Faculty of Engineering
Mass Transfer
Cairo University
Fourth Year
Chemical Engineering Department
Sheet 9: Multi-comp.
Distillation
1. A distillation column is designed for light gases fractionation operates at a reflux ratio of 2.5, if
the condenser temperature is 60oC and it’s required to get a top product of the following specs:
C3
Distillate flow rate
(mol/sec)
5
0.111
i-C4
15
0.333
n-C4
24
0.532
i-C5
1
0.022
n-C5
0.1
0.002
Component
xD
The temperature of the first plate is 65oC and that of the second plate is 70oC, the data for K values
for the components are as follows:
Component
C3
i-C4
n-C4
i-C5
n-C5
K at 65oC
2.36
1.19
0.86
0.42
0.32
K at 70oC
2.58
1.37
0.9
0.51
0.4
Estimate the composition of vapour and liquid leaving the second stage.
2. Estimate the number of ideal stages needed in the butane-pentane splitter defined by the
compositions given in the table below. The column will operate at a pressure of 8.3 bar, with a
reflux ratio of 2.5. The feed is at its boiling point.
Compositions of feed, top and bottom products are shown in table below:
Propane, C4
i-Butane, i-C4
n- Butane, n-C4
i-Pentane, i-C5
n-Pentane, n-C5
Total, kmol
Feed (F)
5
15
25
20
35
100
Tops (d)
5
I5
24
1
0
45
Bottoms (w)
0
0
1
19
35
55
Equilibrium constants were calculated and found to be:
Page 1
MASS TRANSFER
Component
C3
iC4
nC4
iC5
nC5
SHEET 9
Average value of K
5.0
2.6
2.0
1.0
0.85
3. A mixture of Hexane, Heptane, and Octane is to be separated to give the following products.
Use the shortcut method to:
a) Calculate the approximate minimum number of stages
b) Calculate the approximate minimum reflux ratio
c) Show how to get the approximate number of stages
Note: The feed is liquid at its bubble point.
component
Hexane
Heptane
Octane
Feed
F (kmole)
xf
40
0.4
35
0.35
25
0.25
Distillate
D (kmole)
xD
40
0.534
34
0.453
1
0.013
Residue
W (kmole)
xW
0
0
1
0.04
24
0.96
Relative
Volatility
2.7
2.22
1
January 2009:
4. A distillate product containing 0.02 mole fraction n-heptane, 0.7 n-hexane and 0.28 n-pentane is
obtained from a distillation column operating at 1.25 atm at a reflux ratio of 2.5 using hot reflux.
Assume constant molar flow rates and constant molar latent heats.
Calculate:
iTemperature of the top theoretical plate.
iiComposition of the liquid and vapour leaving the top theoretical plate.
iiiComposition of vapour entering the top theoretical plate.
Equilibrium relations:
Temperature, oC
60
70
80
90
100
110
Vapour pressure, atm
n-pentane
2
2.7
3.5
4.5
5.8
7
Vapour pressure, atm
n-hexane
0.73
1
1.4
1.7
2.3
3
Vapour pressure, atm
n-heptane
0.28
0.4
0.55
0.77
1.05
1.4
January 2010:
5. A mixture of A, B, C at its boiling point is to be continuously distilled in a plate column where
the pressure drop across each plate is 5kN/m2 and the pressure in the still is 105kN/m2. The
reflux ratio is 1.5 and compositions of feed, distillate and bottom streams are given in the table
below.
Calculate the composition of the liquid on the second plate above the still (use LewisMatheson method)
Page 2
MASS TRANSFER
Component
A
B
C
Vapour pressure data:
Temp., K
V.P. of A (kN/m2)
V.P. of B (kN/m2)
V.P. of C (kN/m2)
SHEET 9
Feed (xf)
0.5
0.36
0.14
386
159
107
48.7
Distillate (xD)
0.9
0.1
-
388
253
113
51.3
Page 3
390
279
119
54
Residue (xW)
0.05
0.65
0.3
391
285
123
56
393
295
132
60