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

Given 1000 kmolh of 20 mol acetic acid in water find a

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Unformatted text preview: ussed in Section 11.6, it appears possible, using the three column scheme of Fig. 11.31a (without the first column because the feed is already at or near the azeotropic composition of the isopropanol-water mixture), to obtain nearly pure isopropanol from the bottom of the second column and nearly pure water from the bottom of the third column. Runs with ChemCad were made with a feed of 120 mol/s of near-azeotropic composition of 67 mol% P and 33 mol% W at the bubble point temperature, sent directly to the azeotropic column in Fig. 11.31a. An estimate of the total entrainer flow rate and composition (streams L2 and D3 combined) was made by locating a composition point near the distillation boundary between Regions 1 and 2 and near the binodal curve separating the two-liquid-phase region from the one-liquid-phase region. The composition was taken as 8.5 mol% W, 56 mol% B, and 35.5 mol% P. To determine an initial approximation of the total flow rate of the entrainer, the overhead vapor from the azeotropic column was assumed to be near the ternary azeotropic composition. By material balance, the entrainer flow rate was computed to be 240 mol/s. The azeotropic column (column 2 in Fig. 11.31a) was then computed with the SCDS model in ChemCad, using the following specifications, with the pressure = 1 atm everywhere: Total number of equilibrium stages (including partial reboiler) = 20 Feed stage = stage 5 from the top No condenser Bottoms flow rate = 80.4 mol/s (the flow rate of P in the feed) Assumed distillate rate = 239.6 mol/s Assumed reflux rate = 200 mol/s Assumed stage temperatures = 61, 82, 62, and 78OC The converged results showed a large pinch region where little or no separation occurred, and the bottoms purity with respect to isopropanol was not acceptable. Accordingly, a number of additional runs were made with varying conditions for the total number of stages, feed stage location, and total entrainer flow rate. A bottoms product of isopropanol purity of better than 99.5 mol/s at a flow rate of 80.4 mol/s was achieved with 20 equilibrium stages, feed to the top stage, and a total entrainer flow rate of 300 mol/s. The entire separation process was then simulated with Chemcad, according to the flowsheet in Fig. 11.31a, but without the first column (Preconcentration). The flowsheet included the azeotropic column (column 2) with recycle entrainer from the entrainer recovery column (column 3), and a condenser-decanter to condense the overhead vapor from column 2 and split the condensate into two liquid phases, with the organic-rich phase sent as entrainer reflux to the top of column 2 and the water-rich phase sent to column 3. To achieve a nearly pure water bottoms product from column 3, only 4 equilibrium stages (including a total condenser and partial reboiler) were needed with the feed to stage 2 (the top stage of the column) and a reflux ratio of 1. The distillate from column 3 was recycled to the top of column 2. Several loops were needed to...
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