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

From the plots it is seen that 20 theoretical stages

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Unformatted text preview: umn. Exercise 7.23 Subject: Distillation of a mixture of methanol and water with a subcooled liquid feed. Given: Liquid feed of 14,460 kg/h methanol and 10,440 kg/h water at q = 1.12. Distillate of 99 mol% methanol and a bottoms of 99 mol% water are desired. Column has a total condenser and a partial reboiler. Operation at 1 atm with a reflux ratio of L/D = R = 1.0. Vapor-liquid equilibrium data in Exercise of 7.19. Assumptions: Constant molar overflow. Find: Feed stage location and number of equilibrium stages. Analysis: First, determine feed composition in mol%. Using molecular weights of 32.04 for methanol and 18.02 for water, Component Methanol Water Total: kg/h 14,460 10,440 24,900 kmol/h 451.3 579.4 1,030.7 Mol% 43.79 56.21 100.00 Using the vapor-liquid equilibrium data, a y-x plot for the McCabe-Thiele method is made and smoothed with a spreadsheet, noting that methanol is the more volatile. Therefore, xF = 0.4379, xD = 0.99, xB = 0.01. From Eq. (7-26), slope of q-line = q/(q - 1) = 1.12/(1.12 - 1) = 9.333. Therefore, on the McCabeThiele diagram, a line is drawn with a slope of 9.333 that intersects the point {0.438,0.438}. From Eq. (7-7), the slope of the rectifying section operating line = L/V = R/(R + 1) = 1/(1 + 1) = 0.5. This line is drawn on the McCabe-Thiele diagram with a slope of 0.5 that intersects the point {0.99,0.99}. The stripping section operating line is drawn to intersect the point {0.01,0.01} and the point where the q-line intersects the rectifying section operating line. In the McCabe-Thiele graphs below, the first for the high mole-fraction region, it is seen that the specified reflux ratio is, fortunately, above the minimum value for the specified distillate mole fraction. From the plots, it is seen that 20 theoretical stages plus a partial reboiler are needed. The optimal feed stage is number 17 down from the top. Analysis: (continued) Exercise 7.23 (continued) Analysis: (continued) Exercise 7.23 (continued) Exercise 7.24 Subject: condenser. Partial separation of a benzene-toluene mixture with a partial reboiler and a partial Given: Saturated liquid feed of 69.4 mol% benzene (B) in toluene (T) fed to a partial reboiler. Vapor from the reboiler passes to a partial condenser. Vapor from the partial condenser passes to a total condenser. Reflux from the partial condenser is sent to the partial reboiler. Distillate is to contain 90 mol% benzene (yD = 0.9) at a rate of 25 moles per 100 moles of feed. The relative volatility of benzene with respect to toluene = α = 2.5. Find: Moles of vapor generated in the reboiler per 100 moles of feed by analytical and graphical methods. Analysis: First compute overall material balance. The total condenser need not be considered. Basis: 100 moles of feed. Overall total material balance: F = 100 = D + B = 25 + B (1) Solving Eq. (1), B = 75 moles Overall benzene material balance: FxF = 69.4 = DyD + BxB = 25(0.9) + 75xB (2) Solving Eq. (2), xB = 0.625 Analytical Method: Write material balances around the par...
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