U.S. patent application number 10/356330 was filed with the patent office on 2004-06-17 for method and device for producing spherical particles from a polymer melt.
This patent application is currently assigned to Buhler AG. Invention is credited to Geier, Rudolf, Jurgens, Theodor.
Application Number | 20040113300 10/356330 |
Document ID | / |
Family ID | 7654229 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040113300 |
Kind Code |
A1 |
Jurgens, Theodor ; et
al. |
June 17, 2004 |
Method and device for producing spherical particles from a polymer
melt
Abstract
The invention relates to a method and a device for the
production of spherical particles, whereby a molten prepolymer or
precondensate is transformed into droplets by means of a drip
nozzle, the droplets are subjected to a countercurrent with a gas
in a precipitation column until at least partial crystallization is
achieved and are then subjected to an additional
post-crystallization phase. In order to economically produce higher
quality particles at a high flow rate, the molten prepolymer is
transformed into droplets by means of a vibrating nozzle plate
and/or direct vibration of the molten prepolymer or polymer and
resulting droplets are subjected to an air and gas
countercurrent.
Inventors: |
Jurgens, Theodor;
(Catrop-Rauxel, DE) ; Geier, Rudolf; (Essen,
DE) |
Correspondence
Address: |
DARBY & DARBY P.C.
Post Office Box 5257
New York
NY
10150-5257
US
|
Assignee: |
Buhler AG
Uzwil
CH
|
Family ID: |
7654229 |
Appl. No.: |
10/356330 |
Filed: |
January 31, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10356330 |
Jan 31, 2003 |
|
|
|
PCT/EP01/00518 |
Jan 18, 2001 |
|
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Current U.S.
Class: |
264/9 ; 264/12;
425/7 |
Current CPC
Class: |
B29B 2009/165 20130101;
B29B 2009/166 20130101; B29B 9/10 20130101; B29B 9/16 20130101;
B29K 2067/006 20130101; B01J 2/04 20130101; B29C 2791/008 20130101;
B29K 2067/00 20130101 |
Class at
Publication: |
264/009 ;
264/012; 425/007 |
International
Class: |
B29B 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2000 |
DE |
100 42 476.7 |
Claims
What is claimed:
1. A method of producing spherical particles from a polymer melt,
particularly made of polyfunctional carboxylic acids and alcohols,
such as PET or PBT pellets, a molten prepolymer and/or
precondensate and/or non-stringy polymer being dripped into
droplets using a drip nozzle, the droplets having a gas applied to
them in counterstream in a fall tower for at least partial
crystallization and then being transported to a further
post-crystallization stage, characterized in that the molten
polymer is dripped using a nozzle plate set into vibration and/or
through direct vibration excitation of the molten prepolymer and/or
polymer and the droplets thus formed have air applied to them as
the gas in counterflow, the air being supplied to the fall tower at
a temperature T.sub.1, which lies above the glass transition point
of the prepolymer and/or polymer melt to be dripped and/or the air
being supplied to the fall tower at a temperature such that the air
is heated to at most a temperature T.sub.2.ltoreq.210.degree. C.
through thermal transfer from the droplets.
2. The method according to claim 1, characterized in that the air
is supplied to the fall tower at a temperature T.sub.1 of
T.sub.1.ltoreq.160.degree., particularly
T.sub.1.ltoreq.120.degree..
3. The method according to claim 1 or 2, characterized in that, for
the production of PET pellets, the air is supplied to the fall
tower at a temperature such that the air is heated to at most a
temperature T.sub.2.ltoreq.160.degree. C. through thermal transfer
from the droplets.
4. The method according to claim 1 or 2, characterized in that, for
the production of PBT pellets, the air is supplied to the fall
tower at a temperature such that the air is heated to at most a
temperature T.sub.2.ltoreq.140.degree. C. through thermal transfer
from the droplets.
5. The method according to claim 1, characterized in that, for the
production of the prepolymer and/or precondensate, a catalyst based
on titanium oxide, which increases the polycondensation speed, is
added.
6. The method according to claim 1 or 2, characterized in that the
air is supplied to the fall tower in its lower region, particularly
in the floor region.
7. The method according to claim 1 or 2, characterized in that the
air flows against the droplets in the lower region of the fall
tower at a higher speed than in the upper region.
