Unformatted text preview: was only 79.2 mol% B. Several of the
parameters, including total stages, methanol entry stage, feed entry stage, reflux ratio, methanol
flow rate, reflux ratio, and bottoms flow rate. The best result was obtained with the following
specifications:
Number of equilibrium stages = 100 + total condenser = 101
Methanol flow rate = 115 kmol/h sent to stage 56 from the top, counting the total condenser.
Feed sent to stage 71 from the top, counting the total condenser.
Reflux ratio = 10
Bottoms flow rate = 24.0 kmol/h
Products were as follows, with 99.58 mol% benzene bottoms, but only a 95.6% recovery of
benzene:
Component
Feed
Entrainer
Distillate
Bottoms
B
25
0
1.1
23.9
C
75
0
75.0
0.0
M
0
115
114.9
0.1
Total:
100
115
191.0
24.0 Analysis: (continued) Exercise 11.16 (continued) Exercise 11.17
Subject: Separation of toluene (T) from 2,5dimethylhexane (H) by a sequence that includes
homogeneous azeotropic distillation with methanol (M) as the entrainer.
Given: Feed of 100 mol/s of 50 mol% T and 50 mol% H. Reference to an article by Benedict
and Rubin.
Find: A separation sequence that includes homogeneous azeotropic distillation with M as the
entrainer. A feasible design for the azeotropic distillation column.
Analysis:
The normal boiling points of the three components are:
Component
Normal b. pt., oC T
H
110.64 109.15 M
64.48 From use of the UNIFAC equation for activity coefficients with the Chemcad program, the
following predicted minimum boiling binary azeotropes are found for a pressure of 1 atm.
Binary
mixture:
T
H
T
M UNIFAC, prediction
T, oC Mol%
106.9 47.5
52.5 63.8 11.2
88.8 H
60.3
20.7
M
79.3
No ternary azeotrope is known.
A residue curve map, produced by the Aspen Plus program with the UNIFAC equation,
for the THM system at 1 atm is shown on the next page, where the three binary azeotropes are
marked with dots. There are two distillation boundaries, one from the TM azeotrope to the HM
azeotrope with little curvature, and one from the TH azeotrope to the HM azeotrope, with much
curvature. These two boundaries divide the diagram into three distillation regions. Also
included on the diagram are two straight lines, one from the TM feed of 50 mol% H to pure M.
The composition of the combined feed and entrainer must lie somewhere on this line. If we take
advantage of the high degree of curvature of the second distillation boundary, we can assume that
ideally the bottoms product can be pure toluene, with a distillate of the HM azeotrope. Another
straight line connects these two product points. The intersection of the two lines is the mixing Exercise 11.17 (continued)
Analysis: (continued) Exercise 11.17 (continued)
Analysis: (continued)
point. By the inverse leverarm rule or by material balance, the following preliminary, ideal
material balance is obtained:
mol/s:
Component
Feed
Entrainer Distillate
Bottoms
T
50
0
0
50
H
50
0
50
0
M
0
192
192
0
Total:
100
192
242
50
An initial run is made with the Chemcad SCDS model using the following specifications:
Combined feed and entrainer at 1 atm and the bubble point.
Column ∆P = 0
Top pressure = 1 atm
Condenser ∆P = 0
No. of stages = 31 (includes total condenser and partial r...
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 Spring '11
 Levicky
 The Land

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