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sodium alginate two-active-layer composite membranes
Accepted Manuscript Title: Pervaporation dehydration of ethanol by hyaluronic acid/sodium alginate two-active-layer composite membranes Author: Chengyun Gao Minhua Zhang Jianwu Ding Fusheng Pan Zhongyi Jiang Yifan Li Jing Zhao PII: DOI: Reference:
S0144-8617(13)00841-2 http://dx.doi.org/doi:10.1016/j.carbpol.2013.08.057 CARP 8047
To appear in: Received date: Revised date: Accepted date:
5-4-2013 19-8-2013 22-8-2013
Please cite this article as: Gao, C., Zhang, M., Ding, J., Pan, F., Jiang, Z., Li, Y., & Zhao, J., Pervaporation dehydration of ethanol by hyaluronic acid/sodium alginate two-active-layer composite membranes, Carbohydrate Polymers (2013), http://dx.doi.org/10.1016/j.carbpol.2013.08.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights (for review)
► Two-active-layer
composite
membranes
were
prepared
by
sequential
spin-coating. ► Sorption and diffusion behavior in capping layer and inner layer were studied. ► Positioning of HA layer and NaAlg layer in membrane affected separation factor.
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► HA/Alg/PAN membrane exhibited higher pervaporation performance.
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*Manuscript
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Pervaporation dehydration of ethanol by hyaluronic
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acid/sodium alginate two-active-layer composite membranes
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Chengyun Gaoa,b, Minhua Zhangb, Jianwu Dinga,b, Fusheng Pana, Zhongyi Jianga,
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Yifan Lia, Jing Zhaoa
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a
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Chemical Engineering and Technology, Tianjin University, 92# Road Weijin, District
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Nankai, Tianjin 300072, China
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cr
Key Laboratory for Green Chemical Technology of Ministry of Education, School of
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b
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Petrochemical Technology, Tianjin University, Tianjin 300072, China
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Key Laboratory for Green Technology of Ministry of Education, R & D Center for
M
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To whom correspondence should be addressed. Address: School of Chemical
Engineering & Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin, 300072, P. R. China. Tel: 86-22-2350 0086. Fax: 86-22-2350 0086. E-mail: [email protected]
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Abstract:
The composite membranes with two-active-layer (a capping layer and an inner
20
layer) were prepared by sequential spin-coatings of hyaluronic acid (HA) and sodium
21
alginate (NaAlg) on the polyacrylonitrile (PAN) support layer. The SEM showed a
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mutilayer structure and a distinct interface between the HA layer and the NaAlg layer.
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The coating sequence of two-active-layer had an obvious influence on the
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pervaporation dehydration performance of membranes. When the operation
25
temperature was 80oC and water concentration in feed was 10 wt.%, the permeate
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fluxes of HA/Alg/PAN membrane and Alg/HA/PAN membrane were similar, whereas
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the separation factor were 1130 and 527, respectively. It was found that the capping
28
layer with higher hydrophilicity and water retention capacity, and the inner layer with
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higher permselectivity could increase the separation performance of the composite
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membranes. Meanwhile, effects of operation temperature and water concentration in
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feed on pervaporation performance as well as membrane properties were studied.
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Keywords: Hyaluronic acid; Sodium alginate; Two-active-layer composite membrane;
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Pervaporation; Ethanol dehydration
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Introduction Pervaporation, as a promising membrane-based separation technology for the
41
dehydration of solvents, has undergone a rapid development in recent years.
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Solution-diffusion mechanism is proposed and widely accepted (Binning et al., 1961;
43
Lonsdale, 1982) to elucidate the mass transfer of pervaporation process: selective
44
sorption of feed species on the membrane surface, selective diffusion through the
45
membrane and desorption into a vapor phase on the permeate side. Among these three
46
steps, selective sorption and diffusion in membranes are often the rate-governing steps.
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At present, the composite membrane, which comprises a separation layer (active layer)
48
and a support layer is the most commonly used configuration. Ideally, the separation
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layer of membranes should possess the surface with high sorption capacity and
50
selectivity, and the bulk with low diffusion resistance and high diffusion selectivity.
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Materials for the separation layer of the one-active-layer pervaporation membranes
52
have to bear these properties all-in-one, which limits the selection scope of materials.
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It is well known that the composite membranes assign the separation performance and
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the mechanical strength demands into the separation layer and the support layer
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individually. The performance can be highly improved by choosing the appropriate
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materials for the separation layer and the support layer. It is thus envisaged that if the
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similar strategy is adopted, the separation layer is further divided into the capping
58
layer and the inner layer (two-active-layer) to intensify the adsorption and diffusion
59
individually, as shown in Fig.1, the selection scope of the materials for separation
60
layer may be dramatically expanded and the separation performance may be improved
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in a much broader range, since the surface and bulk properties could be varied by the
62
rational tuning of the capping layer and the inner layer.
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Fig. 1. Schematic structure of the two-active-layer composite membrane.
65
Rational screening of hydrophilic materials for capping layer and inner layer is of
66
great importance for the separation performance of two-active-layer membranes. Till
67
now, several kinds of polymer-based materials have been employed in the dehydration
68
process of ethanol-water aqueous solution by pervaporation. Huang et al. (1999)
69
prepared two-ply membranes by successive castings of NaAlg and CS solutions for
70
the pervaporation dehydration of 95 wt.% ethanol-water mixture. When CS was
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selected as capping layer facing to the feed solution, the separation factors was 147,
72
while that of the membranes with NaAlg facing to the feed solution was 1062. These
73
results suggested that coating sequence had an obvious effect on membrane separation
74
performance. In our previous study (Ding et al., 2012), composite membranes with
75
two-active-layer, HA and CS, were constructed through alternative self-assembly and
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spin-coating. It was found that the separation factor of the HA/CS/PAN membrane
77
(704) with CS as inner layer was remarkably higher than that of CS/HA/PAN
78
membrane (104) with HA as inner layer. A probable explanation was HA layer had
79
higher water sorption selectivity and CS layer had higher diffusion selectivity. In the
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above studies, the capping layer and the inner layer are fabricated from cationic
81
polyelectrolyte and anionic polyelectrolyte, individually. Herein, we want to explore
82
the separation performance of the two-active-layer membranes where the capping
83
layer and the inner layer are both fabricated from anionic polyelectrolytes.
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As one common hydrophilic carbohydrate polymer, sodium alginate (NaAlg),
85
which is extracted from marine brown algae, has exhibited excellent performance as a
86
pervaporation membrane material (Bhat & Aminabhavi, 2006, 2007; Kalyani et al.,
87
2006; Kanti et al., 2004; Kumar Naidu et al., 2005; Kurkuri et al., 2002a; Kurkuri et
88
al., 2002b; Magalad et al., 2010; Uragami & Saito, 1989). The carboxylic and
89
hydroxyl groups play a crucial role in preferential water sorption and diffusion
90
through the membranes (Yeom & Lee, 1998) .Since alginate has the excellent
91
permselectivity toward water, it may be the suitable material to offer higher separation
92
performance (Huang et al., 1999; Shi et al., 1998). As another common hydrophilic
93
carbohydrate polymer, hyaluronic acid (HA) (Kim et al., 2004; Ma et al., 2010;
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Tomihata & Ikada, 1997) has demonstrated high water sorption selectivity in ethanol
95
aqueous solution and high water retention capacity (500mL/g) due to a large number
96
of hydroxyl groups and carboxyl groups. Therefore, HA may have potential to
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intensify the sorption/dissolution of water.
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In this study, two-active-layer composite membranes were fabricated by sequential
99
spin-coatings of HA and NaAlg on the PAN support layer. Effect of coating sequences
100
and operation conditions on structure and performance variation of the
101
two-active-layer membranes for dehydration of 90 wt.% ethanol-water mixture were
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102
explored.
103
2. Experiments
104
2.1 Materials The flat-sheet ultrafiltration membrane, which had a molecular weight cut-off of
106
100,000 and was made up of PAN porous layer and polyethylene terephthalate
107
nonwoven fabric substrate, was obtained from Shanghai MegaVision Membrane
108
Engineering & Technology Co. Ltd. (Shanghai, China). NaAlg (viscosity of 1 wt.%
109
aqueous solution at 20oC: 0.02Pa•s ) was purchased by Tianjin Guangfu Ltd. (Tianjin,
110
China). HA (MW 1,200,000 Da, sodium salt) was obtained by Qufu Haixin Industrial
111
Co. Ltd. (Qufu, China). Glutaraldehyde (GA; 25 wt.%), hydrochloric acid (HCl),
112
calcium chloride and absolute ethanol were purchased from Tianjin Kewei Ltd.
113
(Tianjin, China). Deionized water was used in all experiments.
114
2.2 Membrane preparation
ed
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The flat-sheet PAN membrane was immersed into deionized water for two days to
116
remove glycerol on the membrane surface, and air-dried at room temperature. The
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NaAlg solution was prepared by dissolving NaAlg into deionized water with stirring
118
for two hours, and then kept aside for one hour to remove the bubble in the solution.
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HA solution was prepared by dissolving hyaluronic acid into deionized water with
120
stirring for two hours, and then added GA solution to initiate cross-linking and
121
adjusted pH to about 4.5 with 0.01M HCl. After half an hour, stirring was stopped and
122
the sample was kept aside for degassing
123
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Membranes were prepared by two-step spin-coating. NaAlg solution was firstly
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coated on PAN support layer by spin-coater and dried at room temperature. Then the
125
NaAlg membrane was cross-linked by immersing into the CaCl2 aqueous solution for
126
ten minutes and then rinsed with deionized water and air-dried. After that, the second
127
coating was carried on. HA solution was spin-coated on the membrane surface and
128
air-dried. The resultant membrane was denoted as HA/Alg(X)/PAN, where X (unit:
129
mol/L) meant Ca2+ concentrations of solutions that were used to cross-link NaAlg
130
membrane. Similarly, the Alg(X)/HA/PAN membrane was prepared by sequential
131
spin-coatings of first hyaluronic acid and then sodium alginate.
