Prented at the International Congress on Membranes and Membrane Process (ICOM), Seoul, Korea,
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the ceiling
21–26 August 2005.
Desalination 193 (2006) 171–181
Chitosan-poly (vinyl alcohol)/poly (acrylonitrile)
(CS–PVA/PAN) composite pervaporation membranes for the
paration of ethanol–water solutions
Bing-Bing Li a,c , Zhen-Liang Xu a,b*, F. Alsalhy Qusay a , Ran Li c
a
Chemical Engineering Rearch Center, b Key Laboratory for Ultrafine Materials,
East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, PR China
Tel. +86 (21) 6425-3061; Fax: +86 (21) 6425-2989; email: chemxuzl@ecust.edu ;
c
Department of Chemical Engineering, Changchun University of Technology, 17 Yanan Road, Changchun, PR China
Received 1 February 2005; accepted 20 August 2005
Abstract
Pervaporation (PV) membranes made of a chitosan (CS) homogeneous membrane, poly (vinyl alcohcvh
ol)-poly (acrylonitrile) (PVA–PAN) and chitosan-poly (vinyl alcohol)/poly (acrylonitrile) (CS–PVA/PAN) composite membranes for the paration of ethanol–water solutions were manufactured. The swelling behaviors of CS homogeneous membranes and CS–PVA/PAN composite membranes were measured. Effects of membrane thickness,CS–PVA concentrations in the coating solution, PVA concentrations in the CS–PVA blend polymer, ethanol concentrations in the feed solutions and feed solution temperatures on pervaporation performance for the ethanol–water mixtures are discusd. The experimental results showed that the paration factor (α) of CS–PVA/PAN composite membranes incread with an increa of PVA concentration in the CS–PVA blend polymer from 0 to 40 wt%. With an increa in the membrane thickness from 12 to 18 µm, the paration factor (α) of the CS–PVA/PAN composite membrane incread while permeation flux (J ) decread. With an increa of ethanol–water solution temperature, the paration factor (α) of CS membranes decread and the permeation flux (J ) of CS membrane incread while for the PVA–PAN and CS–PVA/PAN composite membranes they incread. The apparent activation energy (∆Ea ) of water for CS membranes was 34.3–59.5KJ/mol and less than that of ethanol with 39.4–71.4 KJ/mol. The apparent activation energy (∆Ea ) of water for both PVA–PAN and CS–PVA/PAN membranes was twice as high as that of ethanol. The ∆Ea of water and ethanol for PVA/PAN and CS–PVA/PAN membranes was 13.6–61.6 KJ/mol, 3.3–15.1 KJ/mol,41.0–60.6 KJ/mol, and 20.9–31.3 KJ/mol, respectively.
Keywords: Pervaporation; Composite membrane; Chitosan; Polyvinyl alcohol; Polyacrylonitrile
*Corresponding author.
0011-9164/06/$– See front matter © 2006 Published by Elvier B.V.
doi:10.1016/j.desal.2005.08.021
B.-B. Li et al. / Desalination 193 (2006) 171–181 172
1. Introduction
Polymeric pervaporation (PV) membranes have been studied widely becau of their poten-tial industrial viability for breaking azeotropes, dehydration of solvents and other volatile organics and organic/organic parations. Addi-tionally, pervaporation has emerged as a good choice for parating heat-nsitive products.
Using chitosan membranes for the paration of ethanol–water solutions has been investigated by many rearchers [1–14]. For examples, Akira et al. [1] showed that the degree of deacetylation (72–98%) of the chitosan did not affect the lectivity of the membrane in the paration of ethanol–water solutions. Tadashi and Katsumi [2] reported that the evapomeation method was better than the pervaporation method for the permeation and paration of aqueous ethanol solutions. Lee and Shin [3] investigated the pervaporation performance of novel phosphorylated chitosan membranes to parate water from aqueous etha-nol solutions. The phosphorylated PCS-30 membranes containing 56 mg/m2 P showed that the flux was ~0.2 kg/m2×h and the lectivity towards water was ~600 for 90% ethanol at 70E C.
The structure of chitosan membranes chemic-ally modified with aldehydes, such as glutaral-dehyde and n-butyl aldehyde, was analyzed by Tadashi et al. [4]. Wu et al. [5–7] studied per-vaporation membranes made by filling polyvinyl alcohol (PVA)-chitosan with zeolite A (Baylith L). Lee et al. [8] prepared a blended PVA–CS membrane using a solvent casting technique for effective paration of an ethanol–water mixture. Haruhiko et al. [9,10] also reported the paration of ethanol–water mixtures by CS homogeneous membranes. Wang et al. [11] studied a novel alcohol dehydration membrane with a three-layer structure having a top layer which was a thin, den CS film and a sup
port layer which was microporous polyacrylonitrile, and between the den and microporous layer, there was an inter-molecular crosslinking layer. Shieh and Huang [12,13] prepared blended membranes of chitosan and N-methylol nylon 6 by solution blending and blended chitosan–polyacrylic acid membranes for the paration of ethanol–water mixtures. Jirara-tananon et al. [14] prepared composite hydro-philic pervaporation membranes from chitosan blended with hydroxyethylcellulo using cellu-lo acetate as a porous support.
