Gassed and ungassed power draw in a pilot scale 550 litre fermentor retrofitted with up-pumping hydrofoil B2 impellers in media of different viscosity and with very high power draw

A number of modern impellers have been designed in the pursuit of an alternative to the traditional Rushton turbine, which has a number of well recognized deficiencies. A dual up-pumping combination of the Hayward Tyler B2 (former APV-B2 or simply B2), a high solidity ratio hydrofoil impeller, was retrofitted using traditional methods to a pilot scale fermentor based on cited reference studies of the impeller performance. Using water as a media and comparatively low power draws, the B2 impeller has previously been shown to have good gas handling properties and a low ungassed power number allowing for use at high impeller-to-tank diameter ratio. In the present study a power characterization of the B2 impeller was undertaken in order to extend the available data to viscous media resembling fermentation broths and to very high power draws. Power characterizations were carried out with specific energy input rates up to 12.9 kW/m(3) using different shear-thinning media. The ungassed power number of the B2 impeller was determined (3.3 for power draws in the range 0-11.6 kW/m(3)) and our findings confirm those of previous studies and extend the trends to media of high viscosity. Upon aeration the B2 impellers loose little power compared to the traditional impeller even when applying a very high power draw (10-20% power loss at 450 rpm and 1.28vvm corresponding to 11.6 kW/m(3)). Torque fluctuations are found to be small (< 5%) for this impeller at high power draw and high viscosity media. Finally it is shown that the B2 impeller can be retrofitted to pilot scale fermentors using traditional methods with a high degree of confidence. (c) 2008 Elsevier Ltd. All rights reserved.