Entwicklung von Mikrostrukturreaktoren zum thermisch instationären Betrieb chemischer Reaktionen

Abstract

From literature it is known that some chemical processes may benefit from periodic changes of one or more process parameters due to non-linear behaviour. Usually, the benefit can be obtained with reasonably short period times. Thus, experimental systems with reduced response times are necessary to examine the possible effects. A periodic change of the process temperature is predicted to have the strongest influence to the process. The maximum frequency of the temperature modulation strongly depends on the thermal inertia of the reactor system. This inertia is dominated by the thermal mass of the reactor system. However, other parameters like the residence time of a reactand mixture may also have to be considered. Recent developments in microstructure technology allow the design of microstructure devices with low thermal masses. These devices permit the thermally unsteady state (periodic) operation of chemical reactions. Metallic microstructure devices are suitable for operation in wide ranges of temperature and pressure. Moreover, it is possible to integrate porous layers to increase the inner surface of the device and to apply catalyst coatings. The objective of the present thesis was to show the possibility of a thermally unsteady state operation with period times down to the subsecond range by using a specifically designed microstructure device for Fast Temperature Cycling (FTC). For a simple heterogeneously catalysed gas phase reaction, the process performance was compared between steady state and FTC operation. New microstructure devices for thermal unsteady state operation and FTC were designed and manufactured. The devices were designed to provide an optimum combination of resistance against high pressures and temperatures and minimized thermal masses. Catalytically active materials were integrated into a number of devices. A special experimental setup was build for the FTC, using conventional hardware providing high measurement accuracy and reliability under harsh operating conditions. A new measurement and control software was developed. With this software, the experimental setup could be operated up to a FTC frequency of 1,6 Hz. The software allows to either preset heating and cooling period times indepently, or to define upper and lower temperature boundaries for an automatic FTC processing. In the latter case, the heating and cooling period times will assume different values. All microstructure devices were designed to be continuously heated with high power resistor heating cartridges and to be periodically cooled by intermittent injection of a coolant liquid. In the work reported, deionised water was chosen as coolant in order to increase the cooling by (partial) evaporation. The thermal behaviour of the latest microstructure design was simulated by CFD methods for unsteady state processing. Agreement with experimental results was excellent. A maximum mean heating / cooling rate of 140 K.s - 1 was reached in FTC reactors. Under these conditions,,,hot spots occured inside the microstructure devices, and prolonged operation with these parameters can result in irreversible damage. A safe continuous operation with constant heating power can be achieved. Under these conditions, a periodic temperature change of 100 K over a partial period time of 2,1 seconds is obtainable, corresponding to a mean heating / cooling rate of approximately 48 K s - 1 . The safety limits defined in the measurement and control software allowed a maximum temperature difference of ′173 K.