EP0807698A2 - Process and apparatus for the preparation of fibrets from cellulose derivatives - Google Patents

Abstract

In order to ensure a more economical production of fibrets with better quality, a method is described in which a dope containing a cellulose derivative and a suitable solvent is introduced into a precipitant and exposed there to a shear field, the dope being precipitated and the fibrets being formed will. The fibrets, the solvent and the precipitant are then separated. The suspension formed from the dope and the precipitant is alternately accelerated and decelerated at least once in the shear field. The suspension is preferably accelerated and decelerated alternately at least twice. As a result, a high degree of turbulence is maintained over a long distance, so that a dope with high viscosity can be processed. The device for producing fibrets provides a dispersing device (40) having at least one nozzle (46-49) in a rotor (44) and stator (43, 45), which has at least two toothed rings (50-53), at least of which a ring gear (50) is part of the rotor (44) of the dispersing device (40).

Description

The invention relates to a method for producing fibrets according to the preamble of claim 1 and a device according to the preamble of claim 21.

Fibrets are understood in the following as very fine fibers, which are characterized by very fine fiber diameters and thus by a very high mass-specific surface. Fibrets are typically produced by means of a precipitation process or by extrusion, in which case precipitation is usually integrated as a sub-process. Fibrets are partly present in the fiber composite or fiber network as a result of the production. The diameters of the individual fibers are generally less than 5 µm, mostly less than 1 µm. The dimensions of the fiber networks, which are referred to as agglomerates and which can be varied over a wide range by the conditions during production and by further processing steps, are up to 1 mm. However, agglomerate sizes of less than 200 µm are aimed for. With the dimensions mentioned, mass-specific surfaces of over 20 m ² / g are achieved.

The fibrets are mainly used in depth filters for liquid filtration, whereby these filters are also used to prove the quality of the fibrets. For optimal operation of depth filters, it is crucial to realize small pore sizes with high porosity. In terms of filtration technology, this results in high separation achieved low differential pressures. Embedding the fibrets in fiber networks also has the advantage over particulate material and shortened staple fibers that the fibers are very tightly integrated in the filter and detachments during the filtration can be largely excluded. The fiber structure in the agglomerate composite gives the filters high strength and flexibility, which is advantageous for pleating.

The use of fibrets is not limited to depth filters for liquid filtration. In nonwovens for air filtration, for example, fibrets can replace the glass fibers, the harmful effects of which are known when they enter the lungs. Since the very large surface area of most polymers requires intensive white coloring, fibrets can be used as optical brighteners in the paper industry. Small amounts of remaining solvent can cause the fibers to fuse during drying, so that e.g. in nonwovens the strength can be increased significantly. Due to the large specific surface, which is largely accessible to a medium flowing through, fibrets can be used advantageously for adsorption processes, including the chromatography processes. This effect can be supported by the introduction of surface-active substances or by a chemical modification of the starting material before the fibrin production or in the subsequent production process.

Fibrets can generally be made from a variety of materials. The only limits are set by the solvent and the viscosity of the solution. As a result of their advantages in the choice of the solvent, fibrets made of cellulose esters, in particular cellulose acetate, with mass-specific surfaces over 20 m ² / g have mainly been presented in the literature. For use in depth filters for liquid filtration, fibrets made from cellulose acetate also have the advantage that, together with the pulp used as matrix material largely achieved material homogeneity is achieved. This ensures problem-free disposal. Compared to the currently preferred diatomaceous earth, perlites and / or metal oxides, the advantages of the very low ion emission and the complete biodegradability must be emphasized.

The production of fibrets from cellulose esters with solvents is basically e.g. known from US 3,342,921, US 3,441,473, US 3,785,918, US 3,842,007, US 3,961,007, US 4,040,856, US 4,047,862, US 4,192,838, US 5,071,599 and US 5,175,276. A solution (dope) is usually prepared from a cellulose ester and a suitable solvent. The non-solvent for the cellulose ester, which is completely miscible with the solvent, can be present in proportions which do not significantly affect the dissolving behavior of the cellulose ester in the solvent. This solution is usually precipitated under the action of shear forces in a non-solvent or precipitant for the cellulose ester, which is completely miscible with the solvent.

