Process Control

1. Process Control Spring, 1999 Professor: In-Beum Lee Department of Chemical Engineering

2. Text and References • Text • Su whan Sung and In-Beum Lee, “PID Controllers and Automatic Tuning” Ajin Press, 1998 • References • Seborg D. E., T. F. Edgar, and D. A. Mellichamp, “Process Dynamics and Control,”John Wiley & Sons, New York, 1989 • Stephanopoulos G., “Chemical Process Control-An Introduction to Theory and Practice,”Prentice -Hall, New Jersey, 1984. • Luyben W. L., “Process Modeling, Simulation and Control for Chemical Engineers,” McGraw-Hill, New York, 2nd Ed., 1990

3. Contents of the Lecture 1. Introduction to Process Control. 2. Mathematical Modeling of Chemical Processes. 3. Laplace Transforms. 4. The Transfer Function. 5. Dynamic Behavior of the Processes. 6. Feedback Controllers. 7. Closed-loop Control Systems. • Dynamic Behavior. • Stability. 8. Feedback Controller design and Tuning. 9. Process Identification and Controller Design.

4. 1. Introduction to Process Control • ‘Process Control’ makes processes satisfy following requirements. • Safety • Production specifications • Environmental regulations • Operational constraints • Economics • Mathematical model of the process should be known(Process Modeling & Identification).

5. 1.1 Illustrative Examples • EX 1. Continuous stirred-tank heater • Question ; Assume that inlet temperature changes with time. How can we ensure that T remains at or near the set point TR? Figure 1.1. Continuous stirred-tank heater.

6. Possible Strategies 1. Measure T and adjust Q . 2. Measure Ti and adjust Q. 3. Measure T and adjust w. 4. Measure Ti and adjust w. 5. Measure T and Ti and adjust Q. 6. Measure T and Ti and adjust w. 7. Place a heat exchanger on the inlet stream. 8. Use a large tank. • Classification • 1 & 3; Feedbackcontrol • 2 & 4; Feedfoward control • 5 & 6; Feedfoward-Feedback control • 7 & 8; Design change

7. EX 2. Furnace using electrical power • Question ; Can you find any problem in controlling the following furnace process? Figure 1.2. Furnace using electrical power

8. EX 2. Furnace using electrical power • Question ; Can you find any problem in controlling the following furnace process? Figure 1.2. Furnace using electrical power • There is no way to decrease the temperature! • The power cannot be negative. • The heat loss is nearly zero.

9. 1.2 Classification of the variables • Input variables ; denote the effect of the surroundings on the chemical process. 1. Manipulated variable (MV or Control variable) ; Its value can be adjusted freely by the human operator or a control mechanism. Ex) In heated tank, the amount of heat added(Q) or mass flow rate(w). 2. Disturbance variable(DV) ; Its value is not the result of the adjustment by an operator or a control system. Ex) In heated tank, inlet temperature, because we can’t usually control the temperature of inlet water.

10. Output variables ; denote the effect of the process oh the surroundings. 1. Measured output variable or Controlled variable(CV) ; Its value is known by directly measuring it. Ex) In heated tank, outlet temperature. 2. Unmeasured output variables ; It is not or cannot be measured directly.

11. 1.3 Classification of Control Strategies • Feedfoward Control ; The disturbance variable is measured and the measurement is used to manipulate MV. • Advantages • If all sources of the disturbances are known and these values can be measured accurately.  Perfect Control ! • Disadvantages • No corrective action for unmeasured disturbances. • In industrial applications, feedfoward control should be used in combination with feedback control.

12. Feedback Control ; The process variable to be controlled is measured and used to adjust another process variable which can be manipulated. • Advantage • Corrective action is taken regardless of the source of the disturbance. • Disadvantage • The controlled variable must be deviate from the set point before corrective action is taken.  NotPerfect Control! • Classification • Negative feedback ; The desirable situation where the corrective action taken by the controller tends to move the controlled variable toward set point. • Positive feedback ; The controller tends to make things worse by forcing the controlled variable farther away from the set point.

13. 1.4 Block Diagram • Schematic diagram ; Physical connection between the components of the control system. Figure 1.3. Schematic diagram of a temperature feedback control system for a stirred-tank heater. ---, Electrical instrument line; TT, temperature transmitter;TC, temperature controller.

14. Block diagram ; Flow of information within the control system. Each block represents a dynamic or static process elements. • Dynamic elements; variable which depends on time. • Static elements; variable which is independent of time. Figure 1.4. Block diagram for temperature feedback control system in Figure 3.

15. 1.5 Control and Modeling Philosophies • Two approaches to design control system. 1. Traditional Approach ; Control strategy and system hardware are selected based on the knowledge of process, experience and insight. After the control system is installed in the plant, the controller settings are adjusted, that is, the controller is tuned. 2. Model-Based Approach ; A process model is developed and based on the model, suitable control strategy and system hardware are selected. • The model-based approach is more advantageous.

16. Usage of the Process Model • It can be used as the basis for classical controller design methods. • It can be incorporated directly in the control law, an approach that now is the starting point for many advanced control techniques. • It can be used to develop a computer simulation of the process to allow exploration of alternative control strategies and to calculate preliminary values of controller setting.

17. 1.6 Analog and Digital Control • Analog controller ; Controller that has continuous input and output signals. • Digital controller ; Controller which involves input input and output signals that change only at discrete instants in time, the so-called sampling instants. • Merits ; increased flexibility and accuracy, and improved monitoring of the plant through data acquisition, storage, and analysis.

18. 1.7 Economic Justification of Process Control • Justification based on • Safe operation • Satisfying environmental constraints • Economic benefit(ex.increased production level, reduced raw material costs or enhanced production quality) • Extended equipment life Example) Figure 1.5. Production validity over time: (a) before improved control; (b) after. The operating variable is % ethane.