Presentation on process dynamics and control

Presentation on process dynamics and control.

1. CHEN502
PROCESS DYNAMICS AND CONTROL
Module 2:
Design and Hardware Aspects of Process
Control System
Prof. Momoh Omuya Raheem
Room 55, PTDF Building
Email: [email protected]
June, 2024

2. Learning Outcome
It is the intent of this Module to help the student to:
1.Classify the variables in chemical processes.
2.Identify the design elements of a control system.
3.Develop control aspects of a complete chemical
plant.
4.Highlight control system development.
5.Discuss hardware for a process control system.
2

3. 1.CLASSIFICATIONOFTHEVARIABLESIN
ACHEMICALPROCESS
•Inputvariablesdenotetheeffectof
surroundingonthechemicalprocess.
•Outputvariablesdenotetheeffectofthe
processonthesurrounding.

4. Example 1
ForaCSTRreactor
•Input Variables: C
Ai, T
i, F
i, Tc
i, Fc
i, (F)
•Output Variables: C
A, T, F, T
Co, V
F
C, T
Ci
C
A, T, F
F
Co, T
Co
Product
F
i,C
Ai,T
i
A B Exothermic

5. Example 2
•For the tank heater
•Input Variables: F
i, T
i,F
st, (F)
•Output Variables: F, V, T
F, T
F
i,T
i
Q
T
h
Steam
Condensate

6. The input variables can be further classified into the
following categories:
a)Manipulated (or Adjustable) variables especially
if their values can be adjusted by an operator or
control mechanism.
b)Disturbance if their values are not resultant
effect of an operator or a control mechanism.
The output variables similarly can be further
divided into the followings:
a)Measured output variables especially if their
values are known by directly measuring them.
b)Unmeasured output variables if they are not or
cannot be measured directly.

7. Fig. 2.1: Input & Output Variables around a
Chemical Process System
Externaldisturbances
Processing
System
Unmeasured Output, (Z)
Measured
Output, (y)
Manipulated
Variables (m)
Measured (d)
Unmeasured (d’)
….
…… ……
……
……
…………

8. 2.DESIGNELEMENTSOFACONTROLSYSTEM
(A) Define Control Objectives
Question 1
Whataretheoperationalobjectivesthata
controlsystemiscalledupontoachieve?
1.Ensuringthestabilityofasystem,
2.Suppressingtheinfluenceofexternal
disturbance,
3.Optimizingtheeconomicperformanceofa
plant,
4.Acombinationoftheabove.
Atthebeginningthecontrolobjectivesare
definedqualitatively,subsequently,theyare
quantifiedintermsoftheoutputvariables.

9. 1.ForCSTRdiscussedinExample1,thecontrolobjective
(qualitativelydefined)istoensurethestabilityofunstable
steadystate.Aquantitativecontrolobjectiverequiresthat
thetemperature(anoutputvariable)notdeviatemorethan
5%fromitsnominalvalueattheunstablesteadystate.
2.For the stirred tank heater (suppressing external
disturbances)
Qualitatively:TandV(h)shouldbedesiredvalue
Quantitatively;
T = T
s
V = V
sWhere Ts and Vs are given and they are desired values
3.FortheBatchreactor(optimizingtheeconomicperformance)
Qualitatively: maximizing profit

10. Quantitatively;
It requires the solution of maximization of profit
which yield the value of the steam flow rate, Q(t);
at each instant during the reaction period.
(B) Select Measurement
Whatever our control objective is, we need some
means to monitor the performance of the chemical
process. This is done by measuring the values of
some certain process variables (Temperature,
Pressure, Flow rate etc).
Question 2
What variables should we measure in order to
monitor the operational performance of the plant?

11. Answer:primary measurements are easy but a secondary
measurement gives mathematical relationship between
unmeasured outputs.
For the Stirred Tank
Simple measurement
T = T
sand V = V
s
Difficult Measurements
A mathematical expression relating the two
Unmeasured Output = f(secondary measurements)
Example
Considerasimpledistillationcolumninthedistillate
compositionisset95%pentaneagainst5%hexane.
Feedback control using analysis
Feedforwardcontrol using analysis
Therefore, use a secondary measurement (T)
Analyzer can be
unreliable or very
costly

12. (C) Select the Control Configuration
Question 3
What is the best control configuration for a
given chemical process situation?

