U.S. patent application number 12/598480 was filed with the patent office on 2010-06-03 for method and device for cleaning an absorptive polyester.
This patent application is currently assigned to BOEHRINGER INGELHEIM PHARMA GMBH & CO. KG. Invention is credited to Anja Enderle, Manfred Schmitt.
Application Number | 20100137550 12/598480 |
Document ID | / |
Family ID | 39800573 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100137550 |
Kind Code |
A1 |
Enderle; Anja ; et
al. |
June 3, 2010 |
Method and Device for Cleaning an Absorptive Polyester
Abstract
In a method for cleaning an absorptive polyester, the polymer is
dissolved in a first solvent (12) and subsequently the polymer
solution is brought into tight contact with a second solvent (41)
in a turbulent shear field under the influence of strong shear
forces. Here, the second solvent (41) represents a non-solvent for
the absorptive polyester and is mixable with the first solvent (12)
to an unlimited extent. Subsequently, the polymer suspension
resulting from the addition of the second solvent (41) is conveyed
onto or into a rotating, cylindrical screen body (71) of a drum
shear screen (70) and then the moist polymer mass is removed from
the screen body (71) and subsequently dried. The method is suitable
for the production of absorptive polyester with a high degree of
quality and may be performed in a cost-effective fashion on an
industrial scale as well.
Inventors: |
Enderle; Anja; (Ingelheim,
DE) ; Schmitt; Manfred; (Ingelheim, DE) |
Correspondence
Address: |
MICHAEL P. MORRIS;BOEHRINGER INGELHEIM USA CORPORATION
900 RIDGEBURY ROAD, P. O. BOX 368
RIDGEFIELD
CT
06877-0368
US
|
Assignee: |
BOEHRINGER INGELHEIM PHARMA GMBH
& CO. KG
Ingelheim
DE
|
Family ID: |
39800573 |
Appl. No.: |
12/598480 |
Filed: |
May 2, 2008 |
PCT Filed: |
May 2, 2008 |
PCT NO: |
PCT/EP08/55408 |
371 Date: |
November 2, 2009 |
Current U.S.
Class: |
528/271 ;
422/261 |
Current CPC
Class: |
C08G 63/08 20130101;
C08G 63/90 20130101 |
Class at
Publication: |
528/271 ;
422/261 |
International
Class: |
C08G 63/90 20060101
C08G063/90; B01D 11/00 20060101 B01D011/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 4, 2007 |
DE |
10 2007 020 951.9 |
Claims
1. A process for purifying a resorbable polyester comprising the
steps of: (a) dissolving the resorbable polyester in a first
solvent (12) to form a polymer solution, wherein the first solvent
(12) is a genuine solvent; (b) intimately contacting the polymer
solution with a second solvent (41) under action of high shear
forces in a turbulent shear field to form a polymer suspension,
wherein the second solvent (41) is a non-solvent for the resorbable
polyester and is unlimitedly miscible with the first solvent (12);
(c) conveying the polymer suspension onto or into a rotating,
cylindrical screen body (71) of a drum shear screen (70) to form a
moist polymer mass; (d) separating the moist polymer mass from the
screen body (71); and (e) drying the polymer mass.
2. The process according to claim 1, wherein the first solvent (12)
is acetone, ethyl acetate, 1,4-dioxane, dimethylacetamide,
tetrahydrofuran, toluene, dimethylformamide, dimethylsulphoxide,
hexafluoroisopropanol or another halogenated hydrocarbon or a
mixture of the aforementioned solvents.
3. The process according to claim 2, wherein the first solvent (12)
is acetone, chloroform or dichloromethane.
4. The process according to any one of claims 1 to 3, wherein the
second solvent (41) is ethanol, methanol or water or a mixture of
the aforementioned solvents.
5. The process according to claim 1, wherein the resorbable
polyester dissolved in the first solvent (12) is further filtered
and subsequently mixed with the second solvent (41) via a two-fluid
nozzle (40).
6. The process according to claim 1, wherein the moist polymer mass
is separated by means of gravity and by means of one or more
helically attached conveyor rails and/or guide blades (72) inside
the rotating, cylindrical screen body (71).
7. The process according to claim 1, wherein the drying step
involves passing nitrogen or air through the polymer mass in a
dryer (90).
8. The process according to claim 1, wherein the resorbable
polyester is an amorphous or partially crystalline polyester.
