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Heat-treatment effect on the morphology and pervaporation performances of asymmetric PAN hollow fiber membranes
Journal of Membrane Science 255 (2005) 33–47
Heat-treatment effect on the morphology and pervaporation performances of asymmetric PAN hollow fiber membranes H.A. Tsai a,∗ , Y.S. Ciou b , C.C. Hu c , K.R. Lee c , D.G. Yu a , J.Y. Lai b a
Department of Textile Science, Nanya Institute of Technology, Chung Li 32034, Taiwan R&D Center for Membrane Technology, Chung Yuan University, Chung Li 32023, Taiwan Department of Chemical Engineering, Nanya Institute of Technology, Chung Li 32034, Taiwan b
c
Accepted 24 September 2004 Available online 2 April 2005
Abstract The influence of heat-treatment conditions on the morphology and pervaporation performances of aqueous isopropanol (IPA) solution through wet spinning prepared polyacrylonitrile (PAN) hollow fiber membranes were investigated. At heat-treatment temperature higher than 210 ◦ C, dehydrogenation and cyclization reaction of the PAN molecular occurred during the heat-treatment process. The SEMs observation depicts that the morphology of heat-treated PAN hollow fiber membranes was denser with increasing the heat-treatment temperature. The permeation rate and water concentration in the permeate for a 90 wt.% aqueous isopropanol solution through the heat-treated PAN hollow fiber membranes at 120 ◦ C for 12 h were 186 g/m2 h and 99.2 wt.%, respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Hollow fiber membrane; Heat-treatment; Cyclization; Pervaporation
1. Introduction Separation of azeotropic, close boiling, isomeric, or heat sensitive liquid mixtures by pervaporation operation process has been studied for some time because of the potential energy cost savings [1]. Thus, many researchers investigated the separation and permeation characteristics for the above mixture solution using a variety of membranes [2–4]. For industrial use, the membranes of pervaporation with high flux and high selectivity are desired. Therefore, several methods of membrane preparation may be used to improve the separation performances such as: surface modification, blending, copolymerization, and grafting a selective species on to an inert film [5–9]. In general, asymmetric membranes, which consist of a very thin and dense top layer supported by a porous sub layer, is the better choice for separation operation due to the combination of the high selectivity of a dense membrane ∗
Corresponding author. Tel.: +886 3 4361070x404. E-mail address: [email protected] (H.A. Tsai).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.09.052
with the high permeation rate of a very thin membrane. The phase inversion method, especially the immersion precipitation process has been used for the preparation of asymmetric membrane. In this process, nascent membrane containing polymer, solvent or additive is immersed in a coagulation bath containing a nonsolvent. Precipitation then occurred because of the exchange of solvent and nonsolvent [2]. Asymmetric hollow fiber membranes that were prepared by the immersion precipitation process have been widely applied to separation process owing to possess many advantageous characteristics than the flat ones [10–16]. In the separation operation process, membrane morphology is one of the effective factors to affect the separation performance. The membrane morphology can be affected by changing casting conditions, such as casting solution composition, casting temperature, coagulation bath composition (included bore liquid and external coagulant composition in hollow fiber membrane preparation), etc. The pore size of membrane can be also reduced by post-treatment [17–19]. Polyacrylonitrile (PAN) membranes were widely used as the substrate of ultrafitration (UF), nanofiltration (NF), reverse
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osmosis (RO) and pervaporation [2]. Many researchers, such as Kim et al. [19], Musale et al. [20,21], Yang and Tong [22] and Wang [23] have investigated the effect of the modification of PAN membrane on the separation performances. In the field of carbon fiber production, PAN fibers have been recognized as the most important and promising precursor for producing high performance carbon fiber. It dominates nearly 90% of all worldwide sales of carbon fibers [24]. PAN fibers form a thermally stable, highly oriented molecular structure when subjected to a heat-treatment. By using PAN fiber as the precursor, the fiber was heated to 200–300 ◦ C in air atmosphere, this process is recognized as the PAN stabilization process [25]. During the stabilization process, applying a force along the fiber axial will enhance the polymer chain orientation. Moreover, dehydrogenation and cyclization will occur during stabilization process. The most important types of reactions during the stabilization of PAN as proposed in the literatures were compiled by Fitzer and Muller, as shown in Fig. 1 [25]. In this study, integrally skinned asymmetric PAN hollow fiber membranes were prepared by using wet spinning then heat-treatment process for pervaporation separation of aqueous isopropanol (IPA) solution. The relationship between the heat-treatment conditions and hollow fiber membrane morphology are investigated. Moreover, the separation performances of heat-treated hollow fiber membranes were also studied.
2. Experiment 2.1. Materials PAN polymer (ηinh = 1.1 dl g−1 ) was supplied by the TongHua synthesis fiber Co. Ltd. (Taiwan). The solvent, N,Ndimethylformamide (DMF) was of reagent grade and used without further purification. Water was used as the external and bore liquid coagulant. 2.2. Fabrication of PAN hollow fiber PAN hollow fiber membranes were fabricated by a wetspinning process. The PAN polymer was dissolved in DMF to form a 22 wt.% dope solution. The dope solution was extruded under a pressure of 2.533125 × 105 Pa (2.5 atm) through a spinneret. The dimensions of this spinneret were 0.83 and 0.53 mm for outer diameter (o.d.) and inner diameter (i.d.), respectively. The bore liquid was delivered using a syringe pump (500D from ISCO Inc., USA). The degassed homogenous PAN dope solution and bore liquid were extruded through the spinneret die to form a nascent hollow fiber membrane and then immersed into water bath immediately. The hollow fiber was washed thoroughly with water to remove the residual solvent for at least 3 days. Table 1 reveals the detailed process parameters and spinning conditions.
Fig. 1. Schematic of reactions during heat-treatment of PAN up to 400 ◦ C [24].
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Table 1 Spinning parameter of PAN precursor hollow fiber membrane
2.4. Characterization
Condition
Value
Spinning process Dope solution composition Dope solution temperature Bore liquid External coagulant Dope extrusion pressure Bore liquid flow rate Take up speed Coagulant temperature Spinneret diameter (mm)
Wet spinning PAN (22 wt.%)/DMF 60 ◦ C H2 O H2 O 2.533125 × 105 Pa (2.5 atm) 1 ml/min 2.9 m/min 50 ◦ C o.d./i.d., 0.83/0.53
To explore the characterization of the PAN hollow fiber membranes before and after the heat-treatment, various analytical methods were used. The change of the PAN chemical structure was studied by using FT-IR spectroscopy (PerkinElmer, Model SPECTRUM ONE). The morphology of the PAN hollow fiber membrane was observed with SEM (Hitachi Co., Model S-4700) and atomic force microscope (AFM, Digital Instrument, DI 5000). 2.5. Pervaporation performances test
Fig. 2. Schematic diagram of the hollow fiber in oven for heat-treatment.
