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Standards
Certification
Education & Training
Publishing
Conferences & Exhibits
Fundamentals ofDistillation Column Control
by Terry Tolliver
ISA Automation Week 2011
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PRESENTER Terry Tolliver
Terry is a retired Senior Fellow from Solutia/Monsanto and presentlyprovides contract engineering at ConocoPhillips Wood River refinery.He has 40+ years of experience in process control, simulation,operations, troubleshooting and optimization.
Terry graduated from the Missouri University of Science and
Technology with BS, MS & PhD ChE and was recently inducted intotheir Academy of Chemical Engineers in 2009. He was inducted intothe CONTROL Automation Hall of Fame in 2002.
He has received several ISA awards including the Distinguished SocietyService Award in 1997, the E. G. Baily Award in 1993 and the
Excellence in Documentation award in 1987. Terry has been an adjunct professor for Washington University from
2004-2008, a Lehigh University Biannual Lecturer from 1974-1986, anISA Fellow in 1990, an AIChE Fellow in 2000 and a PE since 1974.
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DISTILLATION CONTROL TOPICS
Levels of control
Classification and pairing of variables
Control objectives and constraints
Dynamic responses Material and energy balances
Separation
Pressure control
Material balance control Temperature control
References
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DISTILLATION CONTROL
BASIC
Inventory Control
Composition Control
OPTIMIZING
Floating Pressure
Maximum Profit
SUBOPTIMIZINGFeedforward Control
Two Point Com position Control
4
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CLASSIFICATION and PAIRING of VARIABLES
CONTROLLED
VARIABLESdistillate composition
bottom compositionaccumulator level
sump level
column pressure
YY
X
X
La
La
Ls
Ls
P
P
5
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MANIPULATED
VARIABLESdistillate flow
bottom flow
reflux flow
reboiler duty
condenser duty
D
DB
B
R
R
Qr
Qr
Qc
Qc
CLASSIFICATION and PAIRING of VARIABLES
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CLASSIFICATION and PAIRING of VARIABLES
DISTURBANCE
VARIABLESfeed flow
feed compositionfeed temperature
reboiler heat supply
condenser cooling
supply and weather
F F
Z Zq
qS
S
W
W
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CLASSIFICATION and PAIRING of VARIABLES
CONTROLLED MANIPULATED DISTURBANCE
VARIABLES VARIABLES VARIABLES
distillate composition distillate flow feed flow
bottom composition bottom flow feed composition
accumulator level reflux flow feed temperature
sump level reboiler duty reboiler heat supply
column pressure condenser duty condenser cooling
supply and weather
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CONTROL OBJECTIVES
$/ F
RECOVERY
Cost of Produc t Lost
Cost of Energ y
Tota l Operating Cost
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COLUMN CONSTRAINTS
Column Pressure
Boiluprate
Vessel Limit
Weep
Flood
Reboiler
Condenser
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COLUMN INTERNALS
TRAYS SIEVE TRAYS
VALVE TRAYS
BUBBLE CAP TRAYS DUAL FLOW TRAYS
PACKING RANDOM PACKING
STRUCTURED PACKING LIQUID & VAPOR DISTRIBUTORS
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COLUMN DYNAMICS
RELATIVE RESPONSE TIMES
time
VAPOR - FAST
LIQUID - MEDIUM
COMPOSITION - SLOW
Response
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TOTAL REFLUX
n = [y(1-x)]/ [x(1-y)]
SEPARATION
NORMAL OPERATION
S = ( , N, NF, V/ F, z, E)f
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MATERIAL BALANCE and SEPARATION EFFECTS
D/F
LightKeyMoleFrac
tion
0 1.00
1.0
y
X
Z
Increasing S
Increasing S
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MATERIAL BALANCE CONTROL
FC
FC
FC
FC
FC
LC
LC
TC
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PRESSURE CONTROL
FCFCLC
VENT
PI
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CONDENSER DUTY
Qcond = UA(Tp-Tc)
VENT
CONDENSATE
COOLANT
Tc
Tp
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BLOCK & BLEED PRESSURE CONTROL
FCFCLC
VACUUM SOURCE
PC
BLOCK VALVE
BLEEDVALVE
INERTS
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MATERIAL BALANCE CONTROL - TYPE 1
FC
FC
FC
FC
FC
LC
LC
TC
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MATERIAL BALANCE CONTROL - TYPE 3
FC
FC
FC
FC
FC
LC
LC
TC
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MATERIAL BALANCE CONTROL - TYPE 4
FC
FC
FC
FC
FC
LC
LC
TC
time
response
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PROCESS SIMULATION
STEADY STATE PARAMETRIC CASES CONTROL STAGE LOCATION
SENSITIVITY ANALYSIS DISTURBANCE ANALYSIS
DYNAMIC STUDY CONTROL RESPONSE STARTUP AND SHUTDOWN
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CONTROL STAGE LOCATION
Benzene-Toluene Column Temperature Profiles
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
70.0 75.0 80.0 85.0 90.0 95.0 100.0 105.0 110.0 115.0 120.0
Temperature, DegC
StageNumber
+1%D/F
-1%D/F
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TEMPERATURE SENSITIVITY
CONTROL STAGE TEMPERATURE, degF
98
98.2
98.4
98.6
98.8
99
99.2
99.4
99.6
99.8
100
85 90 95 100 105 110
COMPOSITION,w
t%
BOTTOM PRODUCT
DISTILLATE PRODUCT
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DISTURBANCE ANALYSIS
Benzene-Toluene Column Load Effects
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
-5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0
Delta Temperature, DegC
TrayNumber
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REFERENCES
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REFERENCES