Note: Descriptions are shown in the official language in which they were submitted.
CA 02527969 2005-12-O1
Depolymerization method and device
Description
The invention relates to a process and an arrangement
for recovery of monomeric esters of substituted or
unsubstituted acrylic acid, of styrene and/or of
monomeric styrene derivatives from polymer material
comprising corresponding structural units.
Acrylate polymers, among which is acrylic sheet
manufactured mainly from polymethyl methacrylate
(PMMA), are used inter alia for production of long-life
consumer products. To this end, molding processes are
often utilized, and waste polymer is produced during
their course. A treatment process is useful not only
for this reason but also for recycling of used
polymers. Similar considerations apply to polystyrene
and to styrene-containing copolymers and their
treatment. Acrylate polymers, especially PMMA,
polystyrene, and styrene-containing copolymers can
advantageously be broken down again completely at
certain temperatures and pressures to give
corresponding monomers.
The introduction to the description of DE 198 43 112 Al
describes a continuous process for PMMA
depolymerization in which the comminuted plastic is
passed into a hot extruder in which two tightly inter-
meshing screws rotate with self-cleaning action. The
thermal and mechanical shear in the extruder cause
depolymerization of the PMMA. The resultant methyl
methacrylate (MMA) is drawn off in the gas phase by way
of a vent dome and condensed. The MMA content in the
condensate in this process varies from 89 percent to
97 percent, and the yield of MMA is smaller than
97 percent . The heating of the PMMA in the extruder in
that process takes place by way of jacket walls.
However, as reactor volume increases, the ratio between
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jacket area (and therefore heatable wall area) and
reactor volume to be heated becomes poorer. For large
systems on an industrial scale, therefore, very high
wall temperatures are required or reduced yield has to
be expected. High wall temperatures can lead to local
overheating which in turn can lead to formation of
undesired by-products which impair the purity of the
monomer.
It is moreover likewise known from the introduction to
the description of DE 198 43 112 A1 that PMMA can be
depolymerized by means of fluidized-bed pyrolysis. The
fluidization material used is quartz sand of grain size
from 0.3 to 0.7 mm. However, a plant with complicated
flow technology is required to maintain fluidized-bed
flow.
DE 198 43 112 A1 proposes bringing the polymer material
into contact with hot mechanically fluidized solid
(heat-transfer medium) in a reactor and drawing off and
condensing the resultant vapor. In this process, the
previously heated heat-transfer medium is continuously
fed at one end of the reactor and the cooled heat-
transfer medium is discharged at the other end. The
heat-transfer media used comprise inorganic fine
particle solids whose grain size is from 0.1 to
5 millimeters or naturally occurring or synthetically
produced oxides based on silicon, aluminum, magnesium,
or zirconium, or else mixtures composed of these
elements.
That process therefore requires heating equipment
separate from the reactor and equipment for charging
and discharging of the heat-transfer medium into the
reactor and, respectively, out of the reactor. The
discharge of the heat-transfer medium out of the
reactor also has to be coordinated with the residence
time of the polymer material and therefore with the
dynamics of the depolymerization process, in order to
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obtain the desired yield of monomer.
It is an object of the present invention to provide a
process and an arrangement for recovery of monomeric
esters of substituted or unsubstituted acrylic acid, of
styrene and/or of monomeric styrene derivatives from
polymer material comprising corresponding structural
units, where these permit, with very low process-
technology costs, achievement of effective heat
transfer to the polymer material and achievement of
substantially homogeneous temperature distribution at
least in subregions of the reactor space.
In the inventive process, the polymer material is
brought into contact with a heat-transfer medium in a
heated reactor, the heat-transfer medium and the
polymer material are moved in the reactor, and gas
which comprises the monomer and is produced in the
reactor is drawn out of the reactor. Surprisingly, it
has been found that the depolymerization process leads
to good results even without complicated fluidized-bed
technology, if the heat-transfer medium comprises a
large number of spherical particles.
