The Separation of Water and Ethanol by Pervaporation
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Transcript of The Separation of Water and Ethanol by Pervaporation
8/12/2019 The Separation of Water and Ethanol by Pervaporation
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Journal ofM embrane Science, 68 (1992) 141-148
Elsevier Science Publishers B.V., Amsterdam
The separation of water and ethanol by pervaporation
141
with PVA-PAN composite membranes
H. Ohya”, K. Matsumoto*, Y . Negishi, T. Hinob and H.S. Choi”
“Department of M aterial Science and Chemical Engineeri ng, Yokohama Nat ional Uni versit y, 156, Tokiw adai, Hodokaya-
ku, Yokohama 230 Japan)
bChiy oda Corporat i on, 12-1, Tsurumi chuo P-chome, Tsurumi- ku, Yokohama 230 Japan)
‘Depart ment of ndustr ial Chemi stry , Kyungpook Sanup U ni versit y, 55, Hy omokdong, Tong-ku, Taegu 701-702 South-
Korea)
(Received May 29,199l; accepted in revised form November 25,199l)
Abstract
Compositemembranes itha thinpoly vinylalcohol)PVA ] layer coated on poly (acrylonitrile ) [PAN]
supportmembranes ereevaluatedor pervaporationeparation f a water-ethanolmixture. he perme-abilityof purewater hrough he PAN supportmembranewas n the range rom0.214 to 4.32 mm3/ ( m2-
set-Pa), and the molecular weight cut-off was in the range of 35 000 to 100 000. The PVA-PAN com-
posite membranes as prepared were water permeable, and the maximum separation factor was 4800.
From the experimental results, a separation model for the composite membrane for pervaporation is
proposed. The function of the PAN support membrane is to restrict physically swelling of the PVA within
the PAN pores at the PVA-PAN interface, thereby maintaining a dense PVA skin and desirable selectivity.
Keywords: composite membrane; pervaporation; ultrafiltration; separation of ethanol-water mixture;
separation factor
1 Introduction
There has been much progress in the re-
search and development of membranes and
their use in pervaporation processes for the
separation of various organic liquid mixtures
and aqueous organic mixtures, especiallyethanol-water solutions. Currently, much at-
tention is being paid to pervaporation as an en-
ergy saving separation method for the replace-
Corr espondence t o: H. Ohya, Department of Material Sci-
ence and Chemical Engineering, Yokohama National Uni-
versity, 156, Tokiwadai, Hodokaya-ku, Yokohama 240
(Japan).
0376-7388/92/ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
ment of distillation of aqueous alcohol solutions
and azeotropic mixtures [l-4].
The driving force in pervaporation is the
concentration difference across the membrane
resulting from a pressure difference; this offers
substantial energy savings. In addition, i t has
the advantage of simplifying process plants. I talso has the advantage of avoiding the pollu-
tants used in distillation processes to breakup
azeotropic mixtures.
Sander [5] investigated a pilot plant com-
bining pervaporation and extraction-disti lla-
tion. e reported that the operating cost of his
hybrid process was l/3 to l/4 less than that of
a conventional extraction-distillation process.
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142 H . Ohya et al./J, M embrane Sci. 68 1992) 141-148
In recent years, priority in pervaporation re-
search has been given to the development of new
polymer membranes which have a high selec-
tivity, acceptable flux rate, and good stability
and/or durabil ity. For example, in the case of
dehydration of a high-concentration ethanol
solution, above 95 wt. , a membrane which
preferrentially allows the passage of water is
needed. The pioneer membrane used ethanol-
water separation by pervaporation is the GFT-
membrane, which was developed by GFT (Ger-
many) at the beginning of the 1980’s. It is a
composite membrane with a poly(viny1 alco-
hol) [PVA] coated on a poly acrylonitrile)
[PAN] ultrafiltration membrane [61.Recently, to improve further the perform-
ance of pervaporation, several researchers have
investigated the performance of membranes
made from other materials. In addition, during
the past several years, pervaporation has been
studied extensively with the aim of industrial
applications. Wesslein et al. [7] studied the
separation of binary mixtures with the GFT-
membrane for 11 kinds of solvents. Huang and
Yeom [B] reported on the effect of the concen-
tration of a crosslinking agent (amic acid) forPVA membranes on membrane performance.
