Aerobic Fermenter Scale-up: Why Equal Mass Transfer Scale-up Doesn't Always Work

Transferring oxygen to the liquid is only one of several steps that must occur in an aerobic fermentation. After the oxygen is transferred to the liquid, the liquid must be thoroughly mixed throughout the vessel. Then the oxygen must diffuse through the cell wall before finally being metabolized within the cell.

For some types of biology, conditions for the production of the desired product must be held within close tolerances. These conditions include but are not limited to DO concentration, nutrient concentration, and pH. If not, some cellular organisms will tend to undergo a number of different actions. None, of which is beneficial in producing the desired product. This paper will provide a number of references that provide evidence to this fact.

When a process is scaled-up via equal mass transfer logic, the blend time continues to increase. As the scale becomes larger and larger, the time to blend at some point may become the limiting scale-up criteria, not mass transfer.

From a gassed blend time standpoint, not all mixing systems are created equal. The authors will identify relative blend times for a number of common systems.

The authors will suggest techniques that could be used on smaller scales to determine when scale-up via equal blend time is required.