process operability class materials operating window copyright © thomas marlin 2013 the copyright...
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Process Operability Class MaterialsOperating Window
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
PROCESS OPERABILITY : 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
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
Feed
Vaporproduct
LiquidproductProcess
fluidSteam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
AC
L. Key
OPERATING WINDOW
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
-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 productvalveopening
Maximum heatingvalve opening
feasible
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
Define Oper. 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
Class Workshop: 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
Define Oper. Window
-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 productvalveopening
Maximum heatingvalve opening
feasible
OPERATING WINDOW
Minimum heating valve opening is “hard”
Maximum feed valve opening is “hard”
Minimum feed valve opening is “soft”(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
Define Oper. Window
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).
Class Workshop: Discuss the operating window for this non-isothermal CSTR.
What do you note about the shapeof 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
Define Oper. Window
OPERATING WINDOW
Class Workshop: 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)
Define Oper. Window
OPERATING WINDOW
Class Workshop: Discuss the operating window for this non-isothermal CSTR.
Design Procedure
• Set goals and design specifications
• Select process technology
• Define process structure (sequence)
• Simulate the flowsheet
• Design equipment
The flowsheet typically involves basic M&E balances, equilibrium and rate processes. It does not consider practical issues for achieving the operation.
Equipment design achieves the base case flowsheet (plus other concerns). This sets the “capacity” of the plant.
The design must define the range of operations (set points and disturbances) to be achieved.
We can accept less than full production rate or top efficiency for extreme situations.
We must document specifications and range or operations and review with all stakeholders!
These “specifications” are in the
Design Basis Memorandum.
Define Oper. Window
OPERATING WINDOW
Determine the constraints (limitations) that define the frame (boundary) of the window
feasible
Process variable 1
Pro
cess
var
iab
le 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.
Define the frame
OPERATING WINDOW
Class Workshop: 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
Define the frame
OPERATING WINDOW
Class Workshop: Distillation Constraints
FR
FV
xB
xD
Pumping, pipe, valve capacity
Maximum cooling capacity
Maximum and minimum liquid and vapor flow rates
Maximum and minimum liquid and vapor 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
Define the frame
OPERATING WINDOW
The design specification will define a boundary of the operating window.
Heat exchanger Q = U AY (T)lm
What are the “worst case” operating conditions we would use to design 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
Size of Oper. Window
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 wedetermine
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
Size of Oper. Window
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
Size of Oper. Window
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 vesselpressure
Highest pressure drop
Highest pressure drop
Highest flow,largest friction
factorLowest 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
Size of Oper. Window
OPERATING WINDOW
In general, we want a large operating window. Why not always design and construct equipment with very large capacities?
Class Workshop: Complete the following table.
Advantages
Disadvantages
Small equipment*
Large equipment
Just 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
Size of Oper. Window
OPERATING WINDOW
Class Workshop: Complete the following table.
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
Size of Oper. Window
OPERATING WINDOW
Class Workshop: Complete the following table.
Advantages
Disadvantages
Large equipment
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
Size of Oper. Window
OPERATING WINDOW
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
Size of Oper. Window
OPERATING WINDOW
Some designs increase the operating window
Centrifugal pumps - Configurations to increase the operating window
Series
Parallel
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
Size of Oper. Window
OPERATING WINDOW
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
Size of Oper. Window
Feed
Vaporproduct
LiquidproductProcess
fluidSteam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
OPERATING WINDOW
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/ controllability
3. Reliability
4. Safety & equipment protection
5. Efficiency & profitability
6. Operation during transitions
7. Dynamic Performance
8. Monitoring & diagnosis
Size of Oper. Window
Feed
Vaporproduct
LiquidproductProcess
fluidSteam
F1
F2 F3
T1 T2
T3
T5
T4
T6 P1
L1
A1
L. Key
OPERATING WINDOW
The vapor flow rate is usually small. However, in some cases, it is 20 times more that its typical value. What do we do?
We provide a larger pipe and valve in parallel. The pressure control will adjust the small valve first, then the large valve.
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
Size of Oper. Window
OPERATING WINDOW
Determine whether the process and equipment function correctly everywhere within the window.
feasible
Process variable 1
Pro
cess
var
iab
le 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
After the frame has been established, we check the internal points. Are there any “donut holes”?
“Holes” in Oper. Window
OPERATING WINDOW
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
“Holes” in Oper. Window
Any concerns about this design?
Porifice=P1 – P3
Distance
pres
sure
Sensors: Principles of the orifice meter
PorificeMeasure pressure drop
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
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
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
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?
Porifice=P1 – P3
Distance
pres
sure
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
OPERATING WINDOW
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
“Holes” in Oper. Window
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
“Holes” in Oper. Window
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
“Holes” in Oper. Window
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 happensin 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
“Holes” in Oper. Window
http://www.britannica.com/EBchecked/topic-art/632655/7035/Volute-centrifugal-pump
http://www.sprayingequipmentsupply.com/pumps/centrifugal-pumps.html
Basic concept of a centrifugal pump
Basic concept of a centrifugal pump
Towler, G. and R. Sinnott (2008) Chemical Engineering Design, Elsevier-Butterworth-Heinemann, page 254
Constant speedImpeller
diameter
Basic concept of a centrifugal pump
http://hiramada.wordpress.com/2009/07/07/introduction-to-centrifugal-pump-technical-selection/
OPERATING WINDOW
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
“Holes” in Oper. Window
Let’s preventbubbles from
forming.
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
“Holes” in Oper. Window
NPSHA
OPERATING WINDOW
Equipment must function correctly within the operating window
NPSHR: 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
“Holes” in Oper. Window
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
“Holes” in Oper. Window
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
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/100) ? (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
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
OPERABILITY : 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
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