1. Plant-wide Control- Part3
CH EN 5253 – Process Design II
Plant-wide Control- Part3

2. Book
Product and Process Design Principles: Synthesis, Analysis and Evaluation
by J. D Seader, Warren D. Seider and Daniel R. Lewin
Chapter 20 (4th Edition)
Individual Control Loops
These are all simple 1 input, 1 output controllers with alarms for safety
Control for Unit Operation
Reactor, Heat Exchanger, Distillation Column
Have Not Talked About
Flash Unit
Furnace
Plant-wide Control

3. Distillation Control Schemes
Use when Feed is Fixed
L, V control (b)
D, V control (a)
Use ratio when Feed is variable
L/F, V control
D/F, V control
DV control
LV control

4. Flash Control Scheme This design works great for a cold liquid feed
Pressure Control
Temperature Control
Level Control
Hot Gas Fed to Flash
Spray of cold liquid to Flash Tank
Heat Exchanger Placed at Vapor Exit
Always use a De-mister at Flash Exit

5. Plant Wide Control
Alan Foss (“Critique of chemical process control theory”, AIChE Journal,1973):
“The central issue to be resolved ... is the determination of control system structure. Which variables should be measured, which inputs should be manipulated and which links should be made between the two sets? “

6. Plantwide Control Individual Control Loops Control for Unit Operation
These are all simple 1 input, 1 output controllers with alarms for safety
Control for Unit Operation
Reactor, Heat Exchanger, Distillation Column, Flash Unit, Furnace
Plant-wide Control

7. Plant-wide Control Plant Wide Control is Different Profit Optimizer
Active Control System Objectives
Meet production rate and product quality targets
Keep system in Safe Operating Range for equipment, catalysts and materials of construction
Minimize Energy Utilized
Minimize Process Variability
Meet environmental regulations
Air quality
Water discharge quality
Profit Optimizer
Provide inputs for Active Control System
Separate Safety Control System
separate sensors, valves, controllers, power system
Identify Unsafe conditions
Take Action to Safely Bring the Plant/System Down to a Safe Point of Operation or Shut Down

8. Qualitative Design Procedure for Plantwide Control
Step 1: Establish Control Objectives
Safe Operation – Process Within its Constraints
Meets Environmental Constraints
Control Production Rate
Feed Flow Controls
Product (on-demand) Flow Controls
Control Product Quality
Step 2: Determine the Process Degrees of Freedom
Position Control Valves
Step 3: Establish Energy Management System
Temperature control for exothermic/endothermic reaction
Step 4: Set the production rate

9. Qualitative Design Procedure for Plantwide Control
Step 5: Control the product quality and handle safety, environmental, and operational constraints
Step 6: Fix a flow rate in every recycle loop and control vapor and liquid Inventories ( vessel pressure and level)
Step 7: Check component balances ( if build up in the process)
Step 8: Control the individual process units
Step 9: Optimize economics and improve dynamic controllability

10. SELECTION OF CONTROLLED VARIABLES
Rule 1: Select variables that are not self-regulating.
Self Regulating dx/dt=f(u)
Non Self Regulating dx/dt=f(u,x)
Rule 2: Select output variables that would exceed the equipment and operating constraints without control.
Rule 3: Select output variables that are a direct measure of the product quality or that strongly affect it.
Also those that are a direct measure of process emissions
Rule 4: Choose output variables that seriously interact with other controlled variables.
Rule 5: Choose output variables that have favorable static and dynamic responses to the available control variables.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

11. SELECTION OF MANIPULATED VARIABLES
Rule 6: Select inputs that significantly affect the controlled variables.
Rule 7: Select inputs that rapidly affect the controlled variables.
Rule 8: The manipulated variables should affect the controlled variables directly rather than indirectly.
Rule 9: Avoid recycling disturbances.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

12. SELECTION OF MEASURED VARIABLES
Rule 10: Reliable, accurate measurements are essential for good control.
Rule 11: Select measurement points that are sufficiently sensitive.
Rule 12: Select measurement points that minimize time delays and time constants.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

13. Example 1
Plant-wide Control System Configuration for an exothermic reaction Process

14. A  B 1 Jacketed CSTR, R-100 1 Flash vessel, V-100 3 Heat Exchanger
Exothermic reaction

15. Nine-step design procedure for plantwide control A  B
Step 1: Establish Control Objectives
Production rate of B at specified level
Conversion in reactor highest possible
Constant composition for B from the flash vessel