132
2.3 Characterization
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FT-IR spectra for each layer of the composite membrane were acquired by using a
134
Nicolet-560 Fourier transform infrared spectrometer with scan range of 4000-600
135
cm-1.The static water contact angles of the membrane surfaces were measured by
136
contact angle goniometer (JC2000C, Powereach Co., Shanghai, China) for the
137
investigation of the hydrophilic/hydrophobic property. The contact angle of each
138
membrane sample was measured ten times at different locations. The cross-section
139
morphology of membranes was observed by a Nanosem 430 field emission scanning
140
electron microscope (FESEM). Samples were freeze-fractured in liquid nitrogen and
141
then were sputtered with gold prior to scanning.
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2.4 Swelling and sorption measurement
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The swelling degree of homogenous HA and NaAlg membranes was measured by
144
immersing membranes into deionized water and 90 wt.% ethanol-water mixture for 24
145
hours. Then the membranes were carefully wiped off the surface liquid with tissue
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146 147
paper. The degree of swelling was calculated as follows: Degree of swelling
Ms Md 100% Md
(1)
The adsorbed liquid in membranes was desorbed by an apparatus with a vacuum
149
pump and condensed in a liquid nitrogen trap, which was analyzed by gas
150
chromatography. All the experiments were repeated three times.
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Sorption selectivity (αs) was calculated by Eq. (2):
152
s
153
Where WM and EM were the weight fractions of water and ethanol in the membrane,
154
respectively; Wf and Ef were the weight fractions of water and ethanol in the feed
155
solution, respectively.
Eq. (3):
158
D
s
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According to solution–diffusion theory, diffusion selectivity (αD) was calculated by
157
159
(2)
(3)
ce pt
156
WM / EM Ef / Ef
cr
151
where α is the separation factor of single-active-layer HA/PAN or CS/PAN membranes.
161
2.5 Pervaporation experiment
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Pervaporation experiments were carried out for dehydration of ethanol/water
163
mixtures. The pervaporation apparatus were described in our previous study (Peng et
164
al., 2005). The effective surface area was 25.6 cm2. The concentrations of the feed
165
solution and the permeate were measured by gas chromatography (USA Agilent 4890).
166
The permeate flux ( J ) and the separation factor ( ) was calculated as follows
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167
J
Q At
(4)
168
Wp / Ep Wf / Ef
(5)
Where Q is the mass of the permeate collected in time t, and A is the effective
170
membrane area. Where Wp and Ep were the weight fractions of water and ethanol in
171
the permeate solution, respectively; Wf and Ef were the weight fractions of water and
172
ethanol in the feed solution, respectively.
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Generally, permeate flux and separation factor are affected strongly by operation
174
conditions (such as temperature and water concentration in the feed), which change
175
vapor pressure difference between two sides of membrane (driving force), as well as
176
membrane intrinsic properties. In order to investigate the intrinsic properties of
177
membranes with variation of feed concentration and temperature, effect of the driving
178
force should be removed. Baker (Baker et al., 2010) presented a method in term of
179
permeability ( Pi ), permeance ( Pi / l ) and selectivity ( ), which normalized the
180
driving force. They were used to evaluate variations of membrane properties with
181
operation conditions.
183
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The component flux can be written as
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173
Ji
Pi L L sat ( i 0 xi 0 pi 0 pil ) l
(6)
184
Where for component i , J i is the partial permeate flux, Pi is the permeability, l
185
is the thickness of the active layer in the composite membranes, i0L is the activity
186
coefficient in the feed solution, xiL0 is the mole fraction in the feed solution, pisat is 0
187
the saturated vapor pressure of pure component i , pil is the partial pressure in the
188
permeate vapor. It is implicit in the equation (6) that the concentration polarization 9
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189
190
effect is negligible. Therefore the permeance ( Pi / l ) was calculated as follows: Pi Ji L L sat l ( i 0 xi 0 pi 0 pil )
(7)
Since the vacuum pump is used in the permeate side and the vapor pressure is
192
approximately vacuum, pil can be considered as zero. The activity coefficient and
193
the pure component vapor pressure can be calculated by computer simulation
194
programs (ChemCad, HYSYS, ProSim or Aspen Plus).
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The selectivity ( ) is defined as the ratio of the permeability or the permeance of two components:
197
ij
198
3. Results and Discussion
199
3.1 Membrane characterization
200
3.1.1 FT-IR
an
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Pi Pi / l Pj Pj / l
(8)
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(d)
(c)
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(b)
(a)
3380cm
4000
201
3500
1570cm-1 1376cm-1
-1
1436cm-1 -1 1615cm-1 - 1050cm (-COO ) (-C-O-C)
(-OH)
3000
2500
2000
1500
1000
Wave numbers (cm-1)
202
Fig. 2. FTIR spectra of (a) Alg(0.5)/PAN; (b) HA/PAN; (c) Alg(0.5)/HA/PAN;
203
and (d) HA/Alg(0.5)/PAN membranes
204
Fig.2 showed FT-IR spectra of Alg(0.5)/PAN, HA/PAN, Alg(0.5)/HA/PAN and
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HA/Alg(0.5)/PAN membranes, respectively. The spectra of Alg(0.5)/PAN, HA/PAN,
206
Alg(0.5)/HA/PAN and HA/Alg(0.5)/PAN membranes showed the same peak at
207
3380cm-1 which was ascribed to –OH, 1050cm-1 which was ascribed to C-O-C(Ma et
208
al., 2010), 1615cm-1 and 1436cm-1 which were ascribed to asymmetric COO-
209
stretching vibration and symmetric COO- stretching vibration(Papageorgiou et al.,
210
2010). In addition, The bending vibration of the N-H(1570cm-1) and –CH3(1376cm-1)
211
were presented in the spectra of HA/PAN and HA/Alg(0.5)/PAN, respectively. It
212
implied that HA was successfully deposited on HA/PAN and HA/Alg(0.5)/PAN.
213
Meanwhile, the absorption peak of Alg(0.5)/HA/PAN was similar to that of
214
Alg(0.5)/PAN which was also indicated that NaAlg was successfully deposited on
215
Alg(0.5)/HA/PAN and Alg(0.5)/PAN.
216
3.1.2 SEM
ed
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Fig. 3 showed SEM cross-section images of Alg(0.5)/PAN, HA/PAN,
218
Alg(0.5)/HA/PAN and HA/Alg(0.5)/PAN membranes, respectively. It was indicated
219
that the active layer with a thickness about 500nm was uniformly and tightly
220
deposited on the porous PAN support layer. The cross-section of Alg(0.5)/HA/PAN
221
and HA/Alg(0.5)/PAN membranes in Fig. 3 (c-d) clearly showed the two-active-layer
222
structure and a distinct interface between those two layers. The thickness of capping
223
layers of HA/Alg/PAN and Alg/HA/PAN membranes was about 150 nm, while the
224
thickness of inner layers was about 400 nm.
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Fig. 3. SEM images of cross-section: (a) Alg(0.5)/PAN; (b) HA/PAN; (c) Alg(0.5)/HA/PAN;
228
and (d) HA/Alg(0.5)/PAN. Scale: 2 μm.
229
3.1.3 Water contact angle
ed
227
Table 1. Water contact angles of Alg(X)/HA/PAN, Alg(X)/PAN, HA/Alg(X)/PAN
231
and HA/PAN membranes.
Membrane
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Contact angle (°)
Membrane
Contact angle (°)
52 ± 3
Alg(0)/PAN
51 ± 3
Alg(0.02)/HA/PAN 34 ± 2
Alg(0.02)/PAN
33 ± 2
Alg(0.1)/HA/PAN
24 ± 1
Alg(0.1)/PAN
25 ± 2
Alg(0.5)/HA/PAN
23 ± 1
Alg(0.5)/PAN
23 ± 1
HA/Alg(0)/PAN
37 ± 2
HA/PAN
36 ± 2
HA/Alg(0.02)/PAN 34 ± 2
--
--
HA/Alg(0.1)/PAN
33 ± 2
--
--
HA/Alg(0.5)/PAN
33 ± 2
--
--
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Alg(0)/HA/PAN
232
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Table 1 showed water contact angles of Alg(X)/HA/PAN, Alg(X)/PAN,
234
HA/Alg(X)/PAN and HA/PAN membranes. It was observed that with the increase of
235
Ca2+ concentrations (X) in the aqueous solutions that used to crosslink NaAlg layer,
236
the contact angle of Alg(X)/HA/PAN and Alg(X)/PAN membranes became smaller,
237
indicating that hydrophilicity was enhanced by Ca2+ crosslinking. When X value was
238
the same, contact angles of Alg(X)/HA/PAN and Alg(X)/PAN membranes were
239
similar, which indicated that alginic acid layer was completely deposited. Similarly,
240
contact angles of HA/Alg(X)/PAN and HA/PAN membranes proved that HA was also
241
completely deposited on membranes.
242
3.2 Pervaporation studies
243
3.2.1 Swelling, sorption and diffusion characteristics
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Table 2(a). The swelling behavior of HA and NaAlg homogenous membranes
245
in 90 wt.% ethanol-water mixture and water.
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Degree of swelling
90 wt.% ethanol-water
Water
HA
0.06
41
NaAlg
0.03
0.64
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244
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The equilibrium sorption percentage of crosslinked HA and NaAlg homogenous
248
membranes, which were cross-linked by glutaraldehyde and 0.5M Ca2+ respectively,
249
were investigated and results were shown in Table 2(a). Both HA and Alg membranes
250
suffered much less swelling in 10 wt.% water-ethanol mixture than in water, which
251
meant that the membranes were stable for pervaporation under low water
252
concentration. Although Ca2+ cross-linked Alg layer showed higher hydrophilicity, HA 13
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swelled more evidently than Alg in both water and the feed solution. The swelling
254
degree of polymer in various solvents was determined not only by the
255
polymer-solvent affinity but also its spatial structure. The three dimentional
256
cross-linking of Alginate acid chains by Ca2+ can effectively suppress the swelling.
257
The extended chains of HA with lower crosslinking degree in the solvent lead to a
258
looser network, which can be accessed by more solvent molecules.
