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( )Computed tomography CT and computer-automated radioacti®e particle tracking ( ) CARPT were used to study the gas distribution and incipient particle motion in a packedrebullated bed in which gas and liquid are in cocurrent upflow. CT

Flow maldistributions and corresponding hot spots may cause adverse effects in the operation of down-flow packed-bed reactors. To avoid these phenomena it is necessary to know when and how they occur. In this work, a three-dimensional model that considers the effects of fluid flow, mass transfer by diffusion, heat transfer by conduction and reactant consumption by an exothermic irreversible first-order reaction is used to analyse an adiabatic packed-bed reactor with down-flow. It is shown that the one-dimensional uniform down-flow can become unstable to three-dimensional stationary and time-dependent perturbations, giving rise to non-uniform flow fields that lead to fixed as well as moving hot spots. The boundary of the region of the operating conditions at which these instabilities occur is determined as a function of the various physicochemical parameters that characterize the packed-bed reactor.

Micromixing Theory via the Two-Environment Model," ibid., 25, 1785 ( 1970).O'Neill, S. P., "Steady-State stability of Substrate Inhibited Enzyme Catalysis in Open Reaction Systems," Biotechnol. Bioeng., 13,493 (1971).