- Email: [email protected]
water mixture
Journal of Membrane Science 325 (2008) 192–198
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Studies on the pervaporation membrane of permeation water from methanol/water mixture Xianhong Liu a,b , Yuan Sun a,b,∗ , Xinhua Deng a,b a b
School of Materials Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China Key Laboratory of Membrane Material and Membrane Process (Tianjin Polytechnic University), Ministry of Education, Tianjin 300160, China
a r t i c l e
i n f o
Article history: Received 28 December 2007 Received in revised form 9 July 2008 Accepted 9 July 2008 Available online 23 July 2008 Keywords: Methanol/water mixture Pervaporation Three-layer sandwich Acrylic acid (AA)/acrylonitrile (AN) copolymer Nanometer SiO2
a b s t r a c t This investigation was performed to find if the nanometer SiO2 added in the membranes can improve the pervaperation performance of the membranes. Acrylic acid (AA) and acrylonitrile (AN) were synthesized by solution polymerization with and without nanometer SiO2 . The copolymer solution was made into main body of the membranes, then composited with the polyvinyl alcohol (PVA) acetal membranes, to make the three-layer sandwich composite pervaporation membranes. The structure and the performance of the membranes were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry (TG), dynamic themomechanical analysis apparatus (DMA) and mechanical property testing. Pervaporation experiments were carried out using these membranes to separate the mixtures of methanol/water over the complete concentration range 70–98%, and results showed that the selectivity of the membranes with nanometer SiO2 had notable improvement. For the 98% mixture at 60 ◦ C, the separate factor is up to 1458, which is improved more than 10 times compared to the membranes without nanometer SiO2 , the permeate flux is up to 325 g/(m2 h). For the 70% mixture at 70 ◦ C, the separate factor arrived at 12, the permeate flux is up to 7097 g/(m2 h), which is improved more than 14 times compared to membranes without nanometer SiO2 . It was concluded that the pervaperation performance of the membranes can improve greatly by nanometer SiO2 . © 2008 Elsevier B.V. All rights reserved.
1. Introduction Pervaporation is a new technology for the separation of many organic aqueous systems – the removal of organics from water, water removal from liquid organics and organic–organic separation. Pervaporation has the specialties such as high separation effect, simply actualization, no-pollution, and energy-saving which are difficult to obtain by other conventional methods [1]. Pervaporation is a chemical unit operation where the mixture to be separated is vaporized at low pressure on the downstream side of the membranes and the separation of the mixtures takes place by preferential sorption and diffusion of the desired component through the dense membranes. This process is mainly used for dehydration of organics, removal of low concentration organics from its aqueous mixtures and organic–organic separation. Dehydration of acetic acid by hydrophilic membranes like acrylonitrile and hydroxy ethyl methacrylate grafted poly (vinyl alcohol) membrane has already been commercialized [2].
∗ Corresponding author at: School of Materials Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China. E-mail address: [email protected] (Y. Sun). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.031
For separation of low concentration organics from its aqueous mixtures organophilic pervaperation membranes are used [3]. Hoshi et al. studied pervaporation recycling phenol from its aqueous mixture by polyurethane membranes [4]. Jin et al. reported removing volatile organic compounds (VOCs) from water by cross-linked PS/poly (acrylate-co-acrylic acid) membrane [5]. For organic–organic separation membrane selection is not so straightforward like using organophilic or hydrophilic membrane. In this case membrane selection is based on relative closeness of solubility parameter of the organic to be selectively permeated with that of the membrane polymer [6]. Niang et al. used a blended membrane of cellulose acetate and cellulose acetate hydrogen phthalate in the separation of methyl tertiary butyl ether/methanol (MTBE/MeOH) mixtures [7]. The dehydration of organic solvents is the most important application of pervaporation. In a recent study, Jonquières et al. concluded that of the 63 units installed by GFT (now Sulzer Chemtech) and associates between 1984 and 1996, 62 units are used for dehydration purposes, including 22 for ethanol and 16 for isopropyl alcohol [8]. The first commercial application of the pervaporation process was the dehydration of ethanol, using a hybrid system with a distillation column. The pervaporation unit can also be used in combination with a reaction, where it constantly removes one of
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
the reaction products to shift the equilibrium reaction to higher yields. For dehydration purposes, hydrophilic membranes are used. Until now, mostly polymeric membranes have been used on an industrial scale. In recent years, research focuses on the development of ceramic membranes. Successful hydrophilic ceramic membranes have been made from silica [9,10] and from zeolites [11]. Some of these are already commercially available (e.g. Pervatech, Enter, The Netherlands). Full-scale plants using zeolite NaA membranes are already described [12]. Since the dewatering of organic solvents, and more specifically of low molecular weight alcohols, is the most common application, there are many publications on the separation of ethanol/water mixtures [13–15]. The active layer of the polymeric membranes consists in most cases of PVA, although other materials are also reported, such as polyacrylonitrile (PAN), polyetherimide (PEI) or 4,4oxydiphenylene pyromellitimide (POPMI) [16]. But there are few reports about the polymer membrane which is modified by nanometer sized inorganic powder. The performance of the polymer can greatly improved by adding nanometer sized inorganic powder which can advance the rigidity, heat resistance, size stability of the polymer, then can enhance the tensile strength, bending strength, impact strength heat distortion temperature, thermal stability of the polymer [17,18]. Although some reference reported that pervaporation technology used for separating ethanol /water mixture is on an industrial scale, it is nearly no study and application on separating methanol/water mixture. Compared with ethanol, methanol is more similar with water in polarity and molecular weight which makes methanol to compete with water on adsorbing in the membrane. So the pervaporation membrane which is available for separating ethanol/water mixture is not ideal for separating methanol/water mixture [19]. In this article, acrylic acid (AA) and acrylonitrile (AN) were synthesized by solution polymerization with or without nanometer SiO2 . The copolymer solution was made into main body of the membranes, then composited with the polyvinyl alcohol acetal membranes, to make the three-layer sandwich composite pervaporation membranes. Pervaporation experiments were carried out using these membranes to separate the mixtures of methanol/water over the complete concentration range 70–98%. The effect of membranes composition, feed temperature and feed concentration on the separation performance was investigated. The results showed that for the 98% mixture at 60 ◦ C, the separate factor is up to 1458, which is improved more than 10 times compared to the membranes without nanometer silica, the permeate flux is up to 325 g/(m2 h) and for the 70% mixture at 70 ◦ C, the separate factor arrived at 12, the permeate flux is up to 7097 g/(m2 h), which is improved more than 14 times compared to membranes without nanometer SiO2 .
