Journal of Membrane Science 198 (2002) 75–85
Performance of ultrafiltration membranes in ethanol–water
solutions: effect of membrane conditioning
Rishi Shukla1 , Munir Cheryan∗
Agricultural Bioprocess Laboratory, Department of Food Science, University of Illinois,
1302 West Pennsylvania Avenue, Urbana, IL 61801, USA
Received 29 May 2001; received in revised form 15 August 2001; accepted 20 August 2001
Abstract
Several commercial polymeric ultrafiltration membranes were screened for their performance with aqueous ethanol solutions. The method of conditioning the membrane has a major effect on solvent flux, membrane integrity and their pressure
ratings. Gradual solvent exchange with successively higher concentrations increased in small doses appears to work best with
completely miscible solvents such as those studied here (ethanol–water mixtures). Rapid solvent exchange between water and
high concentrations of alcohol disrupts the polymer matrix in many cases. The Darcy model was used to correlate the data and
it indicated that viscosity differences of the ethanol solutions could account for part of the variations in solvent flux with some
membranes. Exposure to organic solvents significantly reduces the pressure rating of the membranes. Several membranes
that provided acceptable rejection of ethanol-soluble proteins at low pressures (138 kPa, 20 psi) lose its properties at higher
pressures (413 kPa, 60 psi) if conditioned incorrectly, and vice versa. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ultrafiltration; Organic separations; Ethanol; Protein
1. Introduction
Organic solvents are being increasingly used in
extraction, purification and processing of pharmaceuticals, food, nutraceuticals and flavor compounds.
Membrane technology would be the method of choice
for separating the desired compounds and for recycling the solvent. However, almost all current applications of membrane technology are with aqueous
systems. The few nonaqueous applications discussed
in the literature usually deal with streams containing
organic compounds with concentrations only up to a
∗ Corresponding author. Fax: +1-217-244-2455.
E-mail address: [email protected] (M. Cheryan).
1 Present address: James R. Randall Research Center, Archer
Daniels Midland Co., Decatur, IL, USA.
few thousand of parts per million such as oily waste
streams and cleaning solvents [1]. There are very few
examples of membrane applications with feeds containing 50–100% organic solvents. In addition, much
of the membrane work to date with organic solvents
has been with nanofiltration membranes, e.g. in the
vegetable oil industry for degumming, deacidification
and solvent recovery [2–4].
There are fewer reports on the use of ultrafiltration
(UF) membranes with relatively high concentrations of
organic solvents. In one of the earliest reports on this
subject, Nguyen et al. [5] observed that, in the absence
of solutes, membrane permeability of several commercial UF membranes increased with some solvents
(ethanol, methanol) and decreased with others (chloroform, decane, benzene). Lencki and Williams [6]
studied 10,000 and 30,000 molecular weight cut-off
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 6 3 8 - X
76
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
(MWCO) membranes with methanol, ethanol and acetonitrile. Solvents with solubility parameters similar
to the membrane reportedly led to the greatest change
in flow resistance, but solvents with a similar solubility parameter but low hydrogen bonding capabilities
could disrupt the structure of anisotropic polysulfone
membranes to such an extent that a dramatic drop in
flow resistance is observed. Jaffrin and Charrier [7] observed an 80% drop in flux in 40% ethanol compared
to water in the manufacture of plasma proteins from
human serum. The reduction in flux was explained
by an increase in viscosity when ethanol was added
to water and the formation of a thicker polarization
gel layer due to lower back diffusion of the solutes in
40% ethanol. Gupta et al. [8] reported a 25–35% drop
in UF flux when ethanol concentration was increased
from 20 to 30% with albumin in mineral (Carbosep)
membranes.
Most polymers used in manufacturing membranes
and/or their supports are first dissolved in organic solvents as part of their casting process. Consequently,
they could swell or dissolve in the solvent, leading to
unacceptable changes in solvent flux and solute separation. For example, of the 15 membranes screened
by Koseoglu et al. [4] for vegetable oil processing,
only 5 were found to be stable to hexane. Raman et al.
