U.S. patent application number 10/954349 was filed with the patent office on 2005-05-26 for method and apparatus for making crystalline pet pellets.
Invention is credited to Eloo, Michael.
Application Number | 20050110182 10/954349 |
Document ID | / |
Family ID | 34590927 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050110182 |
Kind Code |
A1 |
Eloo, Michael |
May 26, 2005 |
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. Air is injected into the
slurry line at a velocity of from about 100 to about 175
m.sup.3/hour, or more. Such high-speed air movement forms a vapor
mist with the water and significantly increases the speed of the
pellets into and out of the dryer such that the PET polymer pellets
leave the dryer at a temperature sufficient to self-initiate
crystallization within the pellets. A valve mechanism in the slurry
line after the gas injection further regulates the pellet residence
time and a vibrating conveyor after the dryer helps the pellets to
achieve the desired level of crystallinity and to avoid
agglomeration.
Inventors: |
Eloo, Michael; (Xanten,
DE) |
Correspondence
Address: |
JACOBSON HOLMAN PLLC
400 SEVENTH STREET N.W.
SUITE 600
WASHINGTON
DC
20004
US
|
Family ID: |
34590927 |
Appl. No.: |
10/954349 |
Filed: |
October 1, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10954349 |
Oct 1, 2004 |
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10717630 |
Nov 21, 2003 |
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Current U.S.
Class: |
264/69 ; 264/142;
425/311; 425/445; 425/68; 528/480 |
Current CPC
Class: |
B29B 9/065 20130101;
B29B 13/021 20130101; B29K 2995/0039 20130101; F26B 5/08 20130101;
Y10S 425/23 20130101; B29B 2009/165 20130101; B29C 2793/0027
20130101; B29B 9/16 20130101; B29K 2067/00 20130101; B29K 2995/0041
20130101; B29C 31/00 20130101; F26B 17/00 20130101 |
Class at
Publication: |
264/069 ;
528/480; 264/142; 425/068; 425/311; 425/445 |
International
Class: |
C08F 006/00 |
Claims
What is claimed is:
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 into and out of said dryer; and passing the pellets
through a vibrating unit while said pellets retain sufficient heat
to initiate crystallization of said pellets.
2. The method as claimed in claim 1 wherein said PET pellets exit
said dryer at a mean temperature above about 135.degree. C. and
said gas is injected at a velocity of at least 100
m.sup.3/hour.
3. The method as claimed in claim 1 wherein said PET pellets exit
said dryer at a temperature above about 125.degree. C. and said gas
is injected at a velocity of about 175 m.sup.3/hour.
4. The method as claimed in claim 1 wherein said pressurized gas is
air at a velocity of between 100-175 m.sup.3/hour.
5. The method as claimed in claim 4 wherein said pressurized gas is
injected into said water stream substantially in alignment with
said water stream.
6. 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, an injector to introduce a pellet
speed expediter into said water and pellet slurry to enhance the
speed of said pellets through said processing apparatus, and a post
pelletizer unit to receive pellet output from said dryer, said
pellets exiting said dryer with sufficient internal heat to
initiate crystallization of said pellets.
7. The apparatus as claimed in claim 6, wherein the pellet speed
expediter is an inert gas moving at a velocity of between 100-175
m.sup.3/hour.
8. The apparatus as claimed in claim 6, wherein said post
pelletizer unit is a vibration unit that keeps said pellets in
movement during said crystallization.
9. The apparatus as claimed in claim 6, wherein said post
pelletizer unit is a heat insulating container.
10. The apparatus as claimed in claim 6, wherein a portion of said
slurry line is straight and angled upwardly at an angle between
30.degree. and 60.degree..
11. The apparatus as claimed in claim 7, wherein said slurry line
includes a straight portion and said gas injector introduces said
inert gas at a beginning of said straight portion, a ball valve
serving to regulate residence time of the pellets in said
apparatus.
12. The apparatus as claimed in claim 11, 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.
13. 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; transporting
said pellets using a water stream injected with a high velocity
insert gas to form a vapor mist that reduces pelletization
processing time of the pellets to a degree that said pellets retain
sufficient heat to initiate crystallization of said polymeric
material without the application of external heat; and passing said
pellets onto a vibrating conveyor where said pellets are jostled
while undergoing said crystallization.
14. The method of claim 13, further comprising the step of placing
said pellets into a heat insulating container to complete the
desired crystallization.