9. The process according to claim 1, wherein the resorbable
polyester contains one or more units derived from lactide
(L-lactide, D-lactide, DL-lactide, meso-lactide), glycolide,
trimethylene carbonate, epsilon-caprolactone, gamma-butyrolactone,
dioxanone, delta-valerolactone and/or similar polymerisable
heterocycles and/or polyethylene glycols.
10. The process according to claim 1, wherein the resorbable
polyester is composed of copolymers of lactide and glycolide and/or
polyethylene glycols having any desired composition.
11. The process according to claim 1, wherein resorbable polyester
has a residual monomer content of less than 1%.
12. The process according to claim 1, wherein the resorbable
polyester has a solvent and/or moisture content of less than
2%.
13. A pharmaceutical formulation or a resorbable implant containing
a resorbable polyester made by the process according to claim
1.
14. A purification device for purifying a resorbable polyester that
contains as its main components a dissolving vessel (10) in which
the polymer is dissolved in a first solvent (12), a separation
device for separating a moist polymer mass from a polymer
suspension and a dryer (90) for drying the polymer mass,
characterised in that the separation device is constructed as a
drum shear screen (70) comprising a rotating, cylindrical screen
body (71).
15. The purification device according to claim 14, characterised in
that the cylindrical screen body (71) has in its interior conveyor
rails and/or guide blades (72).
16. The purification device according to claim 14, characterised in
that the drum shear screen (70) has a suction device (74), in
particular comprising a suction nozzle, above the rotating,
cylindrical screen body (71) in an upper housing cover (76).
17. The purification device according to claim 14, characterised in
that the dryer (90) is a fluidised bed dryer, a circulating air
dryer or a tubular-flow dryer.
18. The purification device according to claim 14, characterised in
that the dryer (90) has a conical portion (95) and a cylindrical
portion (96).
19. The purification device according to claim 14, characterised in
that the dryer (90) comprises inside the cylindrical portion (96)
at least one screen insert (94).
20. The purification device according to claim 14, characterised in
that the dryer (90) has a filter bag (92) for collecting the
purified resorbable polyester.
21. The purification device according to claim 14, characterised in
that the dryer (90) is provided with a swivel bearing (93).
22. The purification device according to claim 14, characterised in
that at least the product-guiding parts of the drum shear screen
(70) and of the dryer (90) are made of stainless steel.
Description
[0001] The invention relates to a process for purifying a
resorbable polyester, the polymer being dissolved in a first
solvent and subsequently the polymer solution being brought into
intimate contact with a second solvent under the action of high
shear forces in a turbulent shear field, the first solvent being a
"genuine" solvent and the second solvent being a non-solvent for
the resorbable polyester and being unlimitedly miscible with the
first solvent.
[0002] Furthermore the invention also relates to a resorbable
polyester which is purified using the process and to the use
thereof.
[0003] The invention further relates to a purification device for
purifying a resorbable polyester that contains as its main
components a dissolving vessel in which the polymer is dissolved in
a first solvent, a separation device for separating a moist polymer
mass from a polymer suspension and a dryer for drying the polymer
mass.
[0004] Resorbable polyesters in the sense of the invention are
homopolymers or copolymers based on lactide (L-lactide, D-lactide,
DL-lactide, meso-lactide), glycolide, epsilon-caprolactone,
dioxanone, trimethylene carbonate, delta-valerolactone,
gamma-butyrolactone, and similar polymerisable heterocycles. The
polymers can either be composed of one or else of a plurality of
different monomer modules or optionally contain further modules in
the polymer chain, such as for example units of ethylene
glycol.
[0005] Preferred according to the invention are homopolymers of
D,L-lactide, copolymers of D,L-lactide and glycolide of differing
composition and block copolymers of the aforementioned polyester
units and polyethylene glycol.
[0006] Resorbable polyesters are raw materials which are widely
used for the production of resorbable, surgical implants and also
as a pharmaceutical auxiliary for the formulation of parenteral
release systems. For example, poly(lactides) and other resorbable
polyesters are used in surgical implants for the fixing of bone
fractures, in reticula and membranes for controlled tissue
regeneration and in microcapsules and implants for subcutaneous or
intramuscular injection, in particular for the controlled release
of active ingredients. After implantation or injection into the
body, resorbable polymers are broken down into oligomers in a slow
hydrolytic reaction. Hydrolysis end products such as lactic acid or
glycolic acid are metabolised into carbon dioxide and water.
[0007] The synthesis of resorbable polyesters is known in the art.
