Influence of buoyancy forces on the flow of gases through packed beds at elevated pressures

Hot zones evolution and dynamics in heterogeneous catalytic systems

Stationary and complex moving hot regions formed for temperatures close to the extinction temperature of uniformly ignited states of several catalytic systems, such as thin rings and hollow cylinders, a thin radial flow reactor (RFR) and a shallow packed bed. IR imaging revealed that the hot and cold regions (temperature difference of the order of 100 degrees C) were separated by a sharp (about 3 mm wide) temperature front. The transition from the branch of uniformly ignited to the states with a hot region was usually supercritical. In some experiments a disjoint branch of states with hot regions existed and two qualitatively different states with hot zones existed under the same operating conditions. A very intricate periodic motion of a hot zone was observed in a shallow packed bed reactor. For example, Fig. 16 shows a hot zone which splits and later coalesces several times during the long (14 h) period. Hot pulse motions were observed on a single catalytic pellet. These were caused by global coupling between the surface reaction rate and the ambient reactant concentration and the inherent nonuniformity of the catalytic activity. It is not yet clear what rate processes generate the transversal hot zones in uniform packed bed reactors. (c) 2002 American Institute of Physics.