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Influence of shear rate on the flux during ultrafiltration of colloidal substances
Journal of Membrane Science, 79 (1993) 93-99
93
Elsevier Science Publishers B.V., Amsterdam
Influence of shear rate on the flux during ultrafiltration of colloidal substances Ann-Sofi Jiinsson Department of Chemical Engineering I, Lund University, P.O. Box 124, S-221 00 Lund (Sweden)
(Received September 14,1992; accepted in revised form December 22,1992)
Abstract The shear rate at the surface of a membrane has a distinct influence on the membrane flux in systems containing colloidal substances. If the shear rate is decreased, flux decreases since the thickness of the boundary layer of retained solutes at the membrane surface then increases. When low molecular solutes are used, flux increases again when the shear rate is increased. However, if large solutes are used, polymeric and colloidal substances, for example, it sometimes happens that the flux cannot be increased to its original value again by increasing the shear rate: the flux decline is regarded as irreversible. Experiments, performed in a new type of turbulence-promoting module, which illustrate the influence of shear rate on flux when treating solutions containing colloidal substances, are presented in this paper. It is also shown that a flux decrease which can not be restored by increasing the shear rate, may be completely restored by a temporary interruption of the permeate flow. Keywords: ultrafiltration;
fouling; plant effluent; waste water; turbulence promotion
Introduction
chances of it returning to the bulk solution by diffusion are substantially reduced.
It is a well-known fact that the flux of a solution is, as a rule, substantially lower than the flux of pure water. This has been explained by membrane scientists in several ways; by the gel layer model [ 1,2], the osmotic pressure model [3,4] and the resistance in series model [ 5-71. The application of the different models depends primarily on the diffusivity (i.e. the size) of the solutes. In contrast to small molecules, such as salt and sugar, the diffusivity of large solute molecules, polymeric and colloidal substances, for example, is very limited. This means that once a large molecule has been transported to the membrane surface the
0376-7388/93/$06.00
The resistance in series model Of the above mentioned transport models, the resistance in series model is the most generally applicable one. The resistance in series model is based on the assumption that there are several distinct resistances in series which control the transmembrane flux, J. AD
J=
R, +Rah;Rm
+R,
(1)
where AP is the operating pressure, R, is the hydraulic resistance of the membrane, Radsthe
0 1993 Elsevier Science Publishers B.V. All rights reserved.
AS. &insson/J. Membrane Sci. 79 (1993) 93-99
94
resistance of solutes adsorbed onto the membrane surface or the pore walls, Rep the resistance of a concentration boundary layer and R, the resistance of a layer of concentrate at the membrane surface (often referred to as the gel layer). The influence of the different resistances may be established by varying the shear rate. This has been demonstrated in experiments in a rotary module by Lopez-Leiva and Matthiasson [6]. The influence of the different resistances is schematically shown in Fig. 1. In order to be able to study the influence of the first three resistances it is necessary to have access to equipment which generates very high shear rates at the membrane surface, i.e. a rotary module with the membrane mounted on the rotary part, as in the experiments per-
Shear rate
Fig. 1. Permeate flux as a function of shear rate. The membrane resistance, R,,,, is calculated from the pure water flux, PWF (zone 1). The resistance of adsorbed solute, Rods,is obtained if the shear rate is high enough to avoid a concentration gradient at the membrane surface (zone 2). A concentration boundary layer is established when the shear rate is decreased. As long as it is possible to regain the initial flux by increasing the shear rate the added resistance, R,, is attributed to this concentration boundary layer (zone 3). When the shear rate is further decreased a point is reached where it is not possible to regain the initial flux by increasing the shear rate. The resistance to flow, R,, beyond this point is attributed to the formation of a cake (or gel layer) on the membrane surface (zone 4).
formed by Lopez-Leiva and Matthiasson [6]. Modules of this type are now commercially available and work in this area is in progress in the USA [8 1. However, the experiments presented in the present paper have been performed in a less sophisticated, but very easy to use, commercially available module which is described below. When evaluating the performance of different ultrafiltration membranes in this module a significant flux decrease was noticed when the rotor speed was decreased. This was not surprising, however. The influence of the hydrodynamics in the system on flux has been demonstrated by many scientists (see for example Refs. [ 9-13 ] ). Increasing the stirring speed to its original value only marginally increased the flux. The conclusion was, accordingly, that the flux decline that occurred when the stirring speed was decreased was irreversible. However, it then was found that the flux was easily restored if the permeate flow was temporarily interrupted. These results indicate that what is usually regarded as an irreversible flux decline that needs cleaning to restore the original flux, may be caused by a layer of retained material that, at least in some cases, may be easily rinsed away if the permeate flow is temporarily interrupted. Interrupting the permeate flow and applying a pressure on the permeate side (backflushing), is often used in microfiltration. In this paper it is shown that the flux decrease (and consequently the need of backflushing) is not established if the shear rate is kept high enough. Experiments with two different solutions illustrate that: - the flux decrease is marginal if the shear rate is sufficiently high -the flux decreases when the shear rate is decreased - the flux is only marginally restored when the shear rate is increased again - the initial flux can be restored if the permeate flow is temporarily interrupted.
