lecture 1 procon
TRANSCRIPT
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Introduction toProcess Control
ChE 220
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Objective
To define and determine importance ofProcess Control to Chemical Engineering
To determine the necessity of Control
To determine where Controls areneeded
Feedback and Feed forward ControlSystems
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Importance of ProcessControl
Describe your method indriving a car.
Could you drive a car withoutlooking out from the windshield?
What must be provided by thecar designers?
Can a good design eliminatethe need to steer?
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Importance of ProcessControl
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Importance of ProcessControl
controlled variables - these are the variables whichquantify the performance or quality of the final
product, which are also called output variables.manipulated variables - these input variables are
adjusted dynamically to keep the controlledvariables at their set-points.
disturbance variables - these are also called "load"variables and represent input variables that cancause the controlled variables to deviate from theirrespective set points.
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Importance of ProcessControl
set-point change - implementing a change in theoperating conditions. The set-point signal is changed
and the manipulated variable is adjustedappropriately to achieve the new operatingconditions. Also called servomechanism (or "servo")control.
disturbance change - the process transient behaviorwhen a disturbance enters, also called regulatory
control or load change. A control system should beable to return each controlled variable back to itsset-point.
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Feedback System
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Necessity of Process Control
We want to achieve the following:
Safety
Environmental Protection
Equipment Protection
Smooth Operation
Product Quality
Profit
Monitoring and diagnosis
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Where is Control Done?
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Where is Control Done?
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Feedback Control System
Distinguishing feature: measure the controlled variable
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Feedback Control System
Advantages:
Corrective action is taken regardless of the source
of the disturbance.
Reduces sensitivity of the controlled variable todisturbances and changes in the process
(shown later).
Disadvantages:
No corrective action occurs until after thedisturbance has upset the process, that is,
until afterx differs from xsp
.
Very oscillatory responses, or even instability
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Feed
Forward ControlSystem
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Feed
Forward ControlSystem
Distinguishing feature: measure a disturbance
variable
Advantage:
Correct for disturbance before it upsets theprocess.
Disadvantage:
Must be able to measure the disturbance.
No corrective action for unmeasureddisturbances.