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7/27/2019 Mc-Cabe Thiele Method
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Chapter 7: Distillation of Binary Mixtures 1
Chapter 7
Distillation of Binary Mixtures
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Chapter 7: Distillation of Binary Mixtures 2
Graphical Methods for Analyzing Binary Distillation
In Chapter 5:
We described a graphical method for analyzing multistage separation systems which involved
drawing operating lines and equilibrium curves and stepping off stages. This approach is equivalent tothe algebraic method and group methods. This approach was demonstrated using absorption and
stripping.
Todays lecture will focus on:
Extending these types of analysis to multisection cascades.
We begin by describing a typical binary distillation column.
We then describe the process generally and make important definitions.
We perform mass balances to get operating lines.
We plot equilibrium data to get an equilibrium curve.
We step of stages noting the cross-over between sections.
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Chapter 7: Distillation of Binary Mixtures 3
McCabe-Thiele Method for Trayed Towers
Absorption and stripping cascades are common methods for separating vapor and liquid mixtures. A morecomplete separation can be achieved by combining these processes into a binary distillation column.
Total condenser
Feed
Overhead vapor
BoilupN
2
1
Distillation
f
Reflux drum
Rectifying section stages
Stripping section stages
Feed Stage
Bottoms
Partial reboiler
RefluxDistillate
L0 (absorbent)
VN+1 (vapor to be
separated)
V1
LN
1
2
N1
N
Absorption
LN+1 (liquid to be separated)
V0(stripper)
VN
L1
12
N1
N
Stripping
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Chapter 7: Distillation of Binary Mixtures 4
Distillation Column
Feed
Rectifying section s tages
Stripping section stages
Total condenser
Reflux drum
Reflux Distillate
Boilup
Feed Stage
Bottoms
Partial reboiler
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Chapter 7: Distillation of Binary Mixtures 5
McCabe-Thiele Method for Trayed Towers
The general countercurrent-flow, multistage, binary distillation column shown below consists of
A column of N theoretical stages
A total condenser to produce a reflux liquid to act as an absorbent and a liquid distillate
A partial reboiler to produce boilup vapor to act as a stripping agent and a bottoms product
An intermediate feed stage.
This configuration allows one to achieve a sharp separation, except in cases where an azeotrope
exists where one of the products will approach the azeotropic concentration.
The goal of distillation
is to achieve a distillate
rich in the light key anda bottoms rich in the
heavy key.
Total condenser
Feed
Overhead vapor
BoilupN
2
1
Distillation
f
Reflux drum
Rectifying section stages
Stripping section stages
Feed Stage
Bottoms
Partial reboiler
Reflux Distillate
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Chapter 7: Distillation of Binary Mixtures 6
McCabe-Thiele Method for Trayed Towers
The feed contains a more volatile component (the light key, LK) and a less volatile component (the heavy key, HK).
At the feed temperature and pressure it may consist of a liquid, vapor or mixture of vapor and liquid. The feed
composition is given by the light key mole fraction ZF. The bottoms composition is given by the LK mole fraction
XB, whereas the distillate composition is given by the LK mole fraction XD.
Total condenser
Feed (L/V)
Overhead vapor
BoilupN
2
1
Distillation
f
Reflux drum
Rectifying section stages
Stripping section stages
Feed Stage
Bottoms
Partial reboiler
RefluxDistillate
LK mole fraction zF
LK mole fraction xD
LK mole fraction xB
The difficulty in achieving
the separation is determined
by the relative volatility, between the LK=1, and
the HK=2.
1,2 = K1/K2
If the two components form an
ideal solution then Raoults
Law applies and:
Ki = Pis
/P
The relative volatility is then
just the ratio of the vapor
pressures:
1,2= P
1
s/P2
sOnly a function of T
As T increases (pressure incresaes), decreasesuntil at some point it becomes equal to one and no
separation is possible.
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Chapter 7: Distillation of Binary Mixtures 7
McCabe-Thiele Method: Equilibrium Curve
We can rewrite the relative volatility in terms of the mole fractions of the light key in a binary mixtureas follows:
1,2 = K1/K2 =y1/x1
y2/x2=
y1/x11 y1( )/ 1 x1( )
=y1 1 x1( )
x1 1 y1( )
For close boiling point components the temperature, and thus will be nearly constant in the column.Solving for the mole fraction of the LK in the vapor gives:
For components which do not have close boiling points will vary depending on composition. Theequilibrium curve will appear similar to that of fixed , but wont fit the equation above for constant .
y1 =1,2x1
1+ x1 1,2 1( )
y1
x1
Equilibrium
curve
45 line
y1
x1
45 line
Increasing relative
volatility
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Chapter 7: Distillation of Binary Mixtures 8
Thermodynamic Considerations and Phase Equilibria: Binary Fluids
Lets consider a binary mixture AB, where
B is a heavy component (high boiling point)
and
A is a light component (low boiling point).
