Unformatted text preview: ter method: Rigorous Chemcad:
Component
Feed,
Vapor Stripped
Vapor Stripped
lbmol/h product,
liquid, product,
liquid,
lbmol/h
lbmol/h
lbmol/h
lbmol/h
C1
59.5
59.5
0.0
59.5
0.0
C2
73.6
73.6
0.0
73.6
0.0
C3
153.2
153.1
0.1
153.1
0.1
nC 4
173.5
147.6
25.9
150.8
22.7
nC 5
58.2
12.3
45.9
12.7
45.5
nC 5
33.6
6.2
27.4
2.6
31.0
Total:
551.6
452.3
99.3
452.3
99.3
The Edmister method is uncertain because average temperature and vapor rate are difficult to
estimate. Exercise 9.28
Subject: Comparison of FUG method with Edmister group method for distillation.
Given: Bubblepoint liquid feed in kmol/h of 100 ethylbenzene, 100 paraxylene, 200
metaxylene, and 100 orthoxylene. Column pressures of 25 psia at the top and 35 psia at the
bottom. Distillate to contain 1 kmol/h of orthoxylene and bottoms to contain 2 kmol/h
metaxylene. Column to be equipped with a total condenser and partial reboiler.
Assumption: Applicability of Raoult's law for estimating Kvalues.
Find: (a) Number of stages and reflux ratio by the FUG method for R/Rmin = 1.1
Feedstage location by the Kirkbride equation.
(b) Distribution of components between distillate and bottoms by Edmister method.
Analysis: Need Kvalues at distillate, feed, and bottoms conditions. Because metaxylene and
paraxylene have very close boiling points, assume the d/b ratio of paraxylene is the same as that
for metaxylene. Therefore, have 1 kmol/h of paraxylene in the bottoms. Use Chemcad to obtain
Kvalues by running bubble points on the feed, and estimated distillate and bottoms
compositions. The results are:
Kvalues:
b, 25 psia, 30 psia, 35 psia,
Component
d,
kmol/h
kmol/h 159.3oC 168.4oC 180.5oC
Ethylbenzene, EB
100
0
1.051
1.080
1.206
Paraxylene, PX
99
1
0.986
1.012
1.131
Metaxylene, MX
198
2
0.982
1.012
1.136
Orthoxylene, OX
1
99
0.855
0.884
0.996
Total:
398
101 distillat
feed bottoms
e
(a) Using the Fenske equation (912), with MX as the LK and OX as the HK, with a geometric
mean relative volatility = [(0.982/0.855)(1.136/0.996)]1/2 = 1.144
log
N min = d MX bOX
d OX bMX log ( α MX,OX )avg log
= (198 ) ( 99 )
(1) ( 2 )
log1.144 = 68. 3 To compute the distribution of nonkey components at total reflux, use for the lighter than light
key components, LLK, Eq. (915), with OX as the reference component, r, and the above value
of Nmin. Exercise 9.28 (continued)
Analysis: (a) (continued)
Thus, for the LLK, i, bi = fi
d
N min
1 + OX α i,OX
bOX = fi
1 68.3
1+
α i,OX
99 (1) Using geometric mean values of αi, OX, with Eq. (1) and the material balance, fi = di + bi , the
following results are obtained:
kmol/h:
Component
Feed Distillate Bottoms
EB
100
99.99
0.01
PX
100
99.00
1.00
MX
200
198.00
2.00
OX
100
1.00
99.00
Total:
500
397.99
102.01
Because it is highly likely that PX, and possibly EB, will distribute, use the Class 2 Underwood
equation, Eq. (928),which for a bubblepoint feed, using relative volatilities at the feed
conditions is:
α i ,OX zi , F
12217(0.20) 11448(0.20) 11448(0.40) 100(0.20)
.
.
.
.
(2)
= 1− q = 0 =
+
+
+
12217 − θ
11448 − θ
1144...
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 Spring '11
 Levicky
 The Land

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