[9,10] tested 13 membranes for deacidification of
soybean oil of which only 6 were stable in methanol,
and only one was compatible with hexane. Kuk et al.
[11] reported significant degradation in membrane
performance on exposure to aqueous and anhydrous
ethanol solutions with cellulose acetate and composite reverse osmosis (RO) membranes with vegetable
oil–solvent mixtures. Koike et al. [12] screened 18
membranes (including some developed for gas separations) for separation of fatty acids and glycerides
from lipase hydrolysates of high oleic sunflower oil.
Cellulosic membranes gave good results but suffered
from poor long-term stability. Zwijnenberg et al.
[13] used prototype polyamide and cellulose-based
membranes for deacidification of vegetable oils in
acetone.
This paper reports on our studies on polymeric UF
membranes with ethanol–water solutions. The ultimate goal was to develop a process for the manufacture of zein, which is a hydrophobic, ethanol-soluble
protein in corn with a molecular weight of ∼22,000.
Zein has a variety of industrial uses, from fibers and
adhesives to chewing gums and biodegradable plastics [14]. The commercial application of this natural
polymer is limited by its high manufacturing cost, due
primarily to the high cost of separating and purifying
the zein from the ethanol extract and recovering the
ethanol solvent. Ultrafiltration could be used readily to
recover and purify the zein while simultaneously recycling the ethanol solvent [15]. The best solvent for extracting zein from corn is 70% (v/v) ethanol [16] and
thus our studies were limited to a maximum ethanol
concentration of 70% (v/v). This paper specifically focuses on the effect of conditioning on solvent flux and
rejection of zein.
2. Materials and methods
2.1. Membrane screening
Ultrafiltration membranes were obtained from several manufacturers and are listed in Table 1. Flat sheets
were evaluated in an Amicon dead-end stirred cell
(model 502) which used 62 mm membrane discs of
area 28.7 cm2 . Pressure was provided by a nitrogen
gas cylinder and turbulence was created by a magnetic
stirrer which was operated at 300 rpm. The hollow
fibers were tested as-is in their housings in a recycle
system. A pump provided pressure and the required
cross-flow for turbulence. All experiments were conducted at room temperature (24 ± 2◦ C).
2.2. Ethanol solutions
Ethanol (anhydrous, 200 proof) was obtained from
McCormick Distillation Co., Weston, MO. Aqueous
solutions of ethanol (EtOH) were prepared on a volume/volume basis. Deionized water was used for all
experiments. Ethanol and deionized water were microfiltered through a 0.2 ␮m filter before use. Viscosity of
ethanol solutions (with and without protein) were measured using Ubbelohde viscometers (model OC with
an instrument coefficient of 0.002752 cSt/s and model
1 with a coefficient of 0.00963 cSt/s) obtained from
Cannon Instrument Co., State College, PA. Densities
of the solutions were measured using precalibrated pycnometers. Viscosity measurements were performed
in triplicate while density data are the mean of five
observations.
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
77
Table 1
Ultrafiltration membranes selected for screening studies
Materiala
Membrane
MWCOb
Manufacturerc
Configurationd
Cellulose ester
Cellulose acetate
Regen. cellulose
Regen. cellulose
Compositee
Compositee
Compositee
PAN-m
PAN-based
PES-m
PES-m
PES
PES
PS
PS
PS
PS
PVDF
Type C
Cell
PLGC
YM10
U20S
G80
H051
MX25
U20T
Alpha
Omega
UFC
PES4H
UFP10
PS10
PM10
PM30
AN09
10000
10000
10000
10000
20000
5000
–
25000
20000
10000
10000
10000
10000
10000
10000
10000
30000
25000
Spectrum-Microgon
Pall Filtron
Millipore
Millipore
Koch
Osmonics
Osmonics
Osmonics
Koch
Pall Filtron
Pall Filtron
Hoechst
Hoechst
A/G Technology
Sartorius
Koch
Millipore
Osmonics
FS
FS
FS
FS
FS
FS
FS
FS
FS
FS
FS
FS
FS
HF
FS
HF
FS
FS
a
PAN: polyacrylonitrile, PES: polyethersulfone, PS: polysulfone, PVDF: polyvinylidine fluoride, Regen.: regenerated, m: modified.
