Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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19080-1061 PATENT
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METHOD FOR PRODUCT RECOVERY OF POLYOLEFINS
TECHNICAL FIELD:
The present invention relates to methods for product
recovery of polyolefins, and more specifically to a method
of product recovery for high-molecular-weight amorphous
poly alpha-olefins.
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BACKGROUND OF THE INVENTION
High-molecular-weight amorphous poly alpha-olefins
such as amorphous propylene homo- and co-polymers, are
important for their use in diverse products. The broad
utility of these materials is' due in large part to the
unique combination of chemical and physical properties such
as chemical inertness, softness, flexibility, etc.,
exhibited by these materials.
Conventionally, amorphous polyolefins are formed in a
reactor and mixed with water to deactivate catalysts and
remove any monomer(s). Removing the catalysts and any
monomers) renders wet, granular chunks of the product.
For the material to be shaped into various products, the
chunks must be dried and then extruded or otherwise shaped.
Extrusion of the material typically involves feeding
the dried chunks from a hopper to the feed section of a
screw-type extruder. The polyolefin material is moved
through the extruder by screw flights where it is heated
and mechanically worked before it is pelletized or
otherwise shaped under high pressure. Alternatively, such
materials are also shaped by other high temperature methods
such as injection molding, roll milling and compression
molding. Both lower- and higher-molecular-weight amorphous
poly alpha-olefins are typically processed as outlined
above.
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However, existing methods of product recovery require
the introduction of water to carry the material through the
several stages of recovery. The extensive use of water by
these methods requires that additional storage tanks,
delivery and removal lines and other miscellaneous
equipment be used to introduce, maintain, remove and
recycle the necessary volume of water. Additionally,
existing methods store the material in a chunk form prior
to extrusion into useable products, thus requiring
additional storage tanks and associated maintenance
equipment for this intermediate stage of processing.
Thus a need has arisen for a product recovery method
for polyolefins, particularly high-molecular-weight
amorphous poly alpha-olefins, wherein the use of water
during the product recovery is significantly decreased and
wherein the intermediate stage of storing and drying the
chunk form of the polyolefin is eliminated.
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SUMMARY OF THE INVENTION
The present invention overcomes the foregoing and
other problems associated with the prior art by providing
a product recovery method for polyolefins, particularly
high-molecular-weight amorphous poly alpha-olefins, wherein
the use of water during the method is significantly
decreased, reducing the need for additional equipment such
as storage tanks, lines and valves, and wherein the method
operates such that intermediate storage of the material is
eliminated, thus preventing the need for additional storage
tanks and associated maintenance equipment.
According to the present method of product recovery
for polyolefins, the monomers) for the polyole.fin are fed
into a reactor. The reactor is cooled to maintain the
appropriate temperature necessary for the production of the
desired polyolefin. Depending upon the polyolefin desired,
appropriate catalysts are added to the reactor.
As the polyolefin material is produced, it is
transferred from the reactor to a kneader. This transfer
is accomplished by a number of methods, but is preferably
accomplished via a blipper valve. Inside the kneader, the
polyolefin material is heated to drive off any unreacted
monomers) remaining in the polyolefin material. Sigma
blades are used to mechanically work the product material
to facilitate this removal process.
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The polyolefin material is then transferred via the
screw flights of the kneader to an extruder for further
processing. This step of the method, the direct transfer
of the polyolefin material in a liquid form, provides a
distinct advantage over prior methods of storing the
polyolefin in wet chunk form and drying the chunks at a
later date for extrusion.
In the extruder, the polyolefin material is mixed with
small amounts of water to deactivate any remaining
catalysts) in the material and antioxidants. Heating the
material further drives off any unreacted monomer(s),
antioxidant solvents and excess steam added during this
stage.
Finally, the polyolefin material is transferred to.a
pelletizer where it is pelletized for storage and/or use.
High-molecular-weight amorphous poly alpha-olefins
exhibit increased tackiness and viscosity when compared
with lower-molecular-weight poly alpha-olefins. Product
recovery of these higher-molecular-weight poly alpha-
olefins has proven especially successful utilizing the
present method of recovery.
The present method minimizes the use of water to carry
the product through the several stages of recovery
processing, thereby minimizing the need for storage tanks,
delivery and removal lines, valves and other equipment used
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to introduce, maintain, remove and recycle the water.