8. The method according to claim 1 or 2 characterized in that the
fall tower is supplied air having a dew point temperature T.sub.t
of particularly -10.degree. C..ltoreq.T.sub.t.ltoreq.-40.degree.
C.
9. The method according to claim 1 or 2, characterized in that the
air is guided in a first loop, which includes the fall tower, and a
part of the air is removed from the first loop and supplied to a
second loop, in which reaction substances such as ethylene glycol
and/or butane diol, oligomers, and/or water are removed.
10. The method according to claim 9, characterized in that the
second loop is a spray loop, in which ethylene glycol and/or butane
diol is sprayed, reaction substances condensed out in the second
loop being supplied to an esterification or reesterification stage
positioned upstream from the nozzle plate.
11. The method according to claim 10, characterized in that the air
removed from the first and/or the second loop, having reaction
products contained therein which are not condensable in the second
loop, such as acetaldehyde and/or tetrahydrofuran, is supplied to a
heat transfer facility.
12. The method according to claim 1 or 2, characterized in that the
at least partially crystallized droplets are removed from the fall
tower in the floor region of the fall tower via a slanted surface
having openings which dry air flows through.
13. The method according to claim 12, characterized in that the
droplets, which are at least partially crystallized into the
spheres, are transported floating and/or oscillating along the
slanted surface and/or at least in the region of the openings
present therein.
14. The method according to claim 12, characterized in that the
droplets, which are at least partially crystallized into the
spheres, are classified after leaving the slanted surface.
15. The method according to claim 12, characterized in that the
droplets, which are at least partially crystallized into the
spheres, reach an oversize separator from the slanted surface and
particles separated there are supplied to a precondensation stage
for the prepolymer.
16. The method according to claim 15, characterized in that the
particles are supplied to a precrystallization stage, which is
positioned in a third loop which dry air flows through, after
passing through the oversize separator.
17. The method according to claim 16, characterized in that a
portion is supplied to the second loop from the third loop, which
includes the crystallization stage.
18. A facility for producing spherical particles from polymer,
particularly made of polyfunctional carboxylic acids and alcohols,
particularly for producing PET or PBT pellets, including at least
one nozzle device, which drips a molten prepolymer and/or polymer,
a fall tower (20), positioned downstream thereto, which is
positioned in a gas loop (40) via at least one gas intake opening
at the floor and at least one outlet opening at the nozzle device,
a transport device, positioned downstream from the fall tower, for
the spherical particles at least precrystallized in the fall tower,
and a crystallization stage, positioned downstream from the
transport device, characterized in that the facility has a
vibration exciter and a nozzle plate having nozzles which are
distributed on an area, and the fall tower is positioned in the
loop which guides the air and has a cross-section which is at least
twice as large as the area.
19. The facility according to claim 18, characterized in that the
nozzles of the nozzle plate are distributed on an area which
particularly corresponds to 1/4 or 1/3 of the cross-section of the
fall tower.
20. The facility according to claim 18, characterized in that the
nozzles are distributed on a circular area having a diameter
D.sub.d, the fall tower has a circular cross-section having a
diameter D.sub.f, and 1.5 D.sub.d.ltoreq.D.sub.f, particularly 2.0
D.sub.d.ltoreq.D.sub.f.
21. The facility according to claim 18, characterized in that the
nozzle plate and/or the molten prepolymer and/or polymer may be
excited to vibration directly via the vibration exciter.
22. The facility according to claim 19, characterized in that the
diameter of the area formed by the nozzles of the nozzle plate has
a ratio to the diameter of the fall tower like D.sub.d:D.sub.f of
approximately 1:2 to 1:10, particularly 1:5.
23. The facility according to claim 18, characterized in that at
least one device which increases the air intake speed is positioned
in the fall tower in the region of its air intake opening.
24. The facility according to claim 18, characterized in that the
air outlet opening runs at a distance to the nozzle plate such that
the particles dripped from the nozzle plate are subjected to an
essentially laminar airflow directly after exiting the nozzle
plate.
25. The facility according to claim 18, characterized in that the
transport device positioned in the floor region of the fall tower
has a slanted surface, like a sieve or perforated plate, having
openings which are permeated by dry air in such a way that the
particles may be moved floating and/or oscillating along the
surface.
