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 qline at the point where the rectifying section operating line intersects the qline. 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 highconcentration region, one the middle region, and one the lowconcentration 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 qline are identical to part (a) for a partial reboiler. Thus, the part (a)
McCabeThiele 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) McCabeThiele 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 lowconcentration 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 McCabeThiele
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: Bubblepoint liquid feed containing 10 mol% alcohol. Vapor overhead to...
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 Spring '11
 Levicky
 The Land

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