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Process operability: the operating windowKey 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
• What is an Operating Window?
- Flash drum, Yourself, CSTR
• What defines the “Frame”?- Distillation
• How can we set equipment capacity (the
operating window) to achieve desired
operation?
- Equipment capacity: Heat exchanger, pump
- Alternative equipment: Pump, flash
• How do we determine if operation is possible
within the window?
- Pump, distillation 1
-20
0
20
40
60
80
feed
tem
pera
ture
(C
)
50 70 90 110 130 150 170 190
feed flow
Design
Minimum heating
Maximum
liquid
product
valve
opening
Maximum heating
valve opening
feasible
Vapour
product
Liquid
productProcess
fluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
AC
L. Key
The range of achievable steady-state operations.
This is affected by manipulated and disturbance
variables. The limitations can be due to equipment
(e.g., maximum flow), safety, product quality, etc.
Flash drum example
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
2
Operating window
Operating window
The variables in the plot can be
• Set points of controlled variables
• Disturbance variables
The frames (boundaries) of the window can be
• “hard” constraints that cannot be violated
• “soft” constraints than can be violated at a
(usually large) economic penalty
Determine the category for each of the
constraints for the flash drum.
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
3
-20
0
20
40
60
80
feed
tem
per
atu
re (
C)
50 70 90 110 130 150 170 190
feed flow
Design
Minimum heating
Maximum
liquid
product
valve
opening
Maximum
heating
valve opening
feasible
Minimum heating valve opening is a “hard” constraint
Maximum feed
valve
opening is
“hard” constraint
Minimum feed valve opening is “soft constraint”
(The valve can be fully closed)
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
4
Operating window
A B
-rA = k0 e -E/RT CA
feasible
infeasible
infeasible
T
A
Reactant
Solvent
Coolant
Note:
This shows a
range of set
points that can
be achieved
(without
disturbances).
Discuss the operating window for this non-
isothermal CSTR.
What do you note about the shape
of the operating window?
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
5
Operating window
Discuss the operating window for this non-
isothermal CSTR.
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
• We can determine the operating window using
modelling (flowsheeting)
• If the plant exists, we could determine the
operating window empirically (but maybe make
off-specification products)
• The operating window is not always a polygon
• The operating window is not always 2-
dimensional (can be much higher dimension)
• Operation can occur outside the window during
transients (or when assumptions are violated)6
Operating window
Determine the constraints (limitations) that
define the frame (boundary) of the window
feasible
Process variable 1
Pro
ce
ss v
aria
ble
2
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
The frame defines the “size” of the operating window.
These are typically physical bounds, equipment
operation and stream specifications.
7
Operating window
Determine typical constraints that affect the
operating window for a distillation tower.
FR
FV
xB
xD
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
8
Operating window
Distillation constraints
FR
FV
xB
xD
Pumping, pipe, valve capacity
Maximum cooling capacity
Maximum and
minimum liquid
and vapour flow
rates
Maximum and
minimum liquid
and vapour flow
rates
Flow pipe, valve capacity
Maximum heating
Minimum natural
circulation to
reboiler
Product composition
Product 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
9
Operating window
The design specification will define a boundary of
the operating window.
Heat exchanger Q = U A (T)lm
What are the “worst case” operating conditions
we would use to design (size) the heat
exchanger?
Hot process fluid
into shell
Cooling water into
tubes
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
The exchanger
exists to cool
this stream
10
Operating window
The design specification will define a boundary of
the operating window – The Worst Case gives the
largest area for heat exchange.
Hot process fluid
into shell
Cooling water into
tubes
Highest flow rate,
Highest temperature
Lowest temperature
Lowest flow rate,
Highest temperature
Greatest fouling,
Lowest U
How do we
determine
values?
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
11
Operating window
The design specification will define a boundary of
the operating window.
Consider the flow system. What variables must
we determine? What is the “worst case” we
would use to design the system, specifically the
required pump outlet pressure?
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
12
Operating window
The design will define a boundary of the operating
window - Worst case gives the largest pump.
What variables must we determine?
- Pipe diameter - by guideline (Liq: 1 m/s, Gas: 30 m/s)
- Pump horsepower - from highest flow rate and PP and
the lowest suction pressure
P
Highest vessel
pressure
Highest
pressure drop
Highest
pressure drop
Highest flow,
largest friction
factor
Lowest level
(lowest head)
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
13
Operating window
In general, we want a large operating window. Why
not always design and construct equipment with
very large capacities?
Complete the following table.
