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silicone rubber multilayer composite membrane for hydrogen–nitrogen separation
Materials Chemistry and Physics 94 (2005) 288–291
Preparation of a novel polysulfone/polyethylene oxide/silicone rubber multilayer composite membrane for hydrogen–nitrogen separation Zhen Ye a,b,∗ , Yong Chen b , Hui Li b , Gaohong He b,c , Maicun Deng a,b b
a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China Tianbang National Engineering Research Centre of Membrane Technology, Dalian 116023, PR China c Dalian University of Technology, School of Chemical Engineering, Dalian 116012, PR China
Received 24 January 2005; received in revised form 14 April 2005; accepted 2 May 2005
Abstract A novel polysulfone/polyethylene oxide/silicone rubber (PSf/PEO/SR) multilayer composite membrane was fabricated by double coating polysulfone substrate membrane with polyethylene oxide and silicone rubber. Gas permeation experiments were performed at 30 ◦ C for hydrogen and nitrogen. PSf/PEO/SR membrane displayed high and steady performance for H2 /N2 : permeances of H2 and N2 of 49.51 and 0.601 GPU, respectively, and H2 /N2 ideal separation factor of 82.3. It was explained that layer interfaces due to the introduction of PEO layer act as the permselective media and are responsible for the higher H2 /N2 ideal separation factor which has exceeded the intrinsic permselectivities of the three polymers used in this study. © 2005 Elsevier B.V. All rights reserved. Keywords: Multilayer membrane; Polysulfone; Polyethylene oxide; Hydrogen
1. Introduction In the early 1980s, Henis and Tripodi [1,2] developed composite membrane for gas separation with acceptable permeability and high permselectivity and established the “Henis resistance model” approach to the membrane. Henis resistance composite membrane found commercial applications of membrane separation technology in H2 recovery and purification processes. Henis resistance model in which the mass transport through the membrane was described in analogue to an electric circuit has been used as the basis for gas membrane separation process because it was useful in analyzing the resistance components and the membrane permselectivity. Thereafter other models such as Wheatstonebridge model [3], improved Henis resistance model [4], and
∗
Corresponding author. Tel.: +86 411 84379192; fax: +86 411 84677947. E-mail address: [email protected] (Z. Ye).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.05.001
Series RC circuit model [5] were also brought forward to modify and complement the Henis resistance model. Henis resistance composite membrane generally consists of two layers: top coating layer made by a nonselective and highly permeable material and porous asymmetric substrate with a dense skin layer. The coating layer is used to plug the pores in the skin layer of the substrate, and the substrate made of permselective polymer is responsible for the membrane separation. However, multilayer composite membrane usually has three layers and can be divided into two types based on the membrane configuration: (selective layer)/(gutter layer)/(support substrate) [6,7] and (sealing or protective layer)/(selective layer)/(support substrate) [8–10]. Polymers such as polyacrylonitrile (PAN) [7], poly(4vinylpyridine) (PVP) [8] and cellulose nitrate (CN) [9] have been used as selective layer material to prepare multilayer composite membrane. In this paper we report a novel polysulfone/polyethylene oxide/silicone rubber multilayer composite membrane (PSf/
Z. Ye et al. / Materials Chemistry and Physics 94 (2005) 288–291
289
PEO/SR) [11]. The membrane structure and separation mechanisms for H2 /N2 are discussed. 2. Experimental 2.1. Membrane preparation Polysulfone (PSf) hollow fibers with asymmetric structure were spun by a dry-wet spinning procedure [12] on a laboratory spinning apparatus. A spinning solution of 35 wt.% PSf (UDEL® P-3500) in complex solvents was used. After washing and drying, fibers of typical dimensions (450/150 m, o.d/i.d.) were obtained. Bundles of 50 fibers each with a length of 25 cm were sealed at one end, while the shell side of the other end was glued onto an aluminum holder using epoxy resin. PSf/PEO/SR multilayer membrane was prepared by successively coating the bundles with polyethylene oxide (PEO, Aldrich Catalog No.: 37,277-3, Mw = 4 × 105 ) and silicone rubber (SR, Sylgard® 184) solutions. Hollow fiber bundles were firstly immersed into 0.1 wt.% PEO/water coating solution for 30 min and desiccated at room temperature for 24 h. PEO-coated membrane was then dip-coated with a 3 wt.% SR/pentane coating solution for 8 min with applying vacuum inside the fibers. Silane coupling agent KH-550 was added to the PEO and SR coating solution to improve the adhesive forces of different polymers. PSf/SR composite membrane was also prepared by coating the bundles with the same SR coating solution and methods. PSf/PEO/SR and PSf/SR membranes were stored for at least 48 h at 30 ◦ C before use, and the PSf/PEO/SR membrane was stored in a desiccator in the course of termly stability testing. 2.2. Gas permeation test The membrane was potted into a stainless steel pressure cell for pure gas permeation measurement and the permeate flows from the open end were measured at 30 ◦ C with the permeate side set at atmospheric pressure. Gas permeance across the membrane, J, was obtained using the following equation: J=
Q 273 × A(pf − pp ) T
(1)
where J is the gas permeance in GPU (1 GPU = 10−6 cm3 (STP) cm−2 s−1 cmHg−1 ), Q the flux per unit of time
Fig. 1. Cross-sectional TEM micrograph of PSf/PEO/SR multilayer composite hollow fiber membrane.
