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H2O mixtures
Separation and Purification Technology 100 (2012) 97–105
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Preparation of heat-treated PAN hollow fiber membranes for pervaporation of NMP/H2O mixtures Hui-An Tsai a,⇑, Ya-Ling Chen b, Kueir-Rarn Lee b, Juin-Yih Lai b a b
Dept. of Creative Fashion Design, Taoyuan Innovation Institute of Technology, Chung-Li 32091, Taiwan R&D Center for Membrane Technology, Chung-Yuan University, Chung-Li 32023, Taiwan
a r t i c l e
i n f o
Article history: Received 23 June 2012 Received in revised form 1 September 2012 Accepted 7 September 2012 Available online 16 September 2012 Keywords: N-methyl-2-pyrrolidone Hollow fiber membrane Heat-treatment Dehydration
a b s t r a c t In this study, the dehydration of NMP aqueous solution is achieved through NMP-insoluble PAN hollow fiber membranes by heat-treatment the NMP-soluble PAN hollow fiber membranes for 12 h at temperatures higher than 180 °C. The pervaporation results of a 30 wt% aqueous NMP solution at 25 °C through PAN hollow fiber membranes showed that the permeation flux was higher when the temperature in the range of 180–300 °C at which the heat treatment occurred was higher, and it was almost constant when the temperature at which the heat treatment occurred was higher than 300 °C. As for the water content of permeate, it was all 100 wt% water concentration in permeate for the PAN hollow fiber membranes heattreated in the temperature range from 180 to 360 °C. Furthermore, the permeation rate decreased with increasing NMP concentration in the feed solution and increased with increasing feed solution temperature. The activation energy of the aqueous NMP solution permeating through the PAN hollow fiber membrane heat-treated at 300 °C was 3.65 kcal/mol for a 30 wt% NMP concentration in feed mixtures. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In separation processes, membranes with high separation performances are desired and usually regarded as the aim of membrane scientists’ attempts. In general, asymmetric membranes are a good choice for separation operations due to the combination of the high selectivity of a dense membrane with the high permeation rate of a very thin skin layer. Asymmetric hollow fiber membranes have been widely applied to separation processes such as microfiltration, ultrafiltration, reverse osmosis, membrane distillation, pervaporation, and gas separation, because they possess more advantageous characteristics than flat membranes that are prepared by the immersion precipitation process [1–7]. For pervaporation and gas separation, it is hard to achieve satisfactory separation performances through asymmetric membranes since there might have many defects on the skin layer of hollow fiber membranes that were fabricated through the wet inversion process. To get high separation performances of asymmetric hollow fiber membranes, there are many ways to modify the hollow fiber membranes such as heat treatment [5,8,9], dip coating [3,10–12], additive addition [13,14], blending [15], cross-linking [5,9], and grafting [16,17]. Regardless of the way of modifying hollow fiber
⇑ Corresponding author. Tel.: +886 886 3 4361070x4101. E-mail address: [email protected] (H.-A. Tsai). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.09.005
membranes, a dense defect-free skin layer is necessary. Heat treatment has been considered as an effective method to improve the membrane selectivity and stability. Pristine hollow fiber membranes have a loosely packed morphology relative to heat-treated fibers having a more densely packed morphology. A thermally stable PAN hollow fiber membrane with highly oriented molecular structure was formed as heat-treating the PAN hollow fiber membrane. Fitzer and Muller [18] proposed that the PAN polymer chain orientation will be enhanced and dehydrogenation and cyclization will occur during heat-treatment to 200–300 °C in air atmosphere. The dehydrogenation reaction let the single bond of CAC in the main chain of PAN change to the double bond of C@C, the cyclization reaction let the CAC bond change to the cyclic structure. In our previous work [19,20], we investigated the effect of heat treatment on the morphology and pervaporation performances with aqueous iso-propanol solutions permeating through heattreated PAN hollow fiber membranes. According to the observation based on scanning electron microscopy (SEM) results, it has shown that the morphology of the heat-treated PAN hollow fiber membranes is denser when they are treated at higher temperatures. Furthermore, it has also demonstrated that for the pervaporation operation of a 90 wt% aqueous iso-propanol solution, the permeation flux is lower for the membrane heat-treated at a higher temperature in the range of 60–180 °C. Nevertheless, the permeation flux becomes higher for the membrane heat-treated at a much higher temperature of 210 °C. The water content of permeate is higher for the membrane heat-treated at a higher temperature in
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the range of 60–120 °C, and the water content of permeate remains above 99 wt% for the membrane heat-treated at a temperature higher than 120 °C. Moreover, we also used the positron annihilation spectroscopy (PAS) to identify the effect of heat treatment on the free volume variation in PAN hollow fiber membranes. It demonstrates that the free volume in the PAN polymer chains of the heat-treated PAN hollow fiber membrane is reduced [20]. NMP, with a five-membered lactam structure, is a water-soluble chemical compound. It is used to recover pure hydrocarbons while processing petrochemicals and in the desulfurization of gases. NMP has desirable properties such as good solvency, low volatility, low flammability, and relatively low toxicity; it is used to dissolve a wide range of polymers. However, it has also been identified as a reproductive toxicant by California in 2001 and then by the European Commission in 2003. NMP is a good solvent for preparing many kinds of polymeric membranes. For preparing the polymeric membrane by using NMP and water as the solvent/coagulant pair, the coagulation bath will contain NMP/water mixture after coagulating the casting membrane or hollow fiber. For environmental consciousness, the waste water could be dehydrated to reuse. Separation of the water and NMP is generally achieved by distillation. It is well-known that pervaporation is used for the economical separation of liquid mixtures, such as azeotropes, close-boiling mixtures, isomers, heat sensitive and hazardous chemicals [21–36]. However, there are rarely reports on the dehydration of NMP by pervaporation. Ghosh et al. [37] synthesized hydroxyterminated polybutadiene (HTPB)-based polyurethaneurea membranes and used for pervaporation recovery of NMP from dilute aqueous solution. They found that with decrease in soft segment content of the membrane, the permeation flux decreased slightly but the separation factor increased to some extent. Moreover, with the increase in concentration of NMP in feed solution, permeation flux was improved significantly. Both the permeation flux and the separation factor for NMP increased with increase in operating temperature. Sato et al. [38] revealed on the first practically available calcined high-silica zeolite membranes in an industrial scale synthesized using an organic structure directing agent for an application of dehydration of NMP solution by pervaporation. The results demonstrated that the synthesized high-silica CHA-type membranes could be applied to dehydration of hydrous NMP solution at wide range of water contents in the feed. Hollow fiber membranes possess many advantageous characteristics than the flat ones such as: (1) the membrane packing density (i.e. membrane area per unit module volume) can be much higher than that of the flat membranes; (2) the hollow fiber membranes are self-supporting; (3) the hollow fiber membranes themselves form the vacuum vessel if the shell-fed mode of operation is used. Therefore, in present work, we tried to fabricate heat-treated PAN hollow fiber membranes that were NMP-insoluble. The goal of this investigation was to fabricate a pervaporation membrane for an aqueous NMP solution through heat treatment of NMPsoluble pristine PAN hollow fiber membranes. The effect of feed composition and feed solution temperature on the separation performances were also investigated in this investigation.
