Comparison between average shear rates in conventional bioreactor with Rushton and Elephant ear impellers

As an alternative to the Rushton turbine impeller (RT), the Elephant ear impeller (EE) has been indicated as suitable for the cultivation of shear-sensitive cells such as animal cells and filamentous fungi. In the present work the average shear rate ((gamma) over dot(av)) was evaluated and compared in a conventional bench-scale stirred-tank bioreactor, using different impeller configurations such as the Rushton turbine and the Elephant ear turbine in the down-pumping (EEDP and up-pumping (EEUP) modes. Based on recent methodology, correlations to estimate the average shear rate ((gamma) over dot(av)) were obtained for the different impellers according to the rotational impeller speed (N), specific air flow rate (phi(air)), and rheological properties of non-Newtonian pseudoplastic fluids. Values of (gamma) over dot(av) predicted by the proposed method were within the range of values calculated by the correlations found in the literature, indicating that the proposed method generates reliable values of (gamma) over dot(av). The (gamma) over dot(av) values obtained for the RT and EEDP impellers increase with the increase of N and decrease with the increase of phi(air) for all non-Newtonian fluids. In contrast, for the EEUP impeller (gamma) over dot(av) values decrease with the increase of N and phi(air). The negative influence of phi(air) in (gamma) over dot(av) at constant agitation (N) for all impellers was attributed to the increase of the gas hold-up by the increase of phi(air). EE impellers generated values of (gamma) over dot(av) lower than that observed in the system equipped with the RT impeller. EEUP was the impeller that combined good oxygen mass transfer and low shear, appropriate characteristics for its use in bioprocesses involving shear-sensitive microorganisms. The importance of the correlations developed in the present work is that they allow estimation and comparison of the shear conditions in cultures as a function of the operating conditions (N, phi(air)), rheological properties of the fluid (K and n), and volumetric oxygen transfer coefficient (k(L)a) in conventional bioreactors equipped with different impeller configurations in turbulent flow regime. (C) 2012 Elsevier Ltd. All rights reserved.