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Table 2(b). The sorption and diffusion behavior of HA and NaAlg homogenous
260
membranes in 10 wt.% water-ethanol mixture
ethanol
HA
5.41
0.59
NaAlg
1.42
1.58
s 83
an
water
Di / (10-12m2/s) water
ethanol
1.75
1.12
1.6
3.73
0.05
91.1
M
Ci / (10-2g/g)
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259
8.1
Sorption selectivity indicated the affinity of the membrane toward the components
262
to be separated. The sorption amount of water and ethanol and the sorption selectivity
263
of HA and NaAlg in the feed (10 wt.% water-ethanol mixture) were also shown in
264
Table 2(b). A big difference in the sorption amount with respect to water and ethanol
265
can be observed which indicated that both the HA and NaAlg had high affinity toward
266
water. And the HA ( s=83) exhibited much higher sorption selectivity than that of
267
NaAlg ( s=8.1).
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261
268
To discriminate the effects of solubility and diffusivity on permeability, Diffusion
269
selectivity was calculated by Eq.(3). The flux and separation factor of Alg/PAN
270
one-active-layer composite membranes were 925 g/m2 h and 738, while those of
271
HA/PAN membranes were 1705 g/m2 h and 130. It showed that diffusion selectivity
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of Alg (
=91.2) was higher than that of HA (
=1.6). It can be explained by
273
concentration-averaged diffusion coefficients ( Di ) of water and ethanol in HA and
274
NaAlg layer in the active layer of HA/PAN and Alg(0.5)/PAN composite membrane.
275
According to the Fick’s law, when the concentration profile is assumed linear along
276
the diffusion length in the active layer for simplicity,
277
Di
278
the concentration-averaged diffusion coefficients Di can be calculated by Eq.(9) .
279
Diffusion coefficient of water molecules were higher both in HA and NaAlg, While
280
diffusion coefficients of ethanol molecules in crosslinked alginate were smaller than
281
that in HA. One reason was that the water molecules (dynamic diameter, 0.26nm)
282
were smaller than ethanol molecules (dynamic diameter, 0.52nm). Furthermore, large
283
free volume voids of HA in the feed, indicated by the swelling degree, promoted the
284
diffusion coefficient of ethanol. But the voids was too large to identify water and
285
molecules, thus, the diffusion coefficient of ethanol in HA was much higher than that
286
in alginate. In a word, HA exhibited much higher water sorption selectivity than that
287
of NaAlg, whereas NaAlg exhibited higher water diffusion selectivity than that of
288
HA.
289
3.2.2 Effect of the cross-linking of Alginate layer on the separation performance
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272
Jil Ci
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(9)
290
Ca2+ cross-linking was a common method to improve alginate membrane
291
performance. Fig. 4 showed the effect of Ca2+ concentration on pervaporation
292
performance
293
HA/Alg(X)/PAN membranes for dehydration of 90 wt.% ethanol-water mixture at
of
Alg(X)/PAN,
Alg(X)/Alg(X)/PAN,
Alg(X)/HA/PAN
and
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Page 16 of 29
80oC. The permeate flux of those four membranes displayed the similar trend with the
295
increase of Ca2+ concentration. Membranes made of alginate which was not
296
cross-linked by Ca2+ exhibited lower flux and separation factor, compared with
297
cross-linked membranes. With the increase of Ca2+ concentration, both of the
298
hydrophilicity and the crosslinking degree were increased, which improved the
299
permselectivity of the membranes. Thus separation factors of these membranes were
300
all increased.
us
Alg/PAN Alg/Alg/PAN HA/Alg/PAN Alg/HA/PAN
Separation factor
1000
800
800 600 400
M
600
200
400 0.05
0.10
2+
0.15
0.50
0
0.55
ed
2
Permeation flux (g/(m h))
1000
0.00
301
(b)1200
Alg/PAN Alg/Alg/PAN HA/Alg/PAN Alg/HA/PAN
1200
an
(a)
cr
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294
Ca concentration(mol/L)
0.00
0.05
0.10
0.15
0.50
0.55
2+
Ca concentration(mol/L)
Fig. 4 Effect of Ca2+ concentration on pervaporation performance of membranes, Alg/PAN,
303
Alg/Alg/PAN, Alg/HA/PAN and HA/Alg/PAN, for dehydration of 90 wt.%
304
ethanol-water mixture at 80℃.
3.2.3 Effect of coating sequences on the separation performance
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305
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302
306
Separation performance of various composite membranes was shown in Fig. 4.
307
Comparison between the separation performance of HA/Alg/PAN and Alg/Alg/PAN
308
membranes showed that HA/Alg/PAN possessed a higher separation factor, while
309
their fluxes were similar. It showed that HA with high water sorption selectivity and
310
water retention capacity was more suitable as the capping layer than alginate. When it
311
was used as capping layer, more water was adsorbed by HA layer, which increased the 16
Page 17 of 29
312
driving force of water diffusion through the membranes. It can be concluded that
313
materials as capping layer should possess higher hydrophilicity and larger sorption
314
capacity to intensify solution process. Comparison
between
the
separation
performance
of
Alg/HA/PAN
and
ip t
315
Alg/Alg/PAN membranes showed that Alg/Alg/PAN after crosslinked possessed a
317
higher separation factor, while their fluxes were similar. It showed that alginate was
318
more suitable as the inner layer than HA. Crosslinked alginate had higher diffusion
319
selectivity than that of HA. When it was used as inner layer, the membranes possessed
320
high separation factor, which proved that materials as inner layer should possess
321
higher diffusion selectivity and small swelling degree.
M
an
us
cr
316
Separation performance of HA/Alg/PAN and Alg/HA/PAN two-active-layer
323
composite membranes was studied to verify the validity of the selection rules. It was
324
found that the separation factor of HA/Alg/PAN was higher remarkably than that of
325
Alg/HA/PAN membranes. HA/Alg(0.5)/PAN membrane possessed a separation factor
326
of 1130 and Alg(0.5)/HA/PAN membrane possessed a separation factor of 527, while
327
their permeate fluxes were 972 g/(m2 h) and 948 g/(m2 h) respectively. This further
328
proved that capping layer with higher hydrophilicity and water capacity, and inner
329
layer with higher permselectivity could increase the separation performance of the
330
composite
331
HA/Alg(0.5)/PAN membrane with Alg as the inner layer have higher separation
332
factor(1130) than that of HA/CS/PAN(704) which further verified that Alg have
333
higher diffusion selectivity than that of CS.
Ac
ce pt
ed
322
membranes.
Meantime,
compared
with
our
previous
study,
17
Page 18 of 29
334
3.2.4 Effect of operation temperature on the separation performance
335
1000
600
800 400
600 400
200 200 300
310
320
330
340
0 360
350
80
60
40
20
300
(d)
HA/Alg/PAN Alg/HA/PAN
600
1.0
0.5
300 200
310
320
330
340
M
300
350
7
0
360
Temperature (K)
ed
3
1 2.8
2.9
3.0
3.1
3.2
330
340
350
360
water ethanol Ep,w=36.98kJ/mol
5
4
Ep,e=18.71kJ/mol
2
320
Temperature (K)
6
lnJp
ce pt
4
310
7
Ep,w=43.63kJ/mol
5
300
(f)
water ethanol
6
lnJp
360
100
(e)
Ep,e=-0.71kJ/mol
3
3.3
2.8
2.9
3.0
3.1
3.2
3.3
1000/T,K Alg/HA/PAN
1000/T,K HA/Alg/PAN
Ac
340
350
an
Selectivity
400 1.5
337
339
340
HA/Alg/PAN Alg/HA/PAN
500
2.0
0.0
330
us
2.5
320
Temperature (K)
2
Permeance(ethanol) (g/(m h kPa))
(c)
338
310
Temperature (K)
336
ip t
2
1200
HA/Alg/PAN Alg/HA/PAN
100
cr
1400
800
0
(b)
1600
HA/Alg/PAN Alg/HA/PAN
Separation factor 2 Permeance(water) (g/(m h kPa))
, ,
1000
Permeation flux (g/(m h))
(a)
Fig. 5. Effect of operation temperature on pervaporation performance of HA/Alg/PAN and Alg/HA/PAN membranes for dehydration of 90 wt.% ethanol-water mixture.
341
(a) Effect of operation temperature on permeation flux and separation factor of HA/Alg/PAN and
342
Alg/HA/PAN membranes.(b) Effect of operation temperature on water permeance of HA/Alg/PAN
343
and Alg/HA/PAN membranes. (c) Effect of operation temperature on ethanol permeance of
344
HA/Alg/PAN and Alg/HA/PAN membranes. (d) Effect of operation temperature on selectivity of 18
Page 19 of 29
345
HA/Alg/PAN and Alg/HA/PAN membranes. (e) Arrhenius plots of permeation flux for separation
346
of ethanol/water mixture by the HA/Alg/PAN membrane.(f) Arrhenius plots of permeation flux for
347
separation of ethanol/water mixture by the Alg/HA/PAN Membrane.
Operation conditions had a remarkable influence on flux and separation factor, and
349
its variation not only changed the driving force of species permeating through
350
membranes, but also the intrinsic properties of membranes. Fig. 5 showed effect of
351
operation temperature on pervaporation performance of HA/Alg(0.5)/PAN and
352
Alg(0.5)/HA/PAN membranes for dehydration of 90 wt.% ethanol-water mixture.
353
With increasing temperature, both flux and separation factor of two membranes went
354
up markedly (as shown in Fig.5 (a)). However, the separation factor of HA/Alg/PAN
355
was higher than that of Alg/HA/PAN at all the temperature point, while fluxes of these
356
two membranes were similar.
ed
M
an
us
cr
ip t
348
The flux was mainly affected by ethanol and water vapor pressure (driving force)
358
and membrane properties. With the increase of temperature, ethanol and water vapor
359
pressure increased and the permeating driving force was enhanced, which was
360
favorable to increase of flux. In order to study the effect of temperature on membrane
361
intrinsic properties, driving force normalized permeance and selectivity were
362
presented in Fig. 5(b-d). It could be found that water and ethanol permeances of
363
Alg/HA/PAN membrane varied in a wider range than those of HA/Alg/PAN,
364
indicating
365
temperature-depended. The dependence of permeation flux on temperature could be
366
expressed by Arrhenius equation (Eq.(10)) and the apparent activation energy could
Ac
ce pt
357
that
properties
of
Alg/HA/PAN
membranes
were
more
19
Page 20 of 29
367
be obtained by equation (10).
