Computational and Experimental Characterization of Operating Conditions in Stirred Aerated Vessels for Industrial Large-Scale Cell Cultivation

During the cultivation of mammalian cells in the pharmaceutical industry, both stirring and sparging are required for proper homogenization and aeration. Hydrodynamic stress generated by such conditions may have deleterious effects on the cell viability. Our work focuses on both experimental and computational characterization of the maximum effective hydrodynamic stress τ_max in large scale industrial cultivation vessels (200 & 12000 L).

Shear sensitive aggregates composed of poly(methyl methacrylate) nanoparticles were exposed to the operating conditions in the vessels studied here. In order to characterize τ_max from aggregate breakup, the steady-state size must be reached, indicating that every aggregate passed through the highest shear stress zone at least once. Aggregate size was subsequently converted to the hydrodynamic stress via calibration, during which aggregates were repeatedly passed through well-defined contracting nozzles.

The zone with the maximum hydrodynamic stress occupies a very small volume fraction, close to the impeller. As such, our experiments showed that the time to reach the steady-state particle size is on the order of tens of minutes. This time-scale is longer than the commonly observed mixing time at these operating conditions. This observation leads us to a new definition of mixing time, which is based on the rarest event regarding the particle breakage within the vessel. This particle-based approach, as opposed to simple species homogenization, provides more specific insights into the time required for a system to reach an equilibrated state. Flow properties were estimated using computational fluid dynamic software MStar, which is based on the lattice-boltzmann method.