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Performances of crosslinked asymmetric poly(vinyl alcohol) membranes for isopropanol dehydration by pervaporation
Chemical Engineering and Processing 41 (2002) 693– 698 www.elsevier.com/locate/cep
Performances of crosslinked asymmetric poly(vinyl alcohol) membranes for isopropanol dehydration by pervaporation Jie Yu a, Chul Haeng Lee b, Won Hi Hong b,* b
a Department of Chemical Engineering, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Department of Chemical Engineering, Korea Ad6anced Institute of Science and Technology, 373 -1, Kusong-dong, Yusong-gu, Taejon 305 -701, South Korea
Received 1 August 2001; received in revised form 9 November 2001; accepted 9 November 2001
Abstract Asymmetric poly(vinyl alcohol) (PVA) membranes crosslinked with glutaraldehyde (GA) were prepared by phase inversion technique for pervaporation dehydration of isopropanol (IPA). The crosslinking solutions contain sodium sulfate, sulfuric acid and different contents of GA, wherein sulfuric acid serves as a catalyst. The effects of three variables involved in the membrane preparation, including PVA molecular weight, PVA concentration and GA concentration on pervaporation characteristics were investigated. The results showed that the permeation flux decreases with the increase of these three factors, whereas the selectivity has an opposite trend. The influence of feed IPA concentration and temperature on pervaporation performances was also studied to find optimum operating conditions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Isopropanol dehydration; Poly(vinyl alcohol); Crosslinking; Glutaraldehyde
1. Introduction Pervaporation differs from other membrane processes in that there is a phase change during the pervaporation process. Thus, lots of energy, which at least equals to the evaporation heat of the permeation, is necessary in pervaporation to cause the phase change. However, compared with distillation, it has high separation efficiency and potential savings in energy costs. In addition, it has the advantage of simplifying process plants and avoiding possible pollutants used in distillation process to break up azeotropic. Pervaporation is now considered a basic unit operation with significant potential for the solution of various environmental and energetic processes. According to the solution – diffusion [1,2] model, pervaporation process is a rate-controlled process, and factors which favor the dissolution and diffusion rate of the permeation in the membrane will increase the permeation flux. Thus asymmetric and composite struc* Corresponding author. Tel.: +82-42-869-3919; fax: + 82-42-8693910 E-mail address: [email protected] (W.H. Hong).
tures have been extensively studied in pervaporation process. Most commercially available pervaporation membranes are prepared by phase inversion technique, which including casting a polymer solution on a suitable support and immersing it into a precipitation solution. Precipitation occurs because of the exchange of solvent and nonsolvent. The membrane structure ultimately obtained results from a combination of mass transfer and phase separation. Polymer concentration, polymer molecular weight, composition of precipitation solution, immersion temperature and some other factors will affect the structure of prepared membranes. Poly(vinyl alcohol) (PVA), a 1,3-diglycol polymer with a molecular weight of 44, is a very effective material for pervaporation dehydration because of its good film-forming, highly hydrophilic, and good chemical-resistant properties. Since PVA has a poor stability in aqueous solution, several techniques such as crosslinking and grafting are used to create a stable PVA membrane with good mechanical properties and selective permeability to water [3]. Crosslinking is a commonly used way to stabilize PVA membranes, many kinds of crosslinking agents were studied in Refs. [3–10], as shown in Table 1.
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 8 6 - 6
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
694
Table 1 PVA crosslinking methods in literature Polymer
Crosslinking agents
References
PVA
Poly(acrylic acid) (molecular weight 2000) Glutaraldehyde, acetone, HCl Citric acid, maleic acid Formic acid Amic acid Glutaraldehyde Sulfur–succinic acid Formaldehyde, glutaraldehyde, HCl
[4]
PVA PVA PVA, N-methylol nylon-6 PVA Carboxymethylated PVA PVA PVA
[5] [6] [7] [8] [9] [10] [3]
In this paper, asymmetric PVA membranes were prepared in an aqueous inorganic salt solution by phase inversion technique, and then crosslinked with glutaraldehyde (GA) under the catalysis of sulfuric acid to increase its stability. The pervaporation performances of isopropanol dehydration were investigated with membranes prepared with different PVA molecular weight, PVA concentration and GA concentration. The influence of IPA feed concentration and temperature on pervaporation was also studied.
