Separation Process Principles- 2n - Seader & Henley - Solutions Manual

10 analysis extractive distillation column continued

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Unformatted text preview: roduct of at least 98 mol%, and MEK 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 MEK 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, MEK, 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 as given below. Since acetone and methanol boil at 56.5oC and 64.7oC, respectively, but acetone is closer in structure to MEK than is methanol, it is difficult to predict which of the two feed components will be the more volatile. At high concentrations of acetone, methanol is more volatile, while at high concentrations of methanol, acetone is more volatile. xA 0.475 0.375 0.250 0.125 0.025 KA 1.345 1.310 1.331 1.413 1.524 KM 1.877 1.616 1.415 1.301 1.255 KK 0.629 0.613 0.627 0.671 0.731 αM-A 1.40 1.23 1.06 0.92 0.82 αM,K 2.98 2.64 2.26 1.94 1.72 Of greater importance, however, is the fact that UNIFAC predicts, at 1 atm, that MEK forms a minimum-boiling azeotrope with methanol at 64.0oC, with a mole fraction of methanol equal to 0.844. Thus, MEK is a questionable solvent for extractive distillation because on the ternary diagram for 1 atm pressure, a distillation boundary will connect the A-M azeotrope with the M-K azeotrope, similar to Fig. 11.5, where a distillation boundary connects the benzene-isopropanol Exercise 11.10 Analysis: (Extractive Distillation Column continued) 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. As shown in the following table, based on the UNIFAC equation, the M-K azeotrope shows a favorable shift with pressure, while the A-M azeotrope does not. Above about 6 atm, the methanol-MEK azeotrope disappears. Pressure, atm A-M azeotrope M-K azeotrope 0.3 1 3 5 10 15 Mole% methanol in azeotrope: 6 22 40 50 62 70 74 84 94 99 - 25 80 - Thus, as shown by Knapp and Doherty in Ref. 34 of Chapter 11, a mixture of acetone and methanol can be separated by extractive distillation with MEK by operating the extractive distillation column (Column 1) at an elevated pressure so as eliminate the M-K azeotrope and to obtain, as will be shown, a distillate of 98 mol% methanol, followed by sending the bottoms to ordinary distillation (Column 2) at 1 atm to obtain a distillate of 95 mol% acetone and a b...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.

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