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Unformatted text preview: ollowing two homogeneous ternary azeotropes are formed. No ternary azeotrope is formed
Normal boiling pt.,
45.0 24.5, 29.4, 46.1
50.8 40.7, 34.5, 24.8 Exercise 11.18 (continued)
The calculations can be made with the ChemCad program, using the SCDS model. The results
will depend on the method used to compute K-values. Because neither the NRTL nor the
UNIQUAC methods in ChemCad include parameters for the methanol-normal hexane pair, use
the UNIFAC method. Consider the use of n-hexane as the entrainer. A two-column method
used successfully by Laroche, Bekiaris, Andersen, and Morari in Ind. Eng. Chem. Res., 31, 21902209 (1992), sends to the first column the feed and a recycle of near ternary azeotrope from the
second column. The bottoms is nearly pure methanol. The distillate is fed to the second column,
where the bottoms is nearly pure methyl acetate and the distillate is the near ternary azeotrope,
which is recycled to the first column. Care is needed in using n-hexane because as noted above,
it forms a two-liquid-phase system over part of the composition space as shown in the triangular
diagram on the following page. Operation of both columns must be outside the two-liquid-phase
region. Included on the diagram are the composition points for the azeotropes and approximate
distillation boundary curves. For a first approximation, assume that the recycle distillate from
the second column is 1500 kmol/h of 29 mol% A, 34.5 mol% M, and 36.5 mol% H. With a
reflux ratio of 0.5 and 22 stages (including total condenser and partial reboiler) in the first
column, with the feed and recycle distillate sent to stage 11 from the condenser, a very pure M is
obtained. A 99.97 mol% M bottoms is produced with 14 stages, with stage 7 as the feed stage.
With a reflux ratio of 4 and 42 stages (including total condenser and partial reboiler) in the
second column, with the feed (distillate from the second column) sent to stage 21 from the
condenser, a bottoms of 98.6 mol% A is obtained. This could not be improved by increasing the
reflux, increasing the number of stages, or shifting the feed stage. The liquid composition
profiles are shown on subsequent pages, where it is seen that no pinch zones are evident. The
material balance is as follows:
Feed F1 B1 D1 = F2 B2 231.7 Entrainer,
516.4 230.0 518.1 1.7 122.5
124.1 The material balance lines are included on the triangular diagram on the next page. Exercise 11.18 (continued)
Analysis: (continued) Exercise 11.18 (continued)
Analysis: (continued) Exercise 11.19
Subject: Separation of a mixture of ethanol and water into nearly pure ethanol and pure water,
using a three-column sequence that includes heterogeneous azeotropic distillation with benzene.
Given: 150 mol/s of the ethanol-water azeotrope at 1 atm. Benzene as the entrainer for
heterogeneous azeotropic distillation.
Find: A suitable three-column desi...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
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