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Unformatted text preview: mewhat from the values cited in Section 11.6.
A binodal plot for the liquid-liquid equilibrium of the ternary system, as
computed with ChemCad at a temperature of 64oC, is given on the next page. Many liquid-liquid
phase equilibrium tie lines are shown. Added to the diagram are the above computed
temperatures and compositions for the azeotropes, and estimates of the distillation boundary lines
taken from the above-cited Bekiaris, Meski, and Morari article Exercise 11.19 (continued)
The plot on the previous page differs somewhat from the residue curve map in Figure
11.29, but the same three distillation regions are evident. Therefore, as discussed in Section
11.6, it appears possible, using the three column scheme of Fig. 11.31 (without the first column
because the feed is already close to the azeotropic composition of the ethanol-water mixture), to
obtain nearly pure ethanol from the bottom of the second column and nearly pure water from the
bottom of the third column.
Runs with ChemCad were made by sending the feed at the bubble point temperature
directly to the azeotropic column in Fig. 1131a. Unfortunately, repeated attempts to achieve
nearly pure ethanol, using different combinations of entrainer flow rates and compositions were
unsuccessful. The best ethanol purity achieved was 95 mol%. For that case, 14 equilibrium
stages were used in the azeotropic column plus a decanter-condenser combination and a partial
reboiler, with the feed to stage 4 from the top. The entrainer recovery column had a reflux ratio
of two and 15 stages, including a total condenser and a partial reboiler, with the feed to stage 5
from the condenser. The distillate from the third column and the benzene-rich liquid phase from
the decanter were recycled to the top of the azeotropic column. Convergence of both the column
calculations and the recycle streams was difficult. A reasonably converged material balance,
keyed to the three-column diagram of Fig. 11.31 is as follows:
D1 D3 L2 V2 B2 D2 B3 15
8.2 Exercise 11.20
Subject: Separation of a mixture of isopropanol and water into nearly pure isopropanol and
pure water, using a three-column sequence that includes heterogeneous azeotropic distillation
Given: 120 mol/s of the isopropanol-water azeotrope at 1 atm. Benzene as the entrainer for the
heterogeneous azeotropic distillation step.
Find: A suitable three-column design.
The molecular weights and normal boiling points of the three components of interest are:
Molecular Normal boiling
Isopropyl alcohol, P
From the Handbook of Chemistry and Physics, the experimental binary azeotropes are:
Binary pair Normal
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land