Measurement of Volumetric Mass Transfer Coefficient in Lab-scale Stirred Tank Reactors: Is There a Point of Diminishing Returns for Impeller Speed and Gas Flowrate?

The transfer of species from gas to liquid across the phase interface is generally regarded as the greatest challenge and limitation in bioreactor design and operation. This is true for both oxygen and other gases used in bioreactors, such as methane. In this study, the volumetric oxygen transfer coefficient was observed for a bioreactor at various sparger flowrates and impeller rotational speeds. Specifically targeted was a point at which increasing the impeller speed or gas flowrate resulted in reduced returns on the observed value of the transfer coefficient. This was to be expected, but much greater influence was observed for impeller speed than there was for gas flowrate. At impeller speeds of 600 rpm, quadrupling the gas flowrate from 2.5L/min to 10L/min only resulted in an increase of approximately 40%. At 0 rpm, the quadrupling of the gas flowrate resulted in a nearly quadrupled kLa value, indicating that at no agitation, the gas flowrate is closely tied to the kLa of the bioreactor, if much lower than under agitation. The study thus concludes that the kLa in these bioreactors is nearly directly influenced by gas inlet flowrate under tranquil conditions, but when agitation is present, it is a much more determining factor for kLa than gas inlet flowrate. This is likely due to the ability of the impeller to break up large bubbles introduced by the sparger to increase the area available for mass transfer. This may be used in experiments involving bioreactors to save on gas costs and more appropriately select a rotational speed to target certain bioreactor output parameters.