AS. Jijnsson/J. Membrane Sci. 79 (1993) 93-99
95
Experimental procedure
The experiments were performed in a module manufactured by ABB Flootek, Sweden, equipped with one membrane above and one below a rotor blade (see Fig. 2). The influence of rotary speed on the flux of two solutions was studied. In the first series of experiments, bleach plant effluent from a sulphate pulp mill with a pH of 10 was used (see Table 1) and in the second series oily waste water with a pH of 6 (see Table 2) was studied. A poly (ether sulphone) membrane, PES25, from Hoechst Werk KALLE, with a nominal cut-off of 25,000 was used in the experiments with bleach plant effluent. In the experiments with oily waste water a polysulphone membrane, PSlOO, also from Hoechst Werk KALLE, with a nominal cut-off of 100,000 was used. During the experiments with the bleach plant effluent the operating pressure was 0.2 MPa and the temperature 55°C. The corresponding pa-
rameters during the experiments with oily waste water were 0.1 MPa and 40’ C. The circulation flow through the module was kept at 0.6-0.65 m3/hr. The membranes were cleaned, immediately before and after each test, with the alkaline cleaning agent Ultrasill from Henkel. Results
The influence of shear rate In the first experiment the influence of rotary speed during ultrafiltration of bleach plant effluent was studied. The rotary speed was decreased stepwise from initially 900 rpm to 125 rpm, kept constant at 125 rpm for 14 hr and was then increased stepwise again. When the rotary speed was decreased the flux decreased, as expected. If this flux decline has been the result of concentration polarization the initial flux would have been regained when the rotary speed was increased again. However, this
TABLE 1 Content of components in the bleach plant effluent
Effluent Permeate Retentate
COD” (g/l)
AOXb (mg/l)
TDS’ (g/l)
Mg (mg/l)
Al
(mg/l )
Ca (mg/l)
Fe (mg/l)
4.4 2.3 25.1
84 36 180
6.9 4.8 22.0
9.5 4.2 63.0
0.9 0.4 5.1
15.0 8.5 103.0
0.5 0.2 8.8
“Chemical oxygen demand. bAdsorbable organic halogens. “Total dry solids content. TABLE 2 Content of components in the oily wastewater
Effluent Permeate
Mineral oil (mg/l)
COD” (g/l)
80 2.4
5.5 3.7
“Chemical oxygen demand.
Pb (mg/l)
Cr (mg/l)
Zn
(mg/l 1
Ni (mg/l)
99 <0.2
12 0.24
560 0.23
5.6 < 0.9
A.S. Jiinsson/J. Membrane Sci. 79 (1993) 93-99
96 600 feed
14 h
Fig. 2. Schematic sketch of the module used in the experiments. The area of each membrane is 0.05 m’. The length of the rectangular rotor blade is 287 mm, the width is 60 mm and the thickness 5 mm. The distance between the two membranes is 18 mm.
0.5hbetweeneachmea~"remBnf
Fig. 4. Ultrafiltration of bleach plant effluent. Before variation of the rotary speed the bleach plant effluent was recirculated in the system for 14 hr in order to make sure that the decrease in the flux was due to the decreased rotary speed and not a time-dependent phenomenon. The rotary speed was kept constant for 30 min at each speed.
~.
