Unformatted text preview: gn. However, because the feed is at nearazeotropic
composition, the first (Preconcentrator) column is not needed.
Analysis:
The molecular weights and normal boiling points of the three components of interest are:
Component
Molecular Normal boiling
Weight
Point, oC
Water, W
18.02
100.0
Benzene, B
78.11
80.1
Ethyl alcohol, E
46.07
78.5
From the Handbook of Chemistry and Physics, the binary azeotropes are:
Binary pair Normal
Type
Mol%
Mol% Mol% lower
boiling
Azeotrope
overall
upper liquid phase
pt., oC
liquid phase
WB
69.4 Heterogeneous 29.8% W
0.26% W
99.98% W
70.2% B
99.74% B
0.02% B
WE
78.2
Homogeneous 10.5% W
89.5% E
BE
67.8
Homogeneous
55.2% B
44.8% E
Also, the following heterogeneous ternary azeotrope is formed:
Ternary Normal
Type
Mol%
Mol% Mol% lower
System
boiling
Azeotrope
overall
upper liquid phase
o
pt., C
liquid phase
WBE
64.6 Heterogeneous 23.3% W
5.0% W
66.7% W
53.9% B
76.0% B
1.7% B
22.8% E
19.0% E
31.6% E
The column calculations were made with the ChemCad program. Important decisions were the
choice of activity coefficient method for the liquid and the binary interaction parameters for the
selected method because these decisions can have a significant influence on the results of the
calculations. The UNIQUAC method was selected, with values of the binary interaction
parameters taken from the Appendix of the article by Bekiaris, Meski, and Morari, IEC Res., 35,
207227 (1996). They give the parameters as values of (uij  ujj)/R in K. For the ChemCad
program, the parameters must be given as (uij  ujj) in cal/mol. Therefore, the Bekiaris, Meski, Exercise 11.19 (continued)
Analysis: (continued)
and Morari values were multiplied by 1.987 cal/molK, giving the following values of (uij  ujj)
for input to ChemCad:
i
j
Water
Benzene
Ethanol
Water
719.890
405.036
Benzene
1795.85
764.780
Ethanol
65.995
85.507
With these values, the computed azeotropes at 1 atm, which deviate somewhat from the above
experimental values, were as follows, where the homogeneous azeotropes were determined with
the TPYX plot feature and the heterogeneous azeotropes were determined by running threephase
flash calculations with the LLVF model until the vapor and overall liquid compositions were
equal. The most serious deviation is the lower liquid phase composition of the ternary azeotrope.
From the builtin vapor pressure correlations in ChemCad, the normal boiling points of water and
benzene are the same as the above literature values, but the ethanol value is 78.3oC.
Binary pair Type
Azeotrope Mol%
overall WB Normal
boiling
pt., oC
69.2 Heterogeneous WE 78.2 Homogeneous BE 67.8 Homogeneous 29.8% W
70.2% B
9.7% W
90.3% E
55.0% B
45.0% E Ternary
System
WBE Normal
boiling
pt., oC
63.8 Type
Azeotrope Mol%
overall Heterogeneous 19.7% W
53.1% B
27.2% E Mol%
upper
liquid phase
0.47% W
99.53% B Mol%
upper
liquid phase
2.4% W
81.7% B
15.9% E Mol% lower
liquid phase
99.94% W
0.06% B Mol% lower
liquid phase
47.7% W
7.0% B
45.3% E The above values also differ so...
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 Spring '11
 Levicky
 The Land

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