26. The facility according to claim 25, characterized in that the
slanted surface (30) runs at a distance to the floor of the fall
tower (20), and a first air intake opening is provided between the
floor and the slanted surface and a second air intake opening of
the air loop is provided above the slanted surface.
27. The facility according to claim 25, characterized in that the
slanted surface has an oversize separator positioned downstream,
from which a crystallization stage operated using dry air is
positioned downstream.
28. The facility according to claim 27, characterized in that a
line which guides oversize particles separated by the oversize
separator leads to a precondensation stage positioned upstream from
the nozzle plate.
29. The facility according to claim 18, characterized in that a
part of the air from the first loop, which includes the fall tower,
may be supplied via a line to a purification stage forming a second
loop.
30. The facility according to claim 29, characterized in that the
purification stage, a spray loop as the second loop, is connected
via a line to an esterification stage positioned upstream from the
nozzle plate.
31. The facility according to claim 29, characterized in that a
portion of the air having non-condensable substances may be
supplied from the second loop to a heat transfer device via a
line.
32. The facility according to claim 29, characterized in that the
crystallization stage is positioned in a third loop, which dry air
flows through, connected to the second loop.
33. The facility according to claim 29, characterized in that the
second loop is connected via a line to the air intake openings or
directly to the fall tower above the air intake opening,
particularly above the device.
34. The method according to claim 9, characterized in that the air
removed from the first and/or the second loop, having reaction
products contained therein which are not condensable in the second
loop, such as acetaldehyde and/or tetrahydrofuran, is supplied to a
heat transfer facility.
Description
[0001] This patent application is a continuation of International
application PCT/EP01/00518 filed Jan. 18, 2001, which designated
the United States of America and was published in the German
language, on Mar. 7, 2002, under Publication No. WO02/18113.
[0002] The present invention relates to a method of producing
spherical particles from a prepolymer and/or polymer melt,
particularly made of polyfunctional carboxylic acids and alcohols,
such as PET or PBT particles, the polymer melt being dripped into
droplets using a drip nozzle, the droplets having a gas applied to
them in counterflow in a fall tower for at least partial
crystallization, and the droplets then preferably being transported
to a polycondensation stage. Furthermore, the present invention
relates to a device for producing spherical particles from a
prepolymer and/or polymer, particularly made of polyfunctional
carboxylic acids and alcohols, such as PET or PBT particles,
including at least one nozzle device which drips molten prepolymer
and/or polymer, a fall tower downstream thereto, which is
positioned in a gas loop via at least one gas intake opening at the
floor and at least one gas outlet opening at the nozzle device, a
transport device positioned in the fall tower for particles which
are at least precrystallized in the fall tower, and a
crystallization stage downstream from the transport device.
[0003] To produce PET granulate, supplying a precondensate, after
the esterification and/or reesterification and pre-polycondensation
of ethylene glycol and/or butane diol in the PBT process, and
terephthalic acid to a reactor which has a partial vacuum applied
to it is known. In this way, the viscosity of the largely liquid
and short-chain polymer is increased, and liberated ethylene glycol
and/or butane diol is returned to the esterification and/or
reesterification. After the reactor treatment, the polycondensate
is cooled in water and cut into granulate, in order to obtain
cylindrical pellets which are largely amorphous. However, there is
the disadvantage that the ends have projections which break off and
may therefore lead to the production of dust. This known method
also has the disadvantage that the pellets are in a largely
amorphous state after their granulation, which requires a partial
crystallization in a subsequent separate treatment stage.
Furthermore, the outlay for the facility and energy is a problem,
since separate treatment stages, such as a reactor stage having a
partial vacuum applied to it and partial crystallization, are
necessary.
[0004] In order to avoid these disadvantages, it is suggested in
German Patent Application 198 49 485 Al that molten precondensate
be supplied to a fall tower having a distributor drip nozzle in the
head region, the precondensate emitted from the distributor drip
nozzle being subjected in the fall tower to an inert gas such as
nitrogen in counterflow. In this way, the falling speed is reduced,
with simultaneous acceleration of crystallization of the droplets.
The particles arising on the floor of the fall tower may then be
supplied as dried and partially crystallized pellets to
polycondensation and/or SSP.