Molecular weight cut-off from manufacturers’ specifications.
c A/G Technology, Needham, MA; Hoechst, Wiesbaden, Germany (through US Tech., Cincinnati, OH); Koch Membrane Systems,
Wilmington, MA; Millipore, Bedford, MA; Osmonics, Minnetonka, MN; Spectrum-Microgon, Laguna Hills, CA; Pall Filtron, Northborough,
MA; Sartorius, Edgewood, NY.
d FS: flat sheet, HF: hollow fiber.
e Composition is proprietary.
b
2.3. Membrane conditioning
Most membranes (except Koch U20S) were received from the manufacturer preserved with glycerol
or a similar humectant. Prior to use, the membranes
were conditioned by gradual exposure to solvents
using the following protocol:
1. In each case, the membrane was thoroughly rinsed
with large amounts of the solvent that the membrane first came in contact with (depending on the
method of conditioning as discussed below).
2. The membranes were then soaked in the solvent
for a period of at least 6 h (overnight in most cases)
and then fitted in to the appropriate membrane test
cell. The Amicon cell was then filled with 200 ml
of the conditioning solvent (for the hollow fibers,
the solvent was placed in a separate feed tank connected to the module via a pump).
3. The system was pressurized to 69 kPa (10 psi) in all
cases except with the U20S and G80 membranes
which were pressurized to 138 kPa (20 psi), and
4.
5.
6.
7.
8.
the hollow fibers which were pressurized to 35 kPa
(5 psi).
At least 25% of the initial solvent volume was allowed to permeate through the membrane.
Flux was then measured.
The pressure was then raised to the next higher level
(which varied for each membrane as reported later)
and steps 4 and 5 repeated. The pressures tested
in each case depended on the pressure rating of
the membrane. In most cases, flat sheet polymeric
membranes were tested up to 413 kPa (60 psi) and
hollow fibers up to 138 kPa (20 psi).
After the tests, the membrane was removed from
the cell and stored in a petri dish soaked in the
appropriate solvent for at least 6 h at 24◦ C and the
flux measurements (steps 4–6) repeated.
The membrane was then conditioned in the next
consecutive solvent, which depended on the
method of conditioning, as discussed below.
Four methods of membrane conditioning were evaluated. The upper ethanol concentration was limited to
78
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
70% ethanol in most of our studies since this was the
optimum concentration for the extraction of zein from
corn [16].
Method 1: Gradual change from 0 to 70% ethanol.
The membrane was initially exposed to water and
then conditioned to 70% ethanol in increments of
10% ethanol concentration (i.e. experiments were performed with water, 10% ethanol, 20, 30, 40, 50, 60
and 70% ethanol). At each stage, steps 2–7 described
above were repeated.
Method 2: Direct change from 0 to 70% ethanol.
The membrane was first exposed to water through
steps 2–7 and then solvent-exchanged directly with
70% ethanol without any intermediate ethanol concentrations.
Method 3: 70% ethanol. The membrane was exposed directly to 70% ethanol using steps 2–7.
Method 4: 100 ethanol to 70% ethanol. The membrane was initially exposed to 100% ethanol and then
conditioned to 70% ethanol in steps of 10% ethanol
concentration (i.e. experiments were performed for
100, 90, 80 and 70% ethanol).