25
ot”““‘l”““““‘l”,’ 0 200
ot.“““l”““‘t”‘l”.’ 400 0 200 400 Rotary
600
800
1000
1200
Rotary
600
_. ._ .- __._: 800
1000
._ 1200
speed (rpm)
speed (rpm)
Fig. 3. Ultrafiltration of bleach plant effluent. The poly (ether sulphone) membrane PES25 has a nominal cutoff of 25,000. The operating pressure was 0.2 MPa and the temperature 55 oC. The rotary speed was kept constant for 1 hr at each speed, apart from 125 rpm where the speed was kept constant for 14 hr. The pure water flux before the test was 1,200 l/m2-hr and after cleaning with 0.5% Ultrasill after the experiment the pure water flux was 1,100 l/mZhr.
was not the case, as shown in Fig. 3. In order to investigate whether the flux decrease was irreversible already at the highest speeds an experiment was performed where the rotary speed was increased to the initial value (900 rpm) between each decrease. As shown in Fig. 4, the flux was never completely restored to the initial value.
5. Ultrafiltration of oily waste water. The polysulphone membrane PSlOO has a nominal cut-off of 100,000. The operating pressure was 0.1 MPa and the temperature 40°C. The rotary speed was kept constant for 5 min at each speed. The pure water flux before and after the test was 400 and 120 l/m2-hr, respectively. After cleaning with 1.0% Ultrasil10 the pure water flux was increased to 450 l/m2-hr. Fig.
The specific properties and the concentration of the solution treated, as well as membrane and operating parameters (such as pressure and temperature), affect the influence of the shear rate. However, behaviour very similar to that shown in Fig. 3 was registered when the experimental conditions were quite different; in this experiment oily waste water was ultrafiltered using a membrane with a higher nominal cut-off than the membrane used when
AS. J&won/J.
97
Membrane Sci. 79 (1993) 93-99
bleach plant effluent was treated However, as shown in Fig. 5, the influence of rotary speed on flux was very similar to that observed when bleach plant effluent was treated. Flux recovery by interruption of permeate flow
The thickness of the layer of retained material, and hence the resistance to flow, is primarily affected by the operating pressure (which influences the convective transport to the membrane) and the shear rate at the membrane. When the shear rate is decreased the thickness of the layer increases and flux is reduced, as demonstrated in Figs. 3-5. Flux may be partly recovered by increasing the shear rate. This flux recovery procedure is, however, normally quite time-consuming. Providing the retained material is not adsorbed onto the membrane, the flux may, however, be restored by interrupting the permeate flow. The layer of retained material is then washed away with the retentate flow. Sometimes a low pressure is applied on the permeate side in order to remove the accumulated material at the mem-
600 500
Fig. 7. Ultrafiltration of oily waste water.
brane surface. This backwashing technique is utilized particularly in microfiltration where accumulation of material at the membrane surface is especially pronounced. The recovery of flux when the permeate flow was interrupted during ultrafiltration of bleach plant effluent is shown in Fig. 6. As can be seen in Fig. 6 there was a fast, immediate flux decline when the permeate valve was opened. Starting the rotor partially increased the flux, whereas interruption of the permeate flow completely restored the flux to its initial value. A similar pattern of flux behaviour was observed when the oily waste water was ultrafiltered. An increase in flux was registered, both when the rotor was started and when the permeate flow was interrupted, as shown in Fig. 7.
400
Discussion
300 200 100 0 0
10
20
30
40
Time (min)
Fig. 6. Ultrafiltration of bleach plant effluent. The experiment was performed in the following way: Deionized water was recirculated through the system with the rotor turned off. The pure water permeate flow was collected and registered on a balance connected to a computer. The water was then replaced by the effluent (with the permeate valve closed). The permeate valve was then opened and the permeate flow registered. After 14 min the rotor was started (at 966 rpm). After another 9 min the permeate valve was closed for 2 min and then opened again.
