Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a process for preparing
solutions of cellulose in aqueous tertiary amine oxides
from a suspension of cellulose in an aqueous solution of
the tertiary amine oxide by supplying heat at a reduced
pressure, as well as to an arrangement for carrying out
the process.
A process of this kind is described in PCT
publication WO 83/04415. There, cellulose is suspended in
an aqueous solution of a tertiary amine oxide containing
up to 40 % by mass of water and is heated to temperatures
of between 90 and 120C under stirring. At the same time,
the pressure is reduced to 80 to 150 mbar and water is
drawn off until the cellulose dissolves. In this manner,
spinnable solutions containing up to 15 % by mass of
cellulose are producible.
By shapinq these solutions in water films, threads
or shaped parts based on cellulose are obtained, i.e.,
articles which, today, are produced according to the
viscose method on a large scale. However, as regards
environmental safety, spinnable solutions of cellulose in
aqueous tertiary amine oxides have a decisive advantage
over viscose: While the tertiary amine oxide is capable
of being recovered during spinning and used again, H2S,
COS, CS2 and colloidal sulfur form at the decomposition
of viscose. These substances can be disposed of only at
great expenditures.
Nevertheless, the above-mentioned process using
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tertiary amine oxides as solubilizing agents has not
found general acceptance so far, still involving a number
of disadvantages.
Thus, water can be drawn off a stirrer vessel only
with difficulties on account of the unfavorable ratio of
the liquid surface to the liquid volume, and this results
in long residence times ranging between 2 and 4 hours
within the stirrer vessel. During that time, the polymer
cellulose chain is partially degraded, which is even
further promoted by the elevated temperature. This
partial degradation, in turn, adversely affects certain
properties of the final products after the spinning
process, such as, e.g., strength, elongation and loop
strength. Furthermore, it is known that heating, in
particular to more than 130C, may lead to a strong
discoloration due to the decomposition of the amine oxide
used. With some compounds, such as, e.g., N-methyl
morpholine N-oxide, this degradation even may occur like
an explosion under vigorous gas development such that the
solutions present in the stirrer vessel constitute a
safety risk by their amount.
When carrying out the process on a large scale, one
would, therefore, have to operate at adequate safety
provisions with high pressure autoclaves if stirrer
vessels were used, which autoclaves are out of the
question for continuous operation for economic reasons.
On the other hand, without safety means, only a
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discontinuous operation is possible in the stirrer
vessel, which renders the process rather inflexible,
because it is difficult to vary parameters, such as,
e.g., the temperature and the evaporation rate. Add to
this that, due to the high viscosity of the cellulose
solutions, much spinning mass is retained by the stirrer
vessel, which impairs the cleaning of the vessel and
further reduces the economy of the process.
The invention has as its object to eliminate these
disadvantages and to provide a process for producing
cellulose solutions in aqueous tertiary amine oxides,
which is operable continuously, wherein the heat
treatment of the suspension is to be performed over a
substantially shorter period of time in order to minimize
the thermal load on the cellulose and the tertiary amine
oxide. Moreover, the safety risk inherent in the prior
art is to be avoided. The invention, furthermore, seeks
to provide an arrangement for carrying out the process,
which does not have the disadvantages connected with
stirrer vessels and high-pressure autoclaves.
In accordance with the invention, this object is
achieved in that the suspension is transported over a
heating surface, spread in layers or coats, until a
homogenous solution of the cellulose has formed, which
has a viscosity of between 50 and 15,000 Pas.s, feeding
of the suspension and drawing-off of the homogenous
solution being carried out continuously.
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The spreading of the cellulose suspension in layers
or coats over the heating surface results in a large
liquid surface, which facilitates the removal of water.
At the same time, rapid heating of the suspension to the
temperature required for the preparation of the solution
is feasible. By being transported over the heating
surface continuous mixing of the suspension is ensured,
which further promotes the heat and substance exchanges.
To adjust the viscosity of the solution, which is
measured in the relative system, and to influence the
swelling behavior of the cellulose within the suspension,
a diluent, e.g., ethanol, may be added to the suspension.
Particularly good mixing is ensured if the layer
spread over the heating surface has a thickness of 20 mm
at the most, preferably 1.5 to 5 mm.