They can be prepared by polycondensation from hydroxycarboxylic
acids such as lactic acid and/or glycolic acid. Another frequently
taken synthesis pathway is the ring-opening polymerisation of the
corresponding heterocycles.
[0008] Irrespective of the synthesis pathway, the crude polymers
always contain a specific content of non-converted monomers that
frequently cannot be reduced below a value of from 1 to 3% even if
the corresponding synthesis processes are optimised. The reason for
this is that ring-opening polymerisation is an equilibrium reaction
and even during polycondensation the linear polyester is present in
equilibrium with the corresponding hydroxycarboxylic acids.
[0009] The presence of monomers in the polymers is extremely
problematic for the following reasons: [0010] a) Because cyclic
monomers are much more unstable in relation to hydrolytic
decomposition than linear polyesters, they decompose more rapidly
than polyesters on ingress of moisture. The hydrolytic
decomposition generates acid equivalents which, again, the
hydrolytic decomposition of the polyesters also catalyses. The
implantation of monomer-containing polyesters would therefore lead
to a greatly accelerated breakdown of the material in the body.
[0011] b) For the same reason, the stability in storage of
monomer-containing polyesters and implants or pharmaceutical
formulations produced therefrom is markedly impaired. [0012] c) The
stability of resorbable polyesters is also impaired during
thermoplastic processing if residual contents of monomers are
present. [0013] d) The encapsulation behaviour of non-purified
polyesters is different from that of purified polyesters, as are
the release behaviour and the breakdown behaviour. Encapsulated
active ingredients, such as peptides, can become damaged or
destroyed as a result of the greater amount of free acid compared
to purified polymers. [0014] e) During the synthesis reactions, the
residual monomer content of the crude polymer is often difficult to
control. Variability in the residual monomer content then
automatically also leads to intolerable batch-to-batch variations
in the breakdown rate, the stability in storage and the processing
stability, so materials of reproducible quality cannot be obtained
without a subsequent purification step to reduce the amount of
residual monomers.
[0015] Purification processes for separating residual monomers from
resorbable polyesters are also known in the art.
[0016] Monomers can be removed from partially crystalline
polyesters using extraction processes. Suitable for this purpose
are solvents which dissolve the monomer but do not dissolve the
polymer. Suitable examples include organic solvents such as
n-hexane, cyclohexane, methanol, ethanol, acetone or ethyl acetate.
EP 0456246 discloses for example an extraction process for
resorbable polyesters that uses carbon dioxide as the solvent.
[0017] Amorphous polyester can generally not be purified by
extraction processes, as the relevant solvents either also dissolve
the polymer or at least cause it to swell. If supercritical or
pressure-liquefied carbon dioxide is used, the polymer mass expands
greatly when the pressure is relieved, and this also prevents a
process of this type from being carried out. The prior art
discloses a number of reprecipitation processes for the
purification of amorphous polyesters. These involve dissolution of
the crude polymer in a suitable solvent. The addition of a large
excess of a non-solvent, which is however miscible with the
solvent, causes precipitation of the polymer. For example, the
reprecipitation of a poly(L-lactide)poly(ethylene
glycol)-poly(L-lactide) by dissolution in chloroform and
precipitation in methanol or methanol/chloroform mixtures is
disclosed (J. Matsumotot et al.; Int. J. of Pharm.; 185; 1999;
93-101). The disclosed reprecipitation processes have the drawback
of using a huge amount of organic solvents and, in addition, the
solid/liquid phase separation, and thus the product isolation, is
extremely difficult. This is in particular due to the fact the
polyesters tend to agglomerate at the contact point at which the
polymer solution enters into contact with the non-solvent.
Application on an industrial scale is therefore difficult.
[0018] U.S. Pat. No. 4,810,775 discloses a purification process for
resorbable polyesters having crystallinity of up to 20%, the
polymer being dissolved in a solvent, the polymer solution
subsequently being brought into intimate contact with a precipitant
under the action of high shear forces in a turbulent shear field.
The turbulent shear field is generated by a device consisting of a
two-fluid nozzle and of a container which is filled with
precipitant and into which the two-fluid nozzle protrudes, so the
precipitating polymer is broken down into very small particles.
However, it is not disclosed how a process of this type can be
carried out economically on a large scale. The phase separation of
the polymer suspension formed during the reprecipitation is carried
out either in a centrifuge or by collection in receptacles which,
owing to the large amount of solvent required, must be large even
in relatively small batch sizes.
