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CHAPTER 2 : CONTROL OBJECTIVES
AND BENEFITS
When I complete this chapter, I want to be
able to do the following.
Recognize examples of the seven (7)control objectives in chemical processes
Calculate indicators of variability in a
process variable
Be able to calculate the economic impact
of variability
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Outline of the lesson.
Seven (7) Control Objectives
1. Safety
2. Environmental protection
3. Equipment protection4. Smooth operation
5. Product quality
6. Profit
7. Monitoring and diagnosis
Variability measures
Economic impact of variability
Workshop
CHAPTER 2 : CONTROL OBJECTIVES
AND BENEFITS
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EXAMPLE PROCESS: FLASH SEPARATION
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
Lets discuss
this process
P 1000 kPa
T 298 K
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Give example
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 PC
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
High pressure
in drum is
dangerous
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Give example
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Never release
hydrocarbons
to atmosphere
To flare
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Give example
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
LC
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
No flow could
damage the
pump
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Give example
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
FC
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Always keep
the production
rate smooth
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Give example
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
AC
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Achieve L.Key
by adjusting
the heating
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Give example
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
AC
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Use the least
costly heating
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Give example
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
Calculate & plot
key parameters,
e.g., UA.
time
UA
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Feed
Vapor
product
Liquid
productProcessfluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis
SEVEN CONTROL OBJECTIVES
All seven must be achieved. Failure to do so
will lead to operation that is
unprofitable or worse, unsafe.
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BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the variability of
key variables to achieve the seven objectives.
Without feedback control
0 50 100 150 200 250 300 350 400 450 5002
3
4
5
6
time (min)
outletconcen
tration
0 50 100 150 200 250 300 350 400 450 50049
49.5
50
50.5
51
time (min)
valveposition(%o
pe
n)
Composition (% H. Key)
Reflux valve
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BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the variability ofkey variables to achieve the seven objectives.
With feedback control
0 50 100 150 200 250 300 350 400 450 5002.5
3
3.5
time (min)
outletconcentration
0 50 100 150 200 250 300 350 400 450 50020
40
60
80
100
time (min)
valveposition
(%o
pen)
Composition (% H. Key)
Reflux valve
Variability is moved from controlled to manipulated variable!
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BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the variability of
key variables to achieve the seven objectives.
What statistics
can we calculate
from this data?
How do we relate
variability toprocess performance?
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BENEFITS FROM PROCESS CONTROL
Calculate the process
performance using the
distribution, not theaverage value of the key
variable!
Process performance =
efficiency, yield, production
rate, etc. It measuresperformance for a control
objective.
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Example of Benefits of reduced
variability for chemical reactor
Goal: Maximize conversion of
feed ethane but do not exceed
864C
Which operation, A or B, is better
and explain why.
A
B
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Example of Benefits of reduced
variability for chemical reactor
Goal: Maximize efficiency and
prevent fuel-rich flue gas
Which operation, A or B, is better
and explain why.
A
B
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CHAPTER 2: GOALS AND BENEFITS WORKSHOP 1
Determine one example for each of the seven control objective categories.
FT
1
FT
2
PT
1
PIC
1
AT
1
TI
1
TI
2
TI
3
TI
4
PI
2
PI
3
PI
4
TI
5
TI
6
TI
7
TI
8
TI
9
FI
3
TI
10
TI
11
PI
5
PI
6
air
feed
fuel
product
The feed flows
through a pipe andis heated by the
combustion of fuel
Flue
gas
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CHAPTER 2: GOALS AND BENEFITS WORKSHOP 2
Two process examples show the benefit of reduced variability,
the fired heater reactor and the boiler. Discuss the difference
between the two examples. Can you think of another example
that shows the principle of each?
0
0.1
0.2
0.3
0.4
frequency
of
occur
rence
-3 -2 -1 0 1 2 3
deviation from mean
Squeeze down the variability
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Feed
Vapor
product
Liquid
productProcess
fluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
CHAPTER 2: GOALS AND BENEFITS WORKSHOP 3
In both the flash drum and the fired heater examples,
temperature measurement is very important. Describe
several methods for measuring temperature and recommendthe most appropriate for the flash drum example.
How hot is it?
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CHAPTER 2 : CONTROL OBJECTIVES & BENEFITS
When I complete this chapter, I want to be
able to do the following.
Recognize examples of the seven (7)control objectives in chemical processes
Calculate indicators of variability in a
process variable
Be able to calculate the economic impact
of variability
Lots of improvement, but we need some more study!
Read the textbook
Review the notes, especially learning goals and workshop
Try out the self-study suggestions Naturally, well have an assignment!
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CHAPTER 2: LEARNING RESOURCES
SITE PC-EDUCATION WEB
- Instrumentation Notes
- Interactive Learning Module (Chapter 2)
- Tutorials (Chapter 1/2)
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CHAPTER 2:
SUGGESTIONS FOR SELF-STUDY
1. Discuss the importance of consistent quality in your
decisions to purchase food, clothing, etc.
2. A P&I drawing of a distillation process is given in
Woods*. Determine at least one example of each of the
seven control objectives for this process. Evaluate thecontrol designs given; do they achieve your objectives?
3. Find process examples in your previous textbooks and
determine the advantage for reduced variability in each.
Can you provide quantitative values for the economic
benefit?
* Woods, D.Process Design and Engineering Practice, Prentice-Hall, 1995 (page 2-65).