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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
In this Lesson, we will learn
Importance of process & equipment design
- Review effect of process dynamics on control performance
- Special concerns with recycle processes- Special considerations for capacity of manipulated variables
- Mixing process, series packed bed reactor, recycle
reactor, batch reactor
Control Design Guidelines
- Nine-step design method
- Flash process
PROCESS OPERABILITY:
DYNAMIC PERFORMANCE
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DYNAMIC PERFORMANCE
OPERABILITY provides the capacity and flexibility
to respond to changes in plant operation.
In general, we wish to respond quickly.
Compensate for (reject) disturbances
Followed changes set points
The value of quickly depends upon the specific
process application.
Safety and Equipment protection - extremely fast (& reliable) Product quality - very fast
Production rate - moderate (very fast if load following)
Efficiency and Optimization - can be slower
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Which elements in the process design should be fast,
and which should be slow?
Process - Dynamics
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DYNAMIC PERFORMANCE
Lets recall that the process dynamics introduce the
fundamental limit to feedback control performance.
Dead time in
feedback,
Time constant in
feedback, P
Time constant indisturbance, D
AC
Steady-state Gain, KP
How does each parameter affect the dynamic performance
for this simple mixing process?
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Process - Dynamics
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DYNAMIC PERFORMANCE
Lets recall that the process dynamics introduce the
fundamental limit to feedback control performance.
0 5 10 15 20 25 30 35 40 45 500
0.5
1
1.5S-LOOP plots deviation variables (IAE = 9.6759)
Time
ControlledV
ariable
0 5 10 15 20 25 30 35 40 45 500
0.5
1
1.5
Time
ManipulatedVariable
CV limited set point overshoot, fast damping,
and return to the set point
CV does not change because of dead
time
Set point
response
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Process - Dynamics
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DYNAMIC PERFORMANCE
Lets recall that the process dynamics introduce thefundamental limit to feedback control performance.
Disturbance
response
, deadtime
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Can not prevent
this deviation with
feedback
Effect of disturbance
without control
Process - Dynamics
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DYNAMIC PERFORMANCE
Lets recall that the process dynamics introduce the
fundamental limit to feedback control performance.
Dead time in
feedback,
Time constant in
feedback, P
Time constant in
disturbance, DA
Steady-state
Gain, KP
How does each parameter affect feedback performance?
Large is goodSmall is good
Small is good
Large is good
Large (range),fast element
is good
Fast sensor
is good
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Process - Dynamics
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DYNAMIC PERFORMANCE
CLASS WORKSHOP: The feedback controller is
performing well (its well tuned), but the dynamic
performance is poor. What improvements can we
make?
AC
1
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Hint: Think about the
structure of
(i) the process and
(ii) the control system
Control a chemical reactor
by adjusting its preheat
Disturbances
in feed
Process - Dynamics
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DYNAMIC PERFORMANCE
CLASS WORKSHOP: The feedback controller is performing well but the
dynamic performance is poor. We must change the system structure!
AC
1
AC+
Feedforward
TC
CascadeAC
Faster feedback
LC
Slower
disturbances
Reduced
disturbances
Shorten pipe, faster
feedback
Process - Dynamics
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Recycle processes: In some (many), specific components do not appear in
exit streams. If no action is taken, these components will accumulate.What is needed in the process design?
Recycle loop
Purge
Process - Recycle
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
In some (many) recycle processes, specific components do not appear in
exit streams. If no action is taken, these components will accumulate.What is needed in the process design?
Recycle loop
Purge
Issues with purges
Some valuable materials will be lost in purge (recover?)
May introduce stream that requires treatment before release (environment)
The purge flow can be adjusted to control the concentration of inert
Large inert concentrations might be desired (e.g., to affect reactions)
Process - Recycle
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DYNAMIC PERFORMANCE
FC
1
Cold
feed
Hot effluent
Cold productIs the
design
complete?
Heating
fluid
Exothermic
chemical
reaction
Class Workshop Recycle Processes: How will this recycle
process respond to disturbances? Can the design beimproved?
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Consider a
feed
temperature
increase
Process - Recycle
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DYNAMIC PERFORMANCE
FC
1
Cold
feed
Hot effluent
Cold product
Heating
fluid
Exothermic
chemical
reaction
++ +
++
++
++ ++
++++
++++
The disturbance
grows because
of the recycle
(positive
feedback)!
++++
++++
++++++++
++++++++
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Consider a
feed
temperature
increase
Class Workshop Recycle Processes: How will this recycle
process respond to disturbances? Can the design beimproved?
Process - Recycle
P R l
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DYNAMIC PERFORMANCE
FC
1
Cold
feed
Hot effluent
Cold product
Heating
fluid
Exothermic
chemical
reaction
Conceptually
OK.
Is the feedbackfast enough?
TC
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Class Workshop Recycle Processes: How will this recycle
process respond to disturbances? Can the design beimproved?
Process - Recycle
P R l
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DYNAMIC PERFORMANCE
FC
1
Cold
feed
Hot effluent
Cold product
Heating
fluid
Exothermic
chemical
reaction
Provide fastfeedback!
