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PSF blending membrane
European Polymer Journal 39 (2003) 2367–2374 www.elsevier.com/locate/europolj
Pervaporation separation of water/ethanol mixture by TGN/PSF blending membrane Mu-Ya Hung a, Shih-Hsiung Chen a,*, Rey May Liou a, Ching-Shan Hsu a, H.A. Tsia b, Juin-Yih Lai b a
Department of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science, 60 Erh-Jen Road, Sec. 1, Tainan 717, Taiwan, ROC b Department of Chemical Engineering, Chung Yuan University, Chung Li 320, Taiwan, ROC Received 1 November 2002; received in revised form 25 July 2003; accepted 15 August 2003
Abstract The separation performance of plasticizer/polysulfone (TGN/PSF) pervaporation membrane was studied. The optimum amount of plasticizer (TGN) in PSF membranes improved the diffusion selectivity of water to ethanol, which was due to the increase in the permeate diffusion rate difference between water to ethanol molecules. On the other hand, the solubility selectivity of water to ethanol in PSF membrane showed a minor change with increasing the plasticizer content in TGN/PSF membrane. The feed ethanol concentration showed a significant influence on the degree of swelling as well as the separation performance of TGN/PSF membrane. It was found that the dominant factor of permeate transport through membranes was the diffusion rate difference, especially at high ethanol concentrations in feed. This study indicated that a good separation performance could be achieved at high ethanol concentrations in feed. This investigation also proves that the flexible polymer chain mobility, which was due to both the addition of TGN in the membrane and the swelling effect of the membrane at the high ethanol concentration in feed solution, strongly influences the separation properties of TGN/PSF membrane. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Membrane; Plasticizer; TGN; Pervaporation; Polysulfone
1. Introduction For dehydration of an organic azotropic mixture, the mass transfer barrier of permeates should be considered in the period of permeate transport through a polymer membrane. The solution-diffusion mechanism was generally applied to describe the transport behavior of permeates in pervaporation membranes. Based on the concept of solution-diffusion model, the permeate dissolves into membrane surface and then diffuses through the polymer membrane. Finally, permeates vaporize and
*
Corresponding author. Tel.: +886-6-2660028; fax: +886-62669090. E-mail address: [email protected] (S.-H. Chen).
leave the other side of membrane. The separation performance of polymer membrane was determined by the competition between solution and diffusion of permeates when it was passed through the membrane. In general, a high solubility permeates in pervaporation membrane also owned a high diffusion rate in period of permeate transport through the membrane [1]. It was proposed that the barrier of mass transfer in membranes is dominated by the solution and diffusion step in the period of pervaporation. The vaporization of permeate was proposed as a non-selective step. In many cases, the resistance of diffusion of downstream layer and sorption of upstream determined the high permselectivity membrane [2]. However, for many of the glassy polymer membranes, the permeation rate of pervaporation is determined
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.08.004
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mainly by the diffusion contribution [3]. Most of the permeate diffusion behavior in pervaporation membranes was explained by free volume model [1]. The theory described that the permeate diffusion behavior can be affected by the relative size of permeates to diffusion path and the polymer chain mobility in membranes. Therefore, many investigators have reported [4–7] that the improvement of permeation flux could be achieved by increasing the polymer chain mobility, such as blending or grafting hydrophilic polymer for improving the polymer chain mobility in pervaporation process. In those works, the low glassy transition temperature polymer was used to modify the glassy polymer and separation performance is significantly enhanced in pervaporation process. Recently, many investigations [8–11] explored the relationship between polymer chain packing efficiency of plasticizer additive membrane and separation performance of membranes. It was found that the separation performance of a polymer membrane was strongly affected by the added plasticizer. On the other hand, there are many studies, which show that the plasticizer additive also significantly affects polymer chain mobility and separation performance of gas separation membranes [12–15]. For the purpose of clearing the plasticizer additive effect on the change of properties of pervaporation membrane, this study aimed at preparing TNG/PSF membrane to separate water from a water/ethanol mixture. These plasticizers were applied to enhance the polymer chain mobility of blend membrane and improved the diffusion selectivity of water to ethanol in period of pervaporation. In order to understand the effect of enhancement of polymer chain mobility on pervaporation performance of polysulfonate membrane, we measured the swelling properties of various amounts of TGN and analyzed the sorption amount in TGN/PSF membrane. The permeation properties and degree of
swelling were measured independently. Glass transition temperature (Tg ) measurement analyses were used to characterize the morphology change in the modified membrane. The relationship between microstructure change of TGN/PSF membranes and pervaporation properties is also discussed in this study.
