cbe 417 “chemical engineering equilibrium separations” 1 lecture: 9 28 sep 2012
Post on 17-Jan-2016
220 Views
Preview:
TRANSCRIPT
CBE 417“Chemical Engineering Equilibrium
Separations”
1
Lecture: 9
28 Sep 2012
Overview
2
• McCabe-Thiele graphical technique (binary systems)• Condensers• Reboilers
• Binary Shortcut Methods• AspenPlus:o Shortcut methods: DSTWUo Rigorous method: RADFRAC
• Efficiencies• Introduction to multicomponent distillation
Problem Solving Exercise
3
Problem Solving Exercise
4
McCabe Thiele: Total Condenser
5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Xa
Ya
Y=X line
1R
xD
D
y1
x1
xD
y2
xD
y1 V1
LO
y3
x3
x2
y4
1
3
2
McCabe Thiele: Partial Condenser
6
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Xa
Ya
Y=X line
1R
xD
D
xOy1
x1
x0
y2
y1 V1
LO
y3
x3
x2
y4
1
3
2
yD
McCabe Thiele: Partial Condenser
7
D
xOy1
x1
x0
y2
y1 V1
LO
y3
x3
x2
y4
1
3
2
yD
Why consider using a partial condenser?•Desire vapor distillate?
•Need to control temperature in the column?
Concept: o control distillation column temperatures
by controlling the operating pressure.o Condenser design / column opn. Po Minimum desirable condenser
temperature is ~ 49 oC (120 oF).o Construction of column more expensive
at higher pressures (i.e. > 14.8 bar [200 psig])
o Bottoms T too high (decomposition or Tc approached)
•Running a partial condenser lowers the column operating pressure (i.e. don’t have to fully condense the distillate product
McCabe Thiele: Partial Condenser
8
Seader & Henley, 2006
9
Seader & Henley, 2006
McCabe Thiele: Reboilers
Effect of Different Reflux Ratios
10
• Before we saw that..• increasing reflux resulted in an enrichment of vapor stream
in the “lighter” component
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.80
5
10
15
20
25
30
35
R/Rmin
N [
nu
mb
er
eq
uil
. sta
ge
s]
Effect of Different Reflux Ratios
11
• Increase R --- cost effect??
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8$0
$20,000
$40,000
$60,000
$80,000
$100,000
$120,000
$140,000
$160,000
$180,000
$200,000
$0
$5,000
$10,000
$15,000
$20,000
$25,000
$30,000
$ steam
$ water
R/Rmin
Ste
am C
osts
[$/
yr;
1991
]
Coo
ling
Wat
er C
osts
[$/
yr;
1991
]
Effect of Different Reflux Ratios
12
Seader & Henley, 2006
R = factor * Rmin
RRR optmin
50.105.1min
R
Ropt
Tie Together
13
• Found using McCabe-Thiele (knowing xD , xB , & feed condition):o Nmin (at total reflux)o Rmin (at infinite number of stages)
• If given actual R (or choose by R = factor * Rmin ) can find:o No NF (optimum feed stage number)
Suggests there is a relation between N, Nmin, and Rmin.
Gilliland Correlation
14
61 Data points over ranges:
1. No. components: 2 to 112. q : 0.28 to 1.423. P : vacuum to 42.4 bar4. : 1.11 to 4.055. Rmin : 0.53 to 9.096. Nmin : 3.4 to 60.3Molokanov Eqn:
5.0
1
2.11711
4.541exp1
X
X
X
XY
Seader & Henley, 2006
Binary Shortcut Methods
15
McCabe-Thiele is good at showing procedure and helping explain concepts for binary distillation. It can be extended to multiple feeds and side draws (Wankat).
When consider multicomponent mixtures, McCabe-Thiele is not quite as useful. However, having some shortcut methods to aid in final column design is useful.
Minimum number of stages: Fenske Equation
ln
11
ln
min
B
B
D
D
xx
xx
N
- binary; total condenser; ~ constant
31
BotFeedDistBotDist or
Binary Shortcut Methods
16
Minimum reflux ratio: Underwood Equation
1
11
min
F
D
F
D
zFxD
FzDx
F
L
- binary; assumes rectifying line intersects eq. line at feed line; ~ constant
D
LR min
min
R = factor * Rmin
Approximate number of equilibrium stages (N): Gilliland correlation
Binary Shortcut Methods
17
Optimum feed stage location (NF): Kirkbride Equation
206.02
1
1
D
B
x
x
z
z
N
N
D
B
F
F
S
R
- binary; approximate result
Binary Shortcut Methods
18
Summary Binary Shortcut Methods (FUG)
19
Fenske Equation
ln
11
ln
min
B
B
D
D
xx
xx
N
31
BotFeedDistBotDist or
Underwood Equation
1
11
min
F
D
F
D
zFxD
FzDx
F
L
D
LR min
min
Gilliland correlation (Nequil)
Kirkbride Equation (NF(opt))
206.02
1
1
D
B
x
x
z
z
N
N
D
B
F
F
S
R
R = factor * Rmin
Overview
20
• Questions on homework??• McCabe-Thiele graphical technique (binary systems)• Condensers• Reboilers
• Binary Shortcut Methods• AspenPlus:o Shortcut methods: DSTWUo Rigorous method: RADFRAC
• Efficiencies• Introduction to multicomponent distillation
AspenPlus: DSTWU
21
top related