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

4118322 09956 for the recovery of ethanol to the

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Unformatted text preview: = B2 x D2 − x D1 (3) (4) (5) (6) Combine Eqs. (5) and (6) to eliminate B1. After rearrangement, the recycle ratio is: x B1 − x F x D1 − x B2 D2 = (7) F x B1 − x B2 x D2 − x D1 In the limit, for pure bottoms products and azeotropic distillate products, Eq. (7) reduces to: xAz1 D2 = 1 − xF (8) F xAz 2 − xAz1 Exercise 11.14 (continued) Analysis: (continued) Eq. (8) shows that the recycle ratio is sensitive to the feed composition and the azeotropic compositions at the pressures of Columns 1 and 2. The smaller the difference between the two azeotropic compositions, the larger the recycle ratio. For this exercise, x F = 0.20, x B1 = 0.99, and x B2 = 0.01 Let x D1 = 0.37 , which is slightly greater than the azeotropic mole fraction of 0.357. Let x D2 = 0.44 , which is slightly smaller than the azeotropic mole fraction of 0.452. Substituting these 5 mole-fraction values into Eq. (7) gives D2/F = 4.1457. Therefore D2 = 414.57 mol/s. This a very high recycle ratio. The system is technically feasible, but may not be economically attractive. Using Eqs. (1) to (6), the following material balance is obtained for the system: F Stream: D2 F1 B1 D1 B2 Flow rate, mol/s: Ethanol 20 182.41 202.41 19.19 183.22 0.81 Benzene 80 232.16 312.16 0.20 311.96 79.80 Total: 100 414.57 514.57 19.39 495.18 80.61 mol% Ethanol: 20 44 39.3 99 37 1 The design of the two columns is made as follows with the Chemcad program, using the Shortcut Column model (FUG method) to obtain initial estimates of stage and reflux requirements, followed by the SCDS Column rigorous model to finalize the designs. Column 1: Using the above material balance, with a combined feed, F1, the specifications for the ShortCut Column model are 311.96/312.96 = 0.9968 for the recovery of benzene to the distillate, and 183.22/202.41 = 0.9052 for the recovery to the distillate of ethanol. Thus, it is expected that little rectification is necessary. The results are: Minimum number of equilibrium stages = 5.95 Minimum reflux ratio = very small and probably in error. Therefore, the Gilliland correlation was not reliable. The SCDS Column model of the Chemcad program was then used to make a rigorous calculation. Based on several runs where the number of stages, the feed stage location, and the reflux ratio were varied, it was found that the separation was almost insensitive to the reflux ratio, indicating that little rectification was needed. However, attempts to configure the column as a reboiled stripper met with failure to converge. The following input data resulted in almost satisfying the specifications. Because the separation depended mainly on the number of stages, the specified separation could not be satisfied exactly. Pressures as stated above. Number of stages = 7 (includes the total condenser and the partial reboiler) Feed stage for recycle, D2 = 3 from the top Entry for feed, F = 4 from top Reflux ratio = 0.05 Bottoms mole flow rate = 19.39 mol/s Exercise 11.14 (continued) Analysis: Column 1 (continued) Estimated distillate rate = 495.18 mol/s Estima...
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