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Process Control

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Title: 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.
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