Hydrodynamique des lits fluidisés à trois phases: Mécanismes convectif et dispersif

The two objectives of the thesis were to identify the key hydrodynamic characteristics of the phases in three-phases fluidized bed and to develop a mixing model as simple as possible, but respecting the key hydrodynamic characteristics. In order to reach partially the first objective, a critical literature review, that allowed identifying the key hydrodynamic characteristics for three-phase fluidized bed, was performed. The Radioactive Particle Tracking (RPT) technique was used to reach completely the first objective and to exploit the mixing model developed during the thesis (second objective). Experiments were performed over two fluidized beds of different diameters, i.e., 0.10 and 0.292 m. Most of these experiments were done with abed containing 3 mm diameter glass beads. PVC particles were also fluidized as well as binary systems composed with glass beads of various diameters and with mixtures of glass beads and PVC particles. The gas phase was air, and water was used to fluidize the particles. Superficial gas and liquid velocity ranges were 0.010-0.106 m/s and 0.042-0.065 m/s, respectively.

The critical literature review shown that the Axial Dispersion Model (ADM) was too simple to simulate the phases mixing when the kinetics were fast and the mass transfers limiting. Gas phase follows a convective mixing mechanism, i.e., the mixing is due to the various velocities of a system containing elements that have constant velocity. In other words, the rise of the bubbles may be represented by a series of plug flow reactors in parallel. This mixing mechanism has to be differentiated from the dispersive mixing mechanism, which is characterized by a system where the elements have an erratic movement and a velocity that changes randomly. A fraction of the liquid and the solid particles, which rise with the bubbles in their wake, follows a convective mixing mechanism. A schematic model showing phases interactions and taking into account various solid and liquid fractions (sub-phases) was developed in the critical literature review. This schematic model is in fact an update of the structural wake model.

RPT data have allowed quantifying schematic model parameters for the solid phase. These parameters were linked to relative contribution of the convective mixing mechanism to the dispersive mixing mechanism. (Abstract shortened by UMI.)