Chemistry Reference
In-Depth Information
The other extreme condition is one in which mixing takes place on a molecu-
lar scale. The incoming fluid elements quickly lose their identity as a result of
mass transfer between them, and the reaction medium becomes a
microfluid
.
Real flows in continuous polymerization vessels are always intermediate
between the completely micromixed and completely segregated conditions. Most
studies of CSTR reactions assume one or the other type. Despite this artificiality,
there have been some successes in modeling actual polymerizations.
The degree of micromixing has little effect on the conversion of polymer per
pass through the reactor, except as it may influence the initiator efficiency in
free-radical reactions. Other criteria, such as molecular weight and branching dis-
tributions of the product polymers, can be strongly affected. The number distribu-
tion of molecular weights in a perfectly mixed CSTR will, of course, be the same
as the instantaneous molecular weight distribution in a batch reactor at the same
temperature, initiation rate, and conversion. The molecular weight distribution in
a segregated CSTR is expected to be broader than in a micromixed CSTR. A per-
fectly mixed CSTR should generally produce a product with a narrower
molecular weight distribution than a batch reactor if the lifetime of the growing
macromolecules is short compared to the mean residence time in the reactor. The
perfectly stirred tank will give a broader distribution product if the life of the
growing chain is long compared to
θ
.
The most significant differences between perfectly mixed and segregated flow
in a CSTR occur in copolymerizations. In a batch reaction, the copolymer compo-
sition varies with conversion, depending on the reactivity ratios and initial mono-
mer feed composition. In a perfectly mixed CSTR, there will be no composition
drifts but the distribution of product compositions will broaden as mixing in the
reactor approaches segregated flow.
PROBLEMS
12-1
The equilibrium constant for reaction (12-13) is
½a
CONH
a½
H
2
O
at 280
C
K
1
C
300
NH
2
5
½a
COOH
a½a
6.5 kcal mol
2
1
.
The polymerization reaction in this problem is finished at a fixed steam
pressure (1 atm). The equilibrium concentration of H
2
O in the polymer melt
varies with temperature and steam pressure in this case. The enthalpy of
vaporization of H
2
O is about 8 kcal mol
2
1
. Compare the limiting values of
number average molecular weight of the polyamide produced at 280 and
250
C final polymerization temperatures. [
Hint:
Recall that the variation of
an equilibrium constant
K
with temperature is given by
d
(ln
K
)/
d
(l/
T
)
The enthalpy change for this polymerization is
Δ
H
p
52
52
ΔH
/
R
, where
ΔH
is the enthalpy change of the particular process
