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Fundamentals of Distillation Column Control

by Terry Tolliver

ISA Automation Week 2011

2

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.

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

3

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

5

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

16

SEPARATION CONTROL

FC

FC

FC

FC

FC

LC

LC

TC NOT RECOMMENDED

17

PRESSURE CONTROL

FCFCLC

VENT

PI

18

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

21

MATERIAL BALANCE CONTROL - TYPE 2

FC

FC

FC

FC

FC

LC

LC

TC

22

MATERIAL BALANCE CONTROL - TYPE 3

FC

FC

FC

FC

FC

LC

LC

TC

23

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

28

29

REFERENCES

30

REFERENCES