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

211908 000168 the change to the part a mccabe thiele

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Unformatted text preview: his slope and passes through the point, y=0.675, x=0.675. the stripping section operating line passes through the point, y=0.0023, x=0.0023 and intersects the vertical q-line at the point where the rectifying section operating line intersects the q-line. As seen, the equilibrium stages are stepped off starting at the top, with a switch from the rectifying section to the stripping section to minimize the number of stages and, thus, locating the optimal feed stage. To achieve accuracy, one diagram covers the high-concentration region, one the middle region, and one the low-concentration region. The result is between 14 and 15 equilibrium stages. Call it 14 stages plus a partial reboiler. Exercise 7.33 (continued) Analysis: (a) (continued) Exercise 7.33 (continued) Analysis: (a) (continued) Exercise 7.33 (continued) Analysis: (a) (continued) Exercise 7.33 (continued) Analysis: (a) (continued) Analysis: (continued) Exercise 7.33 (continued) (b) When open (live) steam is used with the same reflux ratio, the rectification section operating line and the q-line are identical to part (a) for a partial reboiler. Thus, the part (a) McCabe-Thiele diagram for the high concentration region applies for open steam. However, the stripping section operating line and the bottoms mole fraction change as follows. The liquid rate in the rectification section = L = 1.314D = 1.314(14.52) = 19.08 kmol/h. The vapor rate in the rectifying section = V = L + D = 19.08 + 14.52 = 33.6 kmol/h. The liquid rate below the feed stage = L' = L + F = 19.08 + 100 = 119.08 kmol/h. The vapor rate in the stripping section = V' = V = 33.6 kmol/h = open steam flow rate. The bottoms rate = B = L' = 119.08 kmol/h. The mole fraction of isopropanol in the bottoms = 0.2/119.08 = 0.00168. The change to the part (a) McCabe-Thiele diagrams on the preceding page for the middle concentration region is extremely small because the location of the stripping section operating line changes only slightly. However, the change is important in the low-concentration region. The new McCabeThiele diagram for the low concentration region is shown below. The operating line for the stripping section has a slope of L'/V' = 119.08/33.6 = 3.54 and passes through the point {y = 0, x = 0.00168}. The number of stages remains about the same as for part (a). Thus, without a reboiler, use 15 equilibrium stages in the column. Exercise 7.33 (continued) Analysis: ( c) (continued) (c) The minimum number of stages is determined as shown in the McCabe-Thiele diagrams on the next page by stepping off stages between the equilibrium curve and the 45o line from xB = 0.0023 and xD = 0.675. The number of minimum equilibrium stages = just more than 8 equilibrium stages. Exercise 7.33 (continued) Analysis: ( c) (continued) Exercise 7.33 (continued) Analysis: ( c) (continued) Exercise 7.34 Subject: Stripping of isopropyl alcohol from water at 1 atm using either a partial reboiler or open (live) steam. Given: Bubble-point liquid feed containing 10 mol% alcohol. Vapor overhead to...
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