Additionally, the present method allows for continuous
processing of the polyolefin material in a substantially
liquid or molten state. This avoids the intermediate step
associated with existing methods of product recovery
wherein the material is stored as wet chunks and dried at
a later stage for further processing. The elimination of
this step also precludes the need for additional equipment
required for the storage, maintenance and drying of the
product material prior to further processing.
More specifically, in a first embodiment, the
invention provides a method for the recovery of a high-
molecular-weight amorphous polyolefin including: reacting
monomers) in a reaction zone to form a high-molecular-
weight amorphous polyolefin; continuously transferring
blips of material containing the polyolefin along with
residual catalyst and unreacted monomers) as a mixture
from the reaction zone to a recovery zone; venting
unreacted monomers) from the recovery zone; controlling
the transfer of the material blips into the recovery zone
without introducing water and while substantially avoiding
carryover of the polyolefin back to the reaction zone
during the venting step; heating the mixture of said
material blips in the recovery zone to a temperature of at
least about 250°F while kneading the mixture for a time
sufficient to remove unreacted monomers) from the
polyolefin and to facilitate transfer of recovered
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polyolefin from the recovery zone to an extrusion zone; and
controlling the material blip transfer, heating, and
kneading steps to maintain a substantially constant
inventory of the polyolefin in the recovery zone.
Furthermore, the invention provides a method for the
recovery of a high-molecular-weight amorphous polyolefin
which includes: reacting monomers) in a reactor to form a
high molecular weight amorphous polyolefin; continuously
transferring blips of material containing the polyolefin
along with residual catalyst and unreacted monomers) as a
mixture from the reactor directly through a conduit which
contains a blipper valve to a kneader, the kneader having
rotatable sigma blades, a kneader vent line and a kneader
vent valve for venting unreacted monomer(s); venting
unreacted monomers) from the kneader through the vent line
and vent valve; controlling the operation of the blipper
valve and the kneader vent valve to transfer material blips
from the reactor to the kneader primarily when the blipper
valve is open and when the kneader vent valve is closed to
substantially avoid carryover of polyolefin into the
kneader vent line during the venting step; heating the
mixture of material blips in the kneader to a temperature
of from about 250°F to 500°F while kneading the mixture for
a time sufficient to remove unreacted monomers) from the
polyolefin and to facilitate transfer of the recovered
polyolefin from the kneader to an extruder; and controlling
the material blip transfer, heating and kneading steps to
",............
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maintain a substantially constant inventory of the
polyolefin in the kneader.
Still further, the invention provides a method for the
recovery of a high-molecular-weight sticky amorphous
polyolefin which consists essentially of: reacting
monomers) in a reactor to form a high molecular weight
amorphous polyolefin; continuously transferring blips of
material containing the polyolefin along with residual
catalyst and unreacted monomers) as a mixture from the
reactor directly through a conduit which contains a blipper
valve to a kneader, the kneader having rotatable sigma
blades, a kneader vent line and a kneader vent valve for
venting unreacted monomer(s); venting unreacted monomers)
from the kneader through the vent line and vent valve;
controlling the operation of the blipper valve and the
kneader vent valve to transfer material blips from the
reactor to the kneader without introducing water and
primarily when the blipper valve is open and when the
kneader vent valve is closed to substantially avoid
carryover of polyolefin into the kneader vent line during
the venting step; heating the mixture of material blips in
the kneader to a temperature of from about 250°F to 500°F
while kneading the mixture for a time sufficient to remove
unreacted monomers) from the polyolefin and to facilitate
transfer of the recovered polyolefin from the kneader to an
extruder; controlling the material blip transfer, heating
and kneading steps to maintain a substantially constant
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inventory of the polyolefin in the kneader; recovering the
polyolefin from the kneader; transferring the recovered
polyolefin from the kneader to an extruder; and
deactivating residual catalyst contained in the polyolefin
by mixing the polyolefin in the extruder with steam in an
amount sufficient only to deactivate the catalyst, but
insufficient to transport the polyolefin.
In further embodiments of the above methods, the
method may further include controlling the temperature of
the polyolefin in the extruder to be substantially the same
as the temperature of the polyolefin in the kneader.
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BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present
invention and the advantages thereof, reference is now made
to the following description taken in conjunction with the
accompanying drawings in which:
FIGURE 1 illustrates the apparatus associated with the
product recovery method of the present invention; and
FIGURE 2 is a flow diagram illustrating the steps
associated with the product recovery method of the present
invention.
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DETAILED DESCRIPTION OF THE DRAWINGS
Referring now to FIGURE 1, there is shown the
apparatus associated with the product recovery method of
the present invention.
