polyethlene glycol blend membranes

polyethlene glycol blend membranes

Desalination 193 (2006) 90–96 Facilitated transport of CO2 through polyvinylamine/polyethlene glycol blend membranes Chunhai Yi, Zhi Wang*, Meng Li, ...

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Desalination 193 (2006) 90–96

Facilitated transport of CO2 through polyvinylamine/polyethlene glycol blend membranes Chunhai Yi, Zhi Wang*, Meng Li, Jixiao Wang, Shichang Wang Chemical Engineering Research Center, School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, China Fax: +86 (22) 2740 4496; email: [email protected] Received 1 February 2005; accepted 7 April 2005

Abstract Poly(vinylamine)/poly(ethylene glycol) blend membranes were prepared. The effects of blend polymer composition on the membrane structure and CO2/CH4 separation performance were investigated. The results show that membrane crystallinity are at a minimum against poly(ethylene glycol)(PEG) content, while the permeation rates of CO2 and CH4 are the maximum. The blend membrane with 10 wt% PEG has the highest pure CO2 permeation rate of 5.8×10!6 cm3 (STP)/cm2.s.cm Hg and the highest selectivity of 63.1 at 25EC and 96 cm Hg of feed pressure. The effects of crosslinking and the coupling effects between CO2 and CH4 were also investigated. The selectivity increased remarkably when the membrane is cross-linked. For mixed gas of constant CH4 partial pressure and changing CO2 partial pressure, the CH4 permeation rate increased with an increase of CO2 partial pressure due to the coupling effects. Keywords: CH4; CO2; Fixed carrier; Gas permeation; Blend membrane

1. Introduction The use of membranes to selectively remove CO2 from mixtures is of interest for a wide variety of applications such as upgrading of natural gas, landfill gas recovery, enhanced oil recovery and global warming prevention [1]. However, one *Corresponding author.

of the major problems confronting the use of membrane-based CO2 separation technology is the lack of membranes with both high permeability and high selectivity. A gas separation membrane prepared with a blended polymer has exhibited improved mechanical properties, better membrane-forming ability and higher gas permeability, which are all very attractive [2]. By blending a useful combination

Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.

doi:10.1016/j.desal.2005.04.139

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of the advantage of each polymer into a new product can be achieved. Furthermore, the blending of polymers can result in properties not found in a single polymer. Compared with other modification technology or even with the synthesis of entirely new materials, polymer blending is preferred due to its simplicity, reproducibility and commercial character [3]. However, most of the research is focused on common polymeric membrane materials, which are based on a solution– diffusion mechanism. For the separation of CO2, fixed carrier membranes have high permselectivity as well as good stability, which is very attractive [4–6]. Matsuyama et al. [6] prepared the polyethylenimine/poly(vinyl alcohol) blend membrane and investigated its CO2/N2 separation performance. Polyethylenimine contains primary and second amino groups that can react with CO2 reversibly. The results show that the selectivity of CO2 over N2 has a maximum with the increase of polyethylenimine content in the blended membrane. In our previous report [7], a fixed carrier composite membrane with PVAm as the separation layer and a polyacrylonitrile ultrafiltration membrane as the support layer was prepared. PVAm has the primary amino groups, which can act as carriers for CO2. This membrane possesses a high CO2 permeation rate as well as high CO2/CH4 selectivity. However, the membranes made from PVAm often have high crystallinity due to the strong intermolecular interaction. The membranes are brittle because of the high crystallinity. Furthermore, the high crystallinity reduces the membrane’s permselectivity. In this work, a poly(vinylamine)(PVAm)/poly (ethylene glycol) (PEG) blended polymer was prepared. Composite membranes were developed with PVAm/PEG blend as the active layer and a polyethersulfone (PES) ultrafiltration membrane as the support. It is expected that the blending of PVAm with PEG can decrease membrane crystallinity, and thus improve the mechanical property and permselectivity of the membrane.

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2. Experimental PVAm was obtained by the Hoffmann degradation of polyacrylamide [8] and purified by the solution–precipitation method three times before use. PEG with the average molecular weight of 20,000 was purchased from Phentex Chemical and used as received without further purification. The aqueous solution of the PVAm–PEG was cast on a PES ultrafiltration membrane by an applicator and was dried in an artificial climate chamber (Climacell 222R, Germany) for more than 24 h. The cross-linked composite membranes were prepared by dipping the membrane into a cross-linking agent solution for 0.5 min and dried in the artificial climate chamber. Crystallinity was determined by a wide-angle C-ray diffractometer (D/max-2500, Japan). The gas permeation of the membranes was measured by using a set of test apparatus. The effective area of the composite membranes used in the test cell is 19.26 cm2. Prior to contacting the membrane, both the feed gas and the sweep gas (H2) were passed through gas bubblers containing water. The outlet sweep gas composition was analyzed by a gas chromatograph equipped with a thermal conductivity detector (HP4890, Porapak N). The fluxes of CO2 (NCO2) and CH4 (NCH4) were calculated from the sweep gas flow rate and its composition. The downstream pressure in the apparatus was 1 atm. The permeation rate and selectivity are given by Ri = Ni /∆Pi, αCO2/CH4 = RCO2/RCH4.

