96 then the stripping section operating line is

Unformatted text preview: quilibrium curve. From the equilibrium data, this intersection is at y = 0.50 and x = 0.293. Then, the slope of the rectifying section operating line, (L/V)min is (0.96 - 0.50)/(0.96 - 0.293) = 0.690. From a rearrangement of Eq. (7-7), Rmin = (L/V)min /[1 - (L/V)min ] = 0.69/(1 - 0.69) = 2.23. (2) Reflux ratio = 2(2.23) = 4.46. From Eq. (7-7), the slope of the rectifying section operating line = L/V = R/(1 + R) = 4.46/5.46 = 0.817. To determine the bottoms composition, use a McCabe-Thiele diagram in terms of benzene, the more volatile component. The q-line and the rectifying section operating line are fixed and 4 trays are stepped off from the top, starting at the distillate mole fraction for benzene, xD , of 0.96. Then, the stripping section operating line is positioned by trial and error so that 3 more stages plus the reboiler stage are stepped off to arrive at the point where the assumed location of the stripping section operating line intersects the 45o line. The result is shown below, where it is seen that xB = 0.08. Analysis: (b) (continued Exercise 7.17 (continued) (3) The products are now computed by overall material balances: F = 100 = D + B and 50 = xDD + xBB = 0.96D + 0.08B. Solving these two equations, D = 47.7 mol/100 mol feed and B = 52.3 mol/100 mol feed. Exercise 7.17 (continued) Analysis: (continued) (c) (1) A saturated vapor feed is fed to the reboiler. The slope of the rectifying section operating line, (L/V), is 0.9. To determine the bottoms composition, use a McCabe-Thiele diagram in terms of benzene, the more volatile component. The q-line and the rectifying section operating line are fixed and 7 trays are stepped off from the top, starting at the distillate mole fraction for benzene, xD , of 0.96. Then, the stripping section operating line is positioned by trial and error so that the reboiler stage is stepped off to arrive at the point where the assumed location of the stripping section operating line intersects the 45o line. The result is shown below, where it is seen that xB = 0.07. (2) The products are now computed by overall material balances: F = 100 = D + B and 50 = xDD + xBB = 0.96D + 0.07B. Solving these two equations, D = 48.3 mol/100 mol feed and B = 51.7 mol/100 mol feed. Exercise 7.18 Subject: Conversion of a distillation column to a reboiled stripper to obtain very pure toluene from a mixture of benzene and toluene at 101 kPa. Given: A column with 8 theoretical plates, a total condenser, and a partial reboiler. Feed contains 36 mol% benzene and 64 mol% toluene. Reboiler produces 100 kmol/h of vapor. To obtain nearly pure toluene bottoms, feed is introduced to the top plate as a saturated liquid, with no reflux. Vapor-liquid equilibrium data are in Exercise 7.13. Assumptions: Constant molar overflow. Find: (a) Minimum feed rate and corresponding bottoms composition. (b) Bottoms rate and composition for a feed rate 25% above the minimum. Analysis: (a) The minimum feed rate corresponds to a rate equal to the boilup rate...
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