(cm3 s−1 ), A the effective area of the membrane (cm2 ), pf − pp the pressure difference between the feed and the permeate (cmHg), and T the operation temperature (K).
3. Results and discussion 3.1. Membrane morphology Cross-sectional morphology of PSf/PEO/SR membrane was determined by transmission electron micrograph (TEM) and the image was displayed in Fig. 1. Three layers and two obvious interfaces indicated by two straight lines are observed. TEM image demonstrates that PEO and SR have been coated on the PSf hollow fiber substrate and the multilayer composite membrane has been successfully prepared. 3.2. Gas permeation performance Pure gas permeation results of PSf/PEO/SR and PSf/SR membranes are listed in Table 1. PSf/PEO/SR membrane has an H2 permeance of 49.51 GPU while that of PSf/SR membrane is 41.50 GPU. Meanwhile, the H2 /N2 ideal separation factor of the membrane (defined as the ratio of their pure gas permeances, JH2 /JN2 ) has nearly increase of 30% from 63.8 to 82.3. PSf/PEO/SR membranes show better permeation performance than PSf/SR membranes.
Table 1 Pure gas permeation properties through PSf/PEO/SR and PSf/SR membranes at 30 ◦ C PSf/PEO/SR
JH2 (GPU) JN2 (GPU) JH2 /JN2 a Feed
PSf/SR
1a
2a
3a
Average
4a
5a
6a
Average
49.35 0.596 82.8
49.11 0.602 81.6
50.08 0.606 82.6
49.51 0.601 82.3
41.38 0.649 63.8
41.41 0.646 64.1
41.72 0.656 63.6
41.50 0.650 63.8
pressure (Gauge): 5 atm.
Z. Ye et al. / Materials Chemistry and Physics 94 (2005) 288–291
290
Fig. 2. Stability measurements of PSf/PEO/SR membrane permeation properties (Sample 3# ).
Stability of PSf/PEO/SR membrane permeation properties was tested every 15 days for 2 months at 30 ◦ C, and the results are shown in Fig. 2. There was a slight decrease in H2 permeance and increase in ideal separation factor JH2 /JN2 which reached 85.0 two months later. Such membrane is applicable in H2 purification and recovery in refinery tail gas, ammonia production and syngas composition adjustment process. The intrinsic permeation properties of H2 and N2 in PSf, SR and PEO materials are listed in Table 2. As shown in Table 2, each of the ideal separation factors of the three
Table 2 The intrinsic permeability coefficients of polymeric materials for H2 and N2 used in this study and the ideal separation factor PH2 /PN2 Materials
T (◦ C)
PH2 (Barrerb )
PH2 /PN2
Reference
PSf SR PEOa
35 35 35
5–200 500–3000 1.8
25.0–75.0 1.5–3.0 7.2
[13] [13] [14]
a b
Mw = 1 × 106 . 1 Barrer = 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1 .
