Dense Metal Membranes for the Production of High-Purity Hydrogen
- PDF / 495,359 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 13 Downloads / 245 Views
Dense Metal Membranes for the Production of High-Purity Hydrogen David S. Sholl and Y.H. Ma Abstract Dense metal membranes are a well-developed technology for the production of high-purity hydrogen. The physical mechanism of hydrogen transport across metal films—dissociation of molecular hydrogen, diffusion of interstitial atomic hydrogen, and subsequent recombinative desorption of molecular hydrogen—means that metal membranes can have extremely high selectivities for hydrogen transport relative to other gases. We describe current experimental and theoretical trends in the development of metal alloy membranes for hydrogen purification in practical, chemically robust processes. Keywords: hydrogen purification, diffusion, film, metal alloy.
Hydrogen purification using films of Pd and Pd alloys has a long history, with commercial applications going back at least as far as the 1950s. A detailed review of this area has been presented recently by Paglieri and Way.1 Metal membranes are in many respects ideal for high-temperature applications. The physical mechanism of H2 permeation through metal membranes is quite different from porous membranes. As illustrated in Figure 1, metal membranes function by adsorbing and dissociating gaseous H2 on the metal surface exposed to the feed stream and subsequent diffusion of atomic H through interstitial sites in the metal. Recombination of atomic H into H2 on the membrane’s downstream side completes the transport of H2 across the membrane. Because of this mechanism, metal membranes can have essentially perfect selectivity for H2 when exposed to gas mixtures; only defects in the film allow species other than H2 through. Selectivities in excess of 1000 for H2 have been reported.2,3 Because of the mechanism by which H2 permeates through metal membranes, these membranes do not exhibit the tradeoff between selectivity and permeability that
770
is common for polymeric gas-separation membranes. The permeability of metal layers to H2 also makes metal films ideal for use in membrane reactors for hydrogenation or dehydrogenation reactions.4–9 Pure Pd can be an effective membrane for temperatures above ⬃300°C, but at lower temperatures, membrane cracking is common after temperature cycling. This problem occurs because the phase diagram of Pd-H has two phases in this regime that differ considerably in their lattice constant.10,11 At elevated temperatures, pure Pd membranes can be strongly poisoned by chemical impurities, particularly sulfur-containing species.12,13 Both of these difficulties can in principle be overcome by using metal alloys instead of pure Pd as membranes. Because of the extremely high selectivity of metal membranes for H2 purification, an important focus for large-scale applications is to create devices with high H2 fluxes. A common description of H2 transport through metal membranes is to assume that diffusion through the bulk metal dominates the overall mass-transfer resistance and that interstitial H in the membrane is dilute. In this case, the overall
H2 flux J throug
Data Loading...
