Opaque Multiphase Reactors: Experimentation, Modeling and Troubleshooting

Abstract

Multiphase reactors are widely used in petroleum, chemical, petrochemical, pharmaceutical and metallurgical industries as well as in materials processing and pollution abatement. Most reactors of interest in industrial practice (slurry bubble columns, gas-solid risers and fluidized beds, ebullated beds and stirred tanks) are opaque as they contain a large volume fraction of the dispersed phase. All the physical phenomena that affect the fluid dynamics of such systems are not yet entirely understood. This makes a priori predictions of important process parameters (pressure drop, velocity and holdup profiles, degree of backmixing, etc. ) very difficult. Industry relies on correlations, and these are prone to great uncertainty as one departs from the operating conditions contained in the available limited data base. Prediction of the needed process parameters based on fundamental fluid dynamic models would be most welcome, yet even the best models (that can treat large vessels or conduits that are of interest) require closure forms for phase interaction terms which are still subject to uncertainty and debate. Hence, there is a need to verify such models; verification can only be accomplished if we measure precisely those quantities that we would like the model to ultimately predict, i. e. phase holdup and holdup profiles, velocity profiles, backmixing, etc. However, the systems are opaque and we cannot seeinto them, and so it seems that a vicious circle has been closed and that model predictions are destined to remain unchecked. Fortunately, as two extensive recent reviews point out (Chaouki et al. , 1997a, 1997b) there are techniques which can provide us with the desired information. In this paper, we review two of them: gamma ray assisted tomography (CT) for measurement of holdup profiles and computer aided radioactive particle tracking (CARPT) for measurement of velocity profiles and backmixing parameters. We then show how these techniques can be used to obtain information in systems with moving catalysts of industrial interest such as gas-solid riser, liquid-solid riser and gas-liquid bubble column. The ability of the available CFD (Computational Fluid Dynamics) codes to correctly predict the observed hydrodynamic quantities is also briefly discussed. We then address the issue of two-phase flow in packed beds and the evolution of the experimental techniques and models used to quantify these reactors better. Finally, troubleshooting on industrial scale reactors and use of tracer methods to accomplish this are briefly mentioned.