Introduction to process control 2

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CHE334 Instrumentation and

Process Control

Week 2Chapter 1 Introduction to Inst and PC

By Dr. Maria MustafaDepartment of Chemical Engineering

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Lecture Content

Optimize the Performance of a Chemical

Process

Terminology used in Chemical ProcessControl

Hardware for a Process Control System

Sensors

2


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Optimize the Performance of a

Chemical Process

Goal is to make plant operation moreprofitable.

Which Means we can maximize theprofit by control.

To learn how controller can be used tooptimize the economic performance of asingle unit ( increase profit)

Example of batch chemical reactor.


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Case Study II

Optimizing the performance of Batch

Reactor

4

cA, Ti, Fi

Stream

Controller

Condenser

4

A B C (endothermic

Reaction)

Reaction1 2

Desired

undesired


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Optimizing the performance of Batch

Reactor Maximize=

{

+

0

Q

Qmin

Qmax

tr Time

Steam flow rate

0

Minimium utilization

Maximum utilization


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Common Terminologies Process Variables: Conditions of process fluid

that can change manufacturing process insomeway.

Input variable which denotes the effect of thesurroundings on the chemical Process.

Output Variable which denote the effect of thechemical process on the surroundings.

Input Variables

Manipulated Variables Disturbances

Output Variables Measured Output

Unmeasured Output


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Example of Stirred Tank Heater

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Fs

Fi, Ti

h

F, T

T

Q

Input Variable

Fi

Ti Fst

Output Variables

F ( if notmanipulated)

V or h

T


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Example

Controlling the Operation of Unstable

Reactor

8

cA, Ti, Fi

Tci, Fc

Tco, Fc

cAT, F

A + B ---> C ( exothermic reaction)

Chemical Process Operation


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Controlling the Operation of Unstable

Reactor

Input Variable

Disturbances ( Measured & Unmeasured)

cA, Ti, Fi, Tci

Manipulated Variable

Fc

Output Variables

Measured Variables F (if not Manipulated Variable), T, Tco, V

Unmeasured Variable

CA


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Terminologies

Setpoint

The setpoint is a value for a process variable that

is desired to be maintained.


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Design Elements of a Control System

Define Control Objective ( central element)

Select measurements

Measuring and monitoring the process variables

Primary Measurements

Secondary Measurements

Unmeasured output = f ( secondary

measurements)

Select Manipulate Variables

Select Control Configuration


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Terminologies Control Configuration

A control configuration is the information

structure that is used to connect the available

measurements to the valuable manipulated

variables.

Two types of CC

SIS0 = single inputsingle output configuration ,

example controlling the level of the liquid in the tank MIMO= multiple inputmultiple out configuration =

Example controlling the level and temperature of the

liquid in the tank by changing output flow rate and

steam flowrate.


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Terminologies

General Type of Control Configuration

FeedBack Control ConfigurationUse the direct measurements of the controlled variable to

adjust the manipulated variables. The control action is taken

after the disturbances effect the controlled output variables.


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Control Configuration Inferential Control Configuration: Uses secondary

measurements ( because the controlled variables can notbe measured ) to adjust the manipulated. The controlobjective is to keep the un measured controlled variable atdesired levels.


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Design Elements of a Control System

Design the Controller: In every configuration,

the controller Is the active element that

receive information from the measurements

and takes appropriate control actions toadjust the value of manipulated variables. It

implements the control law automatically.


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Summarize: Design Elements of a

Control System

1. Define Control Objective ( central element)

2. Select measurements

a. Measuring and monitoring the process variables

b. Primary Measurements

c. Secondary Measurements

d. Unmeasured output = f ( secondary

measurements)

3. Select Manipulate Variables

4. Select Control Configuration

5. Design the Controller


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Hardware Elements for a Process

Control

In Control configuration , there aremainly two categories of hard wareelements

The chemical process. It represents thematerial equipment together with physicaland chemical operation occur there

The measuring instruments or sensors:Instruments used to measure thedisturbances, the controlled outputvariable, secondary output variables andare the main source of information.