13. (i) Feed back Control Configuration
Fig. 2.2: General Structure of Feedback Control Configuration
Process
Controller
Disturbance
Unmeasured Output
Set Point (desired variable)
Measured Output
(controlled variables)
Manipulated
Variables

14. (ii) Inferential Control Configuration
Fig. 2.2: General Structure of Inferential Controlled Configuration
Process
Disturbance
Unmeasured Output
Controller
Estimator for
unmeasured variable
Set points
Measured
Outputs
Manipulated
Variables

15. (iii) Feed forward Control Configuration
Fig. 2.3 General Structure of Feedforward
Control Configuration
Process
Measured
Output
Unmeasured Output
Controller
Manipulated
Variables
External Disturbance

16. (D) Design the Controller
In every control configuration the controller is the
active element that receives the information from the
measurements and takes appropriate control action(s) to
adjust the values of the manipulated variables. For the
design of a controller one must answer the next question.
Question 4
How is the information taken from the measurements
used to adjust the values of the manipulated variables?
Answer
Use the control law.
E.g. For the Stirred tank heater
At steady state,
The energy balance of the heater
0 = FρCp(T
i,s–T
s) + Q
s-------------------------------------------(1)

17. Diagrammatically
HowTchangeswithtimewillbegivenbythe
transientenergybalancearoundthetank;thatis
Subtracting eqn1 from eqn2, we have
Note that, since Ts = constant
Time
T
s
T
i,s
T
idT/dt=))/dtT-(d(T s (3) --- Qs)-(Q +Ts)]-(T -s)Ti,-Cp[(TiF=Ts))/dt-Cp(d(TV  (2) ---- Q +T)-Cp(TiF=d(T)/dt CpV 

18. The difference, ϵ = T –T
sdenotes the error or
deviation of liquid, temperature from the
desired value Ti. We want to drive this error to
zero by manipulating appropriately the value of
heat input, Q. The simplest control law is to
require that Q changes proportionally to the
error T –T
s, hence
Q = -α(T –T
s) + Q
s----------------------(4)
This law is known as proportional control and
parameter α is called proportional gain.
Substituting eqn4 into eqn3, we have

19. VρCpd(T–T
s)/dt=FρCp[(T
i–T
i,s)–(T–T
s)]–α(T–T
s)--------(5)
Eqn5issolvedfor(T–T
s),andforvariousvalues
ofgainandyieldthesolutionsshowninFig..
below.It’snoticedthatmoreofthesolutionsis
satisfactorysinceT–T
s≠0.Thusoneconcludes
thattheproportionalcontrollawisnot
acceptable.

20. Fig. 2.4: Temperature response under
proportional feedback control
Error
Time
(T –T
s)
No Control
α= 0
α= 1
α= 2

21. However,considerableimprovementinthe
qualityoftheresultingcontrolcanbeobtained
ifoneusesadifferentcontrollawknownas
integralcontrol.InthiscaseQisproportionalto
thetimeintegralof(T–T
s);thus,
SubstitutingagainQfromeqn6intoeqn3,we
have(6) ---------Qs+Ts)dt-(T '
t
0
Q )7(Ts)dt-(T '-] Ts)-(T -s)Ti,-Cp[(TiF=Ts))/dt-(d(T CpV
t
0



22. The solution of eqn7 for various values of the
parameters α
’
is shown in Fig… below
Time
Error
(T –Ts)
α
’
= 1
α
’
= 2
α
’
= 3
α
’
= 0
No Control
Fig. 2.5: Temperature response under Integal feedback control

23. Fromabove,wenoticethatintegralcontrolis
acceptablesinceitdrivestheerror,T–Tsto
zero.However,wealsonoticedthatdepending
onthevaluesofα
’
,theerrorT–Tsreturnsto
fasterorslower,oscillatesforlongerorshorter
time,andsoon.Inotherwords,thequalityof
controldependsonthevalueofα
’
.
Combiningtheproportionalwiththeintegral
action,wetakeanewcontrollawknownas
proportionalintegralcontrol.Accordingtothis
law,thevalueofheatinput,Qisgivenby,

24. Weshallseethisandothertypesofcontrol
laws,somewherelater.However,itshouldbe
rememberedthattheselectionofappropriate
controllawisanimportantquestiontobe
answeredbythechemicalengineercontrol
design. Qs Ts)dt-(T '-Ts)-(T-=Q
t
0



25. 3.0 Control Aspect Of a Complete Chemical Plant
The examples that we have been discussing in the
previous sections were concerned with the control
of single units such as CSTR, a tank heater, and a
batch reactor. It should be emphasized that rarely if
ever a chemical process composed of one unit only.
On the contrary, a chemical process is composed of
a large number of units (reactors, separators, heat
exchangers, tanks, pumps, etc.) which are inter-
connected with each other through the flow of
materials and energy. For such a process the
problem of designing a control system is not simple
but requires experience and good chemical
engineering background. Consider this simple
example.