2. Experimental
2.1. Materials Two kinds of 99% + hydrolyzed PVA with average molecular weight of 89 000– 98 000 and 124 000– 186 000 were used. IPA was guaranteed gradient grade. Glutaraldehyde (25% concentration in water), sodium sulfate anhydrous, sodium hydroxide pellets and 95% sulfuric acid were extra pure grade. The water used was deionized water. All chemicals were used without any further purification.
2.2. Membrane preparation The transparent PVA aqueous solution was cast onto a stainless steel plate with a knife. Evaporated at room temperature for 10 min, the membrane was immersed into an aqueous precipitation solution containing 16 wt.% Na2SO4 and 2 wt.% NaOH at room temperature for 1 h to form an asymmetric structure. After washed with deionized water, the membrane was reacted with a crosslinking solution containing Na2SO4, H2SO4 and GA at 40 °C for 1 h. In all crosslinking solutions used, Na2SO4 and H2SO4 were kept constant at 45 and 5 g/l, respectively, the concentration of GA was changed from 10 to 25 g/l to prepare membranes with different crosslinking degree. The prepared membrane was
Fig. 1. Schematic diagram of the experimental apparatus.
washed with pure water after crosslinking to eliminate any possible residual GA and H2SO4, and dried at room temperature.
2.3. Per6aporation experiments A schematic diagram of the pervaporation apparatus used in this study is presented in Fig. 1. The membrane with an effective area of 15.9 cm2 was supported by a sintered aluminum filter. The operation temperature was kept constant by water bath and the permeate pressure was maintained below 4 mTorr by vacuum pump. The IPA solution was stirred well during experiments to make its concentration homogeneous. The permeated product was condensed and collected in a liquid nitrogen cold trap. Each experimental result was obtained until the operation reached steady state. Concentrations of feed and permeate were analyzed by gas chromatography, and the flux was obtained by measuring the weight of the liquid collected in the cold trap during a fixed time. The selectivity was defined by the following equation: hw/i =
yw/yi xw/xi
where xw, xi, yw and yi are water and IPA concentrations in the feed and permeate, respectively.
3. Results and discussion A desirable pervaporation membrane should have high permeation flux and good selectivity. Although the choice of the polymer is essential to prepare pervaporation membrane by phase inversion technique, factors involved in membrane preparation such as polymer concentration will determine the membrane structure
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
695
Fig. 2. Effect of PVA molecular weight on the permeation flux at 70 °C, PVA concentration 10 wt.%, GA concentration 10 g/l.
Fig. 4. Effect of PVA concentration on the permeation flux with HPVA membranes crosslinked with 10 g/l GA at 60 °C.
and performance. Optimum operating conditions are also important for pervaporation. Therefore, effects of membrane preparation factors as well as operating conditions on pervaporation performance were investigated.
