7/30/2019 Ms Exercise06
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Prof M. MazzottiThermische Verfahrenstechnik I binary
distillation
Binary distillation
In a continuous distillation column the liquid mixture
benzol(1)-toluene(2) is separated at atmo-spheric pressure. The
composition of the boiling feed stream is x1,F = 0.4. The
composition of theproduct has to be x1,D = 0.96 while in the
Bottoms product it is required 98% of toluene. The
mixture has an ideal behavior with a thermodynamic split factor
of 2.4 (w12).
1. What is the minimum reflux ratio Rmin?
2. How many stages should the column have when the column
efficiency is 60% and the columnruns with R = 2.5 Rmin? The
reboiler is a total reboiler. (The determination of the numberof
stages has to be done in the McCabe-Thiele diagram.)
Figure 1: McCabe-Thiele diagram
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7/30/2019 Ms Exercise06
3/4
Prof M. MazzottiThermische Verfahrenstechnik I binary
distillation
(D + Li+1) y1,i = Li+1 x1,i+1 + D x1,D (5)
Using equation 3 and 4:
(D + L0) y1,i = L0 x1,i+1 + D x1,D (6)
y1,i =L0 x1,i + D x1,D
D + L0=
R x1,i+1 + x1,D1 + R
(7)
with reflux ratio=R = L0D
.
Operating line rectification section:
y1,i =R
1 + R x1,i+1 +
1
1 + R x1,D (8)
Graphical solution using the McCabe-Thiele diagram:
1. draw x1,F, x1,D, x1,B
2. feed line (saturated vapor infinite slope)
3. Rmin operating line intersects feed line on the equilibrium
line
(L
V)min =
Rmin
1 + Rmin=
y1D y
1F
x1D x1F(9)
with y1D = x1D
Rmin =x1D y
1D
y
1F
x1F
(10)
and
y1F =w12 x1F
1 x1F + x1F w12= 0.6154 (11)
Rmin = 1.6
Number of stages
R = 2.5 Rmin = 4 (12)
1) new rectification line through (xD, yD). Slope: (
L
V) =
R
1+R = 0.8, intersection point withy-axis: y(x = 0) = x1D1+R
= 0.192
2) stripping line through (xB, 0) and through intersection point
of rectification and feed line.
3) step-by-step construction until specification is reached
4) Amount of points on the equilibrium line is equal the number
of theoretical stages; rectificationpart = 6 stages, stripping part
= 5 stages
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