1

Evaluation of Dephlegmation as anAlternative Separation Process

to Distillation

Stathis Skouras

7. May 2004

Department of Chemical Engineering, NTNU

NTNU

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Keywords

Dephlegmation

Distillation

Evaluation: dephlegmation vs. distillation

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Presentation Overview

• Dephlegmation– Process description, columns, applications

• Distillation– Process description, columns

• Comparisons– Columns, energy considerations, separation achieved,

practical considerations, flexibility, etc

• Case study– EtOH/H2O separation

• Summary– Dephlegmation: for which applications?

• Concluding remarks

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Dephlegmation: process description

Partial condenser or Dephlegmator

cooling water

Vin

yin

Lout, 1

Xout, 1

Vout, 1, yout, 1

SOLUTIONReflux unwanted liquid products

Establish counter-current flowAllow one vapour product, one liquid product

Two dephlegmators connected in series

(multistage condensation)

Lout, 2

Xout, 2

Vout, 2, yout, 2

cooling water1 2

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Dephlegmation: process description (cont.)

• Reminds of heat exchanger

• But HEAT and MASS transfer

• Temperature and composition profile established

• Terminology:- reflux condenser- run-back condenser- fractionating condenser- dephlegmator (in cryogenics)

COOLANT

VAPOUR FEED

STRIPPED CONDENSATE

ENRICHEDVAPOUR

HEATREMOVAL

Vin

Vout

Lout

V

L

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Dephlegmation: process description (cont.)

The Dephlegmation Principle

Ref: Minkkinen et al.

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Dephlegmation: columns

High-surface-area heat exchanger

Partial reflux condenser (top)

+High-surface-area

heat exchanger

Partial reflux condenser (top)

+High-surface-area

packing

Ref: Vane et al., (2004)

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Dephlegmation: columns (cont.)

Ref: Minkkinen et al.

Chart Industries Inc.

Vapour/condensate regions (dark regions)Coolant regions (lighter regions)

State-of-the-art dephlegmator Compact brazed aluminium plate-fin

heat exchanger (configuration A)

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History

- Partial condensers (dephlegmators) at top of distillation columns

- Georges Claude (1903) for air separation columns

- Abandoned as specifications became stricter

Cryogenic separations (most applications)- Natural gas processing (NGL recovery, helium recovery)

- Petrochemical plants (ethylene recovery)

- Well integrated to refrigerated or turbo expanded cold separator process (cold boxes)

- Not directly competitive to distillation process (mostly in synergy with distillation)

Future- Bio-engineering

- Recovery of fermentation products from biological media (bio-ethanol from biomass)

- In combination with membrane separations (vapour permeate from pervaporation)

- Directly competitive to distillation (maybe batch distillation)

Dephlegmation: applications

10Ref: Minkkinen et al.

Simplified IFPEXOL process (licensed by IFP, France)

Key Units

• IFPEX-1 contactor: removes some water from feed

• Cold Box: condenses MeOH/H2O/hydrocarbons to <-90°C

• 3-phase low temperature separator (LTS): MeOH/H2O (liquid phase 1) recycled, residual gas (vapour phase) top product + hydrocarbons (liquid phase 2) bottom product

Dephlegmation: applications (cont.)

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Dephlegmation: applications (cont.)

Ref: Minkkinen et al.

Simplified Dephlexol process (licensed by IFP France)

New Units

• Gas/gas heat exchange: Pre-cooling the gas top product from IFPEX-1

• Dephlegmator + low temperature separator (LTS):

Refrigeration duties significantly reduced

NGL product carries very little of light components (lean gas)

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Distillation: process description

Reflux unwanted vapour and liquid products Establish counter-current flowAllow one top product, one bottom product

steam

F0

V0

L0

cooling water

steam

V1´

L1´

V1

L1

Ref: King, C.J., (1980)

1

2

2´

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Distillation: columns

Ref: C. Judson King, "Distillation", in AccessScience@McGraw-Hill, http://www.accessscience.com

Stripping section

Rectifyingsection

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Comparisons: columns

Distillation

Stripping section

Rectifyingsection

HEATREMOVED

HEATINJECTION

Dephlegmation

Rectifyingsection

HEATREMOVED

Vin

Vout

Lout

V

L

Coolant

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Comparisons: energy issues

Ref: Kent, E.R., Pigford, R.L., (1956)

DEPHLEGMATION is

REVERSIBLE DISTILLATION with interstage heat removal?

