1. Process Operability Class Materials Copyright © Thomas Marlin 2013
Dynamic Performance
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: DYNAMIC PERFORMANCE
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:
DYNAMIC PERFORMANCE
In this Lesson, we will learn
Importance of process & equipment design
- Review effect of process dynamics on control performance
- Special concerns with recycle processes
- Special considerations for capacity of manipulated variables
- Mixing process, series packed bed reactor, recycle
reactor, batch reactor
Control Design Guidelines
- Nine-step design method
- Flash process

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
Process - Dynamics
DYNAMIC PERFORMANCE
OPERABILITY provides the capacity and flexibility to respond to changes in plant operation.
In general, we wish to respond quickly.
Compensate for (reject) disturbances
Follow changing set points
The value of “quickly” depends upon the specific process application.
Safety and Equipment protection - extremely fast (& reliable)
Product quality - very fast
Production rate - moderate (very fast if load following)
Efficiency and Optimization - can be slower
Which elements in the process design should be fast, and which should be slow?

4. 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 - Dynamics
DYNAMIC PERFORMANCE
Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance.
How does each parameter affect the dynamic performance for this simple mixing process?
Time constant in disturbance, D
Dead time in feedback,  AC
Time constant in feedback, P
Steady-state Gain, KP

5. 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 - Dynamics
DYNAMIC PERFORMANCE
Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance.
CV limited set point overshoot, fast damping, and return to the set point
S-LOOP plots deviation variables (IAE = )
1.5 1
CV does not change because of dead time
Controlled Variable
0.5  5 10 15 20 25 30 35 40 45 50
Time
1.5 1
Manipulated Variable
0.5
Set point response 5 10 15 20 25 30 35 40 45 50
Time

6. DYNAMIC PERFORMANCE , deadtime
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 - Dynamics
DYNAMIC PERFORMANCE
Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance.
Effect of disturbance without control
Can not prevent this deviation with feedback
, deadtime
Disturbance response

7. How does each parameter affect feedback performance?
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 - Dynamics
DYNAMIC PERFORMANCE
Let’s recall that the process dynamics introduce the fundamental limit to feedback control performance.
How does each parameter affect feedback performance?
Large is good
Small is good
Large (range),
fast element
is good
Fast sensor
Time constant in disturbance, D
Dead time in feedback,  A
Time constant in feedback, P
Steady-state Gain, KP

8. 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 - Dynamics
DYNAMIC PERFORMANCE
CLASS WORKSHOP: The feedback controller is performing well (its well tuned), but the dynamic performance is poor. What improvements can we make? AC 1
Disturbances in feed
Control a chemical reactor by adjusting its preheat
Hint: Think about the structure of
the process and
the control system

9. Process - Dynamics
DYNAMIC PERFORMANCE
CLASS WORKSHOP: The feedback controller is performing well but the dynamic performance is poor. We must change the system structure!
Shorten pipe, faster feedback AC +
Feedforward
Reduce disturbances TC
Cascade AC
Faster feedback LC
Slower disturbances

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
Process - Recycle
DYNAMIC PERFORMANCE
Recycle processes: In some (many), specific components do not appear in exit streams. If no action is taken, these components will accumulate. What is needed in the process design?
Recycle loop
Purge

11. 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 - Recycle
DYNAMIC PERFORMANCE
In some (many) recycle processes, specific components do not appear in exit streams. If no action is taken, these components will accumulate. What is needed in the process design?
Recycle loop
Purge
Issues with purges
Some valuable materials will be lost in purge (recover?)
May introduce stream that requires treatment before release (environment)
The purge flow can be adjusted to control the concentration of inert
Large inert concentrations might be desired (e.g., to affect reactions)

12. 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 - Recycle
DYNAMIC PERFORMANCE
Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved?
Is the
design
complete?
Cold product
Cold feed
Heating fluid
Consider a feed temperature increase
Exothermic chemical reaction
Hot effluent

13. DYNAMIC PERFORMANCE ++++ ++ ++++++++ ++++
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 - Recycle
DYNAMIC PERFORMANCE
Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved?
++++
Cold product ++
++++ + ++
The disturbance
grows because
of the recycle
(positive
feedback)!
Cold feed
Heating fluid
Exothermic chemical reaction
Consider a feed temperature increase
Hot effluent

