Palladium alloy membranes applications

Shown in Table 8.6 arc some literature data on the use of dense membrane reactors for liquid- or multi-phase catalytic reactions. Compared to gas/vapor phase application studies, these investigations are relatively few in number. Most of them involve hydrogenation reactions of various chemicals such as acetylenic or ethylenic alcohols, acetone, butynediol, cyclohexane, dehydrolinalool, phenylacetylene and quinone. As expected, the majority of the materials adopted as membrane reactors are palladium alloy membranes. High selectivities or yields are observed in many cases. A higher conversion than that in a conventional reactor is found in a few cases. 

Corrosive reaction streams. In some application environments, the reactive or corrosive nature of one or more of the reaction components in a membrane reactor can pose a great technical challenge to the selection as well as the design of the membrane element Feed streams often contain some Impurities that may significantly affect the performance of the membrane. Therefore, attention should also be paid to the response of the selected membrane material to certain impurities in the reactant or product streams. Care should be taken to pretreat the feed streams to remove the key contaminants as far as the membrane is concerned in these cases. For example, palladium alloy membranes can not withstand sulfur- or carbon-containing compounds at a temperature higher than, say, 500 C [Kamcyama et al., 1981]. Even at lOO C, the rate of hydrogen absorption (and, therefore, permeation) in a pure palladium disk is... 

Composite membrane catalysts can also be assembled with polymeric supports or intermediate layers [117-119]. These membranes were tested as membrane catalysts for selective hydrogenation of some dienic hydrocarbons and proved to be as selective as monolithic palladium alloy membranes [117]. The use of polyarilyde has been proposed in order to widen the temperature range of polymer-supported membrane application... 

T. L. Ward, S. B. Rathod, A. S. Chel-lappa, T. R. Vencill, Fabrication of thin supported palladium alloy membranes for fuel cell applications, in Proceedings of the 8th International Conference on Inorganic Membranes, Jul. 18-22, 2004, Cincinnati, OH, Adams Press,... 

The data presented in Fig. 5.6 were derived from membrane modules using Pd- Cu foil (planar) membranes with the membrane area sized to deliver 0.78 Nm h i of product hydrogen. Even though palladium alloy membranes are often criticized as being too expensive for commercial applications due to the cost of palladium, it is clear that as membrane thickness is reduced to 10 pm and less, the cost associated with the value of palladium in the membrane can be very reasonable. 

Advanced organic and inorganic membranes and materials include polymers of intrinsic microporosity (PIMs), microporous PVDF, perovskite and palladium alloy membranes [45]. PIM membranes have displayed both high permeability with high selectivity for various gas mixtures. Major commercial and promising applications of membrane GS are delineated below [43—45] ... 

Lanning BR, Ishteiwy O, Way JD, Edlxmd DJ, Coulter KE. Un-supported palladium alloy membranes for the production of hydrogen. In Bose AC, editor. Inorganic membranes for energy and environmental applications. New York Springer Science+Business Media, LLC 2009. p. 203-20. 

Selective gas permeation has been known for generations, and the early use of palladium silver-alloy membranes achieved sporadic industrial use. Gas separation on a massive scale was used to separate U from U using porous (Knudsen flow) membranes. An upgrade of the membranes at Oak Ridge cost 1.5 billion. Polymeric membranes became economically viable about 1980, introducing the modern era of gas-separation membranes. H2 recovery was the first major application, followed quickly by acid gas separation (CO2/CH4) and the production of N2 from air. 

Metallic Palladium films pass H2 readily, especially above 300°C. a for this separation is extremely high, and H2 produced by purification through certain Pd alloy membranes is uniquely pure. Pd alloys are used to overcome the crystalline instability of pure Pd during heating-cooling cycles. Economics limit this membrane to high-purity applications. 

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One of the earliest applications of membrane to shift equilibrium was developed by Wood(5) (1960). He showed that by imposing a nonequilibrium condition on a hydrogen-porous palladium silver alloy membrane, an otherwise stable cyclohexane vapor is rapidly dehydrogenated to cyclohexene. 

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. 

Table 3 Applications of Monolithic Membrane Catalysts Based on Palladium and Palladium Alloys...

The palladium-silver alloy membrane system was successfully commercialized in the early 1960s [12], but the reduction of palladium content by the addition of silver would still not be a cost-effective alternative for large-scale processes [42] unless micron-scale films could be prepared, a goal currently being addressed by many researchers. In recent years, the Pd-Cu system has been the most heavily investigated alloy for hydrogen membrane applications due to the high permeability of select alloys [67, 90, 91], enhanced mechanical properties [92] and reported chemical resistance. The elevated permeability identifled for select Pd-Cu alloys is attributed to an increase in both the solubility and diffusivity of the B2 crystalline phase [86-88] as compared to the face-centered-cubic (fee) phase that exhibits permeability values proportional to the Pd-content [89, 91, 93]. 

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