Characterization of gel layer formed during ultrafiltration of latex emulsion waste waters

Desalination, 94 (1993) 81-100 Elsevier Science Publishers B.V., Amsterdam -

81 Printed in The Netherlands

Characterization of gel layer formed during ultrafiltration of latex emulsion waste waters Michal Bodzek and Krystyna Konieczny Technical Universityof Silesia, Instituteof Water.Sewage and WastesEngineering, 44-101 Gliwice, VI. Kuczewskiego2 (Poland) (Received August 9, 1992; in revised form January 21, 1993) SUMMARY

The results of investigations of the ultrafiltration of latex waste waters, carried out with and without stirring, have been presented. The dependence of the permeate flux versus time profiles, transmembrane pressure and latex particle concentration for three polyacrylonitrile membranes of differing compactness, have been worked out. The basic parameters of the gel layer formed on the membrane surface, i.e., the resistance of the gel layer (G,), specific resistance (a) and mass of gel per unit area of the membrane (m,/A,) have been calculated based on the data of ultrafiltration without stirring. SYMBOLS

A

-

47% 4 ; I5 c cb

-

constant of exponential function membrane area, m2 membrane permeability for latex solution, m/s=Pa specific resistance of the gel layer, m/kg constant of exponential function exponent of power function concentration of latex solution, kg/m’ concentration of latex in bulk solution, kg/m3

001 l-9164/93 /$06.00 01993

Elsevier Science Publishers B.V. All rights reserved.

82

C.9 CP D 47

J?J JUJ k

mg AP A< 4 &A t

V vg s sg E P 77

concentration of latex in the gel layer, kg/m3 - concentration of latex in the permeate, kg/m3 - diffusion coefficient, m3/s - diameter of the latex particle in the gel layer, m - volume permeate flux, m/s - volume water flux, m/s - mass transfer coefficient, m/s - mass of the latex particles in the gel layer, kg - pressure drop across the membrane, Pa - pressure drop across the gel layer, Pa - resistance of the gel layer, m-* - membrane resistance, m-l - time of ultrafiltration - permeate volume, m3 - volume of the gel layer, m3 - thickness of the boundary layer, m - thickness of the gel layer, m - porosity (fraction voids) - latex particle density, kg/m3 - dynamic viscosity of the permeate, Pas

INTRODUCTION

Ultrafiltration trial applications

in the treatment of latex emulsion has three major indus[l-5]:

1. concentration of latex between polymerization usually from 30% to 65% (e.g. PVC latex);

and spray-drying,

2. concentration of dilute latex streams as a pollution control measure, and as a means of recovering valuable dilute latex, usually from 0.5% to 25% (e.g. styrene butadiene rubber latex, poly(viny1 acetate) latex); 3. purification of latex by ultrafiltration nating the impurities.

in a dilute mode due to elimi-

During ultrafiltration of latex emulsion on the membrane surface, a polarization layer appears, which decreases the flux rate and causes trouble in good exploitation of the ultrafiltration device. It is the aim of this paper to present the physico-chemical characterization of the gel layer which is formed on the membrane surface during ultrafiltration of latex emulsion waste waters.

83 THEORY

In the case of ultrafiltration of colloids, the increase of the particle concentration next to the membrane surface causes an aggregation of colloidal particles, or macromolecules and the formation of gel (Fig. 1). Thus a further resistance to the flow of permeate is encountered, in addition to those of the membrane and the boundary layer. The volume flux rate (J,) in pressure-driven membrane processes can be expressed by the relationship used in the theory of filtration [6-81:

Jv

dV =-.-= dt

1 A,

AP (R,+R,h

(1)

According to the same theory of filtration, the resistance of the gel layer (Fig. 1) can be written as: Rg

(2)

=y m

Here a, the specific resistance can be approximately related to the particle properties by [9]: a=

180 * (1 - E) p.3.G

(3)

The resistance of gel layer, Rg , can be also defined using the CarmanKozeny dependence [9], provided that the gel formed on the membrane surface is considered to be a porous layer of small particles through which the permeate flows laminarily:

A?cl= 180(l- d2.-rl- J, c3 d$ 4

_

(4)