Single-substance nozzle systems, stirring systems, two-substance nozzle systems and T-pipe systems are used for the precipitation process.

In single-component nozzle systems, as are described, for example, in US Pat. Nos. 3,441,473 and 4,040,856, the dope is sprayed above the coagulation bath, so that an at least partial spontaneous evaporation of the solvent begins. The shear required to produce fibrets is achieved at the spontaneous change in state, such as volume expansion, evaporation of the solvent and temperature drop, at the nozzle outlet. However, it was shown that the required fiber fineness and homogeneity could not be achieved with this process variant. The fiber diameters are predominantly above 1 µm.

Gew.-%) nach der Fällung daher nicht überschritten werden können. Da die Turbulenzen in steigendem Maße vom Feststoffgehalt abhängig sind, besitzt die Rühranordnung ein schlechtes Teillastverhalten. Ferner wird die Austrittsgeschwindigkeit des Dope aufgrund der Zentrifugalkräfte beeinflußt, weil die Extrusionsdüsen in der rotierenden Scheibe angeordnet sind. Die Fällbedingungen werden dadurch ebenfalls beeinträchtigt.Stirring systems, as described in US Pat. No. 4,047,862, have a rotating disk with radial extrusion openings through which the dope is introduced into the precipitation bath against a spaced, stationary peripheral wall. The dope emerges from the nozzles at a certain speed and is only slowed down on its way. Such systems have the disadvantage that the setting of a defined shear field is difficult. On the one hand, the shear gap must be chosen to be small in order to ensure an intensive shear field. On the other hand, at shorter distances there is a risk of the extrusion nozzles becoming blocked, which can only be prevented by a higher flow rate of the precipitant perpendicular to the shear field. Larger distances in turn cause the shear gradient to be too low. In addition, the resulting fibrets dampen the turbulence to a great extent, since the distance between the extrusion opening and the wall of 1.6 mm is only insignificantly larger than the maximum agglomerate size of 1 mm, so that solids contents exceeding 1% by mass (

% By weight) cannot therefore be exceeded after the precipitation. Since the turbulence is increasingly dependent on the solids content, the stirring arrangement has poor part-load behavior. Furthermore, the exit velocity of the dope is influenced by the centrifugal forces because the extrusion nozzles are arranged in the rotating disk. This also affects the precipitation conditions.

Two-component nozzle systems, as are described in US Pat. No. 4,192,838, US Pat. No. 5,071,599 and US Pat. No. 5,175,276, and the T-pipe systems which are explained, for example, in US Pat. No. 3,961,007, implement a similar principle. With regard to the flow directions of dope and precipitation medium, the precipitation is carried out in two-stream nozzles in cocurrent, in the case of T-pipe systems in countercurrent. In the case of the two-component nozzles, the nozzle diameter - usually> 2.5 mm - must be designed to be large enough to prevent the solution from failing at the nozzle opening. The nozzle diameter is therefore 20,000 times larger than the required one Fiber diameter. Even with a turbulent flow of the precipitating medium in the constriction of the valve nozzle, turbulence and concentration gradients occur, which in addition to fine fibers cause the formation of larger fibers. This can be observed the more the higher the solids content after the precipitation. Solids contents above 1% by mass cannot be exceeded if the required fiber fineness is guaranteed.

The countercurrent process in the T-pipe systems is the more effective variant with regard to vortex formation and thus shear field formation. However, the precipitation bath flow is braked very strongly, so that the precipitation conditions vary to a large extent. In addition to very fine fibers that meet the above-mentioned requirements, the formation of coarser fibers, especially in the case of solids contents after the precipitation of approximately 1% by mass, cannot be avoided. In contrast to the agglomerate size, these dimensions can only be influenced to a minor extent by workup steps after the precipitation. The partial load and clogging behavior of the counterflow variant is also to be assessed negatively.

