Selective permeation of CO2 through new facilitated transport membranes

Selective permeation of CO2 through new facilitated transport membranes

DESALINATION Desalination 145 (2002) 385-388 ELSEVIER www.elsevier.com/locate/desal Selective permeation of CO2 through new facilitated transport ...

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DESALINATION Desalination

145 (2002) 385-388

ELSEVIER

www.elsevier.com/locate/desal

Selective permeation of CO2 through new facilitated transport membranes Y. Zhang, Z. Wang*, S.C. Wang Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Tel. +86 (22) 2789 0923; Fax +86 (22) 2740 4757; email: [email protected]; [email protected]; [email protected] edu.cn Received 30 January 2002; accepted 25 March 2002

Abstract A new membrane material containing facilitated transport groups for carbon dioxide has been synthesized through the hydrolysis of polyvinylpyrrolidone (PVP) obtained by radical polymerization. The composite membrane was prepared with the hydrolysate as the top layer and microporous membrane as the support. The permeation of pure CO2 and CH, as well as a binary mixture of C02/CH4 through the composite membrane was measured. The effects of feed gas pressure, heat treatment and support membranes on the composite membrane performance were studied. The results show that the composite membranes possess better CO2 permeance and selectivity of CO2 over CH4 than that of other fixed carrier membranes reported in literature. Keywords:

Facilitated transport; Carbon (PVP); Poly{N-Vinyl-y-sodium

dioxide; Fixed carrier membrane; aminobutyrate)

1. Introduction

The high selectivity and permeability make the facilitated transport membranes very attractive for gas separation. There are three kinds of facilitated transport membranes, including liquid membranes, ion-exchange membranes and fixed carrier membranes. The fixed carrier mem*Corresponding

Presented at the International July 7-12. 2002.

Congress on Membranes

I 1-9 I 64/02/$See front matter 0 2002 Elsevier Science PII:SOOll-9164(02)004411

00

Polyvinylpyrrolidone

brane is more favorable compared with the two others because of its superior durability.In recent years, some researchers [ 1,2] concentrated on the fixed carrier membranes having amine group for CO;! separation. We think it is beneficial for COz facilitated transport to introduce diversified carriers in a membrane. But until now, no one has reported about fixed carrier

author

Hydrolysis;

membranes

and Membrane

Processes

B.V. All rights reserved

containing (ICOM),

more

than

one

Toulouse, France,

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kind of carrier for CO*. In this paper, the carriers, secondary amine and carboxylate, were into the membranes by the introduced of polyvinylpyrrolidone, hydrolysis (PVP) which was easily polymerized by radical The selective permeation of polymerization. CO2 is based on the reversible reaction between CO* and the active groups.

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2. Experimental The PVP was synthesized through radical in 20% N-vinylpyrrolidone polymerization (NVP) aqueous solution by using azobisisobutyronitrile (AIBN) (it was twice recrystallized from ethanol) as the initiator at 60” in the inert atmosphere of nitrogen gas [3]. The PVP was hydrolyzed in alkali solution at boiling temperature. The resulting polymer Poly aminobutyrate} (PVSA) {N-Vinyl-y-sodium was purified by precipitating from acetone. The PVSA aqueous solution after removing lowmolecular-weight impurities using ion exchange resin was cast on a support membrane to form a composite membrane. The gas permeation experiments were carried out by using a test cell. The effective area of the composite membrane used in the test the cell is 19.26 cm2. Prior to contacting membrane, both the feed and the sweep (Hz) gases were passed through gas bubblers containing water. The downstream pressure in our apparatus is one atmospheric pressure. 3. Results and discussion Figs. 1, 2 show the effects of feed gas pressure on the performance of the composite membrane by using pure CO2 and CH4. Both the permeance of CO2 and selectivities of COKH4 decrease with the increasing of feed gas pressure. They are the characteristics of the facilitated transport mechanism [4]. The carriers are the secondary amine and carboxylate. The facilitated transport mechanism is shown in Fig. 3. CO2 was transformed

into

small

and

1 20

gas

-1 ’

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40

30

pressure

a. 8

50

/ cmHg

Fig. 1. Effect of feed gas pressure on the selectivity of Co, over CH4. Testingte&perature:27’C; support membrane: polysulfone.

