Mass Transfer and Rheological Characteristics in a Stirred Tank Bioreactor for Cultivation of Escherichia coli BL21

With the rapid growth of pharmaceutical and biotechnology industry, stirred tank bioreactors have received much attention due to simple design, easy control of operating conditions, and low operating cost. In the development of commercial processes, however, a transition from laboratory to industrial scale faces great challenges because many properties related to size change nonlinearly as a system increases. In this context, along with an understanding of fluid dynamics, application of an efficient method for scale-up is critical for designing successful industrial process. Particularly in cell cultivation processes, it is important to evaluate the oxygen transfer and viscous properties of liquid medium. In the present study, the effect of various key operating variables such as agitation rate and aeration rate, impeller diameter, and bioreactor working volume for different impellers on the volumetric mass transfer coefficient (k_La) have been investigated in a stirred tank bioreactor for cultivating Escherichia coli BL21. It was found that the k_La tends to increase with the operating variables except the bioreactor working volume. Among the tested impellers, the pitched blade was observed to be most promising because of relatively higher k_La but less shear force owing to its low power number. It was also found that the liquid medium with E. coli behaves as a Newtonian liquid. Compared to conventional designs of Rushton turbines, dislocated Rushton turbine was found to deliver higher k_La. Finally, using dimensional analysis, the k_La for different impeller configurations was correlated in the form of dimensionless groups, suggesting that this approach can be used for predicting k_La in different scales of stirred tank bioreactors.