Power consumption and solid suspension performance of large-scale impellers in gas-liquid-solid three-phase stirred tank reactors

An experimental investigation into power consumption and solid suspension performance of large-scale impellers was carried out under turbulent conditions. Two types of large-scale impellers, i.e. Maxblend and Fullzone impellers, were employed. For reference, a triple-impeller system, i.e. two four-pitched blade downflow disk turbines (DTs) at middle and upper positions and one Pfaudler type impeller at lower position, was also used. The power consumption and the minimum impeller speeds for off-bottom solid suspension and minimum impeller speeds for ultimately homogeneous solid suspension were measured in unaerated and aerated systems. At a given rotational speed, the power consumption of the Maxblend impeller was roughly half of that of the Fullzone impeller. The decrease in power consumption due to aeration for large-scale impellers was smaller as compared with that for the triple-impeller system. The proposed correlation for power consumption of large-scale impellers in three-phase systems fit the experimental data reasonably well. Interesting and unexpected solid movements caused by the large-scale impellers in the vessels having oval bottom were observed. Since the large-scale impellers create strong axial liquid recirculation flowing downward near the impeller shaft and upward near the wall, usually particles are expected to move outward on the tank bottom. On the contrary, however, solid particles near the bottom moved to the center of the base from the side along the oval tank bottom. The large-scale impellers were found to be more efficient for solid suspension than the triple-impeller system. The Maxblend impeller provided the best solid suspension ability among the three impellers used in this work. We proposed a correlation for power consumption of large-scale impellers in gas-liquid-solid three-phase systems. Empirical correlations were also proposed for the minimum impeller speeds for off-bottom solid suspension, minimum impeller speeds for ultimately homogeneous solid suspension and power consumption at the minimum impeller speeds for ultimately homogeneous solid suspension. (C) 2003 Elsevier B.V. All rights reserved.