15. The method of claim 13, wherein said materials include PET
polymer.
16. The method of claim 13, wherein said inert gas is injected into
said pellet and water stream at a velocity above 100
m.sup.3/hour.
17. The method as claimed in claim 1, wherein said injected gas
forms a gas pellet and water vapor mist mixture which is regulated
by a valve mechanism downstream of said air injection.
18. The apparatus as claimed in claim 6, further comprising a valve
mechanism downstream of said injector to further regulate said
pellet speed.
19. 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 into and out of said dryer; and regulating the speed of
said pellets after said gas injection.
Description
RELATED APPLICATIONS
[0001] This is a continuation-in-part application of copending
application, Ser. No. 10/717,630, filed Nov. 21, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] 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 of the PET polymer
pellets in a manner to self-initiate the crystallization process of
the PET particles and produce pellets having a desired level of
crystalline structure rather than an amorphous structure.
[0004] 2. Description of the Prior Art
[0005] 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.
[0006] 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.
[0007] 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 at which crystallization can occur.
[0008] 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.
[0009] As a result, manufacturers of PET pellets must typically
subject the amorphous PET pellets to a secondary heating step of
several hours at very high temperatures, usually in excess of 80 to
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.
[0010] However, it is recognized by the end users and manufacturers
of PET pellets that total (100%) crystallinity of the PET pellets
is not necessarily required in order to dry the PET pellets for
further processing or use in the Solid State Process (SSP). Rather,
a total crystallinity, or crystallinity grade using the Calcium
Nitrate measurement method, above 30%, and preferably above 40%, is
acceptable for the PET end users.
[0011] An alternative approach is disclosed in WO 2004/033174 in
which the polymer is granulated or pelletized in a water bath at a
temperature of more than 100.degree. C. The resulting pellets may
be further treated in the water bath for a defined period of time
thereafter, while retaining the high temperature, in order to
convert the amorphous material into a crystalline material. This
system requires pressurization to maintain the water at the
super-boiling point temperature, followed by a pressure reduction
procedure.
[0012] 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
[0013] The present invention is directed to an underwater
pelletizing system that produces PET pellets in a hot enough
condition to self-initiate the crystallization process therein and
ultimately provide a sufficiently crystalline character such that
the PET pellets do not require a separate heating step in order to
undergo end user processing. It has been discovered that 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 drying stage so that the
crystallization process is initiated from inside the pellets. To do
this, it is necessary to separate the pellets from the water as
soon as possible and to significantly increase the speed of pellet
flow from the exit of the underwater pelletizer and into and
through the dryer. The hot pellets leaving the dryer can then be
carried on a conventional vibrating conveyor or other vibrating or
handling equipment for a time sufficient to achieve the desired
crystallinity and avoid agglomeration. The hot pellets can also be
stored in a heat retaining condition, such as in a heat insulating
container, to complete the desired crystallization process. For
example, coated steel or plastic containers should be acceptable,
instead of the stainless steel boxes conventionally used.
[0014] The early pellet/water separation and increased pellet speed
through the pelletizer system is accomplished in accordance with
the present invention by injecting air or other suitable gas into
the transportation piping leading from the pelletizer 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 serves
to separate the water from the pellets in the transportation piping
by converting the water to a water vapor mist, significantly speeds
up the transport of the pellets to and through the dryer, and can
serve to generate a pellet temperature exiting the dryer that is
sufficiently high to initiate the crystallization process within
the pellets. Specifically, while the PET polymer pellets may come
out of the dryer in an amorphous condition, there is still
sufficient heat remaining inside the pellets for crystallization to
occur. The extent of the crystallization is sufficient to eliminate
the necessity of the second heating stage heretofore required to
make PET pellets using previous underwater pelletizing systems.
[0015] 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 an air volume of from at least 100
cubic meters (m.sup.3)/hour, to about 175 m.sup.3/hour, or more,
through a valve at a pressure of 8 bar and into a 1.5 inch slurry
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. Other pellet speed expediting methods that
would comparably separate the liquid water from the pellets and
accelerate the pellets from the pelletizer to the dryer exit might
also be employed.
[0016] The slurry piping preferably includes a ball valve or other
valve mechanism after the air injection point. The ball valve
allows the operator to better regulate the residence time of the
pellets in the piping and dryer, and serves to significantly reduce
or eliminate any vibrations in the slurry pipe to the dryer. The
ball valve or valve mechanism also appears to provide an improved
water vapor mist condition in the slurry pipe downstream of the
valve mechanism.
