MISS. RAHIMAH BINTI OTHMAN ERT 422/4 Control system instrumentation MISS. RAHIMAH BINTI OTHMAN (Email: rahimah@unimap.edu.my)

OUTLINES Basic concepts of process dynamics and process control in bioprocess plant system. The needs of control system. Types of controller required in the selected plant processes A case study.

TYPES OF CONTROLLER IN PROCESS PLANT

CONTROL TERMINOLOGY 1. Controlled Variables (CV) 3. Disturbance Variables (DV) 2. Manipulated Variables (MV) CONTROL TERMINOLOGY 4. Setpoint 7. Controller 6. Final Control Element 5. Sensor

CONTROL TERMINOLOGY Controlled Variables (CV) - these are the variables which quantify the performance or quality of the final product, which are also called output variables (Set point). Manipulated Variables (MV) - these input variables are adjusted dynamically to keep the controlled variables at their set-points. Disturbance Variables (DV) - these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points (Cannot be manipulated). Setpoint – the desired o specified value for the CV. Sensor – the device that measures a process variable. Final Control Element – the system that changes the level of the MV. The final control element usually involves a control valve and associated equipment or a variable speed pump. Controller – a unit which adjusts the MV level to keep the CV at or near its setpoint.

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

BACKGROUND Normally a chemical or biochemical process has numerous inputs and many outputs. Consider the diagram below: The objective of a control system is to keep the cv’s at their desired values (or setpoints). This is achieved by manipulating the mv’s using a control algorithm.

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

FEEDBACK CONTROL 2. Applications 3. Positive Feedback 1. Objectives 4. Negative Feedback 8. Advantages & Disadvantages 5. Process Variables (CV, MV, DV, SP) 6. Possible Block Diagram 7. Control System Diagram

FEEDBACK CONTROL SYSTEM; (Example: Blending System) * Notation: w1, w2 and w are mass flow rates x1, x2 and x are mass fractions of component A Block Diagram For The Feedback Control System

Assumptions: w1 is constant x2 = constant = 1 (stream 2 is pure A) Perfect mixing in the tank Control Objective: Keep x at a desired value (or “set point”) xsp, despite variations in x1(t). Flow rate w2 can be adjusted for this purpose. Method 1. Measure x and adjust w2. Terminology: Controlled variable (or “output variable”): x Manipulated variable (or “input variable”): w2 Disturbance variable (or “load variable”): x1

FEEDBACK CONTROL SYSTEM; (Example: Blending System)

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

FEEDFORWARD CONTROL 1. Objectives 2. Applications 5. Process Variables (CV, MV, DV, SP) FEEDFORWARD CONTROL 8. Advantages & Disadvantages 6. Possible Block Diagram 7. Control System Diagram

Method 2. Measure x1 and adjust w2. Measure disturbance variable x1 and adjust w2 accordingly Thus, if x1 is greater than , we would decrease w2 so that If x1 is smaller than , we would increase w2.

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

FEEDFORWARD-PLUS-FEEDBACK CONTROL Because of the difficulty of accounting for every possible load disturbance in a feedforward system, this system are often combined with feedback systems. Controller with summing functions are used in these combined systems to total the input from both the feedforward loop and the feedback loop, and send a unified signal to the final control element.  FC TC Process variable need to be controlled = Temperature FT Fluid in TT Y Steam Fluid out LCV-100

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

CASCADE CONTROL; EXAMPLE The normal block diagram representation of a cascade control loop is shown below,

CASCADE CONTROL Cascade Control Cascade Control uses the output of the primary controller to manipulate the set point of the secondary controller as if it were the final control element. Reasons for cascade control: Allow faster secondary controller to handle disturbances in the secondary loop. Allow secondary controller to handle non-linear valve and other final control element problems. Allow operator to directly control secondary loop during certain modes of operation (such as startup).

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

PIPING AND INSTRUMENTATION DIAGRAM (P&ID) Ratio Control (cont…) Application: - Blending two or more flows to produce a mixture with specified composition. - Blending two or more flows to produce a mixture with specified physical properties. - Maintaining correct air and fuel mixture to combustion. FIC FF FT FT Water Acid 2 part of water 1 part of acid

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

PIPING AND INSTRUMENTATION DIAGRAM (P&ID) Split Range Control FC FT Valve A Valve B

TYPE OF PROCESS CONTROL LOOP Feedforward-plus-Feedback Control Feedforward Control Feedforward-plus-Feedback Control Cascade Control Ratio Control Split Range Control Differential Control DESIRED OUTPUT

PIPING AND INSTRUMENTATION DIAGRAM (P&ID) Differential Control Meaning Systems Theory: In differential control, control action is based on the change (derivative) of the control error. The control error is defined as the difference between the set point and the process output. Explanation: Derivative control is used to provide anticipative action. This is done by using the current change (derivative) of the control error to estimate the error some time ahead and act anticipatively on this estimation.