Opaque multiphase reactors: Experimentation, modeling and troubleshooting

Multiphase reactors are widely used in petroleum, chemical, petrochemical, pharmaceutical and metallurgical industries as well as in materials process ing and pollution abatement. Most reactors of interest in industrial practi ce (slurry bubble columns, gas-solid risers and fluidized beds, ebullated b eds and stirred tanks) are opaque as they contain a large volume fraction o f the dispersed phase. All the physical phenomena that affect the fluid dyn amics of such systems are not yet entirely understood. This makes a priori predictions of important process parameters (pressure drop, velocity and ho ldup profiles, degree of backmixing, etc.) very difficult. Industry relies on correlations, and these are prone to great uncertainty a s one departs from the operating conditions contained in the available limi ted data base. Prediction of the needed process parameters based on fundame ntal fluid dynamic models would be most welcome, fyet even the best models (that can treat large vessels or conduits that are of interest) require clo sure forms for phase interaction terms which are still subject to uncertain ty and debate. Hence, there is a need to verify such models; verification c an only be accomplished if we measure precisely those quantities that we wo uld like the model to ultimately predict, i.e. phase holdup and holdup prof iles, velocity profiles, backmixing, etc. However, the systems are opaque a nd we cannot "see" into them, and so it seems that a vicious circle has bee n closed and that model predictions are destined to remain unchecked. Fortunately, as two extensive recent reviews point out (Chaouki et al., 199 7a, 1997b) there are techniques which can provide us with the desired infor mation. In this paper, we review two of them: gamma ray assisted tomography (CT) for measurement of holdup profiles and computer aided radioactive par ticle tracking (CARPT) for measurement of velocity profiles and backmixing parameters. We then show how these techniques can be used to obtain informa tion in systems with moving catalysts of industrial interest such as gas-so lid riser, liquid-solid riser and gas-liquid bubble column. The ability of the available CFD (Computational Fluid Dynamics) codes to co rrectly predict the observed hydrodynamic quantities is also briefly discus sed. We then address the issue of two-phase flow in packed beds and the evo lution of the experimental techniques and models used to quantify these rea ctors better. Finally, troubleshooting on industrial scale reactors and use of tracer methods to accomplish this are briefly mentioned.