Because mg = Vg . p and Vg = Sg. A,, and after taking into consideration the gel layer porosity (1 - E), the thickness of the deposited gel layer can be approximately calculated from the equation:

mg

sg= A m

*Cl-+p

(3

Substituting Eqn. (5) into (4) produces:

(6)

84

SOLUTION

BOUNDARY LAYER

Fig. 1. Scheme of concentration polarization phenomena: (a) without gel formation (b) with gel formation (explanation of the symbols in the text)

85

A general observation would be that the finer the particles the greater is the specific resistance. Equations (l)-(6) apply equally well to particulate filtration and colloidal ultrafiltration. The migration of particles towards the membrane surface (convective transport - J,) brings about the so-called phenomenon of concentration polarization (Fig. l), i.e. and increase of the concentration of particles next to the membrane surface (C,). Due to the concentration gradients, the particles are returned to the bulk solution (J,). The back transport increases with the membrane surface, until the flux of colloidal particles that are being carried towards the membrane surface becomes equal to the diffusion flux of the particles in the opposite direction. When the convective and back transport flux are equal, there occurs a stationary state that can be expressed by the equation:

Because in our experiments C, = 0, the integration of EQn. (7) results in [lo-121:

Jw= (D/S) ln
(8)

From Eqn. (8) it follows that the permeate flux in stationary state conditions does depend neither on the pressure difference nor on the porosity of the membrane, but only on the characteristics of the particles (D and C,) and the thickness of the boundary layer, S. In cases of ultrafiltration without stirring, gel is formed even at very low pressures, because of the low back-transport rate. For constant pressure and m, = V - Cg, the Eqns. (1) and (2) give a well-known filtration equation: t

-_= V

R,-q AP- A, +2-

Cg *a-q AP-A$,

-V

(9)

In the case of ultrafiltration without stirring, the slope of t/V vs. V enables us to determine the value of a, whereas the membrane resistance, R, is calculated from the intercept. Equation (9) is based on the assumption that all particles convected to the gel layer are accumulated. This assumption is valid for unstirred filtration of large particles and may approximate the initial conditions for deadend ultrafiltration of colloids where concentration gradients are sufficiently small to minimize diffusion. For conditions with significant back-transport (i.e. caused by shear-induced concentration gradients) of solids, a balance

86

is rapidly achieved between convective transport and back-transport so that m, NC, and Pqn. (9) no longer applies (although Eqns. (l)-(3) may still apply). The stirred ultrafiltration of solids, small enough to experience diffusive back-transport, is frequently described by the widely accepted relationship [ 81. Stirring increases considerably the diffusion of particles from the membrane surface towards the bulk concentration, so that at low pressures, when the convective transport of particles in small, no gel is formed next to the membrane surface. For the formation of gel, a certain minimum pressure, called the threshold transmembrane pressure, is required. The flux rate in the stationary state increases with the increase of pressure only up to the threshold pressure. Above this value, gel is formed, so that the flux rate no longer depends on pressure, and Eqn. (8) then holds true. The threshold pressure depends on the type and concentration of the solution, the speed of stirring and the porosity of the membrane. The threshold pressure increases with the increase of the speed of stirring and the decrease of solute concentration and the porosity of the membrane [lo]. EXPERIMENTAL

Stirred and unstirred experiments were performed in a low-pressure Amicon-type cell (capacity 1000 cm3) with all effective membrane area 66 cm2. The cell was equipped with a plasticized magnetic stirrer. The initial solution volume amounted in all investigations to 1,000 cm3. In order to obtain a fixed pressure in the cell, compressed nitrogen was used. The pressure was changed within the range 1. 105Pa-3 - IdPa. In all investigations, ultrafiltration was performed until 100 cm3 of the permeate were obtained in a graduated cylinder. All the investigations were performed at a temperature of 22f lo C, and stirred experiments at stirring rate amounted to 31.4 rad/s. In the experiments, the latex waste waters coming from the production of OSATEKS has been used. Osateks is the preparation which is used to screen printing with pigments on all types of textiles and to roller printing also with pigments. The latex emulsion waste water composition is complex, consisting of acrylo-butadiene-vinyl and styrene-butadiene c’opolymers with modifiers and emulsifier additives [ 131. The dry solids content of waste waters amounted to 15 kg/m3. The particle size of the latex was about 0.2 pm.

