Distribution characteristics of temperature and species in
micro-structured heat-exchanger reactors
Microchannel reactor designs suffer from a fundamental limitation resulting from the flow configuration in which a reacting stream flows parallel to a heat transfer surface through which the majority of heat is transferred perpendicular to the direction of fluid flow. The present study aims to provide a unique microchannel fluid processing system for performing chemical reactions with temperature control. The present study relates to a unique method for performing reversible endothermic, exothermic reactions, and competing reactions. The method comprises flowing reactants through a reaction channel in thermal contact with a heat exchange channel, and conducting heat in support of the reaction between the reactants and fluid flowing through the heat exchange channel to substantially raise or lower the temperature of the reactants as they travel through the reaction channel. Particular emphasis is placed upon how to provide improved conversion and selectivity in chemical reactions, provide chemical reactor systems that are compact, and provide thermally efficient chemical reactor systems. The distribution characteristics of temperature and species in micro-structured heat-exchanger reactors are investigated and the reactor performance is evaluated by performing numerical simulations using computational fluid dynamics. The results indicate that microchannel technology is capable of high heat and mass transfer coefficients between a bulk reaction fluid and the catalytic heat exchange surface. Carbon monoxide output from the fuel processor is controlled over the operating range of the processor. When the reaction in the reaction chamber is a reversible exothermic reaction, heat is generated in the reaction chamber and transferred to the heat exchange fluid. Microchannel reactors offer less resistance to heat and mass transfer thus creating the opportunity for dramatic reductions in process hardware volume. While a steam reforming catalyst in the form of a powder or pellets is appropriate in larger devices, diminished performance may result in the form of a powder or pellets in miniature devices and reactors. The steam reforming catalyst contains a suitable amount of at least one metal oxide and cerium to contribute to high methanol conversion properties. The shift reaction increases hydrogen yield while reducing carbon monoxide. Microchannel reactors offer the advantage of exceptional heat exchange integration and can be utilized for approaching optimum temperature trajectories for exothermic, reversible reactions.
Keywords: Distribution characteristics; Chemical kinetics; Temperature trajectories; Fluid streams; Thermal gradients; Reaction selectivity