Two kinds of membranes were prepared with PVA of different molecular weights. LPVA and HPVA indicate lower and higher molecular weight PVA, respectively. The effect of PVA molecular weight on permeation flux and selectivity was studied with these membranes and the results are shown in Figs. 2 and 3. Fig. 2 illustrates the permeation flux with HPVA membrane is lower than with LPVA membrane regardless of feed IPA concentration. This phenomenon can be explained by the effect of molecular weight on free volume. With the increase of PVA molecular weight, the length of polymer chains in the membrane increases, the interaction
between chains increases and the chains are more entangled. Therefore, the resulting free volume between molecules reduces. As the free volume reduces, the sorption and diffusion rate of water and IPA decrease, and thus the permeation flux decreases. IPA is more difficult to permeate through the compact membrane due to its larger molecular size. Therefore, the selectivity increases with PVA molecular weight, as shown in Fig. 3. The selectivity difference between HPVA membrane and LPVA membrane is small when feed IPA concentration is lower than 75 wt.%. Then the difference rises sharply with the increase of feed IPA concentration. The relationship between PVA concentration and pervaporation performance is illustrated in Figs. 4 and 5 with HPVA membranes crosslinked with 10 g/l GA at 60 °C. According to the basic principles of membrane preparation by phase inversion [11], the delay time for liquid–liquid demixing is increased with a higher PVA concentration, and the distance from the film/bath in-
Fig. 3. Effect of PVA molecular weight on selectivity at 70 °C, PVA concentration 10 wt.%, GA concentration 10 g/l.
Fig. 5. Effect of PVA concentration on selectivity with HPVA membranes crosslinked with 10 g/l GA at 60 °C.
3.1. Effects of membrane preparation factors
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Fig. 6. Effect of GA concentration on the permeation flux with 10 wt.% LPVA membranes at 70 °C.
Fig. 7. Effect GA concentration on selectivity with 10 wt.% LPVA membranes at 70 °C.
terface in the film also increases, so that the first formed nuclei of the dilute phase are formed at a greater distance in the film from the film/bath interface. Thus the thickness of the compact top layer increases with increasing PVA concentration. The diffusion rate of water and IPA in the membrane reduces with the increase of membranes’ thickness, and the decrease in the diffusion rate of IPA is more than that of water because of its larger molecular size. Therefore, a lower permeation flux and a higher selectivity are obtained with the membrane prepared by higher concentration PVA. The results of Yeom and Lee showed that the crosslinking degree increases with an increase of the GA concentration in the reaction solutions [5]. At a higher GA concentration or crosslinking degree, the resulting membrane has a more compact network structure and less chain mobility, which makes the solubility and diffusivity of the permeate molecules to decrease. Therefore, the resulting permeate rate through the membrane decreases with increasing GA concentration, as shown in Fig. 6. Fig. 7 shows that the selectivity toward water increases with GA concentration, which is because the permeation flux of IPA decreases more than that of water at higher GA concentration. Figs. 6 and 7 also illustrate that the change in permeation flux and selectivity with GA concentration at 80 wt.% IPA are bigger than that at 90 wt.% IPA, which is caused by different degree of plasticizing effect of water on membranes.
m2 h and 20 –191, respectively, depending on feed IPA concentration. The permeation flux is lower and the selectivity towards water is higher at higher IPA content in the feed, which can be explained in terms of the plasticizing effect of the water on membranes. With lower IPA concentration or higher water concentration in the feed mixture, the amorphous regions of the membrane are more swollen and the polymer chains become more flexible. This makes both water molecules and IPA molecules more easily penetrate through membranes, thus the permeation flux increases and the selectivity decreases with the water concentration in the feed.
3.3. Effects of operation temperature on permeation flux The effect of operating temperature on selectivity is shown in Fig. 9. The selectivity toward water decreases with increasing of temperature regardless of feed IPA concentration. As the temperature increases, the rotat-
3.2. Effects of IPA feed concentration Pervaporation performances of water– IPA mixture with 7 wt.% HPVA membrane crosslinked with 10 g/l GA was carried out over a range of 60– 95 wt.% IPA in the feed at 60 °C. The results are shown in Fig. 8. The permeation flux and the selectivity were 158– 1380 g/
Fig. 8. Effect of IPA concentration on pervaporation characteristics with 7 wt.% HPVA membrane crosslinked with 10 g/l GA at 60 °C.
