U.S. patent application number 09/755411 was filed with the patent office on 2001-07-19 for continuous process and apparatus for the drying and gel formation of solvent-containing gel-forming polymers.
Invention is credited to Gehrmann, Dietrich, Kempkes, Hartwig.
Application Number | 20010008048 09/755411 |
Document ID | / |
Family ID | 7627455 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008048 |
Kind Code |
A1 |
Gehrmann, Dietrich ; et
al. |
July 19, 2001 |
Continuous process and apparatus for the drying and gel formation
of solvent-containing gel-forming polymers
Abstract
A continuous process for the drying and gel formation of
solvent-containing gel-forming polymers, in particular
polysaccharide derivatives, by flash cooling of suspensions of the
polymers, and an apparatus for the drying of solvent-containing
products are described. The process comprises the steps: metering
the solvent-containing polymer having a solids content of 1 to 65%
by weight and at a pressure of in particular ambient pressure to
6000 hPa, at a polymer temperature of 20 to 100.degree. C. into an
evaporation zone (1), the evaporation zone (1) having a pressure of
0.1 hPa to 800 hPa, cooling the polymer, in particular to a
temperature of <93.degree. C., freeing the polymer of 2 to 5% by
weight of solvent with mechanical stirring, mixing and conveying of
the mixture to the lower end of the evaporation zone (1) with
formation of a dense gelled product layer, discharging the polymer
by means of a mechanical discharge apparatus (7), optionally with
further densification of the polymer, and subsequently drying the
gelled polymer in a drying apparatus to a solvent content of not
more than 5% by weight. The apparatus comprises at least one
evaporation container (3) having an evaporation zone (1), a stirrer
and a conveyor means (8), a product inlet (12), a vapour take-off
(5) with connected vacuum generator, and a discharge apparatus (7)
connected to the lower end of the evaporation container (3), one or
more conveyor paddles (9) having thrust blades (10) which pass
across the entrance of the discharge apparatus (7) at a height of
not more than 5 mm, in particular 1 to 3 mm, being provided in the
lowermost plane of the conveyor means (8).
Inventors: |
Gehrmann, Dietrich;
(Leverkusen, DE) ; Kempkes, Hartwig;
(Engelskirchen, DE) |
Correspondence
Address: |
Patent Department
Bayer Corporation
100 Bayer Road
Pittsburgh
PA
15205-9741
US
|
Family ID: |
7627455 |
Appl. No.: |
09/755411 |
Filed: |
January 5, 2001 |
Current U.S.
Class: |
34/114 ;
34/62 |
Current CPC
Class: |
C08B 11/20 20130101;
B01D 3/06 20130101; F26B 5/04 20130101; B01F 27/90 20220101; F26B
11/14 20130101; B01F 35/75455 20220101 |
Class at
Publication: |
34/114 ;
34/62 |
International
Class: |
F26B 011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 14, 2000 |
DE |
10001256.6 |
Claims
1. Continuous process for the drying and gel formation of
solvent-containing gel-forming polymers, in particular
polysaccharide derivatives, particularly preferably of hot, aqueous
cellulose ether suspensions, by flash cooling of suspensions of the
polymers, comprising the steps: metering the solvent-containing
polymer having a solids content of 1 to 65% by weight, preferably
of 5 to 50% by weight, particularly preferably of 10 to 40% by
weight, at a pressure of in particular ambient pressure to 6000
hPa, at a polymer temperature of 20 to 100.degree. C., preferably
of 30 to 95.degree. C., into an evaporation zone (1), preferably a
paste bunker, the evaporation zone (1) having a pressure of 0.1 hPa
to 800 hPa, preferably of 20 hPa to 700 hPa, particularly
preferably of 40 hPa to 200 hPa, cooling the polymer, in particular
to a temperature of <93.degree. C., in particular <60.degree.
