1. Plant-wide Control- Part2
CH EN 5253 – Process Design II
Plant-wide Control- Part2
6th March 2019

2. Books
Product and Process Design Principles: Synthesis, Analysis and Evaluation
by J. D Seader, Warren D. Seider and Daniel R. Lewin
Chapter 20 (4th Edition)
Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design
by Gavin Towler, and Ray Sinnott
Chapter 5 (2nd Edition)

3. Outlines : Plant-wide Control
Control of individual variables
Control of individual units
Control of entire
plant
Pressure
Level
Flow
Ratio
Temperature
Vaporizer
Heat Exchanger Network Control
Reactor Control (e.g., CSTR)
Distillation Column Control
previous class
Example 1
Example 2

4. Control of individual units

5. Degrees of Freedom (DoF) Analysis
Determine number of manipulated variables permissible
Before selecting controlled variables and manipulated variables
DoF = Nvariables – Nequations (Universal definition)
DoF = NManipulated + Nexternally defined
NManipulated = Nvariables – Nequations- Nexternally defined

6. Reactor Control AB, exothermic Reactant Inlet (A) Coolant Inlet
Product Outlet (A+B)
outlet

7. CSTR (AB, exothermic, jacketed cooling)
Total variables: Nv = 10
3 Flows: Fi, Fo, Fc
4 Temperatures: Ti, T, Tc, Tco
2 Concentrations : CA, CAi
1 Liquid level: h
Externally defined variables: Ndef =3
1 Concentrations : CAi
2 Temperatures: Ti, Tco

8. CSTR (AB, exothermic, jacketed cooling)
Total equations Neq=4
Overall mass balance
Mass balance on component
Energy balance on reacting mixture
Energy balance on jacket coolant

9. CSTR (AB, exothermic, jacketed cooling)
Nmanipulated = Nv- Ndef – Neq = = 3
3 manipulated variables (# control valves)
3 controlled variables
3 Controlled variables
CA : product quality
T : safe operation
h : non-self-regulating
3 Manipulated variables
Fi : direct and rapid on conversion, CA
FC : direct on reactor temperature, T
Fo : same reasons, h

10. Distillation Column Control ( Binary and total condenser)

11. Primary objectives of distillation column control
Separating two components
Primary objectives of distillation column control
Maintain specified composition of the top and bottom products ( and any side streams) correcting for the effects of disturbances
Feed flow rate, composition and temperature
Steam or other hot utility supply
Cooling water or air cooler conditions
Ambient conditions

12. Distillation Column Control
Total variables: Nv = 4NT+13
Externally defined variables: Ndef =2
Feed Flow: F
Feed composition ( overall) : xF
yn , xn
Ln , hn
LD, xD
L, D
ysump , xsump LR
B, V
F, xF PD QC
Sump

13. Distillation Column Control
Total equations Neq= 4NT+6
yn = kn xn
Correlations
Jeronimo & Sawistowski (1973)
Bennett (1983)
Nmanipulated = Nv- Ndef – Neq = (4NT+13) -2 –(4NT+6) = 5
5 manipulated variables (# control valves)
5 controlled variables

14. Distillation Column Control
Selection of 5 Controlled variables (out of 4NT+13 total variables)
Variable 1: Condenser pressure, PD (Guidelines 3 & 4)
Direct measure or strongly affect the product quality
Variable 2: Sump liquid level, LR (Guideline 1)
Not self regulating
Variable 3: Reflux drum liquid level, LD (Guideline 1)
Variable 4: Compositions of bottom stream, xB (Guidelines 3)
Direct measure of the product quality
Variable 5: Compositions of distillate, xD (Guidelines 3)

15. Distillation Column Control
Selection of 5 Manipulated variables
Variable 1: Condenser pressure, PD is controlled by QC
Variable 2: Sump liquid level, LR is controlled by B
Variable 4: Compositions of bottom stream, xB is
controlled by QR ( equivalent to V (vapor) control in sump)
Variable 3: Variable 5:
Reflux drum liquid level, LD Compositions of distillate, xD
If controlled by L (R>4) Controlled by D
If controlled by D Controlled by L

16. Distillation Column Control
Desired Separation by purity xB and xD or recovery D/F or B/F
LV control
L controls xD
V ( or QR) controls xB
DV control
D controls xD
V ( or QR) controls xB TC
Alternative control TC TC
Near the top of the column

17. Distillate Composition Distillate Composition
Other Control Schemes
Direct Control of
Bottom Composition
Indirect Control of
Bottom Composition
Direct Control of
Distillate Composition
Indirect Control of
Distillate Composition

18. Control of entire plant
Guidelines
Example 1 : Exothermic Reaction
Example 2: Vinyl Chloride Process

19. 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 control Degrees of Freedom
Position Control Valves
Step 3: Establish Energy Management System
Temperature control for exothermic/endothermic reaction
Step 4: Set the production rate

20. 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

21. Example 1
Plantwide Control System Configuration for an exothermic reaction Process

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

23. 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

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

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

26. A  B Step 2: Determine Degrees of Freedom for Control System
7 DOF for this system, 7 Manipulated Variables
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

27. 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)

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

29. 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

30. 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

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

32. 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

33. Single loop vs. cascade Before moving to last point, Step 9: Optimize
Cascade control should always be used
a process with relatively slow dynamics like level, temperature, composition
a liquid or gas flow relatively-fast process, has to be manipulated to control the slow process. 
Master controller
Slave controller
Single loop control
Cascade control

34. 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

35. Example 2
Plantwide Control System Configuration for a Vinyl Chloride Process

36. Vinyl Chloride Process
Reactor 1 FeCl3 solid Catalyst
Conversion is >90%
C2H4 + Cl2  C2H4Cl2
Fired Heater for Pyrolysis Reaction
Conversion is 60%
C2H4Cl2  C2H3Cl + HCl

37. Vinyl Chloride Process

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 in the pyrolysis furnace (F-100)
Feed flow rate of Ethylene ( V-1 control valve)

39. Vinyl Chloride Process
Step 2: Determine Degrees of Freedom for Control System
20 control valves placed

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

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 range of production levels
Setpoint of recycle flow controller is proportional to feed flow rate