Chemical Processing - April 2008 - (Page 45)

PROCESS PUZZLER < > THIS MONTH’S PUZZLER A viscous ingredient is pumped through a heat exchanger to raise it to a sufficient temperature so that a reaction can occur when it then goes to a batch reactor. The pump is interlocked with the temperature control loop. Unfortunately, frequent nuisance alarms and trips are associated with this system. Typically, start-up requires the operator’s full attention with all controls in manual mode. Can you suggest some ways to reduce or eliminate these nuisances while assuring a heated exit stream? Consider tuning If I interpret this problem statement correctly, the low temperature alarm is shutting down the pump and heater, which results in having to restart the pump and heater. The question is: what causes the alarms and trips? Are they short-term low temperature excursions caused by controller oscillation, fluid property issues or temperature sensing errors? I have several possible solutions: 1) the obvious solution is to put an automatic bypass valve on the heat exchanger discharge to divert low temperature material back to the pump instead of the reactor; 2) if temperature sensing errors are the problem, such as a lag, put a time delay on the trip out; 3) if temperature oscillation is the cause you have a poorly tuned control system — try tuning the proportional and integral constants; 4) if these ideas don’t help, you have some kind of physical problem in your control system, e.g., a broken or sloppy control valve, slow communications to the valve or excessive lag time; or 5) perhaps the heat exchanger itself is not working well. If tuning doesn’t help, you may have a system that is not well suited for the traditional proportional-integral control strategy. Perhaps a strongly proportional control could work, if you can live with an offset error. Maybe start-up could begin with a proportional-only band during start-up and a standard proportional-integral control after a few error cycles. Fortunately, there are new sophisticated control systems available today. Contact a control expert for advice and try one of these advanced control systems. Glen Oswald, engineer advisor, Mosaic Phosphates, Plymouth, Minn. www.chemicalprocessing.com http://www.chemicalprocessing.com

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Chemical Processing - April 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Is It a Tragedy or Comedy for Engineers? Better Understanding Boosts Mixer Scale-up Don't Err With Air Compressors Control Performance Supervision Enhances Revamp Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - April 2008

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