2. Experiments
193
Table 1 The membranes ready for pervaporation Name of the membranes
AA:AN (monomer)
The content of nanometer SiO2
Membrane 1 Membrane 2 Membrane 3 Membrane 4 Membrane 5 Membrane 6
1:1 1:1 1:1 1:1 1:1 1:1
0 0.05% 0.075% 0.1% 0.125% 0.15%
2.2. Membrane preparing PVA was dissolved in the deionized water coupled with a suitable 36% formaldehyde solution and adjusted the PH value to 1–2 using hydrochloric acid at 90 ◦ C, to prepare PVA acetal membrane solution. AA and AN were synthesized at the monomer ratio of 1:1 by solution polymerization without and with nanometer SiO2 using potassium peroxodisulfate as initiator and N,N-methylene acrylamide as cross-linking agent at 70 ◦ C. When the solution reached the adaptable viscosity, stopped the reaction to make the PAA-CoAN membrane solution ready. First, PVA acetal membrane solution was scraped on the non-woven polyester fabric supporter as the first layer, and then under heat treatment at 75 ◦ C in the oven; Second, the PAA-Co-AN membrane solution as the middle layer of the composite membranes on the surface of the PVA acetal membranes; at last the PVA acetal membrane solution scraped on the surface of the PAA-Co-AN membranes went into the three-layer sandwich composite pervaporation membranes: the first layer next to the non-woven polyester fabric was PVA acetal, the middle layer was PAA-Co-AN (or PAA-Co-AN/SiO2 ) and the surface layer was also PVA acetal. The membranes were dried at 120 ◦ C for 1 h, and marinated in the deionized water and ethanol repeatedly to remove the dissolvable impurity. 2.3. Membranes prepared for pervaporation Table 1 shows the PAA-Co-AN membranes compositions prepared by the method introduced in Section 2.2. 2.4. Copolymer (membrane) characterization 2.4.1. Studies of SEM pictures The SEM pictures were taken by a scanning electron microscope (Quanta 200, FEI Limited, Czech). The samples were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity, and dried in a frozen dryer (beta2-8, Martin Christ, Germany), then dug into liquid nitrogen and divided into two parts by tweezers to obtain a natural cross section. 2.4.2. Studies of FT-IR spectroscopy The FT-IR spectra of different composite membranes were recorded on an FT-IR spectroscopy (TENSOR37, BRUKER Company, Germany). FT-IR samples were thin films of polymer. They were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity.
2.1. Materials Analytical reagent (A.R) grade of AA monomer, AN monomer were obtained from Tianjin chemical reagent Ltd. PVA was obtained from Beijing organic chemistry company, the degree of polymerization is 1700, the degree of alcoholysis is 99%; A.R grade of methanol, formaldehyde, potassium peroxodisulfate, N,N-methylene acrylamide came from the third Tianjin chemical reagent plant. Nanometer SiO2 (TS-610) was bought from Cabot Corporation in America, and its average size is 70 nm.
2.4.3. Studies of the heat weight-loss behavior TG curves were produced by an integrative thermal analysis apparatus (STA409PC, NETZSCH Limited, Germany). The samples were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity. 2.4.4. Studies of mechanical properties The tensile strength and elongation at break of the polymer film was determined by an Instron-Tensile tester (Instron4301, Instron
194
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
Fig. 1. Schematics of pervaporation test equipment. (1) Feed flask, (2) transportating pump, (3) pervaporation pool, (4) cold trap, (5) vacuum meter, (6) vacuum pump.
Limited, England). The length of the samples was 250 mm, and the thickness around 0.1 mm in the experiments. 2.4.5. Studies of glass transition temperature (Tg ) The Tg of membranes was studied by DMA (242C, Germany). The samples were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity. 2.4.6. Pervaporation studies Pervaporation experiments were carried out with homemade set-up shown in Fig. 1. Effective membrane area in contact with the feed solution was 0.0127 m2 and the feed compartment volume was 250 ml. Pervaporation experiments were carried out at constant temperature of 40 ◦ C, 50 ◦ C, 60 ◦ C and 70 ◦ C, respectively. The concentration of methanol in the feed was 70%, 75%, 85%, 95% and 98% separately. The separation factor (˛) for methanol is defined as
˛=
(C1p /C2p ) (C1f /C2f )
where C1p , C2p are weight fraction of component 1, 2 in the permeate; C1f , C2f are weight fraction of component of 1, 2 in the feed, respectively. The permeate flux J (g/(m2 h)) is defined as J=
Fig. 2. SEM pictures of the membranes without nanometer SiO2 . The multiplication factors are 1500.
tor were maintained at 120 ◦ C. The methanol weight fraction is determined with the gas chromatograph analysis. 3. Results and discussion 3.1. Copolymer (membrane) characterization 3.1.1. SEM analysis The SEM pictures are shown below. Fig. 2 shows the cross picture of the membrane without nanometer SiO2 and its multiplication factor is 1500, while Fig. 3 shows the cross picture of the membrane with nanometer SiO2 and its multiplication factor is 1500. From the cross section pictures, it can be found that the clearly three-layer sandwich structure, and that the structure of the membranes with nanometer SiO2 is much finer and smoother than that of the membranes without nanometer SiO2 . The cause is that the nanometer SiO2 has been dispersed in the membranes and the hydrogen band association has formed on the surface of nanometer SiO2 . 3.1.2. Studies of FT-IR spectroscopy Fig. 4 shows the FT-IR spectroscopy of nanometer SiO2 . Si O symmetric stretching vibration band is at 810.86 cm−1 , and asym-
M At
where M is the weight of permeate, and A is the effective membrane area, t is the time of the experiment [20]. The separation factor and flux are always contradictive. Due to the influences acted upon each other, permeate separate index (PSI) is introduced. It is defined as PSI = (˛ − 1)J where ˛ and J are defined above. The permeate was collected every 60 min in a glass container using liquid nitrogen in a Dewar flask as a cold trap; for every different mixture, at least three permeate samples were collected. The vacuum was maintained by a vacuum pump at 1 × 10−3 MPa (10 mba). The flux was determined gravimetrically using a balance with an accuracy of 10−4 g. The composition of the methanol/water mixtures was determined with gas chromatography. A HP6890N gas chromatograph (column: J&W Scientific, DB1 30 m × 0.53 ID, THK film 5.0 m; FID detector) was used. During the analysis, the column was held at a constant temperature of 80 ◦ C, while the injector and the detec-
Fig. 3. SEM pictures of the membranes with nanometer SiO2 . The multiplication factors are 1500.
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
Fig. 4. FT-IR spectroscopy of nanometer SiO2 .
195
Fig. 6. TG curves of the membranes.
Table 2 The tensile strength and elongation at break of different membranes
Fig. 5. FT-IR spectroscopy of the membranes. (a) Membrane without nanometer SiO2 , (b) membrane with nanometer SiO2 .
metric stretching vibration at 1110.48 cm−1 , and –OH on the surface of nanometer SiO2 stretching vibration band at 1628.78 cm−1 . In Fig. 5 the FT-IR spectroscopy of the membranes without and with nanometer SiO2 are shown: Fig. 5a is FT-IR spectroscopy of membranes without nanometer SiO2 , while Fig. 5b is FT-IR spectroscopy of membranes with nanometer SiO2 , where C N stretching vibration band is at 2243.9 cm−1 , and C O stretching vibration band at 1701 cm−1 , which demonstrates that the membranes are synthesized with AA and AN. Both of the two curves have the same trend. In Fig. 5b, there is a new peak at around 1110 cm−1 , which is the characteristic peak of nanometer SiO2 . So it can be concluded that nanometer SiO2 has been added in the membranes which makes the structure of the membranes more compact.
