U.S. patent application number 11/607123 was filed with the patent office on 2007-06-14 for method and apparatus for making crystalline pet pellets.
This patent application is currently assigned to Gala Industries, Inc.. Invention is credited to Michael Eloo.
Application Number | 20070132134 11/607123 |
Document ID | / |
Family ID | 34590927 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070132134 |
Kind Code |
A1 |
Eloo; Michael |
June 14, 2007 |
Method and apparatus for making crystalline pet pellets
Abstract
A method and apparatus for underwater pelletizing and subsequent
drying of polyethylene terephthalate (PET) polymers and other high
temperature crystallizing polymeric materials to crystallize the
polymer pellets without subsequent heating. High velocity air or
other inert gas is injected into the water and pellet slurry line
to the dryer near the pelletizer exit. The slurry line has a
substantially straight component, and air is preferably injected at
the end of the straight component nearest the pelletizer exit and
in a direction substantially coincident with the axis of the
straight component. The air injection significantly increases the
speed of the pellets into and out of the dryer such that the PET
polymer pellets leave the dryer above at least 135.degree. C., and
preferably above 145.degree. C., to self-initiate
crystallization.
Inventors: |
Eloo; Michael; (Xanten,
DE) |
Correspondence
Address: |
JACOBSON HOLMAN PLLC
400 SEVENTH STREET N.W.
SUITE 600
WASHINGTON
DC
20004
US
|
Assignee: |
Gala Industries, Inc.
|
Family ID: |
34590927 |
Appl. No.: |
11/607123 |
Filed: |
December 1, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10717630 |
Nov 21, 2003 |
7157032 |
|
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11607123 |
Dec 1, 2006 |
|
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Current U.S.
Class: |
264/143 ;
425/232; 425/289 |
Current CPC
Class: |
B29B 9/065 20130101;
F26B 5/08 20130101; B29K 2995/0041 20130101; B29B 9/16 20130101;
Y10S 425/23 20130101; B29B 2009/165 20130101; B29K 2067/00
20130101; B29K 2995/0039 20130101; F26B 17/00 20130101; B29C 31/00
20130101; B29C 2793/0027 20130101; B29B 13/021 20130101 |
Class at
Publication: |
264/143 ;
425/232; 425/289 |
International
Class: |
B29B 9/06 20060101
B29B009/06 |
Claims
1. A method for processing PET polymers into pellets, which
comprises: extruding strands of PET polymer through a die plate
into an underwater pelletizer; cutting the PET polymer strands into
pellets in said pelletizer; transporting said PET pellets out of
said pelletizer to a dryer using a water stream, and injecting a
high velocity gas into said water stream to enhance the speed of
the pellets to said dryer and with said pellets retaining
sufficient heat inside said pellets to initiate and complete
crystallization of said pellets.
2. The method as claimed in claim 1 wherein said PET pellets
exiting said dryer are placed in a heat insulating container to
complete said crystallization.
3. The method as claimed in claim 1 wherein said PET pellets exit
said dryer at a mean temperature above about 135.degree. C.
4. The method as claimed in claim 3 wherein said PET pellets exit
said dryer at a temperature above about 145.degree. C.
5. The method as claimed in claim 1 wherein said step of
transporting said PET pellets out of said pelletizer to a dryer
includes a substantially straight slurry line angled upwardly from
the vertical between 30.degree. and 60.degree..
6. The method as claimed in claim 5 wherein said angle is about
45.degree..
7. The method as claimed in claim 1 wherein said pressurized gas is
air.
8. The method as claimed in claim 1 wherein said pressurized gas is
injected into said water stream substantially in alignment with
said water stream.
9. An apparatus for processing PET polymers into pellets which
comprises an underwater pelletizer to cut PET polymer stands
extruded into said pelletizer into pellets, piping to introduce
water into said pelletizer and a slurry line to transport a water
and pellet slurry out of said pelletizer and to a centrifugal dryer
for drying said PET pellets, and a gas injector to introduce high
velocity inert gas into said water and pellet slurry to enhance the
speed of said pellets through said processing apparatus with said
pellets exiting said dryer with sufficient internal heat to
initiate and complete crystallization of said pellets.