[0019] The object of the invention is therefore to provide an
improved purification process for purifying a resorbable polyester,
in particular an amorphous polyester, of the type mentioned at the
outset that allows a resorbable polyester of high and reproducible
quality to be obtained even on an industrial scale. It is
furthermore the object of the invention to provide a corresponding
device for the process according to the invention.
[0020] The object concerning the process is achieved according to
the invention in that subsequently the polymer suspension, which is
formed by the addition of the second solvent, is conveyed onto or
into a rotating, cylindrical screen body of a drum shear screen and
subsequently the moist polymer mass is separated from the screen
body and subsequently dried thereon.
[0021] This provides a continuously operating separation process
which allows the polymer suspension to be separated with high
reproducibility into the solid and liquid phase. The continuous
mode of operation allows batches of constant quality to be provided
irrespective of the amount.
[0022] In terms of the process, provision is made for the first
solvent used to be acetone, ethyl acetate, 1,4-dioxane,
dimethylacetamide, tetrahydrofuran, toluene, dimethylformamide,
dimethylsulphoxide, hexafluoroisopropanol or another halogenated
hydrocarbon or a mixture of the aforementioned solvents. A suitable
solvent is thus provided depending on the type of polyester and the
inherent viscosity of the polyester in solution. Acetone,
chloroform or dichloromethane have been found to be particularly
suitable as the first solvent.
[0023] Preferably, the second solvent used is ethanol, methanol or
water or a mixture of the aforementioned solvents. Particularly
effective precipitation reactions can thus be achieved, depending
on the first solvent used, water in particular being used as the
second solvent. Water is non-toxic and non-explosive but
cost-effective and particularly environmentally acceptable.
[0024] According to a development, the resorbable polyester which
is dissolved in the first solvent is filtered and subsequently
mixed with the second solvent via a two-fluid nozzle. This produces
an intimate contact under the action of high shear forces in the
turbulent shear field, as a result of which optimum thorough mixing
is achieved. Alternative intensive thorough mixing can also be
achieved if both media are injected from two separate nozzles into
a flow tube and a fluidised bed is generated at the contact point
by means of a rapidly rotating stirrer.
[0025] Advantageously, the moist polymer mass is separated
effectively by means of gravity and by means of one or more
helically attached conveyor rails and/or guide blades inside the
rotating, cylindrical screen body. This provides restricted
conveyance which allows the polymer mass to be conveyed
continuously, for example into a receptacle.
[0026] In order to obtain a low residual moisture or residual
solvent content, for drying the moist polymer mass, nitrogen or air
is expediently passed therethrough in a dryer.
[0027] The process according to the invention can be used to purify
cost-effectively and with constant quality, in particular,
resorbable polyesters having an amorphous or partially crystalline
structure.
[0028] Resorbable polyesters purified in this way preferably
contain one or more units derived from lactide (L-lactide,
D-lactide, DL-lactide, meso-lactide), glycolide, trimethylene
carbonate, epsilon-caprolactone, gamma-butyrolactone, dioxanone,
delta-valerolactone and/or similar polymerisable heterocycles
and/or polyethylene glycols. Particularly preferred is a resorbable
polyester which is composed of D,L-lactide or copolymers of
D,L-lactide and glycolide having any desired composition or a block
copolymer of D,L-lactide, or D,L-lactideco-glycolide having any
desired composition and polyethylene glycol.
[0029] The residual monomer content, after the purification has
been carried out using the process according to the invention, is
less than 1%, in particular less than 0.5%, a residual monomer
content of below 0.1% being achievable.
[0030] After the drying, the solvent and/or moisture content of the
resorbable polyester is less than 2%, values of below 1%, in
particular values of below 0.5% being achieved under beneficial
settings. If dried particularly intensively, the solvent and/or
moisture content of the resorbable polyester is <0.1%.
[0031] A particularly preferred use of the resorbable polyester
provides for the production of pharmaceutical formulations or
resorbable implants.
[0032] The object concerning the purification device is achieved
according to the invention in that the separation device is
constructed as a drum shear screen comprising a rotating,
cylindrical screen body.
[0033] With this type of separation device, such as has previously
been used for example in other fields for the continuous
dehydration of high solid loads, it has surprisingly been found
that the device allows polymer suspensions formed from a
precipitation reaction to be separated cost-effectively and at
constant quality into a polymer mass and into solvent residues even
if the throughput fluctuates.
