Membrane Pd-based

Dense Pd-based membranes have been first used for CMRs applications [4]. They are indeed highly selective for H2 permeation but are expensive, sensitive to ageing and poisoning and are strongly limited by their low permeabilities. 

Tong, J. et al., Experimental studies of steam reforming of methane in a thin Pd-based membrane reactor, Ind. Eng. Chem. Res., 44, 1454, 2005. 

Conversion of Carbon monoxide with Hydrogen Nonporous Pd-based membranes... 

Reproducibly of results, however, was questioned, because the total oxidation reaction was underestimated [84]. Possibly, these discrepancies derive from the observation of structural changes of the Pd-based membrane during direct hydroxylation of benzene to phenol [85]. The surface state of Pd during the reaction could be divided into two major regions the oxidized region near the gas... 

Barbieri, G., Bernardo, P., Mattia, R., Drioli, E., Bredesen, R., Klette, H., Pd-based membrane reactor for water-gas shift reaction, Chem. Eng. Trans. 2004, 4, 55-60. 

Barbieri, G., Brunetti, A., Tricoli, G. and Drioli, E. (2008) An innovative configuration of a Pd-based membrane reactor for the production of pure hydrogen. Experimental analysis of water gas shift. Journal of Power Sources, 182 (1), 160-167, http //dx.doi.org/10.1016/ j.jpowsour.2008.03.086. 

Metallic membranes for hydrogen separation can be of many types, such as pure metals Pd, V, Ta, Nb, and Ti binary alloys of Pd, with Cu, Ag, and Y Pd alloyed with Ni, Au, Ce, and Fe and complex alloys of Pd alloyed with more than one metal [3], Body-centered cubic metals, for example, Nb and V, have higher permeability than face-centered cubic metals, for instance, Pd and Ni [26-29], Even though Nb, V, and Ta possess a permeability greater than that of Pd, these metals develop oxide layers and are complicated to be used as hydrogen separation membranes [29], Especially, the Pd and Pd-based membranes have in recent times obtained renovated consideration on account of the prospects of a generalized use of hydrogen as a fuel in the future [26], We emphasize on these types of membranes in this chapter. 

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],... 

In addition to the Pd-based membranes, microporous silica membranes for hydrogen permeation [8] can be produced by a special type of chemical vapor deposition [140] named chemical vapor infiltration (CVI) [141], A large amount of studies have been carried out on silica membranes made by CVI for hydrogen separation purposes [8,121], CVI [141] is another form of chemical vapor deposition (CVD) [140] (see Section 3.7.3). CVD involves deposition onto a surface, while CVI implies deposition within a porous material [141], Both methods use almost similar equipment [140] and precursors (see Figure 3.19) however, each one functions using different operation parameters, that is, flow rates, pressures, furnace temperatures, and other parameters. 

When dense Pd-based membranes are used, permeabilities are low. To increase the membrane surface per unit volume of reactor the use of spiral tubes or helix-shaped Pd membranes has been proposed [39]. 

The use of palladium-based membranes results from the 1866 discovery by Thomas Graham(2) that metallic palladium absorbs an unusually lar e amount of hydrogen. Hydrogen permeates through Pd-based membranes in the form of highly active atomic hydrogen which can react with other... 

Transport and therefore separation mechanisms in porous inorganic membranes are distinctively different from and more varying than those prevailing in dense membranes. Because of the variety of the mechanisms that can be operative, these membranes in principle are capable of separating more varieties of compound mixtures. Compared to dense membranes, these porous membranes generally exhibit permeabilities of one or two orders of magnitude higher. For example, Pd-based membranes typically have a... 

The use of Pd-based membrane reactors can increase the hydrogenation rates of several olefins by more than 10 times higher than those in conventional premixed fixed>bed reactors. Furthermore, it has been pointed out that the type and state of the oxygen used to carry out partial oxidation of methane can significantly affect the conversion and selectivity of the reaction. The use of a solid oxide membrane (e.g., a yttria-stabilized zirconia membrane) not only can achieve an industrially acceptable C2 hydrocarbon yield but also may eliminate undesirable gas-phase reactions of oxygen with methane or its intermediates because oxygen first reaches the catalyst through the solid oxide wall [Eng and Stoukides, 1991]. 

As fabricated, a Pd or its alloy membrane suffers from the relatively low catalytic activity of the attached catalyst due to the typical low surface area to volume ratio of the membrane geometry. The catalyst in a dense Pd-based membrane can be the Pd itself or its alloy or some other materials attached to the membrane. Pretreatments to the Pd-based membranes can help alleviate this problem. This and other membrane material and catalysis issues will be further covered in Chapter 9. 

In contrast to the studies on gas- and vapor-phase hydrogenation reactions utilizing dense Pd-based membrane reactors, dehydrogenation reactions have been consistently observed to benefit from the concept of a membrane reactor. In almost all cases the reaction conversion is increased. This is attributed to the well known favorable effect of equilibrium displacement applied to dehydrogenation reactions which are mostly limited by the equilibrium barrier. 

There has been a large volume of data showing the benefit of having thin dense membranes (mostly Pd-based) on the hydrogen permeation rate and therefore the reaction conversion. An example is catalytic dehydrogenation of propane using a ZSM-5 based zeolite as the catalyst and a Pd-based membrane. Clayson et al. [1987] selected a membrane thickness of 100 m and achieved a yield of aromatics of 38% compared to approximately 80% when a 8.6 pm thick membrane is used [Uemiya et al., 1990]. 

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