C., especially of 60.degree. C. to 29.degree. C., freeing the
polymer of 2 to 5% by weight of solvent with mechanical stirring,
mixing and conveying of the mixture to the lower end of the
evaporation zone (1) with formation of a dense gelled product layer
discharging the polymer by means of a mechanical discharge
apparatus (7), in particular with the aid of a single-screw or
multiscrew conveyor (7), optionally with further densification of
the polymer, and subsequently drying the gelled polymer in a drying
apparatus to a solvent content of not more than 5% by weight.
2. Process according to claim 1, characterized in that the
residence time of the polymer in the evaporation zone (1) is from
30 sec to 2 h, preferably from 5 to 30 min.
3. Process according to claim 1 or 2, characterized in that the
drying of the gelled polymer is effected at 100 to 500.degree. C.,
preferably from 100 to 230.degree. C., in a miller-dryer.
4. Process according to any of claims 1 to 3, characterized in that
the drying of the gelled polymer is effected at a pressure of 1000
to 5000 hPa, preferably of 1000 hPa.
5. Process according to any of claims 1 to 4, characterized in that
the drying of the gelled polymer is effected in a superheated steam
miller-dryer.
6. Process according to any of claims 1 to 5, characterized in that
the drying of the gelled polymer is effected to a solvent content
of not more than 4% by weight.
7. Process according to any of claims 1 to 5, characterized in that
the solvent is essentially water.
8. Process according to any of claims 1 to 5, characterized in that
the polymer is a cellulose ether, in particular
methylcellulose.
9. Apparatus for the drying of solvent-containing products, in
particular of solvent-containing gel-forming polymers, comprising
at least one evaporation container (3) having an evaporation zone
(1), a stirrer and a conveyor means (8), a product inlet (12), a
vapour take-off (5) with connected vacuum generator, and a
discharge apparatus (7) connected to the lower end of the
evaporation container (3), one or more conveyor paddles (9) having
thrust blades (10) which pass across the entrance of the discharge
apparatus (7) at a height of not more than 5 mm, in particular 1 to
3 mm, being provided in the lowermost plane of the conveyor means
(8).
10. Apparatus according to claim 9, characterized in that the
discharge apparatus (7) is a single-screw or multiscrew conveyor
(7).
11. Apparatus according to claim 9 or 10, characterized in that the
evaporation container (3) has an additional jacket (11) for passing
through a heat-transfer medium.
12. Apparatus according to any of claims 9 to 11, characterized in
that the thrust blades (10) pass across the entrance of the
discharge apparatus (7) at a height of not more than 2 mm.
13. Apparatus according to any of claims 9 to 12, characterized in
that a throttle means (4), in particular a throttle valve (4) for
maintaining the reduced pressure in the evaporation container (3),
is mounted upstream of the product inlet (12).
14. Apparatus according to claim 10, characterized in that a
solid-blade screw conveyor (7) is provided as the discharge
apparatus (7).
15. Apparatus according to any of claims 9 to 14 for carrying out
the process according to any of claims 1 to 8.
Description
[0001] The invention relates to a continuous process for the drying
and gel formation of solvent-containing gel-forming polymers, in
particular polysaccharide derivatives, by flash cooling of
suspensions of the polymers and to an apparatus for the drying of
solvent-containing products.
[0002] The process comprises the steps: metering the
solvent-containing polymer having a solids content of 1 to 65% by
weight and a pressure of in particular ambient pressure to 6000
hPa, at a polymer temperature of 20 to 100.degree. C., into an
evaporation zone, the evaporation zone having a pressure of 0.1 hPa
to 800 hPa, cooling the polymer, freeing the polymer of 2 to 5% by
weight of solvent with mechanical stirring, mixing and conveying of
the mixture to the lower end of the evaporation zone with formation
of a dense gelled product layer, discharging the polymer by means
of a mechanical discharge apparatus, optionally with further
densification of the polymer, and subsequently drying the gelled
polymer in a drying apparatus to a solvent content of not more than
5% by weight.