Dephlegmation

HEAT

REMOVED

Vin

Vout

Lout

V

LINTERNALREFLUX

Adiabatic distillationVout

HEATREMOVED

V

L

Vin Lout

EXTERNALREFLUX

THERMALINSULATION

LR

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Comparisons: energy issues

2nd Law Efficiency

Dephlegmation• Heat removed at all temperature levels• Thermodynamically efficient• May operated with very close ΔT (advantageous for cryogenics, refrigeration)

Distillation• Heat removed at lowest temperature (condenser)• Heat injected at highest temperature (reboiler)• Low thermodynamic efficiency

1st Law Efficiency

• Only one study: reflux condenser vs. adiabatic distillation (Kent and Pigford, 1956)• Rmin same in both processes• Dephlegmation provides less surface area for mass transfer thus, actual R increases • Distillation seems to require less heat load per unit of product

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Comparisons: separation achieved

Dephlegmation

• Provides only rectification action• Top product with high purity but low recovery• Can give high recovery of heavy components for bottom product• Dephlegmator more attractive when high recovery of heavy components from gas mixtures rich in light components and α >2 (Lucadamo et al., 1987) • Dephlegmation for “rough” separations (preseparations).

Distillation

• Provides both rectification and stripping action• Two products with high purity• Uneconomical when 0.95 < α < 1.05

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Comparisons: separation achieved (cont.)

Optimization of the olefin separation process

Ref: Lee et al., 2003

Solution: 1 dephlegmator upstream,3 distillation columns further processing and final products

Feed: H2, mixture of hydrocarbonsObj. function: Annual capital cost + compression and utilities (MINLP)Processes: dephlegmation, distillation, absorption, membrane

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Comparisons: practical considerations

Distillation

• Well established process• Efficient in separating mixtures into high purity products• Plenty of studies for design, operation, control, etc• High energy requirements, low thermodynamic efficiency

Dephlegmation

• Thermodynamically more efficient for fractionation• Only for vapour feeds• Design of dephlegmation open topic• Review study (UMIST) for reflux condensers (Jibb et al., 1998)Two major challenges

- Accurate prediction of flooding point- Prediction of heat and particularly mass transfer

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Comparisons: flexibility in future modifications

Retrofit study: Improve product purities

Distillation• Increase # stages or better packing (fixed cost )• Long towers with high (175 stages for argon/oxygen)• Increase reflux (energy )

Dephlegmation• Increase reflux should be OK (enhance heat transfer, increase cooling)• Increase # stages can be problematic • BUT, plate-fin heat exchanger (configuration A) limited in height• Height 6m, HETP = 0.30-0.46 m, 13-20 stages

Ref: (Vane al., 2004, Minkinnen et al., )

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Case Study: EtOH / H2O separation

Vane et al., 2004

DataOperation under vacuum (30 Torr)Feed superheated vapour (60°C)Desired EtOH recovery = 90%

Simulations• Dephlegmator modelled as a 4-stage column• User specified heat removal per stage

Experiments• 0.2m × 0.22m × 2.4m dephlegmator (Chart Industries)• Expected to provide 4-6 stages

Vin

Vout

L

pervaporation

F

retentate

dephlegmation

permeate

5% wtEtOH

34.5% wtEtOH

85.4% wtEtOH

5.4% wtEtOH

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Case Study: EtOH / H2O separation (cont.)

Experiments•90% wt purity, 89% recovery could be obtained• Results in agreement with simulations for 6 stages

Ref: Vane et al., (2004)

Simulations• More cooling enhance separation • However, fairly sharp break-point• Purity competes recovery (like distillation)• Operate at the point were recovery and purity is high (90%)

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Case Study: EtOH / H2O separation (cont.)

