(107g) Experimental Determination of Maximum Shear Stress in Multiphase Flow – From Micro Scale to Pilot Scale Reactors

Although a tremendous effort has been made to determine velocity, turbulence and shear level distribution including their maximum values in stirred tanks, resolving the small sizes of the dissipative scales applying  either experimental techniques or computational approaches still remains a challenge, particularly for multiphase systems. Such flow fields occur frequently in bioreactors where mechanical stress caused by stirring or bubble burst, entrainment, coalescence and break-up has been reported to have detrimental effects on mammalian cell cultures and product quality.

In order to construct a rational process design space, we have developed an aggregate – breakup system which is able to determine the maximum shear stress present in any device with arbitrary geometry. The presented methodology is based on the measurement of the steady state aggregates size broken by the hydrodynamic stress generated by stirring, sparging or their combination. Different contractive nozzles characterized by well-defined fluid dynamic conditions are used to calibrate the aggregate size to the maximum hydrodynamic stress. This system has been applied to biotechnological equipment ranging from micro scale bioreactors (0.015 L) up to a pilot scale bioreactor (300 L). The results show that not only the size and geometry of the vessel but also the flow field has a significant impact on the highest shear stress level. A cell will therefore exhibit a profoundly different hydrodynamic stress from expansion to production which should be carefully taken into account during scale-up as well as during the entire bioprocess development.