process operability class materials process flexibility copyright © thomas marlin 2013 the...
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Process Operability Class MaterialsProcess Flexibility
Copyright © Thomas Marlin 2013The copyright holder provides a royalty-free license for use of this material at non-profit
educational institutions
FC1
LC1
FC
1
TC
1
TC
2
T
10
T
12
T
11
T
13
fuel
LC
1
L2
LAHLAL
F
4
Basic flowsheet Design with Operability
In this Lesson, we will learn
• Why do we need flexibility in a design?
- Distillation
• Deciding what to achieve (control)
- Principles:Control Objectives
- Example: Bioreactor
• Locating the flexibility: how many and where?
- Principles: Degrees of freedom and Controllability
- Blending, CSTR, heat exchange, bioreactor
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
PROCESS OPERABILITY: FLEXIBILITY
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
Without flexibility, the process
• Does not respond to changes in set points
• Responds to all disturbances that change product qualities, production rates and can lead to unsafe operation!
With flexibility, the process
• Achieves set points
• Compensates for all disturbances so that product qualities, production rates and safety are achieved.
We need to “steer” the process
unacceptabl
e
Why flexibility?
FLEXIBILITY
Flexibility enables us to adjust the plant operation after the equipment has been designed. It requires spare capacity in selected equipment and extra equipment to adjust operation.
• Spare capacity in pumps, valve, heat exchangers, vessels, motor speed, etc.
• Additional equipment includes pipes and valves
- Adjust flows (especially utility) to equipment; utility = cooling water, steam, fuel, air,
nitrogen, hydrogen, ….
- Enable flow to (partially) by-pass equipment
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Why flexibility?
FLEXIBILITY
FV
We have designed for an operating window. Now we must move around in it to achieve the desired point. What equipment must we add to the distillation tower?
What defines a “point”• Feed flow rate• Pressure• Levels• Distillate composition• Bottoms composition
What uncertainty exists• L-V equil, heat
transfer, flow, etc.
What is adjusted• ??• ??• ??
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
What disturbances occur?• Feed composition,
enthalpy, & rate• CW temperature• Reboiler temperature
Why flexibility?
FLEXIBILITY
What equipment must we add to the distillation tower?
FV
We add a valve to every adjustable flow.
We could have alternative feed trays, with manual valves used to change the tray.
Naturally, the equipment must have capacity
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Why flexibility?
Feed
Vaporproduct
LiquidproductProcess
fluidSteam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
Give example
FLEXIBILITY
1. Safety
2. Environmental Protection
3. Equipment protection
4. Smooth operation production rate
5. Product quality
6. High profit
7. Monitoring & diagnosis
How do we decide what to control?
See Chapter 2 of Marlin (2000) for solution
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
What to achieve?
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
Class Workshop: We are designing a batch bioreactor. Define the the control objectives, specifically the variables to be controlled.
What to achieve?
time
Variables to satisfy desired trajectory
FLEXIBILITY
How much - We provide capacity to achieve an operating window with specified ”size”; see Operating Window topic.
• How many – How many flexible items are needed?
• Where - We need flexibility (adjustable variables) that influence the operating variables that define the point we want to achieve.
We can check a point using a flowsheeting program. We can determine which manipulated variables change and by how much. But, this takes lots of time to check many points.
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
How do we decide what to manipulate?
Flexibility: where & how many?
FLEXIBILITY – How many?
DEGREES OF FREEDOM
How do we determine the maximum number of variables that be controlled in a process?
How do we determine the minimum number of adjustable variables to achieve desired values for specified variables?
v1
Hot Oil
v2
v3
L1
v7
v5 v6
Hot Oil
F1 T1 T3
T2
F2
T4T5
F3 T6
T8
F4
L2
v8
T7
P1F5
F6T9
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY – How many?
DEGREES OF FREEDOM
A requirement for a successful design is:The number of valves (adjustable variables)
number of variables to be achieved (controlled)
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
v1
Hot Oil
v2
v3
L1
v7
v5 v6
Hot Oil
F1 T1 T3
T2
F2
T4T5
F3 T6
T8
F4
L2
v8
T7
P1F5
F6T9
v4
Flexibility: where & how many?
FLEXIBILITY – Where?We need independent causal relationships between the adjusted and controlled variables. Remember, interaction can exist, but desired points must be able to be achieved. See three cases from Process Control.
Two drivers can achieve independent positions without interaction
Connected by spring
Two drivers can achieve independent positions with interaction
Connected by beam
Two drivers cannot achieve independent positions. They are “linked”
Independent Interaction Linearly dependent
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY – Where?
CONTROLLABILITY: A system is controllable if its CVs can be maintained at the set points, in the steady-state, in spite of disturbances entering the system.
DK
K
MV
MV
KK
KK
CV
CV
d
d
2
1
2
1
2221
1211
2
1
0
0
Model for 2x2 system in deviation variables
A system is controllable when the matrix of process gains can be inverted, i.e., when
the determinant of K 0.
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY
Controllability Class Workshop: Can we achieve desired blended flow and composition by adjusting the valves?
FA, xA
FS, xAS = 0FM, xAM
Blending Process
Total flow and composition
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY
FA, xA
FS, xAS = 0FM, xAM
Blending Process
'S
ssAs
A'A
ssAs
SAM
SAM
F)FF(
FF
)FF(
F'x
'F'F'F
22
0)()(
)(22
SA
S
SA
A
FF
F
FF
FKDet
Yes, this system is controllable!
