Loops are Not Just for Continuous Processes

Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at controltalk@putman.net.

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Greg: Fortunately, my experience with the addition of loops to batch reactors and batch distillation columns did not have these issues, and reductions in batch cycle time were immediate and obvious. I have seen equal or better results from the application of a small MPC to batch operations despite the widely held academic opinion that batches are too nonlinear.

Stan: In many cases, the improvements resulted in a more intelligent selection and schedule of targets. It is important in an opportunity assessment to challenge every setpoint and not be deterred by “war stories.” Many reasons for not operating at a setpoint that increases production rate turn out to be no longer valid or can be addressed by an improvement in the automation system (measurements, valves, strategy and tuning).

Greg: Several examples come to mind where moving or adding sensors have increased plant capacity. The relocation of a pH sensor upstream of a heat exchanger in a recirculation line in a batch neutralizer reduced the batch cycle time by 25% by eliminating the overshoot of the end point and the associated manual sampling and addition of reagent. The addition of Coriolis slurry feed meters with centrifuge amp measurements for override control increased centrifuge production rate by 10% through a more complete and consistent basket loading. One of the great opportunities out there is the addition of bioreactor probes (e.g., capacitance, dissolved CO2 and NIR) and at-line analyzers with automated sample systems for broth measurements (e.g. cell density, glucose, glutamine, lactate, and ammonia). These additional measurements should enable feedback control of bioreactor batch substrate concentration, cell growth rate and product formation rate. To be part of this great future, we encourage you to become a member of the Adorable Automators Association (AAA). Here are the top ten reasons.

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This Month's Puzzler

What are the major sources of changes in the total loop dead time?

Send an email with your answer to the Puzzler, Control questions, or comments to controltalk@putman.net.

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