Integration of Design & Control
CHEN 4470 – Process Design Practice
Dr. Mario Richard Eden
Department of Chemical Engineering
Auburn University
Lecture No. 15 – Integration of Design and Control I
February 28, 2013
Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Outline
Part I: This Lecture
 Provide motivation for handling flowsheet
controllability and resiliency as an integral part of
the design process
 Outline qualitative approach for control variable
selection and unit by unit control structuring
Part II – Next Lecture
 Introduce a qualitative plantwide control structure
selection method
Motivation 1:2
•
Importance of Early-Stage Decision
Motivation 2:2
•
Process Design Stages and Tools
Basic Control Concepts 1:7
•
Process Objectives
•
Classification of Variables
•
Degree of Freedom (DOF) Analysis
•
Unit by Unit Control
Basic Control Concepts 2:7
•
Process Objectives
–
The design of a control system for a chemical plant is
guided by the objective to maximize profits by
transforming raw materials into useful products while
satisfying:
•
•
•
•
Product specifications: quality, rate.
Safety
Operational constraints
Environmental regulations - on air and water quality as well
as waste disposal.
Basic Control Concepts 3:7
•
Classification of Variables
–
Variables that effect and are affected by the process
should be categorized as either control (manipulated)
variables, disturbances and outputs.
Manipulated
variables
Process
Outputs
Disturbances
–
–
It is usually not possible to control all outputs (why?)
Thus, once the number of manipulated variables are
defined, one selects which of the outputs should be
controlled variables.
Basic Control Concepts 4:7
•
Selection of Controlled Variables
–
Rule 1: Select variables that are not self-regulating.
–
Rule 2: Select output variables that would exceed the
equipment and operating constraints without control.
–
Rule 3: Select output variables that are a direct
measure 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 favorable
static and dynamic responses to the available control
variables.
Basic Control Concepts 5:7
•
Selection of Manipulated Variables
–
Rule 6: Select inputs that significantly affect the
controlled variables.
–
Rule 7: Select inputs that rapidly affect the controlled
variables.
–
Rule 8: The manipulated variables should affect the
controlled variables directly rather than indirectly.
–
Rule 9: Avoid recycling disturbances.
Basic Control Concepts 6:7
•
Selection of Measured Variables
–
Rule 10: Reliable, accurate measurements are
essential for good control.
–
Rule 11: Select measurement points that are
sufficiently sensitive.
–
Rule 12: Select measurement points that minimize
time delays and time constants.
Basic Control Concepts 7:7
•
Degree of Freedom Analysis
–
Before selecting the controlled and manipulated
variables, one must determine the number of variables
permissible. The number of manipulated variables
cannot exceed the degrees of freedom, which are
determined using a process model according to:
ND = NVariables - NEquations
Degrees of
freedom
Number of
variables
ND = Nmanipulated + NExternally
Number of
equations
Defined
NManipulated = NVariables - Nexternally defined- NEquations
Example 1: CSTR Control 1:6
Fi
Ti, CAi
Tc
h, T, CA, CAi, Ti,
h
Tco
Fi, Fo, Fc, Tc, Tco
T, CA
Fc
Fo
•
T, CA
Classification of Variables
–
ND = NVariables = 10
–
Externally defined (disturbances): CAi, Ti, Tco
Example 1: CSTR Control 2:6
•
Balance Equations
Overall Mass Balance
Fi
Tc
A dh  Fi  Fo
dt
h
Tco
T, CA
Fc
Fo
Component Balance
Ti, CAi
T, CA
A d hCA   Fi CAi  FoCA  Ah  r C A ,T 
dt
Energy Balance Mixture
Energy Balance Coolant
A  Cp d h T   Fi CpTi  Fo CpT
dt
 Ah  r C A ,T  H   UAs T Tc 
Vc Cpc
dTc
 Fc CpcTco  Fc CpcTc  UAs T Tc 
dt
NEquations = 4
Example 1: CSTR Control 3:6
Fi
Ti, CAi
Tc
Thus we need to select 3
variables to be controlled.
Each must be controlled by
varying one or more of the
remaining variables
h
Tco
T, CA
Fc
Fo
•
T, CA
Degrees of Freedom
–
–
–
NManipulated = NVariables – NExt. Defined – NEquations
NManipulated = 10 – 3 – 4
NManipulated = 3
Example 1: CSTR Control 4:6
Fi
Ti, CAi
Tc
h
Tco
T, CA
Fc
Fo
•
T, CA
Selection of Controlled Variables
–
CA should be selected since it directly affects the
–
T should be selected because it must be regulated
–
h must be selected as a controlled output because it is
product quality (Rule 3).
properly to avoid safety problems (Rule 2) and because
it interacts with CA (Rule 4).
non-self-regulating (Rule 1).
Example 1: CSTR Control 5:6
Fi
Ti, CAi
Tc
h
Tco
T, CA
Fc
Fo
•
T, CA
Selection of Manipulated Variables
–
Fi should be selected as it directly and rapidly affects CA
–
Fc should be selected as it directly and rapidly affects T
–
Fo should be selected as it directly and rapidly affects h
(Rules 6, 7 and 8).
(Rules 6, 7 and 8).
(Rules 6, 7 and 8).
Example 1: CSTR Control 6:6
•
Suggested Control Structure
Fi
TC
Tc
h
Tco
Ti, CAi
LC
CC
T, CA
Fc
Fo
T, CA
Summary
Part I: This Lecture
 Provide motivation for handling flowsheet
controllability and resiliency as an integral part of
the design process
 Outline qualitative approach for control variable
selection
Part II – Next Lecture
 Introduce a qualitative plantwide control structure
selection method
Other Business
•
Evonik Site Visit – March 5
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What to bring?
•
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What not to bring?
•
•
•
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Photo ID (drivers license is preferred)
Cameras (including camera phones)
Cell Phone use is only allowed in buildings
Guns, Drugs or Alcohol (I hope this one is obvious)
What to wear?
•
•
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Long Pants
Closed Shoes (no sandals or flip flops)
Shirts with Sleeves (no tank tops)
Safety Glasses
Other Business
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Evonik Site Visit – March 5 (Cont’d)
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Tentative schedule
Other Business
•
Next Lecture – March 7
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–
•
Integration of design and control – Part II (Plantwide)
SSLW 322-340
Progress Report 2
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Turn in Friday March 8
Remember to fill out the team evaluation forms