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Examples of sensors

Thermocouples or resistance thermometers

(T)

Venturi meters (F)

Differential pressure cell (V)

Gas chromatographys ( Compositions)


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Control

Trasducers: They convert one type of

signals into another type of signals .

Examples In strain guage, metallicconductors are present whose electric

resistance changes when they are

subjected to mechanical stress ( appliedpressure) . Thus they convert pressure

signal into electrical signals.


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Control

Transmission lines : These lines carry information(measured signals) from the measuring device to

controller and from controller to the final control

elements. Examples : Electric transmission linesand pneumatic transmission lines.

Controller : Intelligence hardware that receive

information from the measuring devices and

decides what action should be taken.


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Control

Final Control Elements: It implements thedecision taken by the controller. Example is

Control Valve

Relay and switches on- off system Variable speed pumps

Variable speed Compressors

Recording Elements : They visualdemonstrate the dynamic state of chemicalprocess ( chemical variables).


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Sensors for Measurement and

Control


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The Measurement of Temperature

Fundamental understanding of Temperature

Zeroth Law of thermodynamics

When two bodies are in thermal contact witha third body, they are in thermal equilibrium

with each other.

ExampleAB and B - C

Then TA= TC= TB


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Mathematically saying

If temperature is considered linear

function of thermoelectric property (X)

then,

T= aTX+bT Eq1

The value of aTand bTconstants can be

determined by the numbers assigned to the

fixed points, In Celsius scale for ice and

steam case

0= aTX1+ bT .. Eq 2

100 = aTX2+ bT .. Eq3


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Where X1and X2are the values of thermoelectricproperty at the ice and steam points.

By computing equation 1,2 and 3 we haveT(C) = 100 [( XX1)/(X2-X1)]

The above equation can be modified byemploying different properties or substances

For example For Liquid in glass thermometer

T(C) = 100 [( ll1)/(l2-l1)]

Where l is the length of the column of liquid attemperature T .

For constant Volume gas thermometer

T(C) = 100 [( PP1)/(P2-P1)]

where P is the pressure at temperature T


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Type of Temperature Sensors

Thermoelectric Sensors

Thermocouple

Electrical resistance Detector or Resistance

Thermometer

Thermal Radiation Detection

Broadband Radiation Thermometer

Narrowband Radiation Thermometer

Chopped Radiation Thermometers

Optical Pyrometers


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Thermocouple In 1821, Seebeck, an Estonian-German physicist

discovered that when two dissimilar metals areconnected, as shown in Figure 1(a), and one of the

junctions is heated, there is a continuous flow of current

through the loop.


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When the loop is broken and the voltage ismeasured (Figure 1(b)), the measured voltage is

directly related to the temperature differencebetween the two junctions.

This phenomenon where a voltage is producedbecause of the heating of the junction of two

metallic conductors is called thermoelectriceffect or Seebeck effect.

The junction where heat is applied is called thehot or measurement junction. The other

junction is called the cold junction or referencejunction, and the voltage developed is calledthermo-electromotive force (emf).


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The voltage produced is given as follows

eAB= aT

Where:eAB= Seebeck voltage (Emf)

T = temperature at the thermocouple junction

a = Seebeck coefficient= a small change in voltage corresponding to a

small change in temperature

The change in material EMF with respect to achange in temperature is called the Seebeck

coefficient or thermoelectric sensitivity. This

coefficient is usually a nonlinear function of

tem erature.


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Component of thermocouple
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Types of thermocouples


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Advantages of Thermocouple

They are inexpensive.

They are rugged and reliable.

They can be used over a wide temperature range.


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Next Lecture

Sensors

Mathematical Modeling the dynamicand static Behavior of Chemical Process

Introduction to process control 2

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