2. Experimental 2.1. Materials Udel â Polysulfone (PSF) P-3500 and chloroform were obtained from Amoco Performance Products and Merck Chemical Co. Bayer Co., supplied the polymer plasticizer (ULTRAMOLL TGN). The density of polymer plasticizer (ULTRAMOLL TGN) was 1.095 g/cm3 and the viscosity was 2250 mPa at 20 °C. 2.2. Membrane preparation The TGN/PSF blending membranes were prepared from a casting solution in chloroform. The casting solution was casted onto a glass plate to a predetermined thickness of 350 lm using a Gardner Knife. The membrane was dried at ambient temperature for 30 min, then peeled off and immersed in distilled water for 12 h, and finally put into a vacuum oven for 24 h before sorption and pervaporation measurements. 2.3. Pervaporation experiment A traditional pervaporation process was used [16]. The pervaporation apparatus is shown in Fig. 1. In pervaporation, the feed solution of 90 wt.% ethanol solution was in direct contact with the membrane and was kept at 25 °C. The effective membrane area was
Fig. 1. Schematic diagram of pervaporation apparatus.
aA=B ¼ ðYA =YB Þ=ðXA =XB Þ where, XA , XB and YA , YB are the weight fractions of A and B in the feed and permeate, respectively (A being the more permeative species).
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10.2 cm2 . The downstream pressure was maintained at about 5–8 Torr. The permeation rate was determined by measuring the weights of permeate. The compositions of permeate and solution adsorbed in the membranes was measured by gas chromatography (GC, China Chromatography). The separation factor, aA=B , was calculated by the formula:
Permeation flux (g/m2h)
M.-Y. Hung et al. / European Polymer Journal 39 (2003) 2367–2374
2.4. Sorption measurements 0
The membranes were immersed in the ethanol–water mixture for 24 h at 25 °C. They were subsequently blotted between tissue papers to remove the excess solvent and were then placed in the left half of a twin tube setup. The system was evacuated while the tube was heated with hot water for 30 min and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid in the right tube was determined by GC. The separation factor of sorption was calculated by: asorp ¼ ðYw =Ye Þ=ðXw =Xe Þ where, Xe , Xw and Ye , Yw are the weight fractions of ethanol and water in the feed and membranes, respectively.
3. Results and discussion 3.1. Effect of polymer plasticizer (TGN) additive on separation performance of PSF membrane In this study, the polymer plasticizer (TGN) was added to polysulfone membrane for improving the separation performance. The effect of polymer plasticizer content in the membrane on separation performance of TGN/PSF composite membranes is shown in Fig. 2. It was found that the polymer plasticizer (TGN) content in composite membrane significantly affected the separation factor. However, the TGN content was only slightly affected by the permeation flux of composite membrane. It was found that the permeation flux was almost independent of the TGN content in the membrane. However, the separation factor increased with increasing TGN contents in composite membrane and the separation factor increased up to 6.25 wt.% of TGN content in membrane and then decreased with the increase in the TGN composition in composite membranes. Generally, it can be expected that TGN additive enhances the flexibility of polymer chain in composite membrane and it would lead to an increase in permeation flux and a
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TGN composition (wt%)
Fig. 2. Effect of TGN content on pervaporation performance of TGN/PSF membranes for 90 wt.% ethanol solution in feed at 25 °C.
decrease in diffusion selectivity. However, the addition of TGN in polysulfone membrane is against this expectation. The possible reasons to explain this phenomenon is that a good arrangement of polymer chain was formed in blending membrane with the appreciable amount of TGN content in membranes. A good polymer chain arrangement in TGN/PSF membrane can be expected to decrease the permeation rate and enhance the diffusion selectivity of membranes. As shown in Fig. 2, the decrease in permeation flux and an increase in separation factor were found while the TGN contents 6.25% in membranes. With higher TGN contents added in composite membrane, the separation factor decreased and a slight increase in permeation flux can be seen in TGN/PSF membranes. The addition of low molecular plasticizer in the membrane enhanced the polymer chain flexibility. Due to the flexible polymer chain in membranes, the swelling and permeation properties of the membrane would be increased at a higher plasticizer content in membrane. To further prove the above viewpoint, the Tg and swelling properties were measured to clarify the relationship between flexibility of polymer chain and TGN content in membranes. 3.2. Effect of TGN content on polymer chain flexibility The effect of TGN composition on glassy transition temperature (Tg ) of TGN/PSF membranes is shown in Fig. 3. It can be seen that the glassy transition temperature of TGN additive membrane decreases with an increase in the TGN content, it was indicated that a more flexible polymer chain can be achieved by adding TGN into PSF membrane. It was evidenced that the more flexible polymer chain could be obtained by
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at 90 wt.% ethanol solution. Based on these results of Tg and swelling properties, it can be concluded that the polymer chain flexibility would be improved by adding TGN into polymer matrix. It also can be indicated that the free volume of blending membrane would be increased because of the high degree of swelling of blending membrane in feed solution.
