Mixed oxygen ion and electron conducting hollow fiber membranes for oxygen separation
Introduction
Oxygen is ranked among the top five in the production of commodity chemicals in the United States [1]. How to obtain cheap, pure oxygen is a very important issue in the industry. Commercial oxygen is currently produced by cryogenic distillation and pressure swing adsorption (PSA), which are very costly technologies. The membrane applications have drawn great attention in oxygen separation. Polymeric membranes that have sufficient selectivity to oxygen and high flux are intensively investigated. However, since polymeric membranes generally have a low separation factor, the maximum oxygen concentration produced by a single stage system is up to 50% [2]. After the pioneering papers of Teraoka et al. [3], [4] who first reported the oxygen permeation through a perovskite membrane based on La1−xAxCo1−yFeyO3−δ, such perovskite membranes have become of increasing interest as a potentially economical, clean and efficient means to produce pure oxygen by separating it from air or other oxygen containing gases. The oxygen transport through this type of membranes takes place in the form of oxygen ions instead of oxygen molecules, so pure oxygen can be obtained.
In order to develop the ceramic membrane system for industrial applications, it is essential to increase the oxygen permeation flux [5], [6], [7], [8] and, to optimize the membrane configuration such as flat sheet [9], tubular [10] or hollow fiber [11], [12], [13]. So far, mainly disk-shaped membranes with a limited membrane area (< 5 cm2) were employed because they can be easily fabricated by a conventional pressing method. Although a multiple planar stack can be adopted to enlarge the membrane area to an industry plan scale, many problems such as the high temperature sealing, the connection and the pressure resistance have to be faced [14]. Tubular membranes were developed to reduce the engineering difficulties, especially the problems associated with the high temperature seal [10], but their small surface area to volume ratio and their relative thick wall leading to a low oxygen permeation flux, make them unfavorable in practice.
Hollow fiber membranes with an asymmetric cross section can solve the problems mentioned above. Such hollow fiber membranes possess much larger membrane area per unit volume for oxygen permeation [11], [12], [13]. Furthermore, the resistance of the membrane to oxygen permeation is very much reduced due to the thin wall and the asymmetric structure which increases the membrane surface area for oxygen surface exchange. Therefore, it is expected that asymmetric hollow fiber membranes can give higher permeation fluxes than other membrane geometries. By adopting a long hollow fiber and keeping the two sealing ends away from the high temperature zone, the problem of high temperature sealing no longer exists. In this paper, the perovskite composition BaCoxFeyZrzO3−δ (BCFZ, x + y + z = 1.0), which is a novel O2-permeable membrane material with high O2 permeation fluxes and excellent thermal and mechanical stability[14], was chosen to prepare hollow fiber membranes by spinning followed by sintering. The microstructure and oxygen permeability have been investigated.
Section snippets
Experimental
We applied a simple hydrolysis of the corresponding metal nitrates by an ammonium hydroxide solution to get the BCFZ perovskite powder. The powder was mixed with a solution of polysulfone in N-methylpyrrolidone and was ball milled for 16 h. The slurry was spun through a spinneret and the green BCFZ perovskite fiber obtained was cut into 0.5-m pieces before sintering them in a hanging geometry. During the sintering above 1200 °C the length and the diameter of the green fiber was reduced from an
Results and discussion
Fig. 2 shows SEM micrographs of the BCFZ hollow fiber precusor and the hollow fiber sintered above 1200 °C for 5 h. The outer diameter and the inner diameter of the hollow fiber precursor prepared are 1.764 mm and 1.145 mm, respectively. The green hollow fiber shows an asymmetric structure (Fig. 2 (A1) and (C1)). A porous structure in the middle and denser structures at the outer surfaces were found. This asymmetric structure results from the diffusion and phase separation phenomena occurring
Conclusions
Gas tight BCFZ mixed electron and oxygen ion conducting hollow fiber membranes with an asymmetric structure were fabricated by a phase inversion spinning followed by sintering. A high oxygen permeation flux of 7.6 cm3/min cm2 (∼ 110 m3/day m2) was obtained under an oxygen gradient of 0.209 × 105 Pa/0.065 × 105 Pa (P1 = 0.209 × 105 Pa, P2 = 0.065 × 105 Pa) at 900 °C, which is the highest oxygen permeation flux reported in the open literatures so far. Sulphur was found on the tested hollow fiber membrane
Acknowledgements
The authors gratefully acknowledge the financial support of the BMBF for project 03C0343A under the auspices of ConNeCat. H.H. Wang greatly thanks the financial support from the Alexander von Humboldt Foundation.
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