368
J p Ap exp(
369
Where Ap, Jp, Ep, R and T are the pre-exponential factor, permeation flux, apparent
370
activation energy, gas content and feed temperature, respectively.
)
(10)
ip t
RT
The Arrhenius plots of lnJp versus 1000/T for water and ethanol were shown in
cr
371
Ep
Fig. 5(e,f) . A linear relationship was observed, indicating the permeability followed
373
the Arrheninus law. With the temperature ranging from 303 to 353K, the apparent
374
activation energy for water in HA/Alg(0.5)/PAN membrane and Alg/HA/PAN
375
membrane were 43.63 kJ/mol and 36.98 kJ/mol, respectively. Simultaneously, the
376
apparent activation energy for ethanol in HA/Alg(0.5)/PAN membrane and
377
Alg/HA/PAN membrane were 18.71 kJ/mol and -0.71 kJ/mol, respectively. It could be
378
found that the apparent activation energy for water and ethanol in HA/Alg(0.5)/PAN
379
membrane were larger than that in Alg/HA/PAN membrane. Therefore, with the
380
increase of temperature, the increasing rate of water flux and ethanol flux in
381
HA/Alg(0.5)/PAN membrane were greater than that in Alg/HA/PAN membrane. So
382
the data of selectivity showed that the HA/Alg/PAN membrane was more
383
advantageous than that of Alg/HA/PAN membrane for dehydration of 90 wt.%
384
ethanol-water mixture.
385
3.2.5 Effect of water concentration in feed on the separation performance
Ac
ce pt
ed
M
an
us
372
386
20
Page 21 of 29
2500
1250 2000 1000 1500 750 1000
500
500
250 0
10
20
30
40
0
HA/Alg/PAN Alg/HA/PAN
50 45 40 35 30 25 20 15
0
(d)
20
30
40
HA/Alg/PAN Alg/HA/PAN
cr
HA/Alg/PAN Alg/HA/PAN
0.25
1000
0.20
0.15
0.10
us
800
Selectivity
600
400 0.05
an
2
Permeance(ethanol) (g/(m h kPa)
(c)
10
Water concentration in feed
Water concentration in feed
387
200
0.00
0
10
20
30
40
0
10
20
30
40
Water concentration in feed
Water concentration in feed
M
388
3000
1500
0
(b)
3500
HA/Alg/PAN Alg/HA/PAN
ip t
2
Permeation flux (g/(m h))
1750
, ,
Separation factor 2 Permeance(water) (g/(m h kPa))
(a) 2000
Fig. 6. Effect of water concentration in feed on pervaporation performance of HA/Alg/PAN and
390
Alg/HA/PAN membranes for dehydration of ethanol-water mixtures containing 1 wt.%, 5 wt.%,
391
10 wt.%, 20 wt.%, 30 wt.% and 40 wt.% water at 80℃.
ed
389
(a) Effect of water concentration on permeation flux and separation factor of HA/Alg/PAN and
393
Alg/HA/PAN membranes.(b) Effect of water concentration on water permeance of HA/Alg/PAN
394
and Alg/HA/PAN membranes.(c) Effect of water concentration on ethanol permeance of
395
HA/Alg/PAN and Alg/HA/PAN membranes. (d) Effect of water concentration on selectivity of
396
HA/Alg/PAN and Alg/HA/PAN membranes.
Ac
ce pt
392
397
Fig. 6 showed effect of water concentration in feed on pervaporation performance
398
of HA/Alg(0.5)/PAN and Alg(0.5)/HA/PAN membranes for dehydration of
399
ethanol-water mixtures containing 1 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.% and 40
400
wt.% water at 80oC. With increasing water concentration in feed, fluxes of both
21
Page 22 of 29
membranes increased and separation factors decreased, which resulted from enhanced
402
swelling with increasing water concentration. Driving force normalized permeance
403
and selectivity were presented in Fig. 6(b-d). It was shown that permeances of water
404
and ethanol increased remarkably, suggesting that membrane became more relaxed
405
with the increase of water concentration and was favorable to feed species through
406
membrane.
cr
ip t
401
When water concentration was below 20 wt.%, water selectivity of HA/Alg/PAN
408
was higher than that of Alg/HA/PAN; When water concentration was equal to or
409
above 20 wt.%, reverse. It was probably because the swelling of HA capping layer
410
increased with the increase of water concentration, resulting in the decrease of
411
selectivity. So HA/Alg/PAN membranes were advantageous for dehydration of
412
ethanol/water solution with low water concentration, while Alg/HA/PAN membranes
413
were advantageous for high water concentration, because of better anti-swelling
414
property of Ca2+ cross-linked NaAlg layers.
415
4. Conclusion
ce pt
ed
M
an
us
407
The two-active-layer composite membranes, which comprise the capping layer
417
and the inner layer, were prepared by sequential coating of HA and NaAlg on the PAN
418
support layer and utilized for pervaporation dehydration of ethanol-water mixtures. It
419
was shown that membranes with different coating sequences had an obvious
420
difference in the dehydration performance. Under the operation condition of 80oC and
421
10 wt.% water concentration in feed, HA/Alg/PAN membrane possessed a separation
422
factor of 1130 and Alg/HA/PAN membrane possessed a separation factor of 527,
Ac
416
22
Page 23 of 29
while the permeate fluxes of both membranes were similar. It suggested that materials
424
with higher hydrophilicity and larger sorption capacity could be selected for
425
constructing the capping layer to intensify solution process, while materials with less
426
diffusion resistance and high diffusion selectivity could be used for constructing the
427
inner layer to enhance the diffusion process.
ip t
423
Acknowledgements
us
429
cr
428
The authors gratefully acknowledge the financial support from the National Science
431
Fund for Distinguished Young Scholars (21125627), the National basic research
432
program of China (2009CB623404), the Programme of Introducing Talents of
433
Discipline to Universities (No: B06006) and the Program for Changjiang Scholars and
434
Innovative Research Team in University (PCSIRT), Tianjin Natural Science
435
Foundation (10JCZDJC22600).
Ac
ce pt
ed
M
an
430
23
Page 24 of 29
Nomenclature A
effective membrane area (m2)
Ci
concentration of component
J
permeation flux (g/m2 h)
l
thickness of the active layer (m)
P
permeability
Pi / l
permeance
pisat 0
saturated vapor pressure of pure component i
pil
partial pressure of component i in the permeate side
Q
total mass of the permeate solution in time t (g)
t
time (h)
md
weight of the dry membrane (g)
ms
weight of the swollen membrane (g)
xiL0
mole fraction of component i in the feed solution
Wf , Ef
weight fractions of water and ethanol in the feed solution
Wp , Ep
weight fractions of water and ethanol in the permeate solution
WM
weight fractions of water and ethanol in the membrane
selectivity
s
sorption selectivity
D
i0L
ce pt
ed
M
an
us
cr
ip t
(g/m3)
Ac
in the membrane
diffusion selectivity separation factor activity coefficient of component i in the feed solution
24
Page 25 of 29
References
2
1.Baker, R. W. et al. (2010). Permeability, permeance and selectivity: A preferred way
3
of reporting pervaporation performance data. Journal of Membrane Science,
4
348(1-2), 346-352.
ip t
1
2.Bhat, S. D., & Aminabhavi, T. M. (2006). Novel sodium alginate–Na+MMT hybrid
6
composite membranes for pervaporation dehydration of isopropanol, 1,4-dioxane
7
and tetrahydrofuran. Separation and Purification Technology, 51(1), 85-94.
us
cr
5
3.Bhat, S. D., & Aminabhavi, T. M. (2007). Pervaporation Separation Using Sodium
9
Alginate and Its Modified Membranes—A Review. Separation & Purification
12
M
11
Reviews, 36(3), 203-229.
4.Binning, R. et al. (1961). Separation of Liquid Mixtures by Permeation. Industrial & Engineering Chemistry, 53(1), 45-50.
ed
10
an
8
5.Ding, J. et al. (2012). Enhancing the permselectivity of pervaporation membrane by
14
constructing the active layer through alternative self-assembly and spin-coating.
15
Journal of Membrane Science, 390-391, 218-225.
ce pt
13
6.Huang, R. Y. M.et al. (1999). Characteristics of sodium alginate membranes for the
17
pervaporation dehydration of ethanol–water and isopropanol–water mixtures.
18
Journal of Membrane Science, 160(1), 101-113.
Ac
16
19
7.Kalyani, S. et al. (2006). Blend membranes of sodium alginate and
20
hydroxyethylcellulose for pervaporation-based enrichment of t-butyl alcohol.
21
Carbohydrate Polymers, 64(3), 425-432.
22
8.Kanti, P. et al. (2004). Dehydration of ethanol through blend membranes of chitosan
25
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and sodium alginate by pervaporation. Separation and Purification Technology,
24
40(3), 259-266. 9.Kim, S. J. et al. (2004). Electrical behavior of polymer hydrogel composed of
26
poly(vinyl alcohol)–hyaluronic acid in solution. Biosensors and Bioelectronics,
27
19(6), 531-536.
properties
30
Polymers, 61(1), 52-60.
31
of
11.Kurkuri,
M.
sodium
D.
et
alginate/hydroxyethylcellulose
al.
(2002a).
blends.
us
29
cr
10.Kumar Naidu, B. V. et al. (2005). Thermal, viscoelastic, solution and membrane
Synthesis
and
an
28
ip t
25
Carbohydrate
characterization
of
polyacrylamide-grafted sodium alginate copolymeric membranes and their use in
33
pervaporation separation of water and tetrahydrofuran mixtures. Journal of Applied
34
Polymer Science, 86(2), 272-281.
ed
M
32
12.Kurkuri, M. D.et al. (2002b). Syntheses and characterization of blend membranes
36
of sodium alginate and poly(vinyl alcohol) for the pervaporation separation of
37
water + isopropanol mixtures. Journal of Applied Polymer Science, 86(14),
38
3642-3651.
40
Ac
39
ce pt
35
13.Lonsdale, H. K. (1982). The growth of membrane technology. Journal of Membrane Science, 10(2–3), 81-181.
41
14.Ma, J. et al. (2010). Facile fabrication of structurally stable hyaluronic acid-based
42
composite membranes inspired by bioadhesion. Journal of Membrane Science,
43
364(1-2), 290-297.
