Wrong way behavior of a catalytic packed-bed reactor

A sudden decrease in the feed temperature or a sudden increase in the gas flowrate into a catalytic packed bed reactor may lead to a transient temperature rise, which is known as "wrong-way behavior". The transient temperature rise is caused by the different travelling speeds of the concentration and temperature waves in the reactor. The unexpected temperature excursion may damage the catalyst, cause run-away of the reactor, ignite undesired side reactions and complicate the control strategy and shut-down procedures. Mathematical models with different degrees of simplification are used to investigate the behavior of the reactor when the feed temperature is suddenly decreased. Methanation of carbon dioxide on a nickel catalyst is used to study experimentally the wrong-way behavior when the gas flowrate is suddenly increased.;Initially, a first-order exothermic reaction is studied with a one-dimensional heterogeneous dispersion model. The interfacial and intraparticle heat and mass transfer resistances may lead to a higher transient temperature rise than that predicted from a homogeneous model. A catalytic pellet may be ignited from a low temperature steady state to a high temperature steady state when multiple steady states exist in the catalyst. Multiple steady states of the catalyst may widen the region of steady-state multiplicity for the reactor and increase the chance of a reactor run-away.;The wrong-way behavior for two parallel reactions and two consecutive reactions are studied separately using a pseudo-homogeneous plug-flow model and a pseudo-homogeneous dispersion model. The wrong-way behavior may ignite undesired side reactions that do not occur under normal operating conditions. The ignition of a hidden side reaction occurs when the side reaction has large heat of reaction and activation energy than that of the main reaction. The ignition by the wrong-way behavior occurs only when the reactor has moderate conversion in the initial steady state.;An experiment using the methanation of carbon dioxide on a nickel catalyst shows that a wrong-way behavior can occur when the gas flowrate is suddenly increased. Only a single steady state exists for 3% carbon dioxide in this study. The maximum transient temperature rise increases with the initial conversion. (Abstract shortened with permission of author.)...