fractionation tower controls-part 1

8/10/2019 Fractionation Tower Controls-part 1

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DISTILLATION CONTROL

Dr. Prakash KarpeControl & Elec. Eng. Supt.

ConocoPhillips

San Francisco Refinery, Rodeo


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D

B

L

R = L/DF

Distillation Column Control

Control Objectives

V

Rectification

Stages

Stripping

Stages

QH

Qc

Two Control objectives

Inventory control

Composition control


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Degrees of Freedom Analysis

Flash Vessel (Separator)

V

B

F

F,T,P,xi

Disturbances


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Inventory 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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Flash Vessel (Separator)

V

B

F

F,T,P,xi

Disturbances

LC

PC

Degrees of Freedom = 0


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Liquid Inventory Control

Level Control

Reflux drum level control

LD - L or LDD?

Richardsons rule:

Use the largest stream to control level. Guidelines:

L/D < = 1 : Use LDD pairing

L/D > = 5 : Use LDL pairing

For 1 < L/D < 5, use scheme proposedby Rysjkamp

(L+D)D and L/DL pairings


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Two 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 Schemes

Averaging (nonlinear) level control

Used when product is a feed to adownstream 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 holdup

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 thedraw 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 Control

Common Pressure Control Schemes

Partial Condensers

Off gas rate > 0


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Partial Condensers

Common Problems

If off gas is routed to a compressor, refluxdrum P is controlled leading to tower Pswings.

Inert gas, typically noncondesables, cancause downstream process problems

Off gas rate > 0 or = 0


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Total Condensers

Flooded 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 thedrum from top w/o dip leg, vapor in the drumcan collapse.


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Off gas rate = 0


Total Condensers

Hot Vapor Bypass

Common Problems

Bypass line inadequately sized If drum top surface is not insulated, P can

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


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F

LC

D

B

L

V

QH

PC


Typical Distillation Column

Composition Control

Degrees of Freedom = 3

LD

LB

TD

TB


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Composition Control Problem

Number of MVs = 3 Reflux flow: L

Distillate flow: D

Reboiler heat: QH Reflux ratio

Product/ feed ratio

Steam/ feed ratioNeed three controlled variables(CVs)

Possible CVs

Reflux drum level: LD Distillate composition: xD

Appropriate temperature in rectificationsection (TD)

Bottoms composition: xB Appropriate temperature in stripping

section (TB)

Control problem How do we pair CVs and MVs?


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Composition Control

Fundamental manipulated variables

Feed split or cutpoint variable

Fraction of the feed that is takenoverhead 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 bothdistillate 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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Controlled Variables for

Composition Control

Stage temperature (Inferential control)

Useless for aij< 1.2

Online analyzer

High economic gains

aij

< 1.2

Temperature controlSpecial cases

Difficult separations ( 1.2 < aij< 1.5)

Flat temperature profiles Use differential temperatures ( DT =

TmTk) between stages for control

ExampleHVGO quality control

Extremely easy separations (high aij

)

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 sensitivityto one of the available MVs

From simulation calculate (dTi/ dD)L,B,

(dTi/ dL)D,B ,(dTi/ d )L,D and

(dTi

/ dQ)L,D

where Ti

is the temperature

of stage i. Locate TI at the stage

where (dTi/ dD)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 Control

Temperature Sensor Location


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Optimum Temperature Sensor

Location

Most common Mistake!

TC


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Optimum TI Location for Columns with

Side Draws

Locate the TI in the vapor space onetwo

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 Draw

TC


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Special Cases

Draw 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 maximizeduty

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 Cases

Draw Tray Control


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Special Cases

Stripping Steam Flow

Bottom stripping steam

Maximize to 812 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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Deisobutanizer

Fuel Gas

IC4Tray 13

PIC

PIC

FI

IC4

FIC

RFLX

TI

OVHD

TI

13

A

B

SW

LIC

IC4

SS

AI

IC4

Low Level

Override

Tray 1

Tray 45 TIC

45

LIC

COND

Partially

Flooded

Condenser

Steam

PIC

STM

Condensate

FI

STM

Partially

Flooded

Reboiler

Flooded

Accumulator

AI

NC4

NC4

LIC

NC4

FIC

NC4

Tray 25

Tray 37

Tray 60

Feed 1

Feed 2


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Tower 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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Deisobutanizer

Control 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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90.0

92.0

94.0

96.0

98.0

100.0

%

Analyzer IC4 DT Predicted IC4

IC4 ProductOn-line Analyzer Vs. Delta Temperature Correlation


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IC4 Product

IC4 / Delta Temperature

Correlation

%IC4 = 100.31.4464 * (Delta T)

Process Dynamics

Deadtime: 19 minutes

Lagtime: 102 minutes


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Modified Tower Operation


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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Before and After

Before After


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IC4 Product

%IC4

IC4 Product

88.0

90.0

92.0

94.0

96.0

98.0

00.0

IC4Start New Control

Steam Increase

High Pentanes


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fractionation tower controls-part 1

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