2. Experimental 2.1. Materials A PAN polymer was used as the matrix of a hollow fiber membrane, which was supplied by Tong-Hua Synthesis Fiber Co., Ltd. (Taiwan). The solvent N,N-dimethylformamide (DMF) was of reagent grade and used without further purification. Water was used as the external coagulant and bore liquid during the spinning of PAN hollow fiber membranes.
2.2. Fabrication of PAN hollow fiber PAN hollow fiber membranes were fabricated by the wet-spinning process. A PAN polymer was dissolved in DMF to form a 22 wt% dope solution. The degassed dope solution was extruded through an orifice-in-tube spinneret. The dimensions of this spinneret were 0.83 and 0.53 mm in outer diameter (o.d.) and inner diameter (i.d.), respectively. The bore liquid, water, was delivered using a syringe pump (500D from ISCO Inc., USA) with a flow rate of 1.0 ml/min. The degassed homogenous PAN dope solution and bore liquid were extruded through the spinneret die to form a nascent hollow fiber membrane, and then it was immersed in a water bath immediately. The hollow fiber membrane was washed with fresh water for at least three days to remove the residual solvent. Table 1 reveals the detailed process parameters and spinning conditions. 2.3. Heat treatment of PAN hollow fiber PAN hollow fiber membrane precursors were heat treated in an air atmosphere at different temperatures by using an oven. The heat-treatment processes are shown in Table 2. 2.4. Characterization PAN hollow fiber membranes were characterized with various analytical methods. The morphology of the PAN hollow fiber membrane was observed with SEM (Hitachi Co., Model S-4800). Thermogravimetric analysis (TGA, PERKIN ELMER, Model TGA 7) was performed at a rate of 10 °C/min in the temperature range of 35– 500 °C. 2.5. Pervaporation experiments The hollow fiber module for pervaporation test consisted of five hollow fiber membranes. 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 a previous work [4]. An aqueous NMP feed solution was pumped into the shell side of the module, and the permeates came out of the lumen of the fibers. The permeation rate was determined by
P ¼ W=ðA tÞ where P, W, A, and t represent the permeation rate (g/m2 h), weight of permeate (g), effective hollow fiber area based on the outside diameter of the fiber (m2), and operation time (h), respectively. A vacuum pump maintained the downstream pressure at 3–5 mmHg. The permeation rate was determined by dividing the measured weight of the permeate by the sampling time. The compositions
Table 1 Parameters for wet spinning of pristine PAN hollow fiber membranes. Parameter
Value
Dope solution composition Dope solution temperature Bore liquid External coagulant Dope extrusion pressure Bore liquid flow rate Coagulant temperature Spinneret diameter (mm)
22 wt% PAN/DMF 60 °C H2O H2O 2.5 atm 1 ml/min 50 °C OD/ID = 0.83/0.53
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Table 2 The heat-treatment process of PAN hollow fiber membranes.
of the feed solution and permeate were measured by gas chromatograghy (GC, China Chromatography 8700).
3. Results and discussion 3.1. Membrane characterization NMP has a powerful solvency to dissolve many polymers in preparing polymeric membranes such as PAN, polysulfone, polyethersulfone, cellulose acetate, and PVDF. In this paper, we tried to prepare heat-treated PAN hollow fiber membranes to be used to separate an aqueous NMP solution by pervaporation. The thermal properties of PAN hollow fiber membranes tested by an integrated TGA and thermal analysis/mass spectrometry (TA/MS) have been shown in our previous work [20]. Figs. 1 and 2 show TGA and TA/MS spectra for PAN hollow fiber membranes, respectively, which were heated at a heating rate of 10 °C/min. From these figures, it can be found that during heating, the weight of the PAN hollow fiber membrane decreases with increasing temperature. Moreover, there are four peaks at 14, 16, 28, and 32 amu. It depicts that some of the N, CH4, O, N2, CN, or O2 might be burnt away during the heating process from room temperature to 300 °C at a heating rate of 10 °C/min. To evidence the mechanism of heat100
Weight (%)
90 80 70 60 50 40 30 20
0
100
200
300
400
500
O
Temperature ( C) Fig. 1. Thermal analysis of a PAN hollow fiber membrane (heating rate: 10 °C/min).
treatment process, FT-IR spectra of pristine and 210 °C heat-treated PAN hollow fiber membranes are shown in the Fig. 3. As can be seen in Fig. 3, the CH2 bonds at 2940 cm1 and the CN bonds at 2240 cm1 exist in the heat-treatment free PAN hollow fiber membrane, while these peaks disappear in the case of heat-treated at temperatures higher than 210 °C. It depicts that the dehydrogenation and cyclization reactions is occurred during heat-treated at temperatures higher than 210 °C. NMP is a good solvent for the PAN polymer. Thus, it is impossible to separate NMP mixtures through pristine PAN hollow fiber membranes by pervaporation. However, some of the N, CH4, O, N2, CN, or O2 might be burnt away during the heating process; thus, NMP would not dissolve the heat-treated PAN hollow fiber membrane, since an inorganic-like PAN hollow fiber membrane might be made.