44
15.Magalad, V. T. et al. (2010). Preyssler type heteropolyacid-incorporated highly
26
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45
water-selective sodium alginate-based inorganic–organic hybrid membranes for
46
pervaporation dehydration of ethanol. Chemical Engineering Journal, 159(1-3),
47
75-83. 16.Papageorgiou, S. K. et al. (2010). Metal–carboxylate interactions in metal–alginate
49
complexes studied with FTIR spectroscopy. Carbohydrate Research, 345(4),
50
469-473.
cr
ip t
48
17.Peng, F. et al. (2005). Significant increase of permeation flux and selectivity of
52
poly(vinyl alcohol) membranes by incorporation of crystalline flake graphite.
53
Journal of Membrane Science, 259(1-2), 65-73.
an
us
51
18.Shi, Y. et al. (1998). Preparation and characterization of high-performance
55
dehydrating pervaporation alginate membranes. Journal of Applied Polymer
56
Science, 68(6), 959-968.
ed
M
54
19.Tomihata, K., & Ikada, Y. (1997). Crosslinking of hyaluronic acid with
58
glutaraldehyde. Journal of Polymer Science Part A: Polymer Chemistry, 35(16),
59
3553-3559.
ce pt
57
20.Uragami, T., & Saito, M. (1989). Studies on Syntheses and Permeabilities of
61
Special Polymer Membranes. 68. Analysis of Permeation and Separation
62
Characteristics and New Technique for Separation of Aqueous Alcoholic Solutions
63
through Alginic Acid Membranes. Separation Science and Technology, 24(7-8),
64
541-554.
Ac
60
65
21.Yeom, C. K. et al. (1998). Characterization of sodium alginate membrane
66
crosslinked with glutaraldehyde in pervaporation separation. Journal of Applied
27
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67
Polymer Science, 67(2), 209-219. 22. Moon G. Y. et al. (1999). Novel two-ply composite membranes of chitosan and
69
sodium alginate for the pervaporation dehydration of isopropanol and ethanol.
70
Journal of Membrane Science, 156(1), 17-27.
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ce pt
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an
us
cr
ip t
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28
Page 29 of 29
S0144-8617(13)00841-2 http://dx.doi.org/doi:10.1016/j.carbpol.2013.08.057 CARP 8047
To appear in: Received date: Revised date: Accepted date:
5-4-2013 19-8-2013 22-8-2013
Please cite this article as: Gao, C., Zhang, M., Ding, J., Pan, F., Jiang, Z., Li, Y., & Zhao, J., Pervaporation dehydration of ethanol by hyaluronic acid/sodium alginate two-active-layer composite membranes, Carbohydrate Polymers (2013), http://dx.doi.org/10.1016/j.carbpol.2013.08.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights (for review)
► Two-active-layer
composite
membranes
were
prepared
by
sequential
spin-coating. ► Sorption and diffusion behavior in capping layer and inner layer were studied. ► Positioning of HA layer and NaAlg layer in membrane affected separation factor.
Ac
ce pt
ed
M
an
us
cr
ip t
► HA/Alg/PAN membrane exhibited higher pervaporation performance.
Page 1 of 29
*Manuscript
1
Pervaporation dehydration of ethanol by hyaluronic
2
acid/sodium alginate two-active-layer composite membranes
3
Chengyun Gaoa,b, Minhua Zhangb, Jianwu Dinga,b, Fusheng Pana, Zhongyi Jianga,
5
Yifan Lia, Jing Zhaoa
ip t
4
6 7
a
8
Chemical Engineering and Technology, Tianjin University, 92# Road Weijin, District
9
Nankai, Tianjin 300072, China
us
cr
Key Laboratory for Green Chemical Technology of Ministry of Education, School of
10
b
11
Petrochemical Technology, Tianjin University, Tianjin 300072, China
an
Key Laboratory for Green Technology of Ministry of Education, R & D Center for
M
12
ed
13
ce pt
14
15
Ac
16
17
To whom correspondence should be addressed. Address: School of Chemical
Engineering & Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin, 300072, P. R. China. Tel: 86-22-2350 0086. Fax: 86-22-2350 0086. E-mail: [email protected]
1
Page 2 of 29
18
Abstract:
The composite membranes with two-active-layer (a capping layer and an inner
20
layer) were prepared by sequential spin-coatings of hyaluronic acid (HA) and sodium
21
alginate (NaAlg) on the polyacrylonitrile (PAN) support layer. The SEM showed a
22
mutilayer structure and a distinct interface between the HA layer and the NaAlg layer.
23
The coating sequence of two-active-layer had an obvious influence on the
24
pervaporation dehydration performance of membranes. When the operation
25
temperature was 80oC and water concentration in feed was 10 wt.%, the permeate
26
fluxes of HA/Alg/PAN membrane and Alg/HA/PAN membrane were similar, whereas
27
the separation factor were 1130 and 527, respectively. It was found that the capping
28
layer with higher hydrophilicity and water retention capacity, and the inner layer with
29
higher permselectivity could increase the separation performance of the composite
30
membranes. Meanwhile, effects of operation temperature and water concentration in
31
feed on pervaporation performance as well as membrane properties were studied.
cr
us
an
M
ed
ce pt
32
ip t
19
Keywords: Hyaluronic acid; Sodium alginate; Two-active-layer composite membrane;
34
Pervaporation; Ethanol dehydration
Ac
33
35 36 37 38
2
Page 3 of 29
39
Introduction Pervaporation, as a promising membrane-based separation technology for the
41
dehydration of solvents, has undergone a rapid development in recent years.
42
Solution-diffusion mechanism is proposed and widely accepted (Binning et al., 1961;
43
Lonsdale, 1982) to elucidate the mass transfer of pervaporation process: selective
44
sorption of feed species on the membrane surface, selective diffusion through the
45
membrane and desorption into a vapor phase on the permeate side. Among these three
46
steps, selective sorption and diffusion in membranes are often the rate-governing steps.
47
At present, the composite membrane, which comprises a separation layer (active layer)
48
and a support layer is the most commonly used configuration. Ideally, the separation
49
layer of membranes should possess the surface with high sorption capacity and
50
selectivity, and the bulk with low diffusion resistance and high diffusion selectivity.
51
Materials for the separation layer of the one-active-layer pervaporation membranes
52
have to bear these properties all-in-one, which limits the selection scope of materials.
53
It is well known that the composite membranes assign the separation performance and
54
the mechanical strength demands into the separation layer and the support layer
55
individually. The performance can be highly improved by choosing the appropriate
56
materials for the separation layer and the support layer. It is thus envisaged that if the
57
similar strategy is adopted, the separation layer is further divided into the capping
58
layer and the inner layer (two-active-layer) to intensify the adsorption and diffusion
59
individually, as shown in Fig.1, the selection scope of the materials for separation
60
layer may be dramatically expanded and the separation performance may be improved
Ac
ce pt
ed
M
an
us
cr
ip t
40
3
Page 4 of 29
in a much broader range, since the surface and bulk properties could be varied by the
62
rational tuning of the capping layer and the inner layer.
cr
ip t
61
63
Fig. 1. Schematic structure of the two-active-layer composite membrane.
65
Rational screening of hydrophilic materials for capping layer and inner layer is of
66
great importance for the separation performance of two-active-layer membranes. Till
67
now, several kinds of polymer-based materials have been employed in the dehydration
68
process of ethanol-water aqueous solution by pervaporation. Huang et al. (1999)
69
prepared two-ply membranes by successive castings of NaAlg and CS solutions for
70
the pervaporation dehydration of 95 wt.% ethanol-water mixture. When CS was
71
selected as capping layer facing to the feed solution, the separation factors was 147,
72
while that of the membranes with NaAlg facing to the feed solution was 1062. These
73
results suggested that coating sequence had an obvious effect on membrane separation
74
performance. In our previous study (Ding et al., 2012), composite membranes with
75
two-active-layer, HA and CS, were constructed through alternative self-assembly and
76
spin-coating. It was found that the separation factor of the HA/CS/PAN membrane
77
(704) with CS as inner layer was remarkably higher than that of CS/HA/PAN
78
membrane (104) with HA as inner layer. A probable explanation was HA layer had
79
higher water sorption selectivity and CS layer had higher diffusion selectivity. In the
Ac
ce pt
ed
M
an
us
64
4
Page 5 of 29
above studies, the capping layer and the inner layer are fabricated from cationic
81
polyelectrolyte and anionic polyelectrolyte, individually. Herein, we want to explore
82
the separation performance of the two-active-layer membranes where the capping
83
layer and the inner layer are both fabricated from anionic polyelectrolytes.
ip t
80
As one common hydrophilic carbohydrate polymer, sodium alginate (NaAlg),
85
which is extracted from marine brown algae, has exhibited excellent performance as a
86
pervaporation membrane material (Bhat & Aminabhavi, 2006, 2007; Kalyani et al.,
87
2006; Kanti et al., 2004; Kumar Naidu et al., 2005; Kurkuri et al., 2002a; Kurkuri et
88
al., 2002b; Magalad et al., 2010; Uragami & Saito, 1989). The carboxylic and
89
hydroxyl groups play a crucial role in preferential water sorption and diffusion
90
through the membranes (Yeom & Lee, 1998) .Since alginate has the excellent
91
permselectivity toward water, it may be the suitable material to offer higher separation
92
performance (Huang et al., 1999; Shi et al., 1998). As another common hydrophilic
93
carbohydrate polymer, hyaluronic acid (HA) (Kim et al., 2004; Ma et al., 2010;
94
Tomihata & Ikada, 1997) has demonstrated high water sorption selectivity in ethanol
95
aqueous solution and high water retention capacity (500mL/g) due to a large number
96
of hydroxyl groups and carboxyl groups. Therefore, HA may have potential to
97
intensify the sorption/dissolution of water.
Ac
ce pt
ed
M
an
us
cr
84
98
In this study, two-active-layer composite membranes were fabricated by sequential
99
spin-coatings of HA and NaAlg on the PAN support layer. Effect of coating sequences
100
and operation conditions on structure and performance variation of the
101
two-active-layer membranes for dehydration of 90 wt.% ethanol-water mixture were
5
Page 6 of 29
102
explored.
