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

57 51457 1939 49518 8061 mol ethanol 20 44 393 99 37

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Unformatted text preview: l/s Estimated reflux rate = 23 mol/s (corresponding to a small reflux ratio of 0.1) Estimated temperatures: Stage 1, 67oC Stage 6, 80oC Stage 2, 70oC Analysis: Exercise 11.13 (continued) Column 1 (continued) Exercise 11.13 (continued) Analysis: Column 2 (continued) A converged result was obtained in 21 iterations, with a small reflux ratio of 0.16 or 36.9 mol/s. The material balance was in agreement with the above table. Although the feed location could probably be improved, no pinch region is evident, as shown in the liquid-phase composition profile below. Also included are the column profiles for temperature, liquid flow, and vapor flow. Column 2 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 9 feet. The condenser duty was computed to be 9.38 MW, with a reboiler duty of 9.30 MW. For the 4 theoretical plates, the total pressure drop was computed to be 3.9kPa. For a final design, the computations should be repeated taking into account tray pressure drop and tray efficiency. Exercise 11.13 (continued) Analysis: Column 2 (continued) Exercise 11.14 Subject: Separation of a mixture of ethanol and benzene by pressure-swing distillation. Given: Feed of 100 mol/s of 20 mol% ethanol and 80 mol% benzene at the bubble point at 101.3 kPa. Desired products are 99 mol% ethanol and 99 mol% benzene. Assumptions: UNIFAC method for estimating K-values. Find: Feasible design using pressure-swing distillation in the manner of Example 11.5. Analysis: Use the two-column system of Fig. 11.23(b). Ethanol and benzene form a minimumboiling azeotrope. Use the same column pressures as in Example 11.5. Therefore, in Column 1, the pressures are 26 kPa at the condenser outlet, 30 kPa at the top tray, and 40 kPa at the reboiler. The distillate product is the near azeotrope at 30 kPa and the bottoms product is 99 mol% ethanol. In Column 2, the pressures are 106kPa before the condenser, 101.3 kPa after the condenser, and 120 kPa at the reboiler. The distillate product is the near azeotrope at 101.3 kPa and the bottoms product is 99 mol% benzene. Using the Chemcad program with the UNIFAC method, the azeotrope compositions in mol% ethanol are 35.7 at 26 kPa, and 45.2 at 101.3 kPa. A major factor in the design of a pressure-swing distillation system is the recycle-to-feed ratio, D2/F in Fig. 11.23(b), which is related to compositions as follows: Let x be the mole fraction of ethanol in any stream. Referring to Fig. 11.23(b), an overall system material balance for total flows gives: F = B1 + B2 (1) (2) An overall system balance on ethanol gives: x F F = x B1 B1 + x B2 B2 A total material balance around Column 2 gives: D1 = D2 + B 2 An ethanol balance around Column 2 gives: x D1 D1 = x D2 D2 + x B2 B2 Substitute Eq. (1) into (2) to eliminate B1. After rearrangement, x B1 − x B2 F = B2 x B1 − x F Substitute Eq. (3) into (4) to eliminate D1. After rearrangement, x D1 − x B2 D2...
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