Biotechnology and Bioengineering, Vol.44, No.10, 1177-1185, 1994
A Fluid Dynamic Study of the Retrofitting of Large Agitated Bioreactors - Turbulent-Flow
Studies were conducted in three 19-m(3) fermenters (14 m(3) working volume, aspect ratio similar or equal to 3:1), one fitted with four Rushton turbines (D/T = 0.35), one with three Lightnin’ A315 hydrofoil impellers (D/T similar or equal to 0.5), and one with four Prochem Maxflo T hydrofoils (D/T = 0.46). The power drawn under the same aerated conditions relative to the unaerated ones was always greater with the hydrofoils, which gives them the potential for enhanced mass transfer rates under practical operating conditions. However, the power draw was also sensitive to the magnitude of the unaerated power. Indeed, at low unaerated specific power (similar to 0.6 W/kg) and high air flow rates (similar to 1 vvm), the relative power draw with the hydrofoils could be even greater than 1. The hold-up with each of the impellers was broadly similar at the same aeration rate and power input, though the latter had a much smaller impact in these large vessels than has been reported in the literature based on smaller scale work. As usual, repressed coalescence caused increased hold-up, and, with the hydrofoils, this increase was associated with a lower power draw. Because of the greater mechanical vibration of the reactors with the hydrofoils, vibration characteristics of the vessels were measured and they were very similar. The results showed that provided care is taken in the mechanical design of the system, such impellers can operate reliably in large-scale fermentations with the potential for enhanced biological performance.