Unformatted text preview: ues of xD and xB must shift so that 5 stages are
stepped off, with the fifth, which is the reboiler, intersecting the stripping section operating line
at the 45o line. This is shown on the McCabe-Thiele diagram on the following page.
(a) From this diagram, the mole fractions of benzene in the distillate and bottoms are
0.80 and 0.21, respectively.
Case 3: Since the distillate composition in Case 2 is 80 mol% benzene, the results would be the
same as Case 2 if an 80 mol% benzene stream from another column were used as reflux. Exercise 7.16 (continued)
Analysis: Case 1 (continued) Exercise 7.16 (continued)
Analysis: Case 2 (continued) Exercise 7.17
Subject: Distillation of a mixture of benzene and toluene with different feed conditions. Given: Column with 7 equilibrium plates, total condenser, and partial reboiler. Feed is 50
mol% benzene and 50 mol% toluene. Operation at 101 kPa to produce a distillate of 96 mol%
Equilibrium data from Exercise 7.13.
Assumptions: Constant molar overflow. Use a basis of 100 mol/s for the feed.
Find: (a) For a saturated liquid feed sent to tray 5 from the top, (1) minimum reflux ratio, R =
L/D, (2) bottoms composition for twice the minimum reflux ratio, and (3) moles of products per
100 moles of feed.
(b) Same as (a) except feed is saturated vapor to tray 5 from the top.
(c) For a saturated vapor feed to the reboiler and a reflux ratio, L/V, of 0.9, determine, (1)
bottoms composition and (2) moles of products per 100 moles of feed. Analysis:
(1) For a saturated liquid feed, minimum reflux corresponds to a pinch point located at
the intersection of a vertical q-line passing through xF = 0.5 and the equilibrium curve. From the
equilibrium data, this intersection is at y = 0.72 and x = 0.5. Then, the slope of the rectifying
section operating line, (L/V)min is (0.96 - 0.72)/(0.96 - 0.50) = 0.522. From a rearrangement of
Eq. (7-7), Rmin = (L/V)min /[1 - (L/V)min ] = 0.522/(1 - 0.522) = 1.092.
(2) Reflux ratio = 2(1.092) = 2.18. From Eq. (7-7), the slope of the rectifying section
operating line = L/V = R/(1 + R) = 2.18/3.18 = 0.686. 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 on the next page where it is seen that xB = 0.18.
(3) The products are now computed by overall material balances: F = 100 = D + B and
50 = xDD + xBB = 0.96D + 0.18B. Solving these two equations, D = 41.0 mol/100 mol feed and
B = 59.0 mol/100 mol feed. Analysis: (a) (continued) Exercise 7.17 (continued) (b) (1) For a saturated vapor feed, minimum reflux corresponds to a pinch point located at
the intersection of a horizontal q-line passing through xF = y = 0.5 and the e...
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