3. Results and discussion 3.1. Crystallinity of blended membranes For gas permeation the formation of crystals in polymer membranes is generally deleterious since they act as impermeable obstacles to gas molecular transport [9]. As for the fixed carrier membrane, crystallization will lead not only to a decrease of the effective permeation area but also

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Fig. 1. Effect of PEG content on the crystallinity of blended membranes. Membrane preparation conditions: temperature = 30EC, relative humidity = 60%.

to the decrease of the effective carrier content of the membrane. The carriers in the crystallization region cannot make contact with the CO2 molecule. Therefore, it is essential to investigate the crystallization of blended membranes. As shown in Fig. 1, the crystallinity of blended membranes possesses a minimum against the PEG content (CPEG). In the low CPEG region (<15 wt%), crystallinity decreases with the growth of PEG content. This may be due to the entanglement of the PVAm and PEG chains. When CPEG is more than 15 wt%, the blended polymer may be immiscible, which leads to the increase of crystallinity. 3.2. Effect of PEG content on the performance of blend membranes Fig. 2 shows the effects of CPEG on the performance of PVAm/PEG blended membranes by using pure CO2 or CH4 as feed. Both permeation rates of CO2 and CH4 have a maximum against the CPEG. The blended membrane with CPEG of 10 wt% has the highest CO2 permeation rate of 5.8×10!6 cm3 (STP)/cm2.s.cm Hg, while the membrane with CPEG of 15 wt% shows the highest

Fig. 2. Effects of PEG content on the performances of blended composite membranes by using pure feed gases. Feed gas pressure: 962 cm Hg. Membrane preparation conditions: temperature = 30EC, relative humidity = 60%; testing temperature: 25EC.

CH4 permeation rate of 1.13×10!-6 cm3 (STP)/ cm2.s.cm Hg. At CPEG of 10 wt%, the selectivity of CO2 over CH4 (αCO2/CH4) also has a maximum value, which is about 63.1. The primary factor that causes the CH4 permeation rate change is the effective permeation area change due to the crystallization. In the low CPEG region (<15 wt%), the crystallinity decreases with the increase of CPEG, which leads to a higher CH4 permeation rate. When the CPEG is higher than 15 wt%, the CH4 permeation rate decreases with CPEG due to the increase of crystallinity. CO2 permeates the membranes mainly by the interaction between CO2 and carriers. Therefore, for CO2 permeation the leading influence factor is the effective carrier content. The effective carrier content is determined by two parameters, one of which is total carrier content. The total carrier

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content is reduced with increasing CPEG because the carriers (primary amino groups) only exist in the polymer of PVAm. The other is crystallinity, which is shown in Fig. 1. The effective carrier content increases with the drop of crystallinity and vice versa. As a result of combination of the two effects, the membrane with a CPEG of 10 wt% may have the maximum effective carrier content, which leads to the highest CO2 permeation rate. The variety of αCO2/CH4 can also be attributed to the difference of effective carrier content. From that mentioned above, it is seen that the membrane with 10 wt% PEG and 90 wt% PVAm has the highest CO2 permeation rate and αCO2/CH4, and this membrane was used in the following studies.

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(a)

(b)

3.3. Effect of cross-linking on the performance of blended membranes Cross-linking is an effective and convenient method for improving the gas-transport properties of polymeric membranes [10]. By cross-linking, membranes can obtain improved selectivity and better stability. However, cross-linking has been shown to result in a significant decrease in gas permeability. In this work, the membranes prepared were cross-linked with glutaradehyde, and the effect of cross-linking on the performances of membranes was investigated. Fig. 3 shows the effect of glutaradehyde crosslinking on gas permselectivity of the composite membranes. Compared with uncross-linked membranes, the CO2 and CH4 permeation rates of cross-linked membranes significantly decreased. The CO2/CH4 selectivity increased about 20% through cross-linking with glutaradehyde. The glutaradehyde cross-linking, with the mechanism shown in Fig. 4, leads to two results. On one hand, the cross-linkage reduces the mobility of the polymer and produces a denser membrane, which leads to a decrease in diffusion of CH4 and CO2 [8]. On the other hand, the amino group is reduce due to the reaction between amino group

(c)

Fig. 3. Effects of cross-linking on the performances of blended composite membranes by using pure feed gases. (a) CO2 permeation rate, (b) CH4 permeation rate, (c) selectivity. PEG content: 10 wt%; membrane preparation conditions: temperature 30EC, relative humidity 60%; glutaradehyde concentration: 0.1 mol/L; testing temperature, 25EC.

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Fig. 4. Reaction scheme of glutaradehyde cross-linking.