The hollow fiber module for pervaporation test is consisted of five fibers. The fiber bundles were plotted in 5-min rapid solidified epoxy resin binder. The effective length of every hollow fiber for pervaporation was 8 cm. The procedure and the apparatus used in the pervaporation experiments were the same as described in previous work [16]. The feed solution was pumped into the shell side of the module and the permeates came out from the lumen of the fibers. The permeation rate was determined by W At
2.3. Heat-treatment of PAN hollow fiber membrane
P=
The PAN precursor hollow fiber membranes were heattreated in air atmosphere with different temperature by using an oven. During heat-treatment, two extremities of PAN precursor hollow fiber were hanged a weight to apply 10 g constant loading along the fiber axial as shown in Fig. 2. Table 2 shows the step of heat-treatment process. For 60 ◦ C heat-treatment process, PAN precursor hollow fiber was heattreated from room temperature to 60 ◦ C in 1 h, then hold 12 h in this temperature. In the case of 90–210 ◦ C heat-treatment processes, the PAN precursor hollow fiber was heat-treated from room temperature to 80 ◦ C in 1 h, subsequently heated to the demanded temperature in 1 h, then hold 12 h in that temperature.
where P, W, A, and t represent the permeation rate (g/m2 h), weight of permeate (g), the effective hollow fiber area are based on the outside diameter of fiber (m2 ), and operation time (h), respectively. A vacuum pump maintained the down-stream pressure at (3.999672–6.66612) × 102 Pa (3–5 mmHg). The permeation rate was determined by measuring the weights of permeate. The compositions of the feed solutions and permeates were measured by gas chromatography (GC China Chromatography 8700). The separation factor was calculated from: αwater−IPA =
Ywater /YIPA Xwater /XIPA
Table 2 The heat-treatment process of PAN hollow fiber membranes Heat-treatment temperature (◦ C) 60 90 120 150 180 210
Heat-treatment process
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where Xwater , XIPA are the weight fractions of water and isopropanol in the feed, and Ywater , YIPA are the weight fractions of water and isopropanol in the permeate. The product of total permeation rate and separation factor has been defined as the pervaporation separation index (PSI), which is a measure of the separation ability of the membrane. 2.6. Degree of swelling The PAN heat-treated hollow fiber membrane was cut into 5 cm length and sealed the both ends with epoxy resin. The sealed fiber was immersed in IPA water mixtures for 24 h at different temperature. The degree of swelling of the hollow fiber membrane was defined by the following equation: degree of swelling (%) =
Ww − Wd × 100 Wd
where Wd and Ww denote the weight of dry and swollen fiber, respectively. Since the vapor pressure in the bore of sealed hollow fiber membrane may result in the burst of fiber, the swelling degree experiment of 90 wt.% IPA solution can not achieve as increasing the temperature above 45 ◦ C. 2.7. Sorption measurements The sorption apparatus used in this study was the same as reported in the previous study [26]. PAN heat-treated hollow fiber membrane was cut into 5 cm length and sealed the both ends with epoxy resin. The sealed fiber was immersed in IPA water mixtures for 24 h. The epoxy resin sealed parts of immersed fiber were cut off immediately before taking from IPA water mixtures. And the fiber part was subsequently blotted between tissue paper to remove adherent solvent and placed in the container of a twin tube setup. The system was evacuated while the left tube was heated with hot water and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid in the right tube was determined by GC.
3. Results and discussion 3.1. The morphology of PAN precursor hollow fiber membrane Pervaporation performances of membranes are strongly related to their structure. Therefore, SEM was used to investigate the PAN hollow fiber membrane morphologies that were fabricated by using wet-spinning process. The SEM micrographs of the PAN hollow fiber membrane cross-sectional structure is shown in Fig. 3. It can be seen that many macrovoids exist in the outer edge and inner edge of the PAN hollow fiber membrane. In general, high affinity between the solvent and the nonsolvent is a very strong factor in the formation of UF/MF membrane. DMF/water (solvent/nonsolvent) pair exhibits very high mutual affinities and macrovoids can be found in membranes prepared from this system irrespective of the polymer chosen for their preparation [2]. In this study, DMF was used as the solvent of PAN and water was used as the nonsolvent (bore liquid and external coagulant), asymmetric structure consist of a thin top layer supported by macrovoids should be obtained. Fig. 3 also reveals that there exist many pores in the outer surface of hollow fiber (the pore size was ca. 13 nm). This structure leads to a poor pervaporation performance. Thus, how to modify the PAN hollow fiber membrane structures to further improve the pervaporation performance are the key concerns in this article. 3.2. Effect of heat-treatment on the hollow fiber membrane morphology The PAN precursor hollow fiber membranes were heattreated at different temperature in the range of 60–210 ◦ C for 12 h. The SEM pictures of outer surface and outer edge part of PAN hollow fiber membranes cross-section are shown in Figs. 4 and 5, respectively. As can be seen from these figures, there exist porous structure in the cross-section of PAN precursor hollow fiber. The porous structures became denser after heat-treatment and were denser with increasing
Fig. 3. The morphology of PAN hollow fiber membrane.
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Fig. 4. Effect of heat-treatment temperature on the outer surface morphology of PAN hollow fiber membrane. Heat-treatment time: 12 h; heat-treatment temperature: (A) no heat-treatment, (B) 60 ◦ C, (C) 120 ◦ C, (D) 150 ◦ C, (E) 180 ◦ C and (F) 210 ◦ C.
the heat-treatment temperature. Moreover, the SEM pictures also reveal that the dense skin layer formed when the heattreatment temperature was higher than 120 ◦ C. 3.3. Effect of heat-treatment temperature on the pervaporation performance The effect of heat-treatment temperature on the permeation rate and water content in permeate for pervaporation of 90 wt.% aqueous isopropanol solutions at 25 ◦ C through PAN hollow fiber membrane are shown in Table 3. It demonstrates that the permeation rate decreases with
increasing heat-treatment temperature in the range of 60–180 ◦ C. Nevertheless, the permeation rate increases as the heat-treatment temperature further increases (210 ◦ C). The water content in permeate increases with increasing heat-treatment temperature in the range of 60–120 ◦ C. While the water content in permeate kept above 99 wt.% when the heat-treatment temperature was higher than 120 ◦ C. These phenomena might be due to the fact that the dense skin layer thickness increases with increasing the heat-treatment temperature. Since the morphology of heat-treated hollow fiber membranes are denser than the precursor one, the permeation rate decreases with increasing heat-treatment
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Fig. 5. Effect of heat-treatment temperature on the outer edge morphology of PAN hollow fiber membrane. Heat-treatment time: 12 h; heat-treatment temperature: (A) no heat-treatment, (B) 60 ◦ C, (C) 120 ◦ C, (D) 150 ◦ C, (E) 180 ◦ C and (F) 210 ◦ C.