26. Fig. 2.6: A simple Chemical Plant
Cooling Water
Steam
Steam
Condensate
Jacket
CSTR
Distillation
column
C
Fp
F
N
A+B
A+B+C
A+B C
B, F
B,T
B
A, F
A,T
A
Endothermic
rxn

27. The operational objectivesfor this simple plant
are
1.Product Specifications:
a)To keep the flow rate of the desired product
stream F
Pat the desired level.
b)Keep the required purity of C in the product
stream
2.Operational Constraints:
a)Do not overflow the CSTR.
b)Do not flood the distillation column or let it dry
3.Economic Considerations:
Maximizing the profit = minimizing the cost of
production through operating costs such as cost
of raw materials, utilities etc

28. The disturbancethat will affect the foregoing objectives
are:
a)Theflowrates,compositions,andtemperatureofthe
streamsofthetworawmaterials.
b)The pressure in the distillation column.
c)The temperature of the coolant used in the condenser
of the distillation column. (For example, if the coolant
is water. It will have a different temperature during
the day than during the night)
At first glance, the problem of designing a control
system even for this simple plant looks very complex.
Indeed it is. The basically new feature for the control
design of such a system is the interaction between the
units (reactor, column).

29. Theoutputofthereactorsaffectstheoperation
ofthecolumninaprofoundmannerandthe
overheadproductofthecolumninfluencesthe
conversionintheCSTR.Thistighteninteraction
betweenthetwounitsseriouslycomplicatesthe
designofthecontrolsystemfortheoverallprocess.
Supposethatwewanttocontrolthe
compositionofthebottomsproductby
manipulatingthesteaminthereboilerofthe
column.Thiscontrolactionwillaffectthe
compositionoftheoverheadproduct(A+B),which
willinturnaffectthereactionconversioninthe
CSTR.

30. Ontheotherhand,tokeeptheconversionin
theCSTRconstantatthedesignlevel,wetryto
keeptheratioF
A/F
B=constantandthe
temperatureTintheCSTRconstant.Any
changesinF
A/F
BorTwillaffecttheconversion
inthereactorandthusthecompositionofthe
columnwillaffectthepurityofthetwoproduct
streams.
Thecontrolofintegratedprocessesisthe
basicobjectiveforachemicalengineer.Oneto
itscomplexity,though,wewillstartbyanalyzing
thecontrolproblemsforsingleUnitand
eventuallywewilltrytheintegratedprocesses.

31. 4.0 Control System Development

35. 5.0 Hardware for a Process Control System
5.1HardwareElementsofaControlSystem
Ineverycontrolconfiguration,wecandistinguishthe
followinghardwaresystem:
1.Thechemicalprocess:Itmaybeunitoperationequipment,
reactors,mechanical,etc.,
2.Themeasuringinstrumentsorsensors:Examplesare:
thermocouple,venturemeters,gasorliquid
chromatographs,etc.
3.Transducers:Theseareelementsthatcanconvertphysical
measurementsintosignalsforeasyinterpretation,e.g.,
pneumaticorelectricalsignals.
4.Transmissionlines:Theyareeitherpneumaticbutcurrently
deploymentofelectricsignals.

36. 5.Thecontroller:Thisisthehardwareelementthatpossessthe
“intelligence”toreceiveinformationfromthemeasuring
devicesanddecidesappropriateactiontobetaken.Itmay
employsimplecontrollaw,like,“Proportional(P),
Proportional-Integral(PI),Proportional-Integral-Derivative
(PID),Computer,orcomplexcontrollawlike“Artificial
intelligence(AI)”.
6.Thefinalcontrolelements:Thesearetheelementsthat
implementthedecisiontakenbythecontroller.Examples
are:relayswitchesforon-offcontrol,variablespeedpumps,
etc.,
7.Recorders:Theyarenormallyemployedinprovidingthe
visualdemonstrationofhowchemicalprocessbehavesin-
situoratthecontrolroom.Currently,theuseofcomputer
consolelikeVisualDisplayUnit(VDU)hasbecomevery
popularinmonitoringtheperformanceofchemical
processes.

37. Fig. 2.7: Hardware Elements for the Feedback Control of STH

38. 5.2EmploymentofDigitalComputersinProcessControl
Therapidtechnologicaldevelopmentofdigitalcomputers
sincethelastfortyyears,coupledwithsignificantreductionof
theircosts,hashadsignificanteffectonhowchemicalplantscan
becontrolled.Thecommonestexamplesarediscussedasfollow:
1.Directdigitalcontrol(DDC):Refertomypreviousdiscussions.
Fig. 2.8: Typical DDC Configuration

39. 2.Supervisorycomputercontrol:Oneoftheincentivesof
processcontrolistheoptimizationoftheplant’seconomic
performance.Thesupervisorycomputercontrolcan
coordinatetheactivitiesofDDC,analysethesituationand
suggestthebestpolicy.
Fig. 2.9: Structure of Supervisory Control Configuration

40. 3.Schedulingcomputercontrol:Inthiscase,thecomputercan
beusedtoscheduletheoperationofaplantdependingon
theconditionsinthemarket(demand,supply,prices)change
withtimebycuttingproductiontoavoidoverstocking,
increasingproduction,newproductionline,etc.

41. ASK YOUR QUESTIONS
IF ANY !!!

42. Thank U all for listening