Partial and Total Oxidation of Methane in Monolithic Catalysts at Short Contact Times
The feedstocks of natural gas exceed the resources of crude oil by far. Therefore, there is a strong interest in the utilisation of natural gas with its main component methane for the production of more useful chemicals. This leads to a strong demand for compact and low-capital-cost reactors for the conversion of natural gas to methanol and liquid hydrocarbons and the production of H2. The combustion of natural gas is also considered for power generation in catalytic devices, because the flameless catalytic process results in lower emissions of NOx. Noble metals such as Pt, Pd, and Rh are very active catalysts for the conversion of CH4. However, the processes occurring during the partial and complete oxidation of CH4 on these catalysts are not completely understood. A detailed understanding of the reactor behaviour at all possible conditions is crucial for the technical realisation of these catalytic processes, because explosive mixtures are handled. In particular it is necessary to control the operation of the reactors at transient conditions such as light-off and shut-down. The objective of this work is to study total and partial oxidation of methane over Pt, Pd, and Rh catalysts for a wide range of operating conditions. Therefore, an experimental setup, which is easily applicable for modelling and numerical simulation, has to be developed. The experiment and analysis has to be arranged in a way that allows studies of transient processes such as light-off. Based on the experimentally derived conversion and selectivity surface-reaction schemes, available in literature, have to be evaluated. Available computational tools have to be used for numerically predicted reactor performance and compare these results with the experimental data. Crucial conditions, at which the reaction models need improvement, have to be found, and if necessary and possible improved. A flow reactor with associated analysis is designed to meet these requirements. In order to facilitate the simulation of the system catalysts exhibiting a simple geometry such as monolithic structures are used. The reactor consists of a 40 cm long quartz tube with varying inner diameters. Honeycomb catalysts coated with Rh, Pd, or Pt are placed inside. The gas temperature at the exit of the catalytic monolith and outside the quartz tube is. The reaction is ignited by heating up the reactor by a furnace, in which the flow system is integrated. After ignition, in the case of autothermal operation, the furnace can be switched off. The gases are premixed at room temperature and flow into the quartz tube at atmospheric pressure. The product composition is quantitatively analysed by a quadrupole mass spectrometer (QMS) in order to study transient phenomena. For the evaluation of reaction mechanisms, detailed numerical simulations of the physical and chemical processes using the computer program package DETCHEM and the commercially available CFD code FLUENT are applied. The numerical codes take into account detailed mechanisms of surface and gas-phase reactions as well as mass and heat transport processes in the channels and heat transport in the solid monolithic structure. The channels are simulated under steady-state conditions in a 3D elliptic approach using FLUENT and in two dimensions by a parabolic approach using DETCHEMCHANNEL. Transient phenomena are simulated by DETCHEMMONOLITH with 2D parabolic flow field simulations of a representative number of single channels including transient heat balances of the monolithic structure. First, the ignition of catalytic combustion of CH4 on a Pt loaded monolith is investigated. The conversions of CH4 and O2 as well as the product selectivities are experimentally determined and the results numerically simulated. The influence of H2 addition on the combustion is investigated. Reaction-rate oscillations of the catalytic combustion of methane on a Pd based catalyst are found experimentally at lean conditions in Pd coated monoliths and simulated with a surface-reaction mechanism. The second set of experimental focuses on CPO of CH4 over Rh coated monoliths. The experimentally derived conversion and selectivities are used to improve a detailed reaction mechanism. Next, the influence of the support material on the ignition process is investigated. In the experiment, either -alumina or cordierite as support material is applied and coated with Rh. The models are able to describe the light-off behaviour and are applied for catalyst design: The ignition behaviour of a set of virtual catalytic monoliths exhibiting varying physical properties is modelled. In addition, catalysts coated with Rh-nanopowder produced by laser ablation is experimentally examined and compared with wet coated catalysts. Finally, the CPO of methane is carried in a Pt, presenting an even more well-defined configuration for modelling. At CH4/O2 ratios above 1.9 complex patterns of oscillation in conversion and selectivity occur.
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