Catalytic Design of Palladium-Based Membrane Reactors

8 Catalytic Design of Palladium-Based Membrane Reactors 

Not only the permeability, permselectivity and mechanical properties, but also the catalytic properties are affected by the two hydride forms that can exist in palladium. The a phase corresponds to solid solutions with a H/Pd ratio of about 0.1 and the P phase with a H/Pd ratio of about 0.6. The phase change is associated with a large change in lattice constant that often leads to microcracks and distortion in the palladium membrane. As a result, the mechanical properties are reduced. The transformation depends on the operating conditions such as temperature and hydrogen partial pressure. Repeated thermal cycles, for example, between 100 and 250 C under 1 atm of hydrogen pressure can make a 0.1 mm thick Pd foil expand to become 30 times thicker [Armor, 1992]. 

Given a catalytic reaction, one also needs to consider any unintended catalytic effects of the materials of construction including the membrane and processing equipment. 

Relevant to this issue is dehydrogenation of ethylbenzene for the manufacture of styrene which uses alumina supported iron oxide as the preferred catalyst in most cases. Therefore, when an alumina membrane is used in conjunction with stainless steel piping or vessels as the membrane reactor, caution should be exercised. An estimate of the effects of their exposure to the reaction mixuire at the application temperature of 600 to 640 C is desirable. Wu et al. [1990b] estimated that the alumina membrane contributes to less than 5% conversion of ethylbenzene and the stainless steel tubing or piping could account for as much as 20% conversion. The high activity of the stainless steel is attributed to iron and chromium oxide layers that may form on the wetted surface. 

Material and catalysis aspects arc the two foremost important factors when considering inorganic membrane reactors. They need to be evaluated early in the development stage. 

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