Title: Process Control
1Process Control
- Spring, 1999
- Professor In-Beum Lee
- Department of Chemical Engineering
2Text 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
3Contents 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.
41. 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).
51.1 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.
91.2 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.
111.3 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.
131.4 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.
151.5 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.
171.6 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.
181.7 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.