3-42: Agitator Torque Split with Multiple Impellers in Lignocellulosic Slurries with Yield Stress

Several authors have reported the relationship between yield stress and cavern size in Bingham Plastic fluids in a mechanically agitated tank, and minimum torque required to drive cavern size to the wall. However, little has been published for lignocellulosic slurries, which not only have a yield stress, but are pseudoplastic above the yield stress (i.e., they are Herschel-Bulkley fluids.) Moreover, previous studies were done with a single impeller, as opposed to the more common case of multiple impellers. In this paper, we treat the case of multiple impellers. A key result is that more torque is required of the bottom impeller than the upper impellers, due to the need to overcome yield stress on the tank bottom as well as the side wall. The proportion of torque required for each impeller is theoretically derived in this paper, plus empirically tested for several sample lignocellulosic slurries. The result is that the lower impeller needs 2-10 times as much torque to create full motion out to the wall as each upper impeller, depending on the slurry. Without accounting for this result, too much torque overall will be applied, over agitating the top portions of the vessel needlessly and wasting power. In addition, proper accounting for this torque split affects total power required as a function of tank aspect ratio for a given volume.