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What are the best packed column temperature(s) for control?

This guest post is authored by Greg McMillan.

In the ISA Automation Week Mentor Program, I am providing guidance for extremely talented individuals from Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. If you would like to provide additional answers, please send them to Susan Colwell at ISA. The ninth question in the ISA Mentor program is from Danaca Jordan (USA):

“If there are several temperatures measured in a packed column, how do you choose which ones or averages of several you should use for control?”

The temperature showing the largest and most symmetrical change in temperature for a change in reflux is normally the best location. However, with a packed column, poor flow distribution can cause localized hot spots and cold spots. Also, insertion length in a packed column may be less than optimum to avoid disruption to the packing and flow distribution leading to a slower response and heat conduction errors  seen as a lower than actual temperature when the column is higher than ambient  temperature. Un-insulated temperature connections increase the error. For packed bed reactors, the average of multiple temperature sensors in a thermowell traversing the reactor bed are averaged to provide a more representative cross-sectional temperature. If several temperature measurements in your column show a large and nearly symmetrical response but exhibit some temporary inconsistencies, you could try an average. If these inconsistencies are noise (period of a few seconds), you should consider a PV small filter that is about equal to the noise period to get about a 5:1 attenuation of the noise amplitude.

 

Greg McMillan
Greg McMillan
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the digital twin for exploring new opportunities.

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