Table 3 Effect of heat-treatment temperature on the pervaporation performances of PAN hollow fiber membranesa Heat-treatment temperature
Permeation rate (g/m2 h)
Water content in permeate (wt.%)
PAN precursor 60 ◦ C 90 ◦ C 120 ◦ C 150 ◦ C 180 ◦ C 210 ◦ C
4801 3934 240 186 164 120 171
13.6 14.1 65.8 99.2 99.9 99.5 99
a
Feed solution: 90 wt.% IPA(aq) at 25 ◦ C.
temperature. The water molecules having relatively small molecular size can diffuse through the denser membrane easily rather than isopropanol molecules having large molecular size (the molar volumes of water and isopropanol were 18 and 77 cm3 /mol, respectively). Thus, the water content in permeate increases with increasing heat-treatment temperature. Moreover, as the heat-treated temperature higher than 180 ◦ C, an opposite trend of the permeation rate was observed. This phenomenon may be contributed to the change of surface morphology and chemical structure. To evaluate the heat-treatment effect on the PAN hollow fiber membranes morphology, the results were further
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confirmed by AFM analysis. Fig. 6 and Table 4 shows the outer surface image and roughness of PAN hollow fiber membranes before and after heat-treatment, respectively. It can be seen from Fig. 6 and Table 4 that the membrane surface structure varies with the different heat-treatment temperature. The roughness of the outer surface of hollow fiber membranes is proportional to permeation rate and inversely proportional to water content in permeate. The analysis of AFM images showed that the roughness of the hollow fiber membranes
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in terms of root mean square (RMS) of z-values and mean roughness (Ra ) are decreased with the heat-treatment temperature increased in the range of 60–180 ◦ C. However, the roughness of the outer surface of hollow fiber membrane increased with increasing the heat-treatment temperature in the case of high temperature heat-treated (210 ◦ C). After heat-treated (60–180 ◦ C), the porous outer surface of PAN precursor hollow fiber became more dense and smooth resulting in the surface roughness decreased. Nevertheless,
Fig. 6. The AFM image of PAN heat-treated hollow fiber membranes, heat-treatment temperature and time: (A) no heat-treatment, (B) 60 ◦ C/12 h, (C) 120 ◦ C/12 h, (D) 150 ◦ C/12 h, (E) 180 ◦ C/12 h, (F) 210 ◦ C/12 h and (G) 120 ◦ C/24 h.
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the hollow fiber membrane roughness increases at higher heat-treatment condition (210 ◦ C) may be due to the formation of surface microcracks of hollow fiber membrane. These microcracks lead to the permeation rate increase and the water content in permeate decreases slightly. Consequently, the water content kept above 99 wt.% when the heat-treatment temperature was higher than 120 ◦ C. From the viewpoints of energy and separation performance, the 120 ◦ C heat-treated processes displays a best heat-treatment condition in this study.
In addition, another factor that affects the pervaporation performances is the chemical structure variation during the heat–treatment process. The effect of heat-treatment temperature on the chemical structure of precursor, 120, 180 and 210 ◦ C heat-treated PAN hollow fiber membranes were characterized by using FT-IR technology. The FT-IR spectra of these fibers are shown in Fig. 8. Fitzer and Muller [25] had proposed that the decreases of ( CH2 )-bonds at 2940 cm−1 and the ( CN)-bonds at 2240 cm−1 can be used as indicators for the dehydrogenation
Fig. 6. (Continued)
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and cyclization reactions, respectively. The dehydrogenation reaction let the single bond of C C in the main chain of PAN change to the double bond of C C, the cyclization reaction let the C C bond change to the cyclic structure as shown in Fig. 1. It is well known that the molecular chain is hard to rotate or flex if the polymer chain possess double bond or cyclic structure. The unflexible chain causes an immobile channel to let the feed solution to permeate through the membrane. The data in Fig. 7 shows that the CH2 bonds at 2940 cm−1 and the CN bonds at 2240 cm−1 still exist in the case of 120 and 180 ◦ C heat-treated, while these peaks
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disappear in the case of 210 ◦ C heat-treated. It depicts that the dehydrogenation and cyclization reactions is occurred during 210 ◦ C heat-treated. Also, the data in Fig. 7 shows that the chemical structure of PAN hollow fiber almost none change in the case of 120–180 ◦ C heat-treatment. Thus, the permeation rate decreases with increasing heat-treatment temperature in the range of 60–180 ◦ C, resulting from the denser structure of skin layer. The permeation rate increased in the case of 210 ◦ C heat-treated was due to the facts that some microcracks and the unflexible chain formation. In addition, the water content in permeate increases with
Fig. 6. (Continued)
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Fig. 6. (Continued) .
Table 4 The out surface roughness of PAN hollow fiber membranes Heat-treatment temperature/time
RMS (nm)a
Ra (nm)b
PAN precursor 60 ◦ C/12 h 90 ◦ C/12 h 120 ◦ C/12 h 150 ◦ C/12 h 180 ◦ C/12 h 210 ◦ C/12 h 120 ◦ C/0 h 120 ◦ C/24 h
20.542 14.680 11.212 5.749 4.827 4.509 7.649 8.014 11.176
15.799 10.412 7.155 4.418 3.776 3.619 5.151 5.718 8.484
a b
Root mean square (RMS) of z-values. Mean roughness.
Fig. 7. FT-IR spectra of heat-treated PAN hollow fiber, heat-treatment temperature: (A) no heat-treatment, (B) 120 ◦ C/12 h, (C) 180 ◦ C/12 h and (D) 210 ◦ C/12 h.
increasing heat-treatment temperature in the range of 60–120 ◦ C and kept above 99 wt.%. These phenomena might be due to the fact that the relatively small molecular size of water can diffuse through the denser membrane easily than that of the large molecular size of isopropanol molecules. Thus, the water content in permeate increases with increasing heat-treatment temperature in the range of 60–120 ◦ C. Furthermore, from the view point of separation factor toward water, the separation factor of 180 and 210 ◦ C heat-treated hollow fiber membranes were 1791 and 891, respectively. It depicts that the formation of surface microcracks and unflexible polymer chain at 210 ◦ C heat-treated, resulting in the permeation rate increases from 120 to 171 g/m2 h and decreases one half of separation factor.
Fig. 8. Effect of heat-treatment time on the pervaporation performances of 120 ◦ C heat-treated PAN hollow fiber membranes, feed solution: 90 wt.% aqueous isopropanol solution at 25 ◦ C.
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3.4. Effect of heat-treatment time on the pervaporation performances Heat-treatment temperature can influence not only morphology but also pervaporation separation performances
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as discussed above, the heat-treatment time should also affect the morphology and separation performance of PAN heat-treated hollow fiber membranes. The pervaporation performances of PAN heat-treated hollow fiber membranes preparation with different heat-treatment time at the
Fig. 9. The surface and outer edge morphology of 120 ◦ C heat-treated PAN hollow fiber membranes. Heat-treatment time: (A) and (B) 0 h; (C) and (D) 6 h; (E) and (F) 12 h; (G) and (H) 24 h.
