webinar distillation control august 2010

© Petrocontrol1

© Petrocontrol2

Two DCS control

configurations

Mass balance

Heat balance

© Petrocontrol3

Objective: Separate into light

and heavy fractions.

DeC4 separation example

Feed

C2 LLK

C3 LIK

C4 LK

C5 HK

C6 HIK

C7 HHK

C8 HHK

Distillate product

Bottoms product

© Petrocontrol4

Practical separation

Feed Moles Distillate Bottoms

C2 0.1 LLK 0.1

C3 7.9 LIK 7.89 0.01

C4 12 LK 11.7 0.3

C5 14 HK 0.2 13.8

C6 18 HIK 0.02 17.98

C7 22 HHK 22

C8 26 HHK 26

___ ____ ____

100 19.91 80.09

Impurities 1.1% 0.39%

© Petrocontrol5

Mass balance configuration

FC

FC FC

FC

LC

FC

LC

PC

© Petrocontrol6

Two major handles

1. Distillate yield = CUT

2. Reflux

In mass balance configuration those are

set directly. How?

(3) Feed is normally a given

(4) Pressure is normally preset

© Petrocontrol7

Heat balance configuration

FC

FC

FC

LC

DistillateReflux

FC

Feed

Bottoms

LC

FC

PC

© Petrocontrol8

1. CUT

– Distillate flow is dictated by heat balance:

Reboiler duty - reflux duty

2. Reflux (without changing the cut)

– Reflux flow is also dictated by heat balance:

Reboiler duty and reflux duty

If we are to keep the same

handles in heat balance

© Petrocontrol9

Top Yield

Product

contamination

TopBottom Correct

Cut

Incorrect

Cut

Sensitivity of cut control

Mass balance diagram

© Petrocontrol10

• Estimate the bottom contamination with

1% cut error

– There are 50% lights in the feed

– Top yield is 49% -- 1% yield mistake

Quiz

© Petrocontrol11

1% cut mistake

Feed Distillate Bottoms

50 LK 49 1

50 HK 0 50

2% bottom contamination

© Petrocontrol12

• Knowledge of how to correctly set the cut

is essential

– 1% mistake in yield

Roughly 2% contamination

• What about reflux mistakes?

Simple conclusion on

column control

© Petrocontrol13

The effect of reflux

Top Yield

contamination

TopBottom

Low

reflux

Infinite

reflux

© Petrocontrol14

Heat balance

Mass balance

Differences

© Petrocontrol15

Heat balance column

FC

FC

FC

LC

FC FC

LC

PC

© Petrocontrol16

Mass balance column

FC

FC FC

FC

LC

FC

LC

PC

© Petrocontrol17

Which one is more complicated?

Which is more popular?

Heat

Balance

Mass

Balance

Cut

Control

Reboiler duty

- Reflux duty

Direct

Distillate FC

Fractionation

Control

Reboiler &

Reflux duty

Direct

Reboiler duty

© Petrocontrol18

Heat balance with simple inference

Without such inference

the operator is in the dark

LC

FC

FC

PC

LC

FC

TC

FC

FC

© Petrocontrol19

What’s the tray TC advantage?

Heat Balance without tray temperature

Heat balance with tray temperature

Cut Control Reboiler – reflux duty

Direct tray TC

Fractionation control

Reboiler & reflux duty

Direct reflux FC

© Petrocontrol20

• Used on high reflux column because

drum level control is easier (but this is no

better for quality control)

• Mass balance is not sensitive to heat

disturbances -- (Cut doesn’t change)

• Sensitive to feed composition changes --

(Cut should change but doesn’t)

• Columns with a pickling section

When do you use mass balance?

© Petrocontrol21

Column with a pickling section

FC

FC

FC

LC

FC

FC

LC

PC

FC

Main product

© Petrocontrol22

Is this mass or heat balance?

FC

FC

FC

LC

FC

LC

PC

FC

© Petrocontrol23

How about 2 TCs?

Danger of competition

between 2 cut models

Instabilities

TC

LC

FC

FC FC

FC

PC

LC

FC

TC

© Petrocontrol24

Two temperature inferences

• Two cut models are never in agreement

Weeping or flooding

• Dynamic lags between top and bottom

Oscillations

Rectifying temperature

Stripping temperature

© Petrocontrol25

Quiz

What’s your favorite heat

balance DCS control?

© Petrocontrol26

One TC with reflux ratio setting?

LC

FC

FC

FC

PC

LC

FC

TC

Slow ratio

control

FC

© Petrocontrol27

Do not fall into this trap

Reboiler outlet temperature

is sensitive to HHK

Not sensitive to bottom LK

LC

FC

FC FC

FC

PC

LC

FC

TC

© Petrocontrol28

Pressure control

(Closing the heat balance)

© Petrocontrol29

Column with a partial condenser

TC

LC

FC

FC FC

FC

PC

LC

FC

© Petrocontrol30

Total condenser - hot vapor bypass

FC

FC FC

FC

LC

LC

PC

FC

Regulate pressure by

flooding the condenser

© Petrocontrol31

Another common hot vapor bypass

FC

FC FC

FC

LC

LC

PC

FC

PC

© Petrocontrol32

Avoid this trap. Why?

FC

FC FC

FC

LC

LC

PC

FC

DPC

Interactions make

pressure control

unstable

© Petrocontrol33

Better solution

FC

FC FC

FC

LC

LC

PC

FC

DPC

Pressure setpoint

© Petrocontrol34

Condenser manipulation

FC

FC FC

FC

LC

LC

PC

FC

What’s the

problem here?

© Petrocontrol35

Flooded drum

FC

FC

FC

LC

PC

FCFC