Cross-flow heat exchanger-reactors

Despite the geometric similarities, the problem analyzed here is fundamentally different from that of cross-flow heat exchangers or catalytic reactors in that the solid is not only used as a heat-exchange medium or as a catalyst support but also as the electrolyte across which oxygen ion transport occurs. This introduces an integral electron conservation balance which results in an integro-differential problem. 

A purchasable cross-flow heat exchanger for application in laboratory-, pilot- and production-scale plants was developed by FZK. By incorporation of a catalyst on the quadratic plates inside the heat exchanger, it can also be used as a catalytic wall reactor. Operating conditions up to 850 °C (stainless steel) and pressures of more than 100 bar are possible, and the specific inner surface area is up to 30 000 m m. The reactors can be obtained in many materials and three different sizes with a maximum flow of 6500kgh (water). Therefore, the reactors can be adjusted for various processes, and all types of catalyst deposition techniques are possible [111]. This reactor has already been applied to the catalytic oxidation of H2 by Janicke et al. [112], for example. 

Integrated reactors One type of integrated reactor is micro structured heat exchanger/reactor concepts, which may work as cross- or counter-flow reactors. Another type couples endothermic and exothermic reactions in two separate flow paths normally operated in the co-current mode. Both reactor types are designed as prototype components of future fuel processors for mobile applications. 

Cross-flow monoliths have been explored by Degnan and Wei (11-12) as cocurrent and countercurrent reactor-heat exchangers. Four cross-flow monoliths in series were employed the individual blocks were analyzed by a one-dimensional approximation. They found good agreement between theory and experiment. 

Roy and Gidaspow (13-14) developed two-dimensional continuum models to describe cross-flow monolithic heat exchangers and catalytic reactors. 

It should be noted in these two reactor examples that where the cross-flow structure functioned as both a reactor and a heat exchanger, the channel walls separating the flows were not permeable to mass transfer through the walls. 

Mathematical models of cross-flow air-dryers and regenerators differ from those of the cross-flow reactors, since one must consider the fact that the sides of the channel walls are partly made of a reactive solid substance that is capable of exchanging a species with the process streams. These mass and heat balances result in nonlinear Volterra-type integral equations, which have been studied by Roy and Gidaspow [37,70]. 

Stability aspects of the cross-flow reactor-heat exchanger were the aim of a comprehensive study by Dcgnan and Wei [35,71], where the theoretical results were also verified experimentally. Of particular interest was the experimental demonstration of the multiplicity of the steady states for the autothermal countercurrent process case. 

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