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44924-AC9
Transversal Hot Zones Formation in Packed Bed Reactors

Dan Luss, University of Houston

Local hot zones have been observed in both industrial and laboratory packed bed reactors, which are the work-horse of the chemical and petroleum industry. The presence of a hot zone next to the reactor walls may lead to safety hazard as it can decrease the mechanical strength of the wall and lead to a crack. The subsequent release of reactants may lead to an explosion. We conducted a theoretical and experimental study of the formation of such hot spots in order to gain understanding what may lead to their formation and to gain the needed know how to devise control and operation procedures that circumvent their formation.

Our theoretical analysis revealed that stable hot zones may form 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 as well as C2H4 hydrogenation. We found that qualitatively different spatio-temporal temperature patterns may form under the same operating conditions. Their number increases as the reactor diameter is increased. The interaction and conjugation among qualitatively different moving temperature patterns can generate very complex and chaotic motions.

Experimental study of the hot zone formation on the top of a shallow, cylindrical packed bed reactor was conducted using infra-red thermography to follow the temporal and spatial temperatures. The hot zones were, in general, separated by a sharp temperature front from the adjacent colder region and they were not stationary. In order to check whether the motion was affected mainly by the kinetics of the reactions and the adsorption strength of the reactant or by the flow through the bed and its properties we conducted experiments with two different reactions (oxidation of CO and propylene using a Pd catalyst) and mixtures of the two reactants. We found that the period of hot zones oscillations using a mixture of the two reactants was about 20 times shorter than those during CO oxidation and twice shorter than those during propylene oxidation. The mixture of the two reactions led to the formation of hot zones over a much wider range than that of each single reaction as well as under conditions for which neither reaction led to formation of hot zones. These experiments indicated that the motion of the hot zones is affected mainly by the kinetics of the reactions and not by the properties of the properties of the catalytic packed bed.

The selective purification of ethylene from acetylene is an important industrial reaction. Experiments were conducted to determine if this catalytic hydrogenation by a Pd on  alumina can lead to hot zone formation. We found that moving local hot zones (amplitude of up to 30˚ C) were observed for ethylene feed concentrations exceeding 22.5 mol % in both the presence and absence of carbon monoxide in the feed. The oscillatory hot zones were observed under conditions in which all the acetylene was converted. This indicates that they were generated by the exothermic hydrogenation of the ethylene and not by the hydrogenation of the small acetylene impurity that is present in the feeds to polyethylene polymerization reactors.

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