High flow, low shear impellers versus high shear impellers; dispersion of oil drops in water and other examples

Flow patterns generated by two ChemShear impellers, CS 2 and CS 4 have been measured and flow numbers calculated; Fl = 0.04 for both impellers. Transient and equilibrium drop sizes, d32 μm of 3 different viscosity silicone oils agitated by a high-shear Rushton turbine, RT, a low-shear, high-flow HE3 impeller and the two ChemShears were determined. The equilibrium d32 are correlated for all impellers, by d32 = 1300(εT )−0.58 max.svv0.14 with an R2 = 0.94. However, the time to reach steady state and the equilibrium size at the same specific power do not match the above descriptors of each impeller’s characteristics. In other literature, these descriptors are also misleading. In the case of mixing time, a high shear RT of the same size as a high flow HE3 requires the same time at the same specific power in vessels of H/T = 1. In bioprocessing, where concern for damage to cells is always present, free suspension animal cell culture with high shear RTs and low-shear impellers is equally effective; and with mycelial fermentations, damage to mycelia is greater with low shear than high. The problems with these descriptors have been known for some time but mixer manufacturers and ill-informed users and researchers continue to employ them.