A fluid dynamic study using a simulated viscous, shear thinning broth of the retrofitting of large agitated bioreactors

Studies were conducted(1) in 19-m(3) fermentors (14-m(3) working volume) using four Rushton turbines, four Prochem Maxflo Ts, and three Lightnin' A315s and the results in water have been reported earlier. Here, a 1.7 wt/vol% Xanthan solution has been used as the working fluid, simulating viscous broths to give Reynolds numbers (Re) between 1800 and 4500. As predicted from small-scale studies, the power numbers at these values of Re were similar to those in water. The K factor (the ratio of power draw under aerated conditions compared to non-aerated) was the same as in water at the higher values of Re, but at the lower values it fell more rapidly with increasing aeration rate and to a lower value than in water. At all times, K was higher than with Rushton turbines. Vibration characteristics were also measured. Under aerated conditions, the fermentors vibrated with an amplitude 75% to 100% less than in water due to viscous damping. With increasing air flow, the amplitude increased steadily due to the presence of very large and rapidly rising bubbles in such fluids to give values 2.5 to 3 times those in water. Nevertheless, these mechanical problems can be overcome, allowing such agitators to be used successfully in high viscosity mycelial fermentations. (c) 1996 John Wiley & Sons, Inc.