[0003] The apparatus described comprises at least an evaporation
container having an evaporation zone, a stirrer and a conveyor
means, a product inlet, a vapour take-off with connected vacuum
generator, a discharge apparatus connected to the lower end of the
evaporation container, one or more conveyor paddles having thrust
blades which pass across the entrance of the discharge apparatus at
a height of not more than 5 mm, preferably 1 to 3 mm, being
provided in the lowermost plane of the conveyor means.
[0004] German Offenlegungsschrift 196 02 227 A1 discloses a storage
container (paste bunker) having a discharge apparatus. In
particular, the described stirrer of the storage container has a
sleeve design with variable stirring arm geometry for adaptation to
the product consistency.
[0005] German Patent Specification 952 264 discloses a three-stage
process for converting moist, fibrous methylcellulose into powder
form of high dissolution rate. Here, the product containing 50 to
70% by weight of water is first homogenized to give a plastic
material and is cooled to 10 to 20.degree. C., a cooled screw press
being used; the material is then milled using a hammer mill and is
dried in a forced-circulation dryer. It is the object of the
invention to provide a process and an apparatus which avoid the
disadvantages of the known processes and apparatuses and permit
comparably safer continuous operation.
[0006] Prior gelling of the cellulose ethers is necessary for the
drying and for the millability of the cellulose ethers in a
miller-dryer.
[0007] In particular, it is therefore the object of the invention
to develop a drying process which is suitable for
solvent-containing gel-forming polysaccharide derivatives, in
particular for a cellulose ether suspension, it being necessary to
cool the cellulose ether suspension in order to effect gelling and
to meter the gel continuously into a downstream apparatus, in
particular into a miller-dryer.
[0008] In particular, a cellulose ether suspension must be capable
of being converted into a homogeneous, gel-like consistency in
order to be able to meter the material into a superheated steam
miller-dryer.
[0009] The object is achieved, according to the invention, if the
preheated polymer suspension is introduced into an evaporation zone
and the evaporation zone, for example a cylindrical container, is
present under a reduced system pressure and the polymer suspension
experiences a cooling effect down to below the flocculation point
as a result of flashing into the cylindrical container.
[0010] The invention relates to a continuous process for the drying
and gel formation of solvent-containing gel-forming polymers, in
particular polysaccharide derivatives, particularly preferably of
hot, aqueous cellulose ether suspensions, by flash cooling (abrupt
let-down with cooling) of suspensions of the polymers, comprising
the steps: metering the solvent-containing polymer having a solids
content of 1 to 65% by weight, preferably of 5 to 50% by weight,
particularly preferably of 10 to 40% by weight, at a pressure of in
particular ambient pressure to 6000 hPa, at a polymer temperature
of 20 to 100.degree. C., preferably of 30 to 95.degree. C., into an
evaporation zone, preferably a paste bunker, the evaporation zone
having a pressure of 0.1 hPa to 800 hPa, preferably of 20 hPa to
700 hPa, particularly preferably of 40 hPa to 200 hPa, cooling the
polymer, in particular to a temperature of <93.degree. C., in
particular <60.degree. C., especially of 60.degree. C. to
29.degree. C., freeing the polymer of 2 to 5% by weight of solvent
with mechanical stirring, mixing and conveying of the mixture to
the lower end of the evaporation zone with formation of a dense
gelled product layer, discharging the polymer by means of a
mechanical discharge apparatus, in particular with the aid of a
single-screw or multiscrew conveyor, optionally with farther
densification of the polymer, and subsequently drying the gelled
polymer in a drying apparatus to a solvent content of not more than
5% by weight.
[0011] The residence time of the polymer in the evaporation zone is
preferably from 30 sec to 2 h, particularly preferably from 5 to 30
min.
[0012] Preferably, the drying of the gelled polymer is effected at
100 to 500.degree. C., particularly preferably from 100 to
230.degree. C., in a miller-dryer.