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
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4. Conclusions
Fig. 9. Effect of operation temperature on selectivity with 7 wt.% HPVA membrane crosslinked with 10 g/l GA.
ing frequency and amplitude of the chain increase and the resulting free volume becomes larger. Thus, more IPA molecules penetrate through expanding free volume isolated or associated, and this leads to lower selectivity at higher temperature. The temperature dependence of the permeation flux can be expressed by an Arrhenius type relationship: Jp = Ap exp(− Ep/RT) where Jp is the total permeation flux, Ap and Ep are the pre-exponential factor and the apparent activation energy of permeation, respectively. Arrhenius plots of total permeation flux are illustrated in Fig. 10 which can be used to calculate the apparent activation energy of permeation Ep. The apparent activation energy at 90 wt.% IPA is higher than at 80 wt.% IPA. This is because the membrane is less swollen at higher IPA concentration and more energy is needed for the permeate to penetrate the membrane.
Fig. 10. Arrhenius plots of flux with 7 wt.% HPVA membrane crosslinked with 10 g/l GA.
Phase inversion technique was used to prepare asymmetric PVA pervaporation membranes crosslinked with GA. Pervaporation characteristics of IPA dehydration were measured with membranes prepared under different conditions. The results showed that membrane preparation conditions have great influence on pervaporation performance. The permeation flux decreases with the increase of PVA molecular weight, PVA concentration and GA concentration, and there is a tradeoff trend between the permeation flux and selectivity. The effects of operating conditions on pervaporation were also studied and the permeation flux is larger at higher feed water concentration and operation temperature. Acknowledgements This work was supported by LG Chem. Ltd/BK 21 project. The authors wish to express their thanks. Dr Yu is also grateful to the Korea Foundation for Advanced Studies for a fellowship. References [1] T. Tataoka, T. Tsuru, S. Nakao, S. Kimura, Permeation equations developed for prediction of membrane performance in pervaporation, vapor permeation and reverse osmosis based on the solution – diffusion model, J. Chem. Eng. Jpn. 24 (1991) 326 – 333. [2] J.G. Wijmans, R.W. Baker, The solution – diffusion model: a review, J. Membr. Sci. 107 (1995) 1 – 21. [3] N.D. Hilmioglu, A.E. Yildirim, A.S. Sakaoglu, S. Tulbentci, Acetic acid dehydration by pervaporation, Chem. Eng. Process. 40 (2001) 263 – 267. [4] K.H. Lee, H.K. Kim, J.W. Rhim, Pervaporation separation of binary organic – aqueous liquid mixtures using crosslinked PVA membranes. III. Ethanol – water mixtures, J. Appl. Polym. Sci. 58 (1995) 1707 – 1712. [5] C.K. Yeom, K.H. Lee, Pervaporation separation of water –acetic acid mixtures through poly(vinyl alcohol) membranes crosslinked with glutaraldehyde, J. Membr. Sci. 109 (1996) 257 – 265. [6] M.C. Burshe, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Sorption and permeation of binary water – alcohol systems through PVA membranes crosslinked with multifunctional crosslinking agents, Sep. Purif. Technol. 12 (1997) 145 – 156. [7] R.Y.M. Huang, J.J. Shieh, Crosslinked blended poly(vinyl alcohol)/N-methylol nylon-6 membranes for the pervaporation separation of ethanol – water mixtures, J. Appl. Polym. Sci. 70 (1998) 317 – 327. [8] C.H. Lee, W.H. Hong, Influence of different degrees of hydrolysis of poly(vinyl alcohol) membrane on transport properties in pervaporation of IPA/water mixture, J. Membr. Sci. 135 (1997) 187 – 193. [9] S.Y. Nam, H.J. Chun, Y.M. Lee, Pervaporation separation of water – isopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes, J. Appl. Polym. Sci. 72 (1999) 241 – 249.
698
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
[10] J.W. Rhim, C.K. Yeom, S.W. Kim, Modification of poly(vinyl alcohol) membranes using sulfur – succinic acid and its application to pervaporation separation of water – alcohol mixtures, J.
Appl. Polym. Sci. 68 (1998) 1717 – 1723. [11] M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic, Netherlands, 1996, pp. 89 – 140.
