Unformatted text preview: s in Example 5.4 for distillation at 400 psia of a superheated vapor feed
at 105oF and containing, lbmol/h of 160 C1, 370 C2, 240 C3, 25 nC4, and 5 nC5 in a
column with a partial condenser, 5 equilibrium rectification stages, equilibrium feed
stage, 5 equilibrium stripping stages, and a partial reboiler, for an external reflux rate of
1,000 lbmol/h.
Assumptions: No changes in absorption and stripping factors when numbers of
equilibrium stages are changed.
Find: Molar ratio of C3 flow rate in the distillate to that in the bottoms, with a plot, for
(a) 10 rectification and 10 stripping stages, and (b) 15 rectification and 15 stripping
stages.
Analysis: Using the Edmister group method, the ratio of d to b for any component is
given by a rearrangement of Eq. (566),
d1
=
b AF
From Eqs. (548) and (550), S B φ AX + 1 / φ SX A −1
A N +1 − 1
S −1
φ S = N +1
S
−1
φA = (1) AC φ SE + 1 / φ AE
(2)
(3) where A = L/KV and S = KV/L.
In Eq. (1), φAX and φSX pertain to the stripping (exhauster)section,
φAE and φSE pertain to the rectifying (enricher) section. Assume that all absorption and stripping factors are unchanged from the values given for
Example 5.4. Recalculate the φ factors from Eqs. (2) and (3) for Case (a), where N = 10
for each section and Case (b), where N = 15 for each section. Analysis: (continued) Exercise 5.22 (continued) Factor
φAE
φAX
φSE
φSX N =5
0.0289
0.028
0.435
0.439 N = 10
0.00185
0.00176
0.420
0.424 N = 15
0.000123
0.000113
0.419
0.423 Using Eq. (1) with the above phi factors and the absorption and stripping factors
from Example 5.4, as given below, the following d/b ratios for C3 are obtained:
Factor
Example 5.4
AC
10.48
AE
1.72
SE = 1/AE
0.581
AF
1.108
AX
1.73
SX = 1/AX
0.577
SB
2.78 Case
d/b for C3
Example 5.4 0.0114
(a)
0.000733
(b)
0.0000487
As the number of stages increases, the separation is greatly enhanced. Exercise 5.23
Subject: Light hydrocarbon distillation by the Edmister group method. Given: Column feed at 225oF and 250 psia, corresponding to approximately 23 mol%
vapor, with component flow rates below. Distillate rate is 230 kmol/h to give a
separation between C3 and nC4. Column operates at 250 psia with a partial condenser
and a reflux ratio of 5.0. Column contains 14 theoretical stages, with feed to stage 8,
counting up from the bottom.
Find: Compositions of the distillate and bottoms.
Analysis: Using the DISTL (Edmister method) model of ASPEN PLUS with the ChaoSeader method for Kvalues, the results, with a computed 25.4 mol% vaporization, are:
kmol/h:
Component
Feed Distillate Bottoms
C2
30
29.999
0.001
C3
200
192.663
7.337
nC 4
370
7.325
362.674
nC 5
350
0.012
349.988
nC 6
50
0
50.000
Total
1,000
229.999
770.000 Exercise 5.24
Subject: Light hydrocarbon distillation by the Edmister group method. Given: Bubblepoint column feed at 120 psia, with component flow rates below.
Distillate rate is 45 kmol/h to give a separation between nC4 and iC5. Column operates at
120 psia with a partial condenser and a boilup of 200 kmol/h Column contains 20
theoretical stages, with feed to stage 10, counting up from the bottom.
Find: Compositions of the distillate and bottoms.
Analysis: Using the DISTL (Edmister method) model of ASPEN PLUS with the ChaoSeader method for Kvalues, and a reflux rate of 200  45 = 1...
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 Spring '11
 Levicky
 The Land

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