A disadvantage of the known production process is that large amounts of solvent have to be circulated. For example, US Pat. No. 5,071,599 and US Pat. No. 5,175,276 mention 8 kg of solvent, for example acetone, for the production of 1 kg of fibrets. US Pat. No. 3,842,007 and US Pat. No. 3,961,007, for example, require between 20 and 80 kg or 33 kg of solvents, such as acetone, 1,4-dioxane or methyl acetate, for 1 kg of fibrets.

Since the fibrets have to be provided solvent-free or low in solvents for most applications, a process for separating the solvent from the fibrets is required. First of all, there is a filtration, in which a solution of solvent and non-solvent for the cellulose ester is separated from the fibrets. Because of The very large surface of the fibrets creates a very porous filter cake with a low solids content. A maximum solids content of approximately 12% by mass is usually achieved for fibrets with a specific surface area> 20 m ² / g at a filtration differential pressure of up to 1 bar. At pressures above 1 bar or in a centrifugal force field, the solids content is up to 20% by mass. The amount of solvent remaining in the fibrets is too high for further processing in most applications. In addition, the fibret surface is not fully hardened due to the solvent contained in the solution. The network structure is partially lost when pressure is applied, which leads to clumping at high pressures. For this reason, it is usually filtered under the action of shear forces, with solids contents of at most 4% by mass being achieved with specific surfaces of over 20 m ² / g. In the most favorable case of 8 kg of solvent for 1 kg of fibrets, after filtration, fibrets to solvent are in a ratio of approx. 1: 1, otherwise the solvent predominates.

For the reasons mentioned, a washing process, which in some cases can also be multi-stage, should follow the filtration, as is described, for example, in US Pat. No. 3,961,007. In the course of these processes, however, the concentration of the solvent in the filtrate becomes very low. A low concentration of the solvent and the large amounts of solution to be processed, however, cause the costs for processing the solution to increase exponentially. The workup is necessary because the solvent should be returned to the process of fibrin production for the economic design of the overall process and for reasons of environmental protection. Otherwise, the costs of the at least eight times the amount of solvent would have to be fully apportioned to the fibret costs and disposal costs would also be incurred. High volumes of solution to be worked up with a low concentration of the solvent to be obtained also cause the workup costs to rise sharply. Of the The process of producing the fibrin is therefore economically uninteresting for most applications in both cases.

In addition, filtration alone is not sufficient in most cases to remove the solvent from the fibrets. After the precipitation, the material of the fibrets still contains solvent, which diffuses into the liquid and can be separated from there. Therefore distillation is often used to remove acetone. The cost of this distillation depends heavily on the concentration of the solvents and the volume of the fibret suspension. Both quantities are also reflected in the volume of the distillate and in the concentration of the solvent in the distillate, which affects the cost of working up the distillate. Again, these costs determine the economic uses of fibrets in follow-up products.

The object of the invention is to provide a method and a device which allows a more economical production of the fibrets with better quality.

This object is achieved with a method according to the features of patent claim 1. The device is the subject of claim 21.

The invention is based on the finding that working up the constituents of the coagulation bath separated from the fibrets is only sensible and economical if the coagulant used for the coagulation bath may have as high a proportion of solvent as possible and if at the same time it is permitted that the separated solvent proportion , which is reused for the preparation of the dope, may still have a non-solvent content. The more "impure" the individual components may be after processing, the less effort is required. Contamination of the Solvent due to the non-solvent and vice versa has its limit where the dope prepared with the prepared solvent can no longer be processed. Too high a proportion of non-solvent can lead to premature precipitation on the one hand and to an excessive viscosity of the dope on the other.

It has been found that the precipitation can be carried out in a conventional dispersing device based on the rotor / stator principle. Such dispersing devices are e.g. distributed by the company Ystral under the name "dispersing machine" and by the company IKA Maschinenbau under the name "Dispax-Reaktor". These devices usually contain two to six shear rings, which are preferably designed alternately as stators and rotors. The rotors reach speeds of up to 12,000 revolutions per minute, so that basic flow velocities of up to preferably 100 m / sec can be achieved in the precipitation bath. These known dispersing devices with their high peripheral speeds are usually used for emulsifying, suspending, homogenizing and dissolving dispersions. Although the use of discontinuous dispersion systems is also possible within the scope of the invention, the advantages of continuous dispersion systems lie in the reliable guarantee of a uniform product quality.

