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ChE 479 Process Dynamics and Control

ChE 479 Process Dynamics and Control. www-che.engr.ccny.cuny.edu/courses/che479/ Alexander Couzis Room T307 acouzis@che-mail.engr.ccny.cuny.edu. What Does a Process Control System Do?.

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ChE 479 Process Dynamics and Control

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  1. ChE 479Process Dynamics and Control www-che.engr.ccny.cuny.edu/courses/che479/ Alexander Couzis Room T307 acouzis@che-mail.engr.ccny.cuny.edu

  2. 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.

  3. 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)

  4. Elements of A Process Control System • Sensor/transmitter: Also called the primary and secondary elements • Controller:The “brain” of the system • Final Control Element: The component that imposes the changes decided on by the controller

  5. 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

  6. 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

  7. Process Control Schemes • Feedback Control • Feedforward Control

  8. 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

  9. 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.

  10. 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.

  11. 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

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