Membrane reactor dense metal oxide

Some dense inorganic membranes made of metals and metal oxides are oxygen specific. Notable ones include silver, zirconia stabilized by yttria or calcia, lead oxide, perovskite-type oxides and some mixed oxides such as yttria stabilized titania-zirconia. Their usage as a membrane reactor is profiled in Table 8.4 for a number of reactions decomposition of carbon dioxide to form carbon monoxide and oxygen, oxidation of ammonia to nitrogen and nitrous oxide, oxidation of methane to syngas and oxidative coupling of methane to form C2 hydrocarbons, and oxidation of other hydrocarbons such as ethylene, methanol, ethanol, propylene and butene. 

Enhancement in conversion by the usage of a membrane reactor has been demonstrated for many dehydrogenation reactions. Product selectivity of some hydrogenation and other reactions arc found to improve with a permselective membrane as part of the reactor. Several dense metal as well as solid elecu olyte membranes and porous metal as well as various oxide membranes have been discovered to be effective for the reaction performance. 

Thermal shock resistance. Temperature swing as part of the normal cycles of operation or regeneration of the membranes or membrane reactors can lead to deleterious thermal shock. The materials for the various components in a membrane reactor should be carefully selected to impart good thermal sh k resistance. This is particularly important for high temperature reactions. Also listed in Table 9.5 is a summary of various membrane materials along with qualitative description of their resistance to thermal shock. Again, the available data apply to dense materials. While various metal oxides have been made into commercial inorganic membranes, they tend to be affected by thermal shock much more than other ceramic materials. 

A commercial nickel catalyst was used for methane steam reforming performed at a 500 °C reaction temperature, a S/C ratio of 3.0 and atmospheric pressure, while the permeate side was evacuated. The performance of the vapour deposited platinum membrane was similar to the plated dense palladium membrane. In the permeate of the deposited ruthenium and palladium membranes, small amounts of carbon oxides and also methane were observed. While it was expected that all these species had passed through the membranes by diffusion, in addition some methane was converted into carbon dioxide over the noble metals of the membranes. Kikuchi et al. demonstrated by simulations that conversion and hydrogen permeation in a membrane reactor is higher, where the first portion of the catalyst bed is not coupled to the membrane. Such an arrangement as shown in Figure 7.16 would clearly save expensive membrane surface area. Experimental work by Itoh et al. performed for methanol steam reforming [521] confirmed the assumptions of Kikuchi et al. 

Membrane reaction processes are systems where separation and reaction are carried out simultaneously, and the continuous extraction of one of the products can shift the equilibrium, enhancing yield and selectivity as compared with a traditional system. The development of membrane reactors has gone hand-in-hand with innovations in membrane materials and catalysts. Specifically, in the case of membranes, the same type of materials used to obtain them can also be adapted to support different catalysts. In terms of the separation and catalysis functions, porous membranes with permeance superior to dense membranes are the preferred candidates for use in membrane reactors these include porous oxide, zeolite, glass, metal, and, more recently, carbon membranes. Although carbon membranes are still in their infancy and have some serious challenges, such as weak mechanical strength as unsupported membranes and bad controllability and reproducibility of manufacture as supported membranes, they are believed to be promising... 

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