10. The apparatus as claimed in claim 9 wherein said apparatus
further comprises one or more heat insulating containers for
receiving said pellets out of said dryer for completing
crystallization of said pellets.
11. The apparatus as claimed in claim 1 wherein a portion of said
slurry line is straight and angled upwardly at an angle between
30.degree. and 60.degree..
12. The apparatus as claimed in claim 1 wherein said slurry line
includes a straight portion and said gas injector introduces said
inert gas at a beginning of said straight portion.
13. The apparatus as claimed in claim 12 wherein said gas injector
introduces said inert gas into said water and pellet slurry
substantially in alignment with a longitudinal axis of said slurry
line straight portion.
14. The apparatus as claimed in claim 9 wherein said slurry line
includes an exit end with an enlarged diameter before entry into
said centrifugal dryer.
15. The apparatus as claimed in claim 9 wherein said slurry line
includes a generally vertical section from said pelletizer, a
generally angled straight section from said generally vertical
section, and an enlarged section at an outer end of said generally
angled straight section to reduce potential agglomeration of said
pellets entering said dryer.
16. A method for processing high temperature crystallizing
polymeric materials into pellets, which comprises: extruding a high
temperature crystallizing polymeric material into strands; water
cooling and cutting the extruded strands into pellets; and
transporting said pellets using a water stream and a high velocity
gas with said pellets retaining sufficient heat to initiate and
complete crystallization of said polymeric material without the
application of external heat.
17. The method of claim 16, wherein said material includes PET
polymer.
Description
RELATED APPLICATIONS
[0001] This is a continuation application of co-pending application
Ser. No. 10/717,630, filed Nov. 21, 2003, and hereby claims the
priority thereof.
FIELD OF THE INVENTION
[0002] The present invention generally relates to a method and
apparatus for underwater pelletizing and subsequent drying of
polyethylene terephthalate (PET) polymers. More specifically, the
present invention relates to a method and apparatus for underwater
pelletizing PET polymers and subsequent drying the PET polymer
pellets in a manner to self-initiate the crystallization process of
the PET particles and produce pellets having a crystalline
structure rather than an amorphous structure.
BACKGROUND OF INVENTION AND PRIOR ART
[0003] Underwater pelletizing systems for producing pellets of
polymeric or other plastic materials has been known for many years.
The starting materials such as plastic polymers coloring agents,
additives, fillers and reinforcing agents, and modifiers, are mixed
in kneaders. In the process, a melt is produced which is extruded
or pressed through dies to form strands which are immediately cut
by rotating cutter blades in the water box of the underwater
pelletizer. Water with or without additives is continuously flowing
through the water box to cool and solidify the polymer strands and
pellets and carry the pellets out of the water box through
transport piping to a dryer, such as a centrifugal dryer, where the
water is removed from the pellets.
[0004] For quite some time, the polymer industry has sought to
process PET polymers into a pellet shape using underwater
pelletizer systems. A major drawback of using underwater
pelletizing, as well as other pelletizing systems, for processing
PET into pellet shapes is the typically amorphous condition of
these pellets when they leave the dryer of the underwater
pelletizing system. The amorphous nature of the resulting pellet is
caused by the fast cooling of the PET material once introduced into
the water flow in the water box of underwater pelletizer and while
the water and pellet slurry is being transported by appropriate
piping to the dryer.
[0005] End users of PET polymer pellets typically require that the
pellets be in a crystalline state, rather than an amorphous state,
principally for two reasons, both relating to the fact that the end
user wants to process the PET pellets in a substantially dry
condition, with zero or near zero water content. First, PET
polymers are very hygroscopic, and crystalline PET pellets absorb
considerably less moisture during shipment and storage than
amorphous PET pellets. Accordingly, crystalline PET pellets can be
dried to the requisite zero or near zero moisture content more
easily by the end user. Second, the temperature required to
completely dry PET polymers is higher than the temperature at which
amorphous PET pellets convert to the crystalline form. Therefore,
when drying amorphous PET pellets, it is necessary to first achieve
crystallization at the requisite lower temperature before raising
the temperature to the drying temperature. Otherwise, the amorphous
PET polymer pellets may agglomerate and destroy the pellet
form.
