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Pharmaceutical Technology BIOLOGICS AND STERILE DRUG MANUFACTURING 2018 23 suggested that f was more perti- nent than P/V, meaning that β is greater than α in bioreactors. Because the superficial gas ve- locity appeared to dictate mass transfer performance, the agita- tor was removed from the system leaving the gas to act as a bubble column. The mean k L a 20 dropped to three inverse hours, a third of the dual down-pumping base case mean k L a 20 . The results demon- strated that gas input without dis- persion by an agitator drastically reduced mean k L a 20 . The bubble column at 0.1 VVM even had a lower mean k L a 20 than the half- gas case with dual down-pumping A320s at 0.05 VVM. Three impeller configurations (single down-pumping A320, dual up-pumping A340s, and triple up-pumping A340s) were com- pared to the dual down-pumping A320 base case. All were operated at 0.4 HP/kgal and 0.1 VVM. The single A320 generated a mean k L a 20 in the range of 6–8 inverse hours while the dual A340s gener- ated a mean k L a 20 in the range of 8–9 inverse hours. Both were lower than the mean k L a 20 range exhib- ited by the dual A320 base case. The triple A340 configuration generated a mean k L a 20 in the range of 9–11 inverse hours and was statistically similar to the dual A320 case. The dual down-pumping A320 and triple up-pumping A340 configurations generated greater mean k L a 20 and were more ef- ficient with power and gas input than the single down-pumping A320 and dual up-pumping A340 configurations. 3D CFD results Time-averaged velocity contour plots were rendered from the transient simulations for the dual down- pumping A320, single down-pumping A320, dual up-pumping A340, and triple up-pumping A340 configurations to illustrate time-averaged f low patterns and areas of greater velocity, turbulence, and energy dissipation. The time averaged veloc- ity contour plots for the down-pumping cases are shown in Figure 4. The dual A320 configuration exhibited more consistent velocities throughout the vessel whereas the single A320 configuration exhibited higher ve- locities in the bottom of the vessel and lower ve- locities in the upper half. The time-averaged velocity contour plots for the up-pumping cases are shown in Figure 5. Figure 4: Time-averaged velocity contour plots at 0.4 HP/gal for the dual down-pumping A320 configuration (left) and single down-pumping A320 configuration (right). Figure 5: Time-averaged velocity contour plots at 0.4 HP/gal for the dual up-pumping A340 configuration (left) and triple up-pumping A340 configuration (right).