Multiple-impeller systems with a special emphasis on bioreactors: a critical review.

The multiple-impeller agitated systems are compared with single-impeller agitated systems with a special focus on its applications for bioreactors. Correlations reported in the literature for gas phase hold-up, mass transfer coefficient and power consumption under gassed and ungassed conditions are compared and recommendations have been made regarding their suitability for design and scale-up of bioreactors. The multiple-impeller systems are found to be superior as compared to single-impeller systems in all the above mentioned aspects, except liquid mixing. For all kinds of reactors where the sole purpose is mass transfer, multiple-impeller systems are advantageous and there would be large savings on an industrial scale, especially for the bioreactors where the reaction periods are long and the power consumption cost could be a significant component to the overall production costs.