87

In all investigations polyacrylonitrile ultrafiltration membranes, made in our laboratory [ 141,were used. Asymmetric membranes were formed by means of the phase-inversion method [ 15, 161 consisting of casting a film 200 pm thick from homogeneous polymer solution in dimethylformamide of 13-17.5% concentration on a glass plate. The plate with the membrane was then immersed as quickly as possible into a water coagulation bath. A detailed description of how to prepare the membrane and its characterization have been given in [ 141. Such a membrane completely retained the colloidal latex particles. Table I gives the casting solution composition and details of membrane forming parameters, and Fig. 2 - the water transport characteristics of the membranes used. I

1

PRESSURE*

40m5,

Pa

Fig. 2. Dependence of volume water flux on pressure for three polyacrylonitrile membranes differing in compaction.

The membranes differed in compactness and were characterized by a linear dependence of the water flux (J,) on pressure (A P) according to the following equations:

J7u = 1.25 - lo-” Jul = 8 .38 - lo-” JUJ = 6.00 - lo-”

. -

AP AP AP

PAN

- 13 membr. (open membr.)

PAN - 15 membr. (medium compactness) PAN - 17.5 membr. (compact membr.)

when J, is expressed in m/s and A P in Pa.

.

88 TABLE I Composition of casting solutions and conditions for preparation of PAN membranes Composition of casting solution, mass % Membrane type

Polymer

Dimethylformamide

PAN-13 open PAN- 15 medium compactness

13 15

87 85

PAN-17.5 compact

17.5

82.5

Preparationconditions: Time of solvent evaporation: 25 s Temp. of casting solution and forming atmosphere: 298” K Gelling agent: water Time of gelling: 900 s Temp. of gelling bath: 293’ K DISCUSSION OF THE RESULTS

Influence of time, pressure andfeed concentration on jh.x Figures 3 and 4 show the dependence of the permeate volume vs. time for latex particles of different concentration in the range of 0.15-15 mg/ m3 (A P = 2 - 105Pa) and different transmembrane pressure in the range 1. 105, 2 . lo5 and 3 - 105Pa (concentration 1.5 kg/m3) as well as for three types of membranes used with and without stirring. It is evident that the permeate volume increases with the increase of the pressure and speed of stirring and decreases with latex concentration. For ultrafiltration with stirring a linear dependence of the permeate volume with time is obtained which indicates that during the stirring, a stationary sate, that means a constant value of the flux rate, is quickly established. Without stirring, the flux rate gradually decreases and a stationary state is practically never established_ This is caused by the increase of the thickness of the gel layer which, according to Eqn. (2), brings about an increase of the resistance of the gel layer. Because the resistance of the membrane does not change, the increase of the resistance of the gel layer, according to Eqn. (l), brings about a decrease of the flux rate. Differences observed in the flux rate for various pressures and concentrations are more prominent in the case of ultrafiltration without stirring and for compact membranes.

89

60_

t 2

4

I

L

8

6 TIME

* 40-3,

S

Fig. 3. Permeate volume vs. time for latex waste water ultrafdtration with a concentration of 1.5 kg/m3 1 - transmembrane pressure: 1 . 1dPa 2 - transmembrane pressure: 2.ldPa 3 - transmembrane pressuure: 3 . 1dPa x - unstirred mode of ultrafiltration l - stirred mode of ultrafiltration.

TIME.

‘0~‘,

s

Fig. 4. Permeate volume vs. time for latex waste water ultrafiltration of various concentrations (A P = 2 10’) 1 - concentration: 15 kg/m3 2 -concentration: 7.5 kg/m3 3 - concentration: 5.0 kg/m3 4 - concentration: 3.0 kg/m3 5 - concentration: 1.5 kg/m3 6 - concentration: 0.15 kg/m3 x -unstirred mode of ultrafiltration l - stirred mode of ultrafiltration.

91

The dependence of the flux rate on the transmembrane pressure for three different compactnesses of the membranes and for ultrafiltration with and without stirring is given in Fig. 5. A mathematical analysis of experimental

-r? E

I

‘4r

“0‘ z

-

PAN-I3

_--_-

MEMBRANE

PAN- 15 HEMBRANE

-.-.-

?iN-175

MEMBRANE

i2_

3 u.