At the end of each conditioning experiment, the
membranes were tested for flux and zein rejection with
a model protein solution using the method described
below. This would provide an indication of the effect
of membrane conditioning on membrane integrity and
performance.
Conditioning data were expressed in terms of a convective transport model:
J =
LP PT
µ
(1)
where J is the flux, PT the transmembrane pressure, µ
the viscosity of the permeate and LP is the permeability
coefficient of the membrane. Membranes not affected
by the solvent should give a linear plot of flux versus
1/µ. In cases where the solvent does have an effect
on the membrane (e.g. swelling of polymer, dilating
of pores), the plot will not be linear. All polymeric
membranes listed in Table 1 were first tested using
method 1 for solvent stability. Membranes that gave
linear or nearly linear plots were then evaluated with
methods 2–4.
In order to include pressure effects, the data are also
plotted in terms of relative resistance (R/Rw ), where R
is the membrane resistance in the presence of ethanol
solutions, and Rw is the membrane resistance mea-
sured with pure water. From Eq. (1), the membrane
resistance (R or Rw ) can be calculated as follows:
R=
1
PT
=
LP
µJ
(2)
2.4. Membrane screening with zein solutions
Flux and rejection of an ethanol-soluble protein was
measured using a solution of 5 g/l zein (F4000, Freeman Industries, Tuckahoe, NY) made up in 70% (v/v)
aqueous ethanol. For each experiment, the preconditioned membrane was placed in the Amicon stirred cell
with 250 ml of the zein solution. The cell was pressurized and flux measurement started. Flux measurement
continued at each pressure until at least three consecutive values were constant. This is reported as the
steady-state flux. Permeate samples (approximately
2–3 g each) were collected and analyzed for protein
content by the Kjeldahl method. Rejection is defined
as:
CP
R = 1−
× 100
(3)
CR
where CP and CR are the concentrations of zein in permeate and retentate, respectively. Zein concentrations
for the model solutions were measured spectrophotometrically by the procedure of Craine et al. [17].
All experiments were repeated within 24 h with a
fresh membrane disc. The membranes were cleaned
after the zein experiments by soaking in a solution of
5 g/l NaOH in 70% ethanol for periods up to 6 h. The
membrane was then thoroughly rinsed with several
volumes of fresh 70% ethanol.
2.5. Static swelling
Membrane swelling was determined as an increase
in weight due to solvent absorption (Sw ) (%):
Ww − Wd
Sw =
× 100
(4)
Wd
where Wd and Ww are the weights of dry and wet
membrane samples, respectively. The wet weight, Ww ,
was obtained by incubating membranes at 24◦ C for
up to 24 h in the appropriate solvent. All swelling experiments were replicated with at least five samples of
each membrane and the mean value is reported.
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
79
3. Results and discussion
3.1. Membrane conditioning
Figs. 1–4 show typical flux behavior as a function
of ethanol concentration in the solvent and transmembrane pressure when membranes were conditioned using method 1. Many membranes showed a minimum
in flux at 25–50% (v/v) ethanol (0.12–0.26 M ethanol)
and the flux was higher at higher pressures. Due to
space limitations, data for all combinations of membranes and pressures are not shown here. The complete data set for all membranes studied is available
in Shukla [18].
Each data point in Figs. 1–4 was repeated within
24 h with a fresh membrane disc. Reproducibility
of flux data was high (±5%) for membranes that
were minimally or not affected by ethanol solutions,
while reproducibility was poor for those membranes
affected by the solvent. As a group, the composite
Fig. 2. Effect of ethanol concentration on flux of polysulfone and
polyethersulfone membranes. Membranes were conditioned using
method 1. Pressure was 275 kPa except for PM10 membrane,
which was 69 kPa.
membranes (G80, H051 and U20S) shown in Fig. 3
displayed lower solvent fluxes of <50 l/m2 h (LMH)
than most other membranes. Among the polysulfone and polyethersulfone family of membranes, the
PES4H had much lower flux (5–15 LMH) than others
Fig. 1. Effect of ethanol concentration and transmembrane pressure on flux of a modified PAN and cellulose ester membrane.