[0005] In order to produce spheres of uniform geometry made of
plastic, it is suggested according to German Patent 43 38 212 C2
that plastic in a molten consistency be dripped from a nozzle
device by exciting vibrations, the droplets produced in this way
being cooled in a liquid.
[0006] The present invention is based on the problem of refining a
method and the device of the type initially cited in such a way
that spheres, made of polymers, having a desired size and uniform
geometry may be produced at a large scale. Simultaneously, the
production of the particles is to be energetically more favorable
and have a simpler facility and therefore be more cost-effective.
Furthermore, more rapid melting of the spheres is to be
possible.
[0007] According to the present invention, the problem is solved by
a method of the type initially cited essentially in that the
prepolymer and/or polymer melt is dripped, by a nozzle plate
excited into vibration and/or by exciting vibrations of the
prepolymer and/or polymer melt itself, into droplets, which have
air applied to them as the gas in counterflow, the air being
supplied to the fall tower at a temperature such that the air is
heated to at most a temperature T.sub.1 of
T.sub.1.ltoreq.160.degree. C. through heat transfer from the
droplets. The air is particularly supplied at a temperature T.sub.2
of T.sub.2.ltoreq.150.degree. C., particularly
T.sub.2.ltoreq.110.degree..
[0008] For the production of PBT spheres, the air is to be heated
to at most a temperature T.sub.1 of T.sub.1.ltoreq.140.degree. C.
In this case, air is supplied at a temperature T.sub.2 of
T.sub.2.ltoreq.130.degree. C., particularly
T.sub.2.ltoreq.80.degree. C. However, the air is preferably
supplied to the fall tower at a temperature T.sub.1 which lies
above the glass transition point of the polymer to be dripped.
[0009] Furthermore, the air is introduced in the lower region of
the fall tower, particularly in the floor region, in such a way
that air flows against the droplets in the lower region of the fall
tower at a higher speed than in the upper region.
[0010] Independently of this, the air intake temperature is to be
set in such a way that oxidative damage of the dripped polymer is
avoided and sufficient solidification and/or precrystallization is
provided.
[0011] In order that the air entering the fall tower may absorb
reaction substances such as ethylene glycol and/or butane diol or
water to a sufficient extent, the air is to have a low dew point
upon entering the fall tower, preferably in the range between
-10.degree. C. and -40.degree. C.
[0012] In order to be able to separate and reuse reaction products
dissolved in the air flowing through the fall tower, a refinement
of the present invention provides that a portion--approximately
10%-30%--of the air flowing through the loop is removed and
supplied to a spray loop, in which the reaction products are
removed. In particular, fresh and cold ethylene glycol and/or
butane diol are sprayed in the purification loop, through which
reaction substances such as ethylene glycol and/or butane diol,
oligomers, and water, which are diffused in the dry air, condense
out of the air loop and may be reused as valuable raw materials,
for example, the ethylene glycol and/or butane diol for
esterification for the process using TPA and/or for
reesterification for a process using DMT. The air purified in this
way retains its low dew point and may again be supplied to the loop
flowing through the fall tower.
[0013] To remove acetaldehyde and/or THF (tetrahydrofuran) in the
PBT process, which may not be removed in a corresponding ethylene
glycol and/or butane diol spray loop, a quantity of the intake air
charged with acetaldehyde is mixed into a heat transfer facility
such as a furnace and thus combusted. The combusted quantity of air
is constantly, particularly continuously, replaced by an equal
quantity of air. In this way, the need for combustibles such as
heating gas and/or oil is reduced.
[0014] A downstream precrystallization stage, which is particularly
important in the processing of comonomers and which is also
operated using dry air, may also be included in the purification
loop.
[0015] Since the polymer melt, which is not stringy, and/or the
precondensate is dripped by a nozzle plate set into vibration,
uniform and identically sized and/or identically shaped droplets
result. These droplets first fall through a region of the fall
tower in which there is an essentially laminar flow. Therefore,
sufficient external solidification of the droplets may occur, so
that the danger of collision is minimized, through which otherwise
agglutination of droplets would occur.
[0016] Instead of excitation of vibration of the nozzle plate, or
as a supplement thereto, the prepolymer and/or polymer melt may be
excited into vibration and dripped using a vibration generator, for
example.