3.2. Morphology studies Pristine PAN hollow fiber membranes were heat treated in an air atmosphere at different temperatures by using a furnace. Figs. 4 and 5 represent the overall and outer-edge cross-sectional morphology of PAN hollow fiber membranes, which were heat treated for 12 h at various temperatures. The diameter of these hollow fiber membranes is listed in Table 3. From Figs. 4 and 5, it can be found that the macrovoids morphology of the pristine PAN hollow fiber membrane does not disappear even for the membranes heat-treated at temperatures higher than 300 °C. However, it is obviously to find that the morphology becomes dense after the heat treatment as comparing Fig. 5A with B–H. In our previous work [20], we have demonstrated the Doppler broadened energy spectrum (DBES) data of PAN hollow fiber membranes obtained by PAS experiment with a variable monoenergy slow positron beam. It has shown that the S parameter and R parameter variation of heat-treated PAN hollow fiber membranes decreases with increasing heat-treatment temperature. That is to say, the free volume of heat-treated PAN hollow fiber membranes decreases with increasing heat-treatment temperature. The S parameter variation of heat-treated PAN hollow fiber membranes which heat-treatment temperature were high than 210 °C are almost no change at positron incident energies of 0– 30 keV. These phenomena might be due to the fact that the positron
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Fig. 2. TA/MS spectra for a PAN hollow fiber membrane (heating rate: 10 °C/min).
T%
B
A
4000
3500
3000
2500
2000
1500
-1
Wavenumber (cm ) Fig. 3. FT-IR spectra of PAN hollow fiber membranes (A) without heat-treatment, (B) 210 °C heat-treatment.
was annihilated directly on the surface of PAN hollow fiber membrane due to high surface electron density resulted from the highly carbon cyclic structure after high temperature heat-treatment. Furthermore, as depicted in Table 3, the diameter of the PAN hollow fiber membranes is smaller when the temperature at which the heat treatment occurs is higher. These phenomena after the heat treatment might be due to the following: the morphology of the PAN hollow fiber membranes becomes denser, and the free volume in the PAN polymer chain is reduced [19,20]. 3.3. Effect of heat treatment on the pervaporation performances Because NMP is a good solvent for the PAN polymer, the pristine PAN hollow fiber membrane cannot be used for the pervaporation of NMP mixtures; hence, we determined the pervaporation possi-
bility of NMP mixtures through heat-treated PAN hollow fiber membranes. However, as we used PAN hollow fiber membranes heat-treated at 120 °C and 160 °C to test the pervaporation performance with an aqueous NMP solution, the modules were broken resulting from the dissolution of the organic-rich heat-treated PAN hollow fiber membranes in the NMP solution. The pervaporation performance with a 30 wt% aqueous NMP solution permeating through the PAN hollow fiber membranes heat-treated at temperatures higher than 160 °C is shown in Fig. 6. As can be seen in Fig. 6, the permeation flux is higher when the temperature in the range of 180–300 °C at which the heat treatment occurs is higher, and it is almost constant when the temperature at which the heat treatment occurs is higher than 300 °C. As for the water content of permeate, it is all 100 wt% water concentration in permeate, regardless of the temperature at which the heat treatment occurs. Theoretically, the morphology of heat-treated PAN hollow fiber membranes should become denser at higher temperatures at which the heat treatment occurs, resulting in a lower permeation flux. However, the permeation flux is higher when the temperature in the range of 180–300 °C at which the heat treatment occurs is higher, as shown in Fig. 6. These phenomena might be contributed by the change in the morphology. The surface morphology of PAN hollow fiber membranes heat-treated at different temperatures is shown in Fig. 7. As can be found in Fig. 7, one can observe that there are many cracks on the surface of PAN hollow fiber membranes heat-treated at higher temperatures, resulting in a higher permeation flux. Furthermore, the thickness data of heat-treated PAN hollow fiber membranes listed in Table 1 also depict the decreased thickness of the heat-treated PAN hollow fiber membranes when the temperature at which the heat treatment occurs is increased. The resistance experienced by the permeant will decrease as the thickness of PAN hollow fiber membranes decreases, leading to the permeation flux increase.
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Fig. 4. The overall cross-sectional morphology of PAN hollow fiber membranes heat-treated at different temperatures. (100) (A) Pristine, (B) 180 °C, (C) 210 °C, (D) 240 °C, (E) 270 °C, (F) 300 °C, (G) 330 °C, (H) 360 °C.
Fig. 6 also represents the water content of permeate kept at 100 wt% water concentration in permeate, regardless of the temperature at which the heat treatment occurs. These phenomena might be due to the following factor: the molar volume of the water molecules is smaller than that of the NMP molecules. (The molar volume of water and NMP molecules are 18 and 96.4 cm3/ mol, respectively.) Thus, it is hard for the NMP molecule to permeate through the membrane compared to the water molecule.
3.4. Effect of feed concentration on the vapor permeation performances In the process of pervaporation, the feed solution contacts the membrane surface directly. Vapor permeation can avoid the disadvantages caused by pervaporation. Through the vapor permeation technique, the feed solution vaporizes first and then permeates through the membrane. Consequently, swelling or shrinking of
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Fig. 5. The outer-edge cross-sectional morphology of PAN hollow fiber membranes heat-treated at different temperatures. (10 k) (A) Pristine, (B) 180 °C, (C) 210 °C, (D) 240 °C, (E) 270 °C, (F) 300 °C, (G) 330 °C, (H) 360 °C.
Table 3 List of the diameter and thickness of PAN hollow fiber membranes heat-treated at various temperatures. Temperature (°C) Diameter (lm) Thickness (lm)
Pristine 738 86.3
180 570 61.8
210 513 60.9
240 458 58.3
270 423 53.9
300 383 53.6
330 328 45.7
360 273 43.1
polymer membranes caused by the direct contact with the feed solution can be prevented. To prevent the high concentration of NMP from dissolving or swelling the hollow fiber membrane module during pervaporation, vapor permeation was performed to investigate the effect of feed concentration on the separation performance.