POESSlJRE*

10-5,

Pa

Fig. 5. Permeate flux vs. transmembrane pressure at the ultrtiltration Oflatex Waste WatefS (concentration: 1.5 kg/m3) x - unstirred mode of ultrafiltration l - stirred mode of ultrafiltration.

data leads to the conclusion that the dependence of the volume permeate flux (J,) on the pressure ( A P) is expressed by the function:

Jv

=A,

-(AP)b

(10)

Table II contains the values of the equation constants for all types of membranes and for two modes of ultrafiltration procedure, calculated on the basis of experimental data. The permeability constant (A,) decrease with the growth of the membrane compactness and increases with the speed of stirring. The exponent b increases with the membrane compactness for both modes of ultrafiltration procedure, and has higher values for unstirred mode of ultrafiltration. The data contained in Table II were calculated, approximating the experimental results to Eqn. (10). The obtained mean relative errors of approximation testify a good conformity with the results of measurements basing on Eqn. (10) (Table II).

92

TABLE II Values of A, and b in equation J, = A, . (A P)’ and mean relative error of approximation (E) for ultrafiltration of latex emulsion (J” is expressed in m/s and A P in Pa) Membrane me

Mode OfUF

A, . 10”

b

E

PAN-13 open

Stirred

1.50

0.293

0.00365

Unstirred

0.863

0.695

0.00460

Stirred

1.25

0.358

0.00452

Unstirred

0.468

0.498

0.00347

stirred

1.21

0.375

0.00419

Unstirred

0.368

0.563

0.00332

PAN-15 medium compactness PAN-17.5 compact

The effect of pressure on flux can be explained by “film theory” model [ 1, 17-221 and can be summarized as follows: at low pressure (till 0.5 - l@1 s l@Pa) and high feed velocity (stirring), i.e., under conditions where concentration polarization effects are minimal, flux will be affected by the transmembrane pressure. Deviation from linear flux-pressure model can be observed at higher pressures (regardless of other operating parameters) due to consolidation of gel-polarized layer. Pressure independence occurs at higher pressure, because solute concentration in the gel layer reaches a maximum, i.e., no more solute molecules can be accommodated due to the “close-packed” arrangement and restricted mobility of the solute molecules (C,) (Fig. lb). Increasing the transmembrane pressure merely results in a thicker or dense solute layer. After a momentary rise, the flux will drop back to the previous value. Thus flux in the operation region will be controlled more by the efficiency of minimizing boundary layer thickness and enhancing the rate of back-transfer of polarized molecules. The gel-polarized layer is assumed to be dynamic. Changing the operating conditions (pressure, concentration, feed velocity) will enable us to revert back to the pressure controlled operating regime. The film-theory model (Eqn. 8) states the flux will decrease exponentially with increasing feed concentration. Figure 6 presents the results of permeate flux (J,,) for various latex concentration (C) and for experiments with and without stirring and for all three types of membranes. Mathematical analysis of the results gives, in fact, exponential dependence of J, on c:

93

i1.0. fill_

50..

4.0_

al_

2.0 -

4.0 -

2.

e

3

4

5

6 189Kl’

L

3

2

5

678!

CONCENTRATION,lq/m’

Fig. 6. Permeate flux vs. concentration at the ultrafiltration of latex waste waters 1 - PAN- 13 membrane 2 -PAN-l5 membrane 3 -PAN-17.5 membrane.

Jtl = B

.

,A’C

(11)

Table III presents the results of A and B values with mean relative error of approximation. The constant B decreases with the membrane compactness and also with feed velocity (stirring). The constant A, on the other hand, does not practically depend on membrane compactness for stirred mode of

94 TABLE III

Values of A and B in equation J, = B . eA’C and mean relative error of approximation (E) for ultrtiltration of latex emulsion (.I” is expressed in m/s and C in kg/m3) Membrane tw

Mode OfUF

B alo6

A.ld

E

PAN-13 open

Stirred

5.25

- 8.62

0.00136

Unstirred

1.45

- 8.76

0.00642

PAN-15 medium compactness

Stirred

4.80

- 10.7

0.00161

Unstirred

1.30

-12.8

0.00064

PAN-17.5 compact

Stirred

4.05

-10.1

0.00157

Unstirred

1.33

-17.3

0.00022

ultrafiltration, and decreases with membrane compactness for unstirred experiments. Figure 6 makes possible determining the gel-layer concentration (C,), because the film theory model implies that at J, = 0, C = Cg. Parameters of latex gel layer