Since the suspension formed from dope and precipitant is accelerated and decelerated alternately at least once, preferably at least twice, a high degree of turbulence is maintained over a long distance, so that a dope with high viscosity can be processed. The suspension is alternately preferably subjected to a radial and a transverse flow.

The dope is preferably introduced into the precipitant through stationary nozzles, means for generating a flow being moved past their outlet openings.

When running through several acceleration and deceleration fields, the dope emerging from the nozzles is detected and deducted very quickly, so that nozzle clogging cannot occur even at higher viscosities. Furthermore, the fibrets are already largely homogenized by the alternating acceleration and deceleration, so that under certain circumstances. a subsequent homogenization treatment can be dispensed with. This is obviously due to the fact that a high mean turbulence can be maintained over a long distance, and that during the entire residence time in the dispersing device, which is usually between 0.03 and 0.5 sec. The turbulence itself is only slightly dampened by the fibrets that form, since the turbulence is generated by rotating machine parts and not by a turbulent flowing medium. This has the further advantage that the partial load behavior of the entire arrangement is very good. This means that in large areas the fibret quality is independent of the total throughput of the precipitation bath and the dope and of their relationship to one another.

The dope is preferably prepared with cellulose esters or cellulose ethers. Cellulose diacetate, cellulose triacetate, cellulose acetate butyrate, cellulose acetate propionate, benzyl cellulose or ethyl cellulose and other suitable cellulose derivatives or mixtures of these materials are preferred. A cellulose diacetate with an acetyl value between 54 and 56% is preferably used. The proportion of cellulose derivatives in the dope is preferably 3-20% by mass. Lower concentrations are usually not economically attractive, larger proportions require too high a viscosity. Depending on the proportion of cellulose derivatives in the dope, the proportion of solvent must also be adjusted. Acetone can be used as the solvent. Acetic acid, methyl acetate, methyl ethyl ketone, 1,4-dioxane, acetaldehyde, ethyl acetate, tetrahydrofuran or methyl isopropyl ketone or mixtures of the solvents mentioned can be used. The solvent acetone is particularly preferred.

Since it is only possible with great effort to remove completely pure solvent from the precipitation bath, which in turn is to be used to prepare the dope, it is economically desirable if the solvent can still contain non-solvent, which is the main component of the Precipitates.

Preferred non-solvents for the cellulose ester are water, ethanol or methanol, the proportion of which in the dope can be up to 40% by mass.

The maximum content of non-solvents based on the ratio of solvent to cellulose derivative depends on the precipitation point. The higher the non-solvent content in the dope, the faster the precipitation point is reached. The maximum salary is determined by the precipitation point, which i.a. is temperature-dependent. A non-solvent content of 2-20% by mass below the concentration of the non-solvent at the respective precipitation point is preferred. In contrast, in US 5,071,599 and US 5,175,276, which describe two-component nozzle systems, only up to 20% by weight of non-solvent are permitted in the dope, so that this proportion in the system cellulose acetate, acetone and water is between 8.5% and 21% below Precipitation point.

Regardless of the setting of the dope near the precipitation point, the ratio of solvent to cellulose derivative should also be minimized. According to the prior art, as is known from US 4,192,838, US 5,071,599 and US 5,175,276, the ratio can be a minimum of 4.4. Lower ratios usually cause the viscosity of the dope to be too high, which has a negative effect on the fibret fineness in the known processes. Solves for example, cellulose diacetate in acetone in a mass ratio of 1: 3, a gel-like consistency is obtained. However, the viscosity can be reduced by adding water as the non-solvent. In this regard, it is desirable to add water to just below the precipitation point. However, this behavior can only be observed at high levels of cellulose derivative in the dope and / or at low solvent: cellulose derivative ratios. At low levels of cellulose ester in the dope, as is the case, for example, in US Pat. No. 4,192,838, US Pat. No. 5,071,599 and US Pat. No. 5,175,276, water usually increases the viscosity because of the low viscosity of acetone.

