cbe 417 “chemical engineering equilibrium separations” 1 lecture: 8 24 sep 2012
TRANSCRIPT
CBE 417“Chemical Engineering Equilibrium
Separations”
1
Lecture: 8
24 Sep 2012
Overview
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• Multicomponent Flash• Flash Unit Operation (AspenPlus)• Staged systems• McCabe-Thiele
Distillation Column
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Distillation Column
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Design (binary)• Specify Za, XD, XB (more volatile) Reflux Ratio (Lo/D) Optimum feed stage location P column (condenser) F (feed flowrate) Feed condition• Find N (number of stages) Nfeed (feed stage) D, and B (flowrates) Heat duties Diameter, height
XB
XD
Za
P
Simulation (binary)• Use existing column Simulate to see performance Any needed modifications?
Distillation Column
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Overall Column Balances (SS)
XB
XD
Za
P
hD
hF
hB
QC
QR
Material Balance (MB):
BDF BxDyFz aaa
Energy Balance (EB):
BhDhQQFh BDRCF
• heat added is (+)• heat removed (-)• adiabatic (well insulated)
McCabe-Thiele Graphical Method (binary)
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Used to simplify analysis of binary distillation (ease of understanding)
Assumptions:• Pure components a, b have equal latent heats of vaporization / mole ( ) and they
stay constant.• are much larger than
• Sensible heat changes• Heats of mixing
• Column is adiabatic (well – insulated)• Constant pressure (P) throughout the column (i.e. no P in the column)
i
i
Called Constant Molal Overflow (CMO)• Assumes for every 1 mole of light material vaporized that 1 mole of heavy material
condenses from the vapor phase• Net result:
• Total molar flowrates (i.e. L and V) remain constant within that column section (rectifying or stripping, or other)• Do not need a stage by stage energy balance
McCabe-Thiele is done with MB and thermodynamic information.
MB on Rectifying Section
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At Steady State (SS) for light material (LK) MB: Moles In = Moles Out
DxLxVy DNNNN 11
DN
NN
NN x
V
Dx
V
Ly
111
For CMO
VVLL NN 1;
General Operating
LineDNN x
V
Dx
V
Ly 1
D
L
D
LR
RRatioReflux
o
1
11
RV
DR
R
V
L
111 R
xx
R
Ry D
NN
Equilibrium Stage
Rectifying Section
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Operating Line:• Relates composition of liquid leaving
stage N (i.e. xN) to the composition of vapor entering stage N (yN+1)
General Operating Line: Rectifying Section
111 R
xx
R
Ry D
NN
?:: 1 NDN ythensoxxLet
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
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Xa
Ya
Y=X line
(xD, y1)
1R
xD
Equilibrium Line:• Relates composition of liquid leaving
stage N (i.e. xN) to the composition of vapor leaving stage N (yN)
yN
xN
xN-1
yN+1
Tie Together Equilibrium & Operating Lines
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
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Xa
Ya
Y=X line
(xD, y1)
1R
xD
D
y1
x1
xD
y2
xD
y1 V1
LO
y3
x3
x2
y4
(x2, y3)
(x1, y1)
(x1, y2)(x3, y3)
(x2, y2)
1
3
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MB on Stripping Section
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At SS and CMO still assumed, so:
Bmm BxyVxL 1
constantVL &
Operating Line Stripping Section
B
VV
VRatioBoilup
B
B
Bmm x
V
Bx
V
Ly 1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
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Xa
Ya
Y=X line
(xB)
(xB, yB)
(xN, yN)
(xN, yB)
(xN-1, yN)
Feed Stage
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Suppose:• q = 1
• q = 0
Depending on Feed “condition” will get changes to vapor and liquid flowrates…
F
LLq
Define q = Moles of liquid flow in Stripping section that result from one mole of feed.
F
ZF
L V
L V
FV
FL
Feed Stage Operating Line
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F
ZF
L V
L V
FV
FL
DNN x
V
Dx
V
Ly 1
Bmm x
V
Bx
V
Ly 1
rectifying section
stripping section
DNN DxLxVy 1
Bmm BxxLyV 1
BD BxDxLLxVVy Feed stage & overall column MBs:
BDF BxDxFz
VLVLF
FFzLLxFLLy
FLLVV
FzF
LLx
F
LLy
1
11 q
zx
q
qy F feed line eqn.
Plot Feed MB Line
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
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Xa
Ya
Y=X line
zF
Feed Condition q Slope
11 q
zx
q
qy F
sat’d liquid = 1
sat’d vapor = 0 0
sat’d liq.q = 1
mixed V & L 0 < q < 1 neg. (-)
subcooled L > 1 pos. (+)
superheated V < 0 pos. (+)sat’d vap. q = 0
0<q<1
subcooled liq.
q>1
Superhtd vapor
q<0
partial V&L
Lets put all three lines together:• rectifying section• stripping section• feed line
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
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X
Y
Operating Lines (McCabe-Thiele)
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zF
R incr.q is constant
Rectifying & stripping lines must intersect at the same point on the feed line.
Consider limits:• R = • R where rectifying line
intersects the equil. curve
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
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X
Y
Operating Lines (McCabe-Thiele)
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zF
q incr.
R is constant
Rectifying & stripping lines must intersect at the same point on the feed line.
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
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0.5
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1
X
Y
McCabe-Thiele Graphical Method
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Binary Distillation
Feed stage location: point where switch from rectifying operating line to the stripping operating line.zF
“step off” equilibrium stages on the XY diagram.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
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1
X
Y
McCabe-Thiele Graphical Method
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Binary Distillation
Feed stage location: point where switch from rectifying operating line to the stripping operating line.zF
Optimum feed stage location: switching point to obtain smallest number of stages. Switch when intersection of 3 operating lines is first crossed.
“step off” equilibrium stages on the XY diagram.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
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0.4
0.5
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1
X
Y
McCabe-Thiele Graphical Method
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Binary Distillation
Total reflux; so D = ?and R = ??
zF
Minimum Number of Stages
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
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1
X
Y
McCabe-Thiele Graphical Method
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Binary Distillation
One or both operating lines intersect the equilibrium line.
zF
Result: infinite number of stages.
Minimum Reflux Ratio
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
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1
X
Y
McCabe-Thiele Graphical Method
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Binary Distillation
One or both operating lines intersect the equilibrium line.
zF
Result: infinite number of stages.
Minimum Reflux Ratio
McCabe-Thiele Graphical Method (binary)
21
Used to simplify analysis of binary distillation (ease of understanding)
Assumptions:• Pure components a, b have equal latent heats of vaporization / mole ( ) and they
stay constant.• are much larger than
• Sensible heat changes• Heats of mixing
• Column is adiabatic (well – insulated)• Constant pressure (P) throughout the column (i.e. no P in the column)
i
i
Called Constant Molal Overflow (CMO)• Assumes for every 1 mole of light material vaporized that 1 mole of heavy material
condenses from the vapor phase• Net result:
• Total molar flowrates (i.e. L and V) remain constant within that column section (rectifying or stripping, or other)• Do not need a stage by stage energy balance
McCabe-Thiele is done with MB and thermodynamic information.
Problem Solving Exercise
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Questions?
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