Bewertung des Einsatzes von Mikrostrukturen in mobilen Brenngaserzeugungseinheiten

In this thesis the application of microstructures for heat exchangers and reactors in the gas treatment system of a fuel processing unit for mobile applications based on catalytic autothermal reforming of diesel-like fuels was experimentally and theoretically investigated. The experimental work involved simplified experimental setups and prototypes of microchannel heat exchangers. The theoretical part dealt with the fluid dynamic modelling of the transport phenomena that are relevant in microstructured and monolithic apparatus. The thermohydraulic characterisation of microchannels shows a sharp increase in the pressure drop if the characteristic length of the channels is reduced. The transition from laminar to turbulent flow occurs at lower Re-numbers than on a larger scale. The heat transfer coefficients improve, if the channel size is reduced, but this enhancement is less pronounced than the increasing pressure drop. Therefore very tiny structures are not optimal as far as heat exchanger equipment is concerned. The optimal size is in a range of approximately 0.4 to 1 mm. The prototypes studied indicate the potential for reducing the weight and the volume of the required heat exchanger equipment by the incorporation of microstructure technology. However, for further optimisation there is a need for a thorough revision of the construction principle. Application-oriented, catalytic investigations were performed for all reactions in the gas treatment. The catalysts for the water-gas shift reaction show only a moderate catalytic activity. The catalyst based on precious metals is more active than a base metal catalyst, but lacks the required long-term stability if hydraulically coated on a metallic substrate. The catalyst used for the selective oxidation of CO shows a poor performance and a narrow temperature window for optimal reactor operation. The oxidation of hydrogen can be accomplished at high space velocities, low reaction temperatures or low loadings with catalytically active component. For the oxidation of methane longer residence times, higher reaction temperatures and higher loadings with catalytically active component are needed if complete conversion of methane is required. A platinum-based catalyst is very well suited for the catalytic combustion of hydrogen. In the case of methane, the palladium-based catalyst tested is more active in comparison to the platinum catalyst. For all reactions under consideration here conversion at constant reaction temperature increases if the channel size is reduced. The comparison of experimental data with results of computer simulations shows that fluid dynamic modelling can be used for dimensioning and optimisation of micro reactors and heat exchangers concerning all relevant figures such as pressure drop, heat transfer and chemical reactions. The model for mass transport by mulitcomponent diffusion, which differentiates between the channel and the porous catalyst layer, is suitable for describing the relevant transport phenomena. None of the catalysts under consideration in this thesis are active enough to justify an integration of a heat exchanger into a reactor of the gas treatment system of fuel processors for mobile applications. This can be shown by simulation results and experimental data