Agitation requirements for complete drawdown of floating solids on stirred tanks

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University of Dar es Salaam
The agitation requirements for complete drawdown of floating solids in mechanically agitated tank vessels has been studied both theoretically and experimentally. A semi-theoretical equation has been derived on the basis of Kolmogoroff's theory of isotropic turbulence to determine the minimum impeller speed required for drawdown. The equation contains one adjustable parameter which has been found to be a function of the impeller type and position in the tank vessel. The equation was tested using various vessels and impeller configurations. The solid phase consisted of high density polyethylene (density = 897kg/m3), low density polyethylene (density = 840kg/m3) and cork material (density = 510kg/m3) with particle sizes ranging from 300um to 2200/um. The liquid phase consisted of water and aqueous solutions of zinc chloride in different concentrations so that the liquid density could be varied in the range 996kg/m3-1180kg/m3. The effect of impeller position and pumping direction has been extensively examined, as well as the use of non-conventional baffling systems to facilitate the drawdown of floating particles into the liquid. It was concluded that impeller clearance and pumping direction have a considerable influence over the minimum drawdown speed and its corresponding power consumption. A partial baffling system consisting of four half baffles has been found to have the lowest power requirements. Noticeable similarities exist between settling solids suspensions and floating solids drawdown, particularly for floating particles which, at rest, are almost completely immersed in the liquid. For cases different from this, the suspension of floating solids becomes a three phase system with entrapped air playing a significant role in particle drawdown. The proposed model works well within the experimental range covered (i.e. small density difference (<340kg/m3) and medium particle size (300 - 2500um) and can be used to predict the performance of floating solid-liquid systems.
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Mmbaga, J.P (1989) Agitation requirements for complete drawdown of floating solids on stirred tanks, Masters dissertation,University of Dar es Salaam. Available at ( )