1. Plant-wide Control- Part2
CH EN 5253 – Process Design II
Plant-wide Control- Part2

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. Symbols in P&ID International Society of Automation (ISA)

4. Outlines of the topic: Plant-wide Control
Basics
Objectives
Controlled /Manipulated variables
Sensors
Instrumentation
Few guidelines
Fail safe strategy
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

5. Control of individual variables

6. Flow Control Flow Transducer/Transmitter
Orifice, venturi, many others
Flow Indicator Controller – FIC
PID
Centrifugal /reciprocating Pump
Fixed speed/constant volume output
If Valve fails
Dead-heading
Check valve to prevent back flow
By-pass loop to prevent dead-heading pump

7. Flow Control : Energy efficient way
Variable Frequency Drive (VFD) can be costly ( higher capital cost)
Energy saving ( lower operating cost)

8. Temperature Control with Utility
The Simplest arrangement in process plant
Control the flow rate of cooling or heating medium
When is bypass line of utility stream used?

9. Temperature Control Arrangement for cooler Cooling medium : Air
Temperature of air varies
seasonal basis ( even hourly basis)

10. Temperature Control Arrangement for cooler Cooling medium : Air
Temperature of air varies seasonal basis ( even hourly basis)
For fixed fan speed

11. Ratio Control Ratio Control Valve – FFV – fail Closed
May have two control valves – one for each stream
2 Flow Transducers/transmitters
Orifice, venturi, many others
Example
Reactor Feeds ( stoichiometric ratio)
Distillation column reflux
2H2+O2  2H2O

12. Control of individual units

13. 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

14. DOF Analysis : Implications on control system design
Complex Systems
Heat Exchanger Networks Control
Reactor Control
Distillation Column Control

15. Heat Exchanger Network Control

16. Heat Exchanger Network Control
Total variables: Nv = 15
3 Flows: F1, F2, F3
9 Temperatures: T0, T1, T2, T3, 0, 1, 2, 3, 4
3 Heat duties of 3 Hx : Q1, Q2, Q3
Externally defined variables: Ndef =4
1 Flow: F1
3 Temperatures: T0, 0, 1
Total equations Neq=3x3=9
for each heat exchanger
Q1=F1Cp1(To-T1)
Q1=F3Cp3(4 - 3)
Q1= U1A1ΔTLMTD
DoF = Nv- Ndef – Neq = = 2
2 Control valves for 2 manipulated variables
2 manipulated variables : F2 and F3

17. Heat Exchanger Network Control
Controlled variables: 3 Target temperatures T3, 2, 4
Manipulated variables: 2 flow rates of two cold streams F2, F3
Conclusion: One temperature can not be controlled
Controlling any two temperatures
3 possible configurations
F2- 2 , F3- 4 ( shown here)
F2-4, F3- 2 ( reverse, unstable)
F2- 2 , F3- T3
Solutions to control all three target temperatures?

18. Solution: HEN Control with bypass stream
Total variables: Nv = 17
3 Flows: F1, F2, F3
1 bypass flow fraction: 
10 Temperatures: T0, T1, T2, T3, 0, 1, 2, 3, 3’, 4
3 Heat duties of 3 Hx : Q1, Q2, Q3
Total equations Neq=3x3+1=10
𝜃 3 = 1− 𝜃 3 ′ + 𝜃 0
With By-pass, DoF = Nv-NDef-Neq= =3

19. Solution: HEN Control with bypass stream
Controlled variables: 3 Target temperatures T3, 2, 4
Manipulated variables: 3
2 flow rates of two cold streams F2, F3
1 flow fraction of bypass stream 
F2  HE2 directly
F3  HE1 and HE3
  T3 directly

20. Reactor Control

21. 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

22. 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

23. CSTR (AB, exothermic, jacketed cooling)
DoF = 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
FA : direct on reactor temperature, T
Fo : same reasons, h

24. Distillation Column Control

25. Distillation Column Control ( Binary and total condenser)
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

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

27. Distillation Column Control
5 Controlled variables -PD, LR, LD, xB, xD
5 manipulated variables -L, D, B, QC, QR
PD is controlled with CV QC
LR is controlled by CV B
xB is controlled by CV QR
LD is controlled by
4/5: CV L if so what does CV D control??
4/5 : CV D if so what does CV L control??

28. With Composition Analysis
LV control
DV control

29. Distillation Control Schemes
L, V control
D, V control
L/F, V control
D/F, V control

30. Control of entire plant

31. Next class ( 5-Mar-2018)