[0013] Advantageously, the drying of the gelled polymer is carried
out in particular at a pressure of about 1000 hPa (i.e. ambient
pressure) to 5000 hPa, preferably at 1000 hPa.
[0014] Particular advantages are obtained if the drying of the
gelled polymer is effected in a superheated steam miller-dryer.
[0015] Preferably, the drying of the gelled polymer is effected to
a solvent content of not more than 4% by weight.
[0016] The polymer particularly preferably to be used in the
process is a cellulose ether, in particular methylcellulose.
[0017] The solvent used in the process is preferably essentially
water.
[0018] The invention is distinguished from the prior art in
particular that described in German Patent Specification 952 264,
in that the cooling of the polymer suspension takes place not by
contact cooling of the suspension but by the flash effect, i.e.
during the transition from the higher pressure level in the
suspension line to the reduced system pressure in the paste bunker
(evaporation zone). In contrast to the screw press, this process
takes place spontaneously with simultaneous concentration of the
polymer suspension. As described in the present patent, the further
gelling is achieved by the available residence time in the paste
bunker. In contrast to the screw press, the paste bunker
additionally has the desirable function of a buffer. The gelling of
the polymer (methylcellulose) takes place at a temperature of 10 to
20.degree. C. according to the teaching of German Patent
Specification 952 264 but in the present process it takes place at
a substantially higher temperature, but typically at <60.degree.
C.
[0019] The invention furthermore relates to an apparatus for the
drying of solvent-containing products, in particular of
solvent-containing gel-forming polymers, comprising at least one
evaporation container having an evaporation zone, a stirrer and a
conveyor means, a product inlet, a vapour take-off with connected
vacuum generator, and a discharge apparatus connected to the lower
end of the evaporation container, one or more conveyor paddles
having thrust blades which pass across the entrance of the
discharge apparatus at a height of not more than 5 mm, preferably 1
to 3 mm, being provided in the lowermost plane of the conveyor
means.
[0020] The discharge apparatus is preferably a single-screw or
multiscrew conveyor.
[0021] The evaporation container has an additional jacket for
passing through a heat-transfer medium in order additionally to
promote the devolatilization of the polymer.
[0022] In a particularly preferred variant of the apparatus, the
thrust blades pass across the entrance of the discharge apparatus
at a height of not more than 5 mm, preferably 1 to 3 mm, in order
to ensure complete filling of the screw flights. Uniform product
conveying and at the same time a tight connection between paste
bunker and miller-dryer are thus achieved.
[0023] In a preferred embodiment of the apparatus, a throttle
means, in particular a throttle valve for maintaining the reduced
pressure in the evaporation container, is mounted upstream of the
product inlet.
[0024] A solid-blade screw conveyor has proved particularly useful
as the discharge apparatus.
[0025] The apparatus according to the invention is particularly
suitable for carrying out the above-described process according to
the invention.
[0026] Compared with the arrangement disclosed in German
Specification 195 02 227 A1, the paste bunker in the apparatus
according to the invention has in particular a thrust part,
additionally screwed on in each case, on the lower arms, for
metering the polymer gel.
[0027] The preferred paste bunker shown in FIG. 1 permits gas-tight
continuous metering into a downstream drying apparatus. The flash
cooling makes use of this advantage in order continuously to flash
(to let down) solvent-containing polymers, in particular
polysaccharide derivatives, particularly preferably cellulose ether
suspensions, having a solids content of 1 to 65% by weight,
preferably of 5 to 50% by weight, particularly preferably of 10 to
40% by weight, via an adjustable valve into the cylindrical
container under a pressure of 0.1 hPa to 800 hPa, preferably of 20
hPa to 700 hPa, particularly preferably of 40 hPa to 500 hPa.
[0028] The flash effect takes place spontaneously when the
suspension enters the evaporation zone, if the suspension
temperature is above the saturated vapour temperature of the
solvent, which temperature forms part of the system pressure.
