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

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Unformatted text preview: 8 mol% W, 10 mol% A, and 72 mol% NPA in equilibrium with a saturated water-rich phase containing 97.5 mol% W, 2 mol% A, and 0.5 mol% NPA, which becomes the distillate. The amount of entrainer reflux is fixed so as to give the mixing point, M, such that a line through that point and about 99 mol% A in W will intersect a liquid-liquid tie line that connects the reflux and the nearly pure liquid water distillate in the region close to the binary heterogeneous azeotrope of W and NPA. From this construction, the amount of reflux is determined as 1,000 kmol/h. An initial material balance was then computed, from the feed and mole fraction compositions, with the following result: kmol/h: Component Feed Entrainer Make-up NPA Bottoms Overhead vapor Distillate w/o makeup Water 800 180 0 2 978 798 Acetic acid 200 100 0 185 115 15 NPA 0 720 4 0 720 4 Total: 1000 1000 4 187 1813 817 Analysis: (continued) Exercise 11.21 (continued) Exercise 11.21 (continued) Analysis: (continued) The SCDS distillation model of Chemcad was used with no condenser and a partial reboiler. The overhead vapor was sent a three-phase flash condenser (decanter) operating at the bubble point, modeled with the LLVF unit. The NPA-rich phase would be recycled to the column as entrainer reflux entering the top stage, but in the calculation the guessed amount of 1,000 kmol/h at the above composition was used and then converged by trial and error, accounting for the necessary makeup NPA. An initial guess was made of 20 stages, including the reboiler, with the feed entering stage 6 from the top. Other specifications were 1 atm for the column, feed and entrainer at the bubble-point temperature at 1 atm, 187 kmol/h of bottoms, estimates of 1800 kmol/h for the distillate, 1000 kmol/h for the reflux; 80, 110, 90, and 100 oC for the four temperatures; and a damping factor of 0.3. The calculations converged without difficulty in 14 iterations, but the entrainer flows minus the makeup NPA did not equal the NPArich liquid from the decanter. Also 10 of stages appeared to be pinched. Therefore, the number of stages was reduced to 10, the feed stage was moved to stage 4 from the top, and the component flow rates in the entrainer were revised by trial and error until a reasonable closure of the material balance was obtained. The final material balance was as follows: kmol/h: Component Feed Entrainer Make-up NPA Bottoms Overhead vapor Distillate w/o makeup Water 800 184 0 1.3 982.7 798.4 Acetic acid 200 58 0 183.6 74.4 16.0 NPA 0 760 3.6 0.1 763.5 3.5 Total: 1000 1002 3.6 185.0 1820.6 817.9 Temperatures in the column varied from 85.6oC at the top to 117.5o C at the bottom. The temperature in the decanter was 83.9oC. The following is a plot of the liquid-phase composition profile showing that no pinch existed in the column. The acetic acid product is 99.2 mol% acetic acid, while the bottoms water product is 97.6 mol% water. This could be further purified in a second column, with the small amount of acetic acid-rich product recy...
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