Performances of crosslinked asymmetric poly(vinyl alcohol) membranes for isopropanol dehydration by pervaporation Jie Yu a, Chul Haeng Lee b, Won Hi Hong b,* b
a Department of Chemical Engineering, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Department of Chemical Engineering, Korea Ad6anced Institute of Science and Technology, 373 -1, Kusong-dong, Yusong-gu, Taejon 305 -701, South Korea
Received 1 August 2001; received in revised form 9 November 2001; accepted 9 November 2001
Abstract Asymmetric poly(vinyl alcohol) (PVA) membranes crosslinked with glutaraldehyde (GA) were prepared by phase inversion technique for pervaporation dehydration of isopropanol (IPA). The crosslinking solutions contain sodium sulfate, sulfuric acid and different contents of GA, wherein sulfuric acid serves as a catalyst. The effects of three variables involved in the membrane preparation, including PVA molecular weight, PVA concentration and GA concentration on pervaporation characteristics were investigated. The results showed that the permeation flux decreases with the increase of these three factors, whereas the selectivity has an opposite trend. The influence of feed IPA concentration and temperature on pervaporation performances was also studied to find optimum operating conditions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Isopropanol dehydration; Poly(vinyl alcohol); Crosslinking; Glutaraldehyde
1. Introduction Pervaporation differs from other membrane processes in that there is a phase change during the pervaporation process. Thus, lots of energy, which at least equals to the evaporation heat of the permeation, is necessary in pervaporation to cause the phase change. However, compared with distillation, it has high separation efficiency and potential savings in energy costs. In addition, it has the advantage of simplifying process plants and avoiding possible pollutants used in distillation process to break up azeotropic. Pervaporation is now considered a basic unit operation with significant potential for the solution of various environmental and energetic processes. According to the solution – diffusion [1,2] model, pervaporation process is a rate-controlled process, and factors which favor the dissolution and diffusion rate of the permeation in the membrane will increase the permeation flux. Thus asymmetric and composite struc* Corresponding author. Tel.: +82-42-869-3919; fax: + 82-42-8693910 E-mail address: [email protected] (W.H. Hong).
tures have been extensively studied in pervaporation process. Most commercially available pervaporation membranes are prepared by phase inversion technique, which including casting a polymer solution on a suitable support and immersing it into a precipitation solution. Precipitation occurs because of the exchange of solvent and nonsolvent. The membrane structure ultimately obtained results from a combination of mass transfer and phase separation. Polymer concentration, polymer molecular weight, composition of precipitation solution, immersion temperature and some other factors will affect the structure of prepared membranes. Poly(vinyl alcohol) (PVA), a 1,3-diglycol polymer with a molecular weight of 44, is a very effective material for pervaporation dehydration because of its good film-forming, highly hydrophilic, and good chemical-resistant properties. Since PVA has a poor stability in aqueous solution, several techniques such as crosslinking and grafting are used to create a stable PVA membrane with good mechanical properties and selective permeability to water [3]. Crosslinking is a commonly used way to stabilize PVA membranes, many kinds of crosslinking agents were studied in Refs. [3–10], as shown in Table 1.
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 8 6 - 6
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
694
Table 1 PVA crosslinking methods in literature Polymer
Crosslinking agents
References
PVA
Poly(acrylic acid) (molecular weight 2000) Glutaraldehyde, acetone, HCl Citric acid, maleic acid Formic acid Amic acid Glutaraldehyde Sulfur–succinic acid Formaldehyde, glutaraldehyde, HCl
[4]
PVA PVA PVA, N-methylol nylon-6 PVA Carboxymethylated PVA PVA PVA
[5] [6] [7] [8] [9] [10] [3]
In this paper, asymmetric PVA membranes were prepared in an aqueous inorganic salt solution by phase inversion technique, and then crosslinked with glutaraldehyde (GA) under the catalysis of sulfuric acid to increase its stability. The pervaporation performances of isopropanol dehydration were investigated with membranes prepared with different PVA molecular weight, PVA concentration and GA concentration. The influence of IPA feed concentration and temperature on pervaporation was also studied.