polymers for H2 /N2 separation is less than 81.9, which is the average ideal separation factor of PSf/PEO/SR membrane during 2-month termly measurements. According to the aforementioned resistance model, ideal separation factor of a composite membrane must be lower than the intrinsic selectivity of the most permselective polymer which is used to form the membrane. The intrinsic H2 /N2 permselectivities of PSf is normally regarded as in the scope of 72–80, however, differences between PSf/PEO/SR membrane and other reported multilayer composite membranes are that none of the three layers of PSf/PEO/SR membrane is responsible for the higher ideal separation factor. As a result, neither of foregoing (selective layer)/(gutter layer)/(support substrate) or (sealing or protective layer)/(selective layer)/(support substrate) multilayer membrane structures is accordance with three layers of PSf/PEO/SR membrane. In this case, the contributing permselective media should be attributed to the two layer interfaces due to the introduction of PEO layer. Compared with PSf/SR membrane, the permeance of “fast gas” H2 becomes higher and that of “slow gas” N2 becomes lower in PSf/PEO/SR membrane. The chemical and physical properties of the layer interface region may result in the change tendencies of H2 and N2 permeances in PSf/PEO/SR membrane. The interfacial affinity will affect the membrane performance. Generally speaking, PSf is a kind of hydrophilic polymer compared with hydrophobic SR. It is difficult to obtain a PSf/SR membrane with integrated SR coating layer because it is hard to spread hydrophobic SR on PSf surface very well. In contrast, hydrophilic PEO spread better on PSf surface, and the transient PEO layer may offer a smoother surface for SR coating in PSf/PEO/SR membrane. Meanwhile, the degradation and aggregation of PEO [15] and the interaction of the polymers are involved in the forming of the interfaces. Currently, an in-depth study on the structural characteristic of the membrane interfaces, the H2 /N2 separation mechanism and the application of these novel membranes in H2 membrane separation is in progress in our laboratory.
4. Conclusion In summary, novel PSf/PEO/SR multilayer composite hollow fiber membrane not only provided high performance for H2 /N2 separation but also shows a good stability in H2 permeation. Ideal H2 /N2 separation factor of PSf/PEO/SR membrane has exceeded the intrinsic H2 /N2 permselectivities of PSf which is the most H2 /N2 permselective polymer used in this study, and to our best knowledge, it is the first report that three less permselective polymers can form a more permselective composite membrane. For future studies on multilayer composite membrane a focus on the formation mechanism of the membrane layer interfaces and the effects of the layer interfaces on the composite membrane preparation should be taken into account.
Z. Ye et al. / Materials Chemistry and Physics 94 (2005) 288–291
Acknowledgements Special thanks are due to Ms. ZHANG Guihua, Mr. WANG Yuting and Ms. DOU Hong for their help in gas permeation experiments.
References [1] J.M.S. Henis, M.K. Tripodi, J. Membrane Sci. 8 (1981) 233. [2] J.M.S. Henis, M.K. Tripodi, Sep. Sci. Technol. 15 (1980) 105. [3] A. Fouda, Y. Chen, J. Bai, T. Matsuura, J. Membrane Sci. 64 (1991) 263. [4] G. He, X. Huang, R. Xu, B. Zhu, J. Membrane Sci. 118 (1996) 1.
291
[5] Y.S. Kang, J.M. Hong, J. Jang, U.Y. Kim, J. Membrane Sci. 109 (1996) 149. [6] I. Cabasso, K.A. Lundy, US Patent 4,602,922 (1982). [7] T.S. Chung, E.R. Kafchinski, M. Spak, B.B. Ross, G. Wensley, US Patent 5,324,430 (1994). [8] T.S. Chung, J.J. Shieh, W.W.Y. Lau, M.P. Srinivasan, D.R. Paul, J. Membrane Sci. 152 (1999) 211. [9] J.J. Shieh, T.S. Chung, J. Membrane Sci. 166 (2000) 259. [10] I. Blume, I. Pinnau, US Patent 4,963,165 (1990). [11] Z. Ye, Y. Chen, M.C. Deng, Z.A. Chen, H. Li, G.H. Zhang, G.X. Liu, G.H. He, M. Wu, Chinese Patent, Application No. 10021149 (2004). [12] W.J. Koros, G.K. Fleming, J. Membrane Sci. 83 (1993) 1. [13] J.M.S. Henis, M.K. Tripodi, Science 220 (1983) 4592. [14] H. Lin, B.D. Freeman, J. Membrane Sci. 239 (2004) 105. [15] P. Pang, P. Englezos, Colloids Surf. A: Physiochem., Eng. Aspects 204 (2002) 23.