Particularly for cellulose ethers, the system pressure is chosen so
that the saturated vapour temperature is <60.degree. C. (below
the flocculation point). The vapours formed during the flash effect
(flash evaporation) leave the paste bunker via a connection in the
lid of the cylindrical container. Preferably, the suspension is
concentrated by 2 to 5% and the polymers are cooled to
<60.degree. C., in particular 60.degree. C. to 29.degree. C. In
order to avoid condensation of the vapours on the cylindrical outer
wall, these can preferably be designed to be heatable by means of a
double jacket. In order to achieve an additional cooling effect
after the flash process, the double jacket can in particular also
be cooled, the concentration effect being reduced by the flashing
of the solution due to partial condensation.
[0029] The paste bunker gives a sufficient residence time,
adjustable by means of the height of fill, in order to ensure
complete gelling before the gel leaves the paste bunker through the
conveyor screw. The poorly meterable product, in particular
cellulose ether gel, is fed to the conveyor screw with the aid of a
stirrer and the thrust parts mounted in particular in the lowermost
stirring plane.
[0030] Owing to its design, the paste bunker permits gas tightness
with respect to downstream apparatuses, in particular with respect
to a drying step by means of superheated steam, so that the state
of reduced system pressure in the bunker results in continuous
flashing of the hot cellulose ether suspension with simultaneous
cooling to the saturated vapour temperature forming part of the
system pressure. The system pressure is set so low that the
saturated vapour temperature is below the flocculation point of the
cellulose ether suspension. At the same time, concentration of the
suspension is achieved by the flash effect. On cooling, a
homogeneous gel is produced. The residence time for complete
gelling can be adjusted in the bunker. The paste bunker
simultaneously performs the function of metering the gel into the
superheated steam miller-dryer.
[0031] The use of a paste bunker having a modified sleeve stirrer
is particularly suitable for achieving the technical objects
(gentle gel formation) of the invention. The cooling of the hot
polymer suspension is achieved by "flashing" into the paste bunker
subjected to a vacuum, e.g. at <200 mbar. At the same time,
concentration of the cellulose ether suspension and gelling occur.
Vapours formed are taken off via the vapour connection in the lid
of the paste bunker. Gelling is permitted by a sufficient product
residence time, adjustable by means of the level in the paste
bunker. The preferably used solid-blade screw meters the gel into
the downstream miller-dryer, in particular into a superheated steam
miller-dryer.
[0032] The design of the stirring arms, in particular of the lower
plane, can preferably be adapted to the conveying behaviour of the
product and is all the more important for ensuring gas-tight,
continuous, uniform metering.
[0033] The point for feeding the suspension into the paste bunker
is preferably such that feeding is effected mainly tangentially
into the paste bunker for good separation of the suspension from
the vapours. However, the flashing may also be effected in
particular via the lid of the paste bunker. The height of the feed
point can be variable. In order to avoid condensation of the
vapours, the paste bunker can, as described, be heated by means of
a double jacket. In order to assist the cooling effect during
flashing of the suspension, the paste bunker may be cooled by means
of the double jacket. In this case, the achievable concentration is
reduced by condensing vapours.
[0034] Inter alia, the following advantages are achieved by the
invention:
[0035] 1. Gentle cooling/drying of suspensions, in particular of
solvent-containing gel-forming polysaccharide derivatives,
especially a cellulose ether suspension, by the flash effect.
[0036] 2. Cooling of solvent-containing, gel-forming polysaccharide
derivatives, in particular of the cellulose ether suspension, below
the flocculation point.
[0037] 3. Separation of the resulting vapours from the continuously
flashed-in suspension.
[0038] 4. Sufficient residence time for gelling, adjustable by the
variable level in the paste bunker.
[0039] 5. Formation of a homogeneous gel on cooling and
gelling.
[0040] 6. Continuous gas-tight metering from the paste bunker into
the downstream drying apparatus, in particular into a superheated
steam miller-dryer.
