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Standards
Certification
Education & Training
Publishing
Conferences & Exhibits
Fundamentals of Distillation 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 presently provides 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 into their Academy of Chemical Engineers in 2009. He was inducted into the CONTROL Automation Hall of Fame in 2002.
• He has received several ISA awards including the Distinguished Society Service 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, an ISA 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
SUBOPTIMIZING–Feedforward Control–Two Point Composition Control
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CLASSIFICATION and PAIRING of VARIABLES
CONTROLLEDVARIABLESdistillate compositionbottom compositionaccumulator levelsump levelcolumn pressure
Y Y
X
X
La
La
Ls
Ls
P
P
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MANIPULATEDVARIABLESdistillate flowbottom flowreflux flowreboiler dutycondenser duty
DDB
B
R
R
Qr
Qr
Qc
Qc
CLASSIFICATION and PAIRING of VARIABLES
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CLASSIFICATION and PAIRING of VARIABLES
DISTURBANCEVARIABLESfeed flowfeed compositionfeed temperaturereboiler heat supplycondenser cooling
supply and weather
F FZ Zq qS
S
W
W
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CLASSIFICATION and PAIRING of VARIABLES
CONTROLLED MANIPULATED DISTURBANCEVARIABLES VARIABLES VARIABLES
distillate composition distillate flow feed flowbottom composition bottom flow feed compositionaccumulator level reflux flow feed temperaturesump level reboiler duty reboiler heat supplycolumn pressure condenser duty condenser cooling
supply and weather
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CONTROL OBJECTIVES
$/F
RECOVERY
Cost of Product Lost
Cost of Energy
Total Operating Cost
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COLUMN CONSTRAINTS
Column Pressure
Boilu
p ra
te
Vessel Limit
Weep
FloodReboiler
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 - FASTLIQUID - MEDIUMCOMPOSITION - SLOW
Response
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MATERIAL & ENERGY BALANCES
MATERIAL BALANCEF = D + B
zF = yD + xB
ENERGY BALANCEhF + Qreb = hD + hB + Qcond
D/F = (z-x)/(y-x)
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TOTAL REFLUX
αn = [y(1-x)]/[x(1-y)]
SEPARATION
NORMAL OPERATION
S = (α, N, NF, V/F, z, E)f
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MATERIAL BALANCE and SEPARATION EFFECTS
D/F
Light
Key
Mol
e Fr
actio
n
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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SEPARATION CONTROL
FC
FC
FC
FC
FC
LC
LC
TC NOT RECOMMENDED
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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
BLEE
D VA
LVE
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 2
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
Stag
e N
umbe
r
+1%D/F
-1%D/F
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TEMPERATURE SENSITIVITY
CONTROL STAGE TEMPERATURE, degF
CO
MPO
SITIO
N, 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
Tray
Num
ber
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REFERENCES
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REFERENCES