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8/9/2019 KENDALI_CH1

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Process Control

Spring, 1999

Professor: In-Beum LeeDepartment of Chemical Engineering


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Text and References Text

Su !han Sung an" In-Beum Lee, #PID Controllers an"$utomatic %uning& $'in Press, 199(

References

Se)org D* E*, %* +* E"gar, an" D* $* ellichamp, #Process

Dnamics an" Control,&.ohn /ile 0 Sons, e! 2or3, 19(9 Stephanopoulos 4*, #Chemical Process Control-$n

Intro"uction to %heor an" Practice,&Prentice -5all, e!

.erse, 19(6*

Lu)en /* L*, #Process o"eling, Simulation an" Control

for Chemical Engineers,& c4ra!-5ill, e! 2or3, 7n" E"*,

1998


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Contents of the Lecture

1* Intro"uction to Process Control*7* athematical o"eling of Chemical Processes*

* Laplace %ransforms*

6* %he %ransfer +unction*

* Dnamic Beha;ior of the Processes*


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1. Introduction to Process Control

>Process Control? ma3es processes satisf follo!ingre@uirements*

Safet

Pro"uction specifications

En;ironmental regulations

Aperational constraints

Economics

Mathematical modelof the process shoul" )e

3no!nProcess o"eling 0 I"entification*


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1.1 Illustrative Examples

EX 1. Continuous stirred-tan heater

!uestion $ssume that inlet temperature changes !ith

time* 5o! can !e ensure that % remains at or near the set

point %EF

+igure 1*1* Continuous stirre"-tan3 heater*


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Possi"le #trate$ies

1* easure % an" a"'ust G *

7* easure %ian" a"'ust G*

* easure % an" a"'ust !*

6* easure %ian" a"'ust !*

* easure % an" %ian" a"'ust G*


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EX '. %urnace usin$ electrical po&er

!uestion Can ou fin" an pro)lem in controlling thefollo!ing furnace processF

SC unit J

-

ut

tthermocouple

po!erJ

-

!ell insulate"

SC unit input

temperature

heating coil

+igure 1*7* +urnace using electrical po!er


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EX '. %urnace usin$ electrical po&er

!uestion Can ou fin" an pro)lem in controlling thefollo!ing furnace processF

%here is no !a to decrease the temperatureK

%he po!er cannot )e negati;e*

%he heat loss is nearl ero*

SC unit Jt r c u

-

ut

tthermocouple

po!erJ

t r c u

-

!ell insulate"

SC unit input

temperature

heating coil

+igure 1*7* +urnace using electrical po!er


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1.' Classification of the varia"les

Input varia"les

"enote the effect of the surroun"ings on the chemicalprocess*

1* anipulate" ;aria)le M( or Control ;aria)le

Its ;alue can )e a"'uste" freel ) the human operator

or a control mechanism*EH In heate" tan3, the amount of heat a""e"G or mass

flo! rate!*

7* Distur)ance ;aria)le)(

Its ;alue is not the result of the a"'ustment ) anoperator or a control sstem*

EH In heate" tan3, inlet temperature, )ecause !e can?t

usuall control the temperature of inlet !ater*


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*utput varia"les "enote the effect of the process oh the surroun"ings*

1* easure" output ;aria)le or Controlle" ;aria)leC(

Its ;alue is 3no!n ) "irectl measuring it*

EH In heate" tan3, outlet temperature*

7* nmeasure" output ;aria)les

It is not or cannot )e measure" "irectl*


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1.+ Classification of Control #trate$ies

%eedfo&ard Control %he "istur)ance ;aria)le is measure" an" the measurement

is use" to manipulate M*

,dvanta$es

If all sources of the "istur)ances are 3no!n an" these ;aluescan )e measure" accuratel*Perfect Control

)isadvanta$es

o correcti;e action for unmeasure" "istur)ances*

In industrial applications feedfo&ard control should "eused in com"ination &ith feed"ac control.


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%eed"ac Control

%he process ;aria)le to )e controlle" is measure" an"

use" to a"'ust another process ;aria)le !hich can )emanipulate"*

,dvanta$e

Correcti;e action is ta3en regar"less of the source of the

"istur)ance*

)isadvanta$e

%he controlle" ;aria)le must )e "e;iate from the set point

)efore correcti;e action is ta3en* /otPerfect Control

Classification

/e$ative feed"ac %he "esira)le situation !here the correcti;eaction ta3en ) the controller ten"s to mo;e the controlle" ;aria)le

to!ar" set point*

Positive feed"ac %he controller ten"s to ma3e things !orse )

forcing the controlle" ;aria)le farther a!a from the set point*


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1.0 loc )ia$ram

#chematic dia$ram Phsical connection )et!een the components of the control

sstem*

+igure 1** Schematic "iagram of a temperature fee")ac3 control sstem

for a stirre"-tan3 heater* ---, Electrical instrument line %%, temperature

transmitter%C, temperature controller*


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loc dia$ram

+lo! of information !ithin the control sstem*

Each )loc3 represents a dynamicor staticprocess

elements* )2namic elements ;aria)le !hich "epen"s on time*

#tatic elements ;aria)le !hich is in"epen"ent of time*

+igure 1*6* Bloc3 "iagram for temperature fee")ac3 control sstem in +igure *


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1.3 Control and Modelin$ Philosophies

T&o approaches to desi$n control s2stem.1* %ra"itional $pproach

Control strateg an" sstem har"!are are selecte" )ase" on

the 3no!le"ge of process, eHperience an" insight* $fter the

control sstem is installe" in the plant, the controllersettings are a"'uste", that is, the controller is tuned*

7* o"el-Base" $pproach

$ process mo"el is "e;elope" an" )ase" on the mo"el,

suita)le control strateg an" sstem har"!are are selecte"*

%he mo"el-)ase" approach is more a";antageous*


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4sa$e of the Process Model

It can )e use" as the )asis for classical controller "esign

metho"s*

It can )e incorporate" "irectl in the control la!, anapproach that no! is the starting point for man

a";ance" control techni@ues*

It can )e use" to "e;elop a computer simulation of the

process to allo! eHploration of alternati;e control

strategies an" to calculate preliminar ;alues of

controller setting*


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1.5 ,nalo$ and )i$ital Control

,nalo$ controller

Controller that has continuous input an" output signals*

)i$ital controller

Controller !hich in;ol;es input input an" output signals that

change onl at "iscrete instants in time, the so-calle"

sampling instants*

erits increase" fleHi)ilit an" accurac, an" impro;e"monitoring of the plant through "ata ac@uisition, storage,

an" analsis*


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1.6 Economic 7ustification of Process Control 7ustification "ased on

Safe operation Satisfing en;ironmental constraints

Economic )enefiteH*increase" pro"uction le;el, re"uce"

ra! material costs or enhance" pro"uction @ualit

EHten"e" e@uipment lifeEHample

+igure 1** Pro"uction ;ali"it o;er time: a )efore impro;e" control ) after*

%he operating ;aria)le is N ethane*

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