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Integration of Design & Control
CHEN 4470 Process Design Practice
Dr. Mario Richard EdenDepartment of Chemical Engineering
Auburn University
Lecture No. 19 Integration of Design and Control I
March 27, 2012
Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
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Outline
Part I: This Lecture
Provide motivation for handling flowsheetcontrollability and resiliency as an integral part ofthe design process
Outline qualitative approach for control variableselection and unit by unit control structuring
Part II Next Lecture
Introduce a qualitative plantwide control structureselection method
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Importance of Early-Stage Decision
Motivation 1:2
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Process Design Stages and Tools
Motivation 2:2
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Process Objectives
Classification of Variables
Degree of Freedom (DOF) Analysis
Unit by Unit Control
Basic Control Concepts 1:7
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Process Objectives
The design of a control system for a chemical plant isguided by the objective to maximize profits bytransforming raw materials into useful products whilesatisfying:
Product specifications: quality, rate.
Safety
Operational constraints
Environmental regulations - on air and water quality as well
as waste disposal.
Basic Control Concepts 2:7
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Classification of Variables
Variables that effect and are affected by the processshould be categorized as either control (manipulated)variables, disturbances and outputs.
It is usually not possible to control all outputs (why?)
Thus, once the number of manipulated variables aredefined, one selects which of the outputs should be
controlled variables.
Basic Control Concepts 3:7
Process Outputs
Manipulatedvariables
Disturbances
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Selection of Controlled Variables
Rule 1: Select variables that are not self-regulating.
Rule 2: Select output variables that would exceed theequipment and operating constraints without control.
Rule 3: Select output variables that are a directmeasure of the product quality or that strongly affect it.
Rule 4: Choose output variables that seriously interact
with other controlled variables.
Rule 5: Choose output variables that have favorablestatic and dynamic responses to the available controlvariables.
Basic Control Concepts 4:7
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Selection of Manipulated Variables
Rule 6: Select inputs that significantly affect thecontrolled variables.
Rule 7: Select inputs that rapidly affect the controlled
variables.
Rule 8: The manipulated variables should affect thecontrolled variables directly rather than indirectly.
Rule 9: Avoid recycling disturbances.
Basic Control Concepts 5:7
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Selection of Measured Variables
Rule 10: Reliable, accurate measurements areessential for good control.
Rule 11: Select measurement points that are
sufficiently sensitive.
Rule 12: Select measurement points that minimizetime delays and time constants.
Basic Control Concepts 6:7
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Degree of Freedom Analysis
Before selecting the controlled and manipulatedvariables, one must determine the number of variablespermissible. The number of manipulated variablescannot exceed the degrees of freedom, which are
determined using a process model according to:
Basic Control Concepts 7:7
ND = NVariables - NEquations
ND = Nmanipulated + NExternallyDefined
Degrees offreedom Number ofvariables Number ofequations
NManipulated = NVariables - Nexternally defined- NEquations
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Classification of Variables ND = NVariables = 10
Externally defined (disturbances):CAi,
Ti,Tco
Example 1: CSTR Control 1:6
T, CA
Ti, CAi
c
T, CA
Tc
hTco
o
i
h, T, CA, CAi, Ti,
Fi, Fo, Fc, Tc, Tco
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Balance Equations
Example 1: CSTR Control 2:6
T, CA
Ti, CAi
Fc
T, CA
Tc
hTco
Fo
Fi
oi FFdtdhA
TCrAhCFCFhCdtdA AAoAiiA ,
csA
oii
TTUAHTCrAh
CpTFCpTFThdt
dCpA
,
csccccoccc
cc TTUATCpFTCpFdt
dTCpV
Overall Mass Balance
Component Balance
Energy Balance Mixture
Energy Balance Coolant
NEquations = 4
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Degrees of Freedom NManipulated = NVariables NExt. Defined NEquations NManipulated = 10 3 4
NManipulated
= 3
Example 1: CSTR Control 3:6
T, CA
Ti, CAi
c
T, CA
Tc
hTco
o
i
Thus we need to select 3variables to be controlled.Each must be controlled by
varying one or more of theremaining variables
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Selection of Controlled Variables
CA should be selected since it directly affects theproduct quality (Rule 3).
T should be selected because it must be regulatedproperly to avoid safety problems (Rule 2) and becauseit interacts with CA (Rule 4).
h must be selected as a controlled output because it is
non-self-regulating (Rule 1).
Example 1: CSTR Control 4:6
T, CA
Ti, CAi
Fc
T, CA
Tc
hTco
Fo
Fi
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Selection of Manipulated Variables
Fishould be selected as it directly and rapidly affects CA(Rules 6, 7 and 8).
Fc should be selected as it directly and rapidly affects T(Rules 6, 7 and 8).
Fo should be selected as it directly and rapidly affects h(Rules 6, 7 and 8).
Example 1: CSTR Control 5:6
T, CA
Ti, CAi
Fc
T, CA
Tc
hTco
Fo
Fi
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Suggested Control Structure
Example 1: CSTR Control 6:6
T, CA
Ti, CAi
c
T, CA
Tc
hTco
o
i
TC
CCLC
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Summary
Part I: This Lecture
Provide motivation for handling flowsheetcontrollability and resiliency as an integral part ofthe design process
Outline qualitative approach for control variableselection
Part II Next Lecture
Introduce a qualitative plantwide control structureselection method
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Next Lecture March 29
Integration of design and control Part II (Plantwide)
SSLW 322-340
Milliken Plant Visit March 29
Directions and official excuse placed on website
Leave from Ross Hall at 1:00 PM
No open shoes/sandals/shorts/sleeveless shirts
Limited jewelry 3:00 EST Arrive at plant
3:15 EST Introductions / Safety Review
3:45 EST High level plant tour
4:30 EST Discussion
5:30 EST Wrap up with dinner
Other Business