(71b) Mitigation of Resonance-Induced Fluid Force Amplification to Insure Bioreactor Mechanical Integrity

Developments in the commercial biological production of large-molecule substances requires increased performance of mixing-based processes like blending and mass transfer. New bioreactor construction can readily accommodate these requirements; existing equipment can often be modified to achieve the same level of performance. An existing large-scale bioreactor equipped with angle-mounted, un-baffled, dual low-shear impellers was modified post fabrication with a center-mount dual impeller system with baffles operating at 45% higher speed (>4x power) to accommodate required capacity for planned and future products. Vessel displacement, agitator natural frequency and impeller fluid force were measured to benchmark mechanical performance of the current reactor; these results and confirming finite element analysis (FEA) models indicated that the impeller blades and shaft, drive shaft and mounting all must be strengthened for operation at increased speed and load. Confirming mechanical tests of the strengthened agitator and mounting however yielded impeller fluid force and vessel displacement that were significantly greater than before, rather than diminished as planned. Comprehensive measurement and modeling of the complete bioreactor assembly (i.e. agitator and vessel) revealed that vessel natural frequency and impeller blade passage frequency were near coincident within the range of desired operating speed; the resulting resonance was, in turn, amplifying the impeller fluid force to values much beyond the design limits for the system. Increasing the separation between forcing and natural frequencies was targeted as the approach to reduce impeller fluid force and vessel displacement. FEA was employed to evaluate a variety of approaches to stiffen the vessel mounting; a number of preferred approaches were discarded due to process reasons (e.g. cannot eliminate vessel load cells) or due to implementation difficulty (e.g. obstruction of proposed vessel leg cross-bracing by existing piping and wiring runs). FEA indicated that addition of 1? thick plate to the outside surfaces of the existing 6? square vessel legs would increase the natural frequency of the vessel by >50%, eliminating the observed fluid force amplification due to excitation of the vessel by agitator operation. This solution was implemented; subsequent tests confirmed a dramatic reduction in fluid force and vessel motion.