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

2. Overview 2 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: 3 Seader & Henley (2006)

4. Constant Relative Volatility? 4

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

6. Sizing Flash Drums 6

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

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

9. 9 Sensitivity Analysis: f varies 0.05 – 0.95 Row / CaseStatusVFRACYC2XC2 100.050.9970.474 200.10.9970.445 300.150.9960.413 400.20.9940.376 500.250.9910.336 600.30.9860.292 700.350.9750.244 800.40.9530.198 900.450.9130.162 1000.50.8630.137 1100.550.8100.121 1200.60.7610.108 1300.650.7160.099 1400.70.6750.092 1500.750.6380.086 1600.80.6050.081 1700.850.5750.076 1800.90.5480.072 1900.950.5230.069 In-Class AspenPlus Exercise

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

11. Example: 11 Single flash

12. Simulators 12 How increase overhead purity?

13. Simulators 13 How increase overhead purity?

14. Simulators 14 How increase overhead purity?

15. Simulators 15 How increase overhead purity?

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

17. Simulator: 17 Add 2 nd flash with recycle:

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

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

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

21. Cascade Summary: 21 Cascade Demo Summary Table Fall 2012C6Heat Duty [kW] Flashy outrecoveryreboilercondenser 1 stage0.6969%224.9 2 stages0.8055%224.9-73.2 2 stg w recycle0.7964%267.3-86.8 3 stg w recycle0.8364%276.1-104.7 3 stg w recycle & Q2 = 00.75867%250.9-52.9 " " w/higher "reflux"0.85665%282.3-117.2

22. Cascade Flash Summary: 22 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 (Q in ) and remove heat at top condenser (Q cond ). 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. 23 Add 3 rd stage w/recycle & Middle stage adiabatic & Higher “reflux” & With stage below and recycle:

24. Cascade Summary: 24 Cascade Demo Summary Table SS 2010C6Heat Duty [kW] Flashy outrecoveryreboilercondensor 1 stage0.6969%224.9 2 stages0.8055%224.9-73.2 2 stg w recycle0.7964%267.3-86.8 3 stg w recycle0.8364%276.1-104.7 3 stg w recycle - Q2 = 00.75867%250.9-52.9 " " w/higher "reflux"0.85665%282.3-117.2 " " " and stage below0.81489%420.2-171.6

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

26. Cascade Flash Summary: 26 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” 27 Rectifying (enriching) section of distillation column

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

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

30. Distillation Column 30

31. Distillation Column 31

32. Questions? 32