3.1.3. TG analysis TG curves of the membranes are shown in Fig. 6. The curves without and with nanometer SiO2 have the same trend in the heat weight-loss behavior. However, at the same temperature, the heat weight-loss behavior of the membranes with nanometer SiO2 has weakened, especially at the rather high temperature above 400 ◦ C, which demonstrated that nanometer SiO2 can improve the heat stability of the membranes. And with the nanometer SiO2 concentration increasing, the heat stability is improving.
Name of the membranes
Tensile strength (MPa)
Elongation at break (%)
Membrane 1 Membrane 2 Membrane 3 Membrane 4
54.5 82 105.5 163.8
55.566 36.259 17.248 14.602
3.1.4. Studies of mechanical properties Table 2 shows the mechanical properties of the membranes. Too high tensile strength or elongation at break results in poor quality membrane. Tensile strength and elongation at break have a reciprocal relationship. For a good membrane, there should be an optimum balance between tensile strength and elongation at break [21]. The composite membranes synthesized for this study were found to have a good balance of tensile strength and elongation at break as shown in Table 2. It is observed from Table 2 that the composite membranes fall into acceptable range of tensile strength and elongation at break as required for pervaporation applications. It is also observed from these values that tensile strength increases from membrane 1–4 while elongation at break decreases. After adding nanometer SiO2 m, hydrogen band association may happen. Hydrogen band association is a kind of physical cross-linking. The hydrogen band association of polymer chains prevents high elongation as evidence by decrease of elongation at break and enhancement of tensile strength from membrane 1–4 signifying highest degree of cross-linking in membrane 4 containing maximum amount of SiO2 . 3.1.5. Studies of glass transition temperature The Tg of membranes with different amount of nanometer SiO2 are shown in Table 3. From Table 3, it can be seen that the Tg of the membranes are increasing as the concentration of nanometer SiO2 increasing until 0.1%; and it reaches maximum value when the concentration of nanometer Si02 at 0.1%, and then decreases, which delivered that the rigid nanometer Si02 has great influences on the Tg of polymer. The reason is that the polymer chain segment can not move before the glass transition, and nanometer SiO2 is imprisoned between the chain segment; while the chain segment of polymer is ‘ice-out’ when the temperature increased to Tg , afterwards it starts moving,
Table 3 Tg of membranes with different concentration of nanometer SiO2 under different frequency Membranes
Membrane 1
Membrane 2
Membrane 3
Membrane 4
Membrane 5
Membrane 6
1 Hz 5 Hz 10 Hz
115.2 120.7 123.2
116.7 124.5 127.1
147.1 151.0 153.4
180.2 184.7 186.7
156.5 161.9 164.8
152.0 155.0 157.4
196
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
Fig. 7. Comparison of different membrane separation factor under different temperature.
and then there will be more free space between the polymer chains allowing the nanometer SiO2 to move at this time, but nanometer SiO2 can move easier than the polymer chain segment, and this movement of nanometer SiO2 will limit that of polymer chain segment, so the polymer chain segment can move only when the temperature increased, which lead to the Tg increased. And what’s more, when the concentration of nanometer SiO2 increased, the physical cross-linking point increased, and the limit of nanometer SiO2 movement to chain segment is stronger. When the amount of nanometer SiO2 arrived at 0.1%, the physical cross-linking is saturated, this kind of limit is strongest, so the Tg arrived at maximum. With the concentration of nanometer SiO2 continuously increasing, superfluous nanometer SiO2 has some plasticizing influence, so the Tg is decreased. 3.2. Pervaporation studies 3.2.1. Comparison between the membranes without and with nanometer SiO2 Different membranes are used for separating methanol/water mixture in 98%, and the results under different temperature are shown in Figs. 7–9. Fig. 7 shows the change of separation factor with the temperature increasing, Fig. 8 shows the change of flux, and Fig. 9 shows the change of PSI. In Fig. 7, it can be found that the separation factors of the membranes with nanometer SiO2 are higher than those of the membranes without nanometer SiO2 , and with the concentration
Fig. 8. Comparison of different membrane flux under different temperature.
Fig. 9. The relation between temperature and PSI.
of nanometer SiO2 increasing; the separation factors are increasing too. The cause is that nanometer SiO2 in the membranes change the membrane structure which makes the water molecules can pass much easier than the methanol molecules, and what’s more, there are many hydrophilic groups –OH on the surface of nanometer SiO2 , which makes it easy to sorb and diffuse water molecules. As shown in Fig. 8, the permeate flux does not improve with the concentration of nanometer SiO2 because hydrogen band association makes the structure of the membranes more compact which prevent the transition of methanol and water molecules in a way. It is shown in Fig. 9 that the PSI has the same trend with the separation factor. The membrane has a good pervaporation performance when it contains 0.1% nanometer SiO2 . Dong-mei [22] insisted that the separation factor and flux of membranes with nanometer SiO2 increased due to two causes: one is that nanometer SiO2 is hydrophilic, which is propitious to the adsorption, diffusion and permeate of water molecule; the other is that the highly dispersed nanometer SiO2 powder has active surface, which can change the structure of membranes, and then make the permeation easier for water molecules and more difficult for methanol molecules. 3.2.2. Effect of feed temperature on pervaporation performance of membranes Use the membrane containing 0.1% nanometer SiO2 as the studying object to separate methanol/water mixture at different concentrations, under different temperatures. The results under different temperatures are shown in Figs. 10–12.
Fig. 10. Effect of temperature on the separation factor of membrane.
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
197
Fig. 11. Effect of temperature on the flux of membrane.
Fig. 13. Effect of solution concentration on the separation coefficient of membrane.
Fig. 12. Effect of temperature on PSI of membrane.
Fig. 14. Effect of solution concentration on the flux of membrane.
It is evident in Fig. 10 that with the temperature increasing, the separation factors increase firstly, and reach the maximum value at 60 ◦ C, then decrease. And it can be easily found that the separation factors of all the composite membranes reach maximum values at around 60 ◦ C. In Fig. 11, the permeate fluxes are also increasing with the temperature increasing. PSI has the same trend with separation factor in Fig. 12. According to solution-diffusion mechanism, the increasing of temperature makes the solubility on the surface of membrane and the diffusion rate in the membrane increase. From the microstructure analysis, the free volume of the membranes increases under higher temperature, which makes the interspaces between the polymer chain bigger, and the kinetic energy of permeate are increasing, which make the diffusion easier. Because the coupling effect, the methanol relative concentration are increased, and the separate selectivity are notable decreased with temperature increasing.
ship with the swelling state of membranes. With the increase of methanol in the feed, membranes shrink gradually, and cause the free volume in the membrane to diminish, which lead to the flux decrease. The diminishing of free volume in membrane limits the diffusion and transference of methanol molecules and its association complex, while water molecules can permeate freely because the volume of water molecule is smaller than that of methanol molecule, and nanometer SiO2 makes water molecules permeate easily. So the membrane has larger separation factors and lower
3.2.3. Effect of feed concentration on pervaporation performance of membranes Use the membrane containing 0.1% nanometer SiO2 as the studying object to separate methanol/water mixture at different concentrations under different temperatures. Figs. 13–15 show the effect of feed concentration on the separation factor, flux and PSI. As shown in Fig. 13, with the feed concentration increasing, separation factors are increasing gradually, and beyond 85%, the change is greater. While in Fig. 14, the flux is decreasing with the feed concentration increasing. The explanation over this phenomenon is that the pervaporation behavior of the membrane has relation-
Fig. 15. Effect of solution concentration on PSI of membrane.