1000
100
800
80
2
Permeation rate (g/m h)
900 700 600
60
500 400
40
300 200
20
100 0 160
200
240
280
320
360
0
Water content in permeate (wt.%)
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O
Heat-treatment temperature ( C) Fig. 6. Effect of temperature at which the heat treatment occurs on the pervaporation performance with 30 wt% aqueous NMP solution at 25 °C.
The effect of the feed NMP concentration at 25 °C on the vapor permeation performance of the PAN hollow fiber membranes heat-
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treated at 300 °C is shown in Fig. 8. The data in Fig. 8 demonstrate that the permeation flux decreases with increasing NMP concentration in the feed. These results might be due to the fact that the PAN hollow fiber membranes heat-treated at 300 °C just let only the water molecules to permeate, while the NMP molecules in the feed side are restrained from permeating. As the NMP concentration in the feed increases, the water concentration decreases, resulting in the permeation flux to decrease. 3.5. Effect of feed temperature on the pervaporation performances The effect of the feed temperature on the pervaporation performance with 30 wt% aqueous NMP solution permeating through PAN hollow fiber membranes heat-treated at 300 °C is shown in Fig. 9. The data in Fig. 9 show that the permeation flux increases and the water content of permeate remains almost 100 wt% with increasing feed solution temperature. Generally, the pervaporation operation feed solution temperature can affect not only the permeation transport behavior but also the membrane structure. In general, the driving force for mass transport increases as increasing
Fig. 7. The outer surface morphology of PAN hollow fiber membranes heat-treated at different temperatures. (A) Pristine, (B) 180 °C, (C) 210 °C, (D) 240 °C, (E) 270 °C, (F) 300 °C.
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2
Permeation rate (g/m h )
300
100 80
250
60
200
40 150
20
100
30
40
50
60
70
80
90
0
Water content in permeate (wt.%)
104
Feed NMP concentration (wt%)
100
1600 80
2
Permeation rate (g/m h)
1800
1400
60
1200
40
1000 800
20
600 20
30
40
50
60
70
0
Water content in permeate (wt.%)
Fig. 8. Effect of the feed concentration on the vapor permeation performance of PAN hollow fiber membranes heat-treated at 300 °C.
O
Feed temperature ( C) Fig. 9. Effect of the feed temperature on the pervaporation performance with a 30 wt% aqueous NMP solution permeating through PAN hollow fiber membranes heat-treated at 300 °C.
feed temperature, which represents the concentration gradient resulting from a difference in the partial vapor pressure of the permeants between the feed and permeating mixture. The vapor pressure in the feed compartment increases while the vapor pressure at the permeate side is not affected as increasing the feed temperature. All these results, in an increase of driving force due to increase in temperature. As increasing the feed temperature, the mass transfer coefficients for both water and NMP should increase with increasing feed temperature. However, as shown in Fig. 9, the increase in the mass transfer coefficient for both water and NMP lets only the water molecules to permeate easily; the NMP mole-
cules still cannot permeate through the fiber module. Moreover, the polymer chain of the membrane becomes more flexible with increasing feed temperature, resulting in an increase in the water molecules permeation. The activation energy of diffusion is calculated according to the Arrhenius plot.
J ¼ J 0 exp
Ep RT
where J0 and EP are pre-exponential factor and activation energy for permeation, respectively; R the gas constant and T the temperature in kelvin. From ln(J) versus 1/T plot, the activation energy of permeants in membrane was determined from the slope of Arrhenius plot. The activation energy of an aqueous NMP solution permeating through the PAN hollow fiber membrane heat-treated at 300 °C is 3.65 kcal/mol (15.27 kJ/mole) for 30 wt% NMP concentration in feed mixtures. Higher activation energy means that molecules require higher energy to penetrate through the membrane. Compared with that in literature, as shown in Table 4, the smaller activation energy values observed for the PAN heat-treated hollow fiber membrane than the other membrane suggests that the energy required to cross the barrier decreased, thereby allowing more of water molecules to readily permeate across over the barrier through the PAN heat-treated hollow fiber membrane.
4. Conclusions A high separation performance with an aqueous NMP solution permeating through heat-treated PAN hollow fiber membranes was attained in this study. The as-spun PAN hollow fiber membranes could be dissolved by an NMP solution. However, the heat-treated PAN hollow fiber membranes heat-treated at temperatures higher than 180 °C for 12 h were insoluble in NMP. The pervaporation results of a 30 wt% aqueous NMP solution showed that the permeation flux was higher when the temperature in the range of 180–300 °C at which the heat treatment occurred was higher, and it was almost constant when the temperature at which the heat treatment occurred was higher than 300 °C. As for the water content of permeate, it was all 100 wt% water concentration in permeate for the PAN hollow fiber membranes heat-treated in the temperature range from 180 to 360 °C. In addition, the effect of feed composition and feed solution temperature on vapor permeation and pervaporation performances was also investigated. The permeation rate decreased with increasing NMP concentration in the feed solution and increased with increasing feed solution temperature. The activation energy of the aqueous NMP solution permeating through the PAN hollow fiber membrane heat-treated at 300 °C was 3.65 kcal/mol for 30 wt% NMP concentration in feed mixtures.
Table 4 Comparison of the activation energy data with literature results. Membrane
Feed solution
Activation energy
Ref.
Polyurethaneurea membrane High-silica CHA-type tubular membranes PVA–silicone based hybrid membranes (PVA/TEOS:1/2) CA/PAN blend membranes Sulfonated PVA membrane Chitosan membrane PVA/PAN membrane CS–PVA/PAN membrane CS/PAN hollow fiber membrane CS/PSf hollow fiber membrane PAN heat-treated hollow fiber membrane
1000 ppm NMP 50 wt% NMP 90 wt% Acetic acid 99 wt% pyridine 90 wt% IPA 50 wt% EtOH 50 wt% EtOH 50 wt% EtOH 90 wt% IPA 90 wt% EtOH 30 wt% NMP
24.03 kJ/mole Water: 30 kJ/mole NMP: 12 kJ/mole 19.39 kJ/mole H2O: 29.47 kJ/mole Pyridine: 36.19 kJ/mole 25.65 kJ/mole 30.8 kJ/mole 44.5 kJ/mole 42.7 kJ/mole 22.13 kJ/mole 10.54 kJ/mole 15.27 kJ/mole
[37] [38] [39] [40] [41] [42] [42] [42] [3] [28] This study
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Acknowledgments The authors wish to sincerely thank the National Science Council, the Ministry of Economic Affairs, and the Ministry of Education of Taiwan, ROC, for financially supporting this work.
