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1ICT
Energy efficient distillation
Ivar J. Halvorsen, Maryam Ghadrdan, Deeptanshu Dwivedi, Mohammad Shamsuzzoha and
Sigurd Skogestad
1st Trondheim Gas TechnologyConference,
21 - 22 October 2009
ABCD
A
B
D
C
2ICT
Distillation in the gas value chain
Distillation plays an important role in splitting rawproduction streams into more useful product streams withspecified compositions
Figure, ref http://www.sintef.no/Projectweb/Trondheim_GTS/
3ICT
Distillationconsumes energy
2-5% of the world industry heat consumption
There is a potential for more energy efficient solutions
Picture: Fractionation columnsat the Snøhvit LNG-plant in Hammerfest
Foto: I.Halvorsen
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BEEDIST (Basic Energy Efficient Distillation Technology)
Founded by the Norwegian Research Council through the GASSMAKS programSINTEF/NTNU 2008-2012Objectives
Study new integrated distillation arrangementsFor 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 operationDevelop laboratory2 PhD + post doc
ABCD
CD
A
B
D
C
AB
NTNU lab4-product Kaibel-
column with a dividing wall
ABC
A
B
C
AB
BC
3-product Petlyuk arrangement
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An ideal reversible processrequires:
H
C
C
TTST
Q−
Δ−=
1min
Distillate product
Feed
Heating at TH
Cooling at Tc
Bottom product( )i
iiln xS RxΔ = −∑
Where entropy of mixing is
Q
Q
Teoretical minimum energy
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Increasing purity require energy-Mixing gives irreversible loss
Can we find a minimum enegy route
to pure A?
ABC
AB
A
BC
Remixing at end ofcolumn 1 Position along path
from feed to A outlet
A-composition
Feed C1 Feed C2 A-product
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Minimum energy path to the mountain top
Choose the minimum-energy route
Irreversible losses in potential energy
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Conventional alternatives for 3-product separation: Sequence of binary columns
ABC
BC
A B
C
Direct Split: DS
ABC
AB
A
BC
Indirect split: IS
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ABC
BD
AB
A
B
C
B
Prefractionator arrangement
Alternatives for 3-product separation...
The prefractionatordoes the simple A/C
split
10ICT
Conventional Prefractionator arrangementwith a single main column
ABC
BC
AB
A
B
C
11ICT
Apply full thermal coupling
ABC B
C
AB
A
B
C
The Petlyuk column
removes mixing loss at the interconnections
Saves 20-30% energy
The Petlyuk path
avoids mixing loss
12ICT
The dividing wall column (DWC)
The Petlyuk arrangement in a single shellSeparates a single feed into three separate products in one columnJust a single reboiler and condenserSaves 20-30% in both energy and capital
A
B
C
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The Vmin-diagram –for simple energy asessment
D/F
V/F
ABC
D
F
V
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Industrial DWC/Petlyuk applications
German-speaking community dominatesBASF: 40 DWCs in operation. Increasing. G. Kaibel pionerMonz – main vendor for BASFKrupp-UhdeSulzerRashigLinde
OthersMW Kellogg (UK)UOP (USA)UK, Japan, Indonesia, South Africa
The Kaibel-column4-product DWC!
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Why consider a Petlyuk arrangementLarge potential energy savings compared to conventionalcolumns (20-30%)
Or – increase production for given energy supply
Capital cost savings due to more compact equipment => smaller footprint and removal of reboiler/condenser unitsUsage:
In theory: Anywhere (almost) where distillation is a suitableseparation technology and more than 2 products are produced. In practice: Some cases may be unsuitable due to requiredtemperature/pressure range, height, or if liquid/vapor load in different sections are very different. Practical variations can be made, e.g. side-strippers/rectifiersRevamping of existing conventional columns may have significantpotential
18ICT
Critical for obtaining the teoretical savingsin practice: How to control the splits
ABC
A
B
C
vapor split(Rv)
liquid split(Rl)
Split-ratio
Energy
TeoreticalVmin
Tool: Self optimizing control
19ICT
Extend to 4-product DWC:The Kaibel column – (1987)
ABCD
CD
A
B
D
C
4 products in a single
shell!
