pitch and solvent properties. Coal tar pitches are complex mixtures of polycyclic aromatic compounds, which themselves constitute PO-
lyeutectic solutions of nonelectrolyte components. The solubility of pitches in organic solvents could be explained by the Hildebrand-Scatchard theory of regular solutions (Hildebrand et al., 1970), if they are regular solutions or if they behave like them. According to this theory, there is a relationship between the logarithm of the equilibrium mole fraction of the solute in the solvent, r2, and the solubility parameters of the solute a2, and the solvent
In this equation u2 is the activity coefficient of the solute, V2its molar volume, and the fraction volume of the solvent. The solubility parameter was defined (Hildebrand et al., 1970) as the square root of the cohesive energy density tiH = [AU/Vj1I2,where AU is the energy of vaporization and V is the molar volume of the compound. Prausnitz et al. (Blanks and Prausnitz, 1964) propose that
0888-5886/91/2630-1579$02.50/00 1991 American Chemical Society