Accordingly, it is proposed that this large number of
spherical particles be provided in a heatable reactor
to produce the monomer gas in the inventive
arrangement. A displacer is present here in order to
move firstly the spherical particles and secondly the
material to be moved, comprising the polymer material,
in the reactor.
The reason for the surprising action of the spherical
particles on the depolymerization process is probably
that, in comparison with particles of other shapes, the
spheres can slip with particular ease with respect to
one another, with respect to surfaces of the reactor
and of any equipment arranged therein (e. g. heating
system and/or displacer) and with respect to the
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polymer material, and therefore mix particularly well
with each other and with portions of the polymer
material. It is therefore possible to achieve effective
heat transfer from the heating system to the polymer
material and substantially homogeneous temperature
distribution at least in subregions of the reactor
space.
As far as the size of the spheres is concerned, it has
proven advantageous in experiments for the diameter to
be in the range from 0.075 to 0.25 mm, preferably in
the range from 0.1 to 0.2 mm. Within this size range,
each sphere firstly retains considerable thermal
capacity for the depolymerization process and secondly
has particular ease of slip - like a particle of a
fluid.
Various designs of the displacer are possible. In
particular, use can be made of any of the variants
familiar to the person skilled in the art, e.g. moving
or rotating walls or other moving portions of the
reactor. The displacer can, by way of example, also
have one portion or two or more portions which execute
a mechanical vibration and/or a tenuous linear (or non-
linear) motion, thereby producing and/or maintaining
motion of the material to be moved in the reactor.
Preferred displacers have one or more rotating shafts
in particular provided with mixing elements curved in
the manner of a paddle and/or with other mixing
elements. The shaft(s)~ can extend horizontally or
vertically, for example. By way of example, a reactor
which has a mixing system with shaft running vertically
to which at least one mixing element protruding in the
radial direction of the shaft has been secured is
advantageous for a good mixing result. This embodiment
permits continuous motion of at least one portion of
the material to be moved, and, because the particles
here are spherical, the material to be moved is
subjected to continuous mixing motion.
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The spherical particles preferably remain in the
reactor during the depolymerization process and are
not - as described in DE 198 43 112 A1 - fed at one end
of the reactor and discharged at an opposite end. The
process is considerably simplified by their retention
in the reactor. The process described in
DE 198 43 112 A1 of relatively complicated and wasteful
heating of the heat-transfer medium outside of the
reactor can also be eliminated here (in which
connection see the following paragraph). However, the
invention is not restricted to retention of the
spherical particles in the reactor. Other embodiments
of the invention can also easily achieve better mixing
than with conventional heat-transfer medium, or the
same level of mixing by simpler means. In particular,
lower drive energy levels and a correspondingly lower-
power displacer, and lower heating power are
sufficient. Local overheating with the abovementioned
adverse effects is eliminated.
In one preferred embodiment, the reactor, or at least
portions of the reactor, is/are heated directly,
preferably electrically. By way of example, an area
which is part of an outer wall of the reactor and which
faces inward toward the reactor interior is heated,
and/or at least one portion of a displacer arranged in
the reactor is heated. In particular, at least one
portion of the reactor or within the reactor has been
connected to a heating system in a thermally conductive
manner, and this portion in turn comes into contact
with some of the spherical particles during the
procedure for moving these particles. Good heat
transfer to the polymer material is thus achieved with
the aid of the particles.
If the polymer material comprises acrylic compounds,
the average temperature of the particles of the heat-
transfer medium in the reactor is in particular in the
range from 250 to 600 degrees Celsius, and for recovery
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of MMA it is preferably below 425°C, the auto-ignition
temperature of MMA. Experiments explained at a later
stage below have shown that the inventive process can
achieve high yields and likewise high purity of the
monomer even at these low temperatures.
In one embodiment of the inventive process, the polymer
material is heated and depolymerized during its
residence time in the same reactor. There is no
requirement here - as for example disclosed in
DE 31 46 194 A1 - to preheat the polymer material in a
heating space upstream of the actual reactor space,
because the spherical particles permit particularly
rapid heat transfer from the heat source to the polymer
material, and because particularly uniform temperature
distribution can be achieved.