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Fig. 3. Effect of TGN content on glassy transition temperature of TGN/PSF membranes.
increasing the TGN content in PSF membrane. For the purpose of understanding the swelling behavior of TGN/PSF membrane in 90% ethanol solution, the swelling tests of TGN/PSF membranes were made at the same feed solution. Fig. 4 shows the effect of TGN additive on the degree of swelling of TGN/PSF membrane for a 90 wt.% ethanol solution. It can be seen that the degree of swelling showed a strong dependence on the amount of TGN content in TGN/PSF membranes at 90% ethanol solution. The degree of swelling increased with increasing TGN contents in TGN/PSF membrane. The increase in degree of swelling indicates that the flexibility of polymer chain in blending membrane would be increased due to the effect of permeate plasticization
In order to clarify the influential factors on the separation performance of TGN/PSF membrane, the effect of TGN/PSF composition on water or ethanol content in TGN/PSF membrane was investigated. The effect of TGN composition on ethanol and water content in membrane is shown in Fig. 5. It can be seen that the water content decreased with the increase in the TGN composition in membrane. It was also indicated that the addition of TGN into polysulfone benefited the ethanol content at 90% ethanol solution. The sorption selectivity can be calculated by measuring the permeate content in membrane. If the water content measurement showed a significant large than ethanol content in membrane, it was implied that the sorption selectivity showed be increased due to the higher water sorption in membrane. Based on the viewpoint of sorption difference [17], the difference of water and ethanol content in membrane dominated that sorption selectivity of blending membranes. Based on the result of Fig. 6, the higher the TGN content in the membrane, the more was the ethanol content expected in membrane. The sorption selectivity could be calculated by dividing the ratio of water content to ethanol content to the ratio of feed composition
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Fig. 4. Effect of TGN content on degree of swelling of TGN/ PSF membranes in 90 wt.% ethanol solution.
Fig. 5. Effect of TGN content on permeate content in TGN/ PSF membranes for immersing in 90 wt.% ethanol solution at 25 °C.
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Fig. 6. Effect of TGN content on sorption selectivity of TGN/ PSF membrane for 90 wt.% ethanol solution in feed at 25 °C.
Fig. 7. Effect of TGN content on diffusivity selectivity of TGN/ PSF membrane for 90 wt.% ethanol solution in feed at 25 °C.
of water to ethanol. It was indicated that the sorption selectivity decreased with increasing the TGN content in membrane. Based on the analysis of the permeate content in membranes, it can be noted that the decrease in sorption selectivity was contributed by the increase of TGN composition in membrane. However, as shown in Fig. 2, the separation factor first increased with increasing TGN contents in composite membrane and then decreased with higher TGN contents in membranes. Based on the concept of solution-diffusion mechanism, influential factor on separation factors can be considered to be both the permeate sorption and permeate diffusion properties of TGN/PSF membrane. The diffusion selectivity (ad ) can be defined as the ratio of permeation selectivity (ap ) and sorption selectivity (as ):
selectivity was mainly contributed by the higher polymer chain flexibility. It is worth noting that the effect of TGN contents on separation factor and diffusion selectivity in blending membrane showed the same trend and the strong dependency between separation factor and diffusion behavior can be concluded in this study. In other words, the addition of TGN in PSF membranes enhanced the polymer chain and significantly affect the permeation and diffusion behavior of permeate in pervaporation. It was indicated that the dominant separation factor was diffusion selectivity but not solubility selectivity. It can be concluded that the optimum amount of TGN additive in PSF membrane enhanced both the polymer chain mobility and diffusion selectivity of TGN/PSF membranes.
ad ¼ ap =as
3.4. Effect of feed concentration on pervaporation properties
ð1Þ
Therefore, the decrease of separation factor in higher TGN content may be due to the decrease of diffusion selectivity while the permeate transport through the composite membranes. Fig. 7 shows the relationship between diffusion selectivity and TGN content in composite membranes. The diffusion selectivity first increased and then decreased with increasing TGN contents in composite membrane. This result implies that the diffusion behavior of permeates is strongly affected by the TGN additive membranes in pervaporation process. It was implied that the addition of TGN improved the flexibility of polymer chain in TGN/PSF membrane. However, the higher the TGN content in the membrane, the more flexible was the polymer chain observed in blending membranes with TGN contents higher than 12.5%. The decrease in permeate diffusion
The effect of ethanol composition in feed on permeation flux and separation factor of PEG/PSF membranes with a TGN content 12.5 wt.% is shown in Fig. 8. It can be seen that the permeation flux slightly changed with the increase in ethanol concentration in feed. However, the separation factor increased with an increase in the ethanol concentration in feed. It was indicated that permeate concentration did not affect the permeation flux of blending membrane. It is worth noting that the significant increase in separation factor can be observed at high ethanol composition. The permeation behavior can be strongly affected by the swelling properties of TGN/PSF membranes at various concentrations of feed solution. Fig. 9 shows the effect of feed ethanol concentration on the degree of swelling of
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In order to calculate the permeate content in TGN/ PSF membrane at various ethanol concentrations, the water and ethanol contents in membrane were measured and it is shown in Fig. 10. It was found that the water content in membrane decreased with increasing ethanol concentrations in feed solution. The sorption selectivity and diffusion selectivity can be calculated by the previous equation (Eq. (1)). Figs. 11 and 12 showed the effect of ethanol concentration in feed on the sorption selec-
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Fig. 8. Effect of feed ethanol composition in feed on permeation flux and separation factor of TGN/PSF membrane with 12.5 wt.% TGN content, feed solution was at 25 °C.
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Fig. 10. Effect of feed ethanol composition on sorption content in TGN/PSF membranes with 12.5 wt.% TGN content at 25 °C.