198
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
flux. It is obvious in Fig. 15 that the PSI has the same trend with the separation factors with the feed concentration increasing. Wesslein et al. [23] used GFT Company’s PVA/PAN composite membrane separate methanol/water mixture (XMeOH ≥ 80%). However, the separation factor is so low that it is almost impossible to be applied to do dewatering job. Liping et al. [24] studied on separating methanol/water system by using PVA/PAN composite membrane made by themselves. The separate factor is 40 when the methanol concentration in the feed between 97 and 98%. And the flux is very low, about 200 g/(m2 h). And Fubing [25] did a pervaporation experiment with Polyimide membrane to separate the same mixture, and got the flux about 80 g/(m2 h) and the separate factor 3. Compared with above results, the membrane made in this paper has a much better separate effect.
[6]
[7]
[8]
[9]
[10]
[11]
4. Conclusions [12]
Copolymerization of AA and AN with and without nanometer SiO2 by solution polymerization generated two kinds of membranes. The results of SEM pictures, FI-IR spectra, TG curves and the test of mechanical properties showed that membranes with nanometer SiO2 have better performance in intensity, size stability and heat stability than those membranes without nanometer SiO2 . After adding nanometer SiO2 , there is a hydrogen band association in the membrane structure. These membranes were used for selective separation of water from its mixture with methanol by pervaporation. It was found that they can have high degree of permeation and selectivity for water. All of the membranes have a good performance at around 60 ◦ C, and have the best separation factor when the feed concentration is 98%. References
[13] [14] [15]
[16]
[17] [18]
[19] [20]
[1] J. Neel, Introduction to pervaporation, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, 1991, pp. 36–37. [2] Al-ghezawi, Nawash, et al., Permeation and separation characteristics of acetic acid-water mixtures by pervaporation through acrylonitrile and hydroxy ethyl methacrylate grafted poly(vinyl alcohol) membrane, Sep. Sci. Technol. 41 (2006) 2913–2931. [3] K.W. Boddeker, G. Bengtson, Selective pervaporation of organics from water, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991, p. 452. [4] M. Hoshi, M. Kogure, T. Saitoh, et al., Separation of aqueous phenol through polyurethane membranes by pervaporation, Appl Polym Sci 65 (1997) 469–479. [5] M. Jin, Y. Wu, X.Y. Yang, et al. Cross-linked PS/poly (acrylate-co-acrylic acid) membrane for removal of VOCs from water, China-Japan International Sympo-
[21]
[22] [23] [24] [25]
sium on Membrane Hybrid System applied to Water Treatment Proceedings, J. Tianjin China (1999) 222–227. S.K. Ray, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Development of new synthetic membranes for separation of benzene–cyclohexane mixture by pervaporation – a solubility parameter approach, Ind. Eng. Chem. Res. 36 (1997) 5265. M. Niang, G.S. Luo, P. Schaetzel, Pervaporation separation of methyl tert-butyl ether/methanol mixtures using a high-performance blended membrane, J. Appl. Polym. Sci. 64 (1997) 875–882. A. Jonquières, R. Clément, P. Lochon, J. Néel, M. Dresch, B. Chrétien, Industrial state-of-the-art of pervaporation and vapor permeationin the western countries, J. Membr. Sci. 206 (2002) 87–117. R.W. Van Gemert, F.P. Cuperus, Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation, J. Membr. Sci. 105 (1995) 287–291. H.M. Van Veen, Y.C. van Delft, C.W.R. Engelen, P.P.A.C. Pex, Dewatering of organics by pervaporation with silica membranes, Sep. Purif. Technol. 22–23 (2001) 361–366. D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, Pervaporation of alcohol–water and dimethylformamide-water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results, J. Membr. Sci. 179 (2000) 185–205. Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (1–3) (2001) 251–260. J.H. Kim, K.H. Lee, S.Y. Kim, Pervaporation separation of water from ethanol through polyimide composite membranes, J. Membr. Sci. 169 (2000) 81–93. M. Nomura, T. Yamaguchi, S.I. Nakao, Ethanol/water transport through silicate membranes, J. Membr. Sci. 144 (1998) 161–171. M. Wesslein, A. Heintz, R.N. Lichtenthaler, Pervaporation of liquid mixtures through poly (vinyl alcohol) (PVA) membranes I, Study of water containing binary systems with complete and partial miscibility, J. Membr. Sci. 51 (1990) 169–179. F. Lipnizki, R.W. Field, P.-K. Ten, Pervaporation-based hybrid process: a review of process design, applications and economics, J. Membr. Sci. 153 (1999) 183– 210. E.G. Kenneth, X. Chen, Mechanistic investigation of the preparation of polymer/ceramic nanocomposites, Nanostruct. Mater. 9 (1997) 181–184. X. Chen, K.E. Gonsalves, Synthesis and properties of an aluminum nitride/polyimide nanocomposite prepared by a nonaqueous suspension process, J. Mater. Res 12 (1997) 1274–1286. M. Wesslein, A. Heintz, R.N. Lichtenthaler, Pervaporartion of liquid mixtures through poly(vinyl alcohol)(PVA) membranes, J. Membr. Sci. 51 (1990) 169–179. A. Bakhshi, T. Mohammadi el., Effect of operating conditions on pervaporation of methanol–water mixtures: Part 1, Membr. Technol. 11 (2006) 6–9. S. Ray, S.K. Ray, Synthesis of highly methanol selective membranes for separation of methyl tertiary butyl ether (MTBE)–methanol mixtures by pervaporation, J. Membr. Sci. 278 (2006) 279–289. D.M. Xu, PVA pervaporation membranes modificated by nanometer sized SiO2 , Chem. Eng. Sci. Technol. 11 (2) (2003) 25–27. M. Wesslein, et al., Separation of organic binary mixtures by pervaporation, J. Membr. Sci 51 (1990) 273. L.P. Zhang, Y. Jun, J. Weijun, New studying on methanol dewatering techniques, Modern Chem. 10 (1994) 24–26. P. Fubing, J. Zhongyi, mass transfer model of two-component aqueous in the pervaporation membrane, Chem. Eng. 33 (1) (2005) 60–64.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Studies on the pervaporation membrane of permeation water from methanol/water mixture Xianhong Liu a,b , Yuan Sun a,b,∗ , Xinhua Deng a,b a b
School of Materials Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China Key Laboratory of Membrane Material and Membrane Process (Tianjin Polytechnic University), Ministry of Education, Tianjin 300160, China
a r t i c l e
i n f o
Article history: Received 28 December 2007 Received in revised form 9 July 2008 Accepted 9 July 2008 Available online 23 July 2008 Keywords: Methanol/water mixture Pervaporation Three-layer sandwich Acrylic acid (AA)/acrylonitrile (AN) copolymer Nanometer SiO2
a b s t r a c t This investigation was performed to find if the nanometer SiO2 added in the membranes can improve the pervaperation performance of the membranes. Acrylic acid (AA) and acrylonitrile (AN) were synthesized by solution polymerization with and without nanometer SiO2 . The copolymer solution was made into main body of the membranes, then composited with the polyvinyl alcohol (PVA) acetal membranes, to make the three-layer sandwich composite pervaporation membranes. The structure and the performance of the membranes were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry (TG), dynamic themomechanical analysis apparatus (DMA) and mechanical property testing. Pervaporation experiments were carried out using these membranes to separate the mixtures of methanol/water over the complete concentration range 70–98%, and results showed that the selectivity of the membranes with nanometer SiO2 had notable improvement. For the 98% mixture at 60 ◦ C, the separate factor is up to 1458, which is improved more than 10 times compared to the membranes without nanometer SiO2 , the permeate flux is up to 325 g/(m2 h). For the 70% mixture at 70 ◦ C, the separate factor arrived at 12, the permeate flux is up to 7097 g/(m2 h), which is improved more than 14 times compared to membranes without nanometer SiO2 . It was concluded that the pervaperation performance of the membranes can improve greatly by nanometer SiO2 . © 2008 Elsevier B.V. All rights reserved.