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heat-treatment temperature of 120 ◦ C are shown in Fig. 8. It can be seen from Fig. 8 that the permeation rate almost leveled off below 12 h heat-treatment and slightly increases above this heat-treatment time. Moreover, the water content in permeate imply that there was a certain critical value of heat-treatment time around 12 h, the water content in permeate increased with an increase in heat-treatment time below 12 h and then decreases above this time. These phenomena might be due to the change of morphology. Fig. 9 shows the outer surface and outer edge morphologies of 120 ◦ C heattreated PAN hollow fiber membranes in the heat-treatment time range of 0–24 h. As can be seen in Fig. 9, the porous structure of the PAN heat-treated hollow fiber membranes became a slight denser with increasing the heat-treatment time. Thus, the water content in permeate increases with increasing the heat-treatment time. When the heat-treatment time was prolonged to 24 h, the heat-treatment process was too exceeded, resulted in the formation of surface cracks as can be seen in the AFM of Fig. 6 and the surface roughness data in Table 4. These cracks resulted in the increment of permeation rate and decrement of water content in permeates. Hence, the 12-h heat-treated process displays a best heat-treatment condition as in the case of 120 ◦ C heat-treated. 3.5. Effect of feed composition on pervaporation performance The effect of feed composition on the pervaporation performance for the PAN hollow fiber membranes heat-treated at 120 ◦ C for 12 h is shown in Fig. 10. It shows that an increase in the feed isopropanol concentration results in a decrease in the permeation rate and an increase in the separation factor. These results can be explained by the degree of swelling of the membrane. That is, the degree of swelling of the 120 ◦ C, 12 h heat-treated PAN hollow fiber membrane decreases with increasing the aqueous IPA solution as shown in Fig. 11, and its curve corresponds well with the permeation rate curve of the PAN hollow fiber membranes heat-treated at 120 ◦ C for 12 h. When the degree of swelling of the membrane in the mixture
Fig. 10. Effect of feed composition on the pervaporation performance of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed temperature: 25 ◦ C.
Fig. 11. Effect of feed composition on the swelling degree of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed temperature: 25 ◦ C.
was large, IPA permeated the PAN hollow fiber membrane in spite of its low affinity toward the membrane. That is, excessive swelling due to the selective solvent (water) causes a nonselectivity solvent (IPA) to permeate through the PAN hollow fiber membrane and lower the selectivity. Moreover, to study the effects of solubility and diffusivity on the PAN hollow fiber membrane permselectivity, sorption experiments of the PAN hollow fiber membrane were performed. Fig. 12 shows the effect of IPA concentration in the feed on the sorption selectivity of PAN hollow fiber membrane with 120 ◦ C, 12-h heat-treatment. It can be seen from Figs. 10 and 12 that the permselectivity and sorption selectivity increases with increasing IPA concentration in the feed solution. In addition, the water concentration in membrane and in permeate are higher than that of water concentration in feed solution. These phenomena illustrate that water molecules are selectively dissolved into the PAN hollow fiber membrane and are predominantly permeated through the PAN hollow fiber membrane. Consequently, the diffusivity of water is higher than that of IPA.
Fig. 12. Effect of feed composition on the sorption selectivity of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed temperature: 25 ◦ C.
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Table 5 Comparison of the measured experimental data with literature results Material
IPA in feed (wt.%)
Temperature (◦ C)
Separation factor, α
Flux (g/m2 h)
PSI
References
CMC-CE-01 flat membrane PDMS, 60 wt.% silicalite flat membrane Cross-linked chitosan membrane Cross-linked PVA membrane Silica NaAlg + 5% PVA + 10% PEG Two-ply composite membrane of chitosan/sodium alginate Zeolite Incorporated PVA membrane Carboxymethylated PVA membrane Chitosan/PAN membrane Zeolite-incorporated sodium alginate membrane 120 ◦ C Heat-treated PAN hollow fiber 150 ◦ C Heat-treated PAN hollow fiber
90 90 90 90 95 90 90 90 85 90 90 90 90
45 22.5 30 60 70 30 60 30 80 70 30 25 25
1160 23 1100 190 500 3591 2010 216 362 5000 272 1116 9000
72 29.8 150 155 300 50 554 320 831 430 232.5 186 164
83520 685 165000 29450 150000 179550 1113540 69120 300822 2150000 63240 207600 1476000
[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] This study This study
3.6. Effect of feed solution temperature on pervaporation performance The effect of feed solution temperature on the pervaporation performance of 90 wt.% aqueous isopropanol solution through the 120 ◦ C and 12 h heat-treated hollow fiber membranes is shown in Fig. 13. The data shows that the permeation rate increases and the separation factor decreases with increasing the feed solution temperature. In general, the pervaporation operation feed solution temperature can affect both the permeate transport behavior and the membrane structure. The mass transfer coefficients of both water and iospropanol increased with an increasing feed temperature. Moreover, the polymer chain of the membrane became more flexible with an increasing feed temperature results in an increase in the free volume, frequency and amplitude of the polymer chain motions. And this resulted in a freer space for permeate diffusion through the membrane. This phenomenon can be verified from the data of swelling degree. Fig. 14 shows the swelling degree of 90 wt.% IPA solution in the temperature range of 25–45 ◦ C with 120 ◦ C, 12 h heat-treated hollow fiber membrane. It shows that the swelling degree of these hollow fibers also increases with increasing temperature in the
Fig. 13. Effect of feed solution temperature on the pervaporation performances of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed composition: 90 wt.% IPA.
range of 25–45 ◦ C. Thus, the permeation of the permeating molecules through the hollow fiber membrane becomes easier. Therefore, higher permeation rate and lower separation factor would be expected in the pervaporation system at a higher operating temperature. 3.7. Comparison of membranes The separation performance of investigated PAN heattreated hollow fiber membranes in this study are compared with literature data for aqueous IPA solution. The result of this comparison is shown in Table 5. From this table, it is obvious that our study shows the outstanding performances. Though the PSI value of Two-ply composite membrane of chitosan/sodium alginate [33], carboxymethylated PVA membrane [35], and chitosan/PAN membrane [36] were higher than our 120 ◦ C heat-treated PAN hollow fiber membrane, but it is well known that hollow fiber membrane possess many advantageous characteristics than the flat ones, especially the membrane packing density (i.e., membrane area per unit module volume) can be much higher than
Fig. 14. Effect of feed solution temperature on the swelling degree of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed composition: 90 wt.% IPA.
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that of the flat membranes. Hence, this study displays an excellent pervaporation performance.
4. Conclusion Pervaporation of isopropanol–water mixtures through the wet spinning then heat-treated PAN hollow fiber membranes were investigated in this article. Compared with PAN precursor hollow fiber membrane, the pervaporation performances of heat-treated PAN hollow fiber membranes effectively improved. The porous structure of PAN precursor hollow fiber membranes becomes denser after heat-treatment. The FT-IR spectrum reveals that a dehydrogenation and cyclization reactions occurred when the heat-treatment is higher than 210 ◦ C. The pervaporation results of permeation rate and water content in permeate for a 90 wt.% aqueous isopropanol solution through a 120 ◦ C and 12 h heat-treatment PAN hollow fiber membrane are 186 g/m2 h and 99.2 wt.%, respectively.
Acknowledgements The authors wish to sincerely thank the Ministry of Economic Affairs and the National Science Council of Taiwan, ROC, for financially supporting this work.
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[35] S.Y. Nam, H.J. Chun, Y.M. Lee, Pervaporation separation of water–isopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes, J. Appl. Polym. Sci. 72 (1999) 241. [36] X.P. Wang, Y.F. Feng, Z.Q. Shen, Pervaporation properties of a threelayer structure composite membrane, J. Appl. Polym. Sci. 75 (2000) 740. [37] M.Y. Kariduraganavar, A.A. Kittur, S.S. Kulkarni, K. Ramesh, Development of novel pervaporation membranes for the separation of water–isopropanol mixtures using sodium alginate and NaY zeolite, J. Membr. Sci. 238 (2004) 165.