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Fig. 9. Effect of feed ethanol composition on degree of swelling of TGN/PSF membranes with 12.5 wt.% TGN content at 25 °C.
TGN/PSF membrane, which contains 12.5% TGN. It was indicated that the degree of swelling was less than 10% in all of the concentration range. Based on the swelling test, it can be concluded that the permeation rate was almost independent with the feed ethanol concentration. This phenomenon may be contributed to the low degree of swelling in all concentration ranges of feed solution. On the other hand, the feed concentration showed a significant influence on the separation factor of the blending membranes. In order to distinguish the influent factors of separation factor at various ethanol concentrations in feed, the sorption and diffusion behavior were discussed in the following section.
Sorption selectivity
Ethanol composition (%)
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Fig. 11. Effect of ethanol concentration in feed on sorption selectivity of TGN/PSF membrane with 12.5 wt.% TGN content at 25 °C.
M.-Y. Hung et al. / European Polymer Journal 39 (2003) 2367–2374
amount of water in blending membrane was more than the amount of ethanol in membranes in all concentration range. Therefore, a high sorption selectivity of TGN/PSF membrane can be achieved by controlling the optimum swelling of membranes. It is suggested that the good separation performance of TGN/PSF membrane should be under at high ethanol concentration in feed.
Diffusion selectivity
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4. Conclusions
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Fig. 12. Effect of feed ethanol concentration on diffusion selectivity of TGN/PSF membrane with 12.5 wt.% TGN content at 25 °C.
tivity and diffusivity selectivity of TGN/PSF membrane with 12.5 wt.% TGN content. It was indicated that the sorption selectivity increased with increasing ethanol concentration in the feed solution. On the other hand, the diffusion selectivity of permeates increased with increasing ethanol concentrations in feed solution. It can be seen that both sorption selectivity and diffusion selectivity increased with the increase in the ethanol concentration. It implies that the additive TGN improves permeate selectivity with increasing ethanol concentrations in feed solution. It is interesting to note that both sorption and diffusion selectivity increased in all feed ethanol concentrations. Generally, the selectivity of sorption behavior usually showed a different trend than the diffusion behavior when the permeate passes through the membranes. To further distinguish these results, the swelling test and sorption measurements at different ethanol concentrations in feed were considered. It was shown that the degree of swelling of TGN/PSF membrane only slightly increased with the increase in ethanol concentration. Generally, the polymer chain flexibility of swelling membrane is usually dependent on the degree of swelling. The significant increase in polymer chain flexibility induced an increase in permeate diffusion path when the permeate transfers through the membranes. Based on the result of swelling test, it may be expected that the increase in permeation flux would be slightly increased with the increase in ethanol concentration in feed. The result of permeation test at various ethanol concentrations in feed showed a similar behavior as expected. On the other hand, it can be seen that the increase in sorption selectivity increased with increasing ethanol concentrations in feed. It was found that the
TGN additive successfully improved the pervaporation performance of polysulfone membrane. Tg and swelling measurement proved that the polymer chain flexibility would be improved by the addition of TGN into membrane whether or not the composite membrane was dry or immersed in 90 wt.% ethanol solution. The polymer chain flexibility and packing density of polymer chain in membrane increased with the increase in the TGN content in TGN/PSF membranes at low TGN content. However, the higher polymer packing density can be expected to decrease the permeation rate and enhance the diffusion selectivity of modified membranes. Based on the result of this study, it can be concluded that a high selective membrane could be achieved by controlling the optimum swelling condition in TGN additive membranes.
Acknowledgements The authors wish to thank the National Science Council of ROC (NSC 90-2216-E-041-005) for their financial support for this research.
References [1] Huang RYM. Pervaporation separation process. Elsevier Science Publishers; 1991. [2] Ghazali M, Nawawi M, Huang RYM. Pervaporation dehydration of isorpopanol with chitosan membranes. J Memb Sci 1997;124:53–62. [3] Wang XP, Shen ZQ, Zhang FY, Zhang YF. Preferential separation of ethanol from aqueous solution through hydrophilic polymer membranes. J Appl Polym Sci 1999;73(7):1145–51. [4] Shieh JJ, Huang RYM. Chitosan/N-methylol Nylon 6 blend membranes for the pervaporation separation of ethanol–water mixtures. J Memb Sci 1998;148:243–55. [5] Luo GS, Niang M, Schaetzel P. Sorption and pervaporation separation of ethyl tert-butyl ether and ethanol mixture through a blended membrane. J Appl Polym Sci 1997;66(9):1631–8. [6] Feng X, Huang RYM. Pervaporation with chitosan membranes. I. Separation of water from ethylene glycol
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[8] [9] [10]
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by a chitosan/polysulfone composite membrane. J Memb Sci 1996;116:67–76. Shieh JJ, Huang RYM. Pervaporation with chitosan membranes. II. Blend membranes of chitosan and polyacrylic acid and comparison of homogenous and composite membrane base on polyelectrolyte complexes of chitosan and polyacrylic acid for the separation of ethanol–water mixtures. J Membr Sci 1997;127:185–202. Ghosal K, Freeman BD. Gas separation using polymer membrane: an overview. Polym Adv Technol 1994;5:673. Koros WJ, Fleming GK. Membrane-base gas separation. J Memb Sci 1993;83:1. Morisato A, Ghosal K, Freeman BD, Chern RT, Alvarez JC, dela Campa JG, et al. Gas separation properties of aromatic polyamides containing hexafluroisopropylidene groups. J Memb Sci 1995;104:231. Lee WM. Selection of barrier material from molecular structure. Polym Eng Sci 1980;20:65.
