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n-Pentane
n-Hexane
n-Heptane
n-Octane
Total:
: Feed, kmol/h
2500
400
600
100
200
40
50
40
3930 Distillate, kmol/h
2500
400
594
15
17.9
0
0
0
3526.9 Bottoms, kmol/h
0
0
6
85
182.1
40
50
40
403.1 Exercise 9.12
Subject:
Minimum reflux ratio and minimum number of equilibrium stages as a function of
product purity for the distillation of a binary mixture.
Given: Equimolar bubble-point feed of isobutane and n-butane at column pressure of 100 psia.
Purity of iC4 in the distillate equal to purity of nC4 in the bottoms. Purity range from 90 mol% to
99.99 mol%
Find: Minimum reflux ratio.
Minimum number of equilibrium stages.
Discuss significance of results.
Analysis: From the vapor pressure plot of Fig. 2.4, for 100 psia, the boiling points are 120oF for
iC4 and 146oF for nC4. Take an average temperature of 133oF and assume a relative volatility
equal to the vapor pressure ratio. Thus, again using Fig. 2.4, αiC4, nC4 = 115/85 = 1.35.
For Nmin , use the Fenske equation, (9-11), which simplifies to:
log
N min = xiC 4
xiC 4 2 log D
2 log 135
. B = xiC 4 2 D 1 − xiC 4 2 D (1) 01303
. For Rmin , use the Class 1 form of the Underwood equation, (9-20), which simplifies to: Rmin = xiC 4 , D / 0.5 − 135 1 − xiC4 , D / 0.5
. (2) 0.35 Using a spreadsheet, the following values are computed: % Purity
90
92
94
95
96
98
99
99.5
99.9
99.95
99.99 Nmin
14.6
16.3
18.3
19.6
21.2
25.9
30.6
35.3
46.0
50.7
61.4 Rmin
4.37
4.64
4.91
5.04
5.18
5.45
5.58
5.65
5.70
5.71
5.71 Exercise 9.12 (continued)
Analysis:
(continued)
A plot of the results is shown below. It is clearly evident from the table above or the plot below
that the minimum reflux increases little and less and less as the purity approaches 100%, while
the minimum number of equilibrium stages increases greatly and more and more as the purity
approaches 100%, Exercise 9.13
Subject:
mixture. Reflux ratio by the FUG method for the distillation of a propylene-propane Given: Binary feed of 360 kmol/h of propylene and 240 kmol/h of propane at the bubble point
at column pressure. Distillate to contain 347.5 kmol/h of propylene and 3.5 kmol/h of propane.
Average relative volatility = 1.11. N/Nmin = 2. Total condenser and partial reboiler.
Assumptions: External reflux ratio = internal reflux ratio at the upper pinch.
Find: Operating reflux ratio by the FUG method.
Analysis: By material balance, the bottoms product contains 12.5 kmol/h of propylene and
236.5 kmol/h of propane. Using the Fenske equation (9-12): log
N min = d C = bC
3 3 d C bC = log α C= ,C
3 = 3 3 3 347.5 236.5
35
.
12.5 log log 111
. = 72.2 avg N = 2 Nmin = 2(72.2) = 144.4
For minimum reflux, use the Class 1 Underwood equation (9-20), which applies for a
binary mixture, where the pinch composition is that of the liquid feed. The distillate mole
fractions are 347.5/351 = 0.99 for propylene and 0.01 for propane. In the feed, the mole fractions
are 360/600 = 0.60 for propylene and 0.40 for propane. Thus,
xC = , D
3 Rmin = Lmin
=
D xC = , F − α C = ,C
3 xC
3 3 α C = ,C − 1
3 3 3 ,D xC 3 ,F = 0.99
0.01
− 111...
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- Spring '11
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- The Land
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