Unformatted text preview: from a spreadsheet are,
Equilbrium
stage
1 (still)
2 (bottom plate
3
4
5 (top plate)
reflux drum x y 0.00100
0.00412
0.01679
0.06587
0.22554
0.54603 0.00412
0.01679
0.06587
0.22554
0.54603 By overall total material balance, F = 100 = D + B
(3)
By overall benzene material balance,
FxF = (100)(0.20) = 20 = DxD + BxB = D(0.54603) + B(0.001)
(4)
Solving Eqs. (3) and (4),
D = distillate in reflux drum = 36.51 kmoles
B = bottoms in still = 63.49 kmoles Exercise 7.25 (continued)
Graphical Method:
On the McCabeThiele plot on the next page, the equilibrium curve is computed from Eq.
(1). The rectification section operating line is the 45o line. Five stages are stepped off from the
bottoms of xB = 0.001. The resulting y5 is essentially the same as that for the analytical method.
Thus, again, B = bottoms in still = 63.49 kmoles. Exercise 7.25 (continued) Analysis: Graphical method (continued) Exercise 7.26
Subject: Distillation of acetone and isopropanol, taking into account tray efficiency. Given: Saturated liquid feed of 50 mol% acetone and 50 mol% isopropanol. Column is
equipped with a total condenser, and a partial reboiler. Reflux ratio, L/D, is 0.5. Murphree vapor
efficiency = 50%. Vaporliquid equilibrium data at 1 atm are given, with acetone being the more
volatile component.
Assumptions: Constant molar overflow.
Find: Number of actual trays required to achieve a distillate of 80 mol% acetone and a bottoms
of 25 mol% acetone, inserting the feed at the optimal location.
Analysis: In the McCabeThiele diagram below, the equilibrium curve is plotted from the given
data. The qline is vertical, passing through x = 0.5. The rectification operating line has a slope,
L/V, from Eq. (79), of R/(R + 1) = 0.5/1.5 = 0.333. This operating line passes through the point,
y = 0.8, x = 0.8. From Eq. 79), the equation for the rectifying section operating line is,
y= R
1
x+
x D = 0.333x + 0.667(0.8) = 0.333x + 0.533
R +1
R +1 From this equation, the intersection of the rectifying section operating line and the vertical qline
is at y = 0.333(0.5) + 0.533 = 0.70. The stripping section operating line passes through the {y, x}
points {0.70, 0.50} and {0.25, 0.25}, giving it the equation, y = 1.80x0.20.
Except for the reboiler stage, the stages are stepped off from an efficiency line, which for a
Murphree vapor efficiency of 0.5 is positioned vertically half way between the equilibrium curve
and the operating line, as governed by Eq. (741), EMV = 0.5 = (yn  yn+1)/(yn*  yn+1), where yn+1
is the location on the operating line, yn is the location on the efficiency line, and yn* is the
location on the equilibrium line. However, the reboiler is assumed to have a 100% efficiency.
As seen in the plot, just over 8 trays are required plus the partial reboiler. The feed plate is 4
from the top. Analysis (continued) Exercise 7.26 (continued) Exercise 7.27
Subject: Distillation of carbon disulfide and carbon tetrachloride. Given: Partially vaporized feed (q = 0.5) of 40 mol% CS2. Operation with a reflux ratio, L/D,
of 4 and a Murphree vapor efficiency of 80%. Partial reboiler an...
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 Spring '11
 Levicky
 The Land

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