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Instructor: Chakkrit UmpuchDepartment of Chemical EngineeringFaculty of Engineering

Ubon Ratchathani University

Presented toChemical and Biological Engineering ProgramSenior studentUbon Ratchathani University

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Class: EN6304 1.00-4.00 pm.

Assessment:Homework 10%Quiz 10%Term Project 10%Midterm exam 30%Final exam 40%

Attendance:

Compulsory, students must have attended 80% of lectures in acourse in order to sit for the final exam.

Textbook:Stephanopoulos, G., Chemical Process Control: An Introduction to

Theory and Practice, Prentice-Hall International (1984).

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Chapter 1 Introduction to process controlChapter 2 Dynamics modeling of chemical processesChapter 3 Models in the Laplace domain

Chapter 4 Dynamic behavior of systemsChapter 5 Feedback controlChapter 6 Stability of feedback controlChapter 7 Feedback controller tuning

Chapter 8 Other types of control systems

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1.1 What is Process Control?1.2 Why use Process Control?1.3Terms in Process Control.1.4 Types of Process Control.1.5 Mathematical Domains Used in Process

Control

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Process Control meanscontrol of chemical process by chemical engineer inorder to have the process occurring in the desired

way.

Process in field of chemical engineeringinvolvesMomentum transfer

Heat transferMass transferChemical reaction

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Most courses in Chem. Eng. Design involve Steady State(st.st.)

In fact, no process will ever operate at a true st.st.For instance:1) Start up new plant2) Shut down for maintenance

3) Some process variable can not be controlled to st.st.(raw material from other processes and material fromthe environment (cooling water)) Disturbance

4) Change of product specification Set-point

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Objective: To reject disturbances to the process-Avoid unstable process

To change the set-point of the process-Safety-Environmental Protection and Regulation-Equipment Protection

-Smooth Operation-Product Quality-Profit-Monitoring and Diagnosis

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Type of Variable:

1.Output variables (describe effect of the process has on surrounding)- Measured outputs

- Unmeasured outputs- Controlled outputs- Uncontrolled outputs

2.Input variables (describe the effect of surroundings on the process)- Disturbances (cannot be controlled, move the process away from

set-point)

- Manipulated variables (can be controlled by controller)

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Type of Variable:

3. Set-points (the level of output that we want)

4. Deviation variables:x=x - xs(the value of a variable (x) relative to its initial st.st. value (xs))

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Type of Control Hardware:

1. Sensors (for measurement, it must have a signal)

2. Transducers (convert process measurements into a signal)

3. Transmissions (send the signal from 1 place to another)

4. Controller (make decision based on control signal: brain)

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Type of Control Hardware:

5. Control algorithm (mathematical function: control

calculates signal sent to the final control element)

6. Final control element (a piece of hardware that changesmanipulated variable to change the process to desired

way)

7. Recording elements (a piece of hardware which presentthe information provided by the sensors and the controllerin visual way)

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T T

Temperature controller

Set pointgenerator(cam)

Tset

Reactant chargedinitially

Productswithdrawn finally

Cooling waterinlet

Condensate

Cooling wateroutlet

Temperature transmitter

Stream

A batchreactor,showingnecessary

controlhardware

TVCA

CB

V-3

V-2, FwV-4, Wc

V-1, Fs

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Measured Output: TUnmeasured Outputs: V, CA, CB, WC, etcControlled Output: T

Final Control Elements: V-1, V-2Manipulated Variables: Fs, FwDisturbances: Ps, TwUncontrolled Outputs: V, CA, CB, etc.

Other hardware: show on the diagram

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1.4.1Feedback Control1.4.2 Multiple Loop Control1.4.3 Cascade Control

1.4.4 Feedforward Control1.4.5 Adaptive Control1.4.6 Inferential Control

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Characteristic:1.Measure directly the controlled variable and adjusts manipulatedvariable to be near the set-point.

2. Cannot determine the effect of disturbance until they have changedthe controlled variable.

Main disadvantage:Cannot provide good control when there is a large time delay

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Control

Element

Controlled Output

Process

Controller

ControlledVariable

Comparison and decision

ManipulatedVariable

Sensor

Set point

Disturbances

Feedback Control Loop

measurement

Change manipulatedvariable

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Characteristic:1. It is an extension of simple feedback control.

2. But there are several controlled variables, and each controlled

variable has a different process controller and manipulatedvariable.

Main disadvantage:Change one manipulated variable will also have an effect on the other

controlled variables: this type occurrence is called an Interaction.

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Feed

LC

To containment

Vapor

Liquid

PC

AC

P

Xp

F2

F1

-3 variables to control: Xp, L, P-Needs 3 Loops-Also needs 3 manipulatedvariables

Process

Fluid

Steam

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Characteristic:1. It is an enhancement of feedback control which uses an extrameasurement.

2. Measure controlled output + one important input

3. Uses two feedback controllers which are connected such that theoutput of the primary controller is the set-point for the secondarycontroller.

Main advantage:The second loop is useful since it has very fast dynamics. If feedbackcontrol of the primary loop was used alone the control would occurmuch more slowly.

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T0

F1

F0

TI TC

MV2

CV2

CV2P0

P1

HeatingOil

FC

SP1 fromoperator

SP2=MV1

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Characteristic:Measure inputs.

Main advantage:Give perfect control, where the controlled variable never deviatesfrom the set-point value (Feedback controls cannot achieve this).

Main Disadvantage:1. An inaccurate model will result in poor c0ntrol.

2. Can only compensate for disturbances for which it has a sensor anda model. Other disturbances will have an uncontrollable effect on thecontrolled variable.

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ProductFeedback

Feed

TI

TC

F

P1

Heating Oil

TY2

SP2=MV1

TY1Feedforward

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Characteristic:

1. Be useful for nonlinear process and some process that vary withtime e.g. heat exchanger surface becomes less clean, and catalytic

reactors where the catalyst may degrade over time.

2. The inner loop is ordinary feedback control loop, but the parametersof the feedback equation are adapted to suit the changing system bythe outer control loop.

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set- point

Controller

ControlledOutput

Process

Adjustment

mechanism Auxiliarymeasurements

New values ofcontrollerparameters

Outer loop

Inner loop

Feedback control

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Characteristic:

1. It is not possible (or too costly, or too slow) to measure controlledoutput of the process, which would make a control scheme involvingany type feedback control impossible.

2. The controlled output may be inferred (or estimated from) the valueanother (measured) output and the value of the manipulated variable.

3. It is most often used when composition is the controlled variable

(since composition analyzers are often slow, costly, and unreliable)and in many of these cases temperature will be measured output usedto infer the composition.

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Gp1

Secondarymeasurement

Controlledoutput(unmeasured)

Inner loop

Manipulatedvariable

Gp2

Gd2Gd1

Unmeasureddisturbance

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Time Domain modeling Solving differential equations

-Analytical method-Numerical method

Laplace Domain Transformed linear ordinary differentialequation into algebraic equation s.

Frequency Domain Response of the output of a systemsinusoidal input to the system

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State Variables

The variables which occur in the time differential of the process model.

dCA/dt = f1(CA, T)

dT/dt = f2(CA,T)

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1. What is the difference between manipulated variable, disturbanceand controlled output?

2. What is the difference between feedback control and cascadecontrol?

3. What is the deviation variable?4. What is the state variable and give an example?