2. Experimental
2.1. Materials Two kinds of 99% + hydrolyzed PVA with average molecular weight of 89 000– 98 000 and 124 000– 186 000 were used. IPA was guaranteed gradient grade. Glutaraldehyde (25% concentration in water), sodium sulfate anhydrous, sodium hydroxide pellets and 95% sulfuric acid were extra pure grade. The water used was deionized water. All chemicals were used without any further purification.
2.2. Membrane preparation The transparent PVA aqueous solution was cast onto a stainless steel plate with a knife. Evaporated at room temperature for 10 min, the membrane was immersed into an aqueous precipitation solution containing 16 wt.% Na2SO4 and 2 wt.% NaOH at room temperature for 1 h to form an asymmetric structure. After washed with deionized water, the membrane was reacted with a crosslinking solution containing Na2SO4, H2SO4 and GA at 40 °C for 1 h. In all crosslinking solutions used, Na2SO4 and H2SO4 were kept constant at 45 and 5 g/l, respectively, the concentration of GA was changed from 10 to 25 g/l to prepare membranes with different crosslinking degree. The prepared membrane was
Fig. 1. Schematic diagram of the experimental apparatus.
washed with pure water after crosslinking to eliminate any possible residual GA and H2SO4, and dried at room temperature.
2.3. Per6aporation experiments A schematic diagram of the pervaporation apparatus used in this study is presented in Fig. 1. The membrane with an effective area of 15.9 cm2 was supported by a sintered aluminum filter. The operation temperature was kept constant by water bath and the permeate pressure was maintained below 4 mTorr by vacuum pump. The IPA solution was stirred well during experiments to make its concentration homogeneous. The permeated product was condensed and collected in a liquid nitrogen cold trap. Each experimental result was obtained until the operation reached steady state. Concentrations of feed and permeate were analyzed by gas chromatography, and the flux was obtained by measuring the weight of the liquid collected in the cold trap during a fixed time. The selectivity was defined by the following equation: hw/i =
yw/yi xw/xi
where xw, xi, yw and yi are water and IPA concentrations in the feed and permeate, respectively.
3. Results and discussion A desirable pervaporation membrane should have high permeation flux and good selectivity. Although the choice of the polymer is essential to prepare pervaporation membrane by phase inversion technique, factors involved in membrane preparation such as polymer concentration will determine the membrane structure
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
695
Fig. 2. Effect of PVA molecular weight on the permeation flux at 70 °C, PVA concentration 10 wt.%, GA concentration 10 g/l.
Fig. 4. Effect of PVA concentration on the permeation flux with HPVA membranes crosslinked with 10 g/l GA at 60 °C.
and performance. Optimum operating conditions are also important for pervaporation. Therefore, effects of membrane preparation factors as well as operating conditions on pervaporation performance were investigated.
Two kinds of membranes were prepared with PVA of different molecular weights. LPVA and HPVA indicate lower and higher molecular weight PVA, respectively. The effect of PVA molecular weight on permeation flux and selectivity was studied with these membranes and the results are shown in Figs. 2 and 3. Fig. 2 illustrates the permeation flux with HPVA membrane is lower than with LPVA membrane regardless of feed IPA concentration. This phenomenon can be explained by the effect of molecular weight on free volume. With the increase of PVA molecular weight, the length of polymer chains in the membrane increases, the interaction
between chains increases and the chains are more entangled. Therefore, the resulting free volume between molecules reduces. As the free volume reduces, the sorption and diffusion rate of water and IPA decrease, and thus the permeation flux decreases. IPA is more difficult to permeate through the compact membrane due to its larger molecular size. Therefore, the selectivity increases with PVA molecular weight, as shown in Fig. 3. The selectivity difference between HPVA membrane and LPVA membrane is small when feed IPA concentration is lower than 75 wt.%. Then the difference rises sharply with the increase of feed IPA concentration. The relationship between PVA concentration and pervaporation performance is illustrated in Figs. 4 and 5 with HPVA membranes crosslinked with 10 g/l GA at 60 °C. According to the basic principles of membrane preparation by phase inversion [11], the delay time for liquid–liquid demixing is increased with a higher PVA concentration, and the distance from the film/bath in-
Fig. 3. Effect of PVA molecular weight on selectivity at 70 °C, PVA concentration 10 wt.%, GA concentration 10 g/l.