Membranes were conditioned using method 1.
Fig. 3. Effect of ethanol concentration and transmembrane pressure
on flux of composites, polysulfone and polyethersulfone membranes. Membranes were conditioned using method 1. Pressure
was 275 kPa except for UFP10 membrane, which was 124 kPa
((䊉) UFC; (䊐) UFP10; (䉬) PES4H; () G80; (䊏) H051; (䊊)
U20S).
80
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
Fig. 4. Effect of ethanol concentration on flux of cellulose-based
and PAN-based membranes. Membranes were conditioned using
method 1. Pressure was 275 kPa.
(Figs. 2 and 3). This could be due to differences in
hydrophobicity of the membrane materials, or due
to differences in density of the membrane top layer
which results in higher membrane resistance and
lower permeability. Surfaced-modified PES membranes that are usually cross-linked after membrane
formation displayed lower solvent fluxes.
Since the viscosity of ethanol solutions is maximum
at 50% ethanol concentration (Fig. 5), the flux should
show a corresponding minimum value at this ethanol
concentration, assuming the membrane is not affected
by the solvent in any other manner. This phenomenon
Fig. 5. Effect of ethanol concentration on viscosity and density of
ethanol solutions at 24◦ C.
Fig. 6. Darcy plot for a modified PAN and a PES membrane.
Effect of pressure and ethanol concentration (expressed in terms
of viscosity) on flux. Membranes were conditioned using method
1. (MX25: (䊏) 138 kPa, (䊉) 275 kPa, (䉱) 413 kPa; PES4H: (䊐)
138 kPa, (䊊) 275 kPa, () 413 kPa).
is governed by the Darcy or Hagen–Pouseuille laws
shown in Eq. (1). Several membranes do show a flux
minima in the middle range of ethanol concentrations
(Figs. 1–4). Thus, a linear correlation should be obtained between flux and the reciprocal of viscosity of
the permeating solvent, according to Eq. (1).
A typical Darcy plot for two membranes at different pressures is shown in Fig. 6. Of the 18 membranes
tested, 15 generally followed Darcy’s law in that flux
generally decreased with increasing viscosity of the
ethanol solutions [18]. The three exceptions were the
composite membranes (G80, U20S and H051). This
phenomenon could be explained by viscosity considerations alone, in a manner similar to that observed
by others [6,19,20]. Our results are similar to those
of Machado et al. [21] but in contrast to those of
Iwama and Kazuse [20]. The latter group used binary solvent mixtures that followed the viscosity additive rule, whereas the solvents used by us (aqueous
ethanol solutions) and by Machado et al. [21] (aqueous acetone solutions) do not follow the viscosity additivity rule. Viscosity of acetone–water mixtures also
go through a maximum at ∼0.15 M acetone. However,
acetone–water flux did not go through a minimum at
this acetone–water concentration, indicating that solvent transport is also affected by other parameters such
as surface tension and solubility parameter of the permeating solvent.
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
Fig. 7. Resistance ratio plots for polysulfone and polyethersulfone
membranes. Membranes were conditioned using method 1. ((䊏)
PM10, (䊊) Omega, () UFC, (䊐) PM30, (䉫) PES4H, (䊉) Alpha,
(䉱) PS10).
81
Fig. 9. Resistance ratio plots for hydrophilic polyacrylontrile-based
and cellulose-based membranes. Membranes were conditioned using method 1.
