Aerated and unaerated power consumption and flow patterns in a 0.56 m diameter agitated vessel containing water with dual Rushton turbines have been studied. Under unaerated conditions with a liquid height-to-diameter ratio of 2, an impeller spacing of 2 to 3 times the impeller is required for each to draw an amount of power equal to a single impeller. For aerated conditions, if a similar spacing is used, equations for the flooding-loading transition and for power consumption for a single Rushton impeller can be extended relatively easily to dual systems. All results for this spacing are explained by reference to bulk flow patterns and gassed-filled cavity structures and the proportion of sparged gas flowing through the upper impeller is also estimated. Such a spacing is generally recommended since it maximizes the power draw and hence the potential for oxygen mass transfer. Data are presented for other spacings but the results do not fit in easily with single agitator studies because strong impeller-impeller flow pattern interactions occur.
An attempt is made to describe by use of the onedimensional stochastic model the flow system solid phase-liquid stirred by the mechanical rotary mixer. At the assumption that the solid phase is dimensionally homogenous a diffusion equation has been obtained which is solved under the boundary conditions, characterizing the method of withdrawal of the solid phase from the system. It is demonstrated that the model can express also the relations usually used for description of similar situations (ideal mixing, separation coefficients in stationary state etc.) like individual cases.
Experimental measurements were performed of the time dependence of solid phase concentration in suspension at the outlet from the apparatus with the mechanically stirred charge. Suspension was being withdrawn continuously with fresh water supplied into the charge. Validity of the theoretical approach proposed earlier has been verified by the performed experiments. It has been demonstrated that the studied effect can be under certain conditions described with sufficient accuracy by a simple material balance of the solid phase in the system.
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