(239c) What Generates Transversal Hot Zones in Packed Bed Reactors



Transversal (normal to the flow direction) hot zones have been reported to form in packed-bed reactors. While these hot zones may form due to local non-uniformities in the local activity or porosity, they may form also in a strictly uniform reactor for certain classes of reaction kinetics. Previous modeling attempts failed to predict them without making the unrealistic assumption that the transversal heat dispersion exceeds that of the reactants. It is shown that the formation of transversal hot zones in a uniformly active catalytic reactor is strongly dependent on the reaction kinetics. For example, transversal spatio-temporal concentration and temperature patterns can be predicted to form in an shallow adiabatic packed bed reactor using realistic parameters for a catalytic reaction, the rate of which may oscillate under constant ambient conditions. A large number of different types of stable, transversal patterns may form for a sufficient large reactor diameter, most of which do not exhibit azimuthal symmetry. Surprisingly, the time average effluent reactant concentration and the period of the different spatiotemporal patterns are rather similar and close to those obtained under a uniform oscillating state. Dynamic simulations of a 2-D long packed bed reactor model (one that ignores the azimuthal dependence) reveal the existence of complex periodic motions (period-2, -4, -8) as well as chaos. The transversal features of these periodic motions resemble those of several transversal modes predicted by linear stability analysis. Spatial snapshots of the chaotic motion strongly suggest that it was caused by modulation and juxtaposition among motions corresponding to different modes. Several of the model predictions have been verified by experiments involving the oxidation of either carbon monoxide, or propylene or a mixture of both.