(423b) Harnessing Chemical Energy for Hydrogen Purification In Microreactors

Hydrogen fed fuel cells
are attractive for portable power applications owing to their high conversion
efficiency.  However, hydrogen purification is an important step to avoid fuel
cell catalyst poisoning and improve fuel cell performance. Palladium membranes
which are selectively permeable to hydrogen offer an attractive solution.
Besides making the design compact these membranes yield high hydrogen
selectivity and are chemically resistant to carbon di oxide and carbon
monoxide. While these membranes have been investigated extensively, most
studies report using elaborate electrical heating for reactor heating and thus
pose issues for systems integration [1-5].

In the current work we
propose harnessing chemical energy for sustaining reactor temperature by
coupling an exothermic reaction with hydrogen purification. The design imparts
flexibility to carry multiple reactions simultaneously and compactness for ease
of systems integration. To achieve this goal an integrated hydrogen
purification-burner unit is fabricated using bulk micromachining techniques.
The purification unit consists of a 200 nm palladium-silver membrane while the
burner unit is loaded with platinum catalyst. The palladium membrane separation
process implies an equilibrium amount of hydrogen remains in the exhaust
gas.  This hydrogen can be burned by catalytic combustion to provide the
necessary energy to sustain the reactor temperature and thus impart design
compactness.

The performance of this
integrated hydrogen purification system in terms of energy efficiency and
hydrogen separation is characterized. The minimum hydrogen flow rate at
stoichiometric oxygen hydrogen ratio to maintain a specific target temperature
is investigated.  Further, the permeate side hydrogen flux dependence for
this system is measured. These results compare well with the data obtained
using electrical heating. Finally the CO tolerance of the membrane is also
tested to check device robustness.

References:

1.      K. Deshpande, M.
A. Schmidt, K. F. Jensen ?An Integrated Membrane Microreactor for Elevated
Pressure Hydrogen Purification ? Device Fabrication and Characterization?, In
preparation.

2.      B.A.Wilhite,
M.A.Schmidt, K.F.Jensen ?Palladium-Based Micromembranes for Hydrogen
Separation: Device Performance and Chemical Stability? Ind. Eng. Chem. Res,
2004, 43, 7083-7091.

3.      J. Keurentjes,
F. Gielens, H. Tong, C. Rijn, M. Vorstman ? High-Flux Palladium Membranes Based
on Microsystem Technology? Ind. Eng. Chem. Res, 2004, 43,
4768-4772.

4.      Y. Zhang, J.
Gwak, Y. Murakoshi, T. Ikehara, R. Maeda, C. Nishimura ?Hydrogen Permeation
Characteristics of Thin Palladium Membrane Prepared by Microfabrication
Technology? J. Memb. Sci, 2006, 277, 203-209.

5.      S. Ye, S,
Tanaka, M. Esashi, S. Hamakawa, T. Hanaoka, F. Mizukami ?Thin Palladium
Membrane Microreactors with Oxidized Porous Silicon Support and Their
Application? J. Micromech. Microeng, 2005, 15, 2011-2018.