In this study, chitosan (CS) homogeneous membranes, poly (vinyl alcohol)-poly (acrylo-nitrile) (PVA/PAN) and chitosan/poly (vinyl alcohol)-poly (acrylonitrile) (CS-PVA/PAN) composite membranes for ethanol–water solution paration were prepared. In addition, an attempt was made to study the swelling behavior of water, ethanol and ethanol–water permeation, as well as the effects of ethanol concentrations in the feed solution, feed solution temperature, polymer (PVA) content and CS–PVA concentration in the CS–PVA coating solution and composite mem-brane thickness on the paration factor and permeation flux of different membranes ud for the paration of ethanol–water solutions.
2. Experimental
2.1. Materials
PAN microporous ultrafiltration supported membranes and chitosan (CS) (80% N-deacety-lation degree, M
W
= 2–3×105) were obtained from Hangzhou Water Treatment Center (PR China). PVA (hydrolyzed degree was more than 99% and the average degree of polymerization was 1750), glutaradehyde (GA), and sulfuric acid were obtained from Shanghai Chemical Reagent (PR China). Commercially supplied ethanol was ud for pervaporation experiments without further purification.
2.2. Preparation of CS homogeneous membrane, PVA/PAN and CS-PVA/PAN composite membrane
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2.2.1. Preparation of CS homogeneous membranes
CS was added to 2 wt% acetic acid (HOAc)
B.-B. Li et al. / Desalination 193 (2006) 171–181173 solution to make a 1 wt% chitosan solution, then
the solution was stirred for 24 h to obtain a homo-
genous solution. After that the homogenous
solution was filtered and de-foamed, and then
cast on a glass plate and dried in a baking oven at
wallstreet40E C for 24 h. The resulting membrane was
washed with 2 wt% NaOH solution to neutralize
HOAc, and then with distilled water and kept in
a 5 wt% H
2SO
4
cross-linking solution. After fully
cross-linking, the membrane was washed with distilled water. The CS membrane thickness was 20–45 µm.
2.2.2. Preparation of PVA/PAN composite membranes
PVA was dissolved in hot water at 100E C and stirred for 6 h to prepare a 10 wt% PVA solution, and then cast on a PAN microporous ultrafiltra-tion membrane. The composite membrane was kept in a desiccator at 40E C for 24 h, and then cross-linked using 0.01 wt% GA and 0.5 wt%
sulfuric acid (H
2SO
4
) as a catalyst for 1 h. The
final composite membrane was obtained after washing with water. The membrane thickness was 18–25 µm.
2.2.
3. Preparation of CS–PVA/PAN compo-site membranes
CS and PVA were dissolved in hot water to make a 2 wt% CS–PVA solution. After stirring, filtering and de-foaming, the solution was coated onto a PAN supporting membrane and the solvent was heated in a desiccator at 30E C for 48 h. The prepared composite membrane was cross-linked by a 0.05% GA aqueous solution with 0.5 wt%maken
H
2SO
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at room temperature and then washed with
a 2 wt% NaOH aqueous solution; the cross-linking time was changed from 2 to 4 h. The membrane thickness was 18–25 µm.
Finally, the membrane thickness was adjusted by varying the amount of solution cast on a glass plate with the same surface area. The thickness in the dry state was determined with a micrometer.2.3. Determination of the swelling behavior of CS and CS–PVA/PAN membranes in water–ethanol mixtures
The swelling behaviors of CS and CS/PVA–PAN membranes in water–ethanol mixtures were determined experimentally using the apparatus shown in Fig. 1. The weighted dry CS and CS–PVA/PAN membranes were immerd in water–ethanol mixtures with different ethanol concen-trations for 48 h at 30E C to allow swelling to reach equilibrium. The swollen membranes were rapidly taken out from the mixture and the sur-face was wiped quickly with filter paper and immediately transferred to the half of a twin tube t-up. The left tube was cooled using liquid nitrogen while the system was evacuated under 30 Pa pressure. After 1 h liquid nitrogen was transferred to the right tube for 2 h. The weight and composition of the condend liquid in the right tube were measured by electronic balance Fig. 1. Schematic diagram apparatus of the experimental determination of the swelling behavior of CS homo-genous membranes and CS–PVA/PAN composite mem-branes in water–ethanol mixtures. 1 sample collecting tube, 2 swelling membrane tube, 3 stopcock, 4 swelling
membrane.
B.-B. Li et al. / Desalination 193 (2006) 171–181 174
and gas chromatography (G.C. China Chromato-
graphy GC7890 T), respectively. The swelling
degree of the membrane was calculated with the
following equation:
where W
d and W如何提高英语口语
w
are the weight of dry and
swollen membranes, respectively.
