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

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.

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

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!

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!

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.

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!!