process dynamics and control - جامعة نزوى...7 process dynamics and control •process...
TRANSCRIPT
• 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
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
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
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.