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.

The model predictive control provides faster and tighter control than PID controllers alone. The coordinated flexibility allows control strategies to be changed at any time during fermenting process, making it possible to respond to the ever changing process requirements or customer needs.
For more details of this application, contact ControlSoft for a copy of AN304, "Fermentation Facility using Mantra with Coordinated Controller."

Other Advanced Process Control Topics
Go to www.controlsoftinc.com to request copies of our application notes and e-mails on advanced control applications such as furnace control, fermentation, drinking water chlorination, wastewater treatment, glassmaking, chemical batch reactor control, barrel temperature control for extrusion, moisture control, distillation control, and pH control.

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Request a free copy of PID Loop Tuning Tips Pocket Guide. CLICK HERE for request form.

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