4

Process Dynamics and

Control

Dr Salam Al-Dawery

Chemical Engineering

Department

• Text

– Seborg D. E., T. F. Edgar, and D. A.

Mellichamp, “Process Dynamics and

Control,”John Wiley & Sons, New York, 2nd

Ed., 2004

• References

– Stephanopoulos G., “Chemical Process

Control-An Introduction to Theory and

Practice,”Prentice -Hall, New Jersey, 1984.

– Luyben W. L., “Process Modeling,

Simulation and Control for Chemical

Engineers,” McGraw-Hill, New York, 2nd

Ed., 1990

Learning Objectives

Upon the completion of this chapter, students are able to:

• Describe the importance process control

• Define and identify controlled, manipulated variables and disturbance for a given process

• Explain the implementation of process control in industries

• Explain What is feedback and feedforward control

CHAPTER 1

Introduction

7

Process Dynamics and Control

• Process Dynamics - time varying behaviour

of a process variables

• Process Control - achieve desired conditions and uniform product in a system

by adjusting selected variables

• When I run a kinetics experiment, how do I maintain the temperature and level at desired values?

• How do I manufacture products with consistently high quality when raw material properties change?

• How much time do I have to respond to a dangerous situation?

Why do I learn Process Control and

Instrumentation Course

Why Is Control Necessary?

Control Objective

10

Control Objectives

• Safety

– maintain proper operation to avoid dangerous

situations

– emergency systems

• pressure relief valves

• automatic reactor shutdown systems

• Environmental Protection

– proper operation and containment

• prevent tanks from overflowing

• maintain low concentrations of undesirable

compounds in effluent

11

Control Objectives

• Equipment Protection

– proper operation and shutdown at limiting

conditions

• prevent pumps from running dry

• prevent furnace tubes from getting too hot

• Smooth Operation

– for both inputs and outputs

• be a good neighbour

• minimize disturbances to integrated units

12

Control Objectives

• Product Quality

– Maintain composition, physical properties and

performance properties of products within

customer specifications

• Profit

– reduce costs

• improve efficiency

• consume fewer raw materials and less energy

– increase revenues by achieving higher

production rates or by making specialized

products

13

Control Objectives • Monitoring and Diagnosis

– Collect useful information to improve plant operation

• Immediate and short-term – Identify and potentially dangerous situations and take

appropriate actions

– Provide timely information to operators, supervisors and plant engineers

• Longer-term - identify causes of poor plant operation and find solutions that will prevent future upsets and will reduce variability

– Operators, supervisors, plant engineers and managers use information provided by plant monitoring and control systems to plan process improvements

Control System Process control principle

How to keep the level at

desired value, H?

h

Qin

Qout

H

Manual Control:

• The equipment has the local gauge/ side tube

• Controlled variable-liquid level

• Manipulated variable-flow rate of outlet

• What will cause the level high/ low?

Control System Process control principle

Qin

h

Qout

H

Automatic control

• Machine, electronic, or

computer replace human

operation

h

Qout

Controller

Actuator

Sensor

s

u

Qin

Control System Process control principle

Control Terminology Controlled variables - these are the variables which

quantify the performance or quality of the final product, which

are also called output variables.

Manipulated variables - these input variables are adjusted

dynamically to keep the controlled variables at their set-

points.

Disturbance variables - these are also called "load"

variables and represent input variables that can

cause the controlled variables to deviate from their

respective set points.

Desired Values or Setpoints -target values for flow and

temperature

Measurements - to measure process conditions

or Transmitter (sensors)

18

Controller : to determine corrective action

Final Control Elements : adjusted position of taps

Feedback Control:

• Use measurements of process outputs and some sort of algorithm to determine changes to the inputs of the same process

• Feedback controllers act to reduce differences between the setpoint and the measured values of outputs. This is called negative feedback.

