Spatiotemporal temperature patterns of acetylene hydrogenation in a fixed bed catalytic reactor

The axial temperature profile in a catalytic packed bed reactor undergoing an exothermic reaction can attain a local maximum, which is referred to as a hot zone. Transversal hot zones (normal to the direction of bulk flow) may exist in adiabatic packed bed reactors. The localized high temperature regions may cause operational problems ranging from selectivity losses to a runaway reaction that may cause a serious safety or environmental incident.;Experiments were conducted to enhance our understanding of the formation and dynamics of transversal hot regions in a shallow, adiabatic catalytic packed bed reactor in which the hydrogenation of mixtures of acetylene and ethylene was carried out over a non-promoted palladium/alumina catalyst. Infrared imaging was utilized to determine the transversal temperature patterns on top of the shallow catalyst bed. Two qualitatively different types of non-uniform temperature patterns were observed: (a) anti-phase oscillatory patterns and (b) non-uniform stationary hot zones. The oscillatory patterns consisted of a long period (5 to 50 minutes, depending on residence time) during which the catalyst temperature was essentially at a pseudo stationary state. Then during a rather short period (∼1 to 5 minutes, depending on residence time) an anti-phase oscillation occurred, i.e. the hot region moved from one side of the catalyst bed to another and then returned to the original location. This was followed by a long period during which a pseudo stationary state existed. The temperature oscillations ranged from 3°C to 30°C. They were observed for catalyst temperatures between 78°C and 161°C and ethylene feed concentrations greater than or equal to 22.25 mol%. The oscillations were observed over the entire range of carbon monoxide tested, 0--0.05 mol%. When anti-phase oscillatory behavior did not occur, a non-uniform stationary state was observed in which one or two hot zones existed on the surface of the catalyst bed. A mathematical model is proposed that predicts spatiotemporal oscillations qualitatively similar to those observed in the experiments. The model employs a reversible, blocking mechanism to account for intermediates formed during this reaction. The model predicts that oscillations can form over a rather narrow range of parameter values.;All the experiments were performed under conditions in which the acetylene conversion to ethylene was near 100% and a substantial amount of the ethylene was hydrogenated to ethane. It is unlikely that the dynamic behavior encountered during this research will occur under the conditions employed during industrial acetylene hydrogenation as the non-uniform behavior occurred at high conversions of ethylene to ethane.