Comparison of Traditional Rushton Disc Turbines with up-Pumping Hydrofoil B2 Impellers in 550 L Pilot Scale Aerobic Submerged Fermentations

There has long been a debate concerning the most suitable impeller for aerobic submerged fermentation (and indeed, for gas liquid or 3-phase reactors in general). Much academic work evaluating agitator design with respect to flow pattern, power consumption in gassed and ungassed conditions, kLa, and mixing time has been published. In every single study, the conclusion is that an alternative design to the traditional Rushton Disc Turbine (RDT), offers some kind of advantage; mixer manufacturers have been quick to add such impellers to their portfolio. One configuration currently favoured is the use of multiple up-pumping, large diameter, low power number, high solidity ratio hydrofoil impellers because of axial flow, stable gas dispersion and power characteristics. Interestingly, this impeller design is almost as far as conceptually possible from the RDT.

A program initiated by Novozymes to evaluate the performance of twin up-pumping, high solidity ratio Hayward Tyler (previously APV) B2 impellers included gassed and ungassed power characterization in viscous media and with very high power draw as well as comparison with twin RDTs in pilot scale fermentors of 550 L with commercially exploited fed-batch A. oryzae fermentation expressing recombinant proteins. This work extended previous work with B2s and RDTs to higher power draws, gas rates and velocities that are relevant for modern submerged commercial fermentations at relevant scale.

The experimental design of this investigation was aimed at assessing the ability of the new impellers to improve the yield of productivity at a realistic scale; a most important issue which no studies previously have covered. By variation of agitation and aeration of the process it was possible to determine the influence of these parameters as well as viscosity on the mass transfer capacity, biomass concentration and product concentration. Thus a kLa prediction that includes both impeller types was determined for the system.

A well known problem that often arises when working with filamentous fungi is the resulting viscous fermentation broths, which greatly lowers the maximum rate of oxygen transfer and makes the control of dO2 concentration at that desired difficult. This study resulted in a rheological prediction of the fermentation broth viscosity derived from the power law based on the biomass concentration and the vessel energy dissipation/circulation function.

The results of the comparison of the RDT and B2 impeller performance in pilot scale will be presented. Furthermore using the viscosity and mass transfer prediction it was possible to model the entire fed-batch fermentation process and predict the outcome of varied rates of agitation and aeration. Examples of such simulations will be shown.&'