TC
Build in
efficiency;
use the least
heating
fluid!
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
Class Workshop Recycle Processes: How will this recycle
process respond to disturbances? Can the design beimproved?
Process - Recycle
P R l
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
General principle: When possible, provide an alternative source of
material or energy at the recycle point and control the variable(flow or temperature) after the recycle point.
Process
TC
Adjustable heat exchange
Process
FC
Adjustablefresh feed
Energy recycle
Material recycle
Process - Recycle
Process MV range
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
We usually want extra capacity in the effects of the
manipulated variables. Here,extra means more
than required to achieve the desired steady-state
operating window.
F
L
CW
P
fc
fo
fo
fo
TReactor withexothermic
reaction
Lets concentrate on the
control of temperature.
Process MV range
Process MV range
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
We usually want extra capacity in the effects of the
manipulated variables. (1) MV overshoot
F
L
CW
PT
0 10 20 30 40 50 600
0.5
1
1.5
Time
Contro
lledVariable
0 10 20 30 40 50 600
0.5
1
1.5
Time
ManipulatedVariable
p, feedback dead time
For many processes, some overshoot of the manipulated variable
will improve (speed) control performance.
Temperature
CW flow
Process MV range
Process MV range
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
We usually want extra capacity in the effects of the
manipulated variables. (2) Disturbance response
F
L
CW
PT
In a few processes, much greater MV capacity is required forresponse to disturbances.
For example, a highly exothermic reaction operating at too high a
temperature will tend to run away. A very large exchanger area
and CW flow rate could be required to prevent a hazard.
Process MV range
Process MV range
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
We usually want extra capacity in the effects of the
manipulated variables. (2) Disturbance response
In a few processes, much greater MV capacity is required for
response to disturbances, even a source and sink.
For example, the reactor might produce gas at some times and
condense gas at others times. Pressure control requires an
exhaust line (originally shown) and an inert gas feed line (added
for this figure).
F
L
CW
PT
Inert gas
See Marlin (2000) Chapter 22 for control design
fo
fc
Process MV range
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
For Multiloop PID Feedback Control
1. Define Control Objectives
2. Select measurements
3. Ensure sufficient degrees of freedom exist
4. Ensure the process is controllable
5. Ensure that the operating window is sufficiently large
6. Evaluate interaction
7. Eliminate designs with poor integrity
8. Eliminate designs requiring extensive retuning
9. Apply loop pairing guidelines and unit operation
experience
Control Design
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
Process ExampleFlash Process
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
Vapor
product
Liquid
product
Process
fluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
P 1000 kPaT 298 K
Lets review the process
Control Design
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
1. Define the Control ObjectivesTITLE: Flash drum |ORGANIZATION: McMaster Chemical EngineeringPROCESS UNIT: Hamilton chemical plant |DESIGNER: I. M. LearningDRAWING : Figure 24-1 |ORIGINAL DATE: January 1, 1993
|REVISION No. 1____________________________CONTROL OBJECTIVES:1) SAFETY OF PERSONNEL
a) the maximum pressure of 1200 kPa must not be exceeded under any (conceivable)circumstances
2) ENVIRONMENTAL PROTECTIONa) material must not be vented to the atmosphere under any circumstances
3) EQUIPMENT PROTECTION (see also the safety objective)a) the flow through the pump should always be greater than or equal to a minimum
4) SMOOTH, EASY OPERATIONa) the feed flow should have small variabilityb) the unstable level should be maintained within measurement limitsc) unusual operations should be highlighted (high pressure, high or low level)
5) PRODUCT QUALITYa) the steady-state value of the ethane in the liquid product should maintained atits target of 10 mole% for operating condition changes of +20 to -25% feed flow, 5
mole% changes in the ethane and propane in the feed, and -10 to +50 C in the feedtemperature.
b) the ethane in the liquid product should not deviate more than 1 mole % fromits set point during transient responses for the following disturbances
i) the feed temperature experiences a step from 0 to 30 Cii) the feed composition experiences steps of +5 mole% ethane and -5 mole% ofpropaneiii) the feed flow set point changes 5% in a step
6) EFFICIENCY AND OPTIMIZATIONa) the feed flow rate will vary +10% and -20% from the design (nominal) valueb) the heat transferred should be maximized from the process integration exchangerbefore using the more expensive steam utility exchanger
7) MONITORING AND DIAGNOSISa) sensors and displays needed to monitor the normal and upset conditions of theunit must be provided to the plant operator
b) sensors and calculated variables should be provided for longer term monitoringof the product quality and thermal efficiency of the unit
Control Design
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
2. Select measurements
Required to achieve each of the control objectives
Define the sensor technology (e.g., orifice meter,venturi meter, mass flow) compatible with process
conditions and achieve required accuracy and
reproducibility
Determine if for only local display or fortransmission to centralized control for monitoring,
history and control
Define range (e.g., 150-200 C)
Control Design
Control Design
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DYNAMIC PERFORMANCE
CLASS WORKSHOP on CV selection: We want to know the composition of thebottoms product, but the analyzer is too expensive (or doesnt exist). What do we do?