Nobrega et al. [9] investigated the effect of
chemical and thermal treatments on PVA
membrane performance. Spitzen et al. [lO,ll]
reported on the water permselectivity of PVA
and PAN membranes used in pervaporation
processes. Until now, the results of investiga-
tions on conventional PVA membranes indi-
cate that both its water permselectivity and du-
rability are relatively low. Still, the PVA-PAN
composite membrane is now considered to be
one of the best for aqueous-organic pervapor-
ation processes because of its high water perm-
selectivity and durability. However, the char-
acteristics of PAN ultrafiltration membranes
alone are not yet well understood. Therefore,
this study was carried out to determine the ef-
fects of varying the PAN fabrication param-
eters on the membrane’s pure-water permea-
bility and molecular weight cut-off. We also
examined the effect of the PAN dope compo-
sition on the performance of the PVA-PAN
composite membrane. The composite mem-
branes were also evaluated for pervaporation
separation of a water-ethanol mixture.
2. Experimental
2.1.Preparat i on of he membranes
2.1.1. Materials
The polymers used in this study werepoly acrylonitrile ) , supplied by Aldrich Chem-
icals, and poly (vinyl alcohol) (degree of poly-
merization: 1700, degree of hydrolysis:
99.5 ? 0. 1 , supplied by the Kurare Company
(J apan). The solvent used was analytical grade
dimethylformamide (DMF ) from WAKO
Chemicals (J apan). The non-woven fabrics of
polyester used as a base in the fabrication of
PAN membrane were supplied by the Kanai
J uyo Ind. Co. (J apan).
2.1.2. Suppor t membrane PAN UF-
membrane)
The dope solution consisted of 5, 10 and 15
wt. PAN in DMF. First a non-woven polyes-
ter fabric was fixed on a glass plate (with tape),
and the polymer solution was cast onto the fab-
ric. The solvent was then allowed to evaporate
for a specific period of time at room tempera-
ture or at 60 C in an oven. The glass plate was
subsequently immersed in water gelation bath
(ice water at 2’ C ) for a specific period of time.
Table 1 shows the conditions for preparation of
PAN support membrane.
The membrane was rinsed in tap water for
24 hr, and immersed in an ethanol solution for
24 hr at room temperature to remove the resid-
ual solvent. The membrane was then dried un-
der vacuum at room temperature.
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H. Ohya et al.j J. M embrane Sci. 68 1992) 141-148 143
TABLE 1
Conditions of preparation of PAN support membranes
Mem- Concen- Predrying Predrying Gelation Thicknessbrane tration of temperature time time of top
dope (“C) (min) (min) IayeP
solution (w)
(wt. )
A 5
B 10
C 5 60
D 10 60
E 5 60
“by SEM observation.
30 20
30 26
5 30 27
5 30 26
5 30 41
2.1.3. PVA-PAN composi te membr ane
PVA-PAN composite membranes were made
by the procedure proposed by Brtischke [14].
The concentration of PVA in the coating solu-
tion was 7 wt. in water, and the PVA was
crosslinked by the addition of maleic acid. The
amount of crosslinking agent was nit of mal-
eic acid per 20 units of vinyl alcohol monomer
base, assuming 100 reaction.
The composite membrane was made by ther-
mally treating PVA dip-coated on the PAN UF-
support-membrane. The thermal treatment
consisted of placing the membrane in an oven
at 150” C for 2 hr. The degree of hydration of
the crosslinked PVA layer was 45 .
