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DISTILLATION CONTROLDISTILLATION CONTROL

Dr. Prakash KarpeDr. Prakash KarpeControl & Elec. Eng. Supt.Control & Elec. Eng. Supt.

ConocoPhillipsConocoPhillips

San Francisco Refinery, RodeoSan Francisco Refinery, Rodeo

2

D

B

L

R = L/DF

Distillation Column ControlDistillation Column ControlControl ObjectivesControl Objectives

V

RectificationStages

StrippingStages

QH

Qc

• Two Control objectives– Inventory control

– Composition control

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Degrees of Freedom AnalysisDegrees of Freedom Analysis

• From control perspective, degrees of freedom of a process is defined as number of variables that can or must be controlled.– Helps to avoid over- or under-control of

processes.

• Degrees of freedom (to control) = No. of rationally placed control valves– A control valve represents a

manipulated variable (MV)

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Degrees of Freedom Analysis Degrees of Freedom Analysis Flash Vessel (Separator)Flash Vessel (Separator)

V

B

F

F,T,P,xi

Disturbances

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Inventory ControlInventory Control

• For steady state operation of a process, all inventories must be controlled– Vapor inventories are maintained by

pressure control– Liquid inventories are maintained by

level control

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Degrees of Freedom Analysis Degrees of Freedom Analysis Flash Vessel (Separator)Flash Vessel (Separator)

V

B

F

F,T,P,xi

Disturbances

LC

PC

Degrees of Freedom = 0

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F

LC

D

B

L

V

QH

PC

Degrees of Freedom AnalysisDegrees of Freedom AnalysisTypical Distillation ColumnTypical Distillation Column

Inventory ControlInventory Control

Degrees of Freedom = 3

LD

LB

TD

TB

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Liquid Inventory ControlLiquid Inventory ControlLevel ControlLevel Control

• Reflux drum level control– LD - L or LD – D?

• Richardson’s rule:– Use the largest stream to control level.– Guidelines:

• L/D < = 1 : Use LD – D pairing

• L/D > = 5 : Use LD – L pairing

• For 1 < L/D < 5, use scheme proposed by Rysjkamp

– (L+D) – D and L/D – L pairings

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Two Common Level Control SchemesTwo Common Level Control Schemes

• Level control dilemma– Tight flow control?

• Oscillating level

– Tight level control?• Oscillating product flow

• Averaging or nonlinear level control

• Tight level control

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Common Level Control SchemesCommon Level Control Schemes

• Averaging (nonlinear) level control– Used when product is a feed to a

downstream process• Examples

– Train of lightends columns– Reflux drum level control

• Tight level control– Used when product goes to tankage or a

surge drum or process requires low hold up

• Use P-only controller with KC = 4• Examples

– Reboiler level control– FCC Main Frac and Vacuum

column bottoms (coking concern)– Dirty wash oil draw level control

• Control hydrostatic P in the draw line

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• Common Problems– If off gas is routed to a compressor,

reflux drum P is controlled leading to tower P swings.

Vapor Inventory ControlVapor Inventory ControlCommon Pressure Control SchemesCommon Pressure Control Schemes

Partial CondensersPartial CondensersOff gas rate > 0

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Common Pressure Control SchemesCommon Pressure Control SchemesPartial CondensersPartial Condensers

• Common Problems– If off gas is routed to a compressor, reflux

drum P is controlled leading to tower P swings.

– Inert gas, typically noncondesables, can cause downstream process problems

Off gas rate > 0 or = 0

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Common Pressure Control SchemesCommon Pressure Control SchemesTotal CondensersTotal Condensers

Flooded CondenserFlooded Condenser

Off gas rate = 0

• Common Problems– If P equalizing line is not used, P in the reflux

drum swings.– If condensed liquid is introduced into the drum

from top w/o dip leg, vapor in the drum can collapse.

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Off gas rate = 0

Common Pressure Control SchemesCommon Pressure Control SchemesTotal CondensersTotal CondensersHot Vapor BypassHot Vapor Bypass

• Common Problems– Bypass line inadequately sized– If drum top surface is not insulated, P can

swing with ambient changes. The effect is less pronounced for high P columns.