3.2. Membrane resistance
The influence of different water fluxes of the membranes was factored out by using the resistance ratio (R/Rw ) to compare the membranes, as shown in
Figs. 7–9. As a group, the more hydrophobic membranes, such as PS and PES (Fig. 7) and the composite membranes (Fig. 8) show a decrease in R/Rw
Fig. 8. Resistance ratio plots for composite membranes. Membranes were conditioned using method 1.
with increase in ethanol concentration. The exceptions
in this group are the PM10 and PES4H membranes:
their resistance ratios were about 1 but increased at
higher ethanol concentrations (Fig. 7). However, the
more hydrophilic membranes shown in Fig. 9 did not
behave as a group. Except for the U20T and PLGC,
the others in this group also showed a decrease in
membrane resistance (or the resistance ratio remained
close to 1) with an increase in ethanol concentration.
This suggests that factors other than, or in addition to,
hydrophilicity and viscosity (e.g. surface tension, solubility parameter or dielectric constant in the case of
charged membranes) may be governing solvent flow
through the membrane.
With the composite membranes (G80, H051 and
U20T), resistance was high with water and decreased
when exposed to ethanol (Fig. 8). The pores on the
surface of G80 and U20S membranes, which have
very dense barrier layers, apparently dilated leading to lower membrane resistance and higher fluxes.
This behavior does not appear to be reported in the
literature. With cross-linked PAN-chitosan membranes, Musale and Kumar [22] observed the highest
flux with methanol followed by ethanol and isopropanol. These differences in flux were explained
on the basis of the combined effects of increase in
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R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
molecular weight, viscosity, hydrophobicity and dielectric constant of the alcohol. Reddy et al. [23] observed lower flux of primary and secondary alcohols
with increase in molecular weight and hydrophobicity
of the solvent with a polyamide-polyphenylene sulfone membrane. There was no correlation with viscosity. Only Machado et al. [21] reported an increase in
flux with increase in solvent (acetone) mole fraction.
Their nonlinear exponential increase was thought to
be due to decreasing surface tension at higher acetone
concentrations.
3.3. Membrane swelling
Swelling of several polymeric membranes is shown
in Table 2. Except for two membranes (Alpha and
Omega), water resulted in higher degrees of swelling
by weight while 100% ethanol resulted in the least
weight-swelling. The polysulfone hollow fiber UFP10
showed the greatest degree of swelling in water
(211%) and in neat ethanol (94%). In general, the
dielectric constant of the solvent and difference in
solubility parameters of the polymer and solvent govern swelling behavior. Membrane swelling is higher
in polar solvents like water due to its high dielectric
constant (ε = 80), and less with ethanol (ε = 25)
and nonpolar solvents like hexane (ε = 1.9). Musale
and Kumar [22] observed over 50% swelling by
weight with cross-linked chitosan-PAN membranes
Table 2
Weight swelling of polymeric UF membranesa
Material
Membrane
Water
70%
EtOH
100%
EtOH
Cellulose
Cellulose
Composite
Composite
Composite
PAN-m
PAN
PES-m
PES-m
PES
PS
PVDF
PVDF
Type C
Cell
U20S
G80
H051
MX25
U20T
Alpha
Omega
PES4H
UFP10
AN09
AF5
112.5
25.9
83.7
65.8
31.0
71.2
85.8
44.7
51.0
26.0
211.4
54.3
21.0
60.2
29.9
48.4
61.5
26.4
56.3
39.1
59.9
46.5
18.0
161.4
46.2
9.5
45.9
22.5
34.6
51.3
24.5
46.8
32.5
52.8
56.3
17.1
94.4
34.6
6.1
a Percentage increase in weight after exposure to the solvents
for 24 h.
with ethanol compared to 20% swelling in hexane. It
is interesting that the three membranes manufactured
by Pall Filtron (Alpha, Cell and Omega) showed
essentially no change or no trend in swelling, even
though these membranes are made of two different
materials.
Length-swelling measurements have been reported
in the literature and were also done in this study [18].
However, they were largely inconclusive because the
dimensional changes were so small that measurements
were difficult and imprecise. A typical sample of 5 cm
length experienced a length-swelling of 0.1–0.2 cm.
Consequently this experimental method had high errors and is not useful.