The results shown in Figs. 3-7 demonstrate the important influence of shear rate on flux. These results focus the interest on the following questions that have attracted the attention of membrane scientists since membrane filtration was established as a commercial separation technique: - why does flux decrease when the shear rate is decreased - why is flux not restored when the shear rate is increased again
98
- why is flux regained when the permeate flow is interrupted. It shall be pointed out that the influence of the shear rate on flux observed in these experiments is not a general rule. Experiments with other solutes in this module have demonstrated that even though flux decreases when the rotor speed ia decreased, flux is usually regained when the speed is increased again. The experiments presented in this paper also demonstrate that flux can be kept at the initial high level, if the shear rate is kept high enough. However, in most modules it is usually not feasible to keep the shear rate high enough. In solutions with colloidal substances that are gathered at the membrane surface, a fast initial flux decrease is commonly experienced, followed by a slowly progressing flux decline. The solutions in the actual experiments contain large solute molecules that may build up a cake of retained material at the membrane surface. When the shear rate decreases, the layer of solute molecules accumulated at the membrane surface grows. The flux decrease with time when the rotor is shut off is probably induced by compression and blocking of the flow channels in the layer of retained material at the membrane surface. The reduction in porosity of a compressible material subjected to drag forces of a fluid is treated in Ref. [ 141. In the actual case it is, however, very difficult to theoretically describe the flow resistance of a cake formed upon the membrane as the pore structure is influenced by the nature and concentration of the solutes in the solution, as well as by the initial pure water flux. In this investigation it was experienced that once the flux is decreased it is very difficult to restore it again. However, when the permeate flow is interrupted, and consequently the force pressing the cake against the membrane surface temporarily removed, the cake may be washed away with the retentate stream, as shown in Figs. 6 and 7. However, this implies,
A.S. Jiinsson/J. Membrane Sci. 79 (1993) 93-99
of course, that the material in the cake is not adsorbed at the membrane. Conclusions As soon as transport of solvent through a membrane commences solute is transported by convection towards the membrane and the concentration at the membrane surface is increased as solute is retained. Steady state is established when the convective flow of solute towards the membrane surface is balanced by the diffusive back transport of solute to the bulk solution. When treating solutions containing large aggregates (e.g. polymeric and colloidal substances), the diffusive back transport is almost negligible and a layer of retained material (a cake or gel layer) may be formed which constitutes a considerable hydraulic resistance in series with the membrane. The thickness of this layer is highly susceptible to the shear rate. The experiments presented in this paper indicate that if a cake is formed, an increase in the shear rate only marginally decreases the cake thickness and the flux is accordingly only marginally restored. Flux may, however, be regained by interrupting the permeate flow. As the permeate flow is interrupted the cake is no longer forced against the membrane surface by the permeate flow. Instead the cake is caused to flow laterally over the membrane surface and is discharged with the retentate at the exit of the module. References W.F. Blatt, A. Dravid, A.S. Michael5 and L. Nelsen, Solute polarization and cake formation in membrane ultrafiltration: Causes, consequences and control techniques, in J.E. Flinn (Ed.), Membrane Science and Technology, Plenum Press, New York, NY, 1970, pp. 47-97. D.R. Trettin and M.R. Doshi, Ultrafiltration in an unstirred batch cell, Ind. Eng. Chem. Fundam., 19 (1980) 189.
AS. J&won/J.
99
Membrane Sci. 79 (1993) 93-99
G. Jonsson, Boundary layer phenomena during ultrafiltration of dextran and whey protein solutions, Desalination, 51 (1984) 61-77. J.G. Wijmans, S. Nakao and C.A. Smolders, Flux limitation in uitrafiltration: Osmotic pressure and gel layer model, J. Membrane Sci., 20 (1984) 115-124. J.G. Wijmans, S. Nakao, J.W.A. van den Berg, F.R. Troelstra and C.A. Smolders, Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration, J. Membrane Sci., 22 (1985) 117135. M. Lopez-Leiva and E. Matthiasson, Macromolecular adsorption and fouling in ultrafiltration and their relationships to concentration polarization, Proc. Int. Workshop on the Fundamentals and Applications of Surface Phenomena Associated with Fouling and Cleaning in Food Processing, B. Halistr6m, D.B. Lund and Ch. Trag&d (Eds.), TylSsand, Sweden, 1981, pp. 299-308. G.B. van den Berg and C.A. Smolders, Flux decline in ultrafiltration processes, Desalination, 77 (1990) lOl133.
8
9
10
11
12
13
14
G. Belfort et al., Diagnosis of membrane fouling using a rotating annular filter. 1+2, J. Membrane Sci., 77 (1993) 1,23. M.C. Porter, Concentration polarization with membrane ultrafiltration, Ind Eng. Chem. Prod. Res. Dev., ll(3) (1972)234-248. A, Bottino, G. Capanelli, A. Imperato and S. Munari, Ultratlltration of hydrosoluble polymers. Effect of operating conditions on the performance of the membrane, J. Membrane Sci., 21 (1984) 247-267. F. Vigo, C. Uliana and P. Lupino, The performance of a rotating module in oily emulsions ultrafiitration, Sep. Sci. Technol., 20(2&3) (1985) 213-230. H. de Balmann, P. Aimar and V. Sanchez, Membrane partition and mass transfer in ultrafiltration, Sep. Sci. Technol., 25 (5) (1990) 507-534. M.D. Afonso and M.N. de Pinho, Membrane separation processes in the pulp and paper industry, Desalination, 85 (1991) 53-58. A.-S. JSnsson and B. Jiinsson, Fluid flow in compressible porous media. I. Steady-state conditions, AIChE J., 38(S) (1992) 1340-1348.