Preparation of a novel polysulfone/polyethylene oxide/silicone rubber multilayer composite membrane for hydrogen–nitrogen separation Zhen Ye a,b,∗ , Yong Chen b , Hui Li b , Gaohong He b,c , Maicun Deng a,b b
a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China Tianbang National Engineering Research Centre of Membrane Technology, Dalian 116023, PR China c Dalian University of Technology, School of Chemical Engineering, Dalian 116012, PR China
Received 24 January 2005; received in revised form 14 April 2005; accepted 2 May 2005
Abstract A novel polysulfone/polyethylene oxide/silicone rubber (PSf/PEO/SR) multilayer composite membrane was fabricated by double coating polysulfone substrate membrane with polyethylene oxide and silicone rubber. Gas permeation experiments were performed at 30 ◦ C for hydrogen and nitrogen. PSf/PEO/SR membrane displayed high and steady performance for H2 /N2 : permeances of H2 and N2 of 49.51 and 0.601 GPU, respectively, and H2 /N2 ideal separation factor of 82.3. It was explained that layer interfaces due to the introduction of PEO layer act as the permselective media and are responsible for the higher H2 /N2 ideal separation factor which has exceeded the intrinsic permselectivities of the three polymers used in this study. © 2005 Elsevier B.V. All rights reserved. Keywords: Multilayer membrane; Polysulfone; Polyethylene oxide; Hydrogen
1. Introduction In the early 1980s, Henis and Tripodi [1,2] developed composite membrane for gas separation with acceptable permeability and high permselectivity and established the “Henis resistance model” approach to the membrane. Henis resistance composite membrane found commercial applications of membrane separation technology in H2 recovery and purification processes. Henis resistance model in which the mass transport through the membrane was described in analogue to an electric circuit has been used as the basis for gas membrane separation process because it was useful in analyzing the resistance components and the membrane permselectivity. Thereafter other models such as Wheatstonebridge model [3], improved Henis resistance model [4], and
∗
Corresponding author. Tel.: +86 411 84379192; fax: +86 411 84677947. E-mail address: [email protected] (Z. Ye).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.05.001
Series RC circuit model [5] were also brought forward to modify and complement the Henis resistance model. Henis resistance composite membrane generally consists of two layers: top coating layer made by a nonselective and highly permeable material and porous asymmetric substrate with a dense skin layer. The coating layer is used to plug the pores in the skin layer of the substrate, and the substrate made of permselective polymer is responsible for the membrane separation. However, multilayer composite membrane usually has three layers and can be divided into two types based on the membrane configuration: (selective layer)/(gutter layer)/(support substrate) [6,7] and (sealing or protective layer)/(selective layer)/(support substrate) [8–10]. Polymers such as polyacrylonitrile (PAN) [7], poly(4vinylpyridine) (PVP) [8] and cellulose nitrate (CN) [9] have been used as selective layer material to prepare multilayer composite membrane. In this paper we report a novel polysulfone/polyethylene oxide/silicone rubber multilayer composite membrane (PSf/
Z. Ye et al. / Materials Chemistry and Physics 94 (2005) 288–291
289
PEO/SR) [11]. The membrane structure and separation mechanisms for H2 /N2 are discussed. 2. Experimental 2.1. Membrane preparation Polysulfone (PSf) hollow fibers with asymmetric structure were spun by a dry-wet spinning procedure [12] on a laboratory spinning apparatus. A spinning solution of 35 wt.% PSf (UDEL® P-3500) in complex solvents was used. After washing and drying, fibers of typical dimensions (450/150 m, o.d/i.d.) were obtained. Bundles of 50 fibers each with a length of 25 cm were sealed at one end, while the shell side of the other end was glued onto an aluminum holder using epoxy resin. PSf/PEO/SR multilayer membrane was prepared by successively coating the bundles with polyethylene oxide (PEO, Aldrich Catalog No.: 37,277-3, Mw = 4 × 105 ) and silicone rubber (SR, Sylgard® 184) solutions. Hollow fiber bundles were firstly immersed into 0.1 wt.% PEO/water coating solution for 30 min and desiccated at room temperature for 24 h. PEO-coated membrane was then dip-coated with a 3 wt.% SR/pentane coating solution for 8 min with applying vacuum inside the fibers. Silane coupling agent KH-550 was added to the PEO and SR coating solution to improve the adhesive forces of different polymers. PSf/SR composite membrane was also prepared by coating the bundles with the same SR coating solution and methods. PSf/PEO/SR and PSf/SR membranes were stored for at least 48 h at 30 ◦ C before use, and the PSf/PEO/SR membrane was stored in a desiccator in the course of termly stability testing. 2.2. Gas permeation test The membrane was potted into a stainless steel pressure cell for pure gas permeation measurement and the permeate flows from the open end were measured at 30 ◦ C with the permeate side set at atmospheric pressure. Gas permeance across the membrane, J, was obtained using the following equation: J=
Q 273 × A(pf − pp ) T
(1)
where J is the gas permeance in GPU (1 GPU = 10−6 cm3 (STP) cm−2 s−1 cmHg−1 ), Q the flux per unit of time
Fig. 1. Cross-sectional TEM micrograph of PSf/PEO/SR multilayer composite hollow fiber membrane.
