1. Chapter 12. Controlling the Process
Thermophysical Properties Laboratory
Chapter 12. Controlling the Process
Department of Chemical and Biological Engineering,
Seoul National University

2. Contents 12.1 THE NEEDFOR PROCESS CONTROL 12.2 FEEDBACK CONTROL
12.3 FEEDFORWARD CONTROL
12.4 COMPARISON OF STRATEGIES
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3. 12.1 THE NEED FOR PROCESS CONTROL
Process variables ( e.g., flow rate ) tend to vary with time
 These make it difficult to obtain a product with the desired characteristics
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In order to achieve Figure 12.1 ‘c’, we need a method of controlling the process

4. The HCl entering the acid-neutralization process comes from the existing part of the plant and is received into a holding tank
It is then drawn from that tank via a pump, which delivers it to the acid-neutralization process
If the production rate exceeds the flow rate of the pump for a sufficiently long time, the HCl will overfill the tank
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If the production rate falls below the flow rate of the pump, the tank will run dry
The most common way of avoiding these problems is to place a valve down stream of the centrifugal pump

5. Adding a measurement device or sensor, and a pneumatic valve
Many years ago, processes were controlled manually by people, called operators ( manually open or close the valve )
Adding a measurement device or sensor, and a pneumatic valve
 Remote control ( without ever leaving the control room )
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Controller ( usually a computer ) performs the role of the operator
 A fully automated control system

6. The signal from the level sensor is sent to the controller instead of to the operator
The controller then sends an output signal directly to the valve in order to make the proper adjustment in the flow rate
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Such automatic control systems consisting of hardware and software operate continuously without direct human involvement

7. : desired controlled variable
Set point
: desired controlled variable
Input variables or inputs
:Variables that affect the value of the controlled variable
Disturbances
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: Some of those inputs undergo uncontrolled changes
Error
: a difference between the value of the controlled variable and the set point
Manipulated variable
: the input that is adjusted to reduce the error

8. 12.2 FEEDBACK CONTROL In the feedback control,
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The controlled variable is monitored over time via sensors
Using the measured values of the controlled variable, the feedback controller adjusts an input variable via affectors
The structure consisting of the sensor, controller, and affector, together with the communication lines that connect them, is referred to as the feedback control loop

9. ( i.e., change the parameters in the controller )
In feedback control,
the value of the controlled variable must be different from the setpoint in order for the controller to act
as the value of the controlled variable approaches the set point, the controller will continue to make adjustments in response to the updated value of the error
the engineer can change the response of a control loop by telling the how much of a change to make in the output signal for a given error
( i.e., change the parameters in the controller )
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10. Control the pH of the neutralized acid stream
Figure 12.6 ‘a’
Control the pH of the neutralized acid stream
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Figure 12.6 ‘b’
Control the final outlet temperature

11. 12.3 FEEDFORWARD CONTROL In the feedback control,
the input variables(disturbances) are monitored via sensors
the feedforward controller predicts the effect of the disturbances on the output and adjusts the manipulated variable to offset the predicted effect
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the output variable is not measured but the adjustment of the manipulated variable is based solely on how the process is expected to behave (from mathematical model)
the structure consisting of the sensor, controller, and affector, together with the communication lines that connect them, is commonly referred to as the feedforward control loop

12. If the HCl flow rate from the holding tank increased, the feedforward controller would immediately increase the NaOH flow rate, without waiting for a change in the final pH to occur before responding
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An error or deviation from setpoint is not needed for the feedforward system to respond

13. 12.4 COMPARISON OF STRATEGIES
Feedback control
Advantages
measures the controlled variable and is therefore capable of responding to all disturbances that affect this variable
Disadvantages
Controlled variable must deviate from the desired value before corrective action is take (i.e., there must be an error)
 systems may deviate significantly from the desired setpoint
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14. Feedforward control Advantages
correct for measured disturbances before the controlled variable is affected
 It has the potential of preventing the controlled variable from deviating from the desired value
Disadvantages
does not directly measure the controlled variable, but relies on some kind of model to predict its value from the values and trends of the input variables
 Any errors or imperfections in the model will lead to deviations of the controlled variable from the desired value
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15. The one Feedforward loop is used in combination with the feedback loop
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The three feedback control loops
The level in the HCl holding tank
The pH of the neutralized acid
The cooler outlet temperature
The one feedforward loop
The pH of the neutralized acid