Advantages
Disadvantages
Small
equipment*
Large
equipmentJust satisfies base
case
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* = small equipment just satisfies base case design point
14
Operating window
Advantages
Disadvantages
Small
equipment
Low capital cost
Most efficient at base case
Achieve “precise” operation (smaller equipment to
adjust)
Cannot achieve higher capacity
Cannot compensate for large range of disturbances
Cannot achieve fast transition (no overshoot in
manipulated variable)
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
15
Operating window
High capital cost
Likely lower efficiency at base case and lower
production rates
Might not achieve “precise” operation at base case
Can achieve higher capacity
Can compensate for likely range of disturbances
Can achieve faster transition (allows overshoot in
manipulated variable)
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
16
Operating window
Advantages
Disadvantages
Large (oversized)
equipment
In general, we want a large operating window. Why
not design and construct equipment with very large
capacities?
So, we design plants that have “just the right”
capacity in “the right places”. We have to consider
the Boundaries and the Internal Points of the
operating window.
The following class workshops demonstrate
examples of equipment designs that achieve
operability with acceptable cost through modest
modifications to the process structure.
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
17
Operating window
Some designs that increase the operating window
Centrifugal pumps - Configurations to
increase the operating window
Pumps provide “pressure (head)” and “flow”. How
do we select the correct option, if needed?
Flow rate
Hea
d
Typical pump head curve
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
18
Operating window
Series
Parallel
Some designs that increase the operating window
Centrifugal pumps - Configurations to
increase the operating window
Series
Parallel
Series: This configuration
provides higher pressure at
(approximately) the same
flow rate.
Parallel: This configuration
provides higher flow rate at
(approximately) the same
pump exit pressure.
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
19
Operating window
Feed
Vapor
product
Liquid
productProcess
fluid
Steam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
Some designs that increase the operating window
The vapor flow rate is usually small. However, in
some cases (e.g., start up) , it is 20 times more
that its typical value. What do we do?
Key
Operability
issues
1. Operating
window
2. Flexibility/
controllabilit
y
3. Reliability
4. Safety &
equipment
protection
5. Efficiency &
profitability
6. Operation
during
transitions
7. Dynamic
Performance
8. Monitoring & 20
Operating window
We provide a larger pipe
and valve in parallel. The
pressure control will
adjust the small valve
first, then the large valve.
Equipment must function correctly within the
operating window
heating
FC
Velocity increases;
Bernoulli says that
pressure decreases
Cold
(20C)
liquid
Orifice meter
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
Any concerns about
this design?
22
Operating window
Porifice=P1 – P3
Distance
pre
ssu
re
Sensors: Principles of the orifice meter
PorificeMeasure pressure drop
23
From: Superior Products, Inc. http://www.orificeplates.com/
Sensors: Principles of the orifice meter
Nice visual display of concept.
In practice, pressure difference
is measured by a reliable and
electronic sensor = Porifice
24
Bernoulli’s eqn.
General meter eqn.
Installed orifice meter
(requires density
measurement)
0 = aver. density
C0 = constant for
specific meter
Installed orifice meter
(assuming constant density)31 PPKF
Most common flow
calculation, does not
require density
measurement
v = velocity
F = volumetric flow rate
f = frictional losses
= density
A = cross sectional area
Relate the
pressure drop
to the flow rate
25
P
cooling
K
Take square root of
measurement
Multiply signal by
meter constant K FC
Measure pressure
difference
“Measured value” to flow controller
When an orifice meter is used,
the calculations in yellow are
performed. Typically, they are
not shown on a process drawing.
Sensors: Principles of the orifice meter
liquid
26
General meter eqn.
v = velocity
F = volumetric flow rate
f = frictional losses
= density
A = cross sectional area
Relate the
pressure drop
to the flow rate
Cmeter
Reynolds number
We assume that the meter coefficient is
constant. The flow accuracy is acceptable
only for higher values of flow, typically 25-
100% of the maximum for an orifice
Sensors: Are there limitations to orifices?
27
Porifice=P1 – P3
Distance
pre
ssure
Sensors: Is there a downside to orifices?
What is a key
disadvantage of the
orifice meter?
Pressure loss!
When cost of pressure
increase (P1) by
pumping or compression
is high, we want to avoid
the “non-recoverable”
pressure loss.
Ploss = P1 – P2
Non-
recoverable
pressure
drop
28
Equipment must function correctly within the
operating window
heating
FC
Velocity increases;
Bernoulli says that
pressure decreases
The fluid can partially vaporize.
The pressure difference will
not reliability indicate
the flow rate!
Cold
(20C)
liquid
Orifice meter
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
29
Operating window
Equipment must function correctly within the
operating window
heating
Simple solution
• Locate flow
measurement where the
pressure is highest
and temperature lowest.
• Ensure that flashing does
not occur - design calc’s
FC
Cold
(20C)
liquid
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
30
Operating window
Equipment must function correctly within the
operating window
Bottom tray
Bottoms
product
reboilerCentrifugal pump
Any concerns about this design?
Hint: Describe the condition
of the liquid in the bottom of
the tower Bubble point
What happens
when the
pressure is
reduced?
