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

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

Effect of Pressure: 3 Seader & Henley (2006)

Constant Relative Volatility? 4

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

Sizing Flash Drums 6

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

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 Sensitivity Analysis: f varies 0.05 – 0.95 Row / CaseStatusVFRACYC2XC In-Class AspenPlus Exercise

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

Example: 11 Single flash

Simulators 12 How increase overhead purity?

Simulators 13 How increase overhead purity?

Simulators 14 How increase overhead purity?

Simulators 15 How increase overhead purity?

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

Simulator: 17 Add 2 nd flash with recycle:

Simulator: 18 Add 3 rd stage flash with recycle:

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

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

Cascade Summary: 21 Cascade Demo Summary Table Fall 2012C6Heat Duty [kW] Flashy outrecoveryreboilercondenser 1 stage0.6969% stages0.8055% stg w recycle0.7964% stg w recycle0.8364% stg w recycle & Q2 = % " " w/higher "reflux" %

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

Cascade Summary: 24 Cascade Demo Summary Table SS 2010C6Heat Duty [kW] Flashy outrecoveryreboilercondensor 1 stage0.6969% stages0.8055% stg w recycle0.7964% stg w recycle0.8364% stg w recycle - Q2 = % " " w/higher "reflux" % " " " and stage below %

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

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

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

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

Bottom of “Column” 29 Stripping section of distillation column

Distillation Column 30

Distillation Column 31

Questions? 32