CBE 417
“Chemical Engineering Equilibrium
Separations”
Lecture: 7
17 Sep 2012
1
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
2
Effect of Pressure:
•Seader & Henley (2006)
3
Constant Relative Volatility?
1
0.9
0.8
Y MeOH
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
X MeOH
4
Alternative Thermodynamics
Ki with multicomponent flash:
yi  Ki xi
Into MB:
x
i
1
Zi
xi 
V   L 
Ki     
F F






Zi
Zi
  
1 

V
L




K
f

1

f




K

 i






i
  F   F  
Sequential solution: suggestions p 35-37 (Rachford-Rice Eqn)
Simultaneous solution technique: suggestions p 40-43
Ref: Wankat
5
Sizing Flash Drums
u perm  K drum
htotal
 3to 5
D
V
( mol )
V

 L  V
L
u perm Ac V
MwV
6
Simulators
Flash input: MeOH – Water; 1.013 bar; ZMeOH=0.6;
Find f to give XMeOH < 0.2
AspenPlus Flash
Sensitivity Analysis:
Design Spec:
7
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:
8
In-Class AspenPlus Exercise
Sensitivity Analysis:
f varies 0.05 – 0.95
mol C2 in product
Recovery 
mol C2 in feed
Row
/
Case Status VFRAC
1
0
0.05
2
0
0.1
3
0
0.15
4
0
0.2
5
0
0.25
6
0
0.3
7
0
0.35
8
0
0.4
9
0
0.45
10
0
0.5
11
0
0.55
12
0
0.6
13
0
0.65
14
0
0.7
15
0
0.75
16
0
0.8
17
0
0.85
18
0
0.9
19
0
0.95
YC2
0.997
0.997
0.996
0.994
0.991
0.986
0.975
0.953
0.913
0.863
0.810
0.761
0.716
0.675
0.638
0.605
0.575
0.548
0.523
XC2
0.474
0.445
0.413
0.376
0.336
0.292
0.244
0.198
0.162
0.137
0.121
0.108
0.099
0.092
0.086
0.081
0.076
0.072
0.069
9
Example:
Flash input: n-hexane – n-octane; 1.013 bar; Zhexane = 0.5;
Let f = 0.5
10
Example:
Single flash
11
Simulators
How increase overhead purity?
12
Simulators
How increase overhead purity?
13
Simulators
How increase overhead purity?
14
Simulators
How increase overhead purity?
15
Simulator:
Add 2nd flash onto vapor (V1) stream:
16
Simulator:
Add 2nd flash with recycle:
17
Simulator:
Add 3rd stage flash with recycle:
18
Simulator:
Add 3rd stage w/recycle &
Middle stage adiabatic:
19
Simulator:
Add 3rd stage w/recycle &
Middle stage adiabatic &
Higher “reflux” :
20
Cascade Summary:
Cascade Demo
Summary Table
Fall 2012
C6
Flash
Heat Duty [kW]
y out
recovery
reboiler
condenser
1 stage
0.69
69%
224.9
2 stages
0.80
55%
224.9
-73.2
2 stg w recycle
0.79
64%
267.3
-86.8
3 stg w recycle
0.83
64%
276.1
-104.7
3 stg w recycle & Q2 = 0
0.758
67%
250.9
-52.9
" " w/higher "reflux"
0.856
65%
282.3
-117.2
21
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
22
Add 3rd stage w/recycle &
Middle stage adiabatic &
Higher “reflux” &
With stage below and recycle:
23
Cascade Summary:
Cascade Demo
Summary Table
SS 2010
C6
Flash
Heat Duty [kW]
y out
recovery
reboiler
condensor
1 stage
0.69
69%
224.9
2 stages
0.80
55%
224.9
-73.2
2 stg w recycle
0.79
64%
267.3
-86.8
3 stg w recycle
0.83
64%
276.1
-104.7
3 stg w recycle - Q2 = 0
0.758
67%
250.9
-52.9
" " w/higher "reflux"
0.856
65%
282.3
-117.2
" " " and stage below
0.814
89%
420.2
-171.6
24
Add 3rd stage w/recycle &
Middle stage adiabatic &
Higher “reflux” &
With stage below and recycle
And adiabatic Flash by feed:
25
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
26
Top of “Column”
Rectifying (enriching)
section of distillation
column
27
Equilibrium “Stage”
Liquid and vapor leaving a stage
(tray) are assumed to be in
equilibrium
28
Bottom of “Column”
Stripping section of
distillation column
29
Distillation Column
30
Distillation
Column
31
Questions?
32