93
Elsevier Science Publishers B.V., Amsterdam
Influence of shear rate on the flux during ultrafiltration of colloidal substances Ann-Sofi Jiinsson Department of Chemical Engineering I, Lund University, P.O. Box 124, S-221 00 Lund (Sweden)
(Received September 14,1992; accepted in revised form December 22,1992)
Abstract The shear rate at the surface of a membrane has a distinct influence on the membrane flux in systems containing colloidal substances. If the shear rate is decreased, flux decreases since the thickness of the boundary layer of retained solutes at the membrane surface then increases. When low molecular solutes are used, flux increases again when the shear rate is increased. However, if large solutes are used, polymeric and colloidal substances, for example, it sometimes happens that the flux cannot be increased to its original value again by increasing the shear rate: the flux decline is regarded as irreversible. Experiments, performed in a new type of turbulence-promoting module, which illustrate the influence of shear rate on flux when treating solutions containing colloidal substances, are presented in this paper. It is also shown that a flux decrease which can not be restored by increasing the shear rate, may be completely restored by a temporary interruption of the permeate flow. Keywords: ultrafiltration;
fouling; plant effluent; waste water; turbulence promotion
Introduction
chances of it returning to the bulk solution by diffusion are substantially reduced.
It is a well-known fact that the flux of a solution is, as a rule, substantially lower than the flux of pure water. This has been explained by membrane scientists in several ways; by the gel layer model [ 1,2], the osmotic pressure model [3,4] and the resistance in series model [ 5-71. The application of the different models depends primarily on the diffusivity (i.e. the size) of the solutes. In contrast to small molecules, such as salt and sugar, the diffusivity of large solute molecules, polymeric and colloidal substances, for example, is very limited. This means that once a large molecule has been transported to the membrane surface the
0376-7388/93/$06.00
The resistance in series model Of the above mentioned transport models, the resistance in series model is the most generally applicable one. The resistance in series model is based on the assumption that there are several distinct resistances in series which control the transmembrane flux, J. AD
J=
R, +Rah;Rm
+R,
(1)
where AP is the operating pressure, R, is the hydraulic resistance of the membrane, Radsthe
0 1993 Elsevier Science Publishers B.V. All rights reserved.
AS. &insson/J. Membrane Sci. 79 (1993) 93-99
94
resistance of solutes adsorbed onto the membrane surface or the pore walls, Rep the resistance of a concentration boundary layer and R, the resistance of a layer of concentrate at the membrane surface (often referred to as the gel layer). The influence of the different resistances may be established by varying the shear rate. This has been demonstrated in experiments in a rotary module by Lopez-Leiva and Matthiasson [6]. The influence of the different resistances is schematically shown in Fig. 1. In order to be able to study the influence of the first three resistances it is necessary to have access to equipment which generates very high shear rates at the membrane surface, i.e. a rotary module with the membrane mounted on the rotary part, as in the experiments per-
Shear rate
Fig. 1. Permeate flux as a function of shear rate. The membrane resistance, R,,,, is calculated from the pure water flux, PWF (zone 1). The resistance of adsorbed solute, Rods,is obtained if the shear rate is high enough to avoid a concentration gradient at the membrane surface (zone 2). A concentration boundary layer is established when the shear rate is decreased. As long as it is possible to regain the initial flux by increasing the shear rate the added resistance, R,, is attributed to this concentration boundary layer (zone 3). When the shear rate is further decreased a point is reached where it is not possible to regain the initial flux by increasing the shear rate. The resistance to flow, R,, beyond this point is attributed to the formation of a cake (or gel layer) on the membrane surface (zone 4).
