Mass transfer and hydrodynamic characteristics of unbaffled stirred bio-reactors: influence of impeller design

Unbaffled stirred tanks are increasingly recognized as a viable alternative to common baffled tanks for a range of processes where the presence of baffles is undesirable for some reason. For instance, in the case of shear sensitive cell cultivation (e.g. human cells), unbaffled tanks have been recently found to be able to provide sufficient mass transfer through the free surface vortex. As a consequence the need for bubble formation and subsequent bursting, along with relevant cells damage, is conveniently avoided. In this work the influence of impeller geometry on mass transfer performance and power demand of an unbaffled stirred vessel operating both in sub-critical conditions (the free surface vortex has not yet reached the impeller) and in super-critical conditions (the free surface vortex has reached the impeller and a gas phase is ingested and dispersed inside the reactor) is presented. Experimental results show that the mass transfer performance of unbaffled systems is mainly affected by specific power consumption. Among the stirrer geometries investigated a simple PBT was found to provide the most interesting oxygen transfer performance in the sub-critical regime, and can therefore be regarded as a particularly suitable stirrer for shear sensitive cultures. As regards the super-critical regime, unbaffled tanks are found to provide a performance comparable with that of the standard (baffled) bioreactors, hence resulting in a viable alternative also for fermentations involving robust cells.