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

If this were true then the operating line with a

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Unformatted text preview: so as to give an L / V = 1.0. Thus, the minimum feed rate = 100 kmol/h. Under these conditions, no bottoms product is withdrawn and the reboiler performs as a total reboiler. The vapor leaving the top of the column has the same composition as the feed. Therefore, we consider only a total of 8 equilibrium stages. In the McCabe-Thiele diagram below, the stripping section operating line is coincident with the 45o line. The 8 stages are stepped off from the top at y = 0.36 and x = 0.36. The mole fraction of benzene in the reboiler is found to be 0.008 from the low concentration region plot. Therefore, the corresponding toluene mole fraction = 0.992, which is quite pure. Note that 2 diagrams below are used, with the second, covering the stages at the bottom of the column in the low concentration region. To obtain accuracy in this low-end region, the vaporliquid equilibrium data for x = 0.1 and x = 0.2 were fitted to a quadratic equation passing through the origin, with the result, y = 2.35x - 2.5x2 . Analysis: (a) (continued) Exercise 7.18 (continued) Exercise 7.18 (continued) Analysis: (continued) (b) F = Feed rate = L = 1.25(100) = 125 kmol/h. V = Vapor rate = 100 kmol/h. Therefore, with no condenser and constant molar overflow, B = F - V = 125 - 100 = 25 kmol/h. The slope of the stripping section operating line = L / V = 125/100 = 1.25. The q-line is vertical for a saturated liquid feed, passing through xF = 0.36. The stripping section operating line is positioned by trial and error so that 8 + 1 equilibrium stages will be stepped off. Because part (a) shows that a very low mole fraction of benzene is obtained in the bottoms, it is suspected that the operating line will intersect the 45o line almost at the origin. If this were true, then the operating line, with a slope of 1.25, would intersect the vertical q-line at 1.25(0.36) = 0.45. Use this as a first approximation and adjust it downward until only 9 stages can be stepped off. The final result is shown in the two plots on the next page, where the second plot is for the low concentration region. As seen, the mole percent of benzene in the bottoms = 0.004, giving 99.6 mol% toluene in the bottoms. The overhead vapor contains 0.449 mole fraction benzene. By material balances, F = 125 = D + B and 0.36F = 0.449D + 0.004B, D = 100.0 and B = 25.0 kmol/h. Analysis: (b) (continued) Exercise 7.18 (continued) Low-concentration Region Exercise 7.19 Subject: Normal and abnormal operation of a distillation column separating a methanol water mixture at 101 kPa. Given: Column with 7 theoretical plates, a total condenser, and a partial reboiler. A feed of 100 kmol/h of 50 mol% methanol in water is sent to plate 3 from the bottom. During normal operation, distillate is 90 mol% methanol and bottoms is 5 mol% methanol, with a reflux rate of 1 mole per mole distillate. During abnormal operation, the following data are obtained: Stream kmol/h mol% methanol Feed 100 51 Bottoms 62 12 Distillate 53 80 Reflux 94 Vapor-liquid equilibrium data are given at 101 kPa, where methanol is the more volatile species. Assumptions: Constant molar overflow...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.

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