The prior art processes are unable to produce fibrets with an acetone: water mass ratio of, for example, 2.8 and a non-solvent content of up to 2% by mass below the precipitation point in the dope. The viscosity is still too high despite the addition of water. As a result of the very high turbulence according to the method according to the invention, the main advantage is that higher viscosities of the dope can be processed with satisfactory results.

Since higher-viscosity dopes can be processed, solvent: cellulose derivative ratios between 4.4: 1 and 2.8: 1 can be achieved without the quality of the fibrets deteriorating. Particularly good results could be achieved if the solvent acetone, cellulose ester cellulose diacetate and non-solvent water were used. A reduction in the ratio of acetone: cellulose diacetate from 4.4: 1 to 2.8: 1 means that 36% by mass less solvent has to be used to produce a predetermined amount of fibrets, which considerably reduces the production costs of the fibrets.

The temperature of the dope is largely uncritical for the process. Good results have been obtained at room temperature. To reduce the A higher temperature can be selected for the viscosity of the solution, up to precipitation at overpressure and temperatures above 100 ° C. however, the ambient temperature is preferred as the most economical variant.

The two or three components of the dope are mixed in a suitable manner until a homogeneous solution is obtained. This solution is then optionally fed to the dispersing device via a filter.

The volume flow of the precipitant in the precipitation bath is preferably adjusted so that the fibret content in the precipitation bath is between 0.1 and 2.5% by mass. Differences in the fibret quality could not be determined even with a high fibret content. At concentrations between 2.5 and 3.5% by mass, precipitation is still carried out without clogging of the extrusion nozzles, but there are drawbacks to the fibret fineness and fibrethomogeneity. A range of 1-2.5% by mass is preferred for economic reasons. The process according to the invention is thus significantly better than the processes according to the prior art, according to which the precipitation is only possible at concentrations between 0.1 and 1% by mass. The importance of this change is important because a precipitation of 2.5% by mass compared to a precipitation of 1% by mass results in only 40% of the suspension amount. This means, among other things, that the plant can be designed 60% smaller, which means corresponding savings in investment costs. In the further course, 60% less water / acetone solution has to be separated from the fibrets and processed, which in turn brings savings in energy costs. Furthermore, a filtration process following precipitation can preferably be dispensed with before complete removal of the solvent. This has the advantage that the damage to the morphology of the fibrets which occurs during the filtration before the complete removal of the solvent is avoided because the structure of the fibrets is not completely fixed due to the solvent which is still present.

The methods according to the prior art must work with a large excess of the precipitation medium compared to the dope, in particular at higher viscosities of the dope, in order to generate sufficient turbulence. A minimum ratio of precipitation bath: dope of 11: 1 has been achieved with the two-component nozzle systems according to US Pat. No. 4,192,838 and US Pat. No. 5,071,599 and US Pat. No. 5,175,276. The process according to the invention permits the production of fibrets at a precipitation bath: dope ratio of 10: 1 to 2.5: 1. This has the following advantages: The concentration of the cellulose derivative in the dope can be chosen to be low. This can be useful for setting lower viscosities if fibrets are not to be produced from the cellulose diacetate-acetone-water system. If 5% of the cellulose ester is contained in the dope, after the precipitation with a mass ratio of precipitation bath: dope of 2.5: 1, the suspension consists of 1.43% by weight of fibrets. The advantages of high fibret concentrations can therefore also be used with potentially high dope viscosities. Furthermore, fluctuations in the volume flows of the precipitation bath and / or dope do not influence the fibret quality in wide areas and the part-load behavior is excellent. The reason for these advantages lies in the large decoupling of the volume flow and turbulence.

Finally, a further advantage of the process is that higher solvent concentrations than in the prior art can be present in the precipitation bath. According to US 5,071,599, the acetone content for the acetone-cellulose diacetate-water system in the coagulation bath may not exceed 15% by mass. Above this value, the acetone present in the suspension causes the solidification of the fibers to extend over a longer period. The surfaces of the fibrets that have not yet completely solidified can stick together and form lumps without a porous fibret structure. When using the method according to the invention, however, the shear effect is high enough to separate such lumps until the fibers have completely solidified. The longer solidification time of the fibrets also contributes to finer fibrillation and associated with a higher surface area, because shear forces act on the entire solidification time, intensifying the fibrillation. For example, with the cellulose diacetate - water - acetone system, acetone contents in the precipitation bath of up to 25% by mass are possible.

