Hydrodynamic comparison between a tapered and a cylindrical fluidized bed by measurement of liquid dispersion, bed expansion and particle segregation

A tapered fluidized bed was compared to a cylindrical fluidized bed by measurement of liquid dispersion, bed expansion and particle segregation. This was accomplished by fluidizing coal particles (0.032 cm) and glass particles (0.01 cm) in both a tapered and cylindrical column. The tapered column had a cone angle of 1.47° and a base diameter of 2.54 cm, while the cylindrical column was 5.08 cm in diameter. Liquid dispersion measurements were performed using the axial dispersion model with an imperfect pulse tracer technique. Tracer measurements in the tapered fluidized bed showed that the actual residence time of the fluid was less than it would be for plug-flow. In the cylindrical fluidized bed the two residence times were equal. For this reason dispersion coefficients for the tapered fluidized bed were calculated using the actual residence times and were found to be an average of 11.7 cm²/s for coal and 8.4 cm²/s for glass fluidization. Smaller dispersion coefficients were found for the cylindrical fluidized bed, 4.1 cm²/s for coal and 1.2 cm²/s for glass fluidization, indicating that more fluid mixing occurs in the tapered fluidized bed. This seems to be a result of the downward flow which occurs at the walls of the tapered fluidized bed.

Axial void fraction profiles were measured for the tapered fluidized bed using an electrical resistance technique. These results showed that the particles in the tapered bed fluidize more densely than in the cylindrical bed. The overall bed expansion data also showed this. Particle segregation was also measured for both fluidized beds and more particle segregation occurred for the cylindrical fluidized bed. The fluidization in the tapered column over the range of conditions studied were categorized into three regimes and an empirical flow model was presented to describe the flow.