| Popis: |
The phenomenon of hydraulic transport of a packed bed is observed to occur at the start-up of three-phase fluidized beds operating under high gas hold up conditions. This phenomenon, which deviates from the conventional slugging flow dictated by the column size, may lead to a total or partial entrainment of the particles during the start-up of a fluidized bed reactor system. In this study, flow visualization techniques are used to investigate the bubble dynamics underneath the packed bed at the instant when the liquid flow is introduced. Cylindrical hydrotreating catalysts and 1.5 mm glass beads are used in a rectangular fluidized bed. An aqueous surfactant solution of 0.5 wt. % t-pentanol is employed to yield high gas hold up fluidization conditions. The experimental results indicate that, at a continuous gas flow rate prior to the liquid flow start-up, the small bubbles are attached to the surface of the particles. Consequently, at the start-up, the liquid flow is impeded and the small bubbles accumulate beneath the packed bed, thus forming a stable foam layer and a foam-particle aggregate at the bottom of the bed. The constriction of the liquid flow through the foam layer and foam-particle aggregate results in the formation of a stable moving packed bed. |
