Performance improvement of stirred tank reactors with surface feed

Current best mixing practice requires sub-surface feed of reagents into the turbulent impeller region of stirred reactors to suppress mixing limitations and to minimize byproduct formation. Dip tubes, however, can cause many operational problems. In this thesis alternate surface feed strategies were explored using the third Bourne reaction.; Initial experiments were conducted with the Rushton turbine and the down-pumping pitched blade turbine (PBTD) to establish base case performance. In the first modified configuration the impeller was unchanged but the feed was added at a high velocity to generate additional turbulence, enhance mixing, and transport reagents rapidly to the turbulent impeller region. This strategy failed, since it was very difficult to generate a fully turbulent feed jet and operation was very unstable. Sufficient improvement in performance could also not be achieved due to the macro-scale convective stoichiometric limitations. An analysis of these limitations showed that they are general limitations and this strategy is difficult to implement.; In the second configuration, feed was added slowly at the surface with an up-pumping pitched blade turbine (PBTD). Operation was greatly improved and continued to improve with increasing impeller speed. At very high impeller speeds air bubbles were entrained into the tank. The impeller speed at the onset of air entrainment, NE, gives the upper limit of operation of the PBTD. Reactive experiments showed that the product quality for a PBTU operated close to NE matched the results obtained with dip tubes irrespective of impeller submergence. The up-pumping impeller was identified as a stable and robust impeller configuration suitable for surface feed.; Air entrainment is a result of subtle interactions between subsurface turbulence and the liquid surface. In this investigation a two-step mechanism of air entrainment is proposed: bubbles first form at the surface and are then dragged into the tank by the mean velocity. To validate this model, turbulent (RMS) and mean velocities were measured near the liquid surface with a Laser Doppler Velocimeter (LDV) at N = 0.98NE. The model is able to predict air entrainment using conditions at the surface. The complex relationship between the impeller variables and the turbulent flow-field near the surface, however, is best addressed through full computational simulations.