103
2. Experiments
104
2.1 Materials The flat-sheet ultrafiltration membrane, which had a molecular weight cut-off of
106
100,000 and was made up of PAN porous layer and polyethylene terephthalate
107
nonwoven fabric substrate, was obtained from Shanghai MegaVision Membrane
108
Engineering & Technology Co. Ltd. (Shanghai, China). NaAlg (viscosity of 1 wt.%
109
aqueous solution at 20oC: 0.02Pa•s ) was purchased by Tianjin Guangfu Ltd. (Tianjin,
110
China). HA (MW 1,200,000 Da, sodium salt) was obtained by Qufu Haixin Industrial
111
Co. Ltd. (Qufu, China). Glutaraldehyde (GA; 25 wt.%), hydrochloric acid (HCl),
112
calcium chloride and absolute ethanol were purchased from Tianjin Kewei Ltd.
113
(Tianjin, China). Deionized water was used in all experiments.
114
2.2 Membrane preparation
ed
M
an
us
cr
ip t
105
The flat-sheet PAN membrane was immersed into deionized water for two days to
116
remove glycerol on the membrane surface, and air-dried at room temperature. The
117
NaAlg solution was prepared by dissolving NaAlg into deionized water with stirring
118
for two hours, and then kept aside for one hour to remove the bubble in the solution.
119
HA solution was prepared by dissolving hyaluronic acid into deionized water with
120
stirring for two hours, and then added GA solution to initiate cross-linking and
121
adjusted pH to about 4.5 with 0.01M HCl. After half an hour, stirring was stopped and
122
the sample was kept aside for degassing
123
Ac
ce pt
115
Membranes were prepared by two-step spin-coating. NaAlg solution was firstly
6
Page 7 of 29
coated on PAN support layer by spin-coater and dried at room temperature. Then the
125
NaAlg membrane was cross-linked by immersing into the CaCl2 aqueous solution for
126
ten minutes and then rinsed with deionized water and air-dried. After that, the second
127
coating was carried on. HA solution was spin-coated on the membrane surface and
128
air-dried. The resultant membrane was denoted as HA/Alg(X)/PAN, where X (unit:
129
mol/L) meant Ca2+ concentrations of solutions that were used to cross-link NaAlg
130
membrane. Similarly, the Alg(X)/HA/PAN membrane was prepared by sequential
131
spin-coatings of first hyaluronic acid and then sodium alginate.
132
2.3 Characterization
an
us
cr
ip t
124
FT-IR spectra for each layer of the composite membrane were acquired by using a
134
Nicolet-560 Fourier transform infrared spectrometer with scan range of 4000-600
135
cm-1.The static water contact angles of the membrane surfaces were measured by
136
contact angle goniometer (JC2000C, Powereach Co., Shanghai, China) for the
137
investigation of the hydrophilic/hydrophobic property. The contact angle of each
138
membrane sample was measured ten times at different locations. The cross-section
139
morphology of membranes was observed by a Nanosem 430 field emission scanning
140
electron microscope (FESEM). Samples were freeze-fractured in liquid nitrogen and
141
then were sputtered with gold prior to scanning.
142
2.4 Swelling and sorption measurement
Ac
ce pt
ed
M
133
143
The swelling degree of homogenous HA and NaAlg membranes was measured by
144
immersing membranes into deionized water and 90 wt.% ethanol-water mixture for 24
145
hours. Then the membranes were carefully wiped off the surface liquid with tissue
7
Page 8 of 29
146 147
paper. The degree of swelling was calculated as follows: Degree of swelling
Ms Md 100% Md
(1)
The adsorbed liquid in membranes was desorbed by an apparatus with a vacuum
149
pump and condensed in a liquid nitrogen trap, which was analyzed by gas
150
chromatography. All the experiments were repeated three times.
ip t
148
Sorption selectivity (αs) was calculated by Eq. (2):
152
s
153
Where WM and EM were the weight fractions of water and ethanol in the membrane,
154
respectively; Wf and Ef were the weight fractions of water and ethanol in the feed
155
solution, respectively.
Eq. (3):
158
D
s
us
an
M
ed
According to solution–diffusion theory, diffusion selectivity (αD) was calculated by
157
159
(2)
(3)
ce pt
156
WM / EM Ef / Ef
cr
151
where α is the separation factor of single-active-layer HA/PAN or CS/PAN membranes.
161
2.5 Pervaporation experiment
Ac
160
162
Pervaporation experiments were carried out for dehydration of ethanol/water
163
mixtures. The pervaporation apparatus were described in our previous study (Peng et
164
al., 2005). The effective surface area was 25.6 cm2. The concentrations of the feed
165
solution and the permeate were measured by gas chromatography (USA Agilent 4890).
166
The permeate flux ( J ) and the separation factor ( ) was calculated as follows
8
Page 9 of 29
167
J
Q At
(4)
168
Wp / Ep Wf / Ef
(5)
Where Q is the mass of the permeate collected in time t, and A is the effective
170
membrane area. Where Wp and Ep were the weight fractions of water and ethanol in
171
the permeate solution, respectively; Wf and Ef were the weight fractions of water and
172
ethanol in the feed solution, respectively.
cr
ip t
169
Generally, permeate flux and separation factor are affected strongly by operation
174
conditions (such as temperature and water concentration in the feed), which change
175
vapor pressure difference between two sides of membrane (driving force), as well as
176
membrane intrinsic properties. In order to investigate the intrinsic properties of
177
membranes with variation of feed concentration and temperature, effect of the driving
178
force should be removed. Baker (Baker et al., 2010) presented a method in term of
179
permeability ( Pi ), permeance ( Pi / l ) and selectivity ( ), which normalized the
180
driving force. They were used to evaluate variations of membrane properties with
181
operation conditions.
183
an
M
ed
ce pt
The component flux can be written as
Ac
182
us
173
Ji
Pi L L sat ( i 0 xi 0 pi 0 pil ) l
(6)
184
Where for component i , J i is the partial permeate flux, Pi is the permeability, l
185
is the thickness of the active layer in the composite membranes, i0L is the activity
186
coefficient in the feed solution, xiL0 is the mole fraction in the feed solution, pisat is 0
187
the saturated vapor pressure of pure component i , pil is the partial pressure in the
188
permeate vapor. It is implicit in the equation (6) that the concentration polarization 9
Page 10 of 29
189
190
effect is negligible. Therefore the permeance ( Pi / l ) was calculated as follows: Pi Ji L L sat l ( i 0 xi 0 pi 0 pil )
(7)
Since the vacuum pump is used in the permeate side and the vapor pressure is
192
approximately vacuum, pil can be considered as zero. The activity coefficient and
193
the pure component vapor pressure can be calculated by computer simulation
194
programs (ChemCad, HYSYS, ProSim or Aspen Plus).
cr
us
The selectivity ( ) is defined as the ratio of the permeability or the permeance of two components:
197
ij
198
3. Results and Discussion
199
3.1 Membrane characterization
200
3.1.1 FT-IR
an
196
Pi Pi / l Pj Pj / l
(8)
ce pt
ed
M
195
ip t
191
(d)
(c)
Ac
(b)
(a)
3380cm
4000
201
3500
1570cm-1 1376cm-1
-1
1436cm-1 -1 1615cm-1 - 1050cm (-COO ) (-C-O-C)
(-OH)
3000
2500
2000
1500
1000
Wave numbers (cm-1)
202
Fig. 2. FTIR spectra of (a) Alg(0.5)/PAN; (b) HA/PAN; (c) Alg(0.5)/HA/PAN;
203
and (d) HA/Alg(0.5)/PAN membranes
204
Fig.2 showed FT-IR spectra of Alg(0.5)/PAN, HA/PAN, Alg(0.5)/HA/PAN and
10
Page 11 of 29
HA/Alg(0.5)/PAN membranes, respectively. The spectra of Alg(0.5)/PAN, HA/PAN,
206
Alg(0.5)/HA/PAN and HA/Alg(0.5)/PAN membranes showed the same peak at
207
3380cm-1 which was ascribed to –OH, 1050cm-1 which was ascribed to C-O-C(Ma et
208
al., 2010), 1615cm-1 and 1436cm-1 which were ascribed to asymmetric COO-
209
stretching vibration and symmetric COO- stretching vibration(Papageorgiou et al.,
210
2010). In addition, The bending vibration of the N-H(1570cm-1) and –CH3(1376cm-1)
211
were presented in the spectra of HA/PAN and HA/Alg(0.5)/PAN, respectively. It
212
implied that HA was successfully deposited on HA/PAN and HA/Alg(0.5)/PAN.
213
Meanwhile, the absorption peak of Alg(0.5)/HA/PAN was similar to that of
214
Alg(0.5)/PAN which was also indicated that NaAlg was successfully deposited on
215
Alg(0.5)/HA/PAN and Alg(0.5)/PAN.
216
3.1.2 SEM
ed
M
an
us
cr
ip t
205
Fig. 3 showed SEM cross-section images of Alg(0.5)/PAN, HA/PAN,
218
Alg(0.5)/HA/PAN and HA/Alg(0.5)/PAN membranes, respectively. It was indicated
219
that the active layer with a thickness about 500nm was uniformly and tightly
220
deposited on the porous PAN support layer. The cross-section of Alg(0.5)/HA/PAN
221
and HA/Alg(0.5)/PAN membranes in Fig. 3 (c-d) clearly showed the two-active-layer
222
structure and a distinct interface between those two layers. The thickness of capping
223
layers of HA/Alg/PAN and Alg/HA/PAN membranes was about 150 nm, while the
224
thickness of inner layers was about 400 nm.
Ac
ce pt
217
225
11
Page 12 of 29
ip t cr us an M
226
Fig. 3. SEM images of cross-section: (a) Alg(0.5)/PAN; (b) HA/PAN; (c) Alg(0.5)/HA/PAN;
228
and (d) HA/Alg(0.5)/PAN. Scale: 2 μm.
229
3.1.3 Water contact angle
ed
227
Table 1. Water contact angles of Alg(X)/HA/PAN, Alg(X)/PAN, HA/Alg(X)/PAN
231
and HA/PAN membranes.
