Equilibrium Separations in Chemical Engineering: Flash Units and Cascade Distillation

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?