1. Introduction Pervaporation is a new technology for the separation of many organic aqueous systems – the removal of organics from water, water removal from liquid organics and organic–organic separation. Pervaporation has the specialties such as high separation effect, simply actualization, no-pollution, and energy-saving which are difficult to obtain by other conventional methods [1]. Pervaporation is a chemical unit operation where the mixture to be separated is vaporized at low pressure on the downstream side of the membranes and the separation of the mixtures takes place by preferential sorption and diffusion of the desired component through the dense membranes. This process is mainly used for dehydration of organics, removal of low concentration organics from its aqueous mixtures and organic–organic separation. Dehydration of acetic acid by hydrophilic membranes like acrylonitrile and hydroxy ethyl methacrylate grafted poly (vinyl alcohol) membrane has already been commercialized [2].
∗ Corresponding author at: School of Materials Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China. E-mail address: [email protected] (Y. Sun). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.07.031
For separation of low concentration organics from its aqueous mixtures organophilic pervaperation membranes are used [3]. Hoshi et al. studied pervaporation recycling phenol from its aqueous mixture by polyurethane membranes [4]. Jin et al. reported removing volatile organic compounds (VOCs) from water by cross-linked PS/poly (acrylate-co-acrylic acid) membrane [5]. For organic–organic separation membrane selection is not so straightforward like using organophilic or hydrophilic membrane. In this case membrane selection is based on relative closeness of solubility parameter of the organic to be selectively permeated with that of the membrane polymer [6]. Niang et al. used a blended membrane of cellulose acetate and cellulose acetate hydrogen phthalate in the separation of methyl tertiary butyl ether/methanol (MTBE/MeOH) mixtures [7]. The dehydration of organic solvents is the most important application of pervaporation. In a recent study, Jonquières et al. concluded that of the 63 units installed by GFT (now Sulzer Chemtech) and associates between 1984 and 1996, 62 units are used for dehydration purposes, including 22 for ethanol and 16 for isopropyl alcohol [8]. The first commercial application of the pervaporation process was the dehydration of ethanol, using a hybrid system with a distillation column. The pervaporation unit can also be used in combination with a reaction, where it constantly removes one of
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
the reaction products to shift the equilibrium reaction to higher yields. For dehydration purposes, hydrophilic membranes are used. Until now, mostly polymeric membranes have been used on an industrial scale. In recent years, research focuses on the development of ceramic membranes. Successful hydrophilic ceramic membranes have been made from silica [9,10] and from zeolites [11]. Some of these are already commercially available (e.g. Pervatech, Enter, The Netherlands). Full-scale plants using zeolite NaA membranes are already described [12]. Since the dewatering of organic solvents, and more specifically of low molecular weight alcohols, is the most common application, there are many publications on the separation of ethanol/water mixtures [13–15]. The active layer of the polymeric membranes consists in most cases of PVA, although other materials are also reported, such as polyacrylonitrile (PAN), polyetherimide (PEI) or 4,4oxydiphenylene pyromellitimide (POPMI) [16]. But there are few reports about the polymer membrane which is modified by nanometer sized inorganic powder. The performance of the polymer can greatly improved by adding nanometer sized inorganic powder which can advance the rigidity, heat resistance, size stability of the polymer, then can enhance the tensile strength, bending strength, impact strength heat distortion temperature, thermal stability of the polymer [17,18]. Although some reference reported that pervaporation technology used for separating ethanol /water mixture is on an industrial scale, it is nearly no study and application on separating methanol/water mixture. Compared with ethanol, methanol is more similar with water in polarity and molecular weight which makes methanol to compete with water on adsorbing in the membrane. So the pervaporation membrane which is available for separating ethanol/water mixture is not ideal for separating methanol/water mixture [19]. In this article, acrylic acid (AA) and acrylonitrile (AN) were synthesized by solution polymerization with or without nanometer SiO2 . The copolymer solution was made into main body of the membranes, then composited with the polyvinyl alcohol acetal membranes, to make the three-layer sandwich composite pervaporation membranes. Pervaporation experiments were carried out using these membranes to separate the mixtures of methanol/water over the complete concentration range 70–98%. The effect of membranes composition, feed temperature and feed concentration on the separation performance was investigated. The results showed that for the 98% mixture at 60 ◦ C, the separate factor is up to 1458, which is improved more than 10 times compared to the membranes without nanometer silica, the permeate flux is up to 325 g/(m2 h) and for the 70% mixture at 70 ◦ C, the separate factor arrived at 12, the permeate flux is up to 7097 g/(m2 h), which is improved more than 14 times compared to membranes without nanometer SiO2 .