feed gas pressure I cmHg

Fig. 2. Effect of feed gas pressure on the permeance of CO1 and CH4. Testing temperature: 27°C; support membrane: polysulfone.

mobile ion HCO). CO2 transport is enhanced by the carriers. The effects of heat cross-linking on the membrane properties are shown in Figs. 4 and 5. Compared with the results of uncross-linked membranes, although the CO* permeance of heat cross-linked membrane decreases slightly, the selectivity increases. An explanation for the results is the densification of the polymer matrix arising from the cross-linking. The decrease of the C& permeance due to the densification is more than that of the CO:! permeance, this results in an improved COJC& selectivity of the crosslinked membrane. Furthermore, heat crosslinking reduces segmental mobility and therefore shows a low tendency to be plasticized by CO2 and H20.

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feed

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permeate

feed gas pressure I cmHg

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Fig. 3 The mechanism carrier membrane.

Fig. 5. Effect of heat cross-linking on the permeance of CO2 and CH4. Testing temperature: 26°C; feed gas: 50 ~01% CO,+50 ~01% CH4; support membrane: polysulfone; heat cross-linking temperature: 160°C.

of facilitated transport in fixed

feed

feed gas pressure / cmHg

gas

pressure

I cmHg

Fig. 6. Effect of support membrane on the selectivity of CO2 over CH+ Testing temperature: 27’C; molecular weight cut-off of the support membrane: PS 50,000; PES 30,000; PAN 50,000.

Fig. 4. Effect of heat cross-linking on the selectivity of over CH+ Testing temperature: 26’C; feed gas: 50 ~01% C02+50 ~01% CH4; support membrane: polysulfone; heat cross-linking temperature: 160°C.

The effects of support membrane on the performance of the composite membranes were also investigated, as shown in Fig. 6 and 7. The results showed that the composite membrane using polysulfone support membrane (PVSA/PS) is the best for the C0#2H4 system, the composite membrane using polyethersulfone support membrane (PVSA/PES) is next, while the composite membrane using poly- acrylonitrile support membrane (PVSA/PAN) is the worst.

I. 0

I. 10

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Fig. 7. Effect of support membrane on the permeance of COz. Testing temperature: 27°C; molecular weight cutoff of the support membrane: PS 50,000; PES 30,000; PAN 50,000.

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According to the analysis of the structure, the composite membrane in this work can be simply divided into three layers, dense active layer, interfacial layer arising from the mutual penetration and tight binding of the dense active layer and support membrane, and porous layer. The transport of gas through the dense active layer is described by solution-diffusion which simply depends on the membrane material, the transport of gas in porous layer is a complex combination of Knudsen diffusion, continuum diffusion and viscous flow [5]. The solutiondiffusion of gas in interfacial layer depends on not only the dense active layer material, but also the support membrane material. The influence of the support membrane mainly indicates the influence of the interfacial layer. The gases tend to dissolve in polymeric media of similar chemical structure to them [6]. The structure of CO2 is similar to that of sulfone groups. Therefore, compared with PAN, PS and PES containing sulfone groups favor the solubility of CO;! in the interfacial layers of the composite membranes. In addition, the diffusion coefficient of gas in PAN is lower than that in PS and PES because of high crystallinity of PAN. Consequently, the performance of the PVSA/PAN composite membrane is not as good as that of PVSAJPES and PVSAIPS composite membranes. While the average pore diameter of the PES support membrane is smaller than that of the PS support membrane, owing to its small molecular weight cut-off, the proportion of PVSA in the interfacial layer of the PVSA/PES composite membrane is smaller

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than that in the interfacial layer of PVSA/PS composite membrane. Although PES is a totally amorphous material with a high density of sulfone groups, the solution-diffusion of gas in PVSA is higher than that in PES, thus the CO2 permeance of the PVSA/PES composite membrane is lower than that of the PVSA/PS composite membrane. As a result, the PVSA/PS composite membrane possesses the best performance.

Acknowledgement This research is supported by the National Natural Science Foundation of China (No. 2997603 1, No. 29876027), the Natural Science Foundation of Tianjin and the Foundation of Outstanding Teacher at University.

References PI

PI [31 [41

is1 [61

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