Heat-treatment effect on the morphology and pervaporation performances of asymmetric PAN hollow fiber membranes H.A. Tsai a,∗ , Y.S. Ciou b , C.C. Hu c , K.R. Lee c , D.G. Yu a , J.Y. Lai b a
Department of Textile Science, Nanya Institute of Technology, Chung Li 32034, Taiwan R&D Center for Membrane Technology, Chung Yuan University, Chung Li 32023, Taiwan Department of Chemical Engineering, Nanya Institute of Technology, Chung Li 32034, Taiwan b
c
Accepted 24 September 2004 Available online 2 April 2005
Abstract The influence of heat-treatment conditions on the morphology and pervaporation performances of aqueous isopropanol (IPA) solution through wet spinning prepared polyacrylonitrile (PAN) hollow fiber membranes were investigated. At heat-treatment temperature higher than 210 ◦ C, dehydrogenation and cyclization reaction of the PAN molecular occurred during the heat-treatment process. The SEMs observation depicts that the morphology of heat-treated PAN hollow fiber membranes was denser with increasing the heat-treatment temperature. The permeation rate and water concentration in the permeate for a 90 wt.% aqueous isopropanol solution through the heat-treated PAN hollow fiber membranes at 120 ◦ C for 12 h were 186 g/m2 h and 99.2 wt.%, respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Hollow fiber membrane; Heat-treatment; Cyclization; Pervaporation
1. Introduction Separation of azeotropic, close boiling, isomeric, or heat sensitive liquid mixtures by pervaporation operation process has been studied for some time because of the potential energy cost savings [1]. Thus, many researchers investigated the separation and permeation characteristics for the above mixture solution using a variety of membranes [2–4]. For industrial use, the membranes of pervaporation with high flux and high selectivity are desired. Therefore, several methods of membrane preparation may be used to improve the separation performances such as: surface modification, blending, copolymerization, and grafting a selective species on to an inert film [5–9]. In general, asymmetric membranes, which consist of a very thin and dense top layer supported by a porous sub layer, is the better choice for separation operation due to the combination of the high selectivity of a dense membrane ∗
Corresponding author. Tel.: +886 3 4361070x404. E-mail address: [email protected] (H.A. Tsai).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.09.052
with the high permeation rate of a very thin membrane. The phase inversion method, especially the immersion precipitation process has been used for the preparation of asymmetric membrane. In this process, nascent membrane containing polymer, solvent or additive is immersed in a coagulation bath containing a nonsolvent. Precipitation then occurred because of the exchange of solvent and nonsolvent [2]. Asymmetric hollow fiber membranes that were prepared by the immersion precipitation process have been widely applied to separation process owing to possess many advantageous characteristics than the flat ones [10–16]. In the separation operation process, membrane morphology is one of the effective factors to affect the separation performance. The membrane morphology can be affected by changing casting conditions, such as casting solution composition, casting temperature, coagulation bath composition (included bore liquid and external coagulant composition in hollow fiber membrane preparation), etc. The pore size of membrane can be also reduced by post-treatment [17–19]. Polyacrylonitrile (PAN) membranes were widely used as the substrate of ultrafitration (UF), nanofiltration (NF), reverse
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osmosis (RO) and pervaporation [2]. Many researchers, such as Kim et al. [19], Musale et al. [20,21], Yang and Tong [22] and Wang [23] have investigated the effect of the modification of PAN membrane on the separation performances. In the field of carbon fiber production, PAN fibers have been recognized as the most important and promising precursor for producing high performance carbon fiber. It dominates nearly 90% of all worldwide sales of carbon fibers [24]. PAN fibers form a thermally stable, highly oriented molecular structure when subjected to a heat-treatment. By using PAN fiber as the precursor, the fiber was heated to 200–300 ◦ C in air atmosphere, this process is recognized as the PAN stabilization process [25]. During the stabilization process, applying a force along the fiber axial will enhance the polymer chain orientation. Moreover, dehydrogenation and cyclization will occur during stabilization process. The most important types of reactions during the stabilization of PAN as proposed in the literatures were compiled by Fitzer and Muller, as shown in Fig. 1 [25]. In this study, integrally skinned asymmetric PAN hollow fiber membranes were prepared by using wet spinning then heat-treatment process for pervaporation separation of aqueous isopropanol (IPA) solution. The relationship between the heat-treatment conditions and hollow fiber membrane morphology are investigated. Moreover, the separation performances of heat-treated hollow fiber membranes were also studied.
2. Experiment 2.1. Materials PAN polymer (ηinh = 1.1 dl g−1 ) was supplied by the TongHua synthesis fiber Co. Ltd. (Taiwan). The solvent, N,Ndimethylformamide (DMF) was of reagent grade and used without further purification. Water was used as the external and bore liquid coagulant. 2.2. Fabrication of PAN hollow fiber PAN hollow fiber membranes were fabricated by a wetspinning process. The PAN polymer was dissolved in DMF to form a 22 wt.% dope solution. The dope solution was extruded under a pressure of 2.533125 × 105 Pa (2.5 atm) through a spinneret. The dimensions of this spinneret were 0.83 and 0.53 mm for outer diameter (o.d.) and inner diameter (i.d.), respectively. The bore liquid was delivered using a syringe pump (500D from ISCO Inc., USA). The degassed homogenous PAN dope solution and bore liquid were extruded through the spinneret die to form a nascent hollow fiber membrane and then immersed into water bath immediately. The hollow fiber was washed thoroughly with water to remove the residual solvent for at least 3 days. Table 1 reveals the detailed process parameters and spinning conditions.
Fig. 1. Schematic of reactions during heat-treatment of PAN up to 400 ◦ C [24].
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Table 1 Spinning parameter of PAN precursor hollow fiber membrane
2.4. Characterization
Condition
Value
Spinning process Dope solution composition Dope solution temperature Bore liquid External coagulant Dope extrusion pressure Bore liquid flow rate Take up speed Coagulant temperature Spinneret diameter (mm)
Wet spinning PAN (22 wt.%)/DMF 60 ◦ C H2 O H2 O 2.533125 × 105 Pa (2.5 atm) 1 ml/min 2.9 m/min 50 ◦ C o.d./i.d., 0.83/0.53
To explore the characterization of the PAN hollow fiber membranes before and after the heat-treatment, various analytical methods were used. The change of the PAN chemical structure was studied by using FT-IR spectroscopy (PerkinElmer, Model SPECTRUM ONE). The morphology of the PAN hollow fiber membrane was observed with SEM (Hitachi Co., Model S-4700) and atomic force microscope (AFM, Digital Instrument, DI 5000). 2.5. Pervaporation performances test
Fig. 2. Schematic diagram of the hollow fiber in oven for heat-treatment.
