Scale-up Considerations for Fermentation Mixers

Author:

James Y. Oldshue

This paper discusses key factors in scaling up fermentation mixers, emphasizing the differences in mixing behavior, shear rates, and mass transfer characteristics between small-scale and large-scale fermenters. It examines how variables such as viscosity, gas velocity, impeller design, and power input influence blending efficiency and oxygen transfer in bioreactors. The study concludes that no single mixer design suits all fermentation processes, highlighting the need for tailored approaches based on specific process requirements and fluid dynamics.

Key Learnings

• Scaling up fermentation mixers significantly changes mixing dynamics, including shear rates, blend times, and circulation patterns.
• Larger tanks exhibit greater variability in shear rates and lower pumping capacity per unit volume than smaller systems.
• Gas-liquid mass transfer efficiency depends on parameters such as superficial gas velocity, power input, and impeller-to-tank diameter ratio (D/T).
• Viscosity plays a crucial role in determining mass transfer performance and must be measured under actual mixing conditions.
• Data from pilot plants can guide scale-up, but geometric similarity alone does not ensure accurate replication of full-scale mixing behavior.
• Foam formation and gas velocity must be carefully managed during scale-up to maintain process stability.
• The choice of impeller type—radial, axial, or dual power—significantly affects gas dispersion, shear, and power consumption.
• Sulfite oxidation and oxygen uptake rate (OUR) tests provide valuable metrics for evaluating fermenter mass transfer performance.
• Successful fermenter design requires balancing mixer power, gas input, and process requirements rather than relying on standard configurations.
• Each fermentation process demands customized mixer specifications due to differences in fluid properties, biological activity, and operational goals.