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

Contents 12.1 THE NEEDFOR PROCESS CONTROL 12.2 FEEDBACK CONTROL 12.3 FEEDFORWARD CONTROL 12.4 COMPARISON OF STRATEGIES 512*200 258

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 512*200 258 In order to achieve Figure 12.1 ‘c’, we need a method of controlling the process

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 512*200 258 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

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 ) 512*200 258 Controller ( usually a computer ) performs the role of the operator  A fully automated control system

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 512*200 258 Such automatic control systems consisting of hardware and software operate continuously without direct human involvement

: desired controlled variable Set point : desired controlled variable Input variables or inputs :Variables that affect the value of the controlled variable Disturbances 512*200 258 : 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

12.2 FEEDBACK CONTROL In the feedback control, 512*200 258 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

( 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 ) 512*200 258

Control the pH of the neutralized acid stream Figure 12.6 ‘a’ Control the pH of the neutralized acid stream 512*200 258 Figure 12.6 ‘b’ Control the final outlet temperature

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 512*200 258 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

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 512*200 258 An error or deviation from setpoint is not needed for the feedforward system to respond

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 512*200 258

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 512*200 258

The one Feedforward loop is used in combination with the feedback loop 512*200 258 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