[0034] Conveyor rails and/or guide blades inside the cylindrical
screen body allow, in conjunction with the rotational movement of
the screen body, the polymer mass to be conveyed continuously into,
for example, a receptacle.
[0035] In an advantageous embodiment the drum shear screen has a
suction device, in particular comprising a suction nozzle, above
the rotating, cylindrical screen body in an upper housing cover.
This provides removal of solvent vapours by suction, and this is
particularly advantageous if, for example, acetone, ethanol or
methanol is used, as the suction device allows the explosion
protection class of the surrounding building to be reduced.
[0036] A dryer which is configured as a fluidised bed dryer,
circulating air dryer or tubular-flow dryer is particularly
suitable for effective drying of the moist polymer mass. The dryer
has in a preferred embodiment a conical portion and a cylindrical
portion, intensive swirling of the polymer mass to be dried being
achieved in the conical portion of the dryer owing to the drying
agent, for example nitrogen or air, which flows in from below. In
order to reduce agglomeration, a grater is provided for the
polymer. During drying, the polymer mass to be dried can be removed
from the dryer and grated via the grater, after which the drying is
continued.
[0037] In order to prevent the polymer from passing out of the
dryer into a supply system, the dryer has inside the cylindrical
portion at least one screen insert. For collecting the purified
resorbable polyester, the dryer expediently comprises a filter bag.
A swivel bearing of the dryer in a frame allows the dryer to be
tilted, thus allowing the filter bag which is fastened to the dryer
on the end side and comprises the dried polymer powder easily to be
removed. Moreover the polymer mass to be dried can be mixed
thoroughly more easily during the drying process.
[0038] According to a further configuration at least the
product-guiding parts of the drum shear screen and of the dryer are
made of stainless steel, thus ensuring high product quality with
regard to pharmacological requirements.
[0039] It will be understood that the above-mentioned features and
those which will be described hereinafter can be used not only in
the respectively specified combination but rather also in other
combinations. The scope of the invention is defined merely by the
claims.
[0040] The invention will be described hereinafter in greater
detail using an exemplary embodiment and with reference to the
associated drawings, in which:
[0041] FIG. 1 Is a schematic view of a purification device for
resorbable polyesters for carrying out the process according to the
invention,
[0042] FIG. 2 is a perspective exploded view of the drum shear
screen according to FIG. 1, and
[0043] FIG. 3 is a perspective view of the dryer according to FIG.
1.
[0044] A purification device 1 comprises as its main component for
a first process step a dissolving vessel 10 in which a polyester
crude product 11 to be purified is placed. A typical size of the
dissolving vessel 10 is in the range of from 50 to 1,000 l and can,
in the case of larger systems, be as much as 2,000 l or more. The
addition of a first solvent 12 causes dissolution in the dissolving
vessel 10 of the polyester crude product 11 with the aid of a
stirrer 13 and/or by constant recirculation of the solution.
Impurities in the polyester crude product 11, for example in the
form of lint, are separated, for example by filtration. The
following solvents have, for example, been found to be preferred
first solvents 12: acetone, ethyl acetate, 1,4-dioxane,
dimethylacetamide, tetrahydrofuran, toluene, dimethylformamide,
dimethylsulphoxide, hexafluoroisopropanol or another halogenated
hydrocarbon or a mixture of the aforementioned solvents. In the
resorbable polyesters, acetone, chloroform or dichloromethane have
proven particularly suitable as first solvents 12.
[0045] A pump, for example a diaphragm pump, is used to pump the
polymer solution via a filter 30 containing a fine-meshed screen,
preferably made of stainless steel. In this step insoluble
impurities are separated off. Typical mesh sizes are in this case a
few .mu.m, typically in the range of from 1 to 10 .mu.m.
[0046] Subsequently, the polymer solution is intensively mixed via
a two-fluid nozzle 40 by means of a second solvent 41, which is a
non-solvent for the polymer, and the polymer suspension resulting
therefrom is guided into the interior of a rotating screen body 71
of a drum shear screen 70 via a conveyor 60, in the simplest case
directly or via a flow tube or a pipe, wherein the conveyance can
be carried out by means of gravity, conventional pumps or by
pressurisation by means of a gas. The second solvent 41 used is
ethanol, methanol or water or a mixture of the aforementioned
solvents for precipitation. Water is a particularly preferred
second solvent 41 owing to its toxicological safeness and
environmental acceptability.