(a)

(b)

Fig. 5. Effects of CO2 partial pressure on the performances of blend composite membranes. (a) CO2 and CH4 permeation rates, (b) selectivity; PEG content: 10 wt%. Membrane preparation conditions: temperature = 30EC, relative humidity = 60%; CH4 partial pressure: 882 cm Hg; testing temperature: 25EC.

and glutaradehyde, which will decrease facilitated transport of CO2. It can be seen from Fig. 3 that the decline of the CH4 permeation rate is more obvious than that of CO2. The reason is that the CH4 permeates through the membrane by the solution–diffusion mechanism, while CO2 permeates the membrane mainly by a facilitated transport mechanism. Because the concentration

of a cross-linking agent solution is small and the reaction time is short, in the cross-linking only a small number of amino groups react with glutaradehyde and create a denser structure. This kind of denser membrane possesses relatively lower gas permeation rates but higher selectivity than uncross-linked membranes.

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3.4. Coupling effects between CO2 and CH4 The CH4 feed partial pressure was fixed to 83 ± 2 cm Hg and the feed CO2 partial pressure varied from 0 to 77 cm Hg in order to test the coupling effects between CO2 and CH4. The effects of CO2 partial pressure on the performances of blended composite membranes is shown in Fig. 5. If there are no coupling effects, the CH4 permeation rate remains constant because of the invariability of the CH4 partial pressure. But as can be seen from the figure, the CH4 permeation rate increases quickly with the increase of CO2 partial pressure. The reason is that with the increase of CO2 partial pressure, the membrane is plasticized by CO2. This means that more CO2 dissolves in the membrane, which leads to larger mobility of polymer chains, and with the plasticization of the membrane, the permeation of CH4 will be easier. Thus, the CH4 permeation rate increases with the increase of CO2 partial pressure. Because the transport of CO2 follows the facilitated transport mechanism, the CO2 permeation rate decreases with the CO2 partial pressure [6]. Consequently, the real CO2/CH4 selectivity tested by using mixed gas is lower than the ideal selectivity and decreases with increasing CO2 partial pressure. 4. Conclusions A novel fixed carrier composite membrane was prepared for the first time with a PVAm/PEG blended polymer as the selective layer and a PES ultrafiltration membrane as the substrate. Through blending with PEG, the properties of a fixed carrier membrane can be improved. The crystallinity of blended membranes is minimum against the PEG content (CPEG), while the permeation rates of CO2 and CH4, the selectivity of CO2 over CH4 are maximum. The membrane with a PEG content of 10 wt% has the highest CO2 permeation rate and selectivity.

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Cross-linking can improve the selectivity but leads to decreased gas permeance. The selectivity increases about 20% by dipping the blended membrane into a 0.1 mol/L glutaradehyde solution for 0.5 min. In the testing of coupling effects, the CH4 permeation rate increases with the increase of CO2 partial pressure. The reason is that with the increase of CO2 partial pressure the membrane is plasticized by CO2, which leads to an increased CH4 permeation rate. Acknowledgements This research was supported by the National Basic Research Program (No.2003CB615703) and the Natural Science Foundation of China (No.20476075). References [1] R.W..Baker, Future directions of membrane gas separation technology, Industry Eng. Chem. Res., 41 (2002) 1393–1411. [2] X.-G. Li, I. Kresse, J. Springer, J. Nissen and Y.-L. Yang, Morphology and gas permselectivity of blend membrane of polyvinylpyridine with ethylcellulose, Polymer, 42 (2001) 6859–6869. [3] G.C. Kapantaidakis and G.H. Koops, High flux polyethersufone–polyimide blend hollow fiber membranes for gas separation, J. Membr. Sci., 204 (2002) 153–171. [4] Y. Zhang, Z. Wang and S.C. Wang, Synthesis and characteristics of novel fixed carrier membrane for CO2 separation, Chem. Lett., 4 (2002) 430–431. [5] Y. Zhang, Z. Wang and S.C. Wang, Selective permeation of CO2 through new facilitated transport membranes, Desalination, 145 (2002) 385–388. [6] H. Matsuyama, A. Terada, T. Nakagawara, Y. Kitamura and M. Teramoto, Facilitated transport of CO2 through polyethylenimine/poly(vinylalcohol) blend membrane, J. Membr. Sci., 163 (1999) 221–227. [7] Z. Wang, C.M. Dong, Q. Lu and S.C. Wang, Preparation and CO2 separation performance of polyvinyl-

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amine/polyacrylonitrile composite membranes, J. Chem. Ind. Eng., 54(8) (2003) 1188–1191 [in Chinese]. [8] V.F. Kurenkov, H.G. Harta and F.I. Lobanov, Degradation of polyacrylamide and its derivatives in aqueous solutions, Russ. J. Appl. Chem., 75 (2002) 1039–1050. [9] N.P. Patel, A.C. Miller and R.J. Spontak, Highly

CO2-permeable and selectivity polymer nanocomposite membrane, Adv. Mat., 15(9) (2003) 729– 733. [10] A.F. Ismail and W. Lorna, Penetrant-induced plasticization phenomenon in glassy polymers for gas separation membrane, Sep. Purif. Technol., 27 (2002) 173–194.