Magnetically driven agitation decoupling events in benchtop microbial STR systems

The agitator drive assembly is an essential part of all stir-tank reactor (STR) systems. This component supports the rotation of the internal impellers and is critical for maintaining suitable mass and thermal transfer within the system. To drive impeller rotation, a coupling mechanism is needed to link the external motor and the internally sealed agitator shaft. Both mechanical and magnetic coupling mechanisms are available as solutions to create this linkage. Magnetically coupled drive systems offer several advantages over mechanically coupled systems, including a more robust sanitary barrier, and decreased preventative maintenance requirements.

One of the potential disadvantages of utilizing a magnetically coupled agitation drive system is the risk of high agitation rate decoupling events occurring during processing. The possibility of such events increases for highly viscous bioprocesses, such as some fermentation applications. During magnetic decoupling events, the connection between the motor and the impeller shaft is lost, and system agitation stops until the issue is manually resolved. To reduce the risk of magnetic decoupling events, STR systems can be characterized to define the magnetic decoupling agitation speed thresholds for specific process conditions. Agitation setpoints defined under the decoupling agitation rate threshold would allow end users to operate the system with minimal risk of decoupling events.

In this work, Distek characterized decoupling agitation speed thresholds for a 5-L benchtop fermentation system which was operated under a variety of different media viscosity and bottom air sparge rate process conditions. Results demonstrated that viscosities of ≥ 10 cP were able to decrease the agitation threshold speeds required to induce decoupling events in the system. It was also shown that aeration rates of ≥ 1.5 vvm were able to support near maximum agitation rates, even when the system contained a highly viscous medium (2820 cP). Overall, the data generated in this study provides a robust framework that demonstrates how thorough system characterization can support agitation strategies that are highly suitable for use with magnetically coupled drive systems across a variety of upstream bioprocess applications.