Performance of Pd-Based Membranes and Effects of Various Gas Mixtures on H 2 Permeation

摘要

H 2 permeation and separation properties of two Pd-based composite membranes were evaluated and compared at 400 °C and at a pressure range of 150 kPa to 600 kPa. One membrane was characterized by an approximately 8 μm-thick palladium (Pd)-gold (Au) layer deposited on an asymmetric microporous Al 2 O 3 substrate; the other membrane consisted of an approximately 11 μm-thick pure palladium layer deposited on a yttria-stabilized zirconia (YSZ) support. At 400 °C and with a trans-membrane pressure of 50 kPa, the membranes showed a H 2 permeance of 8.42 × 10 ?4 mol/m 2 ·s·Pa 0.5 and 2.54 × 10 ?5 mol/m 2 ·s·Pa 0.7 for Pd-Au and Pd membranes, respectively. Pd-Au membrane showed infinite ideal selectivity to H 2 with respect to He and Ar at 400 °C and a trans-membrane pressure of 50 kPa, while the ideal selectivities for the Pd membrane under the same operating conditions were much lower. Furthermore, the permeation tests for ternary and quaternary mixtures of H 2 , CO, CO 2 , CH 4 , and H 2 O were conducted on the Pd/YSZ membrane. The H 2 permeating flux decreased at the conclusion of the permeation tests for all mixtures. This decline however, was not permanent, i.e., H 2 permeation was restored to its initial value after treating the membrane with H 2 for a maximum of 7 h. The effects of gas hourly space velocity (GHSV) and the steam-to-carbon (S/C) ratio on H 2 permeation were also investigated using simulated steam methane reforming mixtures. It was found that H 2 permeation is highest at the greatest GHSV, due to a decline in the concentration polarization effect. Variations in S/C ratio however, showed no significant effect on the H 2 permeation. The permeation characteristics for the Pd/YSZ membrane were also investigated at temperatures ranging from 350 to 400 °C. The pre-exponential factor and apparent activation energy were found to be 5.66 × 10 ?4 mol/m 2 ·s·Pa 0.7 and 12.8 kJ/mol, respectively. Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) analyses were performed on both pristine and used membranes, and no strong evidence of the formation of Pd-O or any other undesirable phases was observed.