ChE 479 Process Dynamics and Control

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ChE 479Process Dynamics and Control

• www-che.engr.ccny.cuny.edu/courses/che479/
• Alexander Couzis
• Room T307
• acouzis_at_che-mail.engr.ccny.cuny.edu

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What Does a Process Control System Do?

• The Goal of Process Control is to allow a certain
  process to operate in a safe, profitable, stable
  manner. It should also allow for easy changes of
  specifications of the output, and should allow
  the process to accommodate ambient changes.

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How Does Process Control Achieve its Goals ?

• For every action there is a reaction
• But how does one determine the right reaction?
• Three Key Elements in Every Control Scheme
• Measurement (M)
• Decision (D)
• Action (A)

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Elements of A Process Control System

• Sensor/transmitter Also called the primary and
  secondary elements
• ControllerThe brain of the system
• Final Control Element The component that imposes
  the changes decided on by the controller

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How is Process Control Implemented ?

• Proper Design of the Process
• Controllability of the design
• Response time
• Disturbance Reduction
• Measurement
• Location You can control only what you can
  measure
• Measurement speed
• Accuracy
• Noise Level

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How is Process Control Implemented ? (continued)

• Final Elements
• Manipulate variables that can be adjusted by the
  control calculation
• Improve control performance by adding flexibility
• Control Structure
• Control Decisions
• Pairing
• Control Calculations

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Process Control Schemes

• Feedback Control
• Feedforward Control

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Feedback Control

• Makes use of an output of a system to influence
  an input of the same system
• Benefits
• Does not require model of the process
• Simple
• Disadvantages
• Changes in the output must occur before action
  takes place
• Positive Feedback Does not effect changes as
  there is a tendency to reach set point
• Negative Feedback effects changes in order to
  reach a set value

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Feedforward Control

• Utilize models of the process in order to predict
  changes of the output as a function of changes in
  an input of the system, and take action to
  prevent these changes in the output, before they
  occur.
• Benefits
• Predictive, does not wait for changes in the
  output
• Disadvantages
• Unreliability of model can lead to deviations.
• Uncontrolled variables can cause deviations, that
  cannot be compensated for.

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Process Control Glossary

• Output variable Is the process variable that
  must be maintained, or controlled at some desired
  value.(Often also referred to as process
  variable)
• Set Point Desired Value of Output Variable
• Manipulated variable Is the variable used to
  maintain the controlled variable at its set
  point.

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How Does One Determine The Right Operating
Conditions ?

• Operating Window Defined by
• Physical Principles
• Safety, enironmental, equipment protection
• Equipment Capacity
• Product Quality
• Analyze Plant Economics to Determine Best
  Operation within the Established Operating Window
• Control Must keep Plant Running at most
  Profitable Conditions