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

Trondheim, Norway

NTNU/SINTEF Trondheim

Minimum Energy for the Four-Product Kaibel Distillation Column Introduction What is Petlyuk columns Experimental setup Assessment by the Vmin diagram Rigorous simulation Summary ABCD A B D C

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 ABC A B C AB BC 3-product Petlyuk arrangement ABCD CD A B D C AB

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

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!

Equivalent Petlyuk arrangements Classical Petlyuk arrangement Dividing Wall Column (DWC) ABC A B C AB BC Liquid split Vapor split Fully thermally coupled sections

Conventional alternatives for 3-product separation: Sequence of binary columns Direct Split: DS Indirect split: IS ABC BC A B C ABC AB A B C

Prefractionator arrangement Alternatives for 3-product separation... Prefractionator arrangement A AB ABC B The prefractionator does the simple A/C split while B distributes to both ends B BD C

Conventional Prefractionator arrangement with a single main column AB ABC B BC C

Apply full thermal coupling Petlyuk column AB ABC B BC C

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

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

Extend to 4-product DWC: The Kaibel column – (1987) ABCD CD A B D C AB Separates 4 products in a single shell! Total reflux section Can save 30-40 %

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

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

The Vmin-diagram Operation point f(D/F,V/F) V/F Distillate (D) Binary column – multicomponent feed D/F V/F 1 Feed (F) ABC Vapor rate (V) Feed comp. distribution ? Minimum energy ? Two degrees of freedom – choose D/F,V/F

The Vmin-diagram – 3 component example V/F D F ABC Vmin boundary V Preferred A/C split D/F

Characterisitcs of operation

Schematic diagram of the experimental setup of 4-product Kaibel-arrangement ABCD A B D C

The Kaibel column at NTNU, Trondheim, Norway ABCD CD A B D C AB Lab installation: Height: 8 meters Atmospheric pressure Vacuum glass sections 4 products Contact: Sigurd Skogestad, or Ivar J. Halvorsen B Feed (ABCD) C D

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]

UniSim Simulation for Kaibel Column

Steady-State Simulation for Kaibel Column KAIBEL DISTILLATION COLUMN

Minimum Energy – competition No Configuration Ideal Vmin/F Ideal Savings UniSim Savings 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) 5 Prefractionator+ 2 separate columns 2.62 -10% (loss)

Vmin-diagram for the Kaibel column

Vmin-diagram for the Kaibel column using UniSim Rigorous calculation confirms ideal shortcut method V/F D/F

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

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.

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.