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Optimal operation* of distillation columns using simple control structures

Optimal operation* of distillation columns using simple control structures. Sigurd Skogestad, NTNU, Trondheim. EFCE Working Group on Separations , Gøteborg, Sweden , June 2019. *Optimal in the meaning of economics or sustainability. Distillation is part of the future.

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Optimal operation* of distillation columns using simple control structures

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  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:

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