Dense metal membranes for hydrogen purifying
Fraunhofer Institute for Interfacial Engineering and Biotechnology
A particularly sophisticated technique of hydrogen purification makes use of thin dense metal membranes. Many metals are capable of reacting with hydrogen at moderately elevated temperatures forming metal hydrides. Atomic hydrogen moves fairly easily around the metal lattice thus allowing hydrogen to pass between the two surfaces of dense metal layers while other substances are held back completely. Usually palladium alloys are preferred for this gas separation application since thin layers of most other metals cannot withstand aggressive atmospheres long enough. Palladium, however, is very expensive and the industrial installation of such membranes stipulates layers of less than 10 µm thickness.
Ceramic hollow fiber membranes as porous supports
On the other hand, the mechanical strength of Pd membranes as thin as 10 mm is not sufficient for practical purposes. The solution to this problem is the deposition of Pd layers on porous supports. At working temperatures of several hundred degree celsius inorganic materials are suitable as mechanical backbones. At Fraunhofer IGB a new process technology for production of ceramic hollow fiber membranes has been implemented. With diameters of less than 1 mm and walls as thin as 100 µm these membranes have several advantages over conventional ceramic membranes. Their cross flow resistance is much smaller just as their weight, and they provide very large surface to volume ratios. Thus, compact gas separation modules with large membrane areas become feasible. Naturally, these hollow fibers are very attractive supports for the aforementioned Pd membranes, and very thin metal coatings are developed for them at the IGB.
Dense metal membranes with high permeation rate
- Fig. 1: Scanning electron micrograph of a PdAg coated hollow fiber membrane.
4 µm thin Palladium coatings have been deposited on asymmetric hollow fiber membranes by the electroless plating technique (Fig. 1). These metal coatings are dense and their adhesion to the ceramic support is very good. The hydrogen transport across the membrane is activated and fluxes become technically acceptable only at elevated temperatures. For instance, at 430 °C a H2 flux of over 10 m3m-2h-1bar-1 has been measured. The corresponding N2 permeation rate was 10 l m-2h-1bar-1 yielding a separation factor α(H2/N2) of 1000.
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V. Gepert, M. Kilgus, T. Schiestel, H. Brunner, G. Eigenberger, M. Merten, C.X. Zhang, Z.S. Yuan, N. Liu, S. Wang, S.D. Wang, Ceramics supported capillary Pd membranes for hydrogen separation: potentials and present limitations, Fuel Cells, 2006. 6(6): p. 472-481.