Chemistry Reference
In-Depth Information
catalytic processes and in drying of solids. Fluidization ensures that the gas con-
tacts the solid particles efficiently. Vigorous agitation of the solid materials pre-
vents clumping and minimizes temperature variations.) In this process, ethylene
gas and solid catalysts are fed continuously to a fluidized bed reactor. The fluid-
ized material is polyethylene powder which is produced as a result of the poly-
merization of the ethylene on the catalyst. The ethylene, which is recycled,
supplies monomer for the reaction, fluidizes the solids, and serves as a heat
removal medium. The reaction is exothermic and is run normally at temperatures
25
50
C below the softening temperature of the polyethylene powder in the
bed. This operation requires very good heat transfer to avoid hot spots and means
that the gas distribution and fluidization must be very uniform.
The keys to the process are active catalysts. As mentioned in Chapter 8, these
are organochromium compounds on particular supports. The catalysts will yield up
to about 10
6
kg of polymer per kilogram of metallic chromium. Branching is con-
trolled by use of comonomers like propylene or 1-butene, and hydrogen is used as
a chain transfer agent. The catalyst is so efficient that its concentration in the final
product is negligible. The absence of a solvent and a catalyst removal step gives
the process operating and capital cost advantages over the older slurry processes
for low-pressure polyethylene. However, while the granules produced directly by
gas phase polymerizations are free-flowing, they are smaller than the pellets pro-
duced by the slurry processes. As a result, their bulk density is lower and transpor-
tation costs are correspondingly increased. Because of this and because end-users
frequently have conveying equipment that is tailored to the properties of polymer
pellets, it is not unusual to find that gas phase polyolefins are extruded and pellet-
ized prior to shipment. This illustrates the common wisdom that the costs of pro-
duction of a polymer are
all
those incurred in its synthesis and finishing operations.
Although the density of the polymer can be varied by copolymerization with
higher olefins to match that of polyethylene produced by high-pressure free-
radical processes, the two types differ in branch frequency and character and in
molecular weight distributions. As a result, they do not have comparable proces-
sing and mechanical properties.
Gas phase polymerizations, using other supported catalysts, are also employed
to make isotactic polypropylene, with productivities of the same order as those
reported for polyethylene manufacture.
12.4.2.6
Interfacial Polymerizations
Section 7.5 should be consulted for a general description of this process, which
applies only to the production of condensation polymers.
The growth of macromolecules in interfacial reactions is often observed to be
into the organic phase, indicating that the active hydrogen compound which is ini-
tially in the water phase is the migrating entity. The polymer at the interface
serves primarily to control the penetration of active hydrogen compound into the
organic phase. The rate of mass transfer of the active hydrogen compound is the