Membrane
ce pt
230
Contact angle (°)
Membrane
Contact angle (°)
52 ± 3
Alg(0)/PAN
51 ± 3
Alg(0.02)/HA/PAN 34 ± 2
Alg(0.02)/PAN
33 ± 2
Alg(0.1)/HA/PAN
24 ± 1
Alg(0.1)/PAN
25 ± 2
Alg(0.5)/HA/PAN
23 ± 1
Alg(0.5)/PAN
23 ± 1
HA/Alg(0)/PAN
37 ± 2
HA/PAN
36 ± 2
HA/Alg(0.02)/PAN 34 ± 2
--
--
HA/Alg(0.1)/PAN
33 ± 2
--
--
HA/Alg(0.5)/PAN
33 ± 2
--
--
Ac
Alg(0)/HA/PAN
232
12
Page 13 of 29
Table 1 showed water contact angles of Alg(X)/HA/PAN, Alg(X)/PAN,
234
HA/Alg(X)/PAN and HA/PAN membranes. It was observed that with the increase of
235
Ca2+ concentrations (X) in the aqueous solutions that used to crosslink NaAlg layer,
236
the contact angle of Alg(X)/HA/PAN and Alg(X)/PAN membranes became smaller,
237
indicating that hydrophilicity was enhanced by Ca2+ crosslinking. When X value was
238
the same, contact angles of Alg(X)/HA/PAN and Alg(X)/PAN membranes were
239
similar, which indicated that alginic acid layer was completely deposited. Similarly,
240
contact angles of HA/Alg(X)/PAN and HA/PAN membranes proved that HA was also
241
completely deposited on membranes.
242
3.2 Pervaporation studies
243
3.2.1 Swelling, sorption and diffusion characteristics
M
an
us
cr
ip t
233
Table 2(a). The swelling behavior of HA and NaAlg homogenous membranes
245
in 90 wt.% ethanol-water mixture and water.
ce pt
Degree of swelling
90 wt.% ethanol-water
Water
HA
0.06
41
NaAlg
0.03
0.64
Ac
246
ed
244
247
The equilibrium sorption percentage of crosslinked HA and NaAlg homogenous
248
membranes, which were cross-linked by glutaraldehyde and 0.5M Ca2+ respectively,
249
were investigated and results were shown in Table 2(a). Both HA and Alg membranes
250
suffered much less swelling in 10 wt.% water-ethanol mixture than in water, which
251
meant that the membranes were stable for pervaporation under low water
252
concentration. Although Ca2+ cross-linked Alg layer showed higher hydrophilicity, HA 13
Page 14 of 29
swelled more evidently than Alg in both water and the feed solution. The swelling
254
degree of polymer in various solvents was determined not only by the
255
polymer-solvent affinity but also its spatial structure. The three dimentional
256
cross-linking of Alginate acid chains by Ca2+ can effectively suppress the swelling.
257
The extended chains of HA with lower crosslinking degree in the solvent lead to a
258
looser network, which can be accessed by more solvent molecules.
cr
ip t
253
Table 2(b). The sorption and diffusion behavior of HA and NaAlg homogenous
260
membranes in 10 wt.% water-ethanol mixture
ethanol
HA
5.41
0.59
NaAlg
1.42
1.58
s 83
an
water
Di / (10-12m2/s) water
ethanol
1.75
1.12
1.6
3.73
0.05
91.1
M
Ci / (10-2g/g)
us
259
8.1
Sorption selectivity indicated the affinity of the membrane toward the components
262
to be separated. The sorption amount of water and ethanol and the sorption selectivity
263
of HA and NaAlg in the feed (10 wt.% water-ethanol mixture) were also shown in
264
Table 2(b). A big difference in the sorption amount with respect to water and ethanol
265
can be observed which indicated that both the HA and NaAlg had high affinity toward
266
water. And the HA ( s=83) exhibited much higher sorption selectivity than that of
267
NaAlg ( s=8.1).
Ac
ce pt
ed
261
268
To discriminate the effects of solubility and diffusivity on permeability, Diffusion
269
selectivity was calculated by Eq.(3). The flux and separation factor of Alg/PAN
270
one-active-layer composite membranes were 925 g/m2 h and 738, while those of
271
HA/PAN membranes were 1705 g/m2 h and 130. It showed that diffusion selectivity
14
Page 15 of 29
of Alg (
=91.2) was higher than that of HA (
=1.6). It can be explained by
273
concentration-averaged diffusion coefficients ( Di ) of water and ethanol in HA and
274
NaAlg layer in the active layer of HA/PAN and Alg(0.5)/PAN composite membrane.
275
According to the Fick’s law, when the concentration profile is assumed linear along
276
the diffusion length in the active layer for simplicity,
277
Di
278
the concentration-averaged diffusion coefficients Di can be calculated by Eq.(9) .
279
Diffusion coefficient of water molecules were higher both in HA and NaAlg, While
280
diffusion coefficients of ethanol molecules in crosslinked alginate were smaller than
281
that in HA. One reason was that the water molecules (dynamic diameter, 0.26nm)
282
were smaller than ethanol molecules (dynamic diameter, 0.52nm). Furthermore, large
283
free volume voids of HA in the feed, indicated by the swelling degree, promoted the
284
diffusion coefficient of ethanol. But the voids was too large to identify water and
285
molecules, thus, the diffusion coefficient of ethanol in HA was much higher than that
286
in alginate. In a word, HA exhibited much higher water sorption selectivity than that
287
of NaAlg, whereas NaAlg exhibited higher water diffusion selectivity than that of
288
HA.
289
3.2.2 Effect of the cross-linking of Alginate layer on the separation performance
ip t
272
Jil Ci
Ac
ce pt
ed
M
an
us
cr
(9)
290
Ca2+ cross-linking was a common method to improve alginate membrane
291
performance. Fig. 4 showed the effect of Ca2+ concentration on pervaporation
292
performance
293
HA/Alg(X)/PAN membranes for dehydration of 90 wt.% ethanol-water mixture at
of
Alg(X)/PAN,
Alg(X)/Alg(X)/PAN,
Alg(X)/HA/PAN
and
15
Page 16 of 29
80oC. The permeate flux of those four membranes displayed the similar trend with the
295
increase of Ca2+ concentration. Membranes made of alginate which was not
296
cross-linked by Ca2+ exhibited lower flux and separation factor, compared with
297
cross-linked membranes. With the increase of Ca2+ concentration, both of the
298
hydrophilicity and the crosslinking degree were increased, which improved the
299
permselectivity of the membranes. Thus separation factors of these membranes were
300
all increased.
us
Alg/PAN Alg/Alg/PAN HA/Alg/PAN Alg/HA/PAN
Separation factor
1000
800
800 600 400
M
600
200
400 0.05
0.10
2+
0.15
0.50
0
0.55
ed
2
Permeation flux (g/(m h))
1000
0.00
301
(b)1200
Alg/PAN Alg/Alg/PAN HA/Alg/PAN Alg/HA/PAN
1200
an
(a)
cr
ip t
294
Ca concentration(mol/L)
0.00
0.05
0.10
0.15
0.50
0.55
2+
Ca concentration(mol/L)
Fig. 4 Effect of Ca2+ concentration on pervaporation performance of membranes, Alg/PAN,
303
Alg/Alg/PAN, Alg/HA/PAN and HA/Alg/PAN, for dehydration of 90 wt.%
304
ethanol-water mixture at 80℃.
3.2.3 Effect of coating sequences on the separation performance
Ac
305
ce pt
302
306
Separation performance of various composite membranes was shown in Fig. 4.
307
Comparison between the separation performance of HA/Alg/PAN and Alg/Alg/PAN
308
membranes showed that HA/Alg/PAN possessed a higher separation factor, while
309
their fluxes were similar. It showed that HA with high water sorption selectivity and
310
water retention capacity was more suitable as the capping layer than alginate. When it
311
was used as capping layer, more water was adsorbed by HA layer, which increased the 16
Page 17 of 29
312
driving force of water diffusion through the membranes. It can be concluded that
313
materials as capping layer should possess higher hydrophilicity and larger sorption
314
capacity to intensify solution process. Comparison
between
the
separation
performance
of
Alg/HA/PAN
and
ip t
315
Alg/Alg/PAN membranes showed that Alg/Alg/PAN after crosslinked possessed a
317
higher separation factor, while their fluxes were similar. It showed that alginate was
318
more suitable as the inner layer than HA. Crosslinked alginate had higher diffusion
319
selectivity than that of HA. When it was used as inner layer, the membranes possessed
320
high separation factor, which proved that materials as inner layer should possess
321
higher diffusion selectivity and small swelling degree.
M
an
us
cr
316
Separation performance of HA/Alg/PAN and Alg/HA/PAN two-active-layer
323
composite membranes was studied to verify the validity of the selection rules. It was
324
found that the separation factor of HA/Alg/PAN was higher remarkably than that of
325
Alg/HA/PAN membranes. HA/Alg(0.5)/PAN membrane possessed a separation factor
326
of 1130 and Alg(0.5)/HA/PAN membrane possessed a separation factor of 527, while
327
their permeate fluxes were 972 g/(m2 h) and 948 g/(m2 h) respectively. This further
328
proved that capping layer with higher hydrophilicity and water capacity, and inner
329
layer with higher permselectivity could increase the separation performance of the
330
composite
331
HA/Alg(0.5)/PAN membrane with Alg as the inner layer have higher separation
332
factor(1130) than that of HA/CS/PAN(704) which further verified that Alg have
333
higher diffusion selectivity than that of CS.
Ac
ce pt
ed
322
membranes.