[0047] Inside the rotating screen body 71 of the drum shear screen
70, the polymer suspension can be separated into the solvent
mixture and into the precipitated polymer mass. One or more
helically attached guide rails and/or guide blades 72 inside the
screen body 71 subject the polymer mass to restricted conveyance,
so the polymer mass is transported to a solid outlet 75. The
solvent mixture can in this case flow away downward through a
liquid outlet 73. Solvent vapours can be removed by suction via a
suction device 74, for example a suction nozzle, in a housing cover
76 of the drum shear screen 70 above the screen body 71, and this
is advantageous with regard to the explosion protection class of
the surrounding building.
[0048] The still-moist polymer mass which collects at the solid
outlet 75 is transported using a solid conveyor 80 either directly
into a dryer 90 or into a collection vessel. Batchwise filling of
the dryer 90 from the collection vessel is possible in addition to
continuous feeding. The dryer 90 has a conical portion 95 and a
cylindrical portion 96. At least one screen insert 94 is provided
inside the cylindrical portion 96. The drying agent 91, for example
nitrogen or air, is introduced laterally from below into the
conical portion 95 of the dryer 90, so intensive swirling is
achieved inside the dryer 90. The dryer 90 has a filter bag 92 for
collecting the purified and dried resorbable polyester.
[0049] In the configuration of the drum shear screen 70 according
to FIG. 2, there may be seen on the inside the rotating screen body
71 into which the polymer suspension can be introduced via a
U-shaped channel. The screen body 71 is positioned slightly
obliquely. One or more helically attached guide rails and/or guide
blades 72 inside the screen body 71 cause the restricted conveyance
of the polymer mass to the solid outlet 75. Solvent vapours are
removed by suction via the suction device 74 in the housing cover
76 of the drum shear screen 70 above the screen body 71. The screen
body 71 is closed on its back to prevent the inside of the screen
from becoming contaminated with abraded material from the drive
unit. The back can be opened for purification purposes.
[0050] The dryer 90 according to FIG. 3, which is arranged in a
movable frame 97, can be tilted by means of a swivel bearing 93.
The filter bag 92 is not shown. Moreover at least the
product-guiding parts of the drum shear screen 70 and of the dryer
90 are made of stainless steel.
[0051] The process according to the invention will be described
hereinafter:
[0052] The polyester crude product 11 is dissolved with the
previously calculated amount of acetone as the first solvent 12.
The polyester crude product 11 is weighed out and placed in the
dissolving vessel 10. The calculated amount of acetone is added and
the crude product dissolved by recirculation within approx. 24-72
hours. The mixing ratio is dependent on the starting substances
used (monomers or heterocycles) and the inherent viscosity of the
crude product and is, for example in a copolymer of D,L-lactide and
glycolide; 50:50 mol %; inherent viscosity of 0.5 dl/g measured as
a 0.1% solution in chloroform; solution for precipitation is
dispensed with, 8% by weight of polymer in acetone.
[0053] The polymer solution is conveyed into the two-fluid nozzle
40 by means of the pump 20 via a filter 30 made of stainless steel
having a mesh size of 5 .mu.m and a flowmeter. The flow is
dependent on the nature of the crude product used and is generally
up to 20 l/h. This figure relates to a flowmeter which is adjusted
to the density of acetone. The differing densities of the polymer
solutions to be precipitated prevent precise flow measurement
(except for mass flowmeters). In the two-fluid nozzle 40 the
polymer solution is injected into a water jet at a flow rate of
approx. 700-1000 l/h, the dissolved crude product precipitating
immediately in the form of flakes or fibres.
[0054] The suspension, consisting of water, product flakes, monomer
and acetone, is guided into the drum shear screen 70 via a pipe. In
this case the suspension is guided into the rear region of the drum
shear screen 70. As a result of the rotational movement, the
discharging water/acetone mixture, which contains the monomers to
be separated off, initially forms a product layer at this location.
If the product layer is sufficiently heavy, it becomes detached
from the wall and forms a product cluster (snowball system). As a
result of the guide blades 72, which run obliquely forward, and the
rotational movement, these product clusters are slowly conveyed
toward the solid outlet 75 in the screen body 71. The water/acetone
mixture is separated off, on the one hand, by means of gravity and,
on the other hand, by wedge-shaped screen profile bars and the
Coand{hacek over (a)} effect resulting therefrom.
[0055] The solid is guided into the dryer 90 or the collection
vessel by means of the solid conveyor 80. In the dryer 90, which is
configured as a tubular-flow dryer, the moist polymer mass is dried
by means of a throughflow of air or nitrogen.
* * * * *