(42f) Some Hydrodynamic Characteristics of Gas Fluidized Beds at High Temperatures

In the past decades, chemical, petrochemicals and metallurgical industries have beneficiated from some unique aspects of gas fluidized beds. Good solid mixing, fair mass and heat transfer are some of the advantages of fluidized beds in comparison to fix beds. To this respect, solids and bubbles motions and interactions play a key role in the performance of fluidized bed reactors. Although most industrial reactors operate at high temperatures, most studies of gas-solid fluidization have been carried out at ambient temperature.

In this study, the Radioactive Particle Tracking (RPT) technique is used for the first time at high temperatures to investigate the effect of temperature on the main characteristic of fluidization in a bubbling fluidized bed reactor. Two types of solid particles both belong to group B of Geldart classification (i.e. Alumina with density of 3400 kg/m3 and mean size of 150ìm and sand particles with density of 2600 kg/m3 and size of 250µm) were used in this study. A radioactive particle, with the same properties as the sand and alumina particles, is used as the tracer. The diameter of the column and height of the bed are 78 and 450 mm, respectively. Experiments have been carried out at different temperatures and superficial gas velocities. Temperature and gas velocity are in the range of 25-700^oC and 0.16-0.78 m/s, respectively. RPT technique results in a time array of tracer position in the bed. These results are then analyzed to determine the wake phase velocity, jump time distribution and granular temperature. Ergodicity theory was also applied to calculate particle-particle and particle-wall restitution coefficients. The results show an increase in wake velocity and frequency of jump time with increasing temperature for the both sand and alumina particles. Unlike alumina particles, sand particles, however, show a slight decrease in wake velocity, jump time and granular temperature trends at temperate range of 400-600^oC. This is associated to the different surface properties of alumina and sand.