Meantime,
compared
with
our
previous
study,
17
Page 18 of 29
334
3.2.4 Effect of operation temperature on the separation performance
335
1000
600
800 400
600 400
200 200 300
310
320
330
340
0 360
350
80
60
40
20
300
(d)
HA/Alg/PAN Alg/HA/PAN
600
1.0
0.5
300 200
310
320
330
340
M
300
350
7
0
360
Temperature (K)
ed
3
1 2.8
2.9
3.0
3.1
3.2
330
340
350
360
water ethanol Ep,w=36.98kJ/mol
5
4
Ep,e=18.71kJ/mol
2
320
Temperature (K)
6
lnJp
ce pt
4
310
7
Ep,w=43.63kJ/mol
5
300
(f)
water ethanol
6
lnJp
360
100
(e)
Ep,e=-0.71kJ/mol
3
3.3
2.8
2.9
3.0
3.1
3.2
3.3
1000/T,K Alg/HA/PAN
1000/T,K HA/Alg/PAN
Ac
340
350
an
Selectivity
400 1.5
337
339
340
HA/Alg/PAN Alg/HA/PAN
500
2.0
0.0
330
us
2.5
320
Temperature (K)
2
Permeance(ethanol) (g/(m h kPa))
(c)
338
310
Temperature (K)
336
ip t
2
1200
HA/Alg/PAN Alg/HA/PAN
100
cr
1400
800
0
(b)
1600
HA/Alg/PAN Alg/HA/PAN
Separation factor 2 Permeance(water) (g/(m h kPa))
, ,
1000
Permeation flux (g/(m h))
(a)
Fig. 5. Effect of operation temperature on pervaporation performance of HA/Alg/PAN and Alg/HA/PAN membranes for dehydration of 90 wt.% ethanol-water mixture.
341
(a) Effect of operation temperature on permeation flux and separation factor of HA/Alg/PAN and
342
Alg/HA/PAN membranes.(b) Effect of operation temperature on water permeance of HA/Alg/PAN
343
and Alg/HA/PAN membranes. (c) Effect of operation temperature on ethanol permeance of
344
HA/Alg/PAN and Alg/HA/PAN membranes. (d) Effect of operation temperature on selectivity of 18
Page 19 of 29
345
HA/Alg/PAN and Alg/HA/PAN membranes. (e) Arrhenius plots of permeation flux for separation
346
of ethanol/water mixture by the HA/Alg/PAN membrane.(f) Arrhenius plots of permeation flux for
347
separation of ethanol/water mixture by the Alg/HA/PAN Membrane.
Operation conditions had a remarkable influence on flux and separation factor, and
349
its variation not only changed the driving force of species permeating through
350
membranes, but also the intrinsic properties of membranes. Fig. 5 showed effect of
351
operation temperature on pervaporation performance of HA/Alg(0.5)/PAN and
352
Alg(0.5)/HA/PAN membranes for dehydration of 90 wt.% ethanol-water mixture.
353
With increasing temperature, both flux and separation factor of two membranes went
354
up markedly (as shown in Fig.5 (a)). However, the separation factor of HA/Alg/PAN
355
was higher than that of Alg/HA/PAN at all the temperature point, while fluxes of these
356
two membranes were similar.
ed
M
an
us
cr
ip t
348
The flux was mainly affected by ethanol and water vapor pressure (driving force)
358
and membrane properties. With the increase of temperature, ethanol and water vapor
359
pressure increased and the permeating driving force was enhanced, which was
360
favorable to increase of flux. In order to study the effect of temperature on membrane
361
intrinsic properties, driving force normalized permeance and selectivity were
362
presented in Fig. 5(b-d). It could be found that water and ethanol permeances of
363
Alg/HA/PAN membrane varied in a wider range than those of HA/Alg/PAN,
364
indicating
365
temperature-depended. The dependence of permeation flux on temperature could be
366
expressed by Arrhenius equation (Eq.(10)) and the apparent activation energy could
Ac
ce pt
357
that
properties
of
Alg/HA/PAN
membranes
were
more
19
Page 20 of 29
367
be obtained by equation (10).
368
J p Ap exp(
369
Where Ap, Jp, Ep, R and T are the pre-exponential factor, permeation flux, apparent
370
activation energy, gas content and feed temperature, respectively.
)
(10)
ip t
RT
The Arrhenius plots of lnJp versus 1000/T for water and ethanol were shown in
cr
371
Ep
Fig. 5(e,f) . A linear relationship was observed, indicating the permeability followed
373
the Arrheninus law. With the temperature ranging from 303 to 353K, the apparent
374
activation energy for water in HA/Alg(0.5)/PAN membrane and Alg/HA/PAN
375
membrane were 43.63 kJ/mol and 36.98 kJ/mol, respectively. Simultaneously, the
376
apparent activation energy for ethanol in HA/Alg(0.5)/PAN membrane and
377
Alg/HA/PAN membrane were 18.71 kJ/mol and -0.71 kJ/mol, respectively. It could be
378
found that the apparent activation energy for water and ethanol in HA/Alg(0.5)/PAN
379
membrane were larger than that in Alg/HA/PAN membrane. Therefore, with the
380
increase of temperature, the increasing rate of water flux and ethanol flux in
381
HA/Alg(0.5)/PAN membrane were greater than that in Alg/HA/PAN membrane. So
382
the data of selectivity showed that the HA/Alg/PAN membrane was more
383
advantageous than that of Alg/HA/PAN membrane for dehydration of 90 wt.%
384
ethanol-water mixture.
385
3.2.5 Effect of water concentration in feed on the separation performance
Ac
ce pt
ed
M
an
us
372
386
20
Page 21 of 29
2500
1250 2000 1000 1500 750 1000
500
500
250 0
10
20
30
40
0
HA/Alg/PAN Alg/HA/PAN
50 45 40 35 30 25 20 15
0
(d)
20
30
40
HA/Alg/PAN Alg/HA/PAN
cr
HA/Alg/PAN Alg/HA/PAN
0.25
1000
0.20
0.15
0.10
us
800
Selectivity
600
400 0.05
an
2
Permeance(ethanol) (g/(m h kPa)
(c)
10
Water concentration in feed
Water concentration in feed
387
200
0.00
0
10
20
30
40
0
10
20
30
40
Water concentration in feed
Water concentration in feed
M
388
3000
1500
0
(b)
3500
HA/Alg/PAN Alg/HA/PAN
ip t
2
Permeation flux (g/(m h))
1750
, ,
Separation factor 2 Permeance(water) (g/(m h kPa))
(a) 2000
Fig. 6. Effect of water concentration in feed on pervaporation performance of HA/Alg/PAN and
390
Alg/HA/PAN membranes for dehydration of ethanol-water mixtures containing 1 wt.%, 5 wt.%,
391
10 wt.%, 20 wt.%, 30 wt.% and 40 wt.% water at 80℃.
ed
389
(a) Effect of water concentration on permeation flux and separation factor of HA/Alg/PAN and
393
Alg/HA/PAN membranes.(b) Effect of water concentration on water permeance of HA/Alg/PAN
394
and Alg/HA/PAN membranes.(c) Effect of water concentration on ethanol permeance of
395
HA/Alg/PAN and Alg/HA/PAN membranes. (d) Effect of water concentration on selectivity of
396
HA/Alg/PAN and Alg/HA/PAN membranes.
Ac
ce pt
392
397
Fig. 6 showed effect of water concentration in feed on pervaporation performance
398
of HA/Alg(0.5)/PAN and Alg(0.5)/HA/PAN membranes for dehydration of
399
ethanol-water mixtures containing 1 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.% and 40
400
wt.% water at 80oC. With increasing water concentration in feed, fluxes of both
21
Page 22 of 29
membranes increased and separation factors decreased, which resulted from enhanced
402
swelling with increasing water concentration. Driving force normalized permeance
403
and selectivity were presented in Fig. 6(b-d). It was shown that permeances of water
404
and ethanol increased remarkably, suggesting that membrane became more relaxed
405
with the increase of water concentration and was favorable to feed species through
406
membrane.
cr
ip t
401
When water concentration was below 20 wt.%, water selectivity of HA/Alg/PAN
408
was higher than that of Alg/HA/PAN; When water concentration was equal to or
409
above 20 wt.%, reverse. It was probably because the swelling of HA capping layer
410
increased with the increase of water concentration, resulting in the decrease of
411
selectivity. So HA/Alg/PAN membranes were advantageous for dehydration of
412
ethanol/water solution with low water concentration, while Alg/HA/PAN membranes
413
were advantageous for high water concentration, because of better anti-swelling
414
property of Ca2+ cross-linked NaAlg layers.
415
4. Conclusion
ce pt
ed
M
an
us
407
The two-active-layer composite membranes, which comprise the capping layer
417
and the inner layer, were prepared by sequential coating of HA and NaAlg on the PAN
418
support layer and utilized for pervaporation dehydration of ethanol-water mixtures. It
419
was shown that membranes with different coating sequences had an obvious
420
difference in the dehydration performance. Under the operation condition of 80oC and
421
10 wt.% water concentration in feed, HA/Alg/PAN membrane possessed a separation
422
factor of 1130 and Alg/HA/PAN membrane possessed a separation factor of 527,
Ac
416
22
Page 23 of 29
while the permeate fluxes of both membranes were similar. It suggested that materials
424
with higher hydrophilicity and larger sorption capacity could be selected for
425
constructing the capping layer to intensify solution process, while materials with less
426
diffusion resistance and high diffusion selectivity could be used for constructing the
427
inner layer to enhance the diffusion process.
ip t
423
Acknowledgements
us
429
cr
428
The authors gratefully acknowledge the financial support from the National Science
431
Fund for Distinguished Young Scholars (21125627), the National basic research
432
program of China (2009CB623404), the Programme of Introducing Talents of
433
Discipline to Universities (No: B06006) and the Program for Changjiang Scholars and
434
Innovative Research Team in University (PCSIRT), Tianjin Natural Science
435
Foundation (10JCZDJC22600).
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ce pt
ed
M
an
430
23
Page 24 of 29
Nomenclature A
effective membrane area (m2)
Ci
concentration of component
J
permeation flux (g/m2 h)
l
thickness of the active layer (m)
P
permeability
Pi / l
permeance
pisat 0
saturated vapor pressure of pure component i
pil
partial pressure of component i in the permeate side
Q
total mass of the permeate solution in time t (g)
t
time (h)
md
weight of the dry membrane (g)
ms
weight of the swollen membrane (g)
xiL0
mole fraction of component i in the feed solution
Wf , Ef
weight fractions of water and ethanol in the feed solution
Wp , Ep
weight fractions of water and ethanol in the permeate solution
WM
weight fractions of water and ethanol in the membrane
selectivity
s
sorption selectivity
D
i0L
ce pt
ed
M
an
us
cr
ip t
(g/m3)
Ac
in the membrane
diffusion selectivity separation factor activity coefficient of component i in the feed solution
24
Page 25 of 29
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