Webinar Distillation Control August 2010
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Transcript of 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