A novel geometry for solids drawdown in stirred tanks

Two mechanisms for the drawdown of floating solids in baffled stirred tanks have been identified [Khazam, O. and Kresta, S., 2008, Mechanisms of solids drawdown in stirred tanks. Can J Chem Eng, 86(4): 622–634]: turbulent engulfment and mean drag. Baffling suppresses stable surface vortex formation and increases the intensity of mean drag and turbulence at the surface. Strong top to bottom liquid circulation is also promoted by baffling, however; this circulation rapidly brings particles back to the surface. In this paper, two alternate baffle configurations are proposed: half baffles and surface baffles. The objective is to maintain a high level of turbulence at the surface while reducing the return circulation from the bottom of the tank. The performance of the baffle configurations is compared using the just drawn down speed, Njd, and cloud depth, CD, for the PBTD, PBTU and A340 impellers. CFD simulations and measurements of the power number for the fully baffled and the surface baffled configurations are also reported. It is found that half baffles are very similar to full baffles in performance, but surface baffles offer several advantages: a significant reduction in Njd, more robust performance at large submergences, and a better distribution of solids.