[0041] 7. Stirrer geometry adaptable to the product consistency in
order to ensure constant gas-tight metering.
[0042] The invention is further explained below with reference to
the figures and by means of the examples, which, however, do not
represent any restriction of the invention.
[0043] FIG. 1 shows a schematic diagram of the flash introduction
of the polymer.
[0044] FIG. 2 shows a basic design of the preferably used paste
bunker.
EXAMPLES
[0045] General description
[0046] A methylcellulose suspension prepared in a moist mixer (not
shown) is metered continuously with the aid of a Mohno pump (not
shown) into the paste bunker 1 under a reduced system pressure. The
level in the paste bunker 1 is kept constant by adaptation of the
Mohno pump speed to the gel throughput established by the conveyor
screw 7. The conveyor screw 7 of the paste bunker 1 conveys the gel
into the superheated steam miller-dryer 2 (Ultra-Rotor II type,
manufacturer Jaeckering), where it is dried and milled. The vapours
formed on flashing of the methylcellulose suspension are extracted
with the aid of a vacuum pump 13 via a valve 19 and the connection
5 in the lid 6 of the paste bunker 1. The system pressure in the
paste bunker 1 is set and kept constant by means of a pressure
relief valve 18. A vent valve 20 is also mounted on the same feed
line. In order to avoid crossing of the vacuum into the suspension
line 12 and gelling of the suspension with the danger of blockage
of the suspension line 12, a ball valve 4 is installed before the
entrance into the cylindrical part of the paste bunker 1. The inlet
is arranged tangentially to the housing circumference. The pressure
in the suspension line 12 is set by adjusting the ball valve 4 at
>100 mbar superatmospheric pressure. An inlet 15 and an outlet
16 for a heating fluid are provided on the paste bunker 1.
[0047] Embodiment
[0048] The stirrer 8 of the paste bunker 1 is equipped with four
stirring planes angled so that they press downwards and three
retarders 14 which are fastened in the cylindrical wall 3 and
prevent co-rotation of the product with the stirrer 8. The two
lowermost stirring arms 9 of the lowermost stirring plane are
additionally equipped, in the outer region over a length
corresponding to the screw trough width, with a wedge-shaped thrust
part 10. These thrust parts 10 are 2 mm away from the bottom and
ensure continuous metering of the product to the conveyor screw 7
by the stirrer 8.
[0049] A 21% strength by weight methylcellulose suspension (product
MT 40000, producer Wolff-Walsrode GmbH) at 82.degree. C. was
flashed (metered) continuously into the paste bunker 1 with the aid
of a Mohno pump. The system pressure in the paste bunker 1 was 130
mbar. The pressure in the suspension line 12 was set at 3 bar
superatmospheric pressure with the aid of the ball valve 4. The
double jacket 11 of the paste bunker 1 was not heated and not
cooled. The concentration of the methylcellulose suspension
increased in the moist mixer to 23.5% by weight by removal of water
by evaporation.
[0050] Throughputs of moist material in the range from 57 to 165
kg/h of suspension were operated. The residence time in the paste
bunker 1 was 14 min at low throughput and 6 min at high throughput.
The concentration by the flashing is about 0.054 kg of solvent per
kg of suspension. On increasing the throughput, the stirrer speed
was increased from 15 to 27 rpm and the conveyor screw speed from
12 to 28 rpm. The conveyor screw 7 conveyed the gel into the
miller-dryer 2. By increasing the superheated steam inlet
temperature in the miller-dryer 2 from 133 to 181.degree. C. at
high throughput, the superheated steam outlet temperature of
120.degree. C. was kept constant. The dried product had a residual
moisture content of 2%. The system pressure was 10 mbar
superatmospheric pressure on entry of the superheated steam into
the miller-dryer 2 and 10 mbar reduced pressure on exit from the
dryer 2. The speed of the miller-dryer 2 was set at 1063 rpm.
* * * * *