2. Experiments
193
Table 1 The membranes ready for pervaporation Name of the membranes
AA:AN (monomer)
The content of nanometer SiO2
Membrane 1 Membrane 2 Membrane 3 Membrane 4 Membrane 5 Membrane 6
1:1 1:1 1:1 1:1 1:1 1:1
0 0.05% 0.075% 0.1% 0.125% 0.15%
2.2. Membrane preparing PVA was dissolved in the deionized water coupled with a suitable 36% formaldehyde solution and adjusted the PH value to 1–2 using hydrochloric acid at 90 ◦ C, to prepare PVA acetal membrane solution. AA and AN were synthesized at the monomer ratio of 1:1 by solution polymerization without and with nanometer SiO2 using potassium peroxodisulfate as initiator and N,N-methylene acrylamide as cross-linking agent at 70 ◦ C. When the solution reached the adaptable viscosity, stopped the reaction to make the PAA-CoAN membrane solution ready. First, PVA acetal membrane solution was scraped on the non-woven polyester fabric supporter as the first layer, and then under heat treatment at 75 ◦ C in the oven; Second, the PAA-Co-AN membrane solution as the middle layer of the composite membranes on the surface of the PVA acetal membranes; at last the PVA acetal membrane solution scraped on the surface of the PAA-Co-AN membranes went into the three-layer sandwich composite pervaporation membranes: the first layer next to the non-woven polyester fabric was PVA acetal, the middle layer was PAA-Co-AN (or PAA-Co-AN/SiO2 ) and the surface layer was also PVA acetal. The membranes were dried at 120 ◦ C for 1 h, and marinated in the deionized water and ethanol repeatedly to remove the dissolvable impurity. 2.3. Membranes prepared for pervaporation Table 1 shows the PAA-Co-AN membranes compositions prepared by the method introduced in Section 2.2. 2.4. Copolymer (membrane) characterization 2.4.1. Studies of SEM pictures The SEM pictures were taken by a scanning electron microscope (Quanta 200, FEI Limited, Czech). The samples were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity, and dried in a frozen dryer (beta2-8, Martin Christ, Germany), then dug into liquid nitrogen and divided into two parts by tweezers to obtain a natural cross section. 2.4.2. Studies of FT-IR spectroscopy The FT-IR spectra of different composite membranes were recorded on an FT-IR spectroscopy (TENSOR37, BRUKER Company, Germany). FT-IR samples were thin films of polymer. They were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity.
2.1. Materials Analytical reagent (A.R) grade of AA monomer, AN monomer were obtained from Tianjin chemical reagent Ltd. PVA was obtained from Beijing organic chemistry company, the degree of polymerization is 1700, the degree of alcoholysis is 99%; A.R grade of methanol, formaldehyde, potassium peroxodisulfate, N,N-methylene acrylamide came from the third Tianjin chemical reagent plant. Nanometer SiO2 (TS-610) was bought from Cabot Corporation in America, and its average size is 70 nm.
2.4.3. Studies of the heat weight-loss behavior TG curves were produced by an integrative thermal analysis apparatus (STA409PC, NETZSCH Limited, Germany). The samples were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity. 2.4.4. Studies of mechanical properties The tensile strength and elongation at break of the polymer film was determined by an Instron-Tensile tester (Instron4301, Instron
194
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
Fig. 1. Schematics of pervaporation test equipment. (1) Feed flask, (2) transportating pump, (3) pervaporation pool, (4) cold trap, (5) vacuum meter, (6) vacuum pump.
Limited, England). The length of the samples was 250 mm, and the thickness around 0.1 mm in the experiments. 2.4.5. Studies of glass transition temperature (Tg ) The Tg of membranes was studied by DMA (242C, Germany). The samples were marinated in the deionized water and ethanol for more than 36 h to remove the dissolvable impurity. 2.4.6. Pervaporation studies Pervaporation experiments were carried out with homemade set-up shown in Fig. 1. Effective membrane area in contact with the feed solution was 0.0127 m2 and the feed compartment volume was 250 ml. Pervaporation experiments were carried out at constant temperature of 40 ◦ C, 50 ◦ C, 60 ◦ C and 70 ◦ C, respectively. The concentration of methanol in the feed was 70%, 75%, 85%, 95% and 98% separately. The separation factor (˛) for methanol is defined as
˛=
(C1p /C2p ) (C1f /C2f )
where C1p , C2p are weight fraction of component 1, 2 in the permeate; C1f , C2f are weight fraction of component of 1, 2 in the feed, respectively. The permeate flux J (g/(m2 h)) is defined as J=
Fig. 2. SEM pictures of the membranes without nanometer SiO2 . The multiplication factors are 1500.
tor were maintained at 120 ◦ C. The methanol weight fraction is determined with the gas chromatograph analysis. 3. Results and discussion 3.1. Copolymer (membrane) characterization 3.1.1. SEM analysis The SEM pictures are shown below. Fig. 2 shows the cross picture of the membrane without nanometer SiO2 and its multiplication factor is 1500, while Fig. 3 shows the cross picture of the membrane with nanometer SiO2 and its multiplication factor is 1500. From the cross section pictures, it can be found that the clearly three-layer sandwich structure, and that the structure of the membranes with nanometer SiO2 is much finer and smoother than that of the membranes without nanometer SiO2 . The cause is that the nanometer SiO2 has been dispersed in the membranes and the hydrogen band association has formed on the surface of nanometer SiO2 . 3.1.2. Studies of FT-IR spectroscopy Fig. 4 shows the FT-IR spectroscopy of nanometer SiO2 . Si O symmetric stretching vibration band is at 810.86 cm−1 , and asym-
M At
where M is the weight of permeate, and A is the effective membrane area, t is the time of the experiment [20]. The separation factor and flux are always contradictive. Due to the influences acted upon each other, permeate separate index (PSI) is introduced. It is defined as PSI = (˛ − 1)J where ˛ and J are defined above. The permeate was collected every 60 min in a glass container using liquid nitrogen in a Dewar flask as a cold trap; for every different mixture, at least three permeate samples were collected. The vacuum was maintained by a vacuum pump at 1 × 10−3 MPa (10 mba). The flux was determined gravimetrically using a balance with an accuracy of 10−4 g. The composition of the methanol/water mixtures was determined with gas chromatography. A HP6890N gas chromatograph (column: J&W Scientific, DB1 30 m × 0.53 ID, THK film 5.0 m; FID detector) was used. During the analysis, the column was held at a constant temperature of 80 ◦ C, while the injector and the detec-
Fig. 3. SEM pictures of the membranes with nanometer SiO2 . The multiplication factors are 1500.
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
Fig. 4. FT-IR spectroscopy of nanometer SiO2 .
195
Fig. 6. TG curves of the membranes.
Table 2 The tensile strength and elongation at break of different membranes
Fig. 5. FT-IR spectroscopy of the membranes. (a) Membrane without nanometer SiO2 , (b) membrane with nanometer SiO2 .
metric stretching vibration at 1110.48 cm−1 , and –OH on the surface of nanometer SiO2 stretching vibration band at 1628.78 cm−1 . In Fig. 5 the FT-IR spectroscopy of the membranes without and with nanometer SiO2 are shown: Fig. 5a is FT-IR spectroscopy of membranes without nanometer SiO2 , while Fig. 5b is FT-IR spectroscopy of membranes with nanometer SiO2 , where C N stretching vibration band is at 2243.9 cm−1 , and C O stretching vibration band at 1701 cm−1 , which demonstrates that the membranes are synthesized with AA and AN. Both of the two curves have the same trend. In Fig. 5b, there is a new peak at around 1110 cm−1 , which is the characteristic peak of nanometer SiO2 . So it can be concluded that nanometer SiO2 has been added in the membranes which makes the structure of the membranes more compact.
