Solid behaviour in fluidized beds

The movement of solids in fluidized beds was investigated at different levels by employing the Radioactive Particle Tracking technique (RPT). In the first step, the movement of a single particle in liquid fluidized beds has been studies. The liquid was water and nine different types of particles were used as fluidized particles. A radioactive tracer was dropped into the fluidized bed in order to measure its settling velocity. Emitted γ-rays from the tracer were detected by eight NaI scintillation detectors placed around the bed. The number of counts of γ-rays were then used to calculate the coordinates of the tracer from which the velocity of the tracer and then the effective drag coefficient were calculated. The correction factor, i.e., the ratio of effective drag coefficient over standard drag coefficient, is known to be a function of the porosity and terminal Reynolds number of the falling particle and was found from the present study to be also a function of Archimedes number of the fluidized particles. It was shown in this study that the models which calculate the buoyancy force exerted on a single particle based on the fluid density can predict the slip velocity of particle in the fluidized bed better than those evaluate the buoyancy force based on the bulk density of the bed.

In the second step, the local behavior of fluidized beds was investigated in a gas-solid fluidized bed. The gas was air at room temperature and atmospheric pressure and the solids were sand or FCC powder.

In the third step, the global behavior of fluidized beds in axial direction was studied. The RPT data of the previous section were also used for calculations of this part. A variety of solids behaviors, including natural dispersion, restricted axial movement, and unrestricted axial movement of solids, was studied in this section by processing the data in an appropriate manner. The value of dispersion coefficient of solids was evaluated from the natural dispersion of solids and was found to be constant under the experimental conditions of this work. The variance test was performed on the Residence Time Distribution (RTD) of solids in the restricted upward and downward movement. The results of the variance test clearly showed that upward movement of solids could be explained by a diffusive mechanism while the mechanism of downward movement of solids should be considered as convective. (Abstract shortened by UMI.)