This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.
An exothermic reactor can run away if the catalyst feed flow exceeds a certain flowrate. Excess catalyst can cause the reaction rate to increase, thereby increasing the heat flux inside the reactor vessel.
When tuning the catalyst feed flow controller while in operation, the following tuning method should be used:
a) open-loop, shortcut tuning method
b) general purpose, closed-loop tuning method
c) Lambda tuning method
d) modified Ziegler-Nichols tuning method
e) none of the above
Click Here to Reveal the Answer
Answer A, Open-loop shortcut method, and Answer C, Lambda tuning, are, by definition, open-loop tuning methods. Open-loop tuning methods are suitable only for self-regulating processes.
Answer D, modified Ziegler-Nichols tuning, can be done either open-loop or closed-loop. However, with this method, the gain is adjusted upward until the control output oscillates, which may cause a runaway reaction. This tuning method is not acceptable for this type of process.
The correct answer is B, General purpose, closed-loop tuning method. The key to this problem is to identify that only a closed-loop tuning method can be used with an exothermic reaction where the reactor can “run away” (increasing speed of reaction as the reaction progresses, catalyzed by its own heat of reaction). The tuning, and hence position, of the catalyst flow actuator, is directly related to the ability to maintain reaction stability. Only answer B gives a closed-loop tuning method which maintains the loop in automatic during the tuning process, suitable for this type of reaction.
Reference: Gregory K. McMillan, Good Tuning: A Pocket Guide.
About the Editor
Joel Don is the community manager for ISA and is an independent content marketing, social media and public relations consultant. Prior to his work in marketing and PR, Joel served as an editor for regional newspapers and national magazines throughout the U.S. He earned a master's degree from the Medill School at Northwestern University with a focus on science, engineering and biomedical marketing communications, and a bachelor of science degree from UC San Diego.