However, the invention is not restricted to this
single-stage heating process. Indeed, by way of
example, the polymer material may be introduced after
preheating in a feed vessel allocated to the reactor or
introduced after preheating in this type of feed
vessel.
The spherical particles are preferably composed of a
material not reactively involved in the recovery of the
monomer. This can simplify or even eliminate treatment
of the heat-transfer medium. By way of example, steel
has good suitability as material for the spherical
particles. Particular preference is given to stainless
steel, in particular chromium- and nickel-containing
steel, such as 18/10 Cr/Ni steel (V2A steel) or 17/12/2
Cr/Ni/Mo steel (V4A steel). Even standard steel has
excellent elasticity, and - given appropriate
mechanical excitement via the displacer - easy slip is
also combined with saltation of the individual
particles, and this accelerates heat distribution.
Another reason for the particularly good suitability of
stainless steel as material is that it is resistant to
CA 02527969 2005-12-O1
chemical reactions with a wide variety of the
substances introduced into the reactor in the polymer
material or together with the polymer material. Spheres
composed of V2A steel or V4A steel can moreover be
produced at low cost.
The depolymerization process preferably takes place in
an inert-gas atmosphere, e.g. in a nitrogen atmosphere.
The pressure in the reactor here can be ambient
pressure (generally the same as the pressure of the
earth's atmosphere) or below or above ambient pressure.
If superatmospheric pressure is used, this is by way of
example up to 133.3 hPa (100 tort). Although the
invention also encompasses higher superatmospheric
pressures, in practice they imply higher costs for
technical equipment. The superatmospheric pressure is
preferably in the range from 50 to 80 hPa (from 37.5 to
60 tort), in particular from 65 to 70 hPa (from 48.75
to 52.5 tort). If subatmospheric pressure is used this
can by way of example be from 80 to 133.3 hPa (from 60
to 100 tort) below ambient pressure. Here again, higher
pressures (i.e. lower absolute pressures) are possible.
In one particular embodiment of the invention, airlock
equipment is provided for introduction of the polymer
material into the reactor, and this airlock equipment
comprises an airlock chamber. There is, furthermore, a
first closure arranged at an input side of the airlock
chamber and a second closure arranged at an output side
of the airlock chamber. Evacuation equipment and gas-
charge equipment have been combined with the airlock
chamber so that when the first and second closure have
been closed, it is possible to evacuate gas from the
airlock chamber and to charge an inert gas to the
airlock chamber. In this way it is possible, repeatedly
and respectively, to introduce an amount of polymer
material into the airlock chamber while the first
closure is open, evacuate the airlock chamber, pass the
inert gas into the airlock chamber, and then, after
CA 02527969 2005-12-O1
opening of the second closure, introduce the polymer
material into the reactor.
Because the location of the polymer material even
immediately before it is introduced into the reactor is
in an inert-gas atmosphere, no direct charging of inert
gas to the reactor is needed. In particular, this
permits better insulation of the reactor with respect
to heat losses.
The invention is described in more detail below by way
of example on the basis of the attached drawing.
However, there is no restriction of the invention to
the inventive examples. The individual figures of the
drawing are diagrams showing:
fig. 1 a plant for recovery of monomeric substances
from polymer material,
fig. 2 a heated reactor for production of gas
comprising monomer from the polymer material,
from above,
fig. 3 an arrangement of spherical particles which
are moved by a mixing element,
fig. 4 the arrangement as in fig. 3 at a later
juncture of the motion and
fig. 5 airlock equipment which by way of example has
been installed upstream of the reactor shown
in fig. 1 and serves for passing polymer
material into the reactor via an airlock.
The plant shown in fig. 1 serves by way of example for
depolymerization and recovery of MMA. However, it can -
if appropriate with suitable adjustment of the pressure
and of the temperature in a reactor 1 of the plant -
alternatively be used for recovery of other monomeric
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_ g _
esters of substituted or unsubstituted acrylic acid, of
styrene and/or of monomeric styrene derivatives.