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Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Preparation of heat-treated PAN hollow fiber membranes for pervaporation of NMP/H2O mixtures Hui-An Tsai a,⇑, Ya-Ling Chen b, Kueir-Rarn Lee b, Juin-Yih Lai b a b
Dept. of Creative Fashion Design, Taoyuan Innovation Institute of Technology, Chung-Li 32091, Taiwan R&D Center for Membrane Technology, Chung-Yuan University, Chung-Li 32023, Taiwan
a r t i c l e
i n f o
Article history: Received 23 June 2012 Received in revised form 1 September 2012 Accepted 7 September 2012 Available online 16 September 2012 Keywords: N-methyl-2-pyrrolidone Hollow fiber membrane Heat-treatment Dehydration
a b s t r a c t In this study, the dehydration of NMP aqueous solution is achieved through NMP-insoluble PAN hollow fiber membranes by heat-treatment the NMP-soluble PAN hollow fiber membranes for 12 h at temperatures higher than 180 °C. The pervaporation results of a 30 wt% aqueous NMP solution at 25 °C through PAN hollow fiber membranes showed that the permeation flux was higher when the temperature in the range of 180–300 °C at which the heat treatment occurred was higher, and it was almost constant when the temperature at which the heat treatment occurred was higher than 300 °C. As for the water content of permeate, it was all 100 wt% water concentration in permeate for the PAN hollow fiber membranes heattreated in the temperature range from 180 to 360 °C. Furthermore, the permeation rate decreased with increasing NMP concentration in the feed solution and increased with increasing feed solution temperature. The activation energy of the aqueous NMP solution permeating through the PAN hollow fiber membrane heat-treated at 300 °C was 3.65 kcal/mol for a 30 wt% NMP concentration in feed mixtures. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In separation processes, membranes with high separation performances are desired and usually regarded as the aim of membrane scientists’ attempts. In general, asymmetric membranes are a good choice for separation operations due to the combination of the high selectivity of a dense membrane with the high permeation rate of a very thin skin layer. Asymmetric hollow fiber membranes have been widely applied to separation processes such as microfiltration, ultrafiltration, reverse osmosis, membrane distillation, pervaporation, and gas separation, because they possess more advantageous characteristics than flat membranes that are prepared by the immersion precipitation process [1–7]. For pervaporation and gas separation, it is hard to achieve satisfactory separation performances through asymmetric membranes since there might have many defects on the skin layer of hollow fiber membranes that were fabricated through the wet inversion process. To get high separation performances of asymmetric hollow fiber membranes, there are many ways to modify the hollow fiber membranes such as heat treatment [5,8,9], dip coating [3,10–12], additive addition [13,14], blending [15], cross-linking [5,9], and grafting [16,17]. Regardless of the way of modifying hollow fiber
⇑ Corresponding author. Tel.: +886 886 3 4361070x4101. E-mail address: [email protected] (H.-A. Tsai). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.09.005
membranes, a dense defect-free skin layer is necessary. Heat treatment has been considered as an effective method to improve the membrane selectivity and stability. Pristine hollow fiber membranes have a loosely packed morphology relative to heat-treated fibers having a more densely packed morphology. A thermally stable PAN hollow fiber membrane with highly oriented molecular structure was formed as heat-treating the PAN hollow fiber membrane. Fitzer and Muller [18] proposed that the PAN polymer chain orientation will be enhanced and dehydrogenation and cyclization will occur during heat-treatment to 200–300 °C in air atmosphere. The dehydrogenation reaction let the single bond of CAC in the main chain of PAN change to the double bond of C@C, the cyclization reaction let the CAC bond change to the cyclic structure. In our previous work [19,20], we investigated the effect of heat treatment on the morphology and pervaporation performances with aqueous iso-propanol solutions permeating through heattreated PAN hollow fiber membranes. According to the observation based on scanning electron microscopy (SEM) results, it has shown that the morphology of the heat-treated PAN hollow fiber membranes is denser when they are treated at higher temperatures. Furthermore, it has also demonstrated that for the pervaporation operation of a 90 wt% aqueous iso-propanol solution, the permeation flux is lower for the membrane heat-treated at a higher temperature in the range of 60–180 °C. Nevertheless, the permeation flux becomes higher for the membrane heat-treated at a much higher temperature of 210 °C. The water content of permeate is higher for the membrane heat-treated at a higher temperature in
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the range of 60–120 °C, and the water content of permeate remains above 99 wt% for the membrane heat-treated at a temperature higher than 120 °C. Moreover, we also used the positron annihilation spectroscopy (PAS) to identify the effect of heat treatment on the free volume variation in PAN hollow fiber membranes. It demonstrates that the free volume in the PAN polymer chains of the heat-treated PAN hollow fiber membrane is reduced [20]. NMP, with a five-membered lactam structure, is a water-soluble chemical compound. It is used to recover pure hydrocarbons while processing petrochemicals and in the desulfurization of gases. NMP has desirable properties such as good solvency, low volatility, low flammability, and relatively low toxicity; it is used to dissolve a wide range of polymers. However, it has also been identified as a reproductive toxicant by California in 2001 and then by the European Commission in 2003. NMP is a good solvent for preparing many kinds of polymeric membranes. For preparing the polymeric membrane by using NMP and water as the solvent/coagulant pair, the coagulation bath will contain NMP/water mixture after coagulating the casting membrane or hollow fiber. For environmental consciousness, the waste water could be dehydrated to reuse. Separation of the water and NMP is generally achieved by distillation. It is well-known that pervaporation is used for the economical separation of liquid mixtures, such as azeotropes, close-boiling mixtures, isomers, heat sensitive and hazardous chemicals [21–36]. However, there are rarely reports on the dehydration of NMP by pervaporation. Ghosh et al. [37] synthesized hydroxyterminated polybutadiene (HTPB)-based polyurethaneurea membranes and used for pervaporation recovery of NMP from dilute aqueous solution. They found that with decrease in soft segment content of the membrane, the permeation flux decreased slightly but the separation factor increased to some extent. Moreover, with the increase in concentration of NMP in feed solution, permeation flux was improved significantly. Both the permeation flux and the separation factor for NMP increased with increase in operating temperature. Sato et al. [38] revealed on the first practically available calcined high-silica zeolite membranes in an industrial scale synthesized using an organic structure directing agent for an application of dehydration of NMP solution by pervaporation. The results demonstrated that the synthesized high-silica CHA-type membranes could be applied to dehydration of hydrous NMP solution at wide range of water contents in the feed. Hollow fiber membranes possess many advantageous characteristics than the flat ones such as: (1) the membrane packing density (i.e. membrane area per unit module volume) can be much higher than that of the flat membranes; (2) the hollow fiber membranes are self-supporting; (3) the hollow fiber membranes themselves form the vacuum vessel if the shell-fed mode of operation is used. Therefore, in present work, we tried to fabricate heat-treated PAN hollow fiber membranes that were NMP-insoluble. The goal of this investigation was to fabricate a pervaporation membrane for an aqueous NMP solution through heat treatment of NMPsoluble pristine PAN hollow fiber membranes. The effect of feed composition and feed solution temperature on the separation performances were also investigated in this investigation.