formed by Lopez-Leiva and Matthiasson [6]. Modules of this type are now commercially available and work in this area is in progress in the USA [8 1. However, the experiments presented in the present paper have been performed in a less sophisticated, but very easy to use, commercially available module which is described below. When evaluating the performance of different ultrafiltration membranes in this module a significant flux decrease was noticed when the rotor speed was decreased. This was not surprising, however. The influence of the hydrodynamics in the system on flux has been demonstrated by many scientists (see for example Refs. [ 9-13 ] ). Increasing the stirring speed to its original value only marginally increased the flux. The conclusion was, accordingly, that the flux decline that occurred when the stirring speed was decreased was irreversible. However, it then was found that the flux was easily restored if the permeate flow was temporarily interrupted. These results indicate that what is usually regarded as an irreversible flux decline that needs cleaning to restore the original flux, may be caused by a layer of retained material that, at least in some cases, may be easily rinsed away if the permeate flow is temporarily interrupted. Interrupting the permeate flow and applying a pressure on the permeate side (backflushing), is often used in microfiltration. In this paper it is shown that the flux decrease (and consequently the need of backflushing) is not established if the shear rate is kept high enough. Experiments with two different solutions illustrate that: - the flux decrease is marginal if the shear rate is sufficiently high -the flux decreases when the shear rate is decreased - the flux is only marginally restored when the shear rate is increased again - the initial flux can be restored if the permeate flow is temporarily interrupted.
AS. Jijnsson/J. Membrane Sci. 79 (1993) 93-99
95
Experimental procedure
The experiments were performed in a module manufactured by ABB Flootek, Sweden, equipped with one membrane above and one below a rotor blade (see Fig. 2). The influence of rotary speed on the flux of two solutions was studied. In the first series of experiments, bleach plant effluent from a sulphate pulp mill with a pH of 10 was used (see Table 1) and in the second series oily waste water with a pH of 6 (see Table 2) was studied. A poly (ether sulphone) membrane, PES25, from Hoechst Werk KALLE, with a nominal cut-off of 25,000 was used in the experiments with bleach plant effluent. In the experiments with oily waste water a polysulphone membrane, PSlOO, also from Hoechst Werk KALLE, with a nominal cut-off of 100,000 was used. During the experiments with the bleach plant effluent the operating pressure was 0.2 MPa and the temperature 55°C. The corresponding pa-
rameters during the experiments with oily waste water were 0.1 MPa and 40’ C. The circulation flow through the module was kept at 0.6-0.65 m3/hr. The membranes were cleaned, immediately before and after each test, with the alkaline cleaning agent Ultrasill from Henkel. Results
The influence of shear rate In the first experiment the influence of rotary speed during ultrafiltration of bleach plant effluent was studied. The rotary speed was decreased stepwise from initially 900 rpm to 125 rpm, kept constant at 125 rpm for 14 hr and was then increased stepwise again. When the rotary speed was decreased the flux decreased, as expected. If this flux decline has been the result of concentration polarization the initial flux would have been regained when the rotary speed was increased again. However, this
TABLE 1 Content of components in the bleach plant effluent
Effluent Permeate Retentate
COD” (g/l)
AOXb (mg/l)
TDS’ (g/l)
Mg (mg/l)
Al
(mg/l )
Ca (mg/l)
Fe (mg/l)
4.4 2.3 25.1
84 36 180
6.9 4.8 22.0
9.5 4.2 63.0
0.9 0.4 5.1
15.0 8.5 103.0
0.5 0.2 8.8
“Chemical oxygen demand. bAdsorbable organic halogens. “Total dry solids content. TABLE 2 Content of components in the oily wastewater
Effluent Permeate
Mineral oil (mg/l)
COD” (g/l)
80 2.4
5.5 3.7
“Chemical oxygen demand.
Pb (mg/l)
Cr (mg/l)
Zn
(mg/l 1
Ni (mg/l)
99 <0.2
12 0.24
560 0.23
5.6 < 0.9
A.S. Jiinsson/J. Membrane Sci. 79 (1993) 93-99
96 600 feed
14 h
Fig. 2. Schematic sketch of the module used in the experiments. The area of each membrane is 0.05 m’. The length of the rectangular rotor blade is 287 mm, the width is 60 mm and the thickness 5 mm. The distance between the two membranes is 18 mm.
0.5hbetweeneachmea~"remBnf
Fig. 4. Ultrafiltration of bleach plant effluent. Before variation of the rotary speed the bleach plant effluent was recirculated in the system for 14 hr in order to make sure that the decrease in the flux was due to the decreased rotary speed and not a time-dependent phenomenon. The rotary speed was kept constant for 30 min at each speed.
~.