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F

LC

D

B

L

V

QH

PC

Degrees of Freedom AnalysisDegrees of Freedom AnalysisTypical Distillation ColumnTypical Distillation Column

Composition ControlComposition Control

Degrees of Freedom = 3

LD

LB

TD

TB

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Composition Control ProblemComposition Control Problem

• Number of MV’s = 3– Reflux flow: L– Distillate flow: D– Reboiler heat: QH

– Reflux ratio– Product/ feed ratio– Steam/ feed ratio

• Need three controlled variables (CV’s)

• Possible CV’s– Reflux drum level: LD

– Distillate composition: xD – Appropriate temperature in rectification

section (TD)– Bottoms composition: xB – Appropriate temperature in stripping

section (TB)

• Control problem– How do we pair CV’s and MV’s?

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Composition ControlComposition Control• Fundamental manipulated variables

– Feed split or cutpoint variable• Fraction of the feed that is taken

overhead of out of the bottom– Increasing distillate flow will

increase bottom purity and decrease distillate purity, etc.

– Fractionation variable• Energy that is put into the column to

achieve separation– Increasing the reflux ratio or the

reboiler duty will increase both distillate and bottoms purity

– Feed split has more pronounced impact on product purity than fractionation variable (exception low purity, < 90%, products)

– It is almost impossible to control any composition in the column if the feed split is fixed.

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Manipulation of Fundamental Variables Manipulation of Fundamental Variables for Composition Controlfor Composition Control

• Fractionation Variables– L/D

– QH/ F (steam to feed ratio)

– L/F• High purity columns or dual product

purity columns• DeC3’s, DeC4’s, DIB’s, etc.

• Feed Split Variables– D or B flow (direct control scheme)

• FCC Main Fracs, Crude and Vacuum column side cuts

– L or QH (indirect control scheme)

• Level adjusts the product flow indirectly

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Controlled Variables for Controlled Variables for Composition ControlComposition Control

• Stage temperature (Inferential control)

– Useless for ij < 1.2

• Online analyzer

– High economic gains

– ij < 1.2

• Temperature control – Special cases

– Difficult separations ( 1.2 < ij < 1.5)

• Flat temperature profiles

• Use differential temperatures ( T = Tm – Tk) between stages for control

• Example – HVGO quality control

– Extremely easy separations (high ij)

• Nonlinear in nature

• Steep temperature profile

• Use temperature profile control

• Tavg = (Tk + Tm)/ 2 , etc.

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• Locate TI on the stage whose temperature shows maximum sensitivity to one of the available MV’s

– From simulation calculate (Ti / D)L,B, (Ti / L)D,B , (Ti / )L,D and

(Ti / Q)L,D where Ti is the temperature of stage i. Locate TI at the stage where (Ti / D)L,B , etc., is maximum.

• For calculating the derivatives, vary B, D, L and Q in the column specs only by small amount, e.g., by +0.5% and -0.5%. Calculate average derivative.

• Scale each variable by dividing it by its span in order to calculate the derivatives. The derivative will be a dimensionless number.

• Use high precision numbers

Composition ControlComposition ControlTemperature Sensor LocationTemperature Sensor Location

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Optimum Temperature Sensor Optimum Temperature Sensor LocationLocation

Most common Mistake!

TC

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Optimum TI Location for Columns with Optimum TI Location for Columns with Side DrawsSide Draws

• Locate the TI in the vapor space one – two stages below the product draw for product EP control – This temperature (P-compensated)

correlates well with the product EP– Example

• Atmos column diesel 95% pt control

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DL

F

TI

TI Location for Side DrawTI Location for Side Draw

TC

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Special CasesSpecial CasesDraw Tray ControlDraw Tray Control

• Total Draw Tray– Control tray level by product draw– Control pumpback on flow control– Control p/a on flow control p/a duty as CV

• In fuel vacuum columns maximize duty

LC

FC

LT

FC

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• Partial Draw Tray– Level on the tray is fixed by the outlet

weir height. There is no level control

FC

FC

LT

FC

Special CasesSpecial CasesDraw Tray ControlDraw Tray Control

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Special CasesSpecial CasesStripping Steam FlowStripping Steam Flow

• Bottom stripping steam– Maximize to 8 – 12 lb stm per bbl of

product– Fixed flow control

• Side stripping steam– Minimize to meat front end spec– Use steam/ product ratio control

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Distillation ControlDistillation Control

Case Study:Case Study:Deisobutanizer Control Deisobutanizer Control

Joyce KaumeyerSr. Consulting EngineerSr. Consulting Engineer

Prakash KarpeControl & Elec. Eng. Supt.Control & Elec. Eng. Supt.

