Dynamics of Transversal Hot Zones in Shallow Packed-Bed Reactors

Abstract

Infrared thermography was used to study the evolution and dynamics of hot zones on the top of a shallow packed-bed reactor (10 cm in diameter), packed with spherical catalyst pellets (Pd/Al2O3). The test reaction, the atmospheric oxidation of carbon monoxide, was run under conditions for which steady-state multiplicity exists. Slow cooling of the vessel housing the reactor close to its extinction temperature shifted the reactor from a fully ignited state to one with a hot region, separated by a sharp temperature front from the adjacent colder region (ΔT = 75 °C for a feed of 6 vol % CO). The hot zone exhibited one of three qualitatively different motions: a breathing motion, an antiphase (standing wave) motion, or a hopping motion. In breathing motion, a the sharp temperature front moved back-and-forth. The breathing frequency increased upon cooling of the reactor. In antiphase motion, the hot zone moved rapidly and periodically from one side of the reactor to the other. In hopping motion, the hot zone rotated around the reactor, followed by a periodic rest at some locations. The angular velocity of the rotating hot zone and its size changed with angular position. Slow cooling of the reactor led to the following sequence of bifurcations (breathing → antiphase → hopping) Preliminary experiments suggest that global coupling between the top of the bed and the unconverted reactants stabilized the spatiotemporal patterns, which may not exist in its absence.