The gas-solids fluidized-bed reactors have the ability to process large solid volumes and that is why they have found widespread application in chemical industry. It is very important to measure the radial and axial profiles of solids holdups and particle velocities in fluidized beds. In this work, a sophisticated optical probe was used to measure these important parameters in two different fluidized beds (0.14 m and 0.44 m in ID). The radial and axial solids holdup profiles and particle velocities were studied at the same three axial positions in both units. Air was used as a fluidizing gas and glass beads (ρs=2500 kg/m3, dp=150-210 μm) as a solids material. The bed aspect ratio was set at two. Both fluidized beds were equipped with a porous-plate (pore size: 40 μm) distributor. The effect of reactor scale on solids dynamics was studied at several different superficial gas velocities U.
By using sophisticated optical probe solid concentrations were first measured and then converted by new method to solid holdups. Their spatial distribution was critically analyzed. The sampling frequency was set at 5 kHz. It was found that at the same dimensionless axial position the solids holdups are higher in the bigger unit. In both fluidized beds the solids holdups increased towards the wall. The same trends were reported earlier by Mabrouk et al. (2005). Additionally, it was found that the radial particle velocity profile was nearly parabolic in both fluidized beds.
In the smaller fluidized bed the solids holdup profile was independent of the axial position z up to 0.09 m. Then, at z=0.14 m it exhibited a pronounced decrease. In the bottom of the column there was a clear trend of solids holdup increase towards the wall, whereas at z=0.14 m the solids holdup profile leveled off towards the wall. At higher gas velocities the solids holdup clearly decreased with the axial height. In the bigger fluidized bed at all U values examined always the solids holdup profile decreased with the axial position. In addition, the solids holdup at the wall was always higher than the one in center of the column. Outside the core region, the particle velocities at the different U values examined in the smaller fluidized bed were practically the same. In the center of the column, the particle velocity increased with the superficial gas velocity.
Such exhaustive results have not been reported in the literature hitherto. They will be used for preparing a new scale-up strategy.
 Mabrouk et al., “Scale effects on fluidized bed hydrodynamics” Inter. J. of Chemical Reactor Eng 3, Article A18; 2005.