Saves 30-40 %
ABCD
BCD
CD
A B
D
C
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The Kaibel column at NTNU A
B
C
D
Feed(ABCD)A
BCD
CD
A
B
D
C
AB
H=8m
D=5/8cm
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First really big 4-product DWC
Kaibel column
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Control study (Strandberg 2009)
Need to adjust liquid splitonline in order to stabilizeprefractionator
Stabilizing Control by 4 temperature loops
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Step response test on pilot column
Prefractionatortemperaturebelow feedcontrolled by adjusting theliquid split
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More results:
Master thesis by Martin Kvernland 2009Matlab model exension includes heat loss and vaporbypass for stage inefficiencyImplemented a 4x4 MPC with a reduceded linear statespace model (reduced from original 200 to 15 states)The MPC controller in Matlab can be interfaced to Labview via OPC
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UniSim Simulation for Kaibel Column
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MATLABAdvanced Optimization
& Execution ControlHYSYS/UNISIMRigorous Simulation
Execution control
Variables values
20 21 22 23 24 25 26 27 28 290.75
0.8
0.85
0.9
0.95
1
S1 Rate (kmol/hr)
Com
posi
tions
(mol
e fra
c)
xDxS1xS2xB
20 21 22 23 24 25 26 27 28 290.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
S2 Rate (kmol/hr)
Com
posi
tions
(mol
e fra
c)
xDxS1xS2xB
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.650.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
Liquid split
Com
posi
tions
(mol
e fra
c)
xDxS1xS2xB
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.90.4
0.5
0.6
0.7
0.8
0.9
1
Vavpor split
Com
posi
tions
(mol
e fra
c)
xDxS1xS2xB
3 4 5 6 7 8 9 10
0.4
0.5
0.6
0.7
0.8
0.9
1
Reflux Ratio
Com
posi
tions
(mol
e fra
c)
xDxS1xS2xB
Toolbox for rigorous simulationsby Maryam Ghadrdan
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Further work
Model development and refinement Both in Matlab and Unisim/HysysLab column experimentsControl structure design (including selfoptimising control)Optimizing control / minimum energy controlOptimal process design Extended Petlyuk arrangementsAlternative structures like HIDiC, Heat integrated and other energy efficient arrangements
ABCD
CD
A
B
D
C
AB
28ICT
Extended 4-product Petlyuk DWC with multiple dividing walls
ABCD
A
B
D
C
AB
ABC
BCD
CD
BC
What about complexity in
design and operation?
Teoreticalsavings up
to 50%
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What about refit?A Butane case (butane/C5+) and butane splitter (iC4/nC4):
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Add direct coupling and save 32 %
Simple refit of exisiting columns is equivalent to the full Petlyuk arrangement
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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 stagesConstant relative volatilityConstant molar flowConstant pressureNo internal heat exchange
Then, exact analytic solution is obtained
33ICT
zA:zB:zC:
αA:αB:
qf:
Conventional : Vmin=2.03Petlyuk Column: Vmin=1.37
Reboiler SAVINGS: 32.80%Condenser SAVINGS: 32.80%
C 0.2 0.4 0.6 0.8 A
0.2
0.4
0.6
0.8
B
0 0.2 0.4 0.6 0.8 10
0.5
1
1.5
2
D/F
VT/F
P
R
Savings
0 0.2 0.4 0.6 0.8 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
C3
C2
V(Rl,Rv)
Rl
RC4
P
C1
Characterisitcs of operation
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Feed condition for the Kaibel Distillation
Four components: Methanol+Ethanol+1-Propanol+1-ButanolFlow rate F=1.0 Kgmole/h, q=1 (saturated liquid)Composition z=[0.25, 0.25, 0.25, 0.25]EOS WilsonPressure AtmosphericRelative volatility α= [8.27: 4.84: 2.30: 1.0]
35ICT
Minimum Energy – competition No Configuration Ideal Vmin/F Ideal
SavingsUniSimSavings
1 Four product extended Petlyuk 1.16 51%
33%
16%
0%(reference)
-10%(loss)
2 Kaibel column 1.59 >26%
0%(reference)
3 Prefractionator+single main column
1.98
4 Conventional direct sequence (3 columns)
2.38
5 Prefractionator+2 separate columns
2.62
36ICT
Vmin-diagram for the Kaibel column
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
D/F
V/F
Vmin-diagram
37ICT
How to apply DWC/Petlyuk columns
Make sure to do a reasonable design in terms ofplacement of the dividing wall/design split.Important: Make sure to indentify the optimality region for the expected actual feed variations, and thereby clarifythe requirements for on-line adjustment of:1. None of the split ratios (E.g. in case of quadrangle shaped reg.)2. Just the liquid split (In case of a line segment reg.)3. Both split ratios (in case of a very short line segment)
Determine the final control strategy based the actualproduct value/energy cost, and dynamic controllabilityanalysis.