Post on 11-Jan-2016
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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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