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Chemical Engineering 150B- Fall 2005
Problem Set #11
Solutions
(165 point assignment)
Problem 1.
(40 Points) A distillation operating will separate a mixture of 8 components with the
following feed rates at 700 kPa:
Compound
f
i
(kmol /hr)
C
3
2500
iC
4
400
nC
4
600
iC
5
100
nC
5
200
nC
6
40
nC
7
50
nC
8
40
It is known that the heavy key is iC
5
, which will have a distillate rate of 15 kmol/hr, and the light key is
nC
4
, which will have a bottoms rate of 6 kmol/hr.
Furthermore, assume for all parts of this problem that
only the light key and heavy key distribute; all other components do not distribute.
Determine the
following:
(a)
the minimum number of equilibrium stages and the distribution of nonkey components by the
Fenske equation.
(b)
what class of separation is this?
(c)
the minimum external reflux rate and distribution of nonkey components at minimum reflux by the
appropriate Underwood equation (see class notes from Wed, November 23 for handling
distributing keys and non-distributing other components) if the feed is a bubble-point liquid at
column pressure.
Hint: You may find the following equation useful:
3
12
61
2
ln
ln
P
TT
TP
a
aa
Ka
a
P
P
=+++
+
,
where P is in psia, T is in
o
R, and the constants are the following:
Compound
a
T1
a
T2
a
T6
a
P1
a
P3
C
3
-970,688.5626
0
7.15059
-0.76984 6.90224
iC
4
-1,166,846
0
7.72668
-0.92213
0
nC
4
-1,280,557
0
7.94986
-0.96455
0
iC
5
-1,481,583
0
7.58071
-0.93159
0
nC
5
-1,524,891
0
7.33129
-0.89143
0
nC
6
-1,778,901
0
6.96783
-0.84634
0
nC
7
-2,018,803
0
6.52914
-0.79543
0
nC
8
0
-7,646.81641
12.48457 -0.73152
0
In addition, you might find MathCAD useful to find the bubble-point temperature of the feed, bottoms,
and distillate streams, so that you can determine the K-value of each component in each stream.
This will