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

Item water solvent mek solvent solvent flow rate mols

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Unformatted text preview: ottoms of 99.9 mol% MEK for recycle. The two columns were designed with the SCDS (simultaneous correction) model of Chemcad leading to the following final material balance, which satisfies the product purity specifications: Component Acetone Methanol MEK Total: Temp, oC Mol/s: Feed Solvent 30 10 40 144.9 60 60 145 Column 1 distillate 0.016 8.526 0.158 8.700 144.6 Column 1 bottoms 29.984 1.474 59.842 91.300 169.5 Column 2 distillate 29.924 1.474 0.101 31.499 55.8 Column 2 bottoms 0.060 0.000 59.741 59.801 79.1 The pressure of the extractive distillation column (Column 1) was set at 12 atm, with a solvent rate of 60 mol/s at 145oC. The following Chemcad SCDS results, for the listed conditions, was obtained after 11 iterations. The resulting product compositions are those in the above table. Other combinations of stages, reflux, and solvent rate could also give satisfactory results: Number of stages = 60 (includes total condenser and partial reboiler) Solvent enters stage 20 from the top Feed enters stage 40 from the top Acetone mole fraction in distillate = 0.98 Bottoms flow rate = 91.3 mol/s Solvent flow rate = 60 mol/s at 145oC Initial guesses: Distillate rate = 8 mol/s Reflux rate = 80 mol/s Temperatures: Stage 1, 145oC Stage 60, 170oC Stage 2, 145oC Stage 59, 160oC Exercise 11.10 Analysis: (Extractive Distillation Column continued) The reflux ratio was computed to be 8.083. The liquid phase composition profile is shown below. The extractive distillation column 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 5 feet. For a final design, the computations should be repeated taking into account tray pressure drop, tray efficiency, and impurity in the MEK solvent. Exercise 11.10 (continued) Analysis: (continued) Ordinary Distillation Column: This column, at a pressure of 1 atm, separates the bottoms from the extractive distillation column to obtain 95 mol% acetone and 99.9 mol% MEK. A starting point for this separation is obtained by using the ShortCut Column (FUG) model of Chemcad with an operating reflux ratio of 1.3 times the minimum ratio. The feed is assumed to be at the bubble point. Thus, the bottoms from Column 1 at 169.5oC and 12 atm is cooled and decompressed to 68.7oC and 1 atm. Using the above material balance table, the specifications for the ShortCut Column model are 29.924/29.984 = 0.998 for the recovery of the LK (methanol) to the distillate and 0.101/59.842 = 0.0169 for the recovery of the HK (MEK). The results are: Minimum number of equilibrium stages = 17.7 Minimum reflux ratio = 2.52 Reflux ratio = 1.3(2.52) = 3.28 Number of equilibrium stages = 34.1 Feed stage location (Kirkbride equation) = 18 from the top The SCDS Column model of the Chemcad program was then used to make a rigorous calculation based on the following input data: Pressure constant at 1 atm Number of stages = 35 (includes the total condenser and the partial reboiler) Feed stage = 20 from the top Acetone purity in the distillate = 95 mol% MEK purity in the bottoms = 99.9 mol% Estimated distillate rate = 31.4 mol/s Estimated reflux rate = 109.9 mol/s (reflux ratio of 3.5) Estimated temperatures: Stage 1, 64oC Stage 35, 78oC Stage 2, 65oC Stage 34, 75oC A converged result was obtained in 5 iterations, with a computed reflux rati...
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