The hollow fiber module for pervaporation test is consisted of five fibers. The fiber bundles were plotted in 5-min rapid solidified epoxy resin binder. The effective length of every hollow fiber for pervaporation was 8 cm. The procedure and the apparatus used in the pervaporation experiments were the same as described in previous work [16]. The feed solution was pumped into the shell side of the module and the permeates came out from the lumen of the fibers. The permeation rate was determined by W At
2.3. Heat-treatment of PAN hollow fiber membrane
P=
The PAN precursor hollow fiber membranes were heattreated in air atmosphere with different temperature by using an oven. During heat-treatment, two extremities of PAN precursor hollow fiber were hanged a weight to apply 10 g constant loading along the fiber axial as shown in Fig. 2. Table 2 shows the step of heat-treatment process. For 60 ◦ C heat-treatment process, PAN precursor hollow fiber was heattreated from room temperature to 60 ◦ C in 1 h, then hold 12 h in this temperature. In the case of 90–210 ◦ C heat-treatment processes, the PAN precursor hollow fiber was heat-treated from room temperature to 80 ◦ C in 1 h, subsequently heated to the demanded temperature in 1 h, then hold 12 h in that temperature.
where P, W, A, and t represent the permeation rate (g/m2 h), weight of permeate (g), the effective hollow fiber area are based on the outside diameter of fiber (m2 ), and operation time (h), respectively. A vacuum pump maintained the down-stream pressure at (3.999672–6.66612) × 102 Pa (3–5 mmHg). The permeation rate was determined by measuring the weights of permeate. The compositions of the feed solutions and permeates were measured by gas chromatography (GC China Chromatography 8700). The separation factor was calculated from: αwater−IPA =
Ywater /YIPA Xwater /XIPA
Table 2 The heat-treatment process of PAN hollow fiber membranes Heat-treatment temperature (◦ C) 60 90 120 150 180 210
Heat-treatment process
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where Xwater , XIPA are the weight fractions of water and isopropanol in the feed, and Ywater , YIPA are the weight fractions of water and isopropanol in the permeate. The product of total permeation rate and separation factor has been defined as the pervaporation separation index (PSI), which is a measure of the separation ability of the membrane. 2.6. Degree of swelling The PAN heat-treated hollow fiber membrane was cut into 5 cm length and sealed the both ends with epoxy resin. The sealed fiber was immersed in IPA water mixtures for 24 h at different temperature. The degree of swelling of the hollow fiber membrane was defined by the following equation: degree of swelling (%) =
Ww − Wd × 100 Wd
where Wd and Ww denote the weight of dry and swollen fiber, respectively. Since the vapor pressure in the bore of sealed hollow fiber membrane may result in the burst of fiber, the swelling degree experiment of 90 wt.% IPA solution can not achieve as increasing the temperature above 45 ◦ C. 2.7. Sorption measurements The sorption apparatus used in this study was the same as reported in the previous study [26]. PAN heat-treated hollow fiber membrane was cut into 5 cm length and sealed the both ends with epoxy resin. The sealed fiber was immersed in IPA water mixtures for 24 h. The epoxy resin sealed parts of immersed fiber were cut off immediately before taking from IPA water mixtures. And the fiber part was subsequently blotted between tissue paper to remove adherent solvent and placed in the container of a twin tube setup. The system was evacuated while the left tube was heated with hot water and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid in the right tube was determined by GC.
3. Results and discussion 3.1. The morphology of PAN precursor hollow fiber membrane Pervaporation performances of membranes are strongly related to their structure. Therefore, SEM was used to investigate the PAN hollow fiber membrane morphologies that were fabricated by using wet-spinning process. The SEM micrographs of the PAN hollow fiber membrane cross-sectional structure is shown in Fig. 3. It can be seen that many macrovoids exist in the outer edge and inner edge of the PAN hollow fiber membrane. In general, high affinity between the solvent and the nonsolvent is a very strong factor in the formation of UF/MF membrane. DMF/water (solvent/nonsolvent) pair exhibits very high mutual affinities and macrovoids can be found in membranes prepared from this system irrespective of the polymer chosen for their preparation [2]. In this study, DMF was used as the solvent of PAN and water was used as the nonsolvent (bore liquid and external coagulant), asymmetric structure consist of a thin top layer supported by macrovoids should be obtained. Fig. 3 also reveals that there exist many pores in the outer surface of hollow fiber (the pore size was ca. 13 nm). This structure leads to a poor pervaporation performance. Thus, how to modify the PAN hollow fiber membrane structures to further improve the pervaporation performance are the key concerns in this article. 3.2. Effect of heat-treatment on the hollow fiber membrane morphology The PAN precursor hollow fiber membranes were heattreated at different temperature in the range of 60–210 ◦ C for 12 h. The SEM pictures of outer surface and outer edge part of PAN hollow fiber membranes cross-section are shown in Figs. 4 and 5, respectively. As can be seen from these figures, there exist porous structure in the cross-section of PAN precursor hollow fiber. The porous structures became denser after heat-treatment and were denser with increasing
Fig. 3. The morphology of PAN hollow fiber membrane.
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Fig. 4. Effect of heat-treatment temperature on the outer surface morphology of PAN hollow fiber membrane. Heat-treatment time: 12 h; heat-treatment temperature: (A) no heat-treatment, (B) 60 ◦ C, (C) 120 ◦ C, (D) 150 ◦ C, (E) 180 ◦ C and (F) 210 ◦ C.
the heat-treatment temperature. Moreover, the SEM pictures also reveal that the dense skin layer formed when the heattreatment temperature was higher than 120 ◦ C. 3.3. Effect of heat-treatment temperature on the pervaporation performance The effect of heat-treatment temperature on the permeation rate and water content in permeate for pervaporation of 90 wt.% aqueous isopropanol solutions at 25 ◦ C through PAN hollow fiber membrane are shown in Table 3. It demonstrates that the permeation rate decreases with
increasing heat-treatment temperature in the range of 60–180 ◦ C. Nevertheless, the permeation rate increases as the heat-treatment temperature further increases (210 ◦ C). The water content in permeate increases with increasing heat-treatment temperature in the range of 60–120 ◦ C. While the water content in permeate kept above 99 wt.% when the heat-treatment temperature was higher than 120 ◦ C. These phenomena might be due to the fact that the dense skin layer thickness increases with increasing the heat-treatment temperature. Since the morphology of heat-treated hollow fiber membranes are denser than the precursor one, the permeation rate decreases with increasing heat-treatment
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Fig. 5. Effect of heat-treatment temperature on the outer edge morphology of PAN hollow fiber membrane. Heat-treatment time: 12 h; heat-treatment temperature: (A) no heat-treatment, (B) 60 ◦ C, (C) 120 ◦ C, (D) 150 ◦ C, (E) 180 ◦ C and (F) 210 ◦ C.
