Tomography and Velocimetry for Multiphase Reactors: A USA Perspective

M.H. Al-Dahhan and M.P. Dudukovic Chemical Reaction Engineering Laboratory (CREL)

Washington University, St. Louis, MO 63130, USA

muthanna@wuche.che.wustl.edu, http://crelonweb.wustl.edu/

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. Most the physical phenomena that affect the fluid dynamics of such systems are not yet entirely understood. Hence, the design, scale-up and performance prediction of these reactors are still not an easy task due to the complexity of their flow field. This makes a priori predictions of important process parameters (pressure drop, velocity and holdup profiles, degree of backmixing, etc.) very difficult.

Industry currently relies on correlations, and these are prone to great uncertainty as one departs from the operating conditions contained in the available limited data base. Therefore, predictions of the needed process parameters based on fundamental fluid dynamic models are needed, 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, turbulent parameters, etc. However, the systems are opaque and we cannot "see" into them, and hence that models predictions are destined to remain unchecked. Moreover, the commonly employed for single-phase sophisticated optical techniques such as laser-doppler velocimetry (LDV), digital particle image velocimetry (PIV), etc. cannot be pursued for hydrodynamics and mixing investigations of such opaque systems.

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 presentation, we review two of non-invasive diagnostic techniques which have been developed in our laboratory (CREL) and have been used extensively to characterize different types of multiphase-opaque reactors. These techniques are: gamma ray computed tomography (CT) for measurement of holdup profiles and computer automated radioactive particle tracking (CARPT) for measurement of velocity profiles and backmixing and turbulent parameters. We then show how these techniques can be used to obtain information in systems with moving catalysts of industrial interest such as bubble/slurry bubble columns, liquid-solid risers, gas sparged stirred tanks, etc.

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.