2 2 U lt rafi l t rat ion testi ng of the support
membrane
The pure water flux of the PAN membrane
was determined experimentally, and the mo-
lecular weight cut-off of the membrane was de-
termined with the method reported in Ref. [ 121using aqueous dextran solutions instead of
PEG. In order to keep the levels of concentra-
tion polarization of dextran on the PAN mem-
brane surface the same, the trans-membrane
pressure was chosen to give a similar solution
flux, i.e. 2.85 kPa for the loosest membranes A
and C, 4.55 kPa for membranes B and D, and
35 kPa for the tightest membrane E . The dex-
tran solutes used were T-10, T-40, T-70 and
T-500 from Pharmacia Chem.; the solution was
a multi-component mixture with 100 mg/l of
each component. The permeate samples weretaken after the flux had reached steady state.
The determination of the solution concentra-
tion was carried out with a gel-permeation
chromatograph (Shimadzu, model LC-64 sys-
tem). The columns used were OH Pak B803/s,
804/s and 805 from Shodex.
2.3. Pervaporation
The pervaporation separation of water
(component i) and ethanol (component j)
mixtures was carried out with the PVA-PAN
composite membranes. The details of the per-
vaporation equipment are shown in Fig. 1. The
membrane was placed on the porous stainless
steel plate in the cell. The effective area of the
membrane cell was 23.7 cm2. The feed solution
was circulated with a flow rate of 200 ml/min
from the feed reservoir, which was placed in a
thermostatic bath. The operating temperature
of the equipment was controlled at 25, 40, 50,
or 60°C.The concentrations of the feed solution were
20,40,60,80 or 95 wt. ethanol. The permeate
was condensed in a liquid nitrogen trap, and
the flux was calculated directly by measuring
the permeate weight per unit time. The down
FLI
Pervapnatm cell
B Membrane
C Thermocouple
D Feed reservar
E Thermoset balh
F Feed crculalng pump
G Cold trap
” Prani gauge
I Vacuumump
Fig. 1. Apparatus used for the pervaporation experiments.
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144 H . Ohy a et al ./J. M embrane Sci. 68 1992) 141-148
stream vacuum pressure was maintained be-
tween 0.01 and 1.5 Torr and measured by a Pir-
ani gauge (Okano, model Pg-25). The feed and
permeate composition were measured by gas-
chromatography. The detectors used were TCD
(Shimadzu, model GC-4C) for high concentra-
tions of ethanol and FID (Shimadzu, model GC-
9A) for low concentrations.
The selectivity of the membrane was deter-
mined by the separation factor, Lyi/j,defined for
binary mixtures as follows:
3. Results and discussion
3.1. Ul trafi l t rat i on testi ng of PAN support
membrane
The characteristics of the PAN support
membrane are shown in Table 2. The permea-
bility of pure water at 25 o C was in the range of
0.214 to 4.32 mm”/ (m2 set Pa), and the relative
molecular weight cut-off was between 35 000
and 100 000.
3.2. Pervaporation
3.2.1. Permseparat i on of w at er- et hanol
mixtures
The measured separation for the composite
membrane of PVA coated on UF-membrane A
TABLE 2
Characteristics of PAN supportmembranes
Membrane Pure water Molecular weight
permeability (at 25°C) cut-off
[mm3/(m2-set-Pa)] (g/mol)
A 4.32 100 000
B 3.28 76 000
C 2.63 42 000
D 1.81 50 000E 0.214 35 000
(listed in Table 2) is shown in Fig. 2 together
with that for the GFT membrane [ 151. The UF-
membrane A is known to be water permselec-
tive and the composite membrane (PVA-PAN-
A) shows better behavior than the GFT mem-
brane for the entire ethanol concentration range
investigated in this study [21.
The dependence of the separation factors and
the permeation fluxes on the temperature for
another PVA-PAN composite membrane is
shown in Fig. 3. The permeation flux increased
with increasing temperature, but the separa-
tion factor did not change. A linear relation-
ship is observed between the permeation flux
and the inverse of the absolute temperature.This suggests that the membrane structure did
not change as a result of temperature changes.