In the case of the ultrafiltration of distilled water, Eqn. (1) takes the form:

from which we obtain: (13) Assuming that the viscosity of water is q = 0.93 - 10B3Pas, we can calculate the membrane resistance (R,) from the experimental data of the water flux-pressure dependence (Fig. 2). The value obtained from (Eq. 13) amounted to: PAN- 13 membrane: Rln = (8.60 f 0.011) - lO’*m-’ PAN- 15 membrane: R,,, = (1.28 f 0.009) - 1013m-1 PAN-17.5 membrane: &A = (1.79 f 0.010) - 1013m-1 independently of the conditions of investigation (stirring, pressure). The values obtained from Eqn. (9) are somewhat lower because the deposited gel

95

TABLE IV Resistance of the gel layer, R, for unstirredultrafiltration of latex waste waters (R, . lo- 13, m-l) Pressure A P . 10m5Pa (C = 1.5 kg/m3) 0.5 1.0 1.5 2.0 2.5 3.0

PAN-13 membrane 4.52 4.83 5.82 6.52 7.29 8.52

PAN-15 membrane

PAN-17.5 membrane

4.09 6.07 7.19 9.46 10.1 11.5

4.59 7.39 10.1 12.5 12.5 15.1

Concentration C, kg/m3 (Ap=2.10SPa)

PAN-13 membrane

PAN-15 membrane

PAN-17.5 membrane

1.5 3.0 5.0 7.5 10.0

7.02 16.2 23.7 30.9 51.6

9.89 20.4 31.4 48.1 106

13.1 24.4 37.7 49.6 213

layer influences the obtained results, and straight lines presenting Eqn. (9) for various pressures have different values. The membrane resistance and flux rate being known, the polarization resistance (R,) can be determined from Eqn. (1). Since in the case of unstirred ultrafiltration, the polarization resistance gradually increases, reaching its maximum value at the end of filtering, Rg was determined at the end of the experiment for all unstirred runs. The results have been summarized in Table IV From Table IV it can be seen that the resistance of the gel layer increases with the increase of pressure at the identical latex concentration, and that it rises with the rise of the latex concentration at the same pressure. The increase of Rg is influenced by the decrease of the gel porosity. The gel porosity is smaller if pressure and concentration are greater. The resistance of the gel layer increases also with the membrane compactness. Making use of Eqn. (9) and basing on experimental data, the specific resistance (a) was determined from the slope of the lines t/V = f(V) (Fig. 7). These values of the specific resistance have been presented in Table V. The dependence of the specific resistance on pressure, on the latex concentration and on the membrane compactness is identical to those obtained for

96

Fig. 7. t/V vs. V curves of unstirred ultrafWation experiments carried out with latex waste waters with concentration of 1.5 kg/m3 (V-permeate volume, t - time of experiment) 1 - transmembraue pressure: 1 . 1dPa 2 - transmembrane pressure: 2 x 1dPa 3 - trammembrane pressure: 3 x IdPa.

97

TABLE V Specific resistance of the gel layer, a for unstirred ultrafiltration of latex waste waters (1 . 10-15, m/kg) Pressure A P .lo-‘, Pa (C = 1.5kg/m3 0.5

1.1 1.5 2.0 2.5 3.0

PAN-13 membrane 1.05 1.28 1.45 1.64 2.10 2.46

PAN-15 membrane 1.85 2.18 3.25 4.55 5.01 6.00

PAN-17.5 membrane 2.51 3.27 4.21 5.05 5.71 6.06

Concentration C, kg/m3 (A~=2.ldPa)

PAN-13 membrane

PAN-15 membrane

PAN-17.5 membrane

1.5 3.0 5.0 7.5 10.0

0.78 5.22 6.95 8.98 12.0

4.05 5.15 7.84 11.4 23.8

5.50 6.48 9.21 13.5 28.4

TABLE VI Mass gel per unit area of the membrane for unstirred ultrafiltrationof latex waste waters &/Am. mg/m2) PAN-13 membrane

