2025 AIChE Annual Meeting

Mixing in Aerobic Fermentation Processes

There are several mixing processes that must be accomplished to successfully operate an aerobic fermentation process. The first is to maintain the dissolved oxygen concentration that will enable the micro-organisms to grow and produce the desired product molecule and the second is the provide oxygen at a rate that will allow the organisms to produce the molecule at a rate that is economically viable. The volumetric mass transfer coefficient, or kLa, achieved by the agitator and air flow rate determine the relationship between the dissolved oxygen concentration that can be maintained and the oxygen transfer rate.
The demand for oxygen can depend on which phase the organisms are operating. Generally, in the growth phase, where the concentration of organisms is increased, the oxygen demand is high. Once the concentration reaches the desired value the organisms are switched to production mode and their demand for oxygen is lower. The system must be designed to provide the maximum oxygen transfer rate but can be tuned to reduce the power input by the agitator and / or the air flow rate required to maintain the required dissolved oxygen concentration in each phase.
At the same time, carbon dioxide is produced through the organisms’ respiration and this must be stripped from the broth into the air that is leaving the fermenter. Also, since the organisms are oxidizing sugars to provide energy, fermentations are exothermic and the relationship between the oxygen consumption and the heat transfer capability of the system must be considered during the design of new fermentation equipment or retrofits to existing ones.
In this presentation the general rules describing the selection of equipment configuration for gas-liquid reactions will be discussed and for selection of impeller operating conditions in an agitated vessel, particularly how the impeller type, diameter and operating speed are related to the gas flow rate and mass transfer requirements.
The special considerations that must be taken into account when designing agitators for aerobic fermentations will be reviewed and finally, an example will be presented showing how the design calculations are applied to analysis of data from an existing pilot or full-scale fermenter in order to confidently size and design an agitator for a new fermenter.