Unformatted text preview: o of 3.69, compared
to the above 3.5 estimate by the Underwood equations. The composition profile appears on the
The ordinary distillation column for separating acetone from MEK was sized with Chemcad for
sieve trays, with:
24-inch tray spacing
12% downcomer area
10% hole area
80% of flooding
The resulting column inside diameter was determined to be 7.0 feet. For a final design, the
computations should be repeated taking into account tray pressure drop and tray efficiency.
From the above material balance table, the flow rate of MEK in the bottoms of Column 2
is only 59.80 mol/s, while 60.00 is needed for the extractive distillation column, the makeup
MEK rate is 60.00 - 59.74 = 0.26 mol/s. Analysis: (continued) Exercise 11.10 (continued) Comparison with Example 11.3:
In Example 11.3, water is used as the solvent. The following comparison shows that
water is preferred over MEK.
Solvent flow rate, mol/s
Solvent makeup rate, mol/s
Equilibrium stages in extractive distillation column
Equilibrium stages in ordinary distillation column
Diameter of extractive distillation column, feet
Diameter of ordinary distillation column, feet
Operating pressure in extractive distillation
12 Exercise 11.11
Subject: Separation of acetone from methanol by extractive distillation with toluene
Given: Bubble-point feed of 30 mol/s acetone (A) and 10 mol/s methanol (M) at 1 atm.
Toluene (T) as the solvent. Results of Example 11.3 with a solvent of water.
Assumptions: Because the feed is close to the azeotropic composition of 22 mol% methanol,
use a two-column system, with the first column being extractive distillation and the second
ordinary distillation to recover the solvent. UNIFAC for K-values. Negligible tray pressure
drop. 100% tray efficiency.
Find: Suitable column designs to obtain an acetone product of at least 95 mol%, a methanol
product of at least 98 mol%, and toluene of 99.9 mol% purity for recycle.
Analysis: Of great importance here is the fact that UNIFAC predicts, at 1 atm, that toluene
forms a minimum-boiling azeotrope with methanol at 63.8oC, with a mole fraction of methanol
equal to 0.891. Thus, toluene is a questionable solvent for extractive distillation because on the
residue curve map for 1 atm pressure, shown on the next page, a distillation boundary connects
the A-M azeotrope with the M-T azeotrope, similar to Fig. 11.5, where a distillation boundary
connects the benzene-isopropanol azeotrope with the benzene-n-propanol azeotrope. For
extractive distillation, a residue curve map similar to that in Fig. 11.14 is needed.
As shown in Fig. 11.22, some azeotropes are pressure sensitive, such that if the pressure
is changed sufficiently the azeotrope disappears. Unfortunately, over the pressure range of 0.3
atm to 25 atm, neither the A-M azeotrope nor the M-T azeotrope disappears. Therefore, it is
concluded that the use of toluene can not produce a distillate of either acetone or methanol by
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