Fig. 5. Effect of PVA concentration on selectivity with HPVA membranes crosslinked with 10 g/l GA at 60 °C.
3.1. Effects of membrane preparation factors
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J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
Fig. 6. Effect of GA concentration on the permeation flux with 10 wt.% LPVA membranes at 70 °C.
Fig. 7. Effect GA concentration on selectivity with 10 wt.% LPVA membranes at 70 °C.
terface in the film also increases, so that the first formed nuclei of the dilute phase are formed at a greater distance in the film from the film/bath interface. Thus the thickness of the compact top layer increases with increasing PVA concentration. The diffusion rate of water and IPA in the membrane reduces with the increase of membranes’ thickness, and the decrease in the diffusion rate of IPA is more than that of water because of its larger molecular size. Therefore, a lower permeation flux and a higher selectivity are obtained with the membrane prepared by higher concentration PVA. The results of Yeom and Lee showed that the crosslinking degree increases with an increase of the GA concentration in the reaction solutions [5]. At a higher GA concentration or crosslinking degree, the resulting membrane has a more compact network structure and less chain mobility, which makes the solubility and diffusivity of the permeate molecules to decrease. Therefore, the resulting permeate rate through the membrane decreases with increasing GA concentration, as shown in Fig. 6. Fig. 7 shows that the selectivity toward water increases with GA concentration, which is because the permeation flux of IPA decreases more than that of water at higher GA concentration. Figs. 6 and 7 also illustrate that the change in permeation flux and selectivity with GA concentration at 80 wt.% IPA are bigger than that at 90 wt.% IPA, which is caused by different degree of plasticizing effect of water on membranes.
m2 h and 20 –191, respectively, depending on feed IPA concentration. The permeation flux is lower and the selectivity towards water is higher at higher IPA content in the feed, which can be explained in terms of the plasticizing effect of the water on membranes. With lower IPA concentration or higher water concentration in the feed mixture, the amorphous regions of the membrane are more swollen and the polymer chains become more flexible. This makes both water molecules and IPA molecules more easily penetrate through membranes, thus the permeation flux increases and the selectivity decreases with the water concentration in the feed.
3.3. Effects of operation temperature on permeation flux The effect of operating temperature on selectivity is shown in Fig. 9. The selectivity toward water decreases with increasing of temperature regardless of feed IPA concentration. As the temperature increases, the rotat-
3.2. Effects of IPA feed concentration Pervaporation performances of water– IPA mixture with 7 wt.% HPVA membrane crosslinked with 10 g/l GA was carried out over a range of 60– 95 wt.% IPA in the feed at 60 °C. The results are shown in Fig. 8. The permeation flux and the selectivity were 158– 1380 g/
Fig. 8. Effect of IPA concentration on pervaporation characteristics with 7 wt.% HPVA membrane crosslinked with 10 g/l GA at 60 °C.
