Chemistry Reference
In-Depth Information
instantly with the tank contents in a perfect CSTR, and the effluent composition
and temperature are the same as those of the contents. Neither reactor type is ideal
in actual practice, of course, and finite feed blending times, inhomogeneities, and
short-circuiting and stagnation are observed in the contents of real CSTRs.
In a free-radical polymerization, the molecular weight distribution produced in
a CSTR will be narrower than that made in a comparable batch or tubular reactor.
This will be true in any polymerization where molecular weights are controlled
by mutual termination reactions of macroradicals and chain transfer to polymer is
negligible. If the growth of polymer molecules is halted primarily by chain trans-
fer to monomer or other species the molecular weight distribution will be random
regardless of reactor type. The molecular weight distribution obtained in a series
of CSTRs can be broadened, if desired, by operating the individual reactors in
series or parallel with each unit at a different reaction temperature or mean resi-
dence time. The composition of copolymers is more uniform with CSTR reactors
than with batch, semibatch, or tubular reactors.
In a single CSTR, monomer and other ingredients of the polymerization recipe
are continually fed into the vessel while polymer and the rest of the reaction mix-
ture are removed. The effluent can, of course, serve as the feed to the next CSTR
in a series operation. Problems with heat removal are alleviated to some extent
because of the beneficial effects of cold monomer feed and the removal of reac-
tion heat with the effluent. CSTR reactors are economically attractive for large-
scale production with relatively infrequent changes in product properties.
Consider a mass balance for monomer in a CSTR:
ν½
M
0
2
ν½
M
5
R
p
(12-16)
where
v
[M]
0
is the molar inflow of monomer,
v
[M] is the corresponding outflow,
and
R
p
is the rate of polymerization (all quantities here are in units of mol/time).
Alternatively,
ð
=θÞð½
0
2
½
Þ
5
R
p
1
M
M
(12-17)
where
is the mean residence time equal to the ratio of reactor volume
V
and
volumetric flow rate
v
and [M]
0
and [M] are the molar concentrations of mono-
mer in the influent and effluent, respectively. In terms of conversion
p
(
θ
([M]
0
2
5
[M])/[M]
0
):
p
½
M
0
=θ
5
R
p
(12-18)
The solution of
Eq. (12-18)
depends on the variation of
R
p
with
p
.If
R
p
does
not vary with
p
or decreases with increasing
p
, the equation has a single solution
for a reactor with stipulated residence time. However, if autoacceleration occurs
then the same
R
p
can be observed at different values of [M] in the reactor, provid-
ing these changes also correspond to different conversions.
Multiple steady states are theoretically possible in many free-radical polymeri-
zations, but they are not usually observed in practice because the reaction is
