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 Feedback control
• 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 cant 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. ?
  Not Perfect 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 transmitterTC, 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.