1. CHE412 Process Dynamics and Control
BSc (Engg) Chemical Engineering (7th
Semester)
Dr Waheed Afzal
Associate Professor of Chemical Engineering
wa.icet@pu.edu.pk
Institute of Chemical Engineering and Technology
University of the Punjab, Lahore
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2. 2
• George Stephanopoulos. Chemical process control. Englewood Cliffs,
New Jersey: Prentice-Hall, 1984
• Donald R. Coughanowr and Steven E. LeBlanc. Process Systems Analysis
and Control. McGraw-Hill Science/Engineering/Math, 2008
• William L Luyben. Process modeling, simulation and control for chemical
engineers. 2nd
Edition, McGraw-Hill Higher Education, 1996
• Don Green and Robert Perry. Perry's Chemical Engineers' Handbook,
Eighth Edition McGraw-Hill, New York, 2007
• Dale E. Seborg, Thomas F. Edgar, and Duncan A. Mellichamp. Process
dynamics & control. Wiley. com, 2006.
• Lecture Notes/ Handouts
Text/ Reference Books
3. Place of Process Control in a typical Chemical Plant
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Luyben (1996)
4. Need of a Control
Safety:
Equipment and Personnel
Production Specifications:
Quality and Quantity
Environmental Regulations:
Effluents
Operational Constraints:
Distillation columns (flooding, weeping); Tanks
(overflow, drying), Catalytic reactor (maximum
temperature, pressure)
Economics:
Minimum operating cost, maximum profits
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5. Requirements
1. Suppressing External Disturbances
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Objectives: Achieve Set-point
T = Ts
h = hs
After reaching steady-state
from start-up, disturbances in Fi
and Ti cause changes in F, T.
How to achieve the objective?
Stirred Tank Heater (Stephanopoulos, 1984)
6. Controlling T in a Stirred Tank Heater
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measure T
compare measured T
with Ts
Compute error:
e = Ts - T
e > 0; Ts > T (increase Fst)
e < 0; Ts < T (reduce Fst)
Feedback Control in a Stirred Tank Heater
(Stephanopoulos, 1984)
7. 2. Ensure the Stability of a Process
x (or y) can be T, CA, F; x is disturbed at t0
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x returns to steady-state
without an intervention in a
self-regulating process
y never returns to steady-
state in three different
unstable processes (A, B, C)
8. 3. Optimization of the Performance of a Batch Reactor
Optimization is a major requirement to achieve maximum profit.
A (feed) → B (desired) → C (undesired); endothermic reaction
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Scenarios:
Q(t) is given the largest value
during entire TR to favor A → B
Q(t) is given the smallest value
during entire TR to suppress B→ C
Optimization of Q(t) during TR
Steam
Condensate
Economic Objective
Maximize profit =
ʃ0
tR
f (A, B, steam) dt
9. Visualizing ‘Optimization’ in Chemical Plants
Case: Liquid can be pumped between two points by choosing different
pipe diameters (with right pumping system). The total cost of
transportation includes the pumping (and power) cost and piping cost.
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Cost
/
year/
length
Pipe Diameter
Scenario One:
Pipe with smaller diameters are
cheaper but pumping cost
increases.
Scenario Two:
Pumping cost is small in a pipe
with large diameter but pipes
are expensive.
What is the ‘best’ pipe-
pump combination?
Peters and Timmerhaus (1991)
10. 10
Classification of Variables
Input variables (sometime called as load variables or LV)
Further classified as disturbances and manipulated or control
variables)
Output variables
Further classified into measured and unmeasured variables
Often, manipulated variable effects output variable
(measured) known as controlled variable
When an output variable is chosen as a manipulated variable,
it becomes an input variable.
A manipulated variable is always an input variable.
11. 11
Design Elements in a Control
Objective: h = hs (Controlled Variable or CV)
Scenario C.
Variable
M.
Variable
Input
Variable
Output
Variable
1 (shown) h F Fi h
2 h Fi F, h
Define Control Objective: what are the operational objectives of a control
system
Select Measurements: what variables must be measured to monitor the
performance of a chemical plant
Select Manipulated Variables: what are the manipulated variables to be used
to control a chemical process
Select the Control Configuration: information structure for measured and
controlled variables. Configurations include feedback control, infrential
control, feedforward control
F
h
A
12. 12
Input variables
Fi, Fst, Ti, (F)
Output variables
F, T, h
Control Objective
(a) T = Ts
(b) h = hs
F, T
Fst
h A
F, T
h A
Fst
Temperature and level control in a stirred
tank heater (Stephanopoulos, 1984)
Design Elements in a Control
13. 13
Control Configurations in a Distillation Column
Define Control Objective:
95 % top product
Select Measurements:
composition of Distillate
Select Manipulated variables:
Reflux ratio
Select the Control Configuration:
feedback control
(Stephanopoulos, 1984)
15. 15
Inferential Control in a Distillation Column
(Stephanopoulos, 1984)
Control Objective: xD
Unmeasured input =
f (secondary measurements)
16. • The process (chemical or physical)
• Measuring instruments and sensors (inputs, outputs)
what are the sensors for measuring T, P, F, h, x, etc?
• Transducers (converts measurements to current/ voltage)
• Transmission lines/ amplifier
• The controller (intelligence)
• The final control element
• Recording/ display
elements
Recall Process
Instrumentation
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Hardware for a Process Control System
(Stephanopoulos, 1984)
17. Week 1
Introduction to Process Dynamics and Control
(Stephanopoulos, 1984) Chapter 1-3, Pages 1-41
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Weekly Take-Home Assignment
Problems for Part I (page 36-41) PI.1 to 1.10 of
Stephanopoulos (1984)
Editor's Notes
#3:A chemical plant consists of numerous processing units integrated in a logical way with an over all objective of producing desired products from raw materials in most economic way.