[12] Semenova SI, Ohya H, Smirnov SI. Physical transitions in polymers plasticized by interacting penetrant. J Memb Sci 1997;136:1–11. [13] Semenova SI, Smirnov SI. Mass transfer of a penetrant plasticizer/simple gas mixture in a block copolymer. J Memb Sci 2000;168:1–11. [14] Sun YM, Huang WF, Chang CC. Spray-coated and solution-cast ethylcellulose pseudolatex membrane. J Memb Sci 1999;157:159–70. [15] Houde AY, Kulkarni SS, Kharul UK, Charati SG, Kulkarni MG. Gas permeation in polyarylates: effects of polarity and intersegmental mobility. J Memb Sci 1995;103:167–74. [16] Lee KR, Lai JY. Dehydration of acetic acid–water mixture by pervaporation with a modified poly(4-methyl-1-pentent) membrane. J Polym Res 1994;1:247–54. [17] Van Krevelen DW. Properties of polymers. Amsterdam: Elsevier; 1990.
Pervaporation separation of water/ethanol mixture by TGN/PSF blending membrane Mu-Ya Hung a, Shih-Hsiung Chen a,*, Rey May Liou a, Ching-Shan Hsu a, H.A. Tsia b, Juin-Yih Lai b a
Department of Environmental Engineering and Health, Chia-Nan University of Pharmacy and Science, 60 Erh-Jen Road, Sec. 1, Tainan 717, Taiwan, ROC b Department of Chemical Engineering, Chung Yuan University, Chung Li 320, Taiwan, ROC Received 1 November 2002; received in revised form 25 July 2003; accepted 15 August 2003
Abstract The separation performance of plasticizer/polysulfone (TGN/PSF) pervaporation membrane was studied. The optimum amount of plasticizer (TGN) in PSF membranes improved the diffusion selectivity of water to ethanol, which was due to the increase in the permeate diffusion rate difference between water to ethanol molecules. On the other hand, the solubility selectivity of water to ethanol in PSF membrane showed a minor change with increasing the plasticizer content in TGN/PSF membrane. The feed ethanol concentration showed a significant influence on the degree of swelling as well as the separation performance of TGN/PSF membrane. It was found that the dominant factor of permeate transport through membranes was the diffusion rate difference, especially at high ethanol concentrations in feed. This study indicated that a good separation performance could be achieved at high ethanol concentrations in feed. This investigation also proves that the flexible polymer chain mobility, which was due to both the addition of TGN in the membrane and the swelling effect of the membrane at the high ethanol concentration in feed solution, strongly influences the separation properties of TGN/PSF membrane. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Membrane; Plasticizer; TGN; Pervaporation; Polysulfone
1. Introduction For dehydration of an organic azotropic mixture, the mass transfer barrier of permeates should be considered in the period of permeate transport through a polymer membrane. The solution-diffusion mechanism was generally applied to describe the transport behavior of permeates in pervaporation membranes. Based on the concept of solution-diffusion model, the permeate dissolves into membrane surface and then diffuses through the polymer membrane. Finally, permeates vaporize and
*
Corresponding author. Tel.: +886-6-2660028; fax: +886-62669090. E-mail address: [email protected] (S.-H. Chen).
leave the other side of membrane. The separation performance of polymer membrane was determined by the competition between solution and diffusion of permeates when it was passed through the membrane. In general, a high solubility permeates in pervaporation membrane also owned a high diffusion rate in period of permeate transport through the membrane [1]. It was proposed that the barrier of mass transfer in membranes is dominated by the solution and diffusion step in the period of pervaporation. The vaporization of permeate was proposed as a non-selective step. In many cases, the resistance of diffusion of downstream layer and sorption of upstream determined the high permselectivity membrane [2]. However, for many of the glassy polymer membranes, the permeation rate of pervaporation is determined
0014-3057/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.08.004
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M.-Y. Hung et al. / European Polymer Journal 39 (2003) 2367–2374
mainly by the diffusion contribution [3]. Most of the permeate diffusion behavior in pervaporation membranes was explained by free volume model [1]. The theory described that the permeate diffusion behavior can be affected by the relative size of permeates to diffusion path and the polymer chain mobility in membranes. Therefore, many investigators have reported [4–7] that the improvement of permeation flux could be achieved by increasing the polymer chain mobility, such as blending or grafting hydrophilic polymer for improving the polymer chain mobility in pervaporation process. In those works, the low glassy transition temperature polymer was used to modify the glassy polymer and separation performance is significantly enhanced in pervaporation process. Recently, many investigations [8–11] explored the relationship between polymer chain packing efficiency of plasticizer additive membrane and separation performance of membranes. It was found that the separation performance of a polymer membrane was strongly affected by the added plasticizer. On the other hand, there are many studies, which show that the plasticizer additive also significantly affects polymer chain mobility and separation performance of gas separation membranes [12–15]. For the purpose of clearing the plasticizer additive effect on the change of properties of pervaporation membrane, this study aimed at preparing TNG/PSF membrane to separate water from a water/ethanol mixture. These plasticizers were applied to enhance the polymer chain mobility of blend membrane and improved the diffusion selectivity of water to ethanol in period of pervaporation. In order to understand the effect of enhancement of polymer chain mobility on pervaporation performance of polysulfonate membrane, we measured the swelling properties of various amounts of TGN and analyzed the sorption amount in TGN/PSF membrane. The permeation properties and degree of
swelling were measured independently. Glass transition temperature (Tg ) measurement analyses were used to characterize the morphology change in the modified membrane. The relationship between microstructure change of TGN/PSF membranes and pervaporation properties is also discussed in this study.
