Propagation of Excitation Pulses and Autocatalytic Fronts in Packed-Bed Reactors

Abstract

We experimentally and numerically studied the velocity of excitation pulses in packed bed flow reactors as a function of the fluid flow velocity and the diameter of the glass beads used as packing material. Differential transport was absent. The downstream and upstream propagating pulses were observed to behave in a manner that is strikingly different from the simple Galilean translation expected in the case of an ideal homogeneous plug-flow. Downstream propagating pulses travel faster than anticipated, by a constant factor that depends on the bead size. Upstream propagating pulses travel at a lower velocity and become stationary above a critical value of the fluid flow velocity. Both the width and the intensity of up- and downstream propagating pulses increase when the fluid flow velocity is increased. Model calculations show that the accelerated downstream propagation and the decelerated upstream propagation agree qualitatively with enhanced turbulent diffusion within the packed bed. The formation of stationary pulses can be explained by the existence of a stationary fluid phase of stagnant pockets within the packed bed. Once excited, a stagnant pocket acts as a permanent super-critical perturbation, which causes excitation of the flowing medium and locks the temporal response in space. The dramatic increase of the pulse-intensity remains unexplained by the mentioned models.