Dense metal membrane permeability

Dense metallic membranes have the advantage of very high selectivities since only certain species can be dissolved in their structural lattice. However, the permeabilities are lower by a factor of 100 than those of porous membranes (Ilias and Govind 1989, van Vuren et al. 1987, Itoh 1987, Suzuki, Onozato and Kurokawa 1987). For example, the permeability of... 

To conclude this section, it is necessary to state that Pd and Pd-based membranes are currently the membranes with the highest hydrogen permeability and selectivity. However, the cost, availability, their mechanical and thermal stabilities, poisoning, and carbon deposition problems have made the large-scale industrial application of these dense metal membranes difficult, even when prepared in a composite configuration [26,29,33-37],... 

Dense Metal Membrane Materials, Configurations, Mechanisms of Transport, and Permeability... 

Additionally, the temperature dependence of the permeability of a dense metal membrane is observed to follow an Arrhenius-type expression, Eq. 10.8. Ep and ko represent the activation energy of permeation and the pre-exponential constant respectively. 

The permeability through a composite membrane consisting of several dense metal layers can be described by a resistances in series methodology, similar to that used in heat conduction through a composite wall [100], The total permeability for a composite dense metal membrane can be estimated from Eq. 10.9 where XM.iot is the total thickness of the membrane. 

Dense metal membranes exhibit an absolute permeability to specific species. Clear examples are given by palladium and palladium-alloy membranes, which are exclusively permeable to hydrogen, and by silver membranes. 

However, dense metal membranes present a few challenges that have, to date, prevented their widespread industrial implementation. The best performing materials, Pd and Pd-alloys, are expensive. Pure Pd displays high H2 permeability and can operate at elevated temperatures, but suffers performance degradation because of poisoning and corrosion from exposure to conmum contaminants (H2S, NH3, CO, CO2, and Hg), hydride formation, and other factors, which will be described in following sections. Pd-alloys can display better chemical stability and mechanical strength than pine Pd, but often at the expense of permeability. 

The hydrogen permeability of dense metallic membranes is strictly related to their lattice structure, as well to the presence of lattice defects, and to their reactivity with H2 or other feed stream gases. 

The cold-rolling technique has been applied to synthesize new types of dense metal membranes made of Nb-based alloys. In particular, Luo et al. verified that an Nb-Ti-Ni alloy consisting only of the primary phase and the eutectic phase exhibited high hydrogen permeability and good resistance to hydrogen embrittlement. ... 

Ni is a low-cost metal with lower permeability than Pd it is used for preparing both porous and dense metal membranes. Among its alloys, Ni-Nb-Zr has been investigated, and it has been demonstrated that the Nb in this alloy reduces the embrittlement while the Zr increases the permeability. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

The quantity of ambipolar conductivity is widely used for the analysis of -> electrolytic permeability of -> solid electrolytes, caused by the presence of electronic conductivity. Other important cases include transient behavior of electrochemical cells and ion-conducting solids, dense ceramic membranes for gas separation, reduction/ oxidation of metals, and kinetic demixing phenomena [iv]. In most practical cases, however, the ambipo-... 

Microporous membranes. While dense metal or metal oxide membranes possess exceptionally good peimselectivities, their permeabilities are typically lower than those of porous inorganic membranes by an order of magnitude or more. Commercial availability of porous ceramic membranes of consistent quality has encouraged an ever... 

Silver membranes are permeable to oxygen. Metal membranes have been extensively studied in the countries of the former Soviet Union (Gryaznov and co-workers are world pioneers in the field of dense-membrane reactors), the United States, and Japan, but, except in the former Soviet countries, they have not been widely used in industry (although fine chemistry processes were reported). This is due to their low permeability, as compared to microporous metal or ceramic membranes, and their easy clogging. Bend Research, Inc. reported the use of Pd-composite membranes for the water-gas shift reaction. Those membranes are resistant to H2S poisoning. The properties and performance characteristics of metal membranes are presented in Chapter 16 of this book. 

Now considering dense membranes, attention will be focused only on ceramic membranes, since a detailed description of the preparation and properties of the interesting and promising metal membranes have been described in detail in the preceding chapter of this book. Data concerning the permeability of Ag and Pd-alloy membranes, though, are listed in Table 2 for comparison. 

Figure 8.8. Schematic showing key structural features of the four types of dense, hydrogen-permeable metal membranes.

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