Measurement and modeling of hydrogen transport through high-flux Pd membranes

The permeability of H2-selective palladium membranes fabricated with microsystem technology has been studied. The permeation experiments have been carried out at temperatures between 623 and 873 K at H2 feed partial pressures of 0.2–1.0 bar. At 823 K, a permeance based on the free membrane area of 18 mol H2/m2 s bar0.58 has been measured for a Pd membrane with a thickness of 0.5 μm. From the permeation experiments the rate determining transport step and the stability of the membranes have been determined. At 873 K the H2/He selectivity decreases rapidly, indicating the formation of pinholes at higher temperatures. From a combination of experiments and computer simulations the limiting transport step has been determined to be H-atom diffusion through the membrane at elevated temperatures and H2 surface reactions at the retentate side at lower temperature

Unsteady-state flux behaviour in relation to the presence of a gel layer

The unsteady-state flux behavior for silica and dextran in a stirred ultrafiltration cell was studied. The 2 compds. show clearly different flux behaviors. During the filtration of dextran, only a polarization layer is built up. For silica, a gel layer formation also occurs. As a result, the time to reach steady-state flux is 1 h. The osmotic pressure model provides a good description of the flux for the expts. with dextran. If mass-transfer coeffs. are used that are higher than those electrochem. measured, the transient flux for silica can be rather well predicted by the gel-polarization model. The use of flux measurements under unsteady-state conditions as an indication for the presence of a gel layer is discussed. [on SciFinder (R)