2.4. Pervaporation process of ethanol–water paration
Fig. 2 is the schematic diagram of the pervaporation process apparatus. The effective surface area of the membrane is 28 cm2. The membrane was put in a stainless-steel membrane cell. The feed solution was circulated from the feed tank at a flow rate (180 mL/min). The feed tank was kept in a water bath to control the feed temperature using a temperature controller. The permeate side was maintained at a vacuum pres-sure (30Pa) using vacuum pump. The permeate vapor sample was collected after being condend in a cold trap using liquid nitrogen. Permeation flux (J, g/m2×h) of hollow-fiber membranes was defined as follows:
where W is the total amount of permeate (g), t the experimental time interval (h), and A the mem-brane outer surface area (m2). The permeate solution was analyzed using a gas chromatograph (GC China Chromatography GC7890 T) filled with a 401 packed column at a temperature of 120E C.
The paration factor (α) is defined by:
where , Y
W
, Y
E
, X
W
and X
E
are the weight fractions of water and ethanol in the permeate and feed solutions, respectively.
Fig. 2. Schematic diagram of the pervaporation process for the paration of the ethanol–water solutions.
(1)
(2)
(3)
B.-B. Li et al. / Desalination 193 (2006) 171–181
175
3. Results and discussion
3.1. Swelling behaviors of CS membrane and CS–PVA/PAN membrane
Fig. 3 shows the effect of ethanol concentra-tion on the swelling behaviors of the CS homogeneous membrane and the CS–PVA/PAN composite membrane in the ethanol–water solu-tion at 30E C and 30 Pa downstream pressure. As en in Fig. 3, with the increa of ethanol concentration in the feed solution, the degree of Fig. 3. Effect of ethanol concentration in the ethanol–water solution on the degree of swelling of the CS homogeneous membrane and the CS–PVA/PAN com-posite membrane (feed temperature: 30E C; downstream pressure: 30 Pa; membrane CS thickness: 45 µm; CS–PVA/PAN composite thickness: 22 µm).swelling in water decreas rapidly and the degree of swelling in ethanol ems to remain the same. In addition, the degree of swelling in water is larger than that in ethanol for both the CS homogeneous and CS–PVA/PAN composite membranes. As ethanol concentration increas in the feed solution, the ethanol molecules stay in the mixture becau of the strong interaction between ethanol and water.
In addition, CS and PVA are hydrophilic materials. As shown in Fig. 4, ethanol concen-tration in the s
welled membranes of the CS homogeneous and CS–PVA/PAN membranes is less than water concentration. Therefore, both the membranes have a greater interaction with water than with ethanol. The restriction of ethanol molecules decreas the CS and CS–PVA/PAN swelling as reported by Chuang et al. [15]. Further, ethanol concentration in the CS–PVA/ PAN composite membrane is less than that in the CS homogeneous membrane. This illustrates that the CS–PVA/PAN composite membrane is more suitable than the CS homogeneous membrane for the paration of ethanol–water solutions.
Fig. 4. Relationship between ethanol concentration in an ethanol–water solution and ethanol concentration in the swelled membrane (feed temperature: 30E C; downstream pressure: 30 Pa; membrane CS thickness: 45 µm;
CS–PVA/PAN composite thickness: 22 µm).
B.-B. Li et al. / Desalination 193 (2006) 171–181
schedule1763.2. Effect of membrane thickness (δ) on per-vaporation performance of the CS–PVA/PAN composite membrane
The effect of membrane thickness (δ) on per-vaporation performance of the CS–PVA/PAN composite membrane (CS–PVA, 60:40 weight ratios) at 50E C feed temperature and 30 Pa down-stream pressure with different ethanol concen-trations (50, 80 and 95 wt%) is shown in Fig. 5.As en in Fig. 5, the paration factor (α)increas and the permeation flux (J ) decreas with an increa of the membrane thickness (δ)
Fig. 5. Effect of membrane thickness (δ) on perva-poration performance of the CS–PVA/PAN composite membrane as a function of ethanol concentration in the ethanol–water solution (feed temperature: 50E C; down-stream pressure: 30 Pa).from 12 to 18 µm. As δ exceeds 18 µm, there is no significant change in α while J decreas,except that the ethanol concentration is 95 wt% in the feed solution. For 95 wt% ethanol concen-tration, the paration factor α is 78.4 with a 30.6 kg/m 2×h pervaporation paration index (PSI = α×J ) with an 18 µm membrane thickness.3.3. Effect of CS–PVA concentration in the CS–PVA coating solution and PVA concentration on CS–PVA polymer on pervaporation performance of the CS–PVA/PAN composite membrane Fig. 6 shows the effects of PVA concentration in the CS–PVA polymer on the pervaporation (PV) performance of the CS–PVA/PAN compo-site membrane for the paration of a 95 wt%ethanol–water solution at 50°C and 30 Pa down-stream pressure. As en in Fig. 6, as the PVA concentration in CS-PVA polymer increas from 0 to 20wt%, the α of the CS–PVA/PAN compo-site membrane increas from 53.1 to 104.7,while J decreas strongly from 259.1 to 57.4 g/m 2×h. The α of the CS–PVA/PAN composite membrane increas to a maximum value as PVA concentration in the CS–PVA polymer increas
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Fig. 6. Effects of PVA concentration in CS–PVA polymer on the pervaporation performance of the CS
–PVA/PAN composite membrane (feed temperature:50E C; downstream pressure: 30 Pa; membrane thickness:18 µm).
