1. Process Operability Class Materials Copyright © Thomas Marlin 2013
Operating Window
Basic flowsheet
Design with Operability FC 1 LC
Copyright © Thomas Marlin 2013
The copyright holder provides a royalty-free license for use of this material at non-profit educational institutions

2. 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
PROCESS OPERABILITY :
THE OPERATING WINDOW
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

3. 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 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
feasible

4. OPERATING WINDOW The variables in the plot can be
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.

5. OPERATING WINDOW Minimum heating valve opening is “hard” Maximum 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
Define Oper. Window
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)
feasible

6. 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.
Note:
This shows a range of set points that can be achieved (without disturbances). T A
Reactant
Solvent
Coolant
infeasible
feasible
infeasible
A  B
-rA = k0 e -E/RT CA
What do you note about the shape
of the operating window?

7. 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.
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)

8. Design Basis Memorandum.
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 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.
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.

9. 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 frame defines the “size” of the operating window. These are typically physical bounds, equipment operation and stream specifications.
Determine the constraints (limitations) that define the frame (boundary) of the window
Process variable 2
feasible
Process variable 1

10. 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: Determine typical constraints that affect the operating window for a distillation tower. FR FV xB xD

11. OPERATING WINDOW Class Workshop: Distillation Constraints xD FR FV xB
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
Maximum cooling capacity FR FV xB xD
Maximum and minimum liquid and vapor flow rates
Product composition
Pumping, pipe, valve capacity
Maximum and minimum liquid and vapor flow rates
Maximum heating
Flow pipe, valve capacity
Minimum natural circulation to reboiler
Product composition

12. The exchanger exists to cool this stream
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.
Heat exchanger Q = U AY (T)lm
Hot process fluid into shell
Cooling water into tubes
The exchanger exists to cool this stream
What are the “worst case” operating conditions we would use to design the heat exchanger?

13. 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 – The Worst Case gives the largest area for heat exchange.
Highest flow rate,
Highest temperature
Lowest flow rate,
Highest temperature
Hot process fluid into shell
Cooling water into tubes
Lowest temperature
Greatest fouling,
Lowest U
How do we
determine values?

14. 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?

15. 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.
Highest vessel
pressure
Highest flow,
largest friction
factor
Lowest level
(lowest head) P
Highest
pressure drop
Highest
pressure drop
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

16. 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.
Small equipment*
Large equipment
Just satisfies base case
Advantages
Disadvantages
* = small equipment just satisfies base case design point

17. OPERATING WINDOW Class Workshop: Complete the following table.
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.
Small equipment
Low capital cost
Most efficient at base case
Achieve “precise” operation (smaller equipment to adjust)
Advantages
Cannot achieve higher capacity
Cannot compensate for large range of disturbances
Cannot achieve fast transition (no overshoot in manipulated variable)
Disadvantages

18. OPERATING WINDOW Class Workshop: Complete the following table.
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.
Large equipment
Can achieve higher capacity
Can compensate for likely range of disturbances
Can achieve faster transition (allows overshoot in manipulated variable)
Advantages
High capital cost
Likely lower efficiency at base case and lower production rates
Might not achieve “precise” operation at base case
Disadvantages

19. 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.

20. OPERATING WINDOW Some designs increase 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
Size of Oper. Window
OPERATING WINDOW
Some designs increase the operating window
Centrifugal pumps - Configurations to increase the operating window
Flow rate
Head
Typical pump head curve
Series
Parallel
Pumps provide “pressure (head)” and “flow”. How do we select the correct option, if needed?

21. OPERATING WINDOW Some designs that increase 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
Size of Oper. Window
OPERATING WINDOW
Some designs that increase the operating window
Centrifugal pumps - Configurations to increase the operating window
Series: This configuration provides higher pressure at (approximately) the same flow rate.
Series
Parallel: This configuration provides higher flow rate at (approximately) the same pump exit pressure.
Parallel

22. OPERATING WINDOW Some designs that increase 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
Size of Oper. Window
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?

23. 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
We provide a larger pipe and valve in parallel. The pressure control will adjust the small valve first, then the large valve.
The vapor flow rate is usually small. However, in some cases, it is 20 times more that its typical value. What do we do?