Connected by spring
Controllability Class Workshop: Can we achieve desired blended flow and composition by adjusting the valves?
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY
A B + 2C-rA = k0 e -E/RT CA
A
ACB
CC
v1
v2
Controllability Class Workshop: Can we achieve desired values for the sensors by adjusting the valves?
Non-isothermal Chemical Reactor
Pure A feed
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY
A B + 2C-rA = k0 e -E/RT CA
A
ACB
CC
v1
v2
2
1
1211
1211
220
0
MV
MV
KK
KK
C
C
C
B
Det (K) = 0; No! The system is not controllable!
Connected by beam
Controllability Class Workshop: Can we achieve desired values for the sensors by adjusting the valves?
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY
Goal: Maintain cold effluent Tcold at a desired value
Stream A(cold)
Stream B(hot)
Freedom to adjust flows
Stream A Stream B
1. Constant Adjustable
2. Adjustable Constant
3. Constant Constant
Controllability Class Workshop: Add flexibility to the heat exchanger to achieve the goal for three different scenarios.
T
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY Freedom to adjust flows
Stream A Stream B
1. Constant Adjustable
2. Adjustable Constant
3. Constant ConstantStream A(cold)
Stream B(hot)
TC1
Stream A(cold)
Stream B(hot)
TC
3
Stream A(cold)
Stream B(hot)
TC
2
It is not typical to adjust a stream flow to control its temperature; if the temperature is important, likely the flow rate is also important. But, this design will function.
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITY
Controllability Class Workshop: Add flexibility to the heat exchanger to achieve the goals.
Goals: Maintain cold effluent at Tcold and Maintain hot effluent at Thot
T
Stream A(cold)
Stream B(hot)
T
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
FLEXIBILITYGoals: Maintain cold effluent at Tcold and Maintain hot effluent at Thot
TStream A(cold)
Stream B(hot)
T)(
)(
CinCoutpColdCold
HinHoutpHotHot
TTCFQ
TTCFQ
C
H
Energy balance on each stream
Equipment model with U= f(FH, FC)
lmTUAYQ )(
From an energy balance on entire system:
QHot = Qcold
It is not possible to satisfy both energy balances by adjusting the flows. (We would have to adjust the inlet temperatures, which would seem to defeat the purpose of the heat exchanger.)
Connected by beam
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
Flexibility Workshop: We heat a stream with several process streams, which recovers energy efficiently.
• What disturbances can occur?
• What set point changes can occur?
• No stream flow rate can be manipulated. What flexibility is needed to achieve the desired outlet temperature?
T
Flexibility: where & how many?
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
T
Disturbances:
• Fluid inlet temperatures
• Fluid inlet flow rates
Set points:
• Outlet temperature
Flexibility: where & how many?
Class Exercise: Add flexibility
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
TC
Flexibility: If the final process-fluid heat exchanger has a outlet temperature that is high enough to achieve the desired value, a by-pass could be used.
Flexibility: where & how many?
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
Flexibility: If the final process-fluid heat exchanger has a outlet temperature that is not high enough to achieve the desired value, an additional source is required; here the source is a fired heater.
TC
FT1
FT2
PT1
PIC1
AT1
TI1
TI2
TI3
TI4
PI2
PI3
PI4
TI5
TI6
TI7
TI8
TI9
FI3
TI10
TI11
PI5
PI6
P-27
P-28
fuelair
Flexibility: where & how many?
FLEXIBILITY
Adjust flows (especially utility)
utility = cooling water, steam, fuel, air, nitrogen, hydrogen, ….
disturbance To be achieved disturbance To be achieved
Generalization: From the heat exchanger examples, we see that flexibility can be achieved by adjusting utilities or in some cases, with a by-pass.
Enable flow to (partially) by-pass equipment
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Flexibility: where & how many?
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
Class Workshop: We are designing a batch bioreactor. Define the flexibility required to achieve the control objectives from the previous workshop in this lesson.
What to achieve?
steam
Cooling water
air
Gas exit
substrateacid
base
FLEXIBILITY
INDUSTRIAL PRACTICE
1. Flexibility enables achieving points in the operating window.
2. The choice of adjustable equipment is based on principles and experience.
3. Controllability is often determined by qualitative analysis; however, flowsheeting can be used to see if the dependent variable values can be achieved by changing the selected adjustable variables.
4. Whether the adjustable variable is manipulated by process control or by a person depends on the response time required.
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
FLEXIBILITY
INDUSTRIAL PRACTICE
(1) Model Uncertainty(2) Disturbances(3) Set Point values(4) Production Variation
The plant “Design Specification” must include definitions of items (2) to (4)
The engineer must understand all items when items are significant and must be accommodated with extra capacity or improved sensor technology.
“Know where you are going”
(Remember for 4W04)
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
In this Lesson, we will learn
• Why do we need flexibility in a design?
- Distillation
• Deciding what to achieve (control)
- Principles:Control Objectives
- Example: Bioreactor
• Locating the flexibility: how many and where?
- Principles: Degrees of freedom and Controllability
- Blending, CSTR, heat exchange, bioreactor
Key Operability issues
1. Operating window
2. Flexibility/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
PROCESS OPERABILITY: FLEXIBILITY