Measured Hydrogen Permeability of Bulk Membrane Materials

Measured Hydrogen Permeability of Bulk Membrane Materials 

For hydrogen partial pressures in feeds of above a few torr, and for thicker membranes ( 250 pm) of Nb, Ta, Ti, V and Zr, hydrogen flux can become limited by bulk diffusion, and, in practice, superpermeability is not achieved. Nevertheless, these elements do possess the highest known permeabilities for hydrogen of any of the common elements, far superior to those of palladium and its alloys, the standards in the hydrogen purification membrane industry. 

To calculate the hydrogen flux as a function of temperature, permeabilities at various temperatures are determined to derive an Arrhenius expression of the form g2]. The pre-exponential factor, Pq, and the activation 

Calculated values of Steward have been verified or approached in a number of cases, as is summarized in Table 4.2. However, it should be stressed that the values predicted by Steward are, in general, not achieved if the flux is limited by gas 

Buxbaum and the group at NETL have reported permeabilities for palladium coated tantalum of between 1.07 x 10 and 1.45 x 10 mol m i s Pa for temperatures 623-693 K (350-420 °C) [41]. Weekly backflushes with hydrogen to remove coke and other impurities were recommended [35]. Buxbaum and Marker have reported a permeability for Pd coated Nb of 3.2 x 10 mol m i s i Pa at 698 K (425 °C), which was only one-third of the theoretical value due to gas phase resistance [42]. The Buxbaum group has achieved a permeabihty for Zr of 2.2 X 10 mol m l s i Pa - at 698 K (425 °C) [42]. They state that hydrcgen transport through zirconium is 100 times better than through Pd under similar conditions [42]. 

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Measured Hydrogen Permeability of Bulk Membrane Materials 127... 

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