3.1.3. TG analysis TG curves of the membranes are shown in Fig. 6. The curves without and with nanometer SiO2 have the same trend in the heat weight-loss behavior. However, at the same temperature, the heat weight-loss behavior of the membranes with nanometer SiO2 has weakened, especially at the rather high temperature above 400 ◦ C, which demonstrated that nanometer SiO2 can improve the heat stability of the membranes. And with the nanometer SiO2 concentration increasing, the heat stability is improving.
Name of the membranes
Tensile strength (MPa)
Elongation at break (%)
Membrane 1 Membrane 2 Membrane 3 Membrane 4
54.5 82 105.5 163.8
55.566 36.259 17.248 14.602
3.1.4. Studies of mechanical properties Table 2 shows the mechanical properties of the membranes. Too high tensile strength or elongation at break results in poor quality membrane. Tensile strength and elongation at break have a reciprocal relationship. For a good membrane, there should be an optimum balance between tensile strength and elongation at break [21]. The composite membranes synthesized for this study were found to have a good balance of tensile strength and elongation at break as shown in Table 2. It is observed from Table 2 that the composite membranes fall into acceptable range of tensile strength and elongation at break as required for pervaporation applications. It is also observed from these values that tensile strength increases from membrane 1–4 while elongation at break decreases. After adding nanometer SiO2 m, hydrogen band association may happen. Hydrogen band association is a kind of physical cross-linking. The hydrogen band association of polymer chains prevents high elongation as evidence by decrease of elongation at break and enhancement of tensile strength from membrane 1–4 signifying highest degree of cross-linking in membrane 4 containing maximum amount of SiO2 . 3.1.5. Studies of glass transition temperature The Tg of membranes with different amount of nanometer SiO2 are shown in Table 3. From Table 3, it can be seen that the Tg of the membranes are increasing as the concentration of nanometer SiO2 increasing until 0.1%; and it reaches maximum value when the concentration of nanometer Si02 at 0.1%, and then decreases, which delivered that the rigid nanometer Si02 has great influences on the Tg of polymer. The reason is that the polymer chain segment can not move before the glass transition, and nanometer SiO2 is imprisoned between the chain segment; while the chain segment of polymer is ‘ice-out’ when the temperature increased to Tg , afterwards it starts moving,
Table 3 Tg of membranes with different concentration of nanometer SiO2 under different frequency Membranes
Membrane 1
Membrane 2
Membrane 3
Membrane 4
Membrane 5
Membrane 6
1 Hz 5 Hz 10 Hz
115.2 120.7 123.2
116.7 124.5 127.1
147.1 151.0 153.4
180.2 184.7 186.7
156.5 161.9 164.8
152.0 155.0 157.4
196
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
Fig. 7. Comparison of different membrane separation factor under different temperature.
and then there will be more free space between the polymer chains allowing the nanometer SiO2 to move at this time, but nanometer SiO2 can move easier than the polymer chain segment, and this movement of nanometer SiO2 will limit that of polymer chain segment, so the polymer chain segment can move only when the temperature increased, which lead to the Tg increased. And what’s more, when the concentration of nanometer SiO2 increased, the physical cross-linking point increased, and the limit of nanometer SiO2 movement to chain segment is stronger. When the amount of nanometer SiO2 arrived at 0.1%, the physical cross-linking is saturated, this kind of limit is strongest, so the Tg arrived at maximum. With the concentration of nanometer SiO2 continuously increasing, superfluous nanometer SiO2 has some plasticizing influence, so the Tg is decreased. 3.2. Pervaporation studies 3.2.1. Comparison between the membranes without and with nanometer SiO2 Different membranes are used for separating methanol/water mixture in 98%, and the results under different temperature are shown in Figs. 7–9. Fig. 7 shows the change of separation factor with the temperature increasing, Fig. 8 shows the change of flux, and Fig. 9 shows the change of PSI. In Fig. 7, it can be found that the separation factors of the membranes with nanometer SiO2 are higher than those of the membranes without nanometer SiO2 , and with the concentration
Fig. 8. Comparison of different membrane flux under different temperature.
Fig. 9. The relation between temperature and PSI.
of nanometer SiO2 increasing; the separation factors are increasing too. The cause is that nanometer SiO2 in the membranes change the membrane structure which makes the water molecules can pass much easier than the methanol molecules, and what’s more, there are many hydrophilic groups –OH on the surface of nanometer SiO2 , which makes it easy to sorb and diffuse water molecules. As shown in Fig. 8, the permeate flux does not improve with the concentration of nanometer SiO2 because hydrogen band association makes the structure of the membranes more compact which prevent the transition of methanol and water molecules in a way. It is shown in Fig. 9 that the PSI has the same trend with the separation factor. The membrane has a good pervaporation performance when it contains 0.1% nanometer SiO2 . Dong-mei [22] insisted that the separation factor and flux of membranes with nanometer SiO2 increased due to two causes: one is that nanometer SiO2 is hydrophilic, which is propitious to the adsorption, diffusion and permeate of water molecule; the other is that the highly dispersed nanometer SiO2 powder has active surface, which can change the structure of membranes, and then make the permeation easier for water molecules and more difficult for methanol molecules. 3.2.2. Effect of feed temperature on pervaporation performance of membranes Use the membrane containing 0.1% nanometer SiO2 as the studying object to separate methanol/water mixture at different concentrations, under different temperatures. The results under different temperatures are shown in Figs. 10–12.
Fig. 10. Effect of temperature on the separation factor of membrane.
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
197
Fig. 11. Effect of temperature on the flux of membrane.
Fig. 13. Effect of solution concentration on the separation coefficient of membrane.
Fig. 12. Effect of temperature on PSI of membrane.
Fig. 14. Effect of solution concentration on the flux of membrane.
It is evident in Fig. 10 that with the temperature increasing, the separation factors increase firstly, and reach the maximum value at 60 ◦ C, then decrease. And it can be easily found that the separation factors of all the composite membranes reach maximum values at around 60 ◦ C. In Fig. 11, the permeate fluxes are also increasing with the temperature increasing. PSI has the same trend with separation factor in Fig. 12. According to solution-diffusion mechanism, the increasing of temperature makes the solubility on the surface of membrane and the diffusion rate in the membrane increase. From the microstructure analysis, the free volume of the membranes increases under higher temperature, which makes the interspaces between the polymer chain bigger, and the kinetic energy of permeate are increasing, which make the diffusion easier. Because the coupling effect, the methanol relative concentration are increased, and the separate selectivity are notable decreased with temperature increasing.
ship with the swelling state of membranes. With the increase of methanol in the feed, membranes shrink gradually, and cause the free volume in the membrane to diminish, which lead to the flux decrease. The diminishing of free volume in membrane limits the diffusion and transference of methanol molecules and its association complex, while water molecules can permeate freely because the volume of water molecule is smaller than that of methanol molecule, and nanometer SiO2 makes water molecules permeate easily. So the membrane has larger separation factors and lower
3.2.3. Effect of feed concentration on pervaporation performance of membranes Use the membrane containing 0.1% nanometer SiO2 as the studying object to separate methanol/water mixture at different concentrations under different temperatures. Figs. 13–15 show the effect of feed concentration on the separation factor, flux and PSI. As shown in Fig. 13, with the feed concentration increasing, separation factors are increasing gradually, and beyond 85%, the change is greater. While in Fig. 14, the flux is decreasing with the feed concentration increasing. The explanation over this phenomenon is that the pervaporation behavior of the membrane has relation-
Fig. 15. Effect of solution concentration on PSI of membrane.