[0017] It has been found that crystalline PET pellets can be formed
in accordance with the method and apparatus of the present
invention 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 sufficiently reduced by the injection of high
velocity air or other gas into the slurry line. While larger
pellets lose their heat more slowly so as to retain a high enough
temperature upon exit to undergo crystallization at lower injected
air velocities, such as 100 m.sup.3/hour, as the air velocity
increases smaller pellets with a lower exit temperatures also
exhibit acceptable levels of crystallization. Hence, the rapid
separation of the pellets from the water and the shortened
residence time assures that the PET pellets exit the dryer of the
underwater pelletizing system while retaining sufficient heat
inside the pellets to achieve the desired crystallization in the
amorphous pellets, particularly if the pellets are transported from
the dryer by a heat-retaining vibrating conveyor for a time
sufficient to achieve the desired level of crystallinity, and/or
properly stored in a heat insulating container. As a result, the
necessity of a secondary heating step is eliminated.
[0018] When transported away from the dryer in a vibrating
conveyor, it has been found that transport for a time from about 20
seconds to about 90 seconds, or more, is sufficient to achieve the
desired crystallinity. The preferred transport time is about 30
second to 60 seconds, and the most preferred is about 40
seconds.
[0019] 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
the PET pellets that exit from the dryer.
[0020] 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.
[0021] It is a further object of the present invention to provide a
method and apparatus for the underwater pelletizing of PET polymer
in which an inert 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 in the
transported pellets.
[0022] A still further object of the present invention is to
provide a method and apparatus for underwater pelletizing of PET
polymer in accordance with the preceding object in which the
pellets are rapidly transported through the equipment through the
injection of air at a velocity of at least 100 m.sup.3/hour, to
about 175 m.sup.3/hour or more, so that the residence time of the
pellets before exiting the dryer is sufficiently reduced to
generate crystallization on the order of 30%-40% of total (100%)
crystallization.
[0023] 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 at
least 35% total crystallization of the PET pellets to occur without
subsequent heating.
[0024] 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 one second by
gas injection into the slurry line from the pelletizer to the
dryer.
[0025] A still further object of the present invention is to
provide an underwater pelletizing method and apparatus for
producing PET pellets in accordance with the preceding object in
which the residence time is regulated using a valve mechanism for
improved pressurization of the water vapor mist downstream of the
valve in the slurry line.
[0026] It is another object of the present invention to provide an
underwater pelletizing system in which the hot pellets exiting the
dryer are carried on a vibrating conveyor or other vibrating or
handling equipment to achieve virtually uniform crystallization
throughout a given output pellet volume.
[0027] 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
[0028] FIG. 1 is a schematic illustration of an 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 and vibrating
conveyor in accordance with the present invention.
[0029] FIGS. 2A and 2B are schematic illustrations of side and end
views, respectively, of the vibrating conveyor of FIG. 1.
[0030] FIG. 3 illustrates certain components of the underwater
pelletizing system shown in FIG. 1 during a bypass mode when the
process line has been shut down.
[0031] FIG. 4 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.
[0032] FIG. 5 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 with a ball valve in the
slurry line, in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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 volume of air of at least 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.
[0039] 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 an air velocity of 100 m.sup.3/hour,
but may be lower when the air velocity is increased to 175
m.sup.3/hour. With such high velocity pellet speed expediting
action, the PET pellets retain sufficient heat inside the pellets
to initiate crystallization therein, without the necessity of a
secondary heating step.
[0040] Pellets exiting the dryer are preferably directed through a
vibration unit, such as vibrating conveyor 84, as shown in FIGS. 2A
and 2B. Through agitation and mixing of the crystallizing pellets
in the vibration conveyor 84, variations in the temperatures of
pellets which might otherwise occur through proximity of individual
pellets to a containment wall versus immersion amongst other
pellets, for instance, are avoided. Instead, uniformity in
temperature and in the resulting degree of crystallization is
greatly improved. In addition, stickiness resulting from the
elevated pellet temperatures is countered through the jostling and
relative movement of the pellets which prevents any clumping or
adherence of the pellets to the surrounding wall structure.
