Palladium alloy selective membrane

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

Palladium and selective alloys with other metals can be fabricated into dense membrane reactors in foil or tubular form, mostly in thin layers to reduce permeation resistance. In... 

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... 

Dense metallic membranes, in particular those based on palladium alloys, have been extensively studied for the selective transport of In the case of O2,... 

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... 

The key problem of the cross-flow reactor is not how to construct an effective separation of the two flowing phases. It is instead connected with how to design the porosity and location of the catalytic active zones of the separating walls so that the transport resistance across the wall does not limit the conversion and the selectivity of the chemical reactions. Palladium-alloy membranes, or thin films of these alloys on porous ceramic tubs, seem to have the potential to be good solutions of the separating-wall problem for cross-flow reactors used for hydrogenation reactions. 

Gryaznov ct al. [30-32] have done pioneering work in the study of differently designed palladium-alloy membrane reactors for reactions in both gas phase and liquid phase. Most interest was directed to the composition and properties of the membranes, which were decisive questions at this stage of the development, and still are. Selectivity problems in various organic chemical reactions were also of importance to study. 

Hydrogen is able to permeate selectively through palladium or palladium alloy membranes. This has led to the demonstration of membrane reformers in the labora-... 

Composite metal membranes are most often the structure of choice when a reactive group 3-5 metal or alloy is the principle constituent of the membrane. The relative chemical reactivity of these metals dictates that an inert coating must be applied to at least the feed surface of the membrane. Palladium, or better yet a palladium alloy, customarily serves as the coating layer. If it can be guaranteed that the permeate side of the membrane will never be exposed to reactive gases (e.g., water, carbon oxides, and hydrocarbons), then a two-layer composite membrane is a satisfactory choice. However, normal operating procedures and the potential for process upsets typically favors the selection of a three-layer composite structure. 

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] ... 

Concerning non-porous membranes, these are categorized as dense ceramic electrolytes such as yttria-stabilized zirconia (YSZ) and perovskite membranes [16], which allow only the permeation of ionic oxygen. Permeation through metal membranes such as palladium and a palladium alloy is based on the selective dissolution of hydrogen and diffusion through the metal membrane. 

Membrane reactors (MRs) for fuel processing combine the unit operation of membrane separation with catalytic reactions such as reforming and WGS. The membrane separation process is usually performed by hydrogen removal from the reformate by application of membranes made of ceramics or palladium and palladium alloys, while polymeric membranes are less convenient for systems of smaller than industrial scale, because several separation steps are required owing to their relatively low selectivity of the separation process. In MRs the equilibrium... 

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