A T-x phase diagram of AB mixture, where
x is a mole fraction of component a might
look like this at some constant pressure P.
This phase diagram can be also transformed
in y-x diagram where composition of vapour
phase in terms of mole fraction of
component A is plotted as function of the
liquid phase composition.
x1 y1x2 y2x3 y3x4 y4
T
Tb(B)
Tb(A)
V
L
T1
T2
T3
T4
xA
xA
yA
T1
T2
T3
T4y4
y3
y1
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Chapter 7: Distillation of Binary Mixtures 9
Specifications
F Total Feed Rate
zF Mole fraction composition of the feed
P Column operating pressure (assume uniform in column)
Phase condition of the feed @P
Vapor-liquid equilibrium curve for the binary @P
Type of overhead condenser (total or partial)
xD Mole fraction composition of the distillate
xB Mole fraction composition of the bottoms
R/Rmin Ratio of reflux to minimum reflux
Results
D Distillate flow rate
B Bottoms flow rate
Nmin Minimum number of equilibrium stages
Rmin Minimum reflux ratio, Lmin/D
R Reflux ratio, L/D
VB Boilup ratio, V/B
N Number of equilibrium stages
Optimal feed- stage location
Stage vapor and liquid compositions
Specifications for the McCabe-Thiele Method
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Chapter 7: Distillation of Binary Mixtures 10
McCabe-Thiele Method: Column Mass Balance
FzF = xDD + xBB
Feed (L/V)
BoilupN
2
1
f
Bottoms
Reflux
F, zF
D, xD
B, xB
Distillate
A mass balance in the LK component around the column gives:
A total mass balance around the column gives:
F= D + B
So we know that the mole fraction of the light key of the
feed is between that of the distillate and bottoms:
D = FzF xB
xD xB
If D, F, are zF, specified, then either xD or xB can be specified.
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Chapter 7: Distillation of Binary Mixtures 11
McCabe-Thiele Method: Rectifying Section
Vn+1yn+1 = Lnxn + DxD
Which we can rearrange to find:
The rectifying section extends from stage 1 to the stage just above the feed stage.
yn+1 =Ln
Vn+1xn +
D
Vn+1xD
Feed (L/V)
BoilupN
n
1
f
Bottoms
Reflux
ZF
L, xD= x
0
xB
Distillate
xD
n
1Reflux
L0, x
D= x
0
Distillate
xD
Lxn
Vyn+1
If L and V are constant in the column from
stage to stage, then this is a straight line.
If we perform a material balance in the light key
around the n stages of the rectifying section
including the condenser:
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Chapter 7: Distillation of Binary Mixtures 12
McCabe-Thiele Method: Constant Molar Overflow
If L and V are constant, then this is a straight line.
This requires that:
9 The two components have equal and constant
enthalpies of vaporization
9 The heat capacity changes are negligible compared
to the heat of vaporization
9 The column is well insulated so heat loss is
negligible
9 The pressure in the column is uniform
These conditions lead to the condition ofconstant molar
overflow.
For this condition the amount of vaportransferred to the liquid stream in each stage is
equal to the amount of liquid transferred to the
vapor stream. Thus the liquid and vapor stream
flow rates are constant in the entire section.
Feed (L/V)
Boilup
N
n
1
f
Bottoms
Reflux
ZF
L, xD= x
0
xB
Distillate
xD
yn+1 =Ln
Vn+1xn +
D
Vn+1xD
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Chapter 7: Distillation of Binary Mixtures 13
McCabe-Thiele Method: Rectifying Section Operating Line
y =L
Vx+
D
VxD
The liquid entering stage one is the reflux L and its ratio to the distillate L/D
is the reflux ratio R. If we have constant molar overflow, then R is a constant and
L
V=
L
L + D=
L/D
L/D + D/D=
R
R +1
D
V=
D
L + D=
1
R +1
and
We define this equation as the
operating line of the rectifying
section.
Feed (L/V)
BoilupN
n
1
f
Bottoms
Reflux
ZF
L, xD= x
0
xB
Distillate
xD
In the case of constant molar overflow
we can then drop the stage subscripts:
yn+1 =Ln
Vn+1xn +
D
Vn+1xD
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Chapter 7: Distillation of Binary Mixtures 14
McCabe-Thiele Method: Operating Line
x
Equilibrium
curve
45 line
n
1
f
Reflux
xD= x0
Distillate
xD
L, xn V, yn+1
y = LV
x+ DV
xDWe can then rewrite:
asy =
R
R +1x+
1
R +1xD
x0=xDx1
y
y1
y2
y = 1R +1
xD
Rectifying Section Operating line
Slope=L/V=R/(R+1)
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Chapter 7: Distillation of Binary Mixtures 15
McCabe-Thiele Method: Stripping Section
Lxm = Vym+1 + BxB
Which we can rearrange and use the constant molar overflow assumption to find:
The stripping section extends from the stage just below the feed stage to the bottom stage N.If we perform a material balance in the light key around the bottom stages of the rectifying section
including the condenser we have:
y = LV
x BV
xB
Feed (L/V)
BoilupN
n
1
f
Bottoms
Reflux
zF
L, xD= x0
xB
Distillate
xD
y =VB +1
VBx
1
VBxBand
L
xm
V
ym+1
Boilup
NBottoms
B, xB
m+1
L, xN
V, yB
Since:
L
V=
V+ BV
=VB +1
VB
L = V+ B
ThenVB is called the boilup ratio.