2. Experimental 2.1. Materials Udel â Polysulfone (PSF) P-3500 and chloroform were obtained from Amoco Performance Products and Merck Chemical Co. Bayer Co., supplied the polymer plasticizer (ULTRAMOLL TGN). The density of polymer plasticizer (ULTRAMOLL TGN) was 1.095 g/cm3 and the viscosity was 2250 mPa at 20 °C. 2.2. Membrane preparation The TGN/PSF blending membranes were prepared from a casting solution in chloroform. The casting solution was casted onto a glass plate to a predetermined thickness of 350 lm using a Gardner Knife. The membrane was dried at ambient temperature for 30 min, then peeled off and immersed in distilled water for 12 h, and finally put into a vacuum oven for 24 h before sorption and pervaporation measurements. 2.3. Pervaporation experiment A traditional pervaporation process was used [16]. The pervaporation apparatus is shown in Fig. 1. In pervaporation, the feed solution of 90 wt.% ethanol solution was in direct contact with the membrane and was kept at 25 °C. The effective membrane area was
Fig. 1. Schematic diagram of pervaporation apparatus.
aA=B ¼ ðYA =YB Þ=ðXA =XB Þ where, XA , XB and YA , YB are the weight fractions of A and B in the feed and permeate, respectively (A being the more permeative species).
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10.2 cm2 . The downstream pressure was maintained at about 5–8 Torr. The permeation rate was determined by measuring the weights of permeate. The compositions of permeate and solution adsorbed in the membranes was measured by gas chromatography (GC, China Chromatography). The separation factor, aA=B , was calculated by the formula:
Permeation flux (g/m2h)
M.-Y. Hung et al. / European Polymer Journal 39 (2003) 2367–2374
2.4. Sorption measurements 0
The membranes were immersed in the ethanol–water mixture for 24 h at 25 °C. They were subsequently blotted between tissue papers to remove the excess solvent and were then placed in the left half of a twin tube setup. The system was evacuated while the tube was heated with hot water for 30 min and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid in the right tube was determined by GC. The separation factor of sorption was calculated by: asorp ¼ ðYw =Ye Þ=ðXw =Xe Þ where, Xe , Xw and Ye , Yw are the weight fractions of ethanol and water in the feed and membranes, respectively.
3. Results and discussion 3.1. Effect of polymer plasticizer (TGN) additive on separation performance of PSF membrane In this study, the polymer plasticizer (TGN) was added to polysulfone membrane for improving the separation performance. The effect of polymer plasticizer content in the membrane on separation performance of TGN/PSF composite membranes is shown in Fig. 2. It was found that the polymer plasticizer (TGN) content in composite membrane significantly affected the separation factor. However, the TGN content was only slightly affected by the permeation flux of composite membrane. It was found that the permeation flux was almost independent of the TGN content in the membrane. However, the separation factor increased with increasing TGN contents in composite membrane and the separation factor increased up to 6.25 wt.% of TGN content in membrane and then decreased with the increase in the TGN composition in composite membranes. Generally, it can be expected that TGN additive enhances the flexibility of polymer chain in composite membrane and it would lead to an increase in permeation flux and a
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TGN composition (wt%)
Fig. 2. Effect of TGN content on pervaporation performance of TGN/PSF membranes for 90 wt.% ethanol solution in feed at 25 °C.