3.2.2. M embrane durabi li ty
Pervaporation experiments were carried out
in which the feed concentration of ethanol was
continuously varied from high to low (H-L
segment in Fig. 4) and low to high (L-H seg-
ment ). The change of the permeation flux and
the separation factor with time for a PVA-PAN
composite membrane with support membraneB is shown in Fig. 4. The numbers in the figure
(data point labels) are the days at which the
data were collected. In the H-L segment, the
separation factor for an initial feed concentra-
tion of 95 wt. was 4200, and 70 for 20 wt. at
EtOHCont. in F eed Cwt./J
Fig. 2. Pervaporation curve of composite membranes and
vapor-liquid equilibria for water-ethanol mixtures.
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H. Ohya et al./J. M embrane Sci. 68 1992) 141-148
11 T X103CK- ' II I
60 50 40 25' C
Fig. 3. Dependence of total flux and separation factor on
temperature for a PVA-PAN composite membrane.
(Membrane: PVA-B ) .
v) ' 0 20 40 60 80 100EtOH Cont. in Feed CwtX3
Fig. 4. Permeation flux and separation factor with of time
for a PVA-PAN composite membrane. (Membrane: PVA-
B).
145
the turning point. The concentration of ethanol
in all the permeates was in the range of 0.27-
0.41 wt. for the entire feed concentration
range. Note that hysteresis was observed for
both the separation factor and the flux; the fi-
nal values did not return to their original levels.
The final value of the separation factor in the
L-H segment was approximately l/lOth the
initial value in the H-L segment. The final flux
value was 7.5 times the initial value.
The changes of the permeation fluxes for both
water and ethanol with time and varying feed
concentration are shown in Fig. 5. The differ-
ence in initial value and final value in permea-
tion flux for ethanol was larger than that forwater.
From these results, we believe that the PVA
coating layer of the composite membrane swells
as the ethanol concentration in the feed solu-
tion is decreased, resulting in a more extensive
(more open) polymer network, which remains
open as the ethanol concentration is raised to
its original level.
3.2.3. Compari sons of var i ous supportmembranes
A comparison of the separation factors and
permeate fluxes for PVA-PAN composite
membranes with a different PAN support is
-1 -
k$ 10- l -
so-2 -
;(i l o-3
10-1
' o-5-0
I I I 1 I ,
20 40 60 80 11EtOH Cone. in Feed CwW1
1
Fig. 5. Permeation fluxes for both water and ethanol with
of time for a PVA-PAN composite membrane at 60°C.
(Membrane: PVA-B).
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146 H. Ohya et al /J. M embrane Sci . 68 1992) 141-148
shown in Fig. 6 and 7. Also, the relationship
between the separation factor and the pure
water permeability of the PAN support mem-
brane are shown in Fig. 8. It was found that thePVA-PAN composite membranes have higher
separation factors than conventional non com-
posite PVA membranes, and that the water
Feed Cont. CwlXl
A 60
L
- 0 PVA-A
PVA-B
_ , PVA-C
0 20 40 60 80 100
EtOH Cont. in Feed Cwt”1.1
Fig. 6. A comparison of the separation factors for PVA-
PAN composite membranes with different PAN supportmembranes at 60°C.
_ I3 PVA-D
1o-3; 1 I 8 1;O 40 60 60 100
EtOH Cont. in Feed Cwt l
Fig. 7. A comparison of the permeation fluxes for PVA-
PAN composite membranes with different PAN support
membranes at 60 ’ C.
I I , I , , , , ,
10-l 1 IO
Pure Water Permeability ~VII~/ITI~spa3
Fig. 8. Separation factors of PVA-PAN membranes at 6O’C,
as a function of pure water permeability of PAN support
membranes.
permselectivity of PVA increases with the ad-
dition of a support PAN membrane. Unlike
composite membranes, the PVA membrane
swelled and broke up in a feed with a composi-
tion of less than 60 wt. ethanol, as shown in
Fig. 6. Furthermore, it was observed that thepermeation flux decreased as the densification
of the PAN support membrane increased.
A homogeneous PAN membrane has been
reported to be water permselectivity [ 10,ll 1.
