Optimizing the impeller combination for maximum hold-up with minimum power consumption

To select the optimum operating conditions for a multiple impeller laboratory bio-reactor, nine impeller combinations consisting of three basic impeller types, namely pitched blade turbine downflow (PTD), pitched blade turbine upflow (PTU) and disc turbine (DT), were tested. Power consumption and fractional gas hold-up measurements were taken over a range of 1-4 kw m(-3) of power and 0.5-2 VVM of gas flow rates, for all the nine combinations consisting of any two impellers. A radial Bow impeller (DT) near the sparger, i.e. lower impeller, was found to be a necessity for enhanced gas hold-up. With the DT as a lower impeller, use of PTD as an upper impeller was found to give higher gas hold-up compared with any other combination consisting of both radial Row (DT-DT) or both mixed flow (PTD-PTU, PTD-PTD,PTU-PTD and PTU-PTU) impellers, for equivalent power dissipation. (C) 1998 Elsevier Science S.A.