(cm3 s−1 ), A the effective area of the membrane (cm2 ), pf − pp the pressure difference between the feed and the permeate (cmHg), and T the operation temperature (K).
3. Results and discussion 3.1. Membrane morphology Cross-sectional morphology of PSf/PEO/SR membrane was determined by transmission electron micrograph (TEM) and the image was displayed in Fig. 1. Three layers and two obvious interfaces indicated by two straight lines are observed. TEM image demonstrates that PEO and SR have been coated on the PSf hollow fiber substrate and the multilayer composite membrane has been successfully prepared. 3.2. Gas permeation performance Pure gas permeation results of PSf/PEO/SR and PSf/SR membranes are listed in Table 1. PSf/PEO/SR membrane has an H2 permeance of 49.51 GPU while that of PSf/SR membrane is 41.50 GPU. Meanwhile, the H2 /N2 ideal separation factor of the membrane (defined as the ratio of their pure gas permeances, JH2 /JN2 ) has nearly increase of 30% from 63.8 to 82.3. PSf/PEO/SR membranes show better permeation performance than PSf/SR membranes.
Table 1 Pure gas permeation properties through PSf/PEO/SR and PSf/SR membranes at 30 ◦ C PSf/PEO/SR
JH2 (GPU) JN2 (GPU) JH2 /JN2 a Feed
PSf/SR
1a
2a
3a
Average
4a
5a
6a
Average
49.35 0.596 82.8
49.11 0.602 81.6
50.08 0.606 82.6
49.51 0.601 82.3
41.38 0.649 63.8
41.41 0.646 64.1
41.72 0.656 63.6
41.50 0.650 63.8
pressure (Gauge): 5 atm.
Z. Ye et al. / Materials Chemistry and Physics 94 (2005) 288–291
290
Fig. 2. Stability measurements of PSf/PEO/SR membrane permeation properties (Sample 3# ).
Stability of PSf/PEO/SR membrane permeation properties was tested every 15 days for 2 months at 30 ◦ C, and the results are shown in Fig. 2. There was a slight decrease in H2 permeance and increase in ideal separation factor JH2 /JN2 which reached 85.0 two months later. Such membrane is applicable in H2 purification and recovery in refinery tail gas, ammonia production and syngas composition adjustment process. The intrinsic permeation properties of H2 and N2 in PSf, SR and PEO materials are listed in Table 2. As shown in Table 2, each of the ideal separation factors of the three
Table 2 The intrinsic permeability coefficients of polymeric materials for H2 and N2 used in this study and the ideal separation factor PH2 /PN2 Materials
T (◦ C)
PH2 (Barrerb )
PH2 /PN2
Reference
PSf SR PEOa
35 35 35
5–200 500–3000 1.8
25.0–75.0 1.5–3.0 7.2
[13] [13] [14]
a b
Mw = 1 × 106 . 1 Barrer = 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1 .