decrease in diffusion selectivity. However, the addition of TGN in polysulfone membrane is against this expectation. The possible reasons to explain this phenomenon is that a good arrangement of polymer chain was formed in blending membrane with the appreciable amount of TGN content in membranes. A good polymer chain arrangement in TGN/PSF membrane can be expected to decrease the permeation rate and enhance the diffusion selectivity of membranes. As shown in Fig. 2, the decrease in permeation flux and an increase in separation factor were found while the TGN contents 6.25% in membranes. With higher TGN contents added in composite membrane, the separation factor decreased and a slight increase in permeation flux can be seen in TGN/PSF membranes. The addition of low molecular plasticizer in the membrane enhanced the polymer chain flexibility. Due to the flexible polymer chain in membranes, the swelling and permeation properties of the membrane would be increased at a higher plasticizer content in membrane. To further prove the above viewpoint, the Tg and swelling properties were measured to clarify the relationship between flexibility of polymer chain and TGN content in membranes. 3.2. Effect of TGN content on polymer chain flexibility The effect of TGN composition on glassy transition temperature (Tg ) of TGN/PSF membranes is shown in Fig. 3. It can be seen that the glassy transition temperature of TGN additive membrane decreases with an increase in the TGN content, it was indicated that a more flexible polymer chain can be achieved by adding TGN into PSF membrane. It was evidenced that the more flexible polymer chain could be obtained by
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at 90 wt.% ethanol solution. Based on these results of Tg and swelling properties, it can be concluded that the polymer chain flexibility would be improved by adding TGN into polymer matrix. It also can be indicated that the free volume of blending membrane would be increased because of the high degree of swelling of blending membrane in feed solution.
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3.3. Effect of TGN content on sorption and diffusion behavior of composite membrane
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Fig. 3. Effect of TGN content on glassy transition temperature of TGN/PSF membranes.
increasing the TGN content in PSF membrane. For the purpose of understanding the swelling behavior of TGN/PSF membrane in 90% ethanol solution, the swelling tests of TGN/PSF membranes were made at the same feed solution. Fig. 4 shows the effect of TGN additive on the degree of swelling of TGN/PSF membrane for a 90 wt.% ethanol solution. It can be seen that the degree of swelling showed a strong dependence on the amount of TGN content in TGN/PSF membranes at 90% ethanol solution. The degree of swelling increased with increasing TGN contents in TGN/PSF membrane. The increase in degree of swelling indicates that the flexibility of polymer chain in blending membrane would be increased due to the effect of permeate plasticization
In order to clarify the influential factors on the separation performance of TGN/PSF membrane, the effect of TGN/PSF composition on water or ethanol content in TGN/PSF membrane was investigated. The effect of TGN composition on ethanol and water content in membrane is shown in Fig. 5. It can be seen that the water content decreased with the increase in the TGN composition in membrane. It was also indicated that the addition of TGN into polysulfone benefited the ethanol content at 90% ethanol solution. The sorption selectivity can be calculated by measuring the permeate content in membrane. If the water content measurement showed a significant large than ethanol content in membrane, it was implied that the sorption selectivity showed be increased due to the higher water sorption in membrane. Based on the viewpoint of sorption difference [17], the difference of water and ethanol content in membrane dominated that sorption selectivity of blending membranes. Based on the result of Fig. 6, the higher the TGN content in the membrane, the more was the ethanol content expected in membrane. The sorption selectivity could be calculated by dividing the ratio of water content to ethanol content to the ratio of feed composition
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Fig. 4. Effect of TGN content on degree of swelling of TGN/ PSF membranes in 90 wt.% ethanol solution.
Fig. 5. Effect of TGN content on permeate content in TGN/ PSF membranes for immersing in 90 wt.% ethanol solution at 25 °C.
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Fig. 6. Effect of TGN content on sorption selectivity of TGN/ PSF membrane for 90 wt.% ethanol solution in feed at 25 °C.
Fig. 7. Effect of TGN content on diffusivity selectivity of TGN/ PSF membrane for 90 wt.% ethanol solution in feed at 25 °C.
of water to ethanol. It was indicated that the sorption selectivity decreased with increasing the TGN content in membrane. Based on the analysis of the permeate content in membranes, it can be noted that the decrease in sorption selectivity was contributed by the increase of TGN composition in membrane. However, as shown in Fig. 2, the separation factor first increased with increasing TGN contents in composite membrane and then decreased with higher TGN contents in membranes. Based on the concept of solution-diffusion mechanism, influential factor on separation factors can be considered to be both the permeate sorption and permeate diffusion properties of TGN/PSF membrane. The diffusion selectivity (ad ) can be defined as the ratio of permeation selectivity (ap ) and sorption selectivity (as ):
selectivity was mainly contributed by the higher polymer chain flexibility. It is worth noting that the effect of TGN contents on separation factor and diffusion selectivity in blending membrane showed the same trend and the strong dependency between separation factor and diffusion behavior can be concluded in this study. In other words, the addition of TGN in PSF membranes enhanced the polymer chain and significantly affect the permeation and diffusion behavior of permeate in pervaporation. It was indicated that the dominant separation factor was diffusion selectivity but not solubility selectivity. It can be concluded that the optimum amount of TGN additive in PSF membrane enhanced both the polymer chain mobility and diffusion selectivity of TGN/PSF membranes.