[0041] For crystallization purposes, it is has been found that the
pellets should remain in the vibration conveyor between about 20
and about 90 seconds, or more, preferably between about 30 and
about 60 seconds, and most preferably about 40 seconds. During this
time, sufficient heat is retained by the vibration conveyor to
maintain the pellets at a high enough temperature to complete the
desired crystallization. Larger pellets having an exit temperature
on the order of 145.degree. due to their greater mass may require
only 10 seconds at that temperature within which to achieve 40%
crystallization. With their smaller mass and relatively greater
surface area, smaller pellets having a cooler exit temperature of
about 127.degree. C. may require 20 seconds at that temperature to
complete the desired crystallization. The remaining time in the
vibrating conveyor allows the pellets to cool to a greater or
lesser extent.
[0042] If additional cooling is required due, for example, to the
operator's inability to store, use or transport heated pellets from
the exit of the vibration conveyor, then air blowers may be added
at such exit or the vibration conveyor may be designed to provide a
residence time of up to approximately two minutes. Generally, the
temperature of the pellets is about 128.degree. C. at the entrance
to the vibration conveyor, and between 60.degree. C. and
110.degree. C. at the exit thereof, depending upon whether or not
the operator has provided for additional pneumatic cooling directly
on the conveyor in order to output pellets that are fully cooled
for handling purposes (60.degree. C.) or instead requires only that
the pellets be crystalline (110.degree. C.) upon leaving the
vibrating conveyor. The preferred exit temperature for most
purposes is less than 80.degree. C., while a higher surface tack
temperature (<100.degree. C.) is sufficient for some grades of
PET polymer.
[0043] If a vibrating unit is not used, or in addition to the
vibrating unit, 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.
[0044] In by-pass mode shown in FIG. 3, 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.
[0045] FIG. 4 schematically illustrates one 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 PAC 6, 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.
[0046] As shown in FIG. 4, 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.
[0047] While the angle 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. 4, a preferred range
is 30.degree.-60.degree.. Moreover, the angle 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. 4.
[0048] With the air injection as described, the residence time of
the pellets from the water box to the exit is less than one second
which has been found to produce pellets with the desired
crystallization. However, in another preferred embodiment, a second
ball valve or valve mechanism 150 is positioned after the air
injection point, as shown in FIG. 5. The valve mechanism 150 serves
to better regulate the residence time of the pellets within the
slurry line while retaining sufficient head pressure on the cutting
chamber. This second valve mechanism not only provides for
regulating the residence time of the pellets in the slurry line but
also reduces vibration in the slurry pipe significantly. In
addition, the resulting pressurization of the air injected chamber
seems to improve the water vapor mist generated in the slurry pipe
downstream, enhancing the results obtained with smaller pellets in
particular.
TRIAL EXAMPLES
[0049] First Trial Set
[0050] Molten PET polymer was continuously 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.
[0051] 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. A vibrating unit was not used.
[0052] The air injection flow rate to nozzle or valve 120 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.
[0053] The parameters and results of the first set of trials are
set forth in Table 1 below.
1TABLE 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
[0054] The pellet temperature and percentage crystallinity as set
forth in the last two columns of Table 1 were determined by
examining the product coming out of the dryer 108 at the end of
each trial. Specifically, when the pellets were visually inspected
it was determined approximately how many of 100 pellets had
undergone a color change indicating transformation to a more
crystalline state. For example, in trial 5, about 80 out of 100
pellets indicated a color change. Temperature of the pellets was
also determined on a surface basis using an infrared temperature
gauge. The extent to which the pellets may have been "totally"
crystallized, with "total" crystallization indicating a state in
which each pellet is fully crystalline throughout its individual
structure, could not be determined using these external measuring
techniques. However, for practical application the pellets were
found to be sufficiently crystallized for the purposes of PET end
users, effectively demonstrating at least 30-40% crystallization
during subsequent testing, with no need for any additional
heating/crystallizing processing.
[0055] At an air injection velocity of 100 m.sup.3/hour, it is
preferred that 135.degree. C. be the minimum temperature for PET
polymer pellets to leave the dryer, when the pellets have the sizes
used in the above tests. However, adequate crystallization at lower
exit temperatures may be obtained with this invention if smaller
size PET pellets are made, provided the air injection velocity is
increased.
[0056] Second Trial Set
[0057] Molten PET polymer was continuously 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 plates used were typical for PET polymers. In order to be able
to work with different pellet sizes, die hole diameters and die
hole velocities were varied as well as cutter speeds.
