1 / 22

Optimal Operation of Distillation Columns for Energy Efficiency and Sustainability

Distillation, often seen as energy-intensive, is actually efficient. By improving operation and control, energy and capital can be saved. Close-boiling separations are energy-intensive but suitable for heat pumps, while easy separations require minimal energy. With proper control, significant energy savings are possible. This text explores the myths and realities of distillation efficiency, including ideal work calculations and thermodynamic efficiency. It discusses the role of heat pumps, the Fenske equation, and the trade-off between operation economics and sustainability. Controlled operation in a region of optimal stage efficiency can lead to energy savings. Heat pumps can enhance thermodynamic efficiency, and avoiding over-refluxing can also save energy.

douglasb
Download Presentation

Optimal Operation of Distillation Columns for Energy Efficiency and Sustainability

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Optimal operation* of distillation columns using simple control structures Sigurd Skogestad, NTNU, Trondheim EFCE Working Group onSeparations, Gøteborg, Sweden, June 2019 *Optimal in themeaning of economics or sustainability

  2. Distillation is part of thefuture • It’s a myththatdistillation is bad in terms of energy • Better operation and control can save energy • Integrated schemescan save energy and capital • Divided-wall / Petlyukcolumns

  3. «Distillation is an inefficientprocesswhichuses a lot of energy» • This is a myth! • By itself, distillation is an efficientprocess. • It’s the heat integrationthatmay be inefficient. • Yes, it canuse a lot of energy, but it providesthe same energy at a lowertemperature • Difficultseparations (close-boiling): use a lot of energy -- butwellsuited for heat pumps • Easy separations: Uselittle energy

  4. Typical distillation Case Example 8.20 from Skogestad (2008) Thermodynamicefficiency is 63% Energy efficiency is only 5% (withno heat Integration)

  5. Qc z V Qr King’sformula: (binary, feedliquid, constantα, Infinite* no. Stages, pure products) Qr = reboiler duty [W] *Actual energy only 5-10% higher

  6. Ideal separation work • Minimum suppliedwork (for anyprocess) Ws,id= ΔH - T0ΔS • AssumeΔH=0 for theseparation. Minimum separationwork Ws,id= - F T0 ΔS • Separation of feedinto pure products • This is a negative number so theminimuimseparationworkWs,idis positive!

  7. (g) Tc Distillationwith heat pump Qc Low p (l) Ws (g) (g) High p z V TH Qr Minimum work (Carnot)

  8. Thermodynamic efficiency for conventional distillation • Thermodynamic Efficiency = Ideal work/Actual work: Note that T0 drops out

  9. Thermodynamic efficiencySpecial case: Binary, constant α • King's formula • Ideal binary mixture (Claperyon equation) + no pressure drop. King shows: • So Binary Note thatλ drops out

  10. Thermodynamic efficiency of binary close-boiling mixtures ( Comment: Above 50% for z from 0.2 to 0.8 Peak efficiency is -ln0.5 = 0.693 at z=0.5

  11. Thermodynamic efficiency of binary distillation • High efficiency at small z for easy separations with large α • Reason: Must evaporate light component to get it over top z = fractionlightcomponent in feed

  12. King (1971) Note: Non-idealitydoes not necessarilyimplylowerthermodynamicefficiency

  13. Why is it not perfect – where are the losses? • Irreversible mixing loss at every stage. • Largest losses in the middle of each section – where the bulk separation takes place • Small losses at the high-purity column ends

  14. Reversible binary distillation Reversible binary distillation

  15. HIDiC(Heat Integrated Distillation Column) I don’tbelieve in HIDiC. Too complicated, toomuchinvestment, not enoughsavings

  16. Distillation is unbeatable for high-purityseparations • Operation: Energy usageessentiallyindependent of productpurity • Capital: No. of stages increaseswith log(impurity) Fenske: Nmin = ln S / ln α Actual: N 2.5 Nmin Separationfactor:

  17. OPERATION

  18. Economics and sustainability for operation of distillationcolumnsIs there a trade-off? • No, not as long as thecolumn is operated in a region of constant (optimal) stage efficiency • Yes, ifweoperate at toohigh or tooload so thatthe stage efficiency drops

  19. Can save a lot of energy by improved control • Especially by avoiding over-refluxing

  20. Acknowledgement • Ivar Halvorsen, NTNU (20%) and SINTEF (80%)

  21. Thermodynamic efficiency for conventional distillation • Use heat pumps for reboiler and condenser. Ideal work with surroundings at T0 (Carnot): • Assume feed liquid and constant molar flows so • Thermodynamic Efficiency = Ideal work/Actual work:

More Related