16. Where to set the production rate?
Very important!
Should it be at the inlet or outlet?
Determines structure of remaining inventory (level) control system

17. On-demand Product or Feed Flow Control System
Controlling Outlet (product) flow
Controlling Inlet (feed) flow
On-demand product flow
FC on Product B
Feed of A controlled as needed by reactor
Level Control dictates feed flow rates
FC on Feed A Flow
Level Controls Product B Flow Rate
Level Controls Reactor Product Flow
Note: Reactor Temp is controlled with Cascade Control To meet Composition Requirements!

18. A  B Step 2: Determine Degrees of Freedom for Control System
7 DOF for this system, 7 Manipulated Variables (7 control valvues)
3 Utility streams ( controlled by V-2, V-3, V-6)
1 Feed flow rate (controlled by V-1)
1 Reactor effluent flow rate (controlled by V-4)
2 Product flow rates (controlled by V-5 and V-7)
If on-demand flow control is decided
B Product flow rate is controlled independently by V-7

19. A  B Step 3: Establish Energy Management System
2 Manipulated Variables – critical for safety
Reactor feed temperature (controlled by V-2)
Reactor effluent temperature(controlled by V-3)
Flash Temperature (V-6)

20. A  B Step 4: Set the production rate
If on-demand product flow is selected
Desired flow rate by V-7

21. A  B
Step 5: Control the product quality and handle safety, environmental, and operational constraints
Which parameters determine product quality?
Pressure and temperature of the flash vessel, V-100
Temperature regulated by coolant flow rate through V-6
Pressure is controlled by adjusting flow rate of unreacted A through overhead valve V-5
Rapid and direct effects
Satisfy the Objective 3: constant composition

22. A  B
Step 6: Fix recycle flow rates and vapor and liquid Inventories (vessel pressure and level)
Flash vessel level
Outlet control valve already assigned for product rate
Inlet flow i.e., reactor effluent flow is controlled (controlled by V-4)
Reactor level
Inlet flow i.e., feed flow rate (controlled by V-1)
Both direct and rapid control
Vapor inventory
Control valve V-5, already assigned for pressure

23. A  B Step 7: Check component balances
Not needed- no build up of A and B

24. A  B Step 8: Control the individual process units
No need for individual unit control
All control valves have been assigned
No additional control loops can be designed

25. Single loop vs. cascade Single loop control Cascade control
Master controller
Slave controller
Single loop control
Cascade control

26. PID Tuning – Lamda Tuning
P only P=1
Step Test – CV +/- 10%
Observe increase/decrease in PV
Τd and τ (63.2%) and Kp=%PV/%CV(output)
Kc=(Kp)-1*(Tr)/(λ+Td)
Tr=τ, λ=0.5 to 4 (typically 3) times Max(Td, Tr)
Cascade Control inner loop 5x to 10x faster than outer loop

27. Lamda Tuning Results Max(Td, Tr) = 4 s

28. A  B Step 9: Optimize economics and improve dynamic controllability
2nd Objective: Conversion in reactor highest possible
Temperature control system already established
What is the temperature set point?
Cascade control:
Reactor temperature controller (controlled by V-2)
Temperature Set point adjusted to control
the concentration of B
Irreversible reaction
Highest possible temperature ( safe limit)
Controller unnecessary
Cascade control

29. EXAMPLE 3: CYCLIC PROCESS (Cont’d)
Steps 1 & 2: Establish the control objectives and DOFs:
Maintain the production rate at a specified level.
Keep the conversion of the plant at its highest permissible value.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

30. EXAMPLE 3: CYCLIC PROCESS (Cont’d)
Step 3: Establish energy management system.
Need to control reactor temperature: Use V-2.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

31. EXAMPLE 3: CYCLIC PROCESS (Cont’d)
Step 4: Set the production rate.
For on-demand product: Use V-7.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

32. EXAMPLE 3: CYCLIC PROCESS (Cont’d)
Step 5: Control product quality, and meet safety, environmental, and operational constraints.
To regulate V-100 pressure: Use V-4
To regulate V-100 temperature: Use V-5
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

