Thermal and mechanical analysis of a microreactor for high temperature catalytic gas phase reactions

In this contribution we describe the results of a study into the thermal and thermo-mechanical behavior of a high temperature micro gas reactor for investigation of the intrinsic reaction kinetics of rhodium-catalyzed direct catalytic partial oxidation (CPO) of methane into synthesis gas. The chip comprises a flow channel etched in silicon, capped with a thin composite membrane of heavily boron-doped silicon (p++-Si) and low stress silicon-rich nitride (SiRN), on which thin film heaters and sensors are located. The results of analytical and numerical models, which are verified with experimental results, can be used as general guidelines for the design of membrane-based microreactors. It is found that a membrane composition of 850 nm p++-Si and 150 nm SiRN result in time constants of 1 ms for heating up as well as cooling down, which enables the required fast control of the exothermic reaction. Thermo-mechanical analyses demonstrated that this membrane is mechanically stable for temperatures up to at least 700 °C. Although the shape of the heaters — meander or sinusoidal — does not influence the mechanical stress profiles significantly, a decrease in heater width compared to the membrane width results in a drastically improved thermal efficiency of the microreactor. Furthermore, thermal considerations showed that exact temperature distribution in the composite membrane is mainly determined by the heater width in combination with the thickness of the heavily boron-doped silicon part of the membrane and its (temperature-dependent) thermal conductivity.