2. Experimental 2.1. Materials A PAN polymer was used as the matrix of a hollow fiber membrane, which was supplied by Tong-Hua Synthesis Fiber Co., Ltd. (Taiwan). The solvent N,N-dimethylformamide (DMF) was of reagent grade and used without further purification. Water was used as the external coagulant and bore liquid during the spinning of PAN hollow fiber membranes.
2.2. Fabrication of PAN hollow fiber PAN hollow fiber membranes were fabricated by the wet-spinning process. A PAN polymer was dissolved in DMF to form a 22 wt% dope solution. The degassed dope solution was extruded through an orifice-in-tube spinneret. The dimensions of this spinneret were 0.83 and 0.53 mm in outer diameter (o.d.) and inner diameter (i.d.), respectively. The bore liquid, water, was delivered using a syringe pump (500D from ISCO Inc., USA) with a flow rate of 1.0 ml/min. The degassed homogenous PAN dope solution and bore liquid were extruded through the spinneret die to form a nascent hollow fiber membrane, and then it was immersed in a water bath immediately. The hollow fiber membrane was washed with fresh water for at least three days to remove the residual solvent. Table 1 reveals the detailed process parameters and spinning conditions. 2.3. Heat treatment of PAN hollow fiber PAN hollow fiber membrane precursors were heat treated in an air atmosphere at different temperatures by using an oven. The heat-treatment processes are shown in Table 2. 2.4. Characterization PAN hollow fiber membranes were characterized with various analytical methods. The morphology of the PAN hollow fiber membrane was observed with SEM (Hitachi Co., Model S-4800). Thermogravimetric analysis (TGA, PERKIN ELMER, Model TGA 7) was performed at a rate of 10 °C/min in the temperature range of 35– 500 °C. 2.5. Pervaporation experiments The hollow fiber module for pervaporation test consisted of five hollow fiber membranes. 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 a previous work [4]. An aqueous NMP feed solution was pumped into the shell side of the module, and the permeates came out of the lumen of the fibers. The permeation rate was determined by
P ¼ W=ðA tÞ where P, W, A, and t represent the permeation rate (g/m2 h), weight of permeate (g), effective hollow fiber area based on the outside diameter of the fiber (m2), and operation time (h), respectively. A vacuum pump maintained the downstream pressure at 3–5 mmHg. The permeation rate was determined by dividing the measured weight of the permeate by the sampling time. The compositions
Table 1 Parameters for wet spinning of pristine PAN hollow fiber membranes. Parameter
Value
Dope solution composition Dope solution temperature Bore liquid External coagulant Dope extrusion pressure Bore liquid flow rate Coagulant temperature Spinneret diameter (mm)
22 wt% PAN/DMF 60 °C H2O H2O 2.5 atm 1 ml/min 50 °C OD/ID = 0.83/0.53
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Table 2 The heat-treatment process of PAN hollow fiber membranes.
of the feed solution and permeate were measured by gas chromatograghy (GC, China Chromatography 8700).
3. Results and discussion 3.1. Membrane characterization NMP has a powerful solvency to dissolve many polymers in preparing polymeric membranes such as PAN, polysulfone, polyethersulfone, cellulose acetate, and PVDF. In this paper, we tried to prepare heat-treated PAN hollow fiber membranes to be used to separate an aqueous NMP solution by pervaporation. The thermal properties of PAN hollow fiber membranes tested by an integrated TGA and thermal analysis/mass spectrometry (TA/MS) have been shown in our previous work [20]. Figs. 1 and 2 show TGA and TA/MS spectra for PAN hollow fiber membranes, respectively, which were heated at a heating rate of 10 °C/min. From these figures, it can be found that during heating, the weight of the PAN hollow fiber membrane decreases with increasing temperature. Moreover, there are four peaks at 14, 16, 28, and 32 amu. It depicts that some of the N, CH4, O, N2, CN, or O2 might be burnt away during the heating process from room temperature to 300 °C at a heating rate of 10 °C/min. To evidence the mechanism of heat100
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Temperature ( C) Fig. 1. Thermal analysis of a PAN hollow fiber membrane (heating rate: 10 °C/min).
treatment process, FT-IR spectra of pristine and 210 °C heat-treated PAN hollow fiber membranes are shown in the Fig. 3. As can be seen in Fig. 3, the CH2 bonds at 2940 cm1 and the CN bonds at 2240 cm1 exist in the heat-treatment free PAN hollow fiber membrane, while these peaks disappear in the case of heat-treated at temperatures higher than 210 °C. It depicts that the dehydrogenation and cyclization reactions is occurred during heat-treated at temperatures higher than 210 °C. NMP is a good solvent for the PAN polymer. Thus, it is impossible to separate NMP mixtures through pristine PAN hollow fiber membranes by pervaporation. However, some of the N, CH4, O, N2, CN, or O2 might be burnt away during the heating process; thus, NMP would not dissolve the heat-treated PAN hollow fiber membrane, since an inorganic-like PAN hollow fiber membrane might be made.
