Studies of High Solidity Ratio Hydrofoil Impellers for Aerated Bioreactors. 2. Air—Water Studies

Abstract

AbstractThis second part of a four‐part series of papers reports on the hydrodynamic and power characteristics of two commercial, high solidity ratio, hydrofoil impellers, the Prochem Maxflo T and the Lightnin A315, in an air—water system. Studies were conducted primarily in a mixing vessel of diameter 0.56 m. The influence of operating conditions (impeller speed and gas flow rate) and sparger position and geometry on bulk flow patterns, gas filled cavity formation, and power consumption have been investigated in detail. In the turbulent Reynolds number region, the unaerated impeller power numbers are constant with average values of 1.30 for the Prochem and 0. 76–0.84 for the A315, depending on impeller position relative to the base of the vessel. Gassed power consumption is closely linked to cavity type and flow pattern. In particular, the development of large cavities causes a significant reduction in power drawn, impeller pumping capacity, and gas dispersion capability. As with downward pumping mixed flow impellers, two characteristic gas loading regimes, direct and indirect loading, can be identified. Under certain conditions, severe hydrodynamic instabilities over the transition between the two regimes give rise to very large torque and power fluctuations. Changes in reactor geometry can have a significant effect on impeller performance. Thus, sparger type and position have a strong influence on relative power drawn, gas handling capability, power required to disperse gas and stability of operation. Of the configurations studied, the use of a large ring sparger (DS/D = 0.8D) at a large sparger—impeller separation is suggested, since relative power drawn, gas handling capability, and energy efficiency in dispersing gas are all enhanced.