Transcript Evaluation of Dephlegmation as an Alternative Separation Process to Distillation Stathis Skouras

Evaluation of Dephlegmation as an
Alternative Separation Process
to Distillation
Stathis Skouras
7. May 2004
Department of Chemical Engineering, NTNU
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NTNU
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
practical considerations, flexibility, etc
• Case study
– EtOH/H2O separation
• Summary
– Dephlegmation: for which applications?
• Concluding remarks
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achieved,
Dephlegmation: process description
Partial condenser or
Dephlegmator
1
Vin
yin
4
cooling
water
Vout, 1, yout, 1
Lout, 1
Xout, 1
Two dephlegmators
connected in series
(multistage condensation)
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cooling
water
Vout, 2, yout, 2
Lout, 2
Xout, 2
SOLUTION
Reflux unwanted liquid products
Establish counter-current flow
Allow one vapour product, one liquid product
Dephlegmation: process description (cont.)
COOLANT
Vout
ENRICHED
VAPOUR
• Reminds of heat exchanger
• But HEAT and MASS transfer
L
HEAT
REMOVAL
V
Vin
VAPOUR
FEED
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STRIPPED
CONDENSATE
Lout
• Temperature and composition
profile established
• Terminology:
- reflux condenser
- run-back condenser
- fractionating condenser
- dephlegmator (in cryogenics)
Dephlegmation: process description (cont.)
The Dephlegmation Principle
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Ref: Minkkinen et al.
Dephlegmation: columns
High-surface-area
heat exchanger
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Ref: Vane et al., (2004)
Partial reflux
condenser (top)
+
High-surface-area
heat exchanger
Partial reflux
condenser (top)
+
High-surface-area
packing
Dephlegmation: columns (cont.)
State-of-the-art dephlegmator
Compact brazed aluminium plate-fin
heat exchanger (configuration A)
Vapour/condensate regions (dark regions)
Coolant regions (lighter regions)
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Ref: Minkkinen et al.
Chart Industries Inc.
Dephlegmation: applications
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)
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Dephlegmation: applications (cont.)
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
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Ref: Minkkinen et al.
Dephlegmation: applications (cont.)
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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Ref: Minkkinen et al.
Distillation: process description
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V0
1
F0
V1
cooling
water
steam
L1
2´
L0
steam
V1´
L1´
Reflux unwanted vapour and liquid products
Establish counter-current flow
Allow one top product, one bottom product
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Ref: King, C.J., (1980)
Distillation: columns
Rectifying
section
Stripping
section
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Ref: C. Judson King, "Distillation", in AccessScience@McGraw-Hill, http://www.accessscience.com
Comparisons: columns
Dephlegmation
Coolant
Distillation
Vout
Rectifying
section
L
Rectifying
section
HEAT
REMOVED
V
Stripping
section
Vin
Lout
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HEAT
INJECTION
HEAT
REMOVED
Comparisons: energy issues
Adiabatic distillation
Vout
Dephlegmation
DEPHLEGMATION
is
REVERSIBLE DISTILLATION
Vout with interstage heat removal?
HEAT
REMOVED
LR
EXTERNAL
REFLUX
INTERNAL
REFLUX
L
L
HEAT
V
Vin
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THERMAL
INSULATION
REMOVED
Lout
Ref: Kent, E.R., Pigford, R.L., (1956)
V
Vin
Lout
Comparisons: energy issues
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
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
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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
Feed: H2, mixture of hydrocarbons
Obj. function: Annual capital cost +
compression and utilities (MINLP)
Processes: dephlegmation,
distillation, absorption, membrane
Solution: 1 dephlegmator upstream,
3 distillation columns further
processing and final products
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Ref: Lee et al., 2003
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
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Ref: (Vane al., 2004, Minkinnen et al., )
Case Study: EtOH / H2O separation
Vane et al., 2004
Vout
85.4% wt
EtOH
dephlegmation
Data
Operation under vacuum (30 Torr)
Feed superheated vapour (60°C)
Desired EtOH recovery = 90%
Simulations
• Dephlegmator modelled as a 4stage column
• User specified heat removal
per stage
retentate
F
5% wt
EtOH
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permeate
34.5% wt
EtOH
pervaporation
Vin
Experiments
L
5.4% wt
EtOH
• 0.2m × 0.22m × 2.4m dephlegmator
(Chart Industries)
• Expected to provide 4-6 stages
Case Study: EtOH / H2O separation (cont.)
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Simulations
Experiments
• 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%)
•90% wt purity, 89% recovery could be
obtained
• Results in agreement with simulations
for 6 stages
Ref: Vane et al., (2004)
Case Study: EtOH / H2O separation (cont.)
F=100kg/h, T= 60°C, P=30 Torr, N= 4 stages
Dephlegmation
(Vane et al., 2004)
Distillation
(Hysys)
ybot = 34.5% wt = 17.1% mol
ytop= 85.4% wt = 69.6% mol
xbot = 5.4% wt = 1.5% mol
Ttop= 14.5 °C
Tbot = 23.9 °C
ybot = 34.5% wt = 17.1% mol
xtop= 85.4% wt =69.6% mol
xbot = 5.4% wt = 1.5% mol
Ttop= 14.5 °C
Tbot = 25.4 °C
Cooling duty: 43.6 kW
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Cooling duty: 55.5 kW
• Distillation needs more cooling duty
• Further investigation needed
• This research group claim 50% cost reduction in recovering
bioethanol with dephlegmation (Mairal et al., 2002)
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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Acknowledgements
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
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References
General 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.
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Vane, L.M., Alvarez, F.R., Mairal, A.P., Baker, R.W., Separation of vapor-phase alcohol/water mixtures via fractional condensation using a pilotscale dephlegmator: enhancement of the pervaporation process separation factor, Ind. Eng. Chem. Res., 43, (2004), 173.
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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Thank you for your attention…
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