The monomers) comprising the polyolefin to be
produced are continuously fed into a reactor 10. The
monomers) utilized will naturally depend upon the
polyolefin to be produced. The reactor is cooled and
pressurized to maintain the desired temperature and
pressure for the reaction to occur. The resulting polymer
is continuously transferred to a kneader 20 via a blipper
valve 22. The use of a blipper valve 22 allows for
convenient control of the rate at which the product
material is released from the reactor 10 to the kneader 20.
In the kneader 20, the product material is heated to
a temperature of from about 250° - 500° Fahrenheit,
depending upon the polyolefin to be produced. This
increase in temperature drives off unreacted elements
contained within the product material, such as unreacted
propylene, ethylene, hydrogen and other monomers.
A pair of sigma blades (not shown) within the kneader
20 mechanically works the product material to facilitate
the removal of unreacted monomers and other volatiles from
the product material. The gases produced by this process
are vented through a vent line 30 from the kneader 20 to a
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knockout pot 40. From the knockout pot 40, the vented
gases are purified and returned to the polyolefin plant for
reuse via return line 45. To prevent carryover of the
polyolefin material into the vent line 30, a vent valve 47
closes during each blip of material from the reactor 10 to
the kneader 20. With the vent valve 47 closed, the high
gas velocity within the relatively small volume kneader
does not drive the polyolefin material into the vent line
30. The vent valve 47 is operated by a microprocessor-
based timer (not shown) which controls and coordinates both
the blipper valve 22 and the vent valve 47.
Product material is delivered from the sigma blades to
a variable speed screw (not shown). The screw fights
transfer the product material from the kneader 20 to an
extruder 50. The speed of the screw is adjusted so as to
maintain a constant inventory of product material in the
kneader 20. The barrel of the screw is heated to maintain
the product material at a temperature substantially
consistent with the temperature in the kneader 20.
In the extruder 50, the product material is mixed with
steam to deactivate the catalysts and with additives to
achieve the desired polyolefin material. The steam is
added, via a metered water pump (not shown) to deactivate
the catalysts) added to the reactor 10 to facilitate
formation of the polyolefin. Importantly, this is the only
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place that water is used in the product recovery method of
the present invention.
Additional heating facilitates the removal of any
excess water and further removes any remaining unreacted
monomers or other volatiles from the product material.
Gases produced within the extruder 50 are vented via an
extruder vent line 70 from the extruder 50.
From the extruder 50, the product material is
transferred to a pelletizer 80. The pelletizer 80 includes
a die plate and a set of rotating blades (not shown) driven
by a variable speed motor 90. Heat is provided to the die
plate to maintain the product material at an extrusion
temperature. As the product material emerges from the die
holes, it is cut into pellets sized according to the speed
of the rotating blades and is cooled by circulating water.
The product material rapidly solidifies upon contact with
the water.
The pellets are carried by flowing water to a dryer
100, where the pellets are recovered from the water and
dried with air. The pellets are then packaged and stored
for later use.
FIGURE 2 is a flow diagram illustrating the steps
associated with the product recovery method of the present
invention. The monomer(sj necessary to produce the desired
polyolefin are continuously fed into a reactor during step
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110. The reactor is cooled to facilitate the production of
the polyolefin material. Polyolefin material is then
transferred to a kneader where unreacted monomers) and
volatile(s) are removed 120. The monomers) and
volatile(s) are driven off by heating the polyolefin
material in the kneader. Mechanical working of the
polyolefin material by a pair of sigma blades in this step
enhances the removal of unreacted elements. Gases produced
during this step of the process are vented through a
knockout pot and are purified for reuse 130 and 140.
In step 150, the polyolefin material is transferred to
an extruder where water and desired additives such as
antioxidants are mixed with the polyolefin material 160 and
170. The addition of water deactivates the catalysts in
the polyolefin material. Gases produced during this step
of the product recovery method are also vented 180.
Next, the polyolefin material is transferred from the
extruder to a pelletizer. In the pelletizer, the
polyolefin material is cut via rotating blades into pellets
and is cooled by circulating water 190. The pellets are
transferred by the flowing water to a dryer. The pellets
are removed from the water and dried with air prior to
being packaged or used 200.
Although preferred embodiments of the invention have
been illustrated in the accompanying drawings and described
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in the foregoing Detailed Description, it will be
understood that the invention is not limited to the
embodiments disclosed, but is capable of numerous
rearrangements and modifications of parts and elements
without departing from the spirit of the invention.