[0017] Furthermore, the fall tower has a cross-section,
particularly a diameter, which is significantly larger than the
nozzle plate in regard to an area, essentially the circular area on
which the outlet openings for dripping the prepolymer and/or
polymer melt are positioned. Furthermore, the inner wall of the
fall tower is to be made of a material and/or be coated with a
material which prevents and/or hinders adhesion of droplets.
Teflon.RTM. is an example of a suitable material.
[0018] In order to additionally increase the dwell time of the
droplets, which are formed in a spherical shape, an increase of the
air speed is caused by baffles in the fall tower. The baffles lead
to a change in cross-section of the fall tower and therefore to a
corresponding change of the air speed.
[0019] At the floor of the fall tower, the particles are guided via
a surface, which has openings, to a separating device such as an
oversize separator, in which possible agglomerates are sorted out
and supplied to the starting melt and/or its pre-products. Since
the corresponding agglomerated particles still have a slight
viscosity, rapid and good dissolving in a precondensation stage is
possible.
[0020] It is to be noted in regard to the surface leading to the
oversize separator, which may be implemented as a sieve or a
perforated metal sheet or as a wind sifter, among other things,
that hot air flows through it, the air speed being selected in such
a way that the particles float and oscillate over the surface
and/or its openings. This prevents the particles from being able to
agglutinate. In addition, the dwell time, during which the
particles have air applied to them, is increased.
[0021] After the oversize separator, the particles may be supplied
to a crystallizer, which is also operated using dry air guided in a
loop. Reaction substances enriched in the air may then be separated
in a spray loop in the way previously described and/or
non-separable substances may be supplied to a heat transfer
device.
[0022] A facility for producing spherical particles from a polymer
melt, particularly made of polyfunctional carboxylic acids and
alcohols, such as PET or PBT particles, of the type initially
described is distinguished in that the nozzle device has a nozzle
plate set into vibration and/or a nozzle plate having a vibration
generator acting directly on the melt, with nozzles which are
distributed on a circular area having a diameter D.sub.d, and the
fall tower is positioned in the loop which guides the air and has a
diameter D.sub.f which is at least twice as large as the diameter
D.sub.d. The ratio of the diameter of the active area of the nozzle
plate to the fall tower is particularly 1:2 to 1:10, particularly
approximately 1:5.5. Furthermore, the fall tower is lined on the
inside using an anti-adhesive material or has such a material. The
material is particularly Teflon.RTM..
[0023] In order to adjust the speed of the air which flows through
the fall tower in counterflow to the falling direction of the
droplets using constructively simple means, a refinement of the
present invention provides that the fall tower has baffles which
change the cross-section of the fall tower in the region of the air
intake opening. These baffles may be, for example, conical or
pyramidal stumps, which are coated on the outside with Teflon.RTM.
or another suitable material which prevents adhesion.
[0024] The air outlet opening itself is positioned at a distance to
the nozzle plate such that the particles dripped from the nozzle
plate are subjected to an essentially laminar air flow directly
after they exit the nozzle plate.
[0025] In the floor region of the fall tower, a slanted surface,
such as a sieve or perforated plate, which has openings, is
provided, which has dry air flowing through it in such a way that
the particles may be moved floating and/or oscillating along the
surface, at least in the region of the openings. The surface having
the openings itself leads to an oversize separator, to which a
crystallization stage operated using dry air is connected
downstream.
[0026] Particle agglomerates separated in the oversize separator
may be resupplied to the process via a line leading to the
precondensation stage upstream from the nozzle plate.
[0027] Furthermore, the device includes a purification stage having
a spray loop, which is connected to the first air loop, which
includes the fall tower, and/or a second air loop, which includes
the crystallization stage. Furthermore, connections originate from
the purification stage to one of the esterification and/or
reesterification stages before the precondensation stage and to a
combustion device.
[0028] Further details, advantages, and features of the present
invention result not only from the claims and the features to be
drawn therefrom--alone and/or in combination--but also from a
preferred exemplary embodiment to be drawn from the following
description of the drawing.
[0029] Although the teaching according to the present invention is
particularly intended for the dripping of polyester, there is to be
no restriction of the teaching according to the present invention
therefrom. Rather, it is generally applicable for polymers.