3.2. Morphology studies Pristine PAN hollow fiber membranes were heat treated in an air atmosphere at different temperatures by using a furnace. Figs. 4 and 5 represent the overall and outer-edge cross-sectional morphology of PAN hollow fiber membranes, which were heat treated for 12 h at various temperatures. The diameter of these hollow fiber membranes is listed in Table 3. From Figs. 4 and 5, it can be found that the macrovoids morphology of the pristine PAN hollow fiber membrane does not disappear even for the membranes heat-treated at temperatures higher than 300 °C. However, it is obviously to find that the morphology becomes dense after the heat treatment as comparing Fig. 5A with B–H. In our previous work [20], we have demonstrated the Doppler broadened energy spectrum (DBES) data of PAN hollow fiber membranes obtained by PAS experiment with a variable monoenergy slow positron beam. It has shown that the S parameter and R parameter variation of heat-treated PAN hollow fiber membranes decreases with increasing heat-treatment temperature. That is to say, the free volume of heat-treated PAN hollow fiber membranes decreases with increasing heat-treatment temperature. The S parameter variation of heat-treated PAN hollow fiber membranes which heat-treatment temperature were high than 210 °C are almost no change at positron incident energies of 0– 30 keV. These phenomena might be due to the fact that the positron
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Fig. 2. TA/MS spectra for a PAN hollow fiber membrane (heating rate: 10 °C/min).
T%
B
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Wavenumber (cm ) Fig. 3. FT-IR spectra of PAN hollow fiber membranes (A) without heat-treatment, (B) 210 °C heat-treatment.
was annihilated directly on the surface of PAN hollow fiber membrane due to high surface electron density resulted from the highly carbon cyclic structure after high temperature heat-treatment. Furthermore, as depicted in Table 3, the diameter of the PAN hollow fiber membranes is smaller when the temperature at which the heat treatment occurs is higher. These phenomena after the heat treatment might be due to the following: the morphology of the PAN hollow fiber membranes becomes denser, and the free volume in the PAN polymer chain is reduced [19,20]. 3.3. Effect of heat treatment on the pervaporation performances Because NMP is a good solvent for the PAN polymer, the pristine PAN hollow fiber membrane cannot be used for the pervaporation of NMP mixtures; hence, we determined the pervaporation possi-
bility of NMP mixtures through heat-treated PAN hollow fiber membranes. However, as we used PAN hollow fiber membranes heat-treated at 120 °C and 160 °C to test the pervaporation performance with an aqueous NMP solution, the modules were broken resulting from the dissolution of the organic-rich heat-treated PAN hollow fiber membranes in the NMP solution. The pervaporation performance with a 30 wt% aqueous NMP solution permeating through the PAN hollow fiber membranes heat-treated at temperatures higher than 160 °C is shown in Fig. 6. As can be seen in Fig. 6, the permeation flux is higher when the temperature in the range of 180–300 °C at which the heat treatment occurs is higher, and it is almost constant when the temperature at which the heat treatment occurs is higher than 300 °C. As for the water content of permeate, it is all 100 wt% water concentration in permeate, regardless of the temperature at which the heat treatment occurs. Theoretically, the morphology of heat-treated PAN hollow fiber membranes should become denser at higher temperatures at which the heat treatment occurs, resulting in a lower permeation flux. However, the permeation flux is higher when the temperature in the range of 180–300 °C at which the heat treatment occurs is higher, as shown in Fig. 6. These phenomena might be contributed by the change in the morphology. The surface morphology of PAN hollow fiber membranes heat-treated at different temperatures is shown in Fig. 7. As can be found in Fig. 7, one can observe that there are many cracks on the surface of PAN hollow fiber membranes heat-treated at higher temperatures, resulting in a higher permeation flux. Furthermore, the thickness data of heat-treated PAN hollow fiber membranes listed in Table 1 also depict the decreased thickness of the heat-treated PAN hollow fiber membranes when the temperature at which the heat treatment occurs is increased. The resistance experienced by the permeant will decrease as the thickness of PAN hollow fiber membranes decreases, leading to the permeation flux increase.
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Fig. 4. The overall cross-sectional morphology of PAN hollow fiber membranes heat-treated at different temperatures. (100) (A) Pristine, (B) 180 °C, (C) 210 °C, (D) 240 °C, (E) 270 °C, (F) 300 °C, (G) 330 °C, (H) 360 °C.
Fig. 6 also represents the water content of permeate kept at 100 wt% water concentration in permeate, regardless of the temperature at which the heat treatment occurs. These phenomena might be due to the following factor: the molar volume of the water molecules is smaller than that of the NMP molecules. (The molar volume of water and NMP molecules are 18 and 96.4 cm3/ mol, respectively.) Thus, it is hard for the NMP molecule to permeate through the membrane compared to the water molecule.
3.4. Effect of feed concentration on the vapor permeation performances In the process of pervaporation, the feed solution contacts the membrane surface directly. Vapor permeation can avoid the disadvantages caused by pervaporation. Through the vapor permeation technique, the feed solution vaporizes first and then permeates through the membrane. Consequently, swelling or shrinking of
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Fig. 5. The outer-edge cross-sectional morphology of PAN hollow fiber membranes heat-treated at different temperatures. (10 k) (A) Pristine, (B) 180 °C, (C) 210 °C, (D) 240 °C, (E) 270 °C, (F) 300 °C, (G) 330 °C, (H) 360 °C.
Table 3 List of the diameter and thickness of PAN hollow fiber membranes heat-treated at various temperatures. Temperature (°C) Diameter (lm) Thickness (lm)
Pristine 738 86.3
180 570 61.8
210 513 60.9
240 458 58.3
270 423 53.9
300 383 53.6
330 328 45.7
360 273 43.1
polymer membranes caused by the direct contact with the feed solution can be prevented. To prevent the high concentration of NMP from dissolving or swelling the hollow fiber membrane module during pervaporation, vapor permeation was performed to investigate the effect of feed concentration on the separation performance.
