Process Control: Designing Process and Control Systems for Dynamic Performance

Chapter 2. Control Objectives and Benefits

Copyright © Thomas Marlin 2013The 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

Outline of the lesson.

• Seven (7) Control Objectives1. Safety 2. Environmental protection3. Equipment protection4. Smooth operation5. Product quality

6. Profit7. Monitoring and diagnosis

• Variability measures

• Economic impact of variability

• Workshop

CHAPTER 2 : CONTROL OBJECTIVES AND BENEFITS

EXAMPLE PROCESS: FLASH SEPARATION

Feed

MethaneEthane (LK)PropaneButanePentane

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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 pressurein drum isdangerous

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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 releasehydrocarbons to atmosphere

To flare

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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 coulddamage the

pump

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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 keepthe production

rate smooth

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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.Keyby adjustingthe heating

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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 & plotkey parameters,

e.g., UA.

time

UA

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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.

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)ou

tlet c

once

ntra

tion

0 50 100 150 200 250 300 350 400 450 50049

49.5

50

50.5

51

time (min)

valv

e po

sitio

n (%

ope

n)

Composition (% H. Key)

Reflux valve

BENEFITS FROM PROCESS CONTROL

When we control a process, we reduce the variability of key 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)ou

tlet c

once

ntra

tion

0 50 100 150 200 250 300 350 400 450 50020

40

60

80

100

time (min)

valv

e po

sitio

n (%

ope

n)

Composition (% H. Key), note smaller scale

Reflux valve

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

Calculate the process performance using the distribution, not the average value of the key variable!

Process performance = efficiency, yield, production rate, etc. It measures performance for a control objective.

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.

FT1

FT2

PT1

PIC

1

AT1

TI1

TI2

TI3

TI4

PI2

PI3

PI4

TI5

TI6

TI7

TI8

TI9

FI3

TI10

TI11

PI5

PI6

air

feed

fuel

product

The feed flows through a pipe and is heated by the combustion of fuel

Flue gas

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

freq

uenc

y of

occ

urre

nce

-3 -2 -1 0 1 2 3

deviation from mean

Squeeze down the variability

Feed

Vaporproduct

LiquidproductProcess

fluidSteam

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 recommend the most appropriate for the flash drum example.

How hot is it?

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)

- Tutorials (Chapter 1/2)

www.pc-education.mcmaster.ca/

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).