Dispersion of floating solid particles in aerated stirred tank reactors: Minimum impeller speeds for off-surface and ultimately homogeneous solid suspension and solids concentration profiles

Effects of impeller design, baffle, and gas flow rate on distributions of floating solid particles were examined in a stirred tank of 0.2 m i.d. for solid concentrations up to 50 vol %. Dual small cross-section impeller systems, i.e., dual four-flat blade disk turbines and dual four-pitched blade downflow disk turbines, and large cross-section impellers, i.e., Maxblend impeller and Fulizone impeller, were used. The minimum impeller speeds for off-surface floating-particle suspension decreased with aeration because bubbles rising near the tank wall enhanced the breakup of the floating-particle stagnant layer formed on the liquid surface and then the dispersion of floating particles into the liquid. The minimum impeller speeds for ultimately homogeneous floating-particle suspension also decreased with aeration. These results are contrary to those for the settling particles. The local solid particle concentrations at different heights in the stirred tank were measured. The axial solid particles concentration profiles were examined using the proposed one-dimensional floating-particle dispersion model. The Peclet numbers for floating-particle dispersion in the model were reasonably correlated in terms of impeller speed, power consumption, and forces exerted on floating particles. It was found that the large cross-section impellers could disperse completely floating particles into the liquid with less agitated speed and power consumption as compared with the dual small cross-section impeller systems used in this work.