[0058] 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. A vibrating conveyor 84 was used to receive the pellets
exiting the dryer.
[0059] The air injection flow rate to nozzle or valve 120 was
varied from 0 to a maximum of 175 m.sup.3/hour, as indicated in
Table 2 below, and the water flow and pellet size also varied,
again as indicated in Table 2 below.
[0060] The parameters and results of the second set of trials are
set forth in Table 2 below.
2TABLE 2 Weight Air Amount of of a Water - Water injection Pellet A
- C pellets [%] Pellet pellet temp rate rate temp A = amorphous
Crystallinity Sample size (mm) (g) (.degree. C.) (m.sup.3/h)
(m.sup.3/h) (.degree. C.) C = crystalline grade (%) 10 3.5 .times.
2.6 0.015 77 20 175 147 100% C 43.1 11 2.5 .times. 3.5 0.015 78 22
0 107 10% C 6.9-30.9 11 3.5 .times. 2.5 0.015 78 22 0 107 90% A 3.5
12 2.7 .times. 2.7 0.015 78 17 175 129 100% C 43.9 13 2.4 .times.
3.0 0.015 78 24 0 109 12% C 10.8-35.6 13 2.4 .times. 3.0 0.015 78
24 0 109 88% A 3.7 14 2.6 .times. 3.1 0.012 78 22 175 128 100% C
44.1 15 2.6 .times. 3.1 0.012 78 25 0 95 100% A 3.3 16 2.0 .times.
2.7 0.011 72 20 175 123 100% C 38.9 17 2.4 .times. 2.4 0.010 75 25
175 117 100% C 43.0 18 2.2 .times. 2.2 0.008 79 24 175 116 98% C
38.9
[0061] Samples 10 and 11 were run under the same conditions except
that Sample 10 was conducted with air injection at a rate of 175
m.sup.3/hour and Sample 11 was conducted without any air injection.
Similarly, Samples 12 and 13, and Samples 14 and 15, were conducted
on the same conditions with respect to each pair, with the
exception of the air injection. Samples 16, 17 and 18 had no
corresponding tests in the absence of air because the pellet size
was too small for effective processing without air injection.
[0062] From the results in the second trial set, it can clearly be
seen that the air injection method is essential to maintain a
crystalline pellet, specifically when trying to achieve pellet
weights below 0.015 g/pellet which, in the majority of cases, is
the customer target. As compared to the first trial set which, when
summarized in copending application, Ser. No. 10/717,630
incorporated herein concluded that a minimum exit temperature was
required, the results of the second trial set have clarified the
significance of the air injection velocity to achieving the desired
crystallinity.
[0063] The pellet temperature and percentage crystallinity as set
forth in the second and third to right-most columns of Table 2 were
determined by visual examination and using an infrared temperature
gauge, both as described above in connection with the first trial
set. Subsequent to the time at which the first trial set was
conducted, however, it was determined that total crystallinity, or
crystallinity grade, can be measured using the Calcium Nitrate
measurement method. The right-most column shows the results of such
an evaluation.
[0064] With the air injection method according to the present
invention, PET pellets of various sizes can be produced with an
acceptable crystallinity grade. This is even possible with pellet
weights as low as 0.008 g/pellet provided the air is injected at a
high enough velocity. By contrast, using prior art operating
devices for pelletizing technology including those using extremely
short pipe runs and very high water flows, only a certain
percentage, approximately 10-12%, of crystalline pellets can be
produced. These so-produced pellets, however, contain significant
variation in crystallinity from about 6.9% to up to 35.6%. This
limited degree of homogeneity within the pellets is not acceptable.
Furthermore, if the pellet size is reduced to 0.012 g/pellet or
below, only by the air injection method of the present invention
was it possible to produce a yield in which 100% of the pellets
were crystallized at least to a 35% grade of crystallinity. PET
with a crystallinity percentage of greater than 35% has been found
to be crystalline enough for the Solid State Process (SSP) and
therefore is acceptable for the PET end users.
[0065] As summarized above, the first and second trial sets were
conducted with air flow velocities of 100 m.sup.3/hour and 175
m.sup.3/hour, respectively. Higher air velocities on the order of
200 m.sup.3/hour or higher can also be used, as required by water
flow and pellet rate changes.
[0066] While the present invention is particularly applicable to
the underwater pelletization of PET polymers, it is believed that
other polymers which crystallize 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.
[0067] 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.
* * * * *