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Equilibrium Separations in Chemical Engineering: Flash Units and Cascade Distillation

This lecture provides an overview of flash units and cascade distillation in chemical engineering equilibrium separations. Topics covered include thermodynamics review, binary flash with material and energy balance, sequential and simultaneous solutions, multicomponent flash, AspenPlus simulation, staged systems, and sensitivity analysis.

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Equilibrium Separations in Chemical Engineering: Flash Units and Cascade Distillation

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  1. CBE 417 “Chemical Engineering Equilibrium Separations” Lecture: 7 17 Sep 2012

  2. Overview • Brief thermodynamics review • Binary Flash with material balance and energy balance • Sequential solution • Simultaneous solution • Multicomponent Flash • Flash Unit Operation (AspenPlus) • Staged systems • McCabe-Thiele

  3. Effect of Pressure: • Seader & Henley (2006)

  4. Constant Relative Volatility?

  5. Alternative Thermodynamics Ki with multicomponent flash: Into MB: Sequential solution: suggestions p 35-37 (Rachford-Rice Eqn) Ref: Wankat Simultaneous solution technique: suggestions p 40-43

  6. Sizing Flash Drums

  7. Simulators Flash input: MeOH – Water; 1.013 bar; ZMeOH=0.6; Find f to give XMeOH < 0.2 AspenPlus Flash Sensitivity Analysis: Design Spec:

  8. In-Class AspenPlus Exercise Flash input: Ethane – n-Heptane; 13 bar; Zethane= 0.5; Let f = 0.5 [make Txy and YX diagrams] Sensitivity Analysis: Design Spec:

  9. In-Class AspenPlus Exercise Sensitivity Analysis: f varies 0.05 – 0.95

  10. Example: Flash input: n-hexane – n-octane; 1.013 bar; Zhexane= 0.5; Let f = 0.5

  11. Example: Single flash

  12. Simulators How increase overhead purity?

  13. Simulators How increase overhead purity?

  14. Simulators How increase overhead purity?

  15. Simulators How increase overhead purity?

  16. Simulator: Add 2nd flash onto vapor (V1) stream:

  17. Simulator: Add 2nd flash with recycle:

  18. Simulator: Add 3rd stage flash with recycle:

  19. Simulator: Add 3rd stage w/recycle & Middle stage adiabatic:

  20. Simulator: Add 3rd stage w/recycle & Middle stage adiabatic & Higher “reflux” :

  21. Cascade Summary:

  22. Cascade Flash Summary: • Method to improve vapor purity of light “key” component • Improve overall recovery of light key by recycling liquid from stages above to previous stage • Not practical to have intermediate HX or pump between each flash stage • Assemble stages in vertical column where vapor flows up to next stage, and liquid flows down to stage below. • Preheat feed (Qin) and remove heat at top condenser (Qcond). • Intermediate stages adiabatic • Liquid recycle “enriches” vapor in “lighter” component • Effect enhanced as total liquid recycle flow is increased. Aside: Key components (LK, HK) define where split is to be made. Most of LK in top stream; most of HK in bottom stream

  23. Add 3rd stage w/recycle & Middle stage adiabatic & Higher “reflux” & With stage below and recycle:

  24. Cascade Summary:

  25. Add 3rd stage w/recycle & Middle stage adiabatic & Higher “reflux” & With stage below and recycle And adiabatic Flash by feed:

  26. Cascade Flash Summary: • Additional “flash” stages improve purity, but recovery is poor • Recycle of intermediate streams allows better recovery while preserving good purity • Intermediate stages operated adiabatically – minimizing the need for intermediate HX equipment, pumps, or valves • Heat provided in bottom stage provides vapor “boilup” • Heat removed from top stage provides liquid “reflux” • This allows for a cascade separation to be done in one piece of equipment – called a distillation column

  27. Top of “Column” Rectifying (enriching) section of distillation column

  28. Equilibrium “Stage” Liquid and vapor leaving a stage (tray) are assumed to be in equilibrium

  29. Bottom of “Column” Stripping section of distillation column

  30. Distillation Column

  31. Distillation Column

  32. Questions?

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