Melton1 Control Problem

Control of Reactor Temperature

Dr. Lynn A. Melton
Univ. Texas / Dallas
melton@utdallas.edu

You have just been hired as a process control chemist at BigTime Chemicals and have been assigned to work as an apprentice to Rhonda Awlright, the chemical engineer in charge of the startup process for BigTime's new plant. The plant manager just called the control room and summoned Rhonda to an emergency meeting. You have been left in charge.

The next task in the startup problem is to bring batch reactor #3 up to temperature. It seems like a easy task. There are heating and cooling coils in the reactor, and variable amounts of fluid may be run through these coils. The stirring motor for the tank is already on, so it is unlikely that uneven temperatures will develop as heat is added or removed from the tank.

Just as Rhonda left, she turned and stopped. "Keep your hands off the knobs on Console B", she said. "Those are connected to the plant. On the other hand, Console C is running the simulator we developed while we were designing the unit. You can play with the knobs there. You might want to work out the proper PID parameters for reactor #3. We are stuck with the system response time of three hours, but, even so, we want to get it from 20 oC up to 100 oC operating temperature just as quickly as we can. Plus or minus 1 oC will do."

Information: We do not have Console B. However we can mimic the results of Rhonda's reaction simulation program on Console C through a spreadsheet called CONTROL.WKS which will run under many spread sheets on Windows or Macintosh. Control Spread Sheet The spreadsheet will recalculate the reactor temperature, and redraw a graph, each time you enter a new set of parameters.

The variables you can control are in lines D5 through D11. Do not make changes elsewhere on the spread sheet.

Copy the spreadsheet to some other file, on your own floppy disk, before making any runs. Otherwise there is the possibility that you will mess it up for other users.

Questions:

a. Do you work at Console B or Console C?

b. What is the optimum set of PID parameters (proportional gain, derivative time, and integral time) for this control process?

c. If you were allowed to set the system response time to any value you desired, what would the optimum set of PID parameters be?

d. If you got the wrong answer to part a, what do you tell Rhonda?

e. Write a paragraph of roughly one page length in which you compare and contrast what this problem has taught you about process control with what you know about driving a car on a highway.

Reminder: It is always possible that you can choose some bizarre set of PID parameters which will calculate a great solution but which could not be physically used as a solution to a real problem. Such solutions, while amusing to the professor in limited quantities, will be given very little credit.