Chemistry Reference
In-Depth Information
however, as a model that is derived from accurate knowledge of the polymeriza-
tion process and reactor operation because only the latter tool permits extrapola-
tion to reaction conditions that have not yet been tried.
12.6.1
Batch Reactors
Good mixing is important to ensure uniform temperatures and prevent the occur-
rence of localized inhomogeneities. It is difficult to generalize about mixing in
bulk polymerizations in batch reactors, because the viscosity and density of the
reaction medium are continuously changing as the reaction proceeds. The corre-
sponding changes in emulsion and suspension systems are, of course, much less
pronounced.
The major problem in temperature control in bulk and solution batch chain-
growth reactions is the large increase in viscosity of the reaction medium with
conversion. The viscosity of styrene mixtures at 150
C will have increased about
1000-fold, for example, when 40 wt% of the monomer has polymerized. The heat
transfer to a jacket in a vessel varies approximately inversely with one-third
power of the viscosity. (The exact amount depends also on the nature of the agita-
tor and the speed of fluid flow.) This suggests that the heat transfer efficiency in
a jacketed batch reactor can be expected to decrease by about 40% for every 10%
increase in polystyrene conversion between 0 and 40%.
Heat transfer can be increased up to a point by rotating the agitator faster. An
increase in speed by a factor of 10
3
will increase the heat transfer rate by 10
2
and
the power to the agitator shaft by 10
7
. In viscous systems a speed of maximum
net heat
input
is reached beyond which rate of power input
into the batch
increases faster than the rate of heat removal.
Heat removal is accomplished by transfer to the vessel jacket, use of internal
cooling coils, circulation of the reaction fluid through an external cooling loop, or
by use of an overhead condenser to remove heat from the monomer or diluent in
the vapor phase.
All reactors are jacketed to permit heat removal through the vessel walls. It is
frequently necessary to add extra heat removal means as the reaction vessels are
scaled up because the heat transfer area of the reactor walls increases with reactor
volume to the two-thirds power while the rate of heat generation is proportional
to the volume itself.
The energy balance for an isothermal reaction can be written as
V
ð
2
Δ
H
p
Þ
R
p
5
UA
w
ð
T
T
1
Þ
1
q
E
(12-15)
2
where
V
5
reactor volume,
2
Δ
H
p
5
heat of polymerization, which is negative
for exothermic reactions,
R
p
5
polymerization rate (cf. Chapter 8),
U
5
overall
heat transfer coefficient,
A
w
5
wall area,
T
5
reaction temperature, temperature of
coolant, and
q
E
5
heat removed by the condenser or other auxiliary devices. The
two terms on the right-hand side of this equation represent alternative methods
for removing the exothermic heat of reaction, with
UA
W
(
T
2
T
1
) corresponding