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Heat-treatment temperature ( C) Fig. 6. Effect of temperature at which the heat treatment occurs on the pervaporation performance with 30 wt% aqueous NMP solution at 25 °C.
The effect of the feed NMP concentration at 25 °C on the vapor permeation performance of the PAN hollow fiber membranes heat-
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treated at 300 °C is shown in Fig. 8. The data in Fig. 8 demonstrate that the permeation flux decreases with increasing NMP concentration in the feed. These results might be due to the fact that the PAN hollow fiber membranes heat-treated at 300 °C just let only the water molecules to permeate, while the NMP molecules in the feed side are restrained from permeating. As the NMP concentration in the feed increases, the water concentration decreases, resulting in the permeation flux to decrease. 3.5. Effect of feed temperature on the pervaporation performances The effect of the feed temperature on the pervaporation performance with 30 wt% aqueous NMP solution permeating through PAN hollow fiber membranes heat-treated at 300 °C is shown in Fig. 9. The data in Fig. 9 show that the permeation flux increases and the water content of permeate remains almost 100 wt% with increasing feed solution temperature. Generally, the pervaporation operation feed solution temperature can affect not only the permeation transport behavior but also the membrane structure. In general, the driving force for mass transport increases as increasing
Fig. 7. The outer surface morphology of PAN hollow fiber membranes heat-treated at different temperatures. (A) Pristine, (B) 180 °C, (C) 210 °C, (D) 240 °C, (E) 270 °C, (F) 300 °C.
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0
Water content in permeate (wt.%)
Fig. 8. Effect of the feed concentration on the vapor permeation performance of PAN hollow fiber membranes heat-treated at 300 °C.
O
Feed temperature ( C) Fig. 9. Effect of the feed temperature on the pervaporation performance with a 30 wt% aqueous NMP solution permeating through PAN hollow fiber membranes heat-treated at 300 °C.
feed temperature, which represents the concentration gradient resulting from a difference in the partial vapor pressure of the permeants between the feed and permeating mixture. The vapor pressure in the feed compartment increases while the vapor pressure at the permeate side is not affected as increasing the feed temperature. All these results, in an increase of driving force due to increase in temperature. As increasing the feed temperature, the mass transfer coefficients for both water and NMP should increase with increasing feed temperature. However, as shown in Fig. 9, the increase in the mass transfer coefficient for both water and NMP lets only the water molecules to permeate easily; the NMP mole-
cules still cannot permeate through the fiber module. Moreover, the polymer chain of the membrane becomes more flexible with increasing feed temperature, resulting in an increase in the water molecules permeation. The activation energy of diffusion is calculated according to the Arrhenius plot.
J ¼ J 0 exp
Ep RT
where J0 and EP are pre-exponential factor and activation energy for permeation, respectively; R the gas constant and T the temperature in kelvin. From ln(J) versus 1/T plot, the activation energy of permeants in membrane was determined from the slope of Arrhenius plot. The activation energy of an aqueous NMP solution permeating through the PAN hollow fiber membrane heat-treated at 300 °C is 3.65 kcal/mol (15.27 kJ/mole) for 30 wt% NMP concentration in feed mixtures. Higher activation energy means that molecules require higher energy to penetrate through the membrane. Compared with that in literature, as shown in Table 4, the smaller activation energy values observed for the PAN heat-treated hollow fiber membrane than the other membrane suggests that the energy required to cross the barrier decreased, thereby allowing more of water molecules to readily permeate across over the barrier through the PAN heat-treated hollow fiber membrane.
4. Conclusions A high separation performance with an aqueous NMP solution permeating through heat-treated PAN hollow fiber membranes was attained in this study. The as-spun PAN hollow fiber membranes could be dissolved by an NMP solution. However, the heat-treated PAN hollow fiber membranes heat-treated at temperatures higher than 180 °C for 12 h were insoluble in NMP. The pervaporation results of a 30 wt% aqueous NMP solution showed that the permeation flux was higher when the temperature in the range of 180–300 °C at which the heat treatment occurred was higher, and it was almost constant when the temperature at which the heat treatment occurred was higher than 300 °C. As for the water content of permeate, it was all 100 wt% water concentration in permeate for the PAN hollow fiber membranes heat-treated in the temperature range from 180 to 360 °C. In addition, the effect of feed composition and feed solution temperature on vapor permeation and pervaporation performances was also investigated. The permeation rate decreased with increasing NMP concentration in the feed solution and increased with increasing feed solution temperature. The activation energy of the aqueous NMP solution permeating through the PAN hollow fiber membrane heat-treated at 300 °C was 3.65 kcal/mol for 30 wt% NMP concentration in feed mixtures.
Table 4 Comparison of the activation energy data with literature results. Membrane
Feed solution
Activation energy
Ref.
Polyurethaneurea membrane High-silica CHA-type tubular membranes PVA–silicone based hybrid membranes (PVA/TEOS:1/2) CA/PAN blend membranes Sulfonated PVA membrane Chitosan membrane PVA/PAN membrane CS–PVA/PAN membrane CS/PAN hollow fiber membrane CS/PSf hollow fiber membrane PAN heat-treated hollow fiber membrane
1000 ppm NMP 50 wt% NMP 90 wt% Acetic acid 99 wt% pyridine 90 wt% IPA 50 wt% EtOH 50 wt% EtOH 50 wt% EtOH 90 wt% IPA 90 wt% EtOH 30 wt% NMP
24.03 kJ/mole Water: 30 kJ/mole NMP: 12 kJ/mole 19.39 kJ/mole H2O: 29.47 kJ/mole Pyridine: 36.19 kJ/mole 25.65 kJ/mole 30.8 kJ/mole 44.5 kJ/mole 42.7 kJ/mole 22.13 kJ/mole 10.54 kJ/mole 15.27 kJ/mole
[37] [38] [39] [40] [41] [42] [42] [42] [3] [28] This study
H.-A. Tsai et al. / Separation and Purification Technology 100 (2012) 97–105
Acknowledgments The authors wish to sincerely thank the National Science Council, the Ministry of Economic Affairs, and the Ministry of Education of Taiwan, ROC, for financially supporting this work.
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