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
Process - Recycle
DYNAMIC PERFORMANCE
Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved?
Cold product TC
Cold feed
Heating fluid
Conceptually
OK.
Is the feedback
fast enough?
Exothermic chemical reaction
Hot effluent

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
Process - Recycle
DYNAMIC PERFORMANCE
Class Workshop Recycle Processes: How will this recycle process respond to disturbances? Can the design be improved?
Cold product TC
Build in
efficiency;
use the least
heating
fluid!
Cold feed
Heating fluid
Provide fast
feedback!
Exothermic chemical reaction
Hot effluent

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
Process - Recycle
DYNAMIC PERFORMANCE
General principle: When possible, provide an alternative source of material or energy at the recycle point and control the variable (flow or temperature) after the recycle point.
Energy recycle TC
Process
Adjustable heat exchange
Material recycle FC
Process
Adjustable fresh feed

17. 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 – MV range
DYNAMIC PERFORMANCE
We usually want “extra” capacity in the effects of the manipulated variables. Here,“extra” means more than required to achieve the desired steady-state operating window.
Reactor with exothermic reaction F L CW P fc fo T
Let’s concentrate on the control of temperature.

18. 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 – MV range
DYNAMIC PERFORMANCE
We usually want “extra” capacity in the effects of the manipulated variables. (1) MV overshoot
Temperature 10 20 30 40 50 60
0.5 1
1.5
Time
Controlled Variable
Manipulated Variable
p, feedback dead time F L CW P T
CW flow
For many processes, some overshoot of the manipulated variable will improve (speed) control performance.

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
Process – MV range
DYNAMIC PERFORMANCE
We usually want “extra” capacity in the effects of the manipulated variables. (2) Disturbance response F L CW P T
In a few processes, much greater MV capacity is required for response to disturbances.
For example, a highly exothermic reaction operating at too high a temperature will tend to “run away”. A very large exchanger area and CW flow rate could be required to prevent a hazard.

20. 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 – MV range
DYNAMIC PERFORMANCE
We usually want “extra” capacity in the effects of the manipulated variables. (2) Disturbance response F L CW P T
Inert gas fo fc
In a few processes, much greater MV capacity is required for response to disturbances, even a source and sink.
For example, the reactor might produce gas at some times and condense gas at others times. Pressure control requires an exhaust line (originally shown) and an inert gas feed line (added for this figure).
See Marlin (2000) Chapter 22 for control design

21. Process Control Design Guidelines For Multiloop PID Feedback Control
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
For Multiloop PID Feedback Control
Define Control Objectives
Select measurements
Ensure sufficient degrees of freedom exist
Ensure the process is controllable
Ensure that the operating window is sufficiently large
Evaluate interaction
Eliminate designs with poor integrity
Eliminate designs requiring extensive retuning
Apply loop pairing guidelines and unit operation experience

22. DYNAMIC PERFORMANCE Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
Process Example – Flash Process
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
Vapor
product
Liquid
Process
fluid
Steam F1 F2 F3 T1 T2 T3 T5 T4 T6 P1 L1 A1
L. Key
P  1000 kPa
T  298 K
Let’s review the process

23. Process Control Design Guidelines 1. Define the Control Objectives
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
1. Define the Control Objectives

24. Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
2. Select measurements
Required to achieve each of the control objectives
Define the sensor technology (e.g., orifice meter, venturi meter, mass flow) compatible with process conditions and achieve required accuracy and reproducibility
Determine if for only local display or for transmission to centralized control for monitoring, history and control
Define range (e.g., C)

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
Control Design
DYNAMIC PERFORMANCE
CLASS WORKSHOP on CV selection: We want to know the composition of the bottoms product, but the analyzer is too expensive (or doesn’t exist). What do we do? TC
Inferential/Dominant Variables* - Easily measured and strongly influence product quality and/or profit.
In many (not all) distillation towers, tray temperatures are a good inference of product composition. (The profile moves up/down the column)
Great opportunity to use your process knowledge!
* See Marlin (2000) Chapter 17 for further discussion on inferential variables PC LC LC

26. DYNAMIC PERFORMANCE Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
3. Ensure sufficient degrees of freedom
Degrees of freedom are adjustable variables, i.e., valve openings, motor speeds, etc.
Inputs
Process
Outputs
Controlled variables, related to control objectives
final
element
sensor
Adjustable manipulated variables
final
element
sensor
Majority of variables remain unmeasured (and uncontrolled)
Disturbances variables