Table 3 Effect of heat-treatment temperature on the pervaporation performances of PAN hollow fiber membranesa Heat-treatment temperature
Permeation rate (g/m2 h)
Water content in permeate (wt.%)
PAN precursor 60 ◦ C 90 ◦ C 120 ◦ C 150 ◦ C 180 ◦ C 210 ◦ C
4801 3934 240 186 164 120 171
13.6 14.1 65.8 99.2 99.9 99.5 99
a
Feed solution: 90 wt.% IPA(aq) at 25 ◦ C.
temperature. The water molecules having relatively small molecular size can diffuse through the denser membrane easily rather than isopropanol molecules having large molecular size (the molar volumes of water and isopropanol were 18 and 77 cm3 /mol, respectively). Thus, the water content in permeate increases with increasing heat-treatment temperature. Moreover, as the heat-treated temperature higher than 180 ◦ C, an opposite trend of the permeation rate was observed. This phenomenon may be contributed to the change of surface morphology and chemical structure. To evaluate the heat-treatment effect on the PAN hollow fiber membranes morphology, the results were further
H.A. Tsai et al. / Journal of Membrane Science 255 (2005) 33–47
confirmed by AFM analysis. Fig. 6 and Table 4 shows the outer surface image and roughness of PAN hollow fiber membranes before and after heat-treatment, respectively. It can be seen from Fig. 6 and Table 4 that the membrane surface structure varies with the different heat-treatment temperature. The roughness of the outer surface of hollow fiber membranes is proportional to permeation rate and inversely proportional to water content in permeate. The analysis of AFM images showed that the roughness of the hollow fiber membranes
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in terms of root mean square (RMS) of z-values and mean roughness (Ra ) are decreased with the heat-treatment temperature increased in the range of 60–180 ◦ C. However, the roughness of the outer surface of hollow fiber membrane increased with increasing the heat-treatment temperature in the case of high temperature heat-treated (210 ◦ C). After heat-treated (60–180 ◦ C), the porous outer surface of PAN precursor hollow fiber became more dense and smooth resulting in the surface roughness decreased. Nevertheless,
Fig. 6. The AFM image of PAN heat-treated hollow fiber membranes, heat-treatment temperature and time: (A) no heat-treatment, (B) 60 ◦ C/12 h, (C) 120 ◦ C/12 h, (D) 150 ◦ C/12 h, (E) 180 ◦ C/12 h, (F) 210 ◦ C/12 h and (G) 120 ◦ C/24 h.
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the hollow fiber membrane roughness increases at higher heat-treatment condition (210 ◦ C) may be due to the formation of surface microcracks of hollow fiber membrane. These microcracks lead to the permeation rate increase and the water content in permeate decreases slightly. Consequently, the water content kept above 99 wt.% when the heat-treatment temperature was higher than 120 ◦ C. From the viewpoints of energy and separation performance, the 120 ◦ C heat-treated processes displays a best heat-treatment condition in this study.
In addition, another factor that affects the pervaporation performances is the chemical structure variation during the heat–treatment process. The effect of heat-treatment temperature on the chemical structure of precursor, 120, 180 and 210 ◦ C heat-treated PAN hollow fiber membranes were characterized by using FT-IR technology. The FT-IR spectra of these fibers are shown in Fig. 8. Fitzer and Muller [25] had proposed that the decreases of ( CH2 )-bonds at 2940 cm−1 and the ( CN)-bonds at 2240 cm−1 can be used as indicators for the dehydrogenation
Fig. 6. (Continued)
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and cyclization reactions, respectively. The dehydrogenation reaction let the single bond of C C in the main chain of PAN change to the double bond of C C, the cyclization reaction let the C C bond change to the cyclic structure as shown in Fig. 1. It is well known that the molecular chain is hard to rotate or flex if the polymer chain possess double bond or cyclic structure. The unflexible chain causes an immobile channel to let the feed solution to permeate through the membrane. The data in Fig. 7 shows that the CH2 bonds at 2940 cm−1 and the CN bonds at 2240 cm−1 still exist in the case of 120 and 180 ◦ C heat-treated, while these peaks
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disappear in the case of 210 ◦ C heat-treated. It depicts that the dehydrogenation and cyclization reactions is occurred during 210 ◦ C heat-treated. Also, the data in Fig. 7 shows that the chemical structure of PAN hollow fiber almost none change in the case of 120–180 ◦ C heat-treatment. Thus, the permeation rate decreases with increasing heat-treatment temperature in the range of 60–180 ◦ C, resulting from the denser structure of skin layer. The permeation rate increased in the case of 210 ◦ C heat-treated was due to the facts that some microcracks and the unflexible chain formation. In addition, the water content in permeate increases with
Fig. 6. (Continued)
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Fig. 6. (Continued) .
Table 4 The out surface roughness of PAN hollow fiber membranes Heat-treatment temperature/time
RMS (nm)a
Ra (nm)b
PAN precursor 60 ◦ C/12 h 90 ◦ C/12 h 120 ◦ C/12 h 150 ◦ C/12 h 180 ◦ C/12 h 210 ◦ C/12 h 120 ◦ C/0 h 120 ◦ C/24 h
20.542 14.680 11.212 5.749 4.827 4.509 7.649 8.014 11.176
15.799 10.412 7.155 4.418 3.776 3.619 5.151 5.718 8.484
a b
Root mean square (RMS) of z-values. Mean roughness.
Fig. 7. FT-IR spectra of heat-treated PAN hollow fiber, heat-treatment temperature: (A) no heat-treatment, (B) 120 ◦ C/12 h, (C) 180 ◦ C/12 h and (D) 210 ◦ C/12 h.
increasing heat-treatment temperature in the range of 60–120 ◦ C and kept above 99 wt.%. These phenomena might be due to the fact that the relatively small molecular size of water can diffuse through the denser membrane easily than that of the large molecular size of isopropanol molecules. Thus, the water content in permeate increases with increasing heat-treatment temperature in the range of 60–120 ◦ C. Furthermore, from the view point of separation factor toward water, the separation factor of 180 and 210 ◦ C heat-treated hollow fiber membranes were 1791 and 891, respectively. It depicts that the formation of surface microcracks and unflexible polymer chain at 210 ◦ C heat-treated, resulting in the permeation rate increases from 120 to 171 g/m2 h and decreases one half of separation factor.
Fig. 8. Effect of heat-treatment time on the pervaporation performances of 120 ◦ C heat-treated PAN hollow fiber membranes, feed solution: 90 wt.% aqueous isopropanol solution at 25 ◦ C.
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3.4. Effect of heat-treatment time on the pervaporation performances Heat-treatment temperature can influence not only morphology but also pervaporation separation performances
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as discussed above, the heat-treatment time should also affect the morphology and separation performance of PAN heat-treated hollow fiber membranes. The pervaporation performances of PAN heat-treated hollow fiber membranes preparation with different heat-treatment time at the
Fig. 9. The surface and outer edge morphology of 120 ◦ C heat-treated PAN hollow fiber membranes. Heat-treatment time: (A) and (B) 0 h; (C) and (D) 6 h; (E) and (F) 12 h; (G) and (H) 24 h.
