Abstract
In the present study, we investigated the influences of diameter and impeller position on drawing from the liquid surface and on the dispersion of solid particles lighter than liquid in a vessel stirred by dual impellers. We defined the just drawdown condition as the state in which there are no floating lighter-than-liquid particles on the liquid surface at the just suspension speed for heavier-than-liquid particles. The best type of upper stage impeller, which drew floating particles on liquid surface into the liquid efficiently, was found to be the pitched paddle pumping up and the best impeller-diameter-to-vessel-diameter ratio was 0.5. In addition, the optimal position of the upper side impeller to minimize the just drawdown speed was located at 0.85 of the total liquid level from the bottom of the vessel. We examined the influence of the clearance of dual impellers on the distribution of solid particles in the vessel. The obtained results indicated that the best type of lower stage impeller is the pitched paddle pumping down and the impeller-diameter-to-vessel-diameter ratio is 0.5. The standard deviation of the dispersion of solid particles was found to be lowest when the clearance of the dual impellers to vessel diameter ratio was 0.83.
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