33. EXAMPLE 3: CYCLIC PROCESS (Cont’d)
Step 6: Fix recycle flow rates and vapor and liquid inventories
Need to control recycle flow rate: Use V-6
Need to control vapor inventory in V-100: Use V-4 (already installed)
Need to control liquid inventory in V-100: Use V-3
Need to control liquid inventory in R-100: Cascade to FC on V-1.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

34. EXAMPLE 3: CYCLIC PROCESS (Cont’d)
Steps 7, 8 and 9: Improvements
Install composition controller, cascaded with TC of reactor.
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

35. Example2
Plantwide Control System Configuration for a Vinyl Chloride Process

36. Vinyl Chloride Process
Reactor 1 FeCl3 solid Catalyst
Conversion is >90%
C2H4 + Cl2  C2H2Cl2
Fired Heater for Pyrolysis Reaction
Conversion is 60%
C2H2Cl2  C2H3Cl + HCl

37. Vinyl Chloride Process
C2H4 + Cl2  C2H2Cl2
C2H2Cl2  C2H3Cl + HCl

38. Nine-step design procedure for plantwide control - Vinyl Chloride Process
Step 1: Establish Control Objectives
>90% conversion in dichloroethane reactor, R-100
Fixed conversion (60%) in the pyrolysis furnace (F-100)
Feed flow rate of Ethylene ( V-1 control valve)
C2H4 + Cl2  C2H2Cl2
C2H2Cl2  C2H3Cl + HCl

39. Vinyl Chloride Process
Step 2: Determine Degrees of Freedom for Control System
20 control valves placed
C2H4 + Cl2  C2H2Cl2
C2H2Cl2  C2H3Cl + HCl

40. Vinyl Chloride Process
Step 3: Establish Energy Management System
Reactors temperature control
Temperature of R-100 : V-3
Temperature of F-100 : V-6
Reduction the effects of temperature disturbances
Effluent temperature in the Evaporator, E-101: V-5
Effluent temperature in the Quench tank, V-100: V-7
Effluent temperatures in the partial condenser, E-103: V-8
Effluent temperatures in the recycle cooler, E-108: V-20
Rapid and direct
No recycle of temperature disturbances

41. Vinyl Chloride Process
Step 4: Set the production rate
V1 assigned to control feed
Set point regulates production rate
C2H4 + Cl2  C2H2Cl2
C2H2Cl2  C2H3Cl + HCl

42. Vinyl Chloride Process
Step 5: Control the product quality and handle safety, environmental, and operational constraints
Top products of 2 distillation columns
by adjusting reflux
Distillation column T-100 : V-11
Distillation column T-101 : V-16
Bottom products of 2 distillation columns
by adjusting vapor rate (via stream rate)
Distillation column T-100 : V-13
Distillation column T-101 : V-18
Inlet pressure of distillation column T-100: V-9
Sump level of distillation column T-100 : V-14

43. Vinyl Chloride Process
Step 6: Fix recycle flow rates and vapor and liquid Inventories (vessel pressure and level)
Liquid inventory control
Reactor R-100 : Cascaded V-1
Distillation column T-101 : V-19
Reflux drum V-102 : V-17
Distillation column T-100 : V-14
Reflux drum V-101 : V-10
Vapor inventory control
Distillation column T-101 : V-15
Distillation column T-100 : V-12

44. Vinyl Chloride Process
Step 7 and 8: Check component balances and Control the individual process units
Stoichiometric ratio of feed
Chlorine feed adjusted for >90% conversion : V-2

45. Vinyl Chloride Process
Step 9: Optimize economics and improve dynamic controllability
Improve the toleration for a range of production levels
120% over design, 50% turndown
Setpoint of recycle flow controller is proportional to feed flow rate (cascade loops)

47. DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
SUMMARY
Provided motivation for handling flowsheet controllability and resiliency as an integral part of the design process
Outlined qualitative approach for unit-by- unit control structure selection
Outlined qualitative approach for plantwide control structure selection
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
10 - Design and Control

48. Alternatives structures for optimizing control
Step 3:
What should we control?
Setpoint optimization
RTO controller
Real Time Optimizing

49. Step 3. What should we control? (primary controlled variables)
Intuition: “Dominant variables”
Systematic: Define cost J and minimize w.r.t. DOFs

50. HW 8 1) rewrite Variables L,D  D, L/D then optimize
2) use V=D*(R+1) optimize column to minimize V w.r.t. R and NT
This minimizes capital and operating costs of column.