LC
PC
TC
Inferential/Dominant Variables* -
Easily measured and strongly
influence product quality and/or
profit.
In many (not all) distillation towers,
tray temperatures are a good
inference of product composition.
(The profile moves up/down the
column)
Great opportunity to use your
process knowledge!
* See Marlin (2000) Chapter 17 for further discussion on inferential variables
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
LC
Control Design
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
3. Ensure sufficient degrees of freedom
Degrees of freedom are adjustable variables, i.e., valve
openings, motor speeds, etc.
Inputs Outputs
finalelement sensor
final
elementsensor
Adjustable
manipulated
variables
Disturbances
variables
Controlled
variables,
related to
control
objectives
Majority of
variables
remain
unmeasured
(and
uncontrolled)
Process
Control Design
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
3. Ensure sufficient degrees of freedom
Inputs Outputs
finalelement sensor
final
elementsensor
Adjustable
manipulated
variables
Disturbances
variables
Controlled
variables,
related to
control
objectives
Majority of
variables
remain
unmeasured
(and
uncontrolled)
Process
The system is has sufficient degrees of freedom if
# of manipulated variables # of controlled variables
Is that all
that is
required?
g
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
4. Ensure that the process is controllable
Inputs Outputs
final
elementsensor
final
elementsensor
Adjustable
manipulated
variables
Disturbances
variables
Controlled
variables,
related to
controlobjectives
Majority of
variables
remain
unmeasured
(anduncontrolled)
Process
Controllability ensures that the selected controlled variables can be
moved in desired direction by the manipulated variables
The system is controllable if Det [Kp] 0 (Kp is the gain matrix)
Kp
Is that all
that is
required?
g
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
5. Ensure that the operating window is large enough tosatisfy the design specification, which gives
disturbances and set point changes.
-20
0
20
40
60
80
feedtempera
ture(C)
50 70 90 110 130 150 170 190
feed flow
Design
Minimum heating
Maximum
liquid
productvalve
opening
Maximum heating
valve opening
Ensures that the capacities of the manipulated variables are large enough to move the
process as needed.
g
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
6. Evaluate Interaction: Does it exist?
A multivariable process has interaction when input
(manipulated) variables affect more than one output
(controlled) variable.
0 20 40 60 80 100 1200.98
0.982
0.984
0.986
0.988
0.99
Time (min)
XD
(molfrac)
0 20 40 60 80 100 1200.02
0.025
0.03
0.035
0.04
Time (min)
XB
(molfrac)
0 20 40 60 80 100 1201.12
1.125
1.13
1.135x 10
4
Time (min)
R
(mol/min)
0 20 40 60 80 100 1201.5613
1.5613
1.5614
1.5614
1.5615
1.5615x 10
4
Time (min)
V(mol/min)
Step change to reflux with constant reboiler
g
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
7. Eliminate designs with poor integrity
8. Eliminate designs requiring extensive retuning
This topic requires some additional principles from process control.
See Marlin, Process Control, (2000), Chapter 20
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Process Control Design Guidelines
9. Apply loop pairing guidelines
LOOP PAIRING QUALITATIVE GUIDELINES
CVi - MVj pairing that has strong effect (large Kp)
CVi - MVj pairing that has fast dynamics
CVi - MVj pairing that has large range (MV min to max)
CVi - MVj pairing with causal relationship that is (nearly)
independent of other loops, i.e., reduce interactions
If excess MVs exist, adjust MVj that has lowest cost; keephigh cost MV near zero
Often, these guidelines cannot all be satisfied. In some cases,
they must be violated to achieve good performance.
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
Vapor
product
Liquid
product
Process
fluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
P 1000 kPaT 298 K
Class workshop: Design control loops (select CV-MV pairings) for the
flash process.
Control Design
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
DYNAMIC PERFORMANCE
Class workshop: Design control loops (select CV-MV pairings) for the
flash process.
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
Vapor
product
Liquid
productProcess
fluidSteam
FC-1
F2 F3
T1 T2
T3
T5
TC-6 PC-1
LC-1
AC-1L. Key
PAH
LAL
LAH
cascade
See Marlin (2000), Process Control, Chapter 24 for details
Control Design
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DYNAMIC PERFORMANCE
INDUSTRIAL PRACTICE
Good performance through process design is more
reliable and preferred if the cost is acceptable.
We are guided by control objectives (safety,protection, smooth operation, product quality, and
profit)
We seek to provide good control with simple loop
pairing control strategies, if possible.
We use special knowledge for each unit operation
distillation, reactors, boilers, compressors,
evaporators, etc.
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipmentprotection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
8/3/2019 OPerability Dynamics 2007
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Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipment
protection
5. Efficiency &
profitability
6. Operation
duringtransitions
7. Dynamic
Performance
8. Monitoring &
diagnosis
In this Lesson, we will learn
Importance of process & equipment design
- Review effect of process dynamics on control performance
- Special concerns with recycle processes- Special considerations for capacity of manipulated variables
- Mixing process, series packed bed reactor, recycle
reactor, batch reactor
Control Design Guidelines
- Nine-step design method
- Flash process
PROCESS OPERABILITY:
DYNAMIC PERFORMANCE