(98d) Drawdown of Floating Solids with up-Pumping Agitators

Solids drawdown with down-pumping agitators generally relies on the formation of a vortex that deforms the free liquid surface in the central portion of the vessel.  This reliance on vortex formation requires unusual baffling to optimize performance.  Common approaches include narrow baffles and partial baffles located only in the lower portion of the vessel.  In both cases, reduced baffling permits tangential flow that leads to a surface vortex that pulls the solids into the liquid.

Up-pumping agitators incorporate floating solids via a combination of turbulence, bulk flow, and vortices formed at baffles near the vessel wall.  Thus, standard baffling works well with this configuration.  The use of standard baffling leads to a better understanding of important agitator design parameters such as power draw and mechanical forces on the impeller(s) and shaft.  Additionally, the use of standard baffling for solids drawdown has the benefit of readily being applied in multipurpose vessels or vessels that have been converted from one process to another.  These advantages, combined with generally lower drawdown speeds, have led to the growing popularity of solids drawdown with up-pumping impellers.

Optimal design of up-pumping agitators for floating solids incorporation requires a thorough understanding of the effect of various design parameters on the drawdown speed, torque, and power requirements.  Primary design parameters include impeller type, diameter, and submergence.  This work makes a thorough comparison of pitched-blade turbines and highly axial-flow hydrofoil impellers in this application.  Nine impeller diameter to tank diameter ratios spanning the range of industrial application from twenty to fifty percent are considered, as well as seven submergences ranging from ten to seventy percent of the tank diameter.

Since many floating solids sink after being wetted by liquid, it is common industrially to simultaneously require drawdown of floating solids and suspension of settling solids.  This leads to the use of an up-pumping impeller near the liquid surface and a down-pumping impeller near the tank base.  It is uncommon to have impellers pumping in opposite directions, so little is known about the flow pattern with such an impeller configuration and there might be concern that the presence of a second impeller pumping in the opposite direction would negatively impact performance.  The flow pattern generated by impellers pumping in opposite directions is addressed in this work.

Visual comparison of drawdown of floating solids with up-pumping agitators and suspension of settling solids with down-pumping agitators gives the impression that these two phenomena are very similar.  To determine the extent of this similarity, comparison of the drawdown and suspension speeds is made under equivalent conditions (same impeller type and diameter, same solid, same solid mass, same density difference between solid and liquid, and impeller submergence for drawdown equal to impeller off-bottom clearance for suspension).

Limited scale-up experiments are made to determine the scale-up exponent for solids drawdown with up-pumping agitators.  Knowledge of the scale-up exponent can be indicative of the mechanism of the solids drawdown with up-pumping agitators that appears to be different than the mechanism of solids drawdown with down-pumping impellers.