Numerical simulations are carried out to understand the heat energy transport characteristics of microchannel reactors for hydrogen production by steam-methanol reforming on copper-based catalysts. Enthalpy analysis is performed and the evolution of energy in the oxidation and reforming processes is discussed in terms of reaction heat flux. The effects of solid thermal conductivity, gas velocity, and flow arrangement on the thermal behavior of the reactor is evaluated in order to fully describe the thermal energy change in the reactor. The results indicate that the thermal behavior of the reactor depends upon the thermal properties of the walls. The change in enthalpy is of particular importance in exothermic and endothermic reactions. The net enthalpy change for oxidation and reforming is negative and positive, but the net sensible enthalpy change is always positive in the reactor. The wall heat conduction effect accompanying temperature changes is important to the autothermal design and self-sustaining operation of the reactor. The solid thermal conductivity is of great importance in determining the operation and efficiency of the reactor. The reaction proceeds rapidly and efficiently only at high solid thermal conductivity. The reaction heat flux for oxidation and reforming is positive and negative. The change in flow arrangement significantly affects the reaction heat flux in the reactor. The parallel flow design is advantageous for purposes of enhancing heat transfer and avoiding localized hot spots.

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

Computational fluid dynamics simulations are carried out to better understand how to manage thermally coupled reactors for conducting simultaneous endothermic and exothermic reactions. Particular emphasis is placed upon the mechanisms involved in the heat transfer processes in thermally coupled reactors for hydrogen production by steam reforming. The effects of catalyst layer thickness on the enthalpy of reaction, methanol conversion, and hydrogen yield are delineated. The oxidation and reforming reaction rates involved in the endothermic and exothermic processes are determined. Contour maps denoting temperature, enthalpy, and species mole fractions are constructed and design recommendations are made. The results indicate that the waste heat can efficiently be recovered in a low-temperature region, although the reactivity of a steam reforming reaction is low in such a region. The steam reforming device is configured as to be heated by part of the combustion heat to cause a steam reforming reaction in the device. The steam reforming reaction is endothermic and is therefore typically carried out in an externally heated steam reforming reactor. The incorporation of a simultaneous exothermic reaction to provide an improved heat source can provide a typical heat flux of roughly an order of magnitude above the convective heat flux. Structured catalysts offer heat transfer benefits and extra activity, which is more effective in the inlet zone of the steam reformer. The metallic support is formed substantially to have the same shape as the reactor wall and is arranged in a direct heat conduction relationship with the reactor wall. Desirably all of the tubes contain the same proportions of structured catalyst and particulate catalyst, which provides the benefits of the higher activity, higher heat transfer, and low pressure drop of the structured catalyst at the inlet end and the benefit of the stronger particulate catalyst at the outlet end. Heat transport is more efficient when catalyzed hardware is used in the steam reforming process. Keywords: Heat transfer; Heat management; Heat fluxes; Heat losses; Heat resistances; Heat exchange

Catalytic reactors for carrying out endothermic or exothermic reactions are of great importance in the particular examples being reactors for the endothermic steam reforming of methanol and reactors for the exothermic catalytic combustion reaction. The present study aims to provide a fundamental understanding of the exothermic and endothermic reaction characteristics and operation methods of integrated combustion-reforming reactors. Particular emphasis is placed upon the simultaneous implementation of the endothermic steam reforming and the heat-supplying exothermic catalytic combustion such that the thermal stability of the reaction system is increased. The effect of catalyst layer thickness on the reaction characteristics is investigated in order to understand how to design and operate such reactors with high efficiency. The results indicate that unique jet design features can be implemented in order to suppress homogeneous combustion and promote heterogeneous catalytic combustion on the channel wall. Diffusion within these small pores in the catalyst layers is typically Knudsen in nature for gas phase systems, whereby the molecules collide with the walls of the pores more frequently than with other gas phase molecules. The composition in the combustion chamber is reacted to produce sufficient heat to sustain the micro-combustion process without energy input. The combustion and reforming processes can be stably and efficiently operated at lower temperatures, without the need for energy input to sustain or even to start the combustion process. Since a palladium component is alloyed with the zinc, generation of carbon monoxide due to the methanol decomposition reaction is suppressed. Direct heating is of considerable advantage as it largely overcomes the problems encountered with reaction rates being limited by the rate of heat transfer through the tube wall especially near the reformer entrance. The conventional methods are suitable for large scale hydrogen gas production, but they are not adequate for middle to small scale hydrogen gas production. As the channel dimension nears the quench diameter or drops below, the contribution of the unwanted gas phase homogeneous combustion reaction is reduced. Keywords: Catalytic reactors; Reaction characteristics; Heat exchange; Carbon monoxide; Partial oxidation; Thermal stability

There are significant problems with current methanol steam reformer approaches as applied to vapor phase heterogeneous catalysis. There remains a need for further development in methanol steam reformer processes and systems. The present study aims to provide an improved reactor system and process for the carrying out of vapor phase heterogeneous reactions. The effect of temperature on the methanol mole fraction and effective factor is investigated for a microchannel methanol steam reformer with different shapes of the cross section of the process microchannel. Particular emphasis is placed upon the heat and mass characteristics involved in vapor phase heterogeneous reaction processes in methanol steam reformers. The results indicate that the steam reforming catalyst is adapted to produce a reformate stream from the feed stream, which is delivered to the reforming region at an elevated temperature and pressure. The fuel stream tends to vary in composition and type depending upon the mechanisms used to produce heat. Methanol is a particularly well-suited carbon-containing feedstock for steam reforming reactions. Methanol steam reforming typically takes place at a lower temperature than when other carbon-containing feedstocks are reformed. A methanol steam reforming catalyst is additionally or alternatively not pyrophoric. A benefit of a low temperature shift catalyst is that the reforming catalyst beds do not need to be shielded or otherwise isolated from contact with air to prevent spontaneous oxidation of the catalyst. Improving heat flux from tubular reactor outer environment to inner environment is a critical step to increase reactor efficiency. Smaller diameter catalytic reactors can offer several advantages of improving heat transfer from external heat source to reaction mixture in the tube, enhancing tube life-time by reducing thermal gradients, reducing metal material use, and being applicable for compact steam reformer systems. Keywords: Heterogeneous catalysis; Vapor phases; Reaction processes; Diffusion coefficients; Heat fluxes; Support structures