The plant described below is, as previously mentioned,
an inventive example. One or more constituents of the
plant can be replaced by other constituents. In
particular, the method of introduction described below
of the polymer material into the reactor can be varied,
as can the reactor itself and/or the treatment of the
monomer gas drawn off from the reactor.
The location of the polymer material to be
depolymerized is in a feed vessel 23, whose outlet has
attached metering equipment 21. The polymer material
passes by way of the metering equipment 21 into an
airlock chamber 19. An example of airlock equipment is
explained in more detail using fig. 5. The airlock
equipment and airlock chamber 19 serve to pass the
polymer material into the reactor 1 via an airlock, so
that the depolymerization procedure can take place in
an inert-gas atmosphere.
The polymer material is introduced into the reactor 1
by way of a charging aperture 14 of the reactor 1. The
reactor 1 illustrated in fig. 1 is a heated reactor
with a continuously driven shaft 3 oriented
horizontally, from which a large number of arms 11
protrude in the radial direction of the shaft 3. On
that end of the arms 11 opposite to the shaft 3 there
is in each case a mixing element 13 arranged, which by
way of example has a triangular shape as illustrated in
cross section. The mixing elements, which can also be
regarded as including the arms, can also have a
different design, e.g. a paddle shape and/or a large
number of mixing elements attached at the end of each
arm. A suitable mixing element is selected as a
function of the nature and size of the spherical
particles used (heat-transfer media) and/or as a
function of the nature and size of portions of the
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polymer material. By way of example, the polymer
material can be treated in various ways prior to
introduction into the reactor, in particular comminuted
into varying-size pieces and/or shapes. A familiar and
suitable process is shredding of relatively large
portions of the polymer material to give pieces with
dimensions of from relatively few millimeters to two or
more centimeters. Because the particles are spherical,
the depolymerization process can proceed with good
yields and with high purity using different sizes and
shapes of the polymer material.
Fig. 2 shows an alternative design of the reactor,
namely a reactor 51 with a vertically oriented rotating
shaft 53 and with a plurality (in this case three) of
paddle-shaped mixing elements 63 directly attached to
the shaft 53 and protruding radially from the shaft 53.
The straight-line representation of the mixing elements
63 is to be interpreted diagrammatically. Embodiments
can have vertical and/or horizontal non-linearity of
the mixing element 63.
Fig. 2 shows a material 65 to be moved by the mixing
elements 63 with pieces of polymer material 66 and
spheres 67 (for reasons of clarity of presentation only
in one of the three sectors divided by the mixing
elements 63 in the reactor 51). The shape of the
spheres 67 enables them to move easily with respect to
one another and relative to the pieces of polymer
material 66. No mechanical interlocking or sticking
therefore takes place within the material 67 to be
moved. In order to ensure particularly good motion
within the material to be moved, it is preferable that
the total volume of the spheres in the reactor is
greater than the total volume of the remaining solid
pieces of polymer material, in particular at least
twice as great. This very substantially eliminates
caking or interlocking of the pieces of polymer
material.
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It is also preferable to use spheres composed of
stainless steel, because particularly in the case of
PMMA there is then no caking of the polymer material on
the spheres. However, the high level of freedom of
motion of the spheres always helps to prevent the
polymer material from caking on the mixing elements, on
other portions of the displacer and/or on the reactor
walls.
As indicated in fig. 2, the reactor 51 has an
electrical heating system 59 at least on the reactor
side wall. The reactor side walls and the base are
preferably heated over substantially their entire
surface and/or by means of heating equipment uniformly
distributed across the wall area. By way of example, a
conventional electrical resistance heating system
and/or inductive heating equipment can be used, not
only in this specific example.
In the case of the reactor 1 illustrated in fig. 1, the
outer jacket, by way of example circular, of the
reactor 1 is heated over its entire surface via heating
equipment 9. In order to obtain good long-term running
properties of the shaft 3, the shaft 3 can be combined
with a shaft cooling system 5.