24. 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
After the frame has been established, we check the internal points. Are there any “donut holes”?
Determine whether the process and equipment function correctly everywhere within the window.
Process variable 2
feasible
Process variable 1

25. 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
Orifice meter
heating FC
Cold (20C) liquid
Velocity increases; Bernoulli says that pressure decreases
Any concerns about
this design?

26. Sensors: Principles of the orifice meter
Porifice
Measure pressure drop
                                                                                      
Porifice=P1 – P3
pressure
Distance 

27. Sensors: Principles of the orifice meter
Nice visual display of concept.
In practice, pressure difference is measured by a reliable and electronic sensor
= Porifice
From: Superior Products, Inc.

28. Relate the pressure drop to the flow rate
v = velocity
F = volumetric flow rate
f = frictional losses
= density
A = cross sectional area
Relate the pressure drop to the flow rate
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)
Most common flow calculation, does not require density measurement

29. Sensors: Principles of the orifice meter
When an orifice meter is used, the calculations in yellow are performed. Typically, they are not shown on a process drawing.
“Measured value” to flow controller K FC
Multiply signal by meter constant K
Take square root of measurement 
Measure pressure difference P
liquid
cooling

30. Sensors: Are there limitations to orifices?
v = velocity
F = volumetric flow rate
f = frictional losses
= density
A = cross sectional area
Relate the pressure drop to the flow rate
General meter eqn.
We assume that the meter coefficient is constant. The flow accuracy is acceptable only for higher values of flow, typically % of the maximum for an orifice
Cmeter
Reynolds number

31. Sensors: Is there a downside to orifices?
What is a key disadvantage of the orifice meter?
Ploss = P1 – P2
Non-recoverable pressure drop
Pressure loss!
When cost of pressure increase (P1) by pumping or compression is high, we want to avoid the “non-recoverable” pressure loss.
pressure
Porifice=P1 – P3
Distance 

32. 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
Orifice meter
heating FC
Cold (20C) liquid
The fluid can partially vaporize.
The pressure difference will
not reliability indicate
the flow rate!
Velocity increases; Bernoulli says that pressure decreases

33. 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 FC
heating
Cold (20C) liquid
Simple solution,
Locate flow
measurement where the
pressure is highest
and temperature lowest.
Ensure that flashing does not
occur - design calc’s

34. 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
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?
Bottom tray
Bottoms product
reboiler
Centrifugal pump

35. 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
Centrifugal pump
Bottoms product
Pressure drop due to the velocity increase in the eye of the pump
Pressure drop due to flow frictional losses
reboiler
What happens
in the
pump?

36. Basic concept of a centrifugal pump

37. Basic concept of a centrifugal pump
Constant speed
Impeller diameter
Towler, G. and R. Sinnott (2008) Chemical Engineering Design, Elsevier-Butterworth-Heinemann, page 254

38. Basic concept of a centrifugal pump

39. 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
Let’s prevent
bubbles from
forming.
Equipment must function correctly within the operating window
Centrifugal pump
Bottoms product
reboiler
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.

40. 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
NPSHR: The manufacturer must define the minimum net positive suction head required.
The process engineer must design to provide it. NPSHA>NPSHR
reboiler
This liquid head increases the pressure at the inlet to the pump and prevents cavitation.
Centrifugal pump
Bottoms product
NPSHA

41. 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
NPSHR: The manufacturer must define the minimum net positive suction head required.
The process engineer must design to provide it. How?
Elevate the liquid above the pump (two ways)
Reduce friction losses
Subcool the liquid (careful of added pressure drop)
This is issue when liquid is at (near) its bubble point. Give examples when this is the situation in chemical processes.
From: Woods, D.R., Process Design and Engineering Practice, Prentice -Hall, 1995

42. 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
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
From: Woods, D.R., Process Design and Engineering Practice, Prentice -Hall, 1995

43. Fortunately, engineers have lots of relevant experience!
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
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!

44. “For well tested process, safety factors can approach 0%” *
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: 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)

45. OPERATING WINDOW INDUSTRIAL PRACTICE
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.

46. 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
OPERABILITY : THE OPERATING WINDOW
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