polymers for H2 /N2 separation is less than 81.9, which is the average ideal separation factor of PSf/PEO/SR membrane during 2-month termly measurements. According to the aforementioned resistance model, ideal separation factor of a composite membrane must be lower than the intrinsic selectivity of the most permselective polymer which is used to form the membrane. The intrinsic H2 /N2 permselectivities of PSf is normally regarded as in the scope of 72–80, however, differences between PSf/PEO/SR membrane and other reported multilayer composite membranes are that none of the three layers of PSf/PEO/SR membrane is responsible for the higher ideal separation factor. As a result, neither of foregoing (selective layer)/(gutter layer)/(support substrate) or (sealing or protective layer)/(selective layer)/(support substrate) multilayer membrane structures is accordance with three layers of PSf/PEO/SR membrane. In this case, the contributing permselective media should be attributed to the two layer interfaces due to the introduction of PEO layer. Compared with PSf/SR membrane, the permeance of “fast gas” H2 becomes higher and that of “slow gas” N2 becomes lower in PSf/PEO/SR membrane. The chemical and physical properties of the layer interface region may result in the change tendencies of H2 and N2 permeances in PSf/PEO/SR membrane. The interfacial affinity will affect the membrane performance. Generally speaking, PSf is a kind of hydrophilic polymer compared with hydrophobic SR. It is difficult to obtain a PSf/SR membrane with integrated SR coating layer because it is hard to spread hydrophobic SR on PSf surface very well. In contrast, hydrophilic PEO spread better on PSf surface, and the transient PEO layer may offer a smoother surface for SR coating in PSf/PEO/SR membrane. Meanwhile, the degradation and aggregation of PEO [15] and the interaction of the polymers are involved in the forming of the interfaces. Currently, an in-depth study on the structural characteristic of the membrane interfaces, the H2 /N2 separation mechanism and the application of these novel membranes in H2 membrane separation is in progress in our laboratory.
4. Conclusion In summary, novel PSf/PEO/SR multilayer composite hollow fiber membrane not only provided high performance for H2 /N2 separation but also shows a good stability in H2 permeation. Ideal H2 /N2 separation factor of PSf/PEO/SR membrane has exceeded the intrinsic H2 /N2 permselectivities of PSf which is the most H2 /N2 permselective polymer used in this study, and to our best knowledge, it is the first report that three less permselective polymers can form a more permselective composite membrane. For future studies on multilayer composite membrane a focus on the formation mechanism of the membrane layer interfaces and the effects of the layer interfaces on the composite membrane preparation should be taken into account.
Z. Ye et al. / Materials Chemistry and Physics 94 (2005) 288–291
Acknowledgements Special thanks are due to Ms. ZHANG Guihua, Mr. WANG Yuting and Ms. DOU Hong for their help in gas permeation experiments.
References [1] J.M.S. Henis, M.K. Tripodi, J. Membrane Sci. 8 (1981) 233. [2] J.M.S. Henis, M.K. Tripodi, Sep. Sci. Technol. 15 (1980) 105. [3] A. Fouda, Y. Chen, J. Bai, T. Matsuura, J. Membrane Sci. 64 (1991) 263. [4] G. He, X. Huang, R. Xu, B. Zhu, J. Membrane Sci. 118 (1996) 1.
291
[5] Y.S. Kang, J.M. Hong, J. Jang, U.Y. Kim, J. Membrane Sci. 109 (1996) 149. [6] I. Cabasso, K.A. Lundy, US Patent 4,602,922 (1982). [7] T.S. Chung, E.R. Kafchinski, M. Spak, B.B. Ross, G. Wensley, US Patent 5,324,430 (1994). [8] T.S. Chung, J.J. Shieh, W.W.Y. Lau, M.P. Srinivasan, D.R. Paul, J. Membrane Sci. 152 (1999) 211. [9] J.J. Shieh, T.S. Chung, J. Membrane Sci. 166 (2000) 259. [10] I. Blume, I. Pinnau, US Patent 4,963,165 (1990). [11] Z. Ye, Y. Chen, M.C. Deng, Z.A. Chen, H. Li, G.H. Zhang, G.X. Liu, G.H. He, M. Wu, Chinese Patent, Application No. 10021149 (2004). [12] W.J. Koros, G.K. Fleming, J. Membrane Sci. 83 (1993) 1. [13] J.M.S. Henis, M.K. Tripodi, Science 220 (1983) 4592. [14] H. Lin, B.D. Freeman, J. Membrane Sci. 239 (2004) 105. [15] P. Pang, P. Englezos, Colloids Surf. A: Physiochem., Eng. Aspects 204 (2002) 23.