The mixing motion and the spheres present in the
reactor 1 cause heating and depolymerization of the
polymer material in the reactor 1 in a short time. The
time typically needed for complete conversion of a
piece of PMMA to the monomeric gas phase is in the
range from five to sixty seconds, depending on the
average sphere temperature.
The gaseous MMA is drawn off via a gas aperture 15 of
the reactor 1 and via monomer gas line 25 into a
separator 27 in which additives, e.g. color pigments,
are added. As shown in fig. 1, the separator 27 is in
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particular a cyclone. The additives can be drawn off
from the separator 27 by means of a pump 29 by way of a
draw-off line 28. Inert gas (here nitrogen) also passes
into the separator 27 alongside the MMA and the
additives. The inert gas is subsequently separated from
the MMA or drawn off together with the MMA.
The MMA/inert gas mixture is passed by way of a
connecting line 31 into a cooler 33 (for example a
quencher), in which a portion of previously cooled and
returned condensate is sprayed as in a shower by means
of a nozzle onto the gas mixture, which is still hot,
thus cooling the gas mixture in a very short time. This
can further increase the yield and purity of the
monomer. This markedly reduces the level of solid
deposits which can be produced on other conventional
coolers. For the return process, the cooler 33 has been
connected by way of a monomer draw-off line 35 to a
monomer container 37 into which the cooled monomer is
drawn off. A portion of the monomer located in the
monomer container 37 is returned by means of a pump 41
to the nozzle of the upper apparatus by way of a return
line 40. There is a continuous cooler 43 located in the
return line 40.
The monomer is drawn off from the plant by way of
another line 38 attached to the monomer container 37,
driven via a pump 39. Additional return to the cooler
33 is possible via a connector line connected to the
line 38 and capable of shut-off by means of a valve 47.
For this purpose, the other end of the connector line
46 has been connected to that portion of the return
line 40 which is located on the other side of the
continuous cooler 43 in the direction of flow. The
resultant temperature at the nozzle of the cooler 33
can be controlled by way of control of the valve 47 and
corresponding mixing of returned monomer of varying
temperature
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Fig. 3 and fig. 4 again illustrate a substantial effect
of the inventive spherical shape of the heat-transfer-
medium particles. As indicated via a two-line arrow
pointing downward in the illustration, a sphere 68
adjacent to a mixing element 63 is moved and transfers
the motion to two other spheres 69 , 7 0 adj acent to i t .
The resultant motion of the spheres 69, 70 has been
shown via two arrows. The result is that the spheres
69, 70 are pushed apart with very little resistance to
motion and permit easy passage of the sphere 68 (as
shown in fig. 4). Corresponding motion of the spheres
is also possible relative to pieces of polymer material
with similarly low resistance to motion.
Fig. 5 shows airlock equipment 22 which by way of
example in the arrangement of fig. 1 can be installed
upstream of the reactor 1. A filler neck 20, which by
way of example is attached to the metering equipment 21
shown in fig. 1, opens into the airlock chamber 19 of
the airlock equipment 22. The arrangement has, at the
input side of the airlock chamber 19, a first closure,
in the example an upper closure, which in the inventive
example of the airlock equipment 22 shown can close the
filler neck 20 by means of a closure component 80
capable of forward and backward motion. The design of
the closure 71 is generally such that it can provide
gas-tight sealing of the airlock chamber on the input
side . On the output side of the airlock chamber 19 , in
this case below the airlock chamber, there is a second
closure 72 which likewise by way of example can, by way
of a closure component 81 capable of forward and
backward motion, close a polymer line 17 to the
reactor. The design of the second closure 72 is also
such that it can provide gas-tight closure of the
container 19 (in this case on the output side).
Attached to the airlock chamber 19 there is moreover a
gas line 74, combined with a main valve 78. On that
side of the main valve 78 opposite to the airlock
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chamber 19 there is a T piece 75 at which the gas line
74 branches into an upper branch and a lower branch.
The arrangement has a pump 76 in the upper branch. The
arrangement has an inert-gas valve 79 in the lower
branch. The lower branch opens by way of example into
the inert-gas feed line shown in fig. 1. The upper and
the lower branch do not have to lead upward and,
respectively, downward as shown in the figure, but can
lead in any suitable direction.