27. DYNAMIC PERFORMANCE Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Is that all that is required?
Process Control Design Guidelines
3. Ensure sufficient degrees of freedom
The system is has sufficient degrees of freedom if
# of manipulated variables  # of controlled variables
Inputs
Process
Outputs
Controlled variables, related to control objectives
final
element
sensor
Adjustable manipulated variables
final
element
sensor
Majority of variables remain unmeasured (and uncontrolled)
Disturbances variables

28. DYNAMIC PERFORMANCE Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Is that all that is required?
Process Control Design Guidelines
4. Ensure that the process is controllable
Controllability ensures that the selected controlled variables can be moved in desired direction by the manipulated variables
The system is controllable if Det [Kp]  0 (Kp is the gain matrix)
Inputs
Process
Outputs
Controlled variables, related to control objectives
final
element
sensor
Adjustable manipulated variables Kp
final
element
sensor
Majority of variables remain unmeasured (and uncontrolled)
Disturbances variables

29. Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
5. Ensure that the operating window is large enough to satisfy the design specification, which gives disturbances and set point changes.
Ensures that the capacities of the manipulated variables are large enough to “move” the process as needed.

30. DYNAMIC PERFORMANCE Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
6. Evaluate Interaction: Does it exist?
A multivariable process has interaction when input (manipulated) variables affect more than one output (controlled) variable. 20 40 60 80
100
120
0.98
0.982
0.984
0.986
0.988
0.99
Time (min)
XD (mol frac)
0.02
0.025
0.03
0.035
0.04
XB (mol frac)
1.12
1.125
1.13
1.135
x 10 4
R (mol/min)
1.5613
1.5614
1.5615
V (mol/min)
Step change to reflux with constant reboiler

31. DYNAMIC PERFORMANCE Process Control Design Guidelines
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
Eliminate designs with poor integrity
Eliminate designs requiring extensive retuning
This topic requires some additional principles from process control.
See Marlin, Process Control, (2000), Chapter 20

32. Process Control Design Guidelines 9. Apply loop pairing guidelines
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
Control Design
DYNAMIC PERFORMANCE
Process Control Design Guidelines
9. Apply loop pairing guidelines
LOOP PAIRING QUALITATIVE GUIDELINES
CVi - MVj pairing that has strong effect (large Kp)
CVi - MVj pairing that has fast dynamics
CVi - MVj pairing that has large range (MV min to max)
CVi - MVj pairing with causal relationship that is (nearly) independent of other loops, i.e., reduce interactions
If excess MVs exist, adjust MVj that has lowest cost; keep high cost MV near zero
Often, these guidelines cannot all be satisfied. In some cases, they must be violated to achieve good performance.

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
Control Design
DYNAMIC PERFORMANCE
Class workshop: Design control loops (select CV-MV pairings) for the flash process.
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane
Vapor
product
Liquid
Process
fluid
Steam F1 F2 F3 T1 T2 T3 T5 T4 T6 P1 L1 A1
L. Key
P  1000 kPa
T  298 K

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
Control Design
DYNAMIC PERFORMANCE
Class workshop: Design control loops (select CV-MV pairings) for the flash process.
cascade
Vapor
product
PAH
TC-6
PC-1 T5 T1
Feed
Methane
Ethane (LK)
Propane
Butane
Pentane T2
LAL
LAH
FC-1 T3
LC-1 F2 F3
Liquid
product
AC-1
Process
fluid
Steam
L. Key
See Marlin (2000), Process Control, Chapter 24 for details

35. DYNAMIC PERFORMANCE 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
Control Design
DYNAMIC PERFORMANCE
INDUSTRIAL PRACTICE
Good performance through process design is more reliable and preferred if the cost is acceptable.
We are guided by control objectives (safety, protection, smooth operation, product quality, and profit)
We seek to provide good control with simple “loop pairing” control strategies, if possible.
We use special knowledge for each unit operation – distillation, reactors, boilers, compressors, evaporators, etc.

36. PROCESS OPERABILITY: DYNAMIC PERFORMANCE
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:
DYNAMIC PERFORMANCE
In this Lesson, we will learn
Importance of process & equipment design
- Review effect of process dynamics on control performance
- Special concerns with recycle processes
- Special considerations for capacity of manipulated variables
- Mixing process, series packed bed reactor, recycle
reactor, batch reactor
Control Design Guidelines
- Nine-step design method
- Flash process