[0006] As a result, manufacturers of PET pellets must typically
subject the amorphous PET pellets to a secondary heating step of
several hours at a very high temperatures, usually in excess of
80-100.degree. C., to change the amorphous structure of the pellets
to a crystalline structure. This is a very expensive second step in
order to convert the PET polymer pellets into the desired
crystalline state.
[0007] It is also known generally that air can be injected into the
exit stream of a water and pellet slurry from a pelletizer in order
to enhance the transport of the water/pellet slurry. See, for
example, U.S. Pat. No. 3,988,085.
SUMMARY OF THE INVENTION
[0008] In order to achieve a processed PET polymer pellet having
the desired crystallinity, the pellet must exit the underwater
pelletizing system at a temperature higher than about 135.degree.
C. At temperatures at or above this temperature, PET pellets can
self-initiate the crystallization process and ultimately provide a
crystalline character instead of an amorphous one. Therefore, in
accordance with the present invention, the underwater pelletizing
system should produce PET pellets in a hot enough condition to
self-initiate the desired crystallization. This elevated heat
condition can be accomplished by reducing the residence time of the
pellets in the water slurry in order to leave enough heat in the
PET pellets during the pelletizing and drying stages so that the
crystallization process is initiated from inside the pellets. If
desired, the pellets can be stored in a heat retaining condition,
such as in a heat insulating container, to complete the
crystallization process. For example, coated steel or plastic
containers should be acceptable, instead of the stainless steel
boxes conventionally used.
[0009] Typically, increasing the water flow through the water box
of the underwater pelletizer and increasing the water temperature,
along with pipe dimensional changes and reducing the distance
between the pelletizer and dryer unit, does not help to
sufficiently maintain the pellet temperature. Under such
circumstances, the PET pellets still leave the dryer at a
temperature, usually below 100.degree. C., which is below the
temperature (about 135.degree. C.) at which crystallization can
occur. Accordingly, it is necessary to significantly increase the
speed of pellet flow from the exit of the underwater pelletizer and
into and through the dryer.
[0010] This increased pellet speed is accomplished in accordance
with the present invention by injecting air or other suitable gas
into the transportation piping leading to the dryer just after the
cut pellets and water slurry exit the water box of the pelletizer
unit. It has been found that the injected air helps to separate the
water from the pellets in the transportation piping, significantly
speeds up the transport of the pellets to the dryer and can serve
to generate a pellet temperature exiting the dryer at greater than
about 145.degree. C. While the PET polymer pellets may rcome out of
the dryer in an amorphous condition, there is still sufficient heat
remaining inside the pellets for the crystallization process to
occur without the necessity of the second heating stage heretofore
used to make PET pellets using underwater pelletizing systems.
[0011] The air introduced into the slurry line leading to the dryer
immediately after the exit from the water tank is at a very high
velocity. It has been found that a flow rate of 100 cubic meters
(m.sup.3)/hour through a valve at a pressure of 8 bar and into a
slurry 1.5 inch pipe line produces the requisite air velocity for
the present invention. The volume of air introduced into the
exiting water and pellet slurry produces an overall gas/slurry
mixture in the nature of a mist and is likely to have a gas
component of 98%-99% or more by volume of the overall mixture. The
air injection into the slurry line dramatically increases the speed
of the pellet flow from the water box to the exit of the dryer to a
rate less than one second. While air is the preferred gas in view
of its inert nature and ready availability, other inert gases such
as nitrogen or similar gases could be used.
[0012] It has been found that crystalline PET pellets can be formed
in accordance with the method and apparatus of the present
invention, with a mean temperature of the PET pellets exiting the
dryer above about 145.degree. C., if the residence time of the
pellets from the point of formation by the cutter blades at the die
face to the exit from the centrifugal dryer is reduced by the
injection of high velocity air or other gas into the slurry line.
