The effect of particle diameter and taper angle on the hydrodynamics of a liquid-solid tapered fluidized bed

Liquid dispersion and void fraction measurements were performed for three glass bead sizes (0.0114, 0.044, 0.121 cm) at three taper angles (0°, 0.5°, 1.5°) in order to determine the effect of particle size and taper angle on the hydrodynamics of a tapered liquid-solid fluidized bed. Dispersion coefficients were measured using the imperfect pulse method of Michelsen and Ostergaard (1970) and void fractions were measured locally using an electrical conductivity technique. Dispersion coefficients in the cylindrical bed, averaged over a range of void fractions (0.6 to 0.9), were 5.4, 5.7 and 10.3 cm²/s for 0.0114, 0.044 and 0.121 cm glass beads. Approximately two times more liquid mixing occurs in the 0.5° and three times more liquid mixing occurs in the 1.5° tapered bed than occurs in the 0° (cylindrical) bed. Therefore the advantage of the tapered bed over the cylindrical bed with respect to fluidizing a wider size range of particles is mitigated by increased axial mixing in the tapered bed. Fluidization in the tapered bed for the 0.0114 cm beads was characterized by a symmetric downward flow at the wall and by fluid channelling (the minimum channelling is 20 percent for void fractions greater than 0.75). Downward flow or channelling did not occur in the cylindrical bed fluidizing 0.0114 cm beads or in the tapered bed fluidizing the 0.044 or 0.121 cm beads and is therefore a characteristic of smaller size particle systems which have Reynolds Numbers based on column diameter in the laminar flow region. The 1.5° tapered bed fluidized more densely than the cylindrical bed for all particle sizes while the void fraction distributions in the 0.5° column were similar to those in the cylindrical bed (within 3 percent). Fluid channelling is not the cause of this more dense fluidization. The void fraction data for the 1.5° bed were correlated in a similar manner to the Wen and Yu correlation (1966) for cylindrical beds. The results of this study are of particular interest to those involved in the design of crystallizers and bioreactors.