September/October 2010

ISA Certified Automation Professional (CAP) program

Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

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

CAP answer

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.

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.

Reference: Gregory K. McMillan, Good Tuning: A Pocket Guide, ISA, 2000.