Distillation(Hysys)

ybot = 34.5% wt = 17.1% molxtop= 85.4% wt =69.6% molxbot = 5.4% wt = 1.5% mol

Ttop= 14.5 °C Tbot = 25.4 °C

Cooling duty: 55.5 kW

Dephlegmation (Vane et al., 2004)

ybot = 34.5% wt = 17.1% mol

ytop= 85.4% wt = 69.6% molxbot = 5.4% wt = 1.5% mol

Ttop= 14.5 °CTbot = 23.9 °C

Cooling duty: 43.6 kW

F=100kg/h, T= 60°C, P=30 Torr, N= 4 stages

• Distillation needs more cooling duty • Further investigation needed• This research group claim 50% cost reduction in recovering bioethanol with dephlegmation (Mairal et al., 2002)

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Summary Dephlegmation: for which applications

• Separations where refrigeration needed- Distillation expensive- Dephlegmation thermodynamically efficient (low ΔT)

• Rough separations- Low specifications- Not a lot of stages needed

• Separations where gases are processed- Natural gas processing- Industrial gases- Air separation

• “New” processes- Recovery of products from biomass fermentation- In combination with membrane techniques (pervaporation)- Separation of organics from diluted aqueous solutions- Competitive to batch distillation?

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Concluding Remarks

• Dephlegmation not directly competitive to distillation but in synergy with it

• An additional tool for the engineer in separations of gas streams • Should be considered for low temperature separations (refrigeration) and when thermodynamic efficiency is desired

• Design of dephlegmators should be addressed and efficiency should be discussed openly

• “Dephlegmation technology” is a black box. Many applications –many patents - few publications

• Distillation will continue to be the process for high purity products

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Dr. Leland M. Vane, U.S. Environmental Protection Agency

Prof. Truls Gundersen, Department of Mechanical Engineering, NTNU

Dr. Dag Eimer, Norsk Hydro, F-Senter, Porsgrunn

Acknowledgements

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ReferencesGeneral papers

Baudot, A., Marin, M., Improved recovery of an ester flavor compound by pervaporation coupled with a flash condensation, Ind. Eng. Chem. Res., 38 (11), (1999), 4458.

Di Cave, S., Mazzarotta, B., Sebastiani, E., Mathematical model for process design and simulation of dephlegmators (partial condensers) for binary mixtures, The Canadian Journal of Chemical Engineering, 65, (1987), 559.

Jibb, R.J., Gibbard, I., Polley, G.T., Webb, D.R., The potential for using heat transfer enhancement in vent and reflux condensers, unpublished paper, personal communication with Vane L., U.S. EPA.

Kent, E.R., Pigford, R.L., Fractionation during condensation of vapor mixtures, AIChE J., 2 (3), (1956), 363.

Lee, S., Logsdon, J.S., Foral, M.J., Grossmann, I.E., Superstructure optimization of the olefin separation process, ESCAPE-13 (Proceedings), (2003), 191

Chiu, C-H., Advances in gas separation, Hydrocarbon Processing, (1990), 69.

Lucadamo, G.A., Bernhard, D.P., Rowles H.C., Improved ethylene and LPG recovery through dephlegmator technology, Gas Separation & Purification, 1, (1987), 94.

Mairal, A.P., Ng, A., Vane, L., Alvarez, F., Efficient recovery of bioethanol using novel pervaporation-dephlegmation process, AIChE Annual Meeting 2002, Paper 293e, (2002).

Marin, M., Hammami, C., Beaumelle, D., Separation of volatile organic compounds from aqueous mixtures by pervaporation with multi-stage condensation, Journal of Food Engineering, 28, (1996), 225.

Minkkinen, A., Fischer, B., Wood, T., Avison, C., Deep liquids extraction from natural gas with a synergistic combination of technologies, paper available at: www.gasprocessors.com/GlobalDocuments/E00Feb_02.PDF.

Roehm, H.J., Simulation of the unsteady state behaviour of the dephlegmation of binary vapour mixtures, Letters in Heat and Mass Transfer, 5, (1978), 307.

Roehm, H.J., The simulation of steady state behaviour of the dephlegmation of multi-component mixed vapours, Int. J. Heat Mass Transfer, 23, (1980), 141.

Vane, L.M., Alvarez, F.R., Mairal, A.P., Baker, R.W., Separation of vapor-phase alcohol/water mixtures via fractional condensation using a pilot-scale dephlegmator: enhancement of the pervaporation process separation factor, Ind. Eng. Chem. Res., 43, (2004), 173.

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References (cont.)Books

Isalski, W.H., Separation of Gases, Clarendon Press, Oxford, (1989), 188-190.

King, C.J., Separation processes, McGraw-Hill, (1980), 140-145.

Company/Internet sources

Chart Industries Inc., www.chart-ind.com

Air Products and Chemicals Inc., www.airproducts.com

C. Judson King, "Distillation", in AccessScience@McGraw-Hill, http://www.accessscience.com

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