During the depolymerization procedure, in particular
during operation of the plant shown in fig. 1 or of
another plant, an amount of polymer material is
repeatedly charged to the reactor. For this purpose,
the amount of.~polymer material is first charged via the
filler neck 20 into the airlock chamber 19. The second,
lower closure 72 has been closed here. Once the polymer
material has been introduced, the first, upper closure
71 is also closed. The main valve 78 is then opened
(unless it is already open), and gas (in particular
air) is evacuated from the airlock chamber 19 by means
of the pump 76. The inert-gas valve 79 has been closed
here. The pump 76 is then switched off and, if
necessary, the upper branch of the gas line 74 is also
shut off. Inert gas is then moreover passed into the
airlock chamber 19 while the main valve 78 and the
inert-gas valve 79 have been opened, until a desired
pressure has been reached. The final pressure reached
here in the airlock chamber 19 is preferably higher
than the pressure of the inert gas in the reactor. This
can firstly compensate for inert gas losses from the
reactor via draw-off of the monomer/inert gas mixture
from the reactor and can secondly inhibit escape of the
monomer/inert gas mixture via the polymer feed line
into the airlock chamber 19.
Once the final pressure has been reached in the airlock
chamber 19, the second, lower closure 72 is opened and
the amount of polymer material is thus introduced into
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the reactor.
Experimental examples for design and execution of the
depolymerization procedure in a reactor are now
described below.
Example 1: A paddle reactor of diameter 280 mm and
length 400 mm was selected. Twelve kilograms of steel
spheres of diameter 0.2 mm were charged as heat-
transfer medium to this reactor. During the
depolymerization process, the average temperature of
the spheres was 456 degrees Celsius, and the
superatmospheric pressure of the inert gas in the
reactor (nitrogen in the example) was 66.7 hPa (about
50 torr), based on the pressure of the earth's
atmosphere at sea level, and the rotation rate of the
shaft of the paddle reactor was 100 rpm. An MMA yield
of 97~ with purity of 98.5 was achieved using the
plant structure as shown in fig. 1.
Example 2: The procedure corresponded to that of
example 1, but twenty kilograms of the steel spheres
were charged to the reactor and the average temperature
of the spheres was set at 380 degrees Celsius. MMA
yield was 98~ with purity of 99~.
Example 3: The experiment was carried out as described
in example 2, but the process was run with an average
temperature of only 320 degrees Celsius for the
spheres. MMA yield was 98.5 with purity of 99~.
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Appendix to the letter of March 24, 2005 to the
European Patent Office, Munich, relating to internat.
Patent application No. PCT/EP2004/002954 (Our
reference: OZ 21721
What is claimed is
1. A process for recovery of% monomeric esters of
substituted or unsubstituted acrylic a~d, of
% i
styrene and/or of monom~ric styrene d rivatives
from polymer material;~comprising co~esponding
'
structural units, where,-'~
- the polymer matey al is brought; into contact
with a heat-transfer medium in a heated
reactor ( 1; 51,y',
- the heat-tra~isfer medium a%nd the polymer
material arej~moved in the reactor (1; 51) and
- gas which~~~ comprises the%~' monomer and is
'
produced ~n the reactor (;~; 51) is drawn out
of the reactor (1; 51),
where the hat-transfer medium comprises a large
number of spherical particles (67), in particular
having a diameter in the range from 0.075 mm to
i
0.25 mm. ;
2. The process as claimed in claim 1, where the
polymer material comprises acrylic compounds and
where the average temperature of the particles
i
(67~' of the heat-transfer medium in the reactor
(1j' 51) is in the range from 250 to 600 degrees
Celsius.
i
3. ;The process as claimed in claim 1 or 2, where the
reactor (1; 51; is electrically heated.
Jr
4t The process a:~ claimed in any of claims I to 3,
where the spherical part.~cles (67; are composed of
i
a material not reactivelr;~ involved in the recovers
of the monomer.
