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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. NTNU. Keywords. Dephlegmation Distillation Evaluation: dephlegmation vs. distillation. Presentation Overview. Dephlegmation

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

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  1. Evaluation of Dephlegmation as anAlternative Separation Process to DistillationStathis Skouras 7. May 2004 Department of Chemical Engineering, NTNU NTNU

  2. Keywords Dephlegmation Distillation Evaluation: dephlegmation vs. distillation

  3. Presentation Overview • Dephlegmation • Process description, columns, applications • Distillation • Process description, columns • Comparisons • Columns, energy considerations, separation achieved, practical considerations, flexibility, etc • Case study • EtOH/H2Oseparation • Summary • Dephlegmation: for which applications? • Concluding remarks

  4. Two dephlegmators connected in series (multistage condensation) Partial condenserorDephlegmator cooling water cooling water Vout, 1, yout, 1 Vout, 2, yout, 2 Vin yin Lout, 1 Xout, 1 Lout, 2 Xout, 2 Dephlegmation: process description 1 2 SOLUTION Reflux unwanted liquid products Establish counter-current flow Allow one vapour product, one liquid product

  5. Vout COOLANT ENRICHED VAPOUR L HEAT REMOVAL V Vin STRIPPED CONDENSATE VAPOUR FEED Lout 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)

  6. Dephlegmation: process description (cont.) The Dephlegmation Principle Ref: Minkkinen et al.

  7. Dephlegmation: columns Partial reflux condenser (top) + High-surface-area heat exchanger Partial reflux condenser (top) + High-surface-area packing High-surface-area heat exchanger Ref: Vane et al., (2004)

  8. 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) Ref: Minkkinen et al. Chart Industries Inc.

  9. 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)

  10. 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 Ref: Minkkinen et al.

  11. 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) Ref: Minkkinen et al.

  12. V1 V0 cooling water L1 F0 steam V1´ L0 steam L1´ Distillation: process description 2 1 2´ Reflux unwanted vapour and liquid products Establish counter-current flow Allow one top product, one bottom product Ref: King, C.J., (1980)

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

  14. Dephlegmation Coolant Vout L Rectifying section HEAT REMOVED V Vin Lout Comparisons: columns Distillation HEAT REMOVED Rectifying section Stripping section HEAT INJECTION

  15. Dephlegmation Adiabatic distillation Vout Vout HEAT REMOVED LR EXTERNAL REFLUX L INTERNAL REFLUX HEAT REMOVED L THERMAL INSULATION V V Vin Lout Vin Lout Comparisons: energy issues DEPHLEGMATION is REVERSIBLE DISTILLATION with interstage heat removal? Ref: Kent, E.R., Pigford, R.L., (1956)

  16. 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

  17. 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

  18. 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 Ref: Lee et al., 2003

  19. 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

  20. 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., )

  21. Case Study: EtOH / H2O separation Vane et al., 2004 Data Operation under vacuum (30 Torr) Feed superheated vapour (60°C) Desired EtOH recovery = 90% Vout 85.4% wt EtOH dephlegmation • Simulations • Dephlegmator modelled as a 4-stage column • User specified heat removal per stage retentate • Experiments • 0.2m × 0.22m × 2.4m dephlegmator (Chart Industries) • Expected to provide 4-6 stages Vin permeate L F 5% wt EtOH 5.4% wt EtOH 34.5% wt EtOH pervaporation

  22. 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%) • Experiments • 90% wt purity, 89% recovery could be obtained • Results in agreement with simulations for 6 stages Case Study: EtOH / H2O separation (cont.) Ref: Vane et al., (2004)

  23. Case Study: EtOH / H2O separation (cont.) F=100kg/h, T= 60°C, P=30 Torr, N= 4 stages Dephlegmation (Vane et al., 2004) 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 Cooling duty: 43.6 kW Distillation (Hysys) 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: 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)

  24. SummaryDephlegmation: 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?

  25. 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

  26. 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

  27. 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. 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.

  28. 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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