198
X. Liu et al. / Journal of Membrane Science 325 (2008) 192–198
flux. It is obvious in Fig. 15 that the PSI has the same trend with the separation factors with the feed concentration increasing. Wesslein et al. [23] used GFT Company’s PVA/PAN composite membrane separate methanol/water mixture (XMeOH ≥ 80%). However, the separation factor is so low that it is almost impossible to be applied to do dewatering job. Liping et al. [24] studied on separating methanol/water system by using PVA/PAN composite membrane made by themselves. The separate factor is 40 when the methanol concentration in the feed between 97 and 98%. And the flux is very low, about 200 g/(m2 h). And Fubing [25] did a pervaporation experiment with Polyimide membrane to separate the same mixture, and got the flux about 80 g/(m2 h) and the separate factor 3. Compared with above results, the membrane made in this paper has a much better separate effect.
[6]
[7]
[8]
[9]
[10]
[11]
4. Conclusions [12]
Copolymerization of AA and AN with and without nanometer SiO2 by solution polymerization generated two kinds of membranes. The results of SEM pictures, FI-IR spectra, TG curves and the test of mechanical properties showed that membranes with nanometer SiO2 have better performance in intensity, size stability and heat stability than those membranes without nanometer SiO2 . After adding nanometer SiO2 , there is a hydrogen band association in the membrane structure. These membranes were used for selective separation of water from its mixture with methanol by pervaporation. It was found that they can have high degree of permeation and selectivity for water. All of the membranes have a good performance at around 60 ◦ C, and have the best separation factor when the feed concentration is 98%. References
[13] [14] [15]
[16]
[17] [18]
[19] [20]
[1] J. Neel, Introduction to pervaporation, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, 1991, pp. 36–37. [2] Al-ghezawi, Nawash, et al., Permeation and separation characteristics of acetic acid-water mixtures by pervaporation through acrylonitrile and hydroxy ethyl methacrylate grafted poly(vinyl alcohol) membrane, Sep. Sci. Technol. 41 (2006) 2913–2931. [3] K.W. Boddeker, G. Bengtson, Selective pervaporation of organics from water, in: R.Y.M. Huang (Ed.), Pervaporation Membrane Separation Processes, Elsevier, Amsterdam, 1991, p. 452. [4] M. Hoshi, M. Kogure, T. Saitoh, et al., Separation of aqueous phenol through polyurethane membranes by pervaporation, Appl Polym Sci 65 (1997) 469–479. [5] M. Jin, Y. Wu, X.Y. Yang, et al. Cross-linked PS/poly (acrylate-co-acrylic acid) membrane for removal of VOCs from water, China-Japan International Sympo-
[21]
[22] [23] [24] [25]
sium on Membrane Hybrid System applied to Water Treatment Proceedings, J. Tianjin China (1999) 222–227. S.K. Ray, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Development of new synthetic membranes for separation of benzene–cyclohexane mixture by pervaporation – a solubility parameter approach, Ind. Eng. Chem. Res. 36 (1997) 5265. M. Niang, G.S. Luo, P. Schaetzel, Pervaporation separation of methyl tert-butyl ether/methanol mixtures using a high-performance blended membrane, J. Appl. Polym. Sci. 64 (1997) 875–882. A. Jonquières, R. Clément, P. Lochon, J. Néel, M. Dresch, B. Chrétien, Industrial state-of-the-art of pervaporation and vapor permeationin the western countries, J. Membr. Sci. 206 (2002) 87–117. R.W. Van Gemert, F.P. Cuperus, Newly developed ceramic membranes for dehydration and separation of organic mixtures by pervaporation, J. Membr. Sci. 105 (1995) 287–291. H.M. Van Veen, Y.C. van Delft, C.W.R. Engelen, P.P.A.C. Pex, Dewatering of organics by pervaporation with silica membranes, Sep. Purif. Technol. 22–23 (2001) 361–366. D. Shah, K. Kissick, A. Ghorpade, R. Hannah, D. Bhattacharyya, Pervaporation of alcohol–water and dimethylformamide-water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results, J. Membr. Sci. 179 (2000) 185–205. Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (1–3) (2001) 251–260. J.H. Kim, K.H. Lee, S.Y. Kim, Pervaporation separation of water from ethanol through polyimide composite membranes, J. Membr. Sci. 169 (2000) 81–93. M. Nomura, T. Yamaguchi, S.I. Nakao, Ethanol/water transport through silicate membranes, J. Membr. Sci. 144 (1998) 161–171. M. Wesslein, A. Heintz, R.N. Lichtenthaler, Pervaporation of liquid mixtures through poly (vinyl alcohol) (PVA) membranes I, Study of water containing binary systems with complete and partial miscibility, J. Membr. Sci. 51 (1990) 169–179. F. Lipnizki, R.W. Field, P.-K. Ten, Pervaporation-based hybrid process: a review of process design, applications and economics, J. Membr. Sci. 153 (1999) 183– 210. E.G. Kenneth, X. Chen, Mechanistic investigation of the preparation of polymer/ceramic nanocomposites, Nanostruct. Mater. 9 (1997) 181–184. X. Chen, K.E. Gonsalves, Synthesis and properties of an aluminum nitride/polyimide nanocomposite prepared by a nonaqueous suspension process, J. Mater. Res 12 (1997) 1274–1286. M. Wesslein, A. Heintz, R.N. Lichtenthaler, Pervaporartion of liquid mixtures through poly(vinyl alcohol)(PVA) membranes, J. Membr. Sci. 51 (1990) 169–179. A. Bakhshi, T. Mohammadi el., Effect of operating conditions on pervaporation of methanol–water mixtures: Part 1, Membr. Technol. 11 (2006) 6–9. S. Ray, S.K. Ray, Synthesis of highly methanol selective membranes for separation of methyl tertiary butyl ether (MTBE)–methanol mixtures by pervaporation, J. Membr. Sci. 278 (2006) 279–289. D.M. Xu, PVA pervaporation membranes modificated by nanometer sized SiO2 , Chem. Eng. Sci. Technol. 11 (2) (2003) 25–27. M. Wesslein, et al., Separation of organic binary mixtures by pervaporation, J. Membr. Sci 51 (1990) 273. L.P. Zhang, Y. Jun, J. Weijun, New studying on methanol dewatering techniques, Modern Chem. 10 (1994) 24–26. P. Fubing, J. Zhongyi, mass transfer model of two-component aqueous in the pervaporation membrane, Chem. Eng. 33 (1) (2005) 60–64.