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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/SINTEFTrondheim

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Minimum Energy for the Four-Product Kaibel Distillation Column

IntroductionWhat is Petlyuk columnsExperimental setup Assessment by the Vmin diagram

Rigorous simulation Summary

ABCD

A

B

D

C

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BEEDIST (Basic Energy Efficient Distillation Technology)

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

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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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-wide

=>Need more energy efficient solutions

ABC

A

B

C

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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

Classical Petlyuk arrangement

Dividing Wall Column (DWC)

ABC

A

B

C

AB

BC

ABC

BC

A

B

C

AB Liquid split

Vapor split

Fully thermally coupled sections

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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

A

AB

B

C

B

Prefractionator arrangement

Alternatives for 3-product separation...

The prefractionator does the simple A/C

split while B distributes to both

ends

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Conventional Prefractionator arrangementwith a single main column

ABC

BC

A

AB

B

C

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Apply full thermal coupling

ABC B

C

A

AB

B

C

Petlyuk column

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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 => 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!

ABC

A

B

C

ABC

A

B

C

vapor split (Rv)

vapor split (side draw)

liquid split (side draw) liquid split

(Rl)D1

V1

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Extend to 4-product DWC:The Kaibel column – (1987)

ABCD

CD

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B

D

C

AB

Total reflux section

Separates 4 products in a single shell!

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

( , , )minV f Splits Feed Specs

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The Vmin-diagram

ABC

Distillate (D)

Feed (F)

Vapor rate (V)

D/F

V/F

1

Operation point f(D/F,V/F)

Two degrees of freedom – choose D/F,V/F

Binary column –multicomponent feed

Feed comp. distribution ? Minimum energy ?

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The Vmin-diagram – 3 component example

D/F

V/F

Vmin

boundary

ABC

D

F

V

Preferred A/C split

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zA:

zB:

zC:

A:

B:

qf:

Conventional : Vmin

=2.03Petlyuk Column: V

min=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,R

v)

Rl

RC4

P

C1

Rv

Characterisitcs of operation

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Schematic diagram of the experimental setup of 4-product Kaibel-arrangement

T50

6

2

1

3

T34

T66

VB

Rv

L/D

Rl

S1

S2

F, Z_F, q

x, B

D, x, d

x, S1

x, S2

TS-7

TS-1

TS-2

TS-3

TS-4

TS-5

5

7

4

TS-6

TEE101

TEE100

T34

ABCD

A

B

D

C

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The Kaibel column at NTNU, Trondheim, Norway

Lab installation: Height: 8 meters Atmospheric pressure Vacuum glass sections 4 products Contact: Sigurd Skogestad, or

Ivar J. Halvorsen

A

B

C

D

Feed (ABCD)

ABCD

CD

A

B

D

C

AB

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Feed condition for the Kaibel Distillation

Four components: Methanol+Ethanol+1-Propanol+1-Butanol

Flow rate F=1.0 Kgmole/h, q=1 (saturated liquid) Composition z=[0.25, 0.25, 0.25, 0.25] EOS Wilson Pressure Atmospheric Relative volatility α= [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 IdealSavings

UniSimSavings

1 Four product extended Petlyuk 1.16 51%

2 Kaibel column 1.59 33% >26%

3 Prefractionator+single 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

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

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Vmin-diagram for the Kaibel column using UniSim

Rigorous calculation confirms ideal shortcut method

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

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

ABCD

CD

A

B

D

C

AB

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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: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 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) => 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) => Valid for any operating point Simple sequential calculation sequence Extremely simple assessment for n-product Petlyuk arrangement

based only on feed properties.