This shortened residence time should assure that the PET pellets
will exit the dryer of the underwater pelletizing system at a mean
temperature greater than 145.degree. C. and will retain sufficient
heat inside the pellets to initiate the desired crystallization in
the amorphous pellets, particularly if the pellets are properly
stored in a heat insulating container. Hence, the necessity of a
secondary heating step is eliminated.
[0013] Accordingly, it is an object of the present invention to
provide a method and apparatus for processing PET polymers in an
underwater pelletizing system which can produce crystallization in
PET pellets after exiting from the dryer.
[0014] It is another object of the present invention to provide a
method and apparatus for producing crystallization in PET polymer
pellets utilizing an underwater pelletizing system without the
necessity of an expensive secondary heating stage to convert
amorphous PET pellets to crystalline PET pellets.
[0015] It is a further object of the present invention to provide a
method and apparatus for underwater pelletizing PET polymer in
which the pellets are transported through the equipment at a
sufficiently rapid speed so that the mean temperature of the
pellets exiting the dryer is greater than about 145.degree. C.
[0016] A still further object of the present invention is to
provide a method and apparatus for the underwater pelletizing of
PET polymer in accordance with the preceding object in which a gas
is injected into the water and pellet slurry exiting the pelletizer
to produce a water vapor mist form of slurry handling, thereby
providing better heat retention.
[0017] It is yet another object of the present invention to provide
a method and apparatus for producing PET polymer pellets using an
underwater pelletizing system in which the pellets exiting the
dryer have sufficient heat remaining inside the pellets for
crystallization of the PET pellets to occur, a process that may
include proper conditions of storage, if necessary.
[0018] It is still a further object of the present invention to
provide an underwater pelletizing method and apparatus for
producing PET pellets in which the residence time of the PET
pellets from the time of extrusion at the die face until exit from
the centrifugal dryer is reduced to less than about 1 second by gas
injection into the slurry line from the pelletizer to the
dryer.
[0019] These together with other objects and advantages which will
become subsequently apparent reside in the details of construction
and operation of the invention as more fully hereinafter described
and claimed, reference being had to the accompanying drawings
forming a part hereof, wherein like numerals refer to like parts
throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic illustration of a conventional
underwater pelletizing system, including an underwater pelletizer
and centrifugal dryer as manufactured and sold by Gala Industries,
Inc. ("Gala") of Eagle Rock, Va., with air injection in accordance
with the present invention.
[0021] FIG. 2 illustrates certain components of the underwater
pelletizing system shown in FIG. 1 during a bypass mode when the
process line has been shut down.
[0022] FIG. 3 is a schematic illustration showing a preferred
method and apparatus for air (or gas) injection into the slurry
line from the pelletizer to the dryer in accordance with the
present invention.
DESCRIPTION OF THE INVENTION
[0023] Although only preferred embodiments of the invention are
explained in detail, it is to be understood that the invention is
not limited in its scope to the details of construction and
arrangement of components set forth in the following description or
illustrated in the drawings. The invention is capable of other
embodiments and of being practiced or carried out in various
ways.
[0024] Also, in describing the preferred embodiments, terminology
will be resorted to for the sake of clarity. It is intended that
each term contemplates its broadest meaning as understood by those
skilled in the art and includes all technical equivalents which
operate in a similar manner to accomplish a similar purpose. For
example, the term "water" includes not only water itself, but also
water with one or more additives included, which are added to the
water during the underwater pelletizing step for various purposes
used by those skilled in the art of underwater pelletizing.
[0025] An underwater pelletizing system for use in association with
the present invention is schematically shown in FIG. 1 and is
generally designated by reference number 10. The system 10 includes
an underwater pelletizer 12, such as a Gala underwater pelletizer,
with cutter hub and blades 14 shown separated from the water box 16
and die plate 18. In the underwater pelletizing system 10, PET
polymer is fed from above from a polymer vat (not shown) into a
screen changer 20 which removes any solid particles or other
material. The PET polymer is then fed through gear pump 22 to
control and maintain a smooth flow of the polymer into the polymer
diverter 24 and die plate 18. The PET polymer is typically extruded
through holes in the die plate at a temperature of about
260.degree. C. The PET polymer strands formed by the die holes
enter into the water box 16 and are cut by the cutter hub and
blades 14 into the desired pellets. Cold water flows into the water
box 16 through pipe 26 and the water and cut pellet slurry exits
through pipe 28.
