Dense ceramic membranes modes

Oxygen permeation modes in the dense ceramic membranes (a) electrochemical oxygen pump (b) oxygen permeation together with production of electricity and (c) oxygen permeation in mixed conducting membrane. 

As can be seen, air instead of pure oxygen can be used directly as the oxidant in the dense ceramic membrane reactors because of the 100% oxygen permselectivity of the membrane. This may lead to a dramatic decrease in the operation cost of the membrane reactor. Furthermore, the oxygen can be introduced into the reaction side in a controllable mode. Thus, the excessive temperature excursion can be avoided, making operation of the reactor more safe. 

In order to improve the catalytic activity and selectivity of the membrane reactor, a catalyst has to be used. A simple way is to place the catalyst pellets on/next to the membrane, as shown in Fig. 7.9a. The membrane mainly functions as either a product extractor or a reactant distributor, although it also plays some role in the reaction.The reaction selectivity is mainly determined by the catalyst. This incorporation mode is most popular in practical use and is easily operated. Since the catalyst is physically separated from the membrane, only the separation function of the membrane needs to be controlled. The high selectivity of the dense ceramic membranes leads to highly attractive results (pure Hj extraction in dehydrogenation reactions and direct use of air in partial oxidation reaction). But the permeability of the membrane has to be improved as high as possible. 

Tosti et al. [22] compared a thin-walled dense tube Pd membrane and composite Pd-ceramic tube membranes. They developed a finite elemental model for membrane reactors. Table 6.1 shows the WGS reaction conversion values calculated for the dense and the composite membrane reactors both by taking into account the wall effects ( WE case) resistance and by neglecting such a resistance ( no WE case). The main result is that by increasing the temperature the reactiOTi conversion increases. Table 6.2 shows in co-current mode. Both the membranes show similar activities. 

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