CH4 separation

CH4 separation

Desalination 192 (2006) 112–116 Development of high-performance polysulfone/ poly(4-vinylpyridine) composite hollow fibers for CO2/CH4 separation Jia...

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Desalination 192 (2006) 112–116

Development of high-performance polysulfone/ poly(4-vinylpyridine) composite hollow fibers for CO2/CH4 separation Jian-Jun Qina,b*, Tai-Shung Chunga a

Department of Chemical and Environmental Engineering, National University of Singapore, Singapore 119260 b Centre for Advanced Water Technology, Singapore Utilities International Pte Ltd, Blk 2, #241, Innovation Centre (NTU), 18 Nanyang Drive, Singapore 637723 Tel. +65 (6) 794-1507; Fax +65 (6) 794-2791; email: [email protected] Received 15 March 2005; accepted 7 April 2005

Abstract High-performance polysulfone (PSf)/poly (4-vinylpyridine) (4-PVP)/silicon rubber (SR) multilayer composite membranes were developed for CO2/CH4 separation. PSf hollow-fiber substrates with different pore sizes were manufactured by varying the concentration of nonsolvent additive in the spinning dope. PSf/4-PVP/SR multilayer composite membranes were prepared using dip-coating. The pore size of the PSf substrate increased with an increase of the nonsolvent additive. Selectivity of a composite membrane for CO2/CH4 increased with decreasing pore size of the substrate. The pre-wetting agent played an important role in preventing the penetration of the coating material into the substrate with large pores, but did not show a significant effect on a substrate with small pores. A high-performance multilayer composite membrane with a CO2/CH4 selectivity of 29 and CO2 permeance of 92 GPU was obtained. Keywords: Multilayer membranes; Composite hollow fibers; Poly (4-vinylpyridine); Polysulfone; CO2/CH4 separation

1. Introduction Multilayer composite membranes for gas separation are increasingly attractive due to their advantages over integrally skinned asymmetric *Corresponding author.

membranes: (1) cost-effectiveness when the materials are expensive; (2) use of highly selective but brittle polymers; and (3) ease of being made defect-free [1]. There are two types of multilayer composite membranes: (1) (support substrate)/(selective layer)/(protective or sealing

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.2005.04.132

J.-J. Qin, T.-S. Chung / Desalination 192 (2006) 112–116

layer) [1–3] and (2) (support substrate)/(gutter layer)/(selective layer) [4–6]. The study explored development of a high-performance multilayer composite hollow-fiber membrane of polysulfone (PSf)/poly (4-vinylpyridine) (4-PVP)/silicon rubber (SR), which belongs to the first type mentioned above. Effects of the pore size of substrate fibers and pre-wetting agent FC-72 on the performance of the resultant composite membranes were studied.

2. Experimental PSf hollow-fiber substrates were manufactured using the wet spinning technique. Water was used as the external coagulant to yield an outer skin layer, whereas a mixture of high concentration of solvent (90 wt%) in water was employed as the internal coagulant in order to create an open porous inner surface without a dense skin and to minimize the effect of the substructure of the inner skin on fiber performance. Pore sizes in the outer skin were adjusted by varying the concentration of the nonsolvent additive diethylene glycol (DG) in the spinning dope containing PSf/N-methyl-2-pyrrolidone (NMP)/DG while PSf concentration remained the same at 25 wt%. The as-spun fibers were rinsed in flowing water at room temperature for 48 h, and then they were post-treated by the ethanol–hexane–air drying procedure in order to gradually reduce the surface tension effect during the fiber drying process and to minimize fiber shrinkage/pore collapse. Multilayer composite hollow-fibers were prepared by the dip-coating method. The dried hollow fibers were dipped into the pre-wetting agent, Fluorinert® 72, for 3 min if used before the first coating, and then immediately dipped into the first coating solution composed of 0.2 w% 4-PVP for 2 min and cured for 1 day. After that, it was dipped into the second coating

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solution containing 3 wt% polydimethylsiloxane (Sylgard-184 Silicone elastimer kit, SR) in 97 wt% n-hexane for 2 min and cured for 2 days. Single gas was used to determine the membrane permeance in this study. Five fibers with a length of ~10 cm were assembled into a bundle to make a test module. Three modules were freshly prepared and tested for each membrane sample, and the average of their performance was reported. Pressure-normalized permeance of pure gas was tested in the sequence of CH4 and CO2 at room temperature of 25EC under pressure of 200 psi. Gas permeation rates were measured with bubble flow meters. Detailed descriptions on the manufacture and measurement of gas separation permeance of hollow fibers were given in an earlier study [7].

