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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product is withdrawn and the reboiler performs as a total reboiler.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 McCabeThiele 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 lowend 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 qline 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 qline 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)
Lowconcentration 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
Vaporliquid 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.
 Spring '11
 Levicky
 The Land

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