PAN-15 membrane

PAN-17.5 membrane

0.0430 0.0377 0.0401 0.0398 0.0347 0.0346

0.022 1 0.0278 0.022 1 0.0208 0.0202 0.0192

0.0183 0.0226 0.0240 0.0247 0.0219 0.0249

Concentration C, kg/m3 (A =2.ldPa)

PAN-13 membrane

PAN-15 membrane

PAN-17.5 membrane

1.5 3.0 5.0 7.5 10.0

0.25 1 0.310 0.341 0.344 0.401

0.244 0.404 0.400 0.420 0.045

0.0238 0.0377 0.0409 0.0441 0.0750

Pressure A P .10m5,Pa (C = 1.5 kg/m3) 0.5 1.0 1.5 2.0 2.5 3.0

98

the resistance of gel layer (R,). The previously observed trend [23] with higher a and Rg for higher concentrations and pressures, was confirmed. Using Eqn. (2), the mass of gel per unit area of the membrane (m,/A,) was calculated (Table VI). Analyzing the data gathered in Tables IV-VI it can be seen that the increase of the resistance of the gel layer (R,) with the increase of pressure at the same silica concentration, is not a consequence of the gel mass, but of its specific resistance (a). At the same latex concentration, with the growth of pressure, the rate of gel formation is growing too, but the time of ultrafiltration is reduced. The result of this is that the deposited mass of gel is approximately the same as the one obtained at a lower pressure, where the formation of gel is slower, and ultrafiltration lasts longer. The increase of the resistance of the gel layer, influenced by the increase of the latex concentration at a constant pressure, is the result of the increase of the gel mass and its specific resistance. The size of particles does not considerably influence the mass of gel that has been formed, but it does influence its porosity, and consequently, its specific resistance. Because of the smaller gel porosity in the case of more compact membrane, the specific resistance, i.e., gel resistance, is greater, whereas the flux rate is consequently smaller. Based on the results obtained by means of ultrafiltration with stirring and on data from literature [24], it has been stated that, due to the characteristics of the applied membrane, the pressure threshold was not reached in the course of the experiments. So no gel was formed and a state, during which the permeate flux does not depend on pressure, was not established. Therefore, Eqns. (8) and (9) cannot be applied for data obtained with stirring involved. CONCLUSIONS

1. The dependence of the permeate volume on time is linear for stirring ultrafiltration mode and non-linear for unstirring one. 2. The dependence of the permeate flux on transmembrane pressure can be expressed by the power function: J,, = A, - (A P)b. 3. The dependence of the permeate flux on latex concentration can be described by exponential function: J,, = B - eAeC. 4. The resistance and specific resistance of the latex gel layer increase with the increase of pressure and latex concentration as well as membrane compactness.

99 ACKNOWLEDGEMENTS

The authors are thankful to the Institute of Heavy Organic Synthesis (Qdzierzyn-Ko%le, Poland) for samples of waste waters. The studies were realized within the framework of Project No. 6 027191 Ol/p2 in the years 1991-1994 by the Polish Committee for Scientific Research. REFERENCES M. Cheryan, Ultrafiltration Handbook, Technomic Publishing Co., Lancaster; 1986. A.N. Jonsson and G. Tragardh, Desalination, 77 (1990) 135. M. Bodzek, K. Kominek and J. Zielinski, A new technique in sanitary engineering (in Polish), Wodociagi i Kanalizacja, No. 13, Arkady Warszawa, 1980, pp. 7-l 15. 4

M. Bodzek, Proceedings of the 1st Polish Scientific Conference on Process Engineering in Environmental Control, Warshaw, September 1991, p. 116 (in Polish).

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6

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12

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13

Commercial information obtained from the Institute of Heavy Organic Synthesis (Kedzierzyn-Kotle, Poland) 1992.

14

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15

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16

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17

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18

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100 20

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21

S. Nakao and S. Kimura, in: Characterization of Ultrafiltration Membranes, G. Tragardh (Ed.), Proceedings from an International Workshop arranged by the Department of Food Engineering, Lund Univ., Sweden, 1987, p. 141.

22

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23

M. Bodzek and K. Konieczny, Polish J. Chem., 66 (1992) 1683.

24

R.M. Quinn, Desalination, 46 (1983) 113.