Separation On O2, N2, and SF6 Gas Using Carbon Molecular Sieve Hollow Fiber Membranes

Separation on O2, N2, and SF6 gas using carbon molecular sieve hollow fiber membranes

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

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

haven@airrane.com

Introduction

At the present time, there is a growing interest in the development of gas separation membranes based on materials providing better selectivity, thermal stability and chemical stability than those already existing. Carbon molecular sieve (CMS) membranes are promising materials for membrane-based gas separation because of their excellent permeation and separation properties and thermal and chemical stability under harsh conditions, such as at high pressure and elevated high temperatures [1]. CMS membranes fabricated by pyrolysis of different polymeric materials are characterized by a very narrow microporosity (°3–5Å), which allows very sharp discrimination between gas molecules having different sizes and shape. In particular, thermally stable polyimides acting as organic molecular sieves are excellent precursors for obtaining CMS membranes [2]. Polyimides are rigid, high melting, high Tg, thermally stable polymers formed by the condensation reactions dianhydrides with diamines. Because of their high thermal stabilities, polyimides are used in machined devices and other high temperature applications [3]. The separation of gas molecules with a molecular size under 4Å by CMS membranes can be effectively achieved via a molecular sieve mechanism. The important factors determining the microstructure and gas separation properties of the CMS membranes are the choice of polymeric precursor and the pyrolysis conditions [4-5].

In this study, the precursor hollow fiber membranes were prepared using Matrimid 5218. The precursor was spun into asymmetric hollow fiber form by a dry-wet spinning process. The polymeric precursors were pyrolyzed in a quartz tube furnace. To investigate the gas permeation properties of CMS, O2, N2, and SF6 gas were measured for the gas permeation.

Experimental

A commercially available polyimide-type polymer was chosen as a precursor in this study, which was Matrimid 5218 (Ciba Specialty Chemicals, USA). The polymer was dried at 50°ĘC for 10hr. 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. Polymer dope solution was dissolved using IKA electronic stirrers. The polymer dopes were degassed and filtered. The hollow fiber was formed through a spinneret with outer diameter/inner diameter of the tube of 0.44/0.2mm. 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. The precursor was an integrally-skinned, asymmetric hollow fiber made of a polyimide. 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 precursor polymeric materials were coated with gold in a vacuum atmosphere using a JEOL JFC-1100E (10mV, 300s) before they were placed in the microscope. The carbon yield, defined as the weight relative to the initial precursor weight, was determined using thermo gravimetrical analysis (TGA). TGA was performed using a TA Instruments TGA2050 (TA Instruments, New Castle, DE, USA) to study the thermal properties of the polyimide. CMS membrane was produced from the pyrolysis of polyimide hollow fiber membranes. Hollow fiber membranes were pyrolyzed on alumina tube supports in Lenton 1200°ĘC Gas Atmosphere Tube Furnace with Eurotherm temperature controllers. The best carbon membranes, in terms of both separation and mechanical properties, were produced from the pyrolysis of aromatic polyimides. The carbon membranes were continuously prepared by passing the precursor through the inside of a quartz glass pipe set in the furnace. The precursor was treated thermally in atmospheric air (100cc/min) at 400° for 60 min before the pyrolysis. The treatment contributed to the stabilization of the asymmetric structure of the precursor to temperatures of the pyrolysis. In the case of the precursor without the treatment, the precursor softened during the pyrolysis and the resulting carbon membrane had a low membrane performance. The first step is the annealing of the polyimide hollow fiber at 400°ĘC in air atmosphere (100cc/min), the second is the intermediate heating at 600°ĘC in atmospheric Argon, and the third is the pyrolysis to form at over 600°ĘC in Argon [6]. Hollow fiber membrane modules were one-end-opened types and each module contains 5 pieces of fibers with the length of 20cm. The feed gas was introduced to the outside of the hollow fiber. The permeation experiments for single gases such as O2, N2, and SF6 were performed at feed pressure from 0.5 MPa and at temperatures of 25°ĘC.

Reference

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