Advantaseously, N-methyl morpholine N-oxide is used
as the tertiary amine oxide, preferably in an aqueous
solution containing 40 % by mass of water.
A preferred embodiment of the process according to
the invention is characterized in that the suspension is
brought to a temperature of between 50 and 150C,
preferably of between 60 and 100C, and is subjected to a
pressure of from 0.5 mbar to 1,000 mbar, preferably 50
mbar to 150 mbar.
It has proved particularly favorable if the
suspension is contacted with the heating surface for a
period of from 1 min to 60 min. This period of time, on
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the one hand, is sufficient to produce a homogenous
solution and, on the other hand, is so short that the
decomposition of the tertiary amine oxide and the
degradation of the cellulose are largely prevented.
A suitable arrangement for carrying out the process
of the invention, comprising an indirectly heated
evacuatable vessel provided with a stirring means, is
characterized in that the vessel is designed as a
cylindrical container including a centrically mounted
stirring shaft having agitators joined thereto, the
radial distance of the agitators from the internal wall
of the container being 20 mm at the most, and there are
provided an intake for the cellulose suspension in the
upper part of the container and an outlet for the
homogenous cellulose solution in its lower end.
An advantageous embodiment of the arrangement
according to the invention comprises a distribution ring
provided at the stirring shaft to spread the cellulose
suspension like a layer or coat on the internal wall of
the container.
To control the transportation of the cellulose
suspension along the internal wall of the container, it
has proved advantageous if the agitators have an angle of
inclination relative to the axis of the stirring shaft,
which is adjustable in size.
The process according to the invention to be
performed with the arrangement according to the invention
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is extremely flexible with regard to varying the
operational parameters and involves a substantially lower
safety risk as compared to the prior art, since no large
amount of solvent is heated at once, but only a
comparatively small amount is continuously heated due to
the solvent being spread in layers over the heating
surface.
The arrangement according to the invention will now
be explained in more detail with reference to Figs. 1 and
2, wherein:
Fig. 1 is a partial longitudinal section hrough the
arrangement according to the invention, and
Fig. 2 illustrates a section along line II-II of Fig.
1 on an enlarged scale.
1 denotes the internal wall of a preferably upright
rotational body, which, in the exemplary embodiment
illustrated, is designed as a cylindrical container 2
almost over its entire length. For the major part, the
internal wall 1 is surrounded by a heating jacket 3
including connection means 4 and 5 for heating medium,
the connection means 4 serving to supply, and the
connection means 5 serving to discharge, the heating
medium.
A stirring shaft 7 comprising agitators 8 joined
thereto is centricallly mounted within the container 2
and driven by a motor 6. The agitators 8, which are plain
in the exemplary embodiment illustrated, extend radially
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to the axis, their plane enclosing an angle of
inclination alpha relative to the axis 9 of the stirring
shaft 7, which preferably is adjustable in size. A
distribution ring 10 is attached to the stirring shaft 7
above the agitators 8, spreading the cellulose suspension
introduced through intake 11 on the internal wall 1 in
layers. The distribution ring 10, thus, is provided at
the level of the intake 11.
On its lower end, the container 2 is frustoconically
tapered, including an outlet 12 for the homogenous
cellulose solution. The agitators 8 have a constant
radial distance 13 from the internal wall 1 of the
container 2 over the total extension of the container 2,
which distance is 20 mm at most.
On the upper part of the container 2, i.e., above
the plane of the distribution ring 10, an opening 14 is
provided for evacuating the container 2 and for drawing
off water vapor.
The arrangement functions in the following manner:
The cellulose suspension - if desired in a pre-
heated state - is continuously fed, through the intake
11, into the container 2 being under a reduced pressure,
is seized there by the distribution ring 10, is spread on
the internal wall 1 and is transported by the agitators
8, along the indirectly heated internal wall 1 serving as
heating surface, to the outlet la on the lower end of
the container 2. Heat carrier media, such as water, oil
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or vapor, are suited for indirect heating.
The cellulose suspension is heated during its
transportation along the indirectly heated internal wall
1 and water simultaneously evaporates due to the reduced
pressure, so that the tertiary amine oxide becomes
concentrated until the cellulose dissolves.
