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

11 continued analysis continued case 1 material

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Unformatted text preview: o of 3.69, compared to the above 3.5 estimate by the Underwood equations. The composition profile appears on the next page. 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 1/4-inch holes 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. Item Water solvent MEK solvent Solvent flow rate, mol/s 60 60 Solvent makeup rate, mol/s 1.4 0.26 Equilibrium stages in extractive distillation column 28 59 Equilibrium stages in ordinary distillation column 16 34 Diameter of extractive distillation column, feet 6 5 Diameter of ordinary distillation column, feet 2.5 7 Operating pressure in extractive distillation 1 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 extractive distilla...
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