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CONTROL OBJECTIVES AND BENEFITS
The objectives of this lecture are:
1. Recognize some of the key control strategies and their
differences 2. Learn how to identify controlled outputs,
manipulated inputs and disturbances 3. Recognize examples of the
seven (7) control objectives in chemical processes
a. Safety b. Environmental protection c. Equipment protection d.
Smooth operation e. Product quality f. Profit g. Monitoring and
diagnosis
Control Terminology
1. Controlled variables - these are the variables which quantify
the performance or quality of the final product, which are also
called output variables.
2. Manipulated variables - these input variables are adjusted
dynamically to keep the controlled variables at their
set-points.
3. Disturbance variables - these are also called "load"
variables and represent input variables that can cause the
controlled variables to deviate from their respective set
points.
Overview of Control System Design
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Process variables:
Controlled output Manipulated input Actuator Sensor
Disturbances
CONTROLLED AND MANIPULATED VARIABLE SELECTION
Controlled variables:
All non-self regulating variables Within equipment and operating
constraints Measurable
Manipulated variables:
Large effect on controlled variable Rapid and direct effect
CONTROL STRATEGIES
Feedback Control: The process variable of interest is measured
and used to adjust another process variable.
Feed-forward Control: Measure disturbances and take corrective
action before they upset the process.
TYPES OF FEEDBACK CONTROLLERS
On-Off Control - e.g., room thermostat
Manual Control - Used by operators and based on more or less
open-loop responses
Automatic PID Control (Proportional, integral, derivative)
o Most commonly used controller. Control action based on error
from set-point.
o Advanced PID- Enhancements: ratio, cascade.
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FEEDBACK CONTROL
Example: Stirred tank heater
System components:
Operator dials in a set-point (desired temperature) Thermocouple
measures temperature in tank Measured temperature is compared to
set-point Controller manipulates steam valve based on difference
between set-point and measurement
FEEDFORWARD CONTROL
Example: Stirred tank heater
System components:
Operator develops model of the system Thermocouple measures
temperature in the inlet stream Measured temperature is input to
the process model Controller manipulates the steam valve based on
model prediction
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Control objectives: Lets discuss this example process involving
flash separation!
1. Safety: High pressure drum is dangerous!
2. Environmental protection: Never release hydrocarbons to the
atmosphere!
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3. Equipment protection: No flow could damage the pump!
4. Smooth operation: Always keep the production smooth!
5. Product quality: Achieve L. Key by adjusting the heating!
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6. Profit: Use the least costly heating!
7. Monitoring and diagnosis: Calculate and plot key parameters
(e.g., UA)
All seven must be achieved. Failure to do so will lead to
operation that is unprofitable or worse, unsafe.
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BENEFITS FROM PROCESS CONTROL:
When we control a process, we reduce the variability of key
variables to achieve the seven objectives.
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Variability is moved from the controlled to the manipulated
variable!!