[0026] The water and pellet slurry is then conveyed through the
slurry line 30 into a dryer 32, such as a Gala centrifugal dryer,
at inlet 33. The pellets are dried in the dryer 32 and exit the
dryer at 34. The water removed from the dried pellets exits the
dryer 32 through pipe 38 and is conveyed by pump 40 into a fines
removal sieve 42 and thence into a water tank 44 through pipe 46.
The recycled water leaves water tank 44 through pipe 48 and pump 50
into a water heat exchanger 52 to reduce the water temperature. The
cooled water is recycled through pipe 54 past bypass valve 56 and
pipe 58 to inlet pipe 26 and then into the water box 16.
[0027] In accordance with the present invention, air is injected
into the underwater pelletizing system in slurry line 30 at point
70, preferably near the beginning of the slurry line 30 adjacent
the exit from the water box 16, in order to enhance the transport
of PET pellets in the slurry line 30 and keep the PET pellets at a
high enough temperature to foster the desired crystallization.
[0028] The air is conveniently injected into the slurry line 30 at
point 70 using a conventional compressed air line typically
available in most manufacturing facilities, such as with a
pneumatic compressor, and a standard ball valve sufficient to
produce a high velocity air flow in the slurry line 30. This is
readily achieved by a flow rate in the range of 100 m.sup.3/hour
through a standard ball valve at a pressure of 8 bar into a slurry
line comprising a standard 1.5 inch pipe. This high velocity air
(or other gas) when contacting the water and hot pellets generates
a water vapor mist. The pellets tend to disperse to the inside
circumference of the pipe as they move rapidly therethrough to the
dryer. It is estimated that the volume of air in the overall
gas/slurry mixture is on the order of 98%-99% or more by volume of
the overall mixture. The air injected into the slurry line 30 at
point 70 increases the speed of the pellet flow from the water box
16 to the exit 34 of the dryer 32 to a rate of less than one
second.
[0029] The mean temperature of the PET polymer pellets exiting the
dryer 32 at 34 in accordance with the present invention should be
above about 145.degree. C. At this temperature, the PET pellets
will retain sufficient heat inside the pellets to initiate
crystallization therein, without the necessity of a secondary
heating step. If desired or necessary, the PET polymer pellets
exiting the dryer 32 can be placed in appropriate heat insulating
containers so that the retained heat in the PET pellets is
sufficient to complete the desired crystallization process, before
the pellets cool below the crystallization temperature.
[0030] In by-pass mode shown in FIG. 2, the recycled water goes
through bypass 56 into pipe 60 and then into slurry line 30. In the
bypass mode, the valve 62 is closed and the water/pellet slurry in
line 30 and water box 16, along with the water in inlet line 26 can
drain from the system out of drain valve 64.
[0031] FIG. 3 schematically illustrates a preferred arrangement for
air injection into the slurry line of an underwater pelletizing
system in accordance with the present invention and is generally
designated by reference numeral 100. The underwater pelletizer 102
illustrated is a Gala Model No. A5 PAC6, with water inlet pipe 104
and slurry exit line 106. The dryer 108 illustrated is a Gala Model
No. 12.2 ECLN BF, with the slurry entrance 110 at the top. Inasmuch
as the exit from the underwater pelletizer 102 into slurry line 106
is significantly below the entrance 110 to the centrifugal dryer
108, when both are level on a manufacturing floor, it is necessary
to transport the water and pellet slurry upwardly from the
pelletizer exit to the dryer entrance. The water and pellet slurry
thus moves through valve 112 past angled elbow 114, through angled
slurry line 116 past enlarged elbow 118 and then into the entrance
110 of dryer 108. The air injection is past nozzle or valve 120 and
directly into the angled elbow 114.
