When designing agitators for dispersion of a gas in a liquid, there are three criteria that need to be considered: The minimum rotational speed of impeller � known as the flooding speed.The reduction in power input by the impeller due to the presence of gas � the gassed to ungassed power ratio.The mass transfer rate.For almost 50 years the Rushton turbine was the impeller of choice for gas dispersion and mass transfer in aerobic bioreactors.  In the 1980's research by Smith and Van't Riet in Delft led to the development of impellers with concave blades, essentially half pipes, sold as the Smith turbine or the CD 6.  These could handle more gas at a given impeller speed and experienced a smaller reduction in power on gassing.  This concept was further developed to produce impellers with parabolic blades that could handle an even higher gas flow rate before flooding and experienced a smaller reduction in power input on gassing.This presentation will discuss the development of an impeller that can handle an even higher gas flow rates, experiences minimal reduction in power input on gassing and achieves a mass transfer coefficient that is approximately 20 % than a Rushton turbine when compared at the same power input and gas flow.The reason that the new impeller is more efficient is that, at the same power input, it operates at a higher speed than the Rushton turbine generating higher shear which produces smaller bubbles and a higher interfacial area for mass transfer.

Improving mass transfer efficiency in aerobic bioreactors

247th National Meeting (2014)

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