Therefore, it should be possible to increase the
separation factor by increasing the densifica-
tion of the PAN support membrane. However,
our experimental results in Fig. 6 show that the
opposite is true, i.e. the separation factor de-
creased with increasing densification of the
PAN support membrane (i.e. from PVA-A to
PVA-E). This difference between PVA-A and
PVA-E is the largest at a high feed concentra-
tion. The separation factor has a maximum
value in the vicinity of 80 wt. ethanol and is
lower for 95 wt. ethanol. This fact suggests
that the relative diffusion rates for ethanol and
water through PVA are reversed between
ethanol concentrations of 80 and 95 wt. [lo].
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H. Ohya et al./J. M embrane Sci. 68 1992) 141-148 147
Feed Solution Feed Solution
Permeate
4
Permeate
b)
Fig. 9. Models of PVA-PAN composite membrane during pervaporation.
That is, at 80 wt. ethanol, the ethanol diffu-
sion rate is higher.
From the experimental results of this study,
the model shown in Fig. 9 is proposed. (1) The
feed solution dissolves into the PVA layer, dif-fuses through this layer, and subsequently de-
sorbs, i.e. vaporizes toward the down stream
side through the small pores of the support
membrane. We suppose that a dense, non-
swollen skin forms at the interface between the
PVA coating and the PAN support membrane
[ 131, and that this dense skin increases the se-
lectivity of the composite membrane. (2)
Swell ing of the PVA skin layer is suppressed by
the rigid PAN pore structure in which the PVA
coating is partially imbedded. In the case of a
small pore structure in the support membrane,
the coating is probably not imbedded deeply,
and thus the support only weakly suppresses
swelling (Fig. 9b). This model is consistent with
the selectivities shown in Fig. 8. The increase
in selectivity (separation factor) with respect
to the pure water permeability (pore size) is
relatively large for high feed concentrations, but
small for low feed concentrations. I t may be so
that at the lower ethanol concentrations, moreof the PVA coating layer is swollen, resulting in
a thinner skin.
Furthermore, Figs. 6, 7 and 8 indicate that
highly permeable membranes (PVA-A and
PVA-B ) have relatively high separation factors
at high ethanol concentration, but that their
separation factors drop below those of the less
permeable membranes at middle and low
ethanol concentrations. Possibly, the larger
pore structure of the more permeable mem-
branes does not have sufficient mechanical
strength to suppress the swelling which occurs
at low ethanol concentrations, resulting in the
reduced separation factors discussed above.
4. Conclusions
(1) The permeability of pure water through
a PAN support membrane is in the range from
0.214 to 4.32 mm”/ (m2-set-Pa) and the molec-
ular weight cut-off is in the range from 35 000
to 100 000.
2 ) The PVA-PAN composite membrane as
prepared is water permselective for the ethanol
concentration range of 20 to 95 wt. in the feed,
and the separation factor exhibits a maximum
value of 4800.
(3 ) The dependence of the permeate flux on
temperature is consistent with the Arrhenius
relationship, and the separation factor hardly
varied in the temperature range investigated.
(4) I t is verified that the permselectivity of
the membrane is increased by the addition of
the PAN support membrane, i.e. by making the
PVA-PAN composite membrane. However, the
separation factor decreases with excessively in-creasing densification of the support
membrane.
(5) A function of the PAN support mem-
brane is to suppress the swelling of the PVA
layer at the PVA-PAN interface and to create
a dense skin, thereby improve the selectivity by
allowing PVA to intrude into the micropores of
the PAN support membrane. Therefore, the
support membrane porosity is of great impor-
tance in the selection of a support, capable of
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148 H. Ohya et al./J. M embrane Sci. 68 1992) 141-148
forming the dense (non-swollen) skin. The
support membrane having a molecular weight
cut-off of 100 000 (composite membrane A) to
76 000 (B ) might be the best choice. On theother hand a membrane having a lower pure
water permeability (less than 1.8 mm”/ ( m2-sec-
Pa) might not be good because it is not able to
suppress the swelling of the PVA membrane on
top.
8
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