Membrane dense palladium

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

When considering membrane reactors for dehydrogenation and reforming reactions, three types of membrane are of most interest dense palladium or palladium composite membranes,... 

Dense palladium membranes are the most investigated membranes for hydrogen separation. They are investigated by several research groups as the group of Gryaznov [26-28] and the group of Uemiya and Kikuchi [29,30],... 

Although some inorganic membranes such as porous glass and dense palladium membranes have been commercially available for some time, the recent escalated commercial activities of inorganic membranes can be attributed to the availability of large-scale ceramic membranes of consistent quality. As indicated in Chapter 2, commercialization of alumina and zirconia membranes mostly has been the technical and marketing extensions of the development activities in gas diffusion membranes for the nuclear industry. 

For many industrial bulk processing applications, the purity of the hydrogen required can not justify the use of dense palladium or its alloy membranes due to their low permeabilities. In these cases, porous inorganic membranes are more often considered. 

As mentioned earlier, two compatible reactions may be coupled or conjugated properly by a shared membrane through which the species (as a product on one side of the membrane and a reactant on the other) common to both reactions selectively passes. Summarized in Table 8.5 are some documented studies of reaction coupling using dense palladium-based membranes with the alloying component ranging from nickel, ruthenium, rhodium to silver. 

Dense palladium-based membranes. Shown in Table 10.1 are modeling studies of packed-bed dense membrane shell-and-tube reactors. All utilized Pd or Pd-alloy membranes except one [Itoh et al., 19931 which used yttria-stabilized zirconia membranes. As mentioned earlier, the permeation term used in Ihe governing equations for the tube and shell sides of the membrane is expressed by Equation (10-51b) with n equal to 0.5 [c.g., Itoh, 1987] or 0.76 [e.g., Uemiya et al., 1991]. 

Figure 11.9. Five different flow (or mixing) paliems for two sides of a dense palladium membrane [Itoh ei al., 1990)...

Dense palladium and palladium alloy membranes have been repeatedly demonstrated to show extremely high sclcctiviiies of hydrogen and certain solid electrolyte membranes... 

Examples of dense membranes are palladium foils (selective passage of hydrogen), and films of organic polymers such as polyvinyl alcohol (selective passage of water). 

The methods for preparation of nonporous composite membrane catalyst are discussed in Ref. 10. The porous stainless steel sheets were covered with a dense palladium alloy film by magnetron sputtering [113] or by corolling of palladium alloy foil and porous steel sheet. The electroless plating of palladium or palladium alloy on stainless steel [114] or on porous alumina ceramic [115,116] gives the composite membranes with an ultrathin, dense palladium top layer. 

Ma YH. Dense palladium and perovskite membranes and membrane reactors. MRS Bulletin March 1999 46 9. 

A. Li, W. Liang and R. Hughes, Fabrication of Dense Palladium Composite Membranes for Hydrogen Separation, Calal. Today, 56, 45-51 (2000). 

Among all types of membranes, the inorganic dense membranes have attracted the interest of many researchers due to their capacity to separate completely a product from gaseous mixtures [19]. In particular, the dense palladium membranes are used because of their complete hydrogen perm-selectivity. In the last years, the increasing interest towards this type of membranes is, also, due to hydrogen application as energy carrier. 

In fact, although, niobium (Nb), vanadium (V) and tantalum (Ta) offer higher hydrogen permeability than palladium in a temperature range between 0 and 700°C, as shown in Fig. 2.8, nevertheless these metals give a stronger surface resistance to hydrogen transport than the palladium (Pd). For this reason, dense palladium membranes are preferentially used. 

From Eqs. 4.23 and 4.24, the following equation can be derived for the hydrogen flux at constant temperature through a dense palladium membrane ... 

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