1. 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

2. 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

3. 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

4. EXAMPLE PROCESS: FLASH SEPARATION
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
Vapor
product
Liquid
Process
fluid
Steam F1 F2 F3 T1 T2 T3 T5 T4 T6 P1 L1 A1
L. Key
Let’s discuss
this process
P  1000 kPa
T  298 K

5. SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

6. SEVEN CONTROL OBJECTIVES
High pressure
in drum is
dangerous
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

7. SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

8. SEVEN CONTROL OBJECTIVES
To flare
Never release
hydrocarbons
to atmosphere
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

9. SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

10. SEVEN CONTROL OBJECTIVES
No flow could
damage the
pump
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

11. SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

12. SEVEN CONTROL OBJECTIVES
Always keep
the production
rate smooth
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

13. SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

14. SEVEN CONTROL OBJECTIVES
Achieve L.Key
by adjusting
the heating
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

15. SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

16. SEVEN CONTROL OBJECTIVES
Use the least
costly heating
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

17. SEVEN CONTROL OBJECTIVES
Give example
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

18. SEVEN CONTROL OBJECTIVES
Calculate & plot
key parameters,
e.g., UA.
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis UA
time

19. SEVEN CONTROL OBJECTIVES
All seven must be achieved. Failure to do so
will lead to operation that is
unprofitable or worse, unsafe.
1. Safety
2. Environmental
Protection
3. Equipment
protection
4. Smooth operation
production rate
5. Product quality
6. High profit
7. Monitoring &
diagnosis

20. BENEFITS FROM PROCESS CONTROL Without feedback control
When we control a process, we reduce the variability of key variables to achieve the seven objectives.
Without feedback control
Composition (% H. Key) 50
100
150
200
250
300
350
400
450
500 2 3 4 5 6
time (min)
outlet concentration 49
49.5
50.5 51
valve position (% open)
Reflux valve

21. 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 50
100
150
200
250
300
350
400
450
500
2.5 3
3.5
time (min)
outlet concentration 20 40 60 80
valve position (% open)
Reflux valve
Variability is moved from controlled to manipulated variable!

22. 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?

23. 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!

24. A B 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. B

25. 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.

26. CHAPTER 2: GOALS AND BENEFITS WORKSHOP 1
Determine one example for each of the seven control objective categories.
Flue gas
The feed flows through a pipe and is heated by the combustion of fuel
feed
product
air
fuel

27. CHAPTER 2: GOALS AND BENEFITS WORKSHOP 2 Squeeze down the variability
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?
Squeeze down the variability

28. 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?

29. 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,

30. 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!

31. CHAPTER 2: LEARNING RESOURCES
SITE PC-EDUCATION WEB
- Instrumentation Notes
- Interactive Learning Module (Chapter 2)
- Tutorials (Chapter 1/2)

32. SUGGESTIONS FOR SELF-STUDY
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).