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80% of flooding
The resulting column inside diameter was determined to be 12 feet. The condenser duty was
computed to be 16.76 MW, with a reboiler duty of 16.62 MW. For the 4 theoretical plates, the
total pressure drop was computed to be 4.0 kPa. For a final design, the computations should be
repeated taking into account tray pressure drop and tray efficiency. Exercise 11.14 (continued)
Analysis: Column 2 (continued) Exercise 11.15
Subject: Separation of a mixture of ethanol and water by pressureswing distillation.
Given: Feed of 100 mol/s of 30 mol% ethanol and 70 mol% water. Desired product is 99.8
mol% ethanol..
Assumptions: Feed at the bubble point at 120 kPa. Use of NRTL equation to obtain Kvalues
using modified coefficients given below.
Find: Feasible design using pressureswing distillation in the manner of Fig. 11.23(b).
Analysis: Use the twocolumn system of Fig. 11.23(b), except that for this system, the bottoms
from Column 1 is nearly pure B (water), while the bottoms from Column 2 is nearly pure A
(ethanol). Ethanol and water form a minimumboiling azeotrope. Using Chemcad, the NRTL
equation and the following binary interaction parameters, which were entered to override the
stored parameters (I = ethanol, J= water, Bij = 246, Bji = 586, alpha = 0.3, Aij = 0.8, and
Aji = 3.45), the predicted azeotropes are 89.3 mol% ethanol at 1 atm, 85.6 mol% ethanol at 300
kPa, and 94.3 mol% ethanol at 13 kPa. Agreement with experimental data is excellent at 1 atm,
but not accurate at 13 kPa torr, where the azeotrope composition is about 97 mol%. Thus, the
following calculations using the NRTL equation will not be as accurate as would be desired. For
Column 1, set the pressures at 13 kPa at the condenser outlet, 15 kPa at the top tray, and 30 kPa
at the reboiler. Specify a Column 1 distillation composition of 92.5 mol% ethanol compared to
the predicted azeotropic composition of 94.3 mol%. Let the ethanol composition of the bottoms
from Column 1 be 0.0001 mol% ethanol, because, as will be shown, it is easy to obtain a very
high water purity. For Column 2, set the pressures at 300 kPa at the condenser outlet, 305 kPa at
the top tray, and 375 kPa at the reboiler. Specify a distillation composition of 88 mol% ethanol
compared to the predicted azeotrope of 85.6 mol% ethanol. The bottoms from Column 2 is
specified as 99.8 mol% ethanol.
A major factor in the design of a pressureswing distillation system is the recycletofeed
ratio, D2/F in Fig. 11.23(b), which is related to compositions as follows:
Let x be the mole fraction of ethanol in any stream. Referring to Fig. 11.23(b), an overall system
material balance for total flows gives:
F = B1 + B2
(1)
An overall system balance on ethanol gives: x F F = x B1 B1 + x B2 B2
(2)
A total material balance around Column 2 gives:
D1 = D2 + B 2
An ethanol balance around Column 2 gives: x D1 D1 = x D2 D2 + x B2 B2
Substitute Eq. (1) into (2) to eliminate B1. After rearrangement,
x B1 − x B2
F = B2
(5)
x B1 − x F (3)
(4) Exercise 11.15 (...
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 Spring '11
 Levicky
 The Land

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