44924-AC9
Transversal Hot Zones Formation in Packed Bed Reactors
The proposed research was motivated by reported observations of local hot zones (much smaller than the reactor diameter) in a packed bed reactors (PBR). They have a deleterious impact on the yield of the desired products, may initiate undesired reactions and may lead to safety problems when present next to the reactor walls. We conducted both theoretical and experimental studies in order to gain an understanding what reactions and operating conditions may lead to the formation of such hot zones in the cross-section of the reactor. This information is essential for circumventing their undesired evolution.
Our analysis revealed that a pseudo-homogeneous model of a uniformly active, adiabatic PBR can predict formation of stable hot zones in the cross section of the reactor if the kinetic rate expression can lead to isothermal rate oscillations. This prediction was confirmed by simulations of CO oxidation. We discovered that hot zone formation can be predicted also for C2H4 hydrogenation, the kinetic model of which is structurally different from that of CO oxidation. Qualitatively different spatiotemporal temperature patterns may form under the same operating conditions. Their number increases as the reactor diameter is increased. An increase in the reactor diameter increases the time constant of the transversal heat dispersion and decreases the temperature synchronization among points on the same reactor cross section. The interaction and conjugation among qualitatively different moving temperature patterns can lead to formation of complex motions.
Infrared imaging, used to test these predictions revealed that transversal moving temperature patterns on top of a shallow packed bed during the hydrogenation of mixtures of ethylene and acetylene. Two qualitatively different non-uniform temperature patterns were observed: (a) Non-uniform stationary hot zones. (b) Anti-phase oscillatory patterns, during which the catalyst temperature was essentially at a pseudo stationary state for a long period (35 to 50 minutes). The hot region then moved rapidly from one side of the catalyst bed to another and then returned to the original location (period 3 to 5 minutes). The location of the hot zones was influenced by non-uniformities of the catalyst in the bed and occurred for conditions in which a unique steady state existed. All previous studies of hot zone evolution in a PBR involved a single exothermic reaction. We investigated what new behavioral features will occur if two reactions are carried out simultaneously in a PBR. Infrared thermography was used to study the hot zones formed during the atmospheric oxidation of either propylene or its mixture with CO. The hot regions were separated by a sharp temperature front from the adjacent colder region (DT~50oC). The period of the oscillations of mixtures of propylene and carbon monoxide were about 20 times shorter than those of CO and about 2 times shorter than those of propylene. This indicates that the frequency of the hot zone motions is affected mainly by the kinetics of the catalytic reaction and the strength of adsorption of the organic reactants and not by the properties of the bed and/or the flow through it. The mixture of the two reactants led to formation of moving hot spot over a much wider range than that of either reaction, and under operating conditions for which neither one of the two reactions led to formation of hot regions. The experiments seem to confirm our earlier prediction that transversal hot zones are likely to form in a shallow packed bed reactor for reactions the rate of which may exhibit oscillatory behavior.
