Chemistry Reference
In-Depth Information
Fig. 6.2
Commercial flow
reactor for slurry
polymerization of ethylene
with Ziegler-Natta
catalysts as illustrated in a
British patent # 826 523
In continuous slurry processes, the temperatures are kept between 90 and 100
C and pressures
between 400 and 450 lb/in.
2
. The catalyst concentrations range between 0.004 and 0.03% and typical
diluents are
-hexane. Individual catalyst particles become imbedded in polymer
granules as the reaction proceeds. The granules are removed as slurry containing 20-40% solids.
There are variations in the individual processes. In some procedures, the temperature is kept high
enough to keep the polymer in solution. In others, it is kept deliberately low to maintain the polymer
in slurry. The products are separated from the monomer that is recycled. They are cooled, precipitated
(if in solution), and collected by filtration or centrifugation.
Various reactors were developed to handle different slurry polymerization processes. The slurry is
maintained in suspension by ethylene gas. The gas rises to the top and maintains agitation while the
polymer particles settle to the bottom where they are collected.
Several companies adopted loop reactors. These are arranged so that the flowing reactants and
diluents continuously pass the entrance to a receiving zone. The heavier particles gravitate from the
flowing into the receiving zone while the lighter diluents and reactants are recycled. To accommodate
that, the settling area must be large enough for the heavy polymer particles to be collected and separated.
In addition to suspension, a gas-phase process was developed. No diluent is used in the polymeri-
zation step. Highly purified ethylene gas is combined continuously with a dry-powdery catalyst and
then fed into a vertical fluidized bed reactor. The reaction is carried out at 270 psi and 85-100
C.
The circulating ethylene gas fluidizes the bed of growing granular polymer and serves to remove the
heat [
15
]. Formed polymer particles are removed intermittently from the lower sections of the vertical
reactor. The product contains 5% monomer that is recovered and recycled. Control of polymer
density is achieved by copolymerization with
n
-pentane and
n
-olefins. Molecular weights and molecular weight
distributions are controlled by catalyst modifications, by varying operating conditions, and/or use of
chain transferring agents [
15
], such as hydrogen [
16
]. This is illustrated in Fig.
6.3
.
a