3.4. Effect of methods of conditioning on permeability
The effect of different methods of conditioning
is shown in Table 3 in terms of permeability coefficients of 70% ethanol. Some membranes (Cell,
MX25, Alpha, Omega, PS10) show high LP values (>100 × 10−15 m) at all pressures. On the other
hand, the composite membranes (U20S, G80, H051)
and modified PES membrane (PES4H) had significantly lower LP values. This is expected since
composites and surface-modified membranes have
denser layers and consequently higher membrane
resistance. AN09, which is a PVDF-based high
MWCO membrane, displayed much higher fluxes
when conditioned by methods 2–4. The reason for
this is not clear since PVDF is resistant to ethanol.
Protein rejection values for this membrane were
poor when conditioned by methods 2–4 (Table 4),
indicating that high solvent fluxes might be due
to pore dilation, which also results in low protein
rejection.
If solvent flux increased in proportion to the pressure (i.e. if the value of LP was unaffected by pressure), we can assume the membrane integrity remains
intact. Exposure to water followed by exposure to 70%
ethanol (method 2) appears to cause some damage to
some membranes, e.g. type C, Cell, G80 and PES4H,
which show an increase in LP at higher pressure. The
order of magnitude of the fluxes with method 2 are
comparable to those observed with method 1, except
at the highest pressure in the above cases. However,
some membranes (PS10 and AN09) show a decrease
in LP at higher pressure.
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
83
Table 3
Effect of methods of conditioning and pressure (kPa) on membrane permeability (LP ) with 70% ethanol solventa
Membrane
Method 1
Type C
Cell
PLGC
U20Sb
G80b
H051
MX25
Alpha
Omega
PES4H
PS10
AN09
a
b
Method 2
Method 3
Method 4
138
275
413
138
275
413
138
275
413
138
275
413
68
142
51
23
38
62
247
424
642
21
205
50
58
132
42
31
54
46
235
414
666
16
150
35
58
240
49
28
62
36
220
443
655
14
132
39
75
127
45
50
25
71
254
411
554
20
432
282
150
130
53
50
50
70
261
397
576
20
303
214
256
618
45
51
81
84
271
445
585
37
265
182
62
136
38
22
15
66
241
362
739
18
853
323
140
232
40
38
104
95
249
371
760
49
726
242
489
412
75
38
665
145
337
463
855
136
585
285
71
79
26
21
19
49
137
356
101
16
150
219
141
119
101
22
22
92
164
371
142
18
334
209
370
1112
278
42
21
428
179
655
483
126
445
214
LP values calculated from Eq. (1) (×10−15 m).
Pressures were 275, 310 and 345 kPa.
Direct exposure to 70% ethanol (method 3) seems to
have the most destructive effect on membranes such as
type C, Cell, PLGC, G80, H051, MX25, Omega, PS10
and PES4H, as measured by their higher LP values
compared to method 1 and the increase in LP at higher
pressure. Only AN09 and Alpha appear to remain intact. Method 4 (direct exposure to 100% ethanol) also
appears to damage the polymer matrix in all cases except U20S, G80 and AN09.
3.5. Effect of conditioning methods on protein flux
and rejection
A better indicator of membrane damage is changes
in rejection properties. This was studied using model
zein solutions. As expected, LP values are generally
lower with zein present (Table 4) than in its absence
(Table 3), in some cases by a substantial margin, e.g.
compare Alpha, Omega and MX25 in Tables 3 and 4.
Table 4
Effect of methods of conditioning and pressure (kPa) on membrane permeability (×10−15 m) and rejection (%) of proteina
Membrane
Method 1
LP
Type C
Cell
PLGC
U20Sb
G80b
H051
MX25
Alpha
Omega
PES4H
PS10
AN09
Method 2
Rejection
LP
Method 3
Rejection
LP
Method 4
Rejection
LP
138
275
138
275
138
275
138
275
138
275
138
275
138
275
138
275
56
63
42
N.D.c
8
34
41
70
45
22
114
63
46
55
29
9
15
34
39
68
N.D.
