(62f) Detailed Measurements of Flow Dynamics inside a Dense Gas-Solids Fluidized Bed

Turbulent fluidization is commonly utilized in industrial processes, (e.g. FCC regeneration, Fischer-Tropsch synthesis, acrylonitrile production, silicon chloridization and particle drying) due to its vigorous gas-solid contacting, good mixing, high heat and mass transfer rates, high solids hold-ups (typically 25-35% by volume) and relatively low axial mixing of the gas. Despite its practical importance, turbulent fluidization has received much less attention than other flow regimes and many important aspects of hydrodynamics of fluidized systems are still poorly understood, probably due to the significant complex dynamic flow behavior. This study provides a comprehensive experimental work about flow structure characteristics in both bubbling and turbulent fluidization and the regime transition, and then establish a good description of flow patterns for the turbulent fluidized bed.

Experiments were carried out in a large diameter air-fluidized bed (0.267m i.d. x 2.5m high) using FCC particles (dp = 62 mm, ρp =1800 kg/m3). The flow dynamics were investigated based on local solids concentration and velocity data, as well as differential pressure signals, under various superficial gas velocities of 0.02~1.4 m/s (covering both bubbling and turbulent fluidization regimes). A newly developed optical fiber probe system capable of simultaneously measuring instantaneous solids concentration and velocity at four different locations were used to determine the radial profiles at several axial levels. Results show that in the turbulent fluidized bed the solids holdup is not at all uniform in both radial and axial directions. Although the curves of standard deviation for the differential pressure signals and local solids holdup all peak at the transition velocity from the bubbling to turbulent regimes Uc, the statistical analysis of the radial solids fraction indicates that for local flow regime the transition from bubbling to turbulent fluidization is a gradual process instead of a sudden change reflected by the fluctuations of differential pressure signals. In the turbulent regime, the two-phase flow structure breaks down and the flow structure is basically axis-symmetric with a high-density annulus region near the wall. More detailed differences in the dynamic behaviors between bubbling and turbulent fluidizations were observed. In addition, the effects of static bed height on the local flow structures are also investigated.