Modeling of Surface-bed Reactor for Endothermic and Exothermic Reactions Coupling

Abstract

Hydrogen has acquired great importance in the last decade mainly because its combustion with air does not give rise to pollutant emissions in the atmosphere (either toxic or producing greenhouse effect). Therefore, it is seen as the future transportation fuel at least for urban areas (Jamal & Wyszynski (1994); Moore & Raman, (1998); Kruger, et al., 2003; Lerer et al., 2006). A second reason for the growth of its importance is the proposed use as main energy carrier for fuel cells (Rostrup-Nielsen, 2004). However, hydrogen cannot be considered a fuel in the conventional meaning of the term since it is not available in elemental form in concentrations and amounts suitable for human needs. It is, more appropriately, an energy carrier because it can be produced industrially at reasonable costs from fossil fuels or also from water electrolysis but in a very expensive way (Lincoln, 2006). The needs for hydrogen of the chemical industry are, generally, satisfied by the process of catalytic steam reforming of fossil fuels, which produces synthesis gas (H2 and CO) that in turn is used for many important processes including methanol/dimethyl ether (DME) synthesis, ammonia production, olefins oxosynthesis to aldehydes (Marschner et al., 2005), carbonylation, hydrodesulphurization, hyrdrocracking, etc. (Rostrup-Nielsen, 1995) or as feedstock to the Fischer–Tropsch process for liquid hydrocarbons production (RostrupNielsen et al., 2001; Cao et al., 2005). Commercial processes for methane steam reforming use tubular reactors packed with supported nickel catalysts. In the conventional reactors, strong endothermic reactions occur at high temperatures for high conversion with fast kinetics. In these conditions the reaction becomes diffusion controlled and, therefore, a suitable catalyst structure design play an important role in obtaining high activity and stability. In addition, because of the endothermicity of the steam reforming process, the rate at which heat is transferred from an outside occurring exothermic reaction to the reactor tubes may become controlling. In recent years, microchannel reactors have been developed for process intensification and used in exothermic and endothermic reactions for their noticeable temperature control and improved mass transfer (Ehrfeld et al., 2000; Jensen, 2001; Schubert et al., 2001; Lerou & Ng (1996); Holladay et al., 2004). The use of microchannels potentially minimizes the temperature gradient and allows the reaction occurring at a higher average temperature so that the process efficiency is enhanced. A structured metallic support, with high thermal