23
123
52
92
99
94
N.D.
87
96
98
99
78
88
90
79
97
94
95
93
96
92
99
98
N.D.
95
86
51
21
24
12
91
59
6
18
20
20
7
23
20
81
127
71
726
136
21
82
82
73
40
300
112
91
92
97
18
31
91
95
93
92
79
48
80
94
72
96
11
20
96
58
96
94
95
4
58
56
39
23
14
15
9
26
35
32
10
153
28
199
32
25
17
14
25
24
83
54
13
138
397
96
94
94
78
100
72
96
98
98
91
25
87
55
93
99
80
93
36
97
53
56
90
23
0
68
76
59
18
5
15
51
97
63
20
616
74
89
428
40
16
9
166
87
109
41
19
539
70
93
69
96
98
90
94
84
88
87
84
15
81
99
9
98
87
79
10
52
26
88
96
19
62
The protein was 5 g/l zein in 70% ethanol solvent. LP values were calculated from Eq. (1) (×10−15 m).
Pressures were 275 and 345 kPa.
c N.D.: not determined.
a
b
Rejection
84
R. Shukla, M. Cheryan / Journal of Membrane Science 198 (2002) 75–85
Among the methods, membranes conditioned by
method 1 appear to retain their integrity and provide
reasonably high LP values and high zein rejections.
Membranes conditioned using the other methods
result in lower rejections in almost all cases. The
U20S membrane gave >75% rejection in all cases
except with method 2. This is probably because
the manufacturer ships the U20S membrane in 50%
ethanol, and exposing it first to water, followed by
70% ethanol, could cause drastic structural changes.
This also explains why protein rejection with this
membrane remains high (∼80%) after direct exposure to 70% ethanol even though this conditioning
method 3, damages most of the other membranes
tested.
Some membranes that have high zein rejections at
low pressures (138 kPa) lose their rejection and become permeable to the protein at higher pressures
(>275 kPa). Organic solvents tend to lower the glass
transition temperature of polymers by acting as polymer plasticizers, which in turn reduces their ability to
resist high pressures [6]. For instance, soaking polysulfone in an organic solvent with a similar solubility parameter will lead to cracking of the matrix [24].
There is evidence that hydrogen bonding solvents are
usually much less disruptive to the polymer matrix
than strongly nonpolar solvents like hexane [25]. Consequently, on exposure to organic solvents, the pressure ratings of the membranes are significantly reduced in many cases, except if they are conditioned
using method 1.
4. Summary and conclusions
The method of conditioning has a strong effect
on the solvent flux, membrane integrity and pressure rating of polymeric membranes. Gradual solvent
exchange with successively higher concentrations
increased in small doses appears to work best with
completely miscible solvents such as those studied
here (ethanol–water mixtures). Rapid solvent exchange between water and high concentrations of
alcohol disrupts the polymer matrix in many and
leads to pore degradation. Exposure to organic solvents significantly reduces the pressure rating of the
membranes. A membrane that provides acceptable rejection of protein up to 413 kPa (60 psi) under normal
conditioning may lose its rejection even at 138 kPa
(20 psi) if conditioned incorrectly.
Acknowledgements
This research was supported by the Illinois Corn
Marketing Board, Illinois Department of Commerce
and Community Affairs Bureau of Energy and Recycling, US Department of Agriculture through the
NRICGP program (Award No. 97-35504-4296) and
the Illinois Agricultural Experiment Station. Contributions of membranes by Koch Membrane Systems, Osmonics and Pall Filtron are gratefully acknowledged.
Analytical assistance was provided by Amanda D.
Popp and Melissa M. Stein. Useful discussions were
held with D.A. Musale, M. Balakrishnan, H. Yacubowicz and J. Yacubowicz.
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