Preferred materials may be drawn from U.S. Pat. No. 5,633,018, to
whose disclosure reference is expressly made here.
[0030] The teaching according to the present invention is
particularly also applicable for producing
[0031] PET using esterification of ethylene glycol and PTA,
[0032] PET using reesterification of ethylene glycol and DMT
(dimethyl terephthalate),
[0033] PBT using esterification of butane diol and PTA,
[0034] PBT using reesterification of butane diol and DMT.
[0035] Furthermore, there is the possibility of adding a catalyst
in the form of the compound based on titanium oxide to the starting
product, in order to increase the polycondensation speed, without
having to simultaneously accept that the pellets produced will have
an undesired yellow color. This is because the production process
according to the present invention occurs at relatively low
temperatures in comparison to the related art.
[0036] In the single figure, a facility layout for producing
spherical particles from a polymer, particularly made of
polyfunctional carboxylic acids and alcohols, particularly for
producing spherical PET (polyethylene terephthalate) pellets is
illustrated, purely as an example. In order to produce spherical
pellets, from a paste preparation stage 10, an esterification stage
12 for terephthalic acid and ethylene glycol, and a subsequent
pre-polycondensation stage 14, which has a partial vacuum applied
to it, a polyester precondensate having a product temperature of
approximately 260.degree.-280.degree. C. and an intrinsic viscosity
of 0.1-0.4 is supplied via a heat exchanger 15 and a filter 16 to a
nozzle plate 18, via which the well-filtered precondensate is
dripped. If PBT pellets are produced, the polyester condensate has
a product temperature between 210.degree. C. and 240.degree. C. and
an intrinsic viscosity from 0.3 to 0.6.
[0037] The nozzle plate 18 may be set into vibration and
particularly has outlet openings arranged in concentric circles,
which have an area having a diameter D.sub.d of, for example, 300
mm. The nozzle plate 18 having the openings and/or nozzles may be
inserted elastically in a holder, the nozzle plate itself being
connected to a vibration exciter. The vibration exciter, which is
to be an electromagnetic vibration exciter, is based on a
load-bearing structure, in order to be able to vibrate the nozzle
plate. Frequencies at which the nozzle plate may be set into
vibration may lie in the range between 200 and 2000 Hz. The
diameter of the openings and/or nozzles is to lie in the range
between 0.2 and 0.8 mm. Furthermore, the polyester precondensate is
to be supplied to the nozzle plate 18 at an overpressure of, for
temple, 0.2 to 1.0 bar. The nozzle plate 18 is also uniformly
heated, a temperature in the magnitude between 220 and
250.degree.--for PBT between 190.degree. C. and 220.degree.
C.--particularly being selected.
[0038] As an alternative or supplement, the melt may be excited to
vibration using a vibration exciter for dripping.
[0039] By setting the nozzle plate 18 into vibration, it is ensured
that the molten prepolymer is uniformly dripped in identically
large and identically shaped particles and a fall tower 20, which
is equivalent to a Prill tower. The length of the fall tower 20 may
lie in the range between 10 and 30 meters, particularly in the
range of 20 meters. Of course, tower heights of more than 30 meters
are also technically possible. At a diameter D.sub.d of the active
surface of the nozzle plate 18 of approximately 300 mm, the fall
tower 20 is to have a diameter of 1600 mm. Furthermore, the fall
tower 20 is to be lined on the inside with an anti-adhesive agent,
particularly Teflon.RTM., and/or be made of this material, in order
to ensure that droplets leaving the nozzle plate 18 are not able to
adhere.
[0040] Through the vibration excitation of the nozzle plate 18
and/or direct vibration excitation of the melt and the uniform
distribution of the nozzles on the circular area, it is ensured
that the droplets fall without colliding in the fall tower 20 via a
path in which hardening of the surface of the droplets occurs to an
extent such that agglomeration of droplets is prevented.
Simultaneously, a spherical shape results due to the cohesion
forces.
[0041] Furthermore, to avoid collisions, it is provided that the
droplets in the fall tower 20 fall in an essentially laminar
portion of an air flow, which runs in counterflow to the falling
direction of the droplets, directly after leaving the nozzle plate
18. This air counterflow is used for further solidification of the
spheres and their precrystallization, the flow speed of the
particles which are falling and/or floating downward being adjusted
as a function of their diameter.