Control Terminology

Task 1 You are implementing control manually

Objective:

To keep the water level in tank at

5m

1. Identify control variable,

manipulated variable and

disturbance for this process

2. Explain the correct action if you

want to increase the controlled

variable

Task 2 State your own objective for this chemical reactor.

List the controlled, manipulated variables and disturbance(s)

Automatic Control

Answer for Task 2

Level and Temperature Control

LC

TC

Automatic Control

Answer for Task 2

Composition Control AC

Automatic Control

Reactants

Task 3: Composition Control

Task 3: Composition Control

25

• inputs - concentrated salt flow, exit flow, inlet water flow

• outputs - level, salt concentration

• states - level, salt concentration, temperature, pressure

• input/output designation does not always match physical

direction of flow

water brine solution

effluent

AI

LI

Task 4: Mixing Salt tank

26

• controlled variables

- level

- salt concentration - inferred from

conductivity

• manipulated variables

- concentrated salt flow rate (pump)

- exit flow rate (pump)

Systems Approach Salt Tank Experiment

27

Systems Approach Salt Tank Experiment

brine flow

exit flow

tap water flow

(changes due

to pressure

fluctuations)

{ manipulated

variable

inputs

{ disturbance

input

Salt Tank

salt conc.

level

temperature

pressure

INPUTS OUTPUTS

exit salt conc.

level

STATES

28

Control Engineering addresses...

• Process Design

– controllability and operability.

– how long does it take for disturbances to have an

effect on the output and how long does a problem

persist after the disturbance goes away?

– How responsive is the output to changes in the

inputs?

• Measurements

– selection and location of sensors.

– accuracy and speed of sensors.

– Are existing sensors good enough so that we can

detect problems and remedy them?

29

Control Engineering addresses...

• Final Control Elements

– location, type of variables to manipulate

– flexibility, speed of action

• Control Structure

– Which inputs should be used to control which

outputs?

– We want to get to new setpoints quickly and

take care of disturbances quickly.

– We want to make sure that the adjustments

that our control system makes don’t cause

upsets elsewhere in the plant.

30

Control is necessary for …

• rejecting disturbances

e.g., when someone else brushes or flushes

– adjust manipulated variables to restore and

maintain controlled variables at their setpoints

(feedback control)

– if we know the disturbance is coming we can act

in advance to prevent deviations from setpoint

(feedforward control)

• Disturbance rejection is sometimes called

the “load problem”

31

Control is necessary for ...

• following changes in setpoint

– move to new target values

• change to warmer temperature

– frequently for economic or environmental

reasons

• grade changes to satisfy different customers

• summer vs. winter gasoline

• also known as the “servo problem”

• important in mechanical systems • positioning of robot arms

32

On/off Control

Is on-off control good enough for all variables in a chemical process? If the answer is “yes”, the course is over!

Example Process: Flash Separation

• Case 1: High pressure is dangerous.

• Case 2: No release hydrocarbons to atmosphere

• Case 3: No flow could damage the pump

• Case 4: High temperature may cause thermal

loading in the column

• Case 5: Quality of product at the bottom column

is the most critical

• Case 6: Keep the process at smooth rate, avoid

dry column

• Case 7: Calculate and plot key parameters

37

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Give example

38

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

High pressure

in drum is

dangerous

39

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Give example

40

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Never release

hydrocarbons

to atmosphere

To flare

41

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Give example

42

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

No flow could

damage the

pump

43

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Give example

44

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Always keep

the production

rate smooth

45

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Give example

46

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Achieve L.Key

by adjusting

the heating

47

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Give example

48

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Use the least

costly heating

49

Feed

Vapor

product

Liquid

productProcess

fluid

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

5. Product quality

6. High profit

7. Monitoring &

diagnosis

SEVEN CONTROL OBJECTIVES

Give example

50

Feed

Vapor

product

Liquid

productProcess

fluid

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

51

Feed

Vapor

product

Liquid

productProcess

fluid

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

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.