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Transient Behavior of Hydrogen Permeation Through Pd/Ag Alloy Micro/Nano Membranes

[+] Author Affiliations
Logan S. McLeod, Levent F. Degertekin, Andrei G. Fedorov

Georgia Institute of Technology

Paper No. IMECE2006-14438, pp. 103; 1 page
doi:10.1115/IMECE2006-14438
From:
  • ASME 2006 International Mechanical Engineering Congress and Exposition
  • Heat Transfer, Volume 1
  • Chicago, Illinois, USA, November 5 – 10, 2006
  • Conference Sponsors: Heat Transfer Division
  • ISBN: 0-7918-4784-5 | eISBN: 0-7918-3790-4
  • Copyright © 2006 by ASME

abstract

Separation of hydrogen from the reaction products stream leaving a fuel processor is an essential step prior to its introduction to the fuel cell. To this end, we are developing micromachined Palladium-Silver alloy micro/nano membranes for in-situ hydrogen separation suitable for integration with catalytic fuel reforming microreactors. Although achieving the maximum steady-state hydrogen flux across these membranes is the most important design goal, a detailed understanding and control of the transient process from the starting (no permeation) conditions to the nominal steady-state are critical for effective utilization of H2-separating membranes in a distributed power scheme. In our previous work (McLeod et al., Proceedings of ASME 2005 Summer Heat Transfer Conference, San Francisco, CA) a steady-state model was developed which divides the hydrogen permeation sequence into seven distinct regimes: gas phase mass transport to the surface, adsorption onto the surface, transition into the bulk material, solid-phase diffusion through the bulk material, transition to the effluent surface, desorption into the gas phase and diffusion away from the surface. This presentation investigates the kinetics of each permeation step in order to understand and predict the transient behavior of thin Pd/Ag alloy membranes exposed to varying hydrogen concentration gradients and to establish the rate-limiting step(s). Possible explanations for observed long-term transient behavior, on the order of several hours, will also be discussed. In addition to transient behavior of the Pd/Ag membranes, the design and effects of critical geometric parameters for the membrane test fixture will also be discussed. In order to accurately measure the true performance of a gas permeable membrane, it is critical to minimize changes in partial pressure and the effects of gas-phase mass diffusion across the membrane surface. To quantify these effects three time scales have been defined; flow time, chemical time and diffusion time. The importance of the relative values of these time scales will be discussed for the ideal membrane test fixture.

Copyright © 2006 by ASME

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