Dynamic responses of packed bed reactors

Abstract

Amplification of process disturbances, wrong-way behavior, and extinction waves are responses to inlet disturbances of temperature, concentration or flow velocity in packed bed catalytic reactors. They can result in unexpected high temperatures that might compromise the reactor safety or performance. All of these responses are either manifestations of or are related to differential flow instability. The degree of amplification depends on the width of the reaction zone, which in turn depends on the diameter of catalyst particle. The ratio of the former and the latter determines the relative strength of convective (destabilizing) and diffusive (stabilizing) transport of heat. A modified Peclet number based on the length of the reaction zone is proposed as a criterion for the importance of disturbance amplification. Experiments and numerical analyses indicate that significant amplification under typical operating conditions of packed-bed reactors occurs for gaseous reaction systems at disturbance frequencies 0.0003–0.001 Hz and for liquid-phase reactions around 1 Hz. While simple estimates suggest that amplification that is large enough to threaten the reactor safety or to deactivate catalysts is infrequent, amplification and related responses to disturbance cannot be neglected in reactor design. Despite almost a decade of study, several questions persist about the prediction of these responses to inlet perturbations in industrial reactors.