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Unformatted text preview: The normal boiling points of the three components are:
Normal b. pt., oC B
64.48 From the "Handbook of Chemistry and Physics" and use of the Wilson equation for activity
coefficients with the Chemcad program, the following experimental and predicted binary
azeotropes, respectively, are found, with reasonably good agreement.
Handbook, expt. Wilson, predic.
T, oC Mol% T, oC Mol%
M 57.5 39.0
61.0 58.59 40.9
M 53.9 40.0
60.0 54.97 39.0
61.0 No ternary azeotrope is known.
In the reference article by Ratliff and Strobel of Continental Oil, they describe the use of
homogeneous azeotropic distillation with methanol to purify a benzene-rich stream. The feed to
the tower, including the methanol entrainer, is in vol%, 9.1 M, 86.3 B, 2.4 paraffins-napthenes
(including C), and 2.2 olefins. The amount of C in the feed is not known. From the azeotropic
tower, a bottoms product of 99.0 vol% benzene is obtained, together with 0.3 vol% paraffinsnaphthenes and 0.7 vol% olefins. The distillate in vol% is 36.0 M, 48.6 B, 8.7 paraffinsnaphthenes, and 6.7 olefins. Thus, compared to the design with acetone in Example 11.6, Ratliff
and Strobel report the use of only a small amount of M and a recovery of benzene of only 85%. Exercise 11.16 (continued) Analysis: (continued)
A residue curve map, produced by the Aspen Plus program with the Wilson equation, for
the B-C-M system at 1 atm is shown on the next page, where the three binary azeotropes are
marked with dots. Note that Aspen Plus predicts the B-M azeotrope at about 39 mol% B, which
is in agreement with experiment. There are two distillation boundaries, one from the C-M
azeotrope to the B-M azeotrope with little curvature, and one from the C-M azeotrope to the B-C
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 B-C feed of 75 mol%
C to pure M. The composition of the combined feed and entrainer lies somewhere on this line.
Assume that ideally the bottoms product is pure benzene and that the distillate is the C-M
azeotrope. Another straight line connects these two product points. The intersection of the two
lines is the mixing point. By the inverse lever-arm rule or by material balance, the following
preliminary, ideal material balance is obtained:
Feed Entrainer Distillate Bottoms
Even though the distillate and bottoms compositions appear to lie in a region different from the
combined feed and entrainer, the separation is considered possible because of the strong
curvature of the boundary from the C-M azeotrope to the B-M azeotrope, as discussed on pages
602-604. However, many stages may be necessary to achieve the products. As a first try,
assume a reflux ratio of 5 and 100 equilibrium stages. Mix the feed and entrainer together and
send the combined feed, as a bubble-point liquid, to stage 40 from the top. Specify a bottoms
flow rate of 25 kmol/h. The calculations were made with the Chemsep program, using the
continuation option because of convergence difficulties due to the non-ideality of the system. A
good separation was not achieved since the bottoms...
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