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heat-treatment temperature of 120 ◦ C are shown in Fig. 8. It can be seen from Fig. 8 that the permeation rate almost leveled off below 12 h heat-treatment and slightly increases above this heat-treatment time. Moreover, the water content in permeate imply that there was a certain critical value of heat-treatment time around 12 h, the water content in permeate increased with an increase in heat-treatment time below 12 h and then decreases above this time. These phenomena might be due to the change of morphology. Fig. 9 shows the outer surface and outer edge morphologies of 120 ◦ C heattreated PAN hollow fiber membranes in the heat-treatment time range of 0–24 h. As can be seen in Fig. 9, the porous structure of the PAN heat-treated hollow fiber membranes became a slight denser with increasing the heat-treatment time. Thus, the water content in permeate increases with increasing the heat-treatment time. When the heat-treatment time was prolonged to 24 h, the heat-treatment process was too exceeded, resulted in the formation of surface cracks as can be seen in the AFM of Fig. 6 and the surface roughness data in Table 4. These cracks resulted in the increment of permeation rate and decrement of water content in permeates. Hence, the 12-h heat-treated process displays a best heat-treatment condition as in the case of 120 ◦ C heat-treated. 3.5. Effect of feed composition on pervaporation performance The effect of feed composition on the pervaporation performance for the PAN hollow fiber membranes heat-treated at 120 ◦ C for 12 h is shown in Fig. 10. It shows that an increase in the feed isopropanol concentration results in a decrease in the permeation rate and an increase in the separation factor. These results can be explained by the degree of swelling of the membrane. That is, the degree of swelling of the 120 ◦ C, 12 h heat-treated PAN hollow fiber membrane decreases with increasing the aqueous IPA solution as shown in Fig. 11, and its curve corresponds well with the permeation rate curve of the PAN hollow fiber membranes heat-treated at 120 ◦ C for 12 h. When the degree of swelling of the membrane in the mixture
Fig. 10. Effect of feed composition on the pervaporation performance of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed temperature: 25 ◦ C.
Fig. 11. Effect of feed composition on the swelling degree of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed temperature: 25 ◦ C.
was large, IPA permeated the PAN hollow fiber membrane in spite of its low affinity toward the membrane. That is, excessive swelling due to the selective solvent (water) causes a nonselectivity solvent (IPA) to permeate through the PAN hollow fiber membrane and lower the selectivity. Moreover, to study the effects of solubility and diffusivity on the PAN hollow fiber membrane permselectivity, sorption experiments of the PAN hollow fiber membrane were performed. Fig. 12 shows the effect of IPA concentration in the feed on the sorption selectivity of PAN hollow fiber membrane with 120 ◦ C, 12-h heat-treatment. It can be seen from Figs. 10 and 12 that the permselectivity and sorption selectivity increases with increasing IPA concentration in the feed solution. In addition, the water concentration in membrane and in permeate are higher than that of water concentration in feed solution. These phenomena illustrate that water molecules are selectively dissolved into the PAN hollow fiber membrane and are predominantly permeated through the PAN hollow fiber membrane. Consequently, the diffusivity of water is higher than that of IPA.
Fig. 12. Effect of feed composition on the sorption selectivity of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed temperature: 25 ◦ C.
H.A. Tsai et al. / Journal of Membrane Science 255 (2005) 33–47
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Table 5 Comparison of the measured experimental data with literature results Material
IPA in feed (wt.%)
Temperature (◦ C)
Separation factor, α
Flux (g/m2 h)
PSI
References
CMC-CE-01 flat membrane PDMS, 60 wt.% silicalite flat membrane Cross-linked chitosan membrane Cross-linked PVA membrane Silica NaAlg + 5% PVA + 10% PEG Two-ply composite membrane of chitosan/sodium alginate Zeolite Incorporated PVA membrane Carboxymethylated PVA membrane Chitosan/PAN membrane Zeolite-incorporated sodium alginate membrane 120 ◦ C Heat-treated PAN hollow fiber 150 ◦ C Heat-treated PAN hollow fiber
90 90 90 90 95 90 90 90 85 90 90 90 90
45 22.5 30 60 70 30 60 30 80 70 30 25 25
1160 23 1100 190 500 3591 2010 216 362 5000 272 1116 9000
72 29.8 150 155 300 50 554 320 831 430 232.5 186 164
83520 685 165000 29450 150000 179550 1113540 69120 300822 2150000 63240 207600 1476000
[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] This study This study
3.6. Effect of feed solution temperature on pervaporation performance The effect of feed solution temperature on the pervaporation performance of 90 wt.% aqueous isopropanol solution through the 120 ◦ C and 12 h heat-treated hollow fiber membranes is shown in Fig. 13. The data shows that the permeation rate increases and the separation factor decreases with increasing the feed solution temperature. In general, the pervaporation operation feed solution temperature can affect both the permeate transport behavior and the membrane structure. The mass transfer coefficients of both water and iospropanol increased with an increasing feed temperature. Moreover, the polymer chain of the membrane became more flexible with an increasing feed temperature results in an increase in the free volume, frequency and amplitude of the polymer chain motions. And this resulted in a freer space for permeate diffusion through the membrane. This phenomenon can be verified from the data of swelling degree. Fig. 14 shows the swelling degree of 90 wt.% IPA solution in the temperature range of 25–45 ◦ C with 120 ◦ C, 12 h heat-treated hollow fiber membrane. It shows that the swelling degree of these hollow fibers also increases with increasing temperature in the
Fig. 13. Effect of feed solution temperature on the pervaporation performances of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed composition: 90 wt.% IPA.
range of 25–45 ◦ C. Thus, the permeation of the permeating molecules through the hollow fiber membrane becomes easier. Therefore, higher permeation rate and lower separation factor would be expected in the pervaporation system at a higher operating temperature. 3.7. Comparison of membranes The separation performance of investigated PAN heattreated hollow fiber membranes in this study are compared with literature data for aqueous IPA solution. The result of this comparison is shown in Table 5. From this table, it is obvious that our study shows the outstanding performances. Though the PSI value of Two-ply composite membrane of chitosan/sodium alginate [33], carboxymethylated PVA membrane [35], and chitosan/PAN membrane [36] were higher than our 120 ◦ C heat-treated PAN hollow fiber membrane, but it is well known that hollow fiber membrane possess many advantageous characteristics than the flat ones, especially the membrane packing density (i.e., membrane area per unit module volume) can be much higher than
Fig. 14. Effect of feed solution temperature on the swelling degree of 120 ◦ C/12 h heat-treated PAN hollow fiber membranes, feed composition: 90 wt.% IPA.
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that of the flat membranes. Hence, this study displays an excellent pervaporation performance.
4. Conclusion Pervaporation of isopropanol–water mixtures through the wet spinning then heat-treated PAN hollow fiber membranes were investigated in this article. Compared with PAN precursor hollow fiber membrane, the pervaporation performances of heat-treated PAN hollow fiber membranes effectively improved. The porous structure of PAN precursor hollow fiber membranes becomes denser after heat-treatment. The FT-IR spectrum reveals that a dehydrogenation and cyclization reactions occurred when the heat-treatment is higher than 210 ◦ C. The pervaporation results of permeation rate and water content in permeate for a 90 wt.% aqueous isopropanol solution through a 120 ◦ C and 12 h heat-treatment PAN hollow fiber membrane are 186 g/m2 h and 99.2 wt.%, respectively.
Acknowledgements The authors wish to sincerely thank the Ministry of Economic Affairs and the National Science Council of Taiwan, ROC, for financially supporting this work.
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