ConocoPhillipsConocoPhillipsSan Francisco Refinery, RodeoSan Francisco Refinery, Rodeo

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DeisobutanizerDeisobutanizer

Fuel Gas

IC4Tray 13

PIC

PIC

FIIC4

FICRFLX

TI

OVHD

TI

13

A

B

SW

LIC

IC4

SS

AI

IC4

Low LevelOverride

Tray 1

Tray 45 TIC

45

LIC

COND

PartiallyFlooded

Condenser

Steam

PIC

STM

Condensate

FI

STM

PartiallyFloodedReboiler

FloodedAccumulator

AI

NC4

NC4

LIC

NC4

FIC

NC4

Tray 25

Tray 37

Tray 60

Feed 1

Feed 2

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Tower OperationTower Operation

• Tower Pressure Control– By Overhead Product Rate

• Tower Temperature Control– Tray 45 By Condensate Level (Steam)

• Composition Control– Operator Adjusts Reflux Rate Based on

Lab / On-line Analyzer

• Tower Feed from Various Upstream Units – Large Rate Swings

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DeisobutanizerDeisobutanizerControl ObjectivesControl Objectives

• Control IC4 Product, IC4 Concentration – Reduce Variability & Control Closer to

Specification• Improve Tower Pressure Control

– Reflux / Product Rate = 5 / 1

• Change Existing Temperature / Composition Control

• Reduce NC4 Product, IC4 Concentration

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DeisobutanizerDeisobutanizerModified ControlsModified Controls

Fuel Gas

IC4Tray 13

PIC

PIC

FICIC4

FICRFLX

TI

OVHD

TI

13

A

B

SW

LIC

IC4

SS

AI

IC4

Low LevelOverride

Tray 1

Tray 45 TI

45

LIC

COND

PartiallyFlooded

Condenser

Steam

PIC

STM

Condensate

FI

STM

PartiallyFloodedReboiler

FloodedAccumulator

AI

NC4

NC4

LIC

NC4

FIC

NC4

TDIC

OVHD

UIC

BTU

Tray 25

Tray 37

Tray 60

Feed 1

Feed 2

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90.0

92.0

94.0

96.0

98.0

100.0

%

Analyzer IC4 DT Predicted IC4

IC4 ProductIC4 ProductOn-line Analyzer Vs. Delta Temperature CorrelationOn-line Analyzer Vs. Delta Temperature Correlation

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IC4 ProductIC4 ProductIC4 / Delta TemperatureIC4 / Delta Temperature

CorrelationCorrelation

%IC4 = 100.3 – 1.4464 * (Delta T)

Process Dynamics

• Deadtime: 19 minutes

• Lagtime: 102 minutes

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Modified Tower OperationModified Tower Operation

• Tower Pressure Control– By Reflux Rate

• Tower Heat Input Control– By Condensate Level (Steam)

• Composition Control– Operator Adjusts TDIC Setpoint Based

on Lab / On-line Analyzer

• Tower Feed from Various Upstream Units – Large Rate Swings

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Tower Pressure ControlTower Pressure ControlBefore and AfterBefore and After

Before After

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IC4 ProductIC4 Product%IC4%IC4

IC4 Product

88.0

90.0

92.0

94.0

96.0

98.0

100.0

IC4Start New Control

Steam Increase

High Pentanes

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NC4 ProductNC4 Product%IC4%IC4

Isobutane Giveaway in n-Butane

0.0

1.0

2.0

3.0

4.0

5.0

6.0 Start New Control Operator TrainingInitial Implementation phase%

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FutureFuture

• ARC – Add AIC Cascaded to TDIC

• Requires improved analyzer performance

– Add Heat Input Feed-Forward to AIC

-OR-• DMC

– Requires improved analyzer performance

– Hold for DCS platform conversion to Refinery Standard