Unformatted text preview: xercise 11.8 (continued) Exercise 11.8 (continued)
Analysis: (continued) Exercise 11.9
Subject: Separation of acetone from methanol by extractive distillation with ethanol.
Given: Bubble-point feed of 30 mol/s acetone (A) and 10 mol/s methanol (M) at 1 atm.
Ethanol (E) 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. Column operations at 1 atm. 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 ethanol of 99.9 mol% purity for recycle.
Analysis: Extractive Distillation Column: First compute relative volatilities over a composition
range to determine: (1) which of the two feed components is the most volatile in the presence of
ethanol and (2) potential ease or difficulty of the separations. This is done with the LLVF threephase flash model of Chemcad, which also detects any formation of a second liquid phase, by
conducting a series of flashes at 1 atm and a V/F = 0.0001 (a near bubble-point condition), for
different mole fractions of the two feed components. For all cases, the mole fraction of the
solvent, ethanol, is fixed at 0.5 so as to have a strong effect on the liquid-phase activity
coefficients of acetone and methanol. The flash model computes the K-values, from which the
relative volatilities can be obtained. Since acetone and methanol boil at 56.5oC and 64.7oC,
respectively, and methanol is closer in structure to ethanol than is acetone, it is expected that the
volatility of acetone to methanol will be increased in the presence of ethanol The following
results confirm this expectation:
3.97 Although these relative volatilities are favorable, they are not as high as those
obtained when water is used as the solvent. Nevertheless, an initial run was made using the same
arrangement of stages as in Example 11.3, a solvent rate equal to 60 mol/s, entering as a liquid at
60oC, and a reflux ratio of 4. The following results were obtained using the SCDS model of
Chemcad, with an assumed bottoms rate of 68.774 mol/s, as computed in Example 11.3: Exercise 11.9 (continued)
Ethanol, E The desired separation is not achieved. The distillate contains too much methanol and ethanol,
while the bottoms contains too much acetone. The acetone recovery is only 70.3%. In Example
11.3, it is 99.5%. Therefore, the stage, reflux, and solvent rate were increased in increments until
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