6.8 Large-Scale Preparation of the Supported Metallocene
Catalysts
There are many different ways to combine the three main ingredients (metallocene
molecule(s), MAO, and the silica) chemically to obtain the final supported
metallocene catalysts. One of the most recommended procedures in the patent
literature is to first treat the silica with MAO and then combine it with metallocene
molecules. No matter which procedure is used, one must be aware that the resulting
final heterogeneous metallocene catalyst is very reactive towards moisture, leading
to its destruction, and to polar molecules that act as a very effective poison and
deactivate the catalyst in an irreversible manner. The “long-term” storage of
different batches of supported catalyst under inert atmosphere, in a temperate
environment that excludes excessive heat, and with no exposure to light is essential
for its constant activity and reliability.
6.9 Large-Scale Production of Syndiotactic Polypropylene
The Phillips process technology involving loop reactors, using liquid propylene both
as the reaction medium and monomer, requires like any other continuous process
meticulous control of polymerization conditions. Any deviations in temperature,
pressure, liquid circulation velocity, etc. must be maintained within a very narrow
range during the entire period of production. The process is very sensitive to
formation of fines (very fine polymer particles), which are formed gradually during
the polymerization and adhere to the reactor walls if the catalyst composition is not
well defined or the set polymerization conditions not respected. With time, under the
influence of the reactor wall temperature, these fines are transformed into a thin layer
of polymer film that can act as an insulator and interfere with the reactor’s temperature control system, leading to temperature and pressure fluctuations in the reactor.
In severe cases, this phenomenon can lead to reactor fouling and shut down. The
build-up of fines is generally avoided by the proper choice of silica particles,
adjustment of the heat treatment regime, and the reaction conditions between
silica support and MAO. The use of an antistatic agent is also not uncommon in
moderate cases.
The polymer particle size, morphology, and bulk density are other important
parameters for the maximum efficiency of the loop bulk process. Spherical or semispherical polymer particles of an average size of about 50–100 μm, with narrow
particle size distribution and bulk densities close to or higher than 4 g/ml are
generally desired. This is achieved primarily by the proper choice of silica particle
morphology and the catalyst preparation conditions but also by adding a
pre-polymerization step to the process during which the catalyst is first contacted
with propylene, or other polymerizable monomers, at low concentration and
moderate temperatures, before entering the main reactor.
98
A. Razavi
Catalysts
There are many different ways to combine the three main ingredients (metallocene
molecule(s), MAO, and the silica) chemically to obtain the final supported
metallocene catalysts. One of the most recommended procedures in the patent
literature is to first treat the silica with MAO and then combine it with metallocene
molecules. No matter which procedure is used, one must be aware that the resulting
final heterogeneous metallocene catalyst is very reactive towards moisture, leading
to its destruction, and to polar molecules that act as a very effective poison and
deactivate the catalyst in an irreversible manner. The “long-term” storage of
different batches of supported catalyst under inert atmosphere, in a temperate
environment that excludes excessive heat, and with no exposure to light is essential
for its constant activity and reliability.
6.9 Large-Scale Production of Syndiotactic Polypropylene
The Phillips process technology involving loop reactors, using liquid propylene both
as the reaction medium and monomer, requires like any other continuous process
meticulous control of polymerization conditions. Any deviations in temperature,
pressure, liquid circulation velocity, etc. must be maintained within a very narrow
range during the entire period of production. The process is very sensitive to
formation of fines (very fine polymer particles), which are formed gradually during
the polymerization and adhere to the reactor walls if the catalyst composition is not
well defined or the set polymerization conditions not respected. With time, under the
influence of the reactor wall temperature, these fines are transformed into a thin layer
of polymer film that can act as an insulator and interfere with the reactor’s temperature control system, leading to temperature and pressure fluctuations in the reactor.
In severe cases, this phenomenon can lead to reactor fouling and shut down. The
build-up of fines is generally avoided by the proper choice of silica particles,
adjustment of the heat treatment regime, and the reaction conditions between
silica support and MAO. The use of an antistatic agent is also not uncommon in
moderate cases.
The polymer particle size, morphology, and bulk density are other important
parameters for the maximum efficiency of the loop bulk process. Spherical or semispherical polymer particles of an average size of about 50–100 μm, with narrow
particle size distribution and bulk densities close to or higher than 4 g/ml are
generally desired. This is achieved primarily by the proper choice of silica particle
morphology and the catalyst preparation conditions but also by adding a
pre-polymerization step to the process during which the catalyst is first contacted
with propylene, or other polymerizable monomers, at low concentration and
moderate temperatures, before entering the main reactor.
98
A. Razavi