Process Control: Designing Process and Control
Systems for Dynamic Performance
Chapter 2. Control Objectives and Benefits
Copyright © Thomas Marlin 2013
The copyright holder provides a royalty-free license for use of this material at non-profit
educational institutions
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
CHAPTER 2 : CONTROL OBJECTIVES
AND BENEFITS
Outline of the lesson.
• Seven (7) Control Objectives
1. Safety
2. Environmental protection
3. Equipment protection
4. Smooth operation
5. Product quality
6. Profit
7. Monitoring and diagnosis
• Variability measures
• Economic impact of variability
• Workshop
EXAMPLE PROCESS: FLASH SEPARATION
Let’s discuss
this process
T6
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
T1
P1
P  1000 kPa
T5
T2
Vapor
product
T  298 K
F1
T4
F2
Process
fluid
T3
L1
F3
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
High pressure
in drum is
dangerous
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
PC
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
To flare
Never release
hydrocarbons
to atmosphere
1. Safety
2. Environmental
Protection
3. Equipment
protection
T1
T2
F1
T4
T6
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
No flow could
damage the
pump
1. Safety
2. Environmental
Protection
3. Equipment
protection
T1
T2
F1
T4
T6
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
LC
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Always keep
the production
rate smooth
1. Safety
2. Environmental
Protection
3. Equipment
protection
T1
T2
FC
T4
T6
Vapor
product
P1
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Achieve L.Key
by adjusting
the heating
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
Vapor
product
P1
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
AC
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Use the least
costly heating
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
Vapor
product
P1
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
AC
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
T6
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
SEVEN CONTROL OBJECTIVES
Calculate & plot
key parameters,
T6
e.g., UA.
1. Safety
2. Environmental
Protection
3. Equipment
protection
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
4. Smooth operation
production rate
T3
L1
5. Product quality
F2
F3
6. High profit
7. Monitoring &
diagnosis
Process
fluid
Steam
A1
L. Key
Liquid
product
UA
time
SEVEN CONTROL OBJECTIVES
All seven must be achieved. Failure to do so
will lead to operation that is
1. Safety
unprofitable or worse, unsafe.
2. Environmental
Protection
3. Equipment
protection
T6
4. Smooth operation
production rate
5. Product quality
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
T3
L1
6. High profit
7. Monitoring &
diagnosis
F2
Process
fluid
F3
Steam
A1
L. Key
Liquid
product
BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the variability of
key variables to achieve the seven objectives.
Without feedback control
Composition (% H. Key)
outlet concentration
6
5
4
3
valve position (% open)
2
0
50
100
150
200
250
time (min)
300
350
400
450
500
300
350
400
450
500
51
Reflux valve
50.5
50
49.5
49
0
50
100
150
200
250
time (min)
BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the variability of
key variables to achieve the seven objectives.
With feedback control
Composition (% H. Key), note smaller scale
outlet concentration
3.5
3
valve position (% open)
2.5
0
50
100
150
200
250
time (min)
300
350
400
450
500
350
400
450
500
100
Reflux valve
80
60
40
20
0
50
100
150
200
250
time (min)
300
Variability is moved from controlled to manipulated variable!
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 to
process performance?
BENEFITS FROM PROCESS CONTROL
Process performance =
efficiency, yield, production
rate, etc. It measures
performance for a control
objective.
Calculate the process
performance using the
distribution, not the average
value of the key variable!
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
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
CHAPTER 2: GOALS AND BENEFITS WORKSHOP 1
Determine one example for each of the seven control objective categories.
FT
1
PIC
1
Flue
gas
AT
1
PI
4
TI
1
The feed flows
through a pipe and
is heated by the
combustion of fuel
PI
5
TI
5
feed
TI
2
TI
6
PT
1
product
TI
3
TI
7
TI
4
air
TI
8
FT
2
PI
2
PI
3
TI
9
TI
10
FI
3
TI
11
PI
6
fuel
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?
o
c
c
u
r
r
e
n
c
e
Squeeze down the variability
o
f
0.4
n
c
y
0.3
q
u
e
0.2
f
r
e
0.1
0
-3
-2
-1
0
deviation from mean
1
2
3
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 recommend
the most appropriate for the flash drum example.
How hot is it?
T6
T1
T2
F1
T4
P1
Vapor
product
T5
Feed
F2
Process
fluid
T3
L1
F3
Steam
A1
L. Key
Liquid
product
CHAPTER 2: GOALS AND BENEFITS WORKSHOP 4
Here is your chance to investigate a
problem!
Research the design of a vapor-liquid
separator vessel. Determine
• the equipment included,
• how to determine the diameter and
height,
• how to select the materials of
construction, and
• how to change the operating
variables to achieve the desired
product purity as feed composition
changes
Figure source: Mbeychok, public domain, http://en.wikipedia.org/wiki/File:Vap-Liq_Separator.png
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
Lot’s 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, we’ll have an assignment!
CHAPTER 2: LEARNING RESOURCES
•
SITE PC-EDUCATION WEB
- Instrumentation Notes
- Interactive Learning Module (Chapter 2)
www.pc-education.mcmaster.ca/
- Tutorials (Chapter 1/2)
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 the
control 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).