ad ¼ ap =as
3.4. Effect of feed concentration on pervaporation properties
ð1Þ
Therefore, the decrease of separation factor in higher TGN content may be due to the decrease of diffusion selectivity while the permeate transport through the composite membranes. Fig. 7 shows the relationship between diffusion selectivity and TGN content in composite membranes. The diffusion selectivity first increased and then decreased with increasing TGN contents in composite membrane. This result implies that the diffusion behavior of permeates is strongly affected by the TGN additive membranes in pervaporation process. It was implied that the addition of TGN improved the flexibility of polymer chain in TGN/PSF membrane. However, the higher the TGN content in the membrane, the more flexible was the polymer chain observed in blending membranes with TGN contents higher than 12.5%. The decrease in permeate diffusion
The effect of ethanol composition in feed on permeation flux and separation factor of PEG/PSF membranes with a TGN content 12.5 wt.% is shown in Fig. 8. It can be seen that the permeation flux slightly changed with the increase in ethanol concentration in feed. However, the separation factor increased with an increase in the ethanol concentration in feed. It was indicated that permeate concentration did not affect the permeation flux of blending membrane. It is worth noting that the significant increase in separation factor can be observed at high ethanol composition. The permeation behavior can be strongly affected by the swelling properties of TGN/PSF membranes at various concentrations of feed solution. Fig. 9 shows the effect of feed ethanol concentration on the degree of swelling of
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In order to calculate the permeate content in TGN/ PSF membrane at various ethanol concentrations, the water and ethanol contents in membrane were measured and it is shown in Fig. 10. It was found that the water content in membrane decreased with increasing ethanol concentrations in feed solution. The sorption selectivity and diffusion selectivity can be calculated by the previous equation (Eq. (1)). Figs. 11 and 12 showed the effect of ethanol concentration in feed on the sorption selec-
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Fig. 8. Effect of feed ethanol composition in feed on permeation flux and separation factor of TGN/PSF membrane with 12.5 wt.% TGN content, feed solution was at 25 °C.
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Fig. 10. Effect of feed ethanol composition on sorption content in TGN/PSF membranes with 12.5 wt.% TGN content at 25 °C.
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Fig. 9. Effect of feed ethanol composition on degree of swelling of TGN/PSF membranes with 12.5 wt.% TGN content at 25 °C.
TGN/PSF membrane, which contains 12.5% TGN. It was indicated that the degree of swelling was less than 10% in all of the concentration range. Based on the swelling test, it can be concluded that the permeation rate was almost independent with the feed ethanol concentration. This phenomenon may be contributed to the low degree of swelling in all concentration ranges of feed solution. On the other hand, the feed concentration showed a significant influence on the separation factor of the blending membranes. In order to distinguish the influent factors of separation factor at various ethanol concentrations in feed, the sorption and diffusion behavior were discussed in the following section.
Sorption selectivity
Ethanol composition (%)
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Fig. 11. Effect of ethanol concentration in feed on sorption selectivity of TGN/PSF membrane with 12.5 wt.% TGN content at 25 °C.
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amount of water in blending membrane was more than the amount of ethanol in membranes in all concentration range. Therefore, a high sorption selectivity of TGN/PSF membrane can be achieved by controlling the optimum swelling of membranes. It is suggested that the good separation performance of TGN/PSF membrane should be under at high ethanol concentration in feed.
Diffusion selectivity
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4. Conclusions
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Fig. 12. Effect of feed ethanol concentration on diffusion selectivity of TGN/PSF membrane with 12.5 wt.% TGN content at 25 °C.
tivity and diffusivity selectivity of TGN/PSF membrane with 12.5 wt.% TGN content. It was indicated that the sorption selectivity increased with increasing ethanol concentration in the feed solution. On the other hand, the diffusion selectivity of permeates increased with increasing ethanol concentrations in feed solution. It can be seen that both sorption selectivity and diffusion selectivity increased with the increase in the ethanol concentration. It implies that the additive TGN improves permeate selectivity with increasing ethanol concentrations in feed solution. It is interesting to note that both sorption and diffusion selectivity increased in all feed ethanol concentrations. Generally, the selectivity of sorption behavior usually showed a different trend than the diffusion behavior when the permeate passes through the membranes. To further distinguish these results, the swelling test and sorption measurements at different ethanol concentrations in feed were considered. It was shown that the degree of swelling of TGN/PSF membrane only slightly increased with the increase in ethanol concentration. Generally, the polymer chain flexibility of swelling membrane is usually dependent on the degree of swelling. The significant increase in polymer chain flexibility induced an increase in permeate diffusion path when the permeate transfers through the membranes. Based on the result of swelling test, it may be expected that the increase in permeation flux would be slightly increased with the increase in ethanol concentration in feed. The result of permeation test at various ethanol concentrations in feed showed a similar behavior as expected. On the other hand, it can be seen that the increase in sorption selectivity increased with increasing ethanol concentrations in feed. It was found that the
TGN additive successfully improved the pervaporation performance of polysulfone membrane. Tg and swelling measurement proved that the polymer chain flexibility would be improved by the addition of TGN into membrane whether or not the composite membrane was dry or immersed in 90 wt.% ethanol solution. The polymer chain flexibility and packing density of polymer chain in membrane increased with the increase in the TGN content in TGN/PSF membranes at low TGN content. However, the higher polymer packing density can be expected to decrease the permeation rate and enhance the diffusion selectivity of modified membranes. Based on the result of this study, it can be concluded that a high selective membrane could be achieved by controlling the optimum swelling condition in TGN additive membranes.
Acknowledgements The authors wish to thank the National Science Council of ROC (NSC 90-2216-E-041-005) for their financial support for this research.
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