Preparation and Characterization of Hollow Fiber Membrane for Gas Separation Using Polyimide

Preparation and characterization of hollow fiber membrane for gas separation using polyimide

Hyung Chul Koh, Se Jong Kim, Chung Seop Lee, Seong Yong Ha

Airrane Co., Ltd., 217-2, Shinsung-dong, Yusung-gu, Daejeon 305-805, Korea

haven@airrane.com

Introduction

Gas separation using polymeric membranes has achieved important commercial success in some industrial processes since the first commercial-scale membrane gas separation system was produced in the late of 1970s. In order to extend its application and compete successfully with traditional gas separation processes such as cryogenics, pressure swing adsorption and absorption, great attention has been made in fabricating high separation performance membranes in both academia and industry. Development of ultra-thin skinned asymmetric hollow fiber membranes from highly selective polymers has been a challenge in gas separation membrane processes [1]. Although thousands of polymers exhibit perm-selective properties for gas mixtures, only a few glassy polymers are useful in making asymmetric gas separation membranes, particularly, polyimide (PI), polysulfone (PSf), polyethersulfone (PESf), and  polyetherimide (PEI).

Polyimides have been attractive as membrane materials for gas separation because of their excellent thermal, chemical and mechanical properties, as well as excellent film-forming properties. Relationship between the chemical structure of polyimides and their gas permeation and separation properties has been investigated extensively. For a given polymer, how to make a high separation performance membrane by studying the membrane formation protocol, is an important topic in the membrane research. Formation of asymmetric hollow fiber contains many variables such as the nature of the solvent, the additive and the coagulant, the spinning conditions etc. all of which may affect the structure of the hollow fiber membranes and separation performance. Reviewing advances in preparing asymmetric gas separation membranes, the selection of a suitable solvent, additive and coagulant is the most important. From the view point of economic and environmental basis, the solvents, additives and coagulants used must be commercially available and low toxicity. In addition, the manufacture method must be easy, economic and reliable. However, in practice, the selection of solvent and coagulant is quite limited. Water is the best choice as the coagulant to remove solvent from hollow fiber membrane. For water soluble solvents used, the most common solvents are N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAc). Therefore, the use of a suitable additive became a most efficient tool to prepare desired membranes. In fact, all high separation performance membranes have been prepared by the use of right additives [2–5].

In this study, PI hollow fiber membranes have been prepared NMP/IPA/THF systems. The effect of polymer, solvent, non-solvent, additives concentration, synthesized polyimide molecular weight and air-gap on the membrane structure and gas permeation were investigated.

Experimental

PI (synthesized polyimides) was received from Aekyung Petrochemical Company Limited (Korea). NMP (99%), THF (99%), and Ethanol (99.9%) were supplied by Samchun Chemical Company (Korea) and used without further purification. Tap water was also used as the external coagulant and washing medium. The coating solution was prepared by mixing silicone elastomer and curing agent (Sylgard-184) in n-hexane. Dopes were dissolved using IKA electronic stirrers. Dopes were stirred until homogeneous, typically taking 6 hr. After mixing, the dopes were poured into the 2 L dope reservoir tank and allowed to degas for 12 hr. Bore fluid was delivered by HPLC pump (Gilson, USA). The spinneret had lumen ID, lumen OD, and annular ID measurements of 0.1, 0.2, and 0.44 mm, respectively. The internal coagulant was injected through the tube during the spinning. The hollow fiber membranes were kept in water for at least 24 hours and then solvent exchanged with the ethanol and n-hexane. To investigate the effects of the pyrolysis on the structure of the porous asymmetric support layer of the fibers scanning electron microscopy (SEM) was used. The micrographs were obtained using a JEOL JSM 6390 (Japan) LV-SEM for magnifications and a Philips XL30 S FEG (Netherlands) FE-SEM for higher magnifications. The hollow fiber membranes were permeated with pure gas O2, N2, and mixture gas O2/N2 (air) with feed pressures of 0.5MPa. Fluxes of pure gases were measured with bubble flow meters (Gillian, USA). Effective area of the module used in the experiment is the 1.27m2.

Reference

[1] R.W. Baker, E.L. Cussler, W.J. Koros, H. Strathmann, Membrane Separation Systems: Recent Development and Future Directions, Noyes Data Corp. Park Ridge, NJ, 1991.

[2] R.E. Kesting, A.K. Fritzsche, M.K. Murphy, A.C. Handermann, C.A. Cruse, R.F. Malon, Process for forming asymmetric gas separation membranes having graded density skins, US Patent 4,871,494 (1989).

[3] D. Wang, K. Li, W.K. Teo, Polyethersulfone hollow fibre gas separation membranes prepared from NMP/alcohol solvent systems, J. Membr. Sci. 115 (1996) 85.

[4] D. Wang, K. Li, W.K. Teo, Preparation and characterization of polyetherimide hollow fibre membranes for gas separation, J. Membr. Sci. 138 (1998) 193–201.

[5] W.K. Teo, K. Li, D.Wang, Highly permeable polyethersulfone hollow fibre membranes for gas separation, US Patent 6,017,474 (2000).