25
ot”““‘l”““““‘l”,’ 0 200
ot.“““l”““‘t”‘l”.’ 400 0 200 400 Rotary
600
800
1000
1200
Rotary
600
_. ._ .- __._: 800
1000
._ 1200
speed (rpm)
speed (rpm)
Fig. 3. Ultrafiltration of bleach plant effluent. The poly (ether sulphone) membrane PES25 has a nominal cutoff of 25,000. The operating pressure was 0.2 MPa and the temperature 55 oC. The rotary speed was kept constant for 1 hr at each speed, apart from 125 rpm where the speed was kept constant for 14 hr. The pure water flux before the test was 1,200 l/m2-hr and after cleaning with 0.5% Ultrasill after the experiment the pure water flux was 1,100 l/mZhr.
was not the case, as shown in Fig. 3. In order to investigate whether the flux decrease was irreversible already at the highest speeds an experiment was performed where the rotary speed was increased to the initial value (900 rpm) between each decrease. As shown in Fig. 4, the flux was never completely restored to the initial value.
5. Ultrafiltration of oily waste water. The polysulphone membrane PSlOO has a nominal cut-off of 100,000. The operating pressure was 0.1 MPa and the temperature 40°C. The rotary speed was kept constant for 5 min at each speed. The pure water flux before and after the test was 400 and 120 l/m2-hr, respectively. After cleaning with 1.0% Ultrasil10 the pure water flux was increased to 450 l/m2-hr. Fig.
The specific properties and the concentration of the solution treated, as well as membrane and operating parameters (such as pressure and temperature), affect the influence of the shear rate. However, behaviour very similar to that shown in Fig. 3 was registered when the experimental conditions were quite different; in this experiment oily waste water was ultrafiltered using a membrane with a higher nominal cut-off than the membrane used when
AS. J&won/J.
97
Membrane Sci. 79 (1993) 93-99
bleach plant effluent was treated However, as shown in Fig. 5, the influence of rotary speed on flux was very similar to that observed when bleach plant effluent was treated. Flux recovery by interruption of permeate flow
The thickness of the layer of retained material, and hence the resistance to flow, is primarily affected by the operating pressure (which influences the convective transport to the membrane) and the shear rate at the membrane. When the shear rate is decreased the thickness of the layer increases and flux is reduced, as demonstrated in Figs. 3-5. Flux may be partly recovered by increasing the shear rate. This flux recovery procedure is, however, normally quite time-consuming. Providing the retained material is not adsorbed onto the membrane, the flux may, however, be restored by interrupting the permeate flow. The layer of retained material is then washed away with the retentate flow. Sometimes a low pressure is applied on the permeate side in order to remove the accumulated material at the mem-
600 500
Fig. 7. Ultrafiltration of oily waste water.
brane surface. This backwashing technique is utilized particularly in microfiltration where accumulation of material at the membrane surface is especially pronounced. The recovery of flux when the permeate flow was interrupted during ultrafiltration of bleach plant effluent is shown in Fig. 6. As can be seen in Fig. 6 there was a fast, immediate flux decline when the permeate valve was opened. Starting the rotor partially increased the flux, whereas interruption of the permeate flow completely restored the flux to its initial value. A similar pattern of flux behaviour was observed when the oily waste water was ultrafiltered. An increase in flux was registered, both when the rotor was started and when the permeate flow was interrupted, as shown in Fig. 7.
400
Discussion
300 200 100 0 0
10
20
30
40
Time (min)
Fig. 6. Ultrafiltration of bleach plant effluent. The experiment was performed in the following way: Deionized water was recirculated through the system with the rotor turned off. The pure water permeate flow was collected and registered on a balance connected to a computer. The water was then replaced by the effluent (with the permeate valve closed). The permeate valve was then opened and the permeate flow registered. After 14 min the rotor was started (at 966 rpm). After another 9 min the permeate valve was closed for 2 min and then opened again.
