Page 799 - Carrahers_Polymer_Chemistry,_Eighth_Edition

762 Carraher’s Polymer Chemistry

This was derived assuming uniform concentration so that good mixing is important for this rela-
tionship to hold. It also assumes a constant temperature. Both these assumptions are only approached

in most batch systems. Further, stirring becomes more difficult as conversion increases so that both

control of localized temperature and concentration becomes more difficult. In reality, this relation-
ship holds for only a few percentage points of conversion. Overall, temperature is a major concern
for vinyl polymerizations because they are relatively quite exothermic. This is particularly impor-
tant for bulk polymerizations. This, coupled with the general rapid increase in viscosity leads to the
Trommsdorff-like effects.

M.2 PLUG FLOW (TUBULAR)

A plug flow or tubular flow reactor is tubular in shape with a high length to diameter l/d, ratio. In an

ideal case (as in most ideal cases such as an ideal gas, this only approached reality), flow is orderly
with no axial diffusion and no difference in velocity of any members in the tube. Thus, the time a
particular material remains within the tube is the same as that of any other material. We can derive

relationships for such an ideal situation for a first-order reaction. One that relates extent of conver-
sion with mean residence time, t, for free radical polymerizations is
′′
[M] = [M ] e –k′′t and k = −(1/τ) ln ([M]/[M ])
0 0
Again, while such relationships are important, they are approximate at best. For vinyl polymer-

izations, temperature control is again difficult with temperature increasing from the cooling reactor
wall to the center of the tube, and along with high and different viscosities led to broad molecular
weight distributions. Further, these factors contribute to differences in initiator and monomer con-
centrations again leading to even greater molecular weight distributions.

M.3 CONTINUOUS STIRRED TANK REACTOR

In the continuous stirred tank reactor, instant mixing to achieve a homogeneous reaction mixture
is assumed so that the composition throughout the reactor is uniform. During the reaction, mono-
mer is fed into the system at the same rate as polymer is withdrawn. The “heat” problem is some-
what diminished because of the constant removal of heated products and the addition of nonheated
reactants.
In a CSTR, each reaction mixture component has an equal change of being removed at any time
regardless of the time it has been in the reactor. Thus, in a CSTR, unlike the tubular and batch sys-
tems, the residence time is variable. The residential times can take the exponential form

R(t) = e –t/τ

where R(t) is the residence time distribution, t is the time, and τ is the mean residence time, which is

a ratio of the reactor volume to the volumetric flow rate. The residence time distribution infl uences
the mixing effectiveness which in turn determines the uniformity of the composition and tempera-
ture of the reactants in the reactor and ultimately the primary and secondary polymer structure.
Table M.1 contains a listing of selected polymerization processes and most industrially employed
reactor types.

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