Dissolved oxygen and fermentation control

How to adjust for DO₂

WHETHER YOU'RE brewing beer, creating clean water from wastewater, or producing pharmaceuticals, flexibility is what you need when it comes to controlling fermentation. Flexibility and effective coordination are key to minimizing waste of material and maximizing the quality of the product throughout the several phases of a typical fermentation process (e.g., sterilization followed by cool down and then fermentation).

Proper pH control is vital, for instance, in cultures that produce acid as they grow, because these cultures require the addition of base to maintain neutral or alkaline conditions. In this abbreviated description (full version is AN304), we focus on applying Mantra's advanced process control solution for dissolved oxygen, referred to as DO₂.

What affects the concentration of DO₂ in a culture?
In this example, DO₂ concentration is affected by the speed of the agitator, by air supply, and by the pressure of the vessel. The crucial question is: when and how should these factors be used for control of DO₂?

Model Predictive Control Solution
Since agitator speed, air flow, and pressure interact and affect the DO₂, Mantra's Coordinated Controller (CC) is used as the main supervisory controller for DO₂. In effect, all three loops are coordinated by the CC, which provides the setpoints for the individual PID controllers. Refer to the block diagram below.

In one control strategy, the CC will use the air flow as primary control to maintain DO₂, while target agitator RPM and pressure are maintained at the same time. Agitator speed and pressure are activated only when the air flow is not able to maintain the desired DO₂. Alternatively, agitator speed can be used as primary control while the other two controls will be maintained at certain target values. The CC will effectively coordinate all three manipulated variables. The goal of the CC is three-fold:

1. Reject any disturbances to the process without variable interaction,
2. Respond quickly to setpoint changes, and
3. Optimize the available controller outputs during steady state control.