The results shown in Figs. 3-7 demonstrate the important influence of shear rate on flux. These results focus the interest on the following questions that have attracted the attention of membrane scientists since membrane filtration was established as a commercial separation technique: - why does flux decrease when the shear rate is decreased - why is flux not restored when the shear rate is increased again
98
- why is flux regained when the permeate flow is interrupted. It shall be pointed out that the influence of the shear rate on flux observed in these experiments is not a general rule. Experiments with other solutes in this module have demonstrated that even though flux decreases when the rotor speed ia decreased, flux is usually regained when the speed is increased again. The experiments presented in this paper also demonstrate that flux can be kept at the initial high level, if the shear rate is kept high enough. However, in most modules it is usually not feasible to keep the shear rate high enough. In solutions with colloidal substances that are gathered at the membrane surface, a fast initial flux decrease is commonly experienced, followed by a slowly progressing flux decline. The solutions in the actual experiments contain large solute molecules that may build up a cake of retained material at the membrane surface. When the shear rate decreases, the layer of solute molecules accumulated at the membrane surface grows. The flux decrease with time when the rotor is shut off is probably induced by compression and blocking of the flow channels in the layer of retained material at the membrane surface. The reduction in porosity of a compressible material subjected to drag forces of a fluid is treated in Ref. [ 141. In the actual case it is, however, very difficult to theoretically describe the flow resistance of a cake formed upon the membrane as the pore structure is influenced by the nature and concentration of the solutes in the solution, as well as by the initial pure water flux. In this investigation it was experienced that once the flux is decreased it is very difficult to restore it again. However, when the permeate flow is interrupted, and consequently the force pressing the cake against the membrane surface temporarily removed, the cake may be washed away with the retentate stream, as shown in Figs. 6 and 7. However, this implies,
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of course, that the material in the cake is not adsorbed at the membrane. Conclusions As soon as transport of solvent through a membrane commences solute is transported by convection towards the membrane and the concentration at the membrane surface is increased as solute is retained. Steady state is established when the convective flow of solute towards the membrane surface is balanced by the diffusive back transport of solute to the bulk solution. When treating solutions containing large aggregates (e.g. polymeric and colloidal substances), the diffusive back transport is almost negligible and a layer of retained material (a cake or gel layer) may be formed which constitutes a considerable hydraulic resistance in series with the membrane. The thickness of this layer is highly susceptible to the shear rate. The experiments presented in this paper indicate that if a cake is formed, an increase in the shear rate only marginally decreases the cake thickness and the flux is accordingly only marginally restored. Flux may, however, be regained by interrupting the permeate flow. As the permeate flow is interrupted the cake is no longer forced against the membrane surface by the permeate flow. Instead the cake is caused to flow laterally over the membrane surface and is discharged with the retentate at the exit of the module. References W.F. Blatt, A. Dravid, A.S. Michael5 and L. Nelsen, Solute polarization and cake formation in membrane ultrafiltration: Causes, consequences and control techniques, in J.E. Flinn (Ed.), Membrane Science and Technology, Plenum Press, New York, NY, 1970, pp. 47-97. D.R. Trettin and M.R. Doshi, Ultrafiltration in an unstirred batch cell, Ind. Eng. Chem. Fundam., 19 (1980) 189.
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G. Jonsson, Boundary layer phenomena during ultrafiltration of dextran and whey protein solutions, Desalination, 51 (1984) 61-77. J.G. Wijmans, S. Nakao and C.A. Smolders, Flux limitation in uitrafiltration: Osmotic pressure and gel layer model, J. Membrane Sci., 20 (1984) 115-124. J.G. Wijmans, S. Nakao, J.W.A. van den Berg, F.R. Troelstra and C.A. Smolders, Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration, J. Membrane Sci., 22 (1985) 117135. M. Lopez-Leiva and E. Matthiasson, Macromolecular adsorption and fouling in ultrafiltration and their relationships to concentration polarization, Proc. Int. Workshop on the Fundamentals and Applications of Surface Phenomena Associated with Fouling and Cleaning in Food Processing, B. Halistr6m, D.B. Lund and Ch. Trag&d (Eds.), TylSsand, Sweden, 1981, pp. 299-308. G.B. van den Berg and C.A. Smolders, Flux decline in ultrafiltration processes, Desalination, 77 (1990) lOl133.
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G. Belfort et al., Diagnosis of membrane fouling using a rotating annular filter. 1+2, J. Membrane Sci., 77 (1993) 1,23. M.C. Porter, Concentration polarization with membrane ultrafiltration, Ind Eng. Chem. Prod. Res. Dev., ll(3) (1972)234-248. A, Bottino, G. Capanelli, A. Imperato and S. Munari, Ultratlltration of hydrosoluble polymers. Effect of operating conditions on the performance of the membrane, J. Membrane Sci., 21 (1984) 247-267. F. Vigo, C. Uliana and P. Lupino, The performance of a rotating module in oily emulsions ultrafiitration, Sep. Sci. Technol., 20(2&3) (1985) 213-230. H. de Balmann, P. Aimar and V. Sanchez, Membrane partition and mass transfer in ultrafiltration, Sep. Sci. Technol., 25 (5) (1990) 507-534. M.D. Afonso and M.N. de Pinho, Membrane separation processes in the pulp and paper industry, Desalination, 85 (1991) 53-58. A.-S. JSnsson and B. Jiinsson, Fluid flow in compressible porous media. I. Steady-state conditions, AIChE J., 38(S) (1992) 1340-1348.