Chemistry Reference
In-Depth Information
to heat transfer through the reactor walls. Substitution of representative values for
the various parameters into
Eq. (12-15)
shows that
q
E
increases in relative impor-
tance as the reactor size is increased. Limiting operating conditions for non-
isothermal polymerizations may be estimated by modifying
Eq. (12-15)
to take
account of the activation energy for rate of energy release, which is set equal to
that for rate of polymerization. The modified equation can be solved numerically
for different values of
R
p
,
T
1
, and so on.
Reflux cooling is the most common method for additional heat removal if an
ingredient of the polymerization mixture is volatile at the reaction temperature
and extensive foaming does not occur. External heat exchangers can be used in
some processes in which a portion of the reaction mixture is continuously
removed, pumped through a heat exchanger to cool it, and returned to the reactor.
This method is used sometimes in emulsion processes where the mixture viscosity
is low and the mechanical stability of the latex is good. It is not practical in sus-
pension systems, however, because continuous agitation is required in these reac-
tions to prevent coalescence of the polymer particles. Bulk and solution
polymerizations do not ordinarily rely on external heat exchangers, because the
high viscosity and poor agitation in the heat exchanger lead to polymer build-up
on the cool walls of this unit. Internal cooling coils can be used only in reactions
where the mixture viscosity is low and polymer scale build-up is not a problem.
Otherwise, poor mixing around the coils can result in poor product quality and
long cleanup times between batches.
12.6.2
Tubular Reactors
Tubular reactors consist in principle of unstirred vessels with very high length/
radius ratios. They are attractive reactors for production of some micromolecular
species but are limited in their application to polymer production. This is because
the relatively high viscosities that are encountered at intermediate conversions in
polymer syntheses lead to difficulties in controlling the reaction temperature.
Polymer tends to form a slow-moving layer on the cool reactor walls, reducing
the flow-through capacity of the tube and the effective heat transfer coefficient.
In general in tubular reactors, the material at the tube center will be at a higher
temperature than the reaction mixture at the tube wall. The temperature rise
increases with the tube radius, because heat transfer in this reactor type is entirely
by convection through the reaction mixture. Thus, a larger tube radius increases
production rates because of the greater volumetric flow rate, but there is an addi-
tional augmentation of production resulting from the higher center line tempera-
tures in the larger bore vessel. The broad temperature distribution is reflected, of
course, in a greater polydispersity of polymer molecular weights. As a corollary,
thermal runaways are possible with increasingly larger tube diameters.
For a given tube radius there exists a particular wall temperature that gives
maximum conversions in free-radical polymerizations. This can be seen qualita-
tively from the following considerations. If the tube wall is too cool, the initiator