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
31
Operating window
Equipment must function correctly within the
operating window
Bottoms
product
reboiler
Centrifugal pump
Pressure drop
due to flow
frictional losses
Pressure drop due to
the velocity increase in
the eye of the pump
What happens
in the
pump?
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
32
Operating window
http://www.britannica.com/EBchecked/topic-
art/632655/7035/Volute-centrifugal-pump
http://www.sprayingequipmentsupply.com/pumps/cent
rifugal-pumps.html
Basic concept of a centrifugal pump
33
Basic concept of a centrifugal pump
Towler, G. and R. Sinnott (2008) Chemical Engineering Design, Elsevier-Butterworth-Heinemann, page 254
Constant speedImpeller
diameter
34
Basic concept of a centrifugal pump
http://hiramada.wordpress.com/2009/07/07/introduction-to-centrifugal-pump-technical-selection/
35
Equipment must function correctly within the
operating window
Bottoms
product
reboiler
Centrifugal pump
Cavitation: The liquid partially vaporizes. As the
pressure increases in the pump, the vapor is
subsequently condensed. This collapsing of bubbles
(cavitation ) causes noise, vibration and erosion - all of
which are bad.
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
Let’s prevent
bubbles from
forming.
36
Operating window
Equipment must function correctly within the
operating window
Bottoms
product
reboiler
Centrifugal pump This liquid head increases
the pressure at the inlet to
the pump and prevents
cavitation.
NPSHR: The
manufacturer must
define the minimum
net positive suction
head required.
The process engineer
must design to provide
it. NPSHA>NPSHR
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 NPSHA37
Operating window
Equipment must function correctly within the
operating windowNPSHR: The manufacturer
must define the minimum net
positive suction head required.
From: Woods, D.R., Process Design and Engineering Practice, Prentice -Hall, 1995
The process engineer must
design to provide it. How?
This is issue when liquid is at
(near) its bubble point. Give
examples when this is the
situation in chemical processes.
• Elevate the liquid above the
pump (two ways)
• Reduce friction losses
• Subcool the liquid (careful of
added pressure drop)
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
38
Operating window
Equipment must function correctly within the
operating window
From: Woods, D.R., Process Design and Engineering Practice, Prentice -Hall, 1995
This is issue when liquid is at (near) its bubble point. Give examples when this is
the situation in chemical processes.
We deal with liquids at their
bubble points often, for example,
• Distillation/stripper bottoms
• Distillation/absorber condensers and
OH drums
• Flash drums
• Concentration by boiling
• Vapor compression refrigeration
• Reactor cooling by solvent vaporization
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
39
Operating window
Regrettably, no systematic method is used in practice
First, define the range over which the plant must
operate. Consider most demanding conditions.
Second, solve flowsheet for the limiting cases
Third, design equipment to function for each of the
limiting cases; may have to change structure.
Fourth, ensure that interior is operable.
Fifth, add features to achieve other operability
features (on list at left), as needed
Fortunately, engineers have lots of relevant experience!
INDUSTRIAL PRACTICE
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
40
Operating window
INDUSTRIAL PRACTICE
SAFETY FACTORS: Couldn’t we just design for the
base case and multiply every capacity by a safety
factor, (1+ X)? (where X = 25%, 35%, 50%, …)
This is not engineering! Any single factor would be
too small for some equipment and too large for others.
After applying the proper procedure, a small safety
factor can be employed for modelling uncertainty,
based on experience. Typical values are 10-15%.
“For well tested process, safety factors can approach 0%” *
* Valle-Riestra, J.F. (Dow Chemical Co.), Project Evaluation in the Process Industries, McGraw-Hill, New
York, 1983 (pg 209)
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipment
protection
5. Efficiency &
profitability
6. Operation
during
transients
7. Dynamic
Performance
8. Monitoring &
diagnosis
41
Operating window
INDUSTRIAL PRACTICE
SAFETY FACTORS: Some “safety factor” is built
into the design procedure. After we have calculated
the required pipe diameter, valve diameter, vessel size,
motor power etc., we purchase the closest available
size.
Since the manufactured sizes are discrete, we select
the next largest size.
This provides some safety margin.
Key Operability
issues
1. Operating
window
2. Flexibility/
controllability
3. Reliability
4. Safety &
equipment
protection
5. Efficiency &
profitability
6. Operation
during
transients
7. Dynamic
Performance
8. Monitoring &
diagnosis
42
Operating window
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
• What is an Operating Window?
- Flash Drum, CSTR
• What defines the “Frame”?- Distillation
• How can we set equipment capacity (the operating
window) to achieve desired operation?
- Equipment capacity: Heat exchanger, pump
- Alternative Equipment: Pump, flash
• How do we determine if operation is possible
within the window?
- Pump, distillation
43
Operating window
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