3. Results and discussion Fig. 1 shows SEM images of an example of hollow-fiber substrates that were spun with the dope without DG addition. Fig. 1(c) and (d) indicate that the fiber had a porous inner surface and a relatively dense outer surface, even at a magnification of 10,000. As a consequence, a desired substrate structure was obtained and the resistance of the inner surface to the substrate could be eliminated. Table 1 shows the effect of the nonsolvent additive concentration in the spinning dope on the performance of the resultant PSf substrate fibers and their PSf/4-PVP/SR multilayer composite hollow-fiber membranes. It can be seen that the dope viscosity dramatically increased as the concentration of nonsolvent DG increased. Generally speaking, a higher amount of nonsolvent in the dope will result in a membrane with a larger pore size and/or higher porosity because the polymer-poor phase has a higher fraction when the phase separation happens. However, too high a viscosity of the dope may reduce the precipitation speed and result in a thicker top skin

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Fig. 1. SEM images of a PSf hollow-fiber substrate. (a) Cross-section: 100×; (b) Partial cross-section: 500×; (c) Inner surface: 500×; (d) Outer surface: 10,000×). Table 1 Effect of nonsolvent additive on performance of the resultant substrate and composite hollow fibers Dope composition (wt.%)

PSf

NMP DG

25 25 25

75 70 55

0 5 20

Viscosity (cp)

2,585 8,000 48,400

Substrate fibers

Multilayer composite fibers With pre-wetting agent

Without pre-wetting agent

Permeance

Selectivity

Permeance

Selectivity

Permeance

CO2 (GPU)

CO2/CH4

CO2 (GPU)

CO2/CH4

CO2 (GPU)

980 5,235 2,618

33.3 28.8 1.8

73.6 92.0 70.7

43.8 26.7 —

64.5 20.0 —

Note: 1 GPU = 1×10!6 cm3 (STP)/cm2-s-cmHg.

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Fig. 2. SEM images of outer edge (top layer) of PSf substrates spun with different concentrations of DG in the dope. (a) 0 wt%; (b) 5 wt%; (c) 20 wt%. All magnifications are 10,000×.

of the asymmetric substrate. As a consequence, the addition of DG in the dope obviously enhanced the permeance of the substrate fiber but too much DG addition reduced the permeance

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due to a resultant substrate with a thicker skin layer. SEM images of top layers of PSf substrates in Fig. 2 reveal that when 5 wt% DG was added into the dope, the skin layer in Fig. 2(b) seemed to be thinner compared to that in Fig. 2(a); however, the thickness of the skin layer in Fig. 2(c) was increased significantly when concentration of DG was 20 wt%, which supports the above statements. For the multilayer composite fibers with the pre-wetting agent, the selectivity of CO2/CH4 decreased from 33.3 to 28.8 while permeance of CO2 increased from 73.6 to 92 GPU with increasing the pore size of the substrate fiber. In other words, a less defective composite fiber was obtained if the substrate had smaller pore sizes. However, a defective composite fiber resulted if the substrate pore size was too large (when DG concentration was 20%), even though a prewetting agent was used. In the latter case, the low permeance was due to the penetration of coating material into the substrate. For the multilayer composite fibers without the pre-wetting agent, both the selectivity and permeance dramatically decreased with increased substrate pore size. The former was due to more defective composite fibers from the substrate with large pore size and the latter was caused by serious penetration of the coating material. For the substrate with the largest pore size, the test without the pre-wetting agent was not conducted since the results tested with prewetting agent were bad. When the multilayer composite fibers with the pre-wetting agent were compared to those without the pre-wetting agent, for the substrate fiber with the smallest pore size, the selectivity significantly decreased as the pre-wetting agent was used. This could be attributed to the fact that a defect-free composite membrane was more difficult to form because the pre-wetting agent might affect the adhesive between the selective layer and the substrate. The permeance slightly decreased when the pre-wetting agent was not

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used, which might be due to slight penetration of selective material. However, for the substrate fiber with increased pore size, due to the serious penetration of the coating material, the permeance of the resultant composite fiber without the prewetting agent was much less than that with the pre-wetting agent, although the same selectivity was obtained for both. This means that a prewetting agent had a more obvious effect on the substrate with large pore size than that with small pore size. In order to avoid using a pre-wetting agent and reduce the step to manufacture a PSf/4PVP/SR multilayer composite membrane, an optimum substrate with an appropriately small pore size needs to be used. 4. Conclusions PSf hollow fibers with different pore sizes were made by varying the amount of nonsolvent additive in the spinning dope. PSf/4-PVP/SR multilayer composite membranes were prepared using dip-coating. The pore size of PSf substrate increased with increasing addition of the non-

solvent additive. Selectivity of a composite membrane for CO2/CH4 increased with decreasing pore size of the substrate. The pre-wetting agent played an important role in preventing the penetration of the coating material into a substrate with large pores. A high-performance multilayer composite membrane with the CO2/CH4 selectivity of 29 and the CO2 permeance of 92 GPU was developed in the study.

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