Rigorous Simulation of Divided-wall Columns

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Rigorous Simulation of Divided-wall Columns

• M. Shamsuzzoha, Maryam Ghadrdan, Ivar J.
  Halvorsen, Sigurd Skogestad
• The 15th NPCW'09, Telemark University College,
  Porsgrunn, Norway January 29-30 2009

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Trondheim, Norway
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NTNU/SINTEF Trondheim
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Minimum Energy for the Four-Product Kaibel
Distillation Column

• Introduction
• What is Petlyuk columns
• Experimental setup
• Assessment by the Vmin diagram
• Rigorous simulation
• Summary

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BEEDIST (Basic Energy Efficient Distillation
Technology)
NTNU lab

• Founded by the Norwegian Research Council through
  the GASSMAKS program
• SINTEF/NTNU 2008-2012
• Objectives
• Study new integrated distillation arrangements
• For reduction of capital cost and energy
  consumption ( CO2-emission related to the
  energy).
• 20-40 savings in reach.
• Evaluate application in natural gas processing
  and conversion.
• Design and operation
• Develop laboratory
• 2 PhD post doc

4-product Kaibel-column with a dividing wall
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What is a Petlyuk arrengement ?

• Integrated distillation column arrangement
• Separates a single feed into three separate
  products
• Just a single reboiler and condenser
• Why?
• Saves energy and capital
• Distillation consumes 3-5 of the industry energy
  consumption world-widegtNeed more energy
  efficient solutions

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Industrial DWC/Petlyuk applications

• German-speaking community dominates
• BASF 40 DWCs in operation. Increasing. G. Kaibel
  pioner
• Monz main vendor for BASF
• Krupp-Uhde
• Sulzer
• Rashig
• Linde
• Others
• MW Kellogg (UK)
• UOP (USA)
• UK, Japan, Indonesia, South Africa

The Kaibel-column 4-product DWC!
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Equivalent Petlyuk arrangements
Dividing Wall Column (DWC)
Classical Petlyuk arrangement
Fully thermally coupled sections
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Conventional alternatives for 3-product
separation Sequence of binary columns
Indirect split IS
Direct Split DS
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Prefractionator arrangement
Alternatives for 3-product separation...
A
AB
ABC
B
The prefractionator does the simple A/C split
while B distributes to both ends
B
BD
C
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Conventional Prefractionator arrangementwith a
single main column
A
AB
ABC
B
BC
C
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Apply full thermal coupling
A
Petlyuk column
AB
ABC
B
BC
C
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Why consider a Petlyuk arrangement

• Large potential energy savings compared to
  conventional columns (20-30)
• Or increase production for given energy supply
• Capital cost savings due to more compact
  equipment gt smaller footprint and removal of
  reboiler/condenser units
• Usage
• In theory Anywhere (almost) where distillation
  is a suitable separation technology and more than
  2 products are produced.
• In practice Some cases may be unsuitable due to
  required temperature/pressure range, height, or
  if liquid/vapor load in different sections are
  very different.
• Practical variations can be made, e.g.
  side-strippers/rectifiers
• Revamping of existing conventional columns may
  have significant potential

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Critical How to set the splits

• The potential savings are easily lost unless the
  splits are adjusted properly
• Do it right and obtain all the benefits!

liquid split (side draw)
A
liquid split (Rl)
D1
A
ABC
ABC
B
B
V1
vapor split (Rv)
vapor split (side draw)
C
C
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Extend to 4-product DWCThe Kaibel column
(1987)
Separates 4 products in a single shell!
Total reflux section
Can save 30-40
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Minimum energy-Definitions and assumptions

• Vapour flow rate (V) generated from all reboilers
  is used as the energy measure
• Ideal Assumptions
• Infinite number of stages
• Constant relative volatility
• Constant molar flow
• Constant pressure
• No internal heat exchange
• Then, exact analytic solution is obtained

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Minimum energy (Vapor flow rate V)

• Sharp splits Flat optimum at a line segment
  (optimality region)
• Optimality region depends on feed properties
• Rapid increased vapor flow outside optimality
  region

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The Vmin-diagram
Operation point f(D/F,V/F)
Distillate (D)
Binary column multicomponent feed
Feed (F)
ABC
Vapor rate (V)
Feed comp. distribution ? Minimum energy ?
Two degrees of freedom choose D/F,V/F
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The Vmin-diagram 3 component example
V/F
D
F
ABC
Vminboundary
V
Preferred A/C split
D/F
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Characterisitcs of operation
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Schematic diagram of the experimental setup of
4-product Kaibel-arrangement
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The Kaibel column at NTNU, Trondheim, Norway
A

• Lab installation
• Height 8 meters
• Atmospheric pressure
• Vacuum glass sections
• 4 products
• Contact Sigurd Skogestad, or Ivar J. Halvorsen

B
Feed (ABCD)
C
D
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Feed condition for the Kaibel Distillation

• Four components MethanolEthanol1-Propanol1-But
  anol
• Flow rate F1.0 Kgmole/h, q1
  (saturated liquid)
• Composition z0.25, 0.25, 0.25, 0.25
• EOS Wilson
• Pressure Atmospheric
• Relative volatility a 8.27 4.84 2.30 1.0

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UniSim Simulation for Kaibel Column
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Steady-State Simulation for Kaibel Column
KAIBEL DISTILLATION COLUMN
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Minimum Energy competition
No Configuration Ideal Vmin/F Ideal Savings UniSimSavings
1 Four product extended Petlyuk 1.16 51
2 Kaibel column 1.59 33 gt26
3 Prefractionatorsingle main column 1.98 16
4 Conventional direct sequence (3 columns) 2.38 0 (reference) 0 (reference)
5 Prefractionator 2 separate columns 2.62 -10 (loss)
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Vmin-diagram for the Kaibel column
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Vmin-diagram for the Kaibel column using UniSim

• Rigorous calculation confirms ideal shortcut
  method

V/F
D/F
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Further work

• Status Startup of new PhDs
• Directions
• Further studies with Unisim (extended from ideal
  mixtures)
• Further development of Matlab models
• Alternative structures like HIDiC, Heat
  integrated and other energy efficient
  arrangements
• Dynamic studies
• Optimizing control
• Lab column experiments

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How to apply DWC/Petlyuk columns

• Make sure to do a reasonable design in terms of
  placement of the dividing wall/design split.
• Important Make sure to indentify the optimality
  region for the expected actual feed variations,
  and thereby clarify the requirements for on-line
  adjustment of
• None of the split ratios (E.g. in case of
  quadrangle shaped reg.)
• Just the liquid split (In case of a line segment
  reg.)
• Both split ratios (in case of a very short line
  segment)
• Determine the final control strategy based the
  actual product value/energy cost, and dynamic
  controllability analysis.

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Key issues for full thermal coupling

• Liquid and vapour flows in equilibrium avoids
  irreversible loss due to mixing (Petlyuk 1965) gt
• Explains why Petlyuk columns beat the other
  arrangements
• Require operation of every internal column at its
  preferred split
• Underwood roots carry over the coupling
  (Halvorsen 2001) gt
• Valid for any operating point
• Simple sequential calculation sequence
• Extremely simple assessment for n-product Petlyuk
  arrangement based only on feed properties.