[0032] As shown in FIG. 3, the angled slurry line 116 is preferably
straight and has an enlarged elbow 118 at its exit end. The
enlarged elbow facilitates the transition of the high velocity
water and pellet slurry from the straight slurry line 116 into the
dryer entrance 110 and reduces potential agglomeration into the
dryer 108. Further, the air injection into the angled elbow 114 is
preferably in line with the axis of slurry line 116 to maximize the
effect of the air injection on the water and pellet slurry and to
keep constant aspiration of the air/slurry mixture.
[0033] While the angle .varies. between the vertical axis of slurry
line 116 and the longitudinal axis of angle slurry line 116 is most
preferably about 45.degree., as shown in FIG. 3, a preferred range
is 30.degree.-60.degree.. Moreover, the angle .varies. can be
varied from 0.degree. to 90.degree., and even more in the event the
water and pellet slurry exit from pelletizer 102 is higher than the
entrance 110 to dryer 108 when, for example, the pelletizer and
dryer are placed at different levels in the plant or the heights of
the components are different than shown in FIG. 3.
TRIAL EXAMPLES
[0034] Molten PET polymer was continuous extruded into an overall
underwater pelletizing system as illustrated in FIG. 1, using a
Gala Underwater Pelletizer Model No. A5 PAC 6 and a Gala Model 12.2
ECLN BF Centrifugal Dryer, in the arrangement shown in FIG. 3. The
melt temperature was about 265.degree. C. and the cutter blade
speed in pelletizer 102 was varied between 2500 and 4500 RPM. The
die plate was typical for PET polymers and a typical 3.5 mm die
plate with elongated lands was used. The melt velocity through the
die holes during the trials was constant at 40 kg/hole/hr.
[0035] The pipe for slurry line 116 was a standard 1.5 inch pipe
and its length was 4.5 meters. The speed of centrifugal dryer 108
was kept constant during the trials, and the countercurrent air
flow through the dryer 108 was also kept constant during the
trials.
[0036] The air injection flow rate to nozzle or valve 118 was
varied from 0 to a maximum of 100 m.sup.3/hour, as indicated in
Table 1 below, and the water flow and pellet size also varied,
again as indicated in Table 1 below.
[0037] The parameters and results of the trials are set forth in
Table 1 below. TABLE-US-00001 TABLE 1 Air Weight injec- Crystal-
Pellet of a Water - Water tion Pellet linity size pellet temp rate
rate temp grade Trial (mm) (g) (.degree. C.) (m.sup.3/h)
(m.sup.3/h) (.degree. C.) (%) 1 5.5 .times. 3.0 0.032 76 13 100 155
98 2 4.5 .times. 3.0 0.0299 74 13 100 152 98 3 4.5 .times. 3.0
0.0306 71 19 0 105 0 4 4.0 .times. 2.6 0.0185 64 19 100 130 60 5
3.5 .times. 3.0 0.0256 69 18 100 136 80 6 4.1 .times. 3.1 0.0267 73
18 100 146 98
The pellet temperature and percentage crystallinity as set forth in
the last two columns of Table 1 was determined by examining the
product coming out of the dryer 108 at the end of each trial.
[0038] It is believed that 135.degree. C. is the minimum
temperature for PET polymer pellets to leave the dryer, when the
pellets have the sizes used in the above tests. However, a lower
exit temperature may be possible for this invention if larger size
PET pellets are made.
[0039] While the present invention is particularly applicable to
the underwater pelletization of PET polymers, it is believed that
other polymers which crystalize at elevated temperatures and which
retain heat when subjected to high temperatures may also be
appropriate for the present invention. Such polymers include
certain grades of thermoplastic polyurethane (TPU), PET copolymers
and/or PET blends.
[0040] The foregoing is considered as illustrative only of the
principles of the invention. Since numerous modifications and
changes will readily occur to those skilled in the art, it is not
desired to limit the invention to the exact construction and
operation shown and described. Accordingly, all suitable
modifications and equivalents may be resorted to, falling within
the scope of the invention.
* * * * *