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Chemical Engineering 140
November 18, 2009
Problem Set #13
60
.
from Midterm 3, 2004
A liquid mixture initially containing 60.0 moles of npentane and 40.0 moles of
nheptane and initially at 53
o
C is partially vaporized at P = 1 atm in a batch still, as
illustrated in Figure 2.
A constant heating rate is applied to vaporize the liquid at a
rate
v
n
(mol/s). The vapor product and remaining liquid are always in equilibrium
with each other. The relationship between the liquid and vapor mole fractions of
pentane,
x
and
y
, respectively, is given by
b
x
ax
y
+
=
2
. Assume that
a
and
b
are
constant across the practical range of interest.
N
(the total moles of liquid in the still),
x, y
, and
v
n
(the vaporization rate) all change with time.
(a) Derive the relationship between
a
and
b
in the vaporliquid equilibrium expression.
From this result, find the expression for the
K
factor of pentane only in terms of the
parameter
a
(and the independent variable
x
).
(b) Write the differential equation describing the change of total moles in the still over
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 Fall '09
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