Fig. 2 illustrates in detail how the cellulose
suspension is processed in the container 2. It shows the
stirring shaft 7 with agitators 8, the internal wall 1
and the heating jacket 3, a rotation direction of the
stirring shaft 7 in the clock-wise direction having been
assumed and illustrated by arrow 7'. The spreading in
layers, respectively the thickness of the layer of
cellulose suspension, is provided by the radial distance
13 of the agitators 8 from the heated internal wall 1.
Bow waves of cellulose suspension, which are
schematically illustrated in Fig. 2, are formed at the
agitators by the rotational movement. As indicated in
Fig. 2, the cellulose particles are circulated in these
bow waves, which movement is translated to the suspension
layer spread on the internal wall 1. Thereby, continuous
rearrangement as well as intensive mixing of the
suspension are safeguarded, substantially promoting the
heat and substance exchanges.
It is decisive to the continuous control of the
process according to the invention that the water vapor
separated is drawn off in counterflow relative to the
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suspension transport. Moreover, it is important to the
rapid drawing off of water vapor to provide for a
sufficiently large exhaust vapor space 15, which is the
case if the ratio of the length to the diameter of the
cylindrical part of the container 2 amounts to between 4
and 8.
By means of the invention, it is possible to produce
cellulose solutions containing up to 30 % by mass of
cellulose.
The invention will be explained in even more detail
by the following examples:
Example 1
A suspension of prehydrolysis sulfate cellulose
(degree of polymerization about 1,400) in an aqueous
solution of N-methyl morpholine N-oxide having a water
content of 40 % by mass was heated to 70C and
continuously introduced into the arrangement according to
the invention at an amount of 90 kg/h through intake 11.
The content of prehydrolysis sulfate cellulose in the
suspension was chosen such that a final concentration of
cellulose of 10 % by mass was obtained after evaporation
of excess water.
The stirring shaft 7 was operated at a speed of 450
min 1, the thickness of the layer spread over the
internal wall being 15 mm. The indirectly heated internal
wall 1 had a surface of 0.5 m2 and was heated with heat
carrier oil in a manner that a mean temperature
difference of 83C resulted as a function of the heating
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up of the suspension (in counterflow relative to the heat
carrier oil). In the exhaust vapor space 15, a pressure
of 100 mbar was adjusted.
72 kg of homogenous cellulose solution per hour
were obtainable at the outlet 12, which corresponds to a
residence time of the suspension within the arrangement
of the invention of 3 min. The solution was capable of
being discharged in a degassed form. Its viscosity was
1,500 Pas.s (measured in the relative system). It was
ascertained by microscopic assaying of the solution that
no undissolved cellulose particles were present in the
solution.
The exhaust vapors formed were drawn off in
counterflow at a temperature of 70C and subsequently
were condensed, the distillate stream being 29 kg per
hour.
Example 2_
A suspension of ground prehydrolysis sulfate
cellulose (degree of polymerization about 1,400) in an
aqueous solution of N-methyl morpholine N-oxide having a
water content of 40 % by mass was heated to 80C and
was continuously introduced into the arrangement of the
invention through intake 11 at an amount of 90 kg/h. The
content of prehydrolysis sulfate cellulose was chosen
such that a final concentration of cellulose of 15 % by
mass was obtained after evaporation of excess water.
The stirring shaft 7 was operated at a speed of 450
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min 1, the thickness of the layer spread over the
internal wall 1 amounting to 1.5 mm. The indirectly
heated internal wall 1 had a surface of 0.5 m2 and was
heated with heat carrier oil in a manner that a mean
temperature difference of 112C resulted as a function of
the heating up of the suspension (in counterflow relative
to the heat carrier oil). In the exhaust vapor space lS,
a pressure of 150 mbar was adjusted.
64 kg of homogenous solution per hour were
obtainable at the outlet 12, incurring in a degassed
form. This mass flow corresponded to a residence time of
4 min.
The solution was obtained as a high-viscous mass
(11,000 Pas.s, measured in the relative system), no
undissolved cellulose particles being detectable under
the microscope. The solution was directly supplied to a
spinning machine and spun to cellulosic fibers.
The exhaust vapors formed were drawn off in
counterflow at a temperature of 80C and subsequently
were condensed, the distillate stream being 26 kg per
hour.
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