A CFD study of the drawdown speed of floating solids in a stirred vessel

The drawdown of floating solids is studied through visual observation and CFD simulations in this work. Four different baffle configurations and two different impellers were used to study the effects of the baffle design, the clearance size and the impeller type on the just drawdown speed of floating solids in a stirred vessel. Particles accumulate in the vicinity of the shaft above the impeller level in the Rushton impeller system at higher speeds. A full top-to-bottom circulation loop was found in the A310 impeller system at higher speeds. Increasing the clearance can lead to a more homogeneous particle concentration distribution in the stirred vessel. The shift baffles and hollow-out baffles show superior particle drawdown performances than the standard baffles. It was also found that higher clearances result to a better drawdown rate of floating solids for both impellers. A new method has been proposed to determine the just drawdown speed based on the rotating impeller domain in the CFD simulations. The CFD predicted just drawdown speeds agree well with our experimental observations.