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Unformatted text preview: =
5
1+
α 10.3
=
195 i ,C3
fi (4) 3 Using the above Kvalues to compute geometric mean values of αi, C3= , followed by use of Eqs.
(3) and (4) with the material balance, fi = di + bi , the following results are obtained: Component
Methane
Ethylene
Ethane
Propylene
Propane
nButane αi, C3= at top αi, C3= at bottom 27.8
5.83
3.83
1.00
0.78
0.18 8.70
3.22
2.35
1.00
0.88
0.34 αi, C3=
average
15.55
4.33
3.00
1.00
0.83
0.25 di , lbmol/h bi , lbmol/h 1000
2499.973
1999
5
0.375
8.1x107 2.1x108
0.027
1
195
99.625
50 Exercise 9.9
Subject:
Recovery of a key component as a function of distillate flow rate for the
distillation of a paraffin hydrocarbon mixture when the minimum number of equilibrium stages
is fixed.
Given: Feed composition and flow rate of 1000 lbmol/h. Column pressure of 250 psia.
Minimum equilibrium stages = 15. Kvalues from Figs. 2.8 and 2.9.
Find: Percent recovery of propane in the distillate as a function of distillate flow rate.
Analysis: Assume that the average relative volatility between propane and nbutane is not
sensitive to the separation. If a perfect separation were made between propane and nbutane at
250 psia, the distillate temperature would be approximately 110oF and the bottoms temperature
would be approximately 270oF. Assume an average temperature of 190oF. From Fig. 2.8, the Kvalues and relative volatilities at 250 psia and 190oF, with the light key as propane and the heavy
key as nbutane are:
Component
Ethane
Propane
nButane
nPentane
nHexane Feed rate,
lbmol/h
30
200
370
350
50
1000 Total: Kvalue
4.3
1.74
0.72
0.30
0.133 α referred to
nC4
5.97
2.42
1.00
0.417
0.185 Based on these relative volatility values, assume that little of the nonkey components will
distribute. Therefore, D = d C2 + d C3 + d nC 4 + d nC5 + d nC6 = 30 + d C 3 + d nC 4
(1)
The Fenske equation, (912) becomes:
log
N min = 15 = Rearranging, 370 − d nC4
d nC4 = d C3 bnC4
d nC4 bC 3 log α C3 , nC4
200 − d C 3
d C3 avg log
= dC3 370 − d nC4 d nC 4 200  d C3 log 2.42 2.4215 = 571800 200 − d C 3
d C3 (2)
Assume a value of d for C3. Calculate d for nC4 from Eq. (2). Then compute D from Eq. (1). Then compute recovery of C3 = d for C3/200 and make plot. Exercise 9.9 (continued)
Analysis: (continued)
d of C3, lbmol/h % recovery of C3 d of nC4, lbmol/h
198
99
0.064
190
95
0.012
180
90
0.006
150
75
0.002
100
50
0.00065 D, lbmol/h
228.1
220.0
210.0
180.0
130.0 We see that with 15 minimum stages, the distribution of nC4 to the distillate is almost negligible. Exercise 9.10
Subject:
Minimum reflux ratio by the Underwood equation for the separation of a binary
mixture as a function of feed vaporization.
Given: Binary feed of 30 mol% propane and 70 mol% propylene. Distillate to contain 99
mol% propylene, and bottoms to contain 98 mol% propane. Column pressure of 300 psia. Kvalues in Figs. 2.8 and 2.9.
Find: Minimum reflux ratio by the Underwood equation for:
(a) Bubblepoint liquid feed.
(b) Feed of 50 mol% vapor.
(c) Dewpoint vapor feed.
Analysis:...
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 Spring '11
 Levicky
 The Land

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