[0042] There are air intake openings 22, 24 in the floor region of
the fall tower 20 and, at a distance to the nozzle plate 18, an air
outlet opening 27 to generate the counterflow.
[0043] Furthermore, there are baffles 26, of conical or conical
stump geometry, for example, which change the cross-section,
located in the floor region of the fall tower 20, through which the
flow speed in the floor region of the fall tower 20 is increased in
comparison to the head region, with the consequence that the dwell
time of the droplets reaching the floor region, which are
precrystallized and/or prehardened, is increased. Through the
baffles 26, the airspeed in the floor region may be set to a speed
between 3 and 7 m per second. The baffles 26 themselves are to at
least have an anti-adhesive material such as Teflon.RTM. on the
outside or be made of such a material.
[0044] Furthermore, the air flowing in via the air intake openings
22, 24 in the floor region, which flows against the falling
particles, has a starting temperature between 80.degree. C. and
16.degree. C.--for PBT between 60.degree. C. and 120.degree.
C.--the air temperature in the intake to lie above the glass
transition point of the precondensate (approximately 70.degree.
C.-80.degree. C. for PET and 35.degree. C.-50.degree. C. for PBT).
A temperature of 160.degree. C. for PET and/or 120.degree. C. for
PBT is not, however, to be exceeded, in order to avoid oxidative
damage to the particles, adequate solidification and/or
precrystallization nonetheless to be ensured simultaneously. The
entering air is also to have a low dew point upon entering the fall
tower 20, preferably between -10.degree. C. and -40.degree. C., for
absorbing ethylene glycol, water, etc.
[0045] A slanted surface 30, in the form of a sieve or a perforated
metal sheet, for example, which has passages 28, runs in the floor
region of the fall tower 20. One of the air intake openings, in the
exemplary embodiment the air intake opening 24, discharges into the
space between the floor 32 of the fall tower 20 and the slanted
surface 30. The speed of the dry air 24 flowing through the
openings 28 is selected so that the particles reaching the floor 30
float and/or oscillate at least in the region of the openings 28.
These measures also hinder agglutination of particles.
Simultaneously, the dwell time of the particles in the fall tower
20 through which the air flows is increased.
[0046] Via the slanted surface 30, which is used as a
quasi-transport device, the particles and/or pellets reach an
oversize separator 34, through which the agglomerates are separated
from the particles, in order to be resupplied to the
precondensation stage 14 via a line 36. Due to the slight viscosity
which they still have, possibly occurring agglomerates may be
dissolved without problems in the precondensation stage 14 and may
thus be resupplied to the process.
[0047] The pellets are supplied to a crystallization stage 38,
which is also operated using dry air, from the oversize separator
34 and/or its funnel-shaped floor region 36. From the
crystallization stage 38, the particles may reach a typical SSP
polycondensation stage, which is particularly operated under
partial vacuum.
[0048] At the schematic illustration of the single figure
indicates, the air permeating the fall tower 20 is conveyed in
a--first-loop 40, the intake openings 22, 24 having flaps 42, 44
connected upstream for air quantity regulation. Furthermore, there
is a fan 46 before the control flaps 42, 44.
[0049] The air removed via the outlet opening 27 is charged with
reaction products such as ethylene glycol and/or butane diol,
water, oligomers, or acetaldehyde and/or tetrahydrofuran, which
arise from the dripped precondensate and/or molten prepolymer. In
order to resupply the reaction products, if they are reusable, to
the production process, a portion from the loop 40 is supplied via
a line 48 to a--second-loop 50, a spray loop, which includes a
spray condenser 52, in which fresh and cold ethylene glycol and/or
butane diol, which is supplied via a line 54, is sprayed via a
spray device 56. Through this measure, reaction substances such as
ethylene glycol, butane diol, oligomers, water, etc., are condensed
out of the loop 50, and may be reused as raw materials and supplied
to the esterification and/or reesterification stage 12 via a line
58. To accelerate the condensation, there is a heat exchanger 60 in
the loop 50, through which the temperature of the air flowing
through the loop 50 may be adjusted optimally. There is a pump 62
to convey the loop liquid itself.