VB =V
B
We define this equation as the operating line
of the stripping section.
This is also the operating line of
the stripping section .
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Chapter 7: Distillation of Binary Mixtures 16
McCabe-Thiele Method: Stripping Section
x
Equilibrium
curve
45 line
xNxB
y
yB
yN
Stripping Section Operating Line
Slope=L/V=(VB+1)/VB
If VB and XB are specified then we can graph this as the line shown in the
following plot.
y =VB +1
VBx
1
VBxB
L
xm
V
ym+1
Boilup
NBottoms
B, xB
m+1
L, xN
V, yB
xm
Ym+1
y =VB +1
VBx
1
VBxB
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Chapter 7: Distillation of Binary Mixtures 17
Feed Stage Considerations
In determining the operating lines for the rectifying and stripping sections we needed the bottoms and
distillate compositions and reflux and reboil ratios. The compositions can be independently specified, but
R and VB are related to the vapor to liquid ratio in the feed.
FF
FFF
L
L
L
L
L
V
V< V
V
VV
V
V= VV= VF + V
V= F+ VV> F+ V
L > F+ L L = F+ L L = L + LF
L = L L < L
Subcooled Liquid Bubble Point Liquid Partially Vaporized
Dew Point Vapor Superheated Vapor
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Chapter 7: Distillation of Binary Mixtures 18
Feed Conditions
So except in the cases where the feed is a supercooled liquid or superheated
vapor the boilup is related to the reflux by the material balance:
V= L + D VF
VB V
B=
L + D VFB
Distillation operations can be specified by the reflux ratio or boilup ratio
although the reflux ratio (or R/Rmin) is most often specified.
Dividing by B gives the boilup ratio:
L = B + V
V= D + L
VF
+ LF
= D + B
V= V+ VF
L = L + LFVF + L L = D + B
VF
+ L L = D + L V
V= L + D VF
Consider the cases where the feed is not a supercooled liquid or a superheated vapor:
Mass balance around the reboiler:
Mass balance around the condenser:
Mass balance around the column:
Vapor entering the rectifying section:
Liquid entering the stripping section:
Substitute this into the column balance:
Substitute in the reboiler balance:
In other words, the vapor
entering the rectifying section
is the vapor entering the condenserminus the feed vapor flow rate.
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Chapter 7: Distillation of Binary Mixtures 19
The q-line
First, we define the parameter q by: q = L LF
yV= Lx BxByV= Lx+ DxD
Subtracting the two operating lines:
Gives: y V V( )= L L( )x+ DxD + BxBUsing a material balance in the LK: DxD + BxB = FzF
Using a material balance around the feed stage to elminate vapor flow rates:
F+V+ L = V+ L
Simplifying and using the definition of q results in the q-line:
y =q
q 1
x
zF
q 1
x= zF y = zF
minus
y V V( )= L L( )x+ FzF
V V= F+ L Ly F+ L L( )= L L( )x+ Fz
F
The q-line has slope q/(q-1)
and intercepts the 45 degree
line at y=zF
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Chapter 7: Distillation of Binary Mixtures 20
Construction Lines for McCabe-Thiele Method
Equilibrium
curve
45 line
x=zFxB
y
yB
yN
Stripping Section:Operating line
Slope=L/V=(VB+1) /VB
xD
Rectifying Section:Operating line
Slope=L/V=R/(R+1)
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Chapter 7: Distillation of Binary Mixtures 21
Feed Stage Location Using McCabe-Thiele
Equilibrium
curve
x=zFxB
y
yB
yN
xD
Equilibrium
curve
x=zFxB
y
yB
yN
xD
1
2
3
4
1
2
3
4
5
Feed stage located one tray too low. Feed stage located one tray too high.
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Chapter 7: Distillation of Binary Mixtures 22
Construction Lines for McCabe-Thiele Method
Equilibriumcurve
x=zFxB
y
yB
yN
xD
1
2
3
4
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Chapter 7: Distillation of Binary Mixtures 23
Summary
This lecture:
We extended the analysis used for absorption and stripping to binary distillation.
We described a typical binary distillation configuration.
We made definitions such as reflux ratio, constant molar overflow, etc.
We described operating lines. We plotted the equilibrium curve.
We stepped through stages to show the change in composition as you go through
the column.
Next lecture well continue our discussion of binary distillation and the
McCabe Thiele method.