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
697
4. Conclusions
Fig. 9. Effect of operation temperature on selectivity with 7 wt.% HPVA membrane crosslinked with 10 g/l GA.
ing frequency and amplitude of the chain increase and the resulting free volume becomes larger. Thus, more IPA molecules penetrate through expanding free volume isolated or associated, and this leads to lower selectivity at higher temperature. The temperature dependence of the permeation flux can be expressed by an Arrhenius type relationship: Jp = Ap exp(− Ep/RT) where Jp is the total permeation flux, Ap and Ep are the pre-exponential factor and the apparent activation energy of permeation, respectively. Arrhenius plots of total permeation flux are illustrated in Fig. 10 which can be used to calculate the apparent activation energy of permeation Ep. The apparent activation energy at 90 wt.% IPA is higher than at 80 wt.% IPA. This is because the membrane is less swollen at higher IPA concentration and more energy is needed for the permeate to penetrate the membrane.
Fig. 10. Arrhenius plots of flux with 7 wt.% HPVA membrane crosslinked with 10 g/l GA.
Phase inversion technique was used to prepare asymmetric PVA pervaporation membranes crosslinked with GA. Pervaporation characteristics of IPA dehydration were measured with membranes prepared under different conditions. The results showed that membrane preparation conditions have great influence on pervaporation performance. The permeation flux decreases with the increase of PVA molecular weight, PVA concentration and GA concentration, and there is a tradeoff trend between the permeation flux and selectivity. The effects of operating conditions on pervaporation were also studied and the permeation flux is larger at higher feed water concentration and operation temperature. Acknowledgements This work was supported by LG Chem. Ltd/BK 21 project. The authors wish to express their thanks. Dr Yu is also grateful to the Korea Foundation for Advanced Studies for a fellowship. References [1] T. Tataoka, T. Tsuru, S. Nakao, S. Kimura, Permeation equations developed for prediction of membrane performance in pervaporation, vapor permeation and reverse osmosis based on the solution – diffusion model, J. Chem. Eng. Jpn. 24 (1991) 326 – 333. [2] J.G. Wijmans, R.W. Baker, The solution – diffusion model: a review, J. Membr. Sci. 107 (1995) 1 – 21. [3] N.D. Hilmioglu, A.E. Yildirim, A.S. Sakaoglu, S. Tulbentci, Acetic acid dehydration by pervaporation, Chem. Eng. Process. 40 (2001) 263 – 267. [4] K.H. Lee, H.K. Kim, J.W. Rhim, Pervaporation separation of binary organic – aqueous liquid mixtures using crosslinked PVA membranes. III. Ethanol – water mixtures, J. Appl. Polym. Sci. 58 (1995) 1707 – 1712. [5] C.K. Yeom, K.H. Lee, Pervaporation separation of water –acetic acid mixtures through poly(vinyl alcohol) membranes crosslinked with glutaraldehyde, J. Membr. Sci. 109 (1996) 257 – 265. [6] M.C. Burshe, S.B. Sawant, J.B. Joshi, V.G. Pangarkar, Sorption and permeation of binary water – alcohol systems through PVA membranes crosslinked with multifunctional crosslinking agents, Sep. Purif. Technol. 12 (1997) 145 – 156. [7] R.Y.M. Huang, J.J. Shieh, Crosslinked blended poly(vinyl alcohol)/N-methylol nylon-6 membranes for the pervaporation separation of ethanol – water mixtures, J. Appl. Polym. Sci. 70 (1998) 317 – 327. [8] C.H. Lee, W.H. Hong, Influence of different degrees of hydrolysis of poly(vinyl alcohol) membrane on transport properties in pervaporation of IPA/water mixture, J. Membr. Sci. 135 (1997) 187 – 193. [9] S.Y. Nam, H.J. Chun, Y.M. Lee, Pervaporation separation of water – isopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes, J. Appl. Polym. Sci. 72 (1999) 241 – 249.
698
J. Yu et al. / Chemical Engineering and Processing 41 (2002) 693–698
[10] J.W. Rhim, C.K. Yeom, S.W. Kim, Modification of poly(vinyl alcohol) membranes using sulfur – succinic acid and its application to pervaporation separation of water – alcohol mixtures, J.
Appl. Polym. Sci. 68 (1998) 1717 – 1723. [11] M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic, Netherlands, 1996, pp. 89 – 140.