[0050] The proportion of air transferred out of the first loop 40
is preferably between 10 % and 30 %.
[0051] The air which leads the spray loop 50 via a line 64 is
purified and has a low dew point and may be supplied to the loop 40
flowing through the fall tower 20 via a line 66. Due to the low
temperature of the air leaving the spray loop 50 via the line 66
and its low dew point, the temperature in the loop 50 is adjusted
to a desired intake temperature in the floor region of the fall
tower 20.
[0052] Since small quantities of acetaldehyde and/or THF
(tetrahydrofuran), which are not able to be condensed in the spray
loop 50, are formed both in the fall tower 20 and in the downstream
precrystallization stage 38, small quantities of air are supplied
to the first loop 40 via a connection 68. An identical quantity of
air is removed from a line 64, which connects the spray loop 50
with the loop 40 of the fall tower 20 and/or a third loop 70
including the precrystallization stage 38, via a connection 72, in
order to be mixed into a heat transfer facility for the purpose of
combustion, through which the requirement for external energy such
as heating gas and/or oil may be reduced.
[0053] As mentioned, the precrystallization stage 38, which is
particularly necessary for the processing of comonomers, also
includes a loop 70, in which dry air is conveyed using a fan 74.
The air flowing in the loop 70 may also be heated to the desired
extent via a heating device 76. As shown in the drawing, the loop
70 is connected via a line 78 to the spray loop 50, in order to be
able to condense out reaction substances with which the circulating
air is enriched and resupply them to the esterification and/or
reesterification process.
[0054] A quantity of air, having a low dew point, corresponding to
the quantity of air removed via the line 78 is resupplied via the
line 64 to the loop 70.
[0055] The removal of reaction substances from the loop 70 is
advantageous for economic reasons alone, since due to their
relatively low intrinsic viscosities, a relatively large amount of
ethylene glycol and/or butane diol is still in the air loop 70, so
that, as mentioned, condensing out ethylene glycol and/or butane
diol and returning them to the esterification and/or
reesterification stage 12 suggests itself.
[0056] The following is to be noted in regard to the temperatures
of the particles and/or the air loops permeating the fall tower 20.
The particles leave the nozzle plate at the temperature of
approximately 230.degree. for the PET process and/or 190.degree.
for the PBT process and reach a temperature of approximately
180.degree. in the middle region of the fall tower 20. A
temperature of approximately 160.degree. C. for the PET process
and/or 130.degree. C. for the PBT process exists in the oversize
separator 34.
[0057] The quantity and temperature of air entering the fall tower
20 via the intake openings 22, 24 is adjusted according to the
throughput. The air removed from outlet opening 27 has a
temperature of approximately .ltoreq.160.degree. C. for a PET
process and .ltoreq.130.degree. C. for a PBT process. In the second
loop 50, which includes the spray purification, the air is cooled
to approximately 20.degree. and is supplied at this temperature to
both the first loop 40 and the second 70.
[0058] The air supplied underneath the slanted surface 30, which
exercises the function of a fluidized bed, via the intake opening
24 is to be supplied at a temperature at which the crystallization
speed for the pellets to be produced is optimal. This means
approximately 160.degree. C. for the production of PET spheres and
.ltoreq.130.degree. C. for PBT pellets. The air supplied via the
opening 22 above the surface 30 is to be below the temperatures
indicated previously, since it is heated through heat transfer from
the falling droplets as it flows through the tower 20. In order to
perform an optimization in this regard, a connection 67 leads from
the line 64, which comes from the spray loop 50, to the tower, via
which purified air of relatively low temperature (approximately
20-30.degree. C.) is introduced directly into the tower 20, through
which the temperature of air flowing through the tower 20 is
reduced overall. Therefore, air of a desired relatively high
temperature may be supplied in the region of the fluidized bed 30,
without the optimum crystallization temperature being exceeded
inside the tower 20, since cooler air is mixed in via the line 67,
as described.
[0059] Using the method according to the present invention, uniform
spherical pellets which lie in a narrow grain spectrum may be
produced. Spherical pellets having a diameter of 0.8 millimeters
may be obtained at a nozzle diameter of 0.5 mm, a nozzle plate
frequency of 1000-2000 Hz, and a fall height of 20 meters.
* * * * *