Higher concentrations of solvent in the dope are favorable with regard to further processing. The higher the solvent content in the suspension, by which the precipitation bath is understood after the precipitation, the higher it is also in the distillate (separated solvent plus residual non-solvent), so that correspondingly fewer work-up steps of the distillate are necessary. This procedure does not have any disadvantages with regard to explosion protection, since in practically all variants the lower explosion limit of 2.5 vol.% Acetone is exceeded for an economical design of the process.

The device for producing the fibrets provides that the at least one nozzle, through which the dope is introduced into the precipitant, is arranged in a dispersing device having a rotor and stator, which has at least two toothed rings, of which at least one toothed ring is part of the rotor of the Is dispersing device and that the dispersing device has a feed line for the precipitant.

Precipitation bath and dope are combined in the dispersing device to achieve the precipitation. For this purpose, the precipitation bath flows through the dispersing machine, as is known from homogenizing and dispersing tasks. The feed line for the precipitant preferably encloses the feed line for the dope.

The nozzle is preferably arranged in the interior of the dispersing device and aligned radially outward on the innermost ring gear. In a preferred embodiment, the first is in the flow direction of the precipitation bath Sprocket part of the rotor, followed by a sprocket of a stator, etc. The distance between sprocket and nozzle is small to allow the actual precipitation in the zone of the shear field, which is generated by the teeth moving in front of the nozzle opening. The distance can be between 0.01 and 5 mm, preferably 0.01-0.1 mm. It is also possible to equip the first ring gear in the direction of flow of the precipitation bath with nozzles for adding the dope.

The nozzle diameter can be selected in large areas because this parameter has only a minor influence on the quality of the fibrets in the intended area. A nozzle diameter between 5 and 10 mm is preferred. However, smaller nozzle diameters are possible. Since no blockages of the nozzles occur during the use of the dispersing device, the use of larger nozzles, although possible from the point of view of the fibret quality, is also not necessary. If the volume flow from precipitant to dope is low or the total volume flow is high, it is advisable to divide the dope flow over several nozzles. These nozzles are preferably arranged symmetrically within the innermost ring gear. If, for example, three nozzles are provided, they are arranged in a star shape. In this case, the feed line to the nozzles is preferably located in the center of the dispersing device. In order to find the same conditions with regard to the flow of the precipitation bath at all nozzles, the precipitation agent is also introduced centrally into the dispersing device. This is ensured by the fact that the feed line of the precipitant preferably encloses the feed line of the dope.

It is generally assumed that fibrets develop very quickly after the dope has been introduced into the precipitation bath. The development time is of the order of 0.001 - 0.5 seconds. The fibret morphology should then be determined. However, it is assumed that when the fibrets develop, an outer shell first forms, since the concentration bales in the Scherfeld first reaches the concentration of non-solvent required for the precipitation. The solvent must diffuse outwards and / or the non-solvent inwards through this shell. A time which is greater than the specified precipitation time therefore passes again until the core has hardened. Since the outer morphology has already developed in this time period, the fibers, despite the soft inner core, have a turbulence-inhibiting effect to such an extent that the fibers can no longer be divided. By using a dispersing device it is possible to cut fibers with a soft inner core by means of the high shear effect applied by the rotors and therefore to increase the fineness of the fibrets.

The dispersing device according to the invention also has a positive effect on the agglomerate size of the fibrets. Depending on the choice of rotor-stator geometries and the rotor speed, agglomerate sizes of up to 1 mm can also be achieved. However, preference is given to arrangements and speeds which result in agglomerate sizes below 200 μm. These dimensions are achieved in that, in contrast to the arrangements according to the prior art, sufficient shear forces are still available in the end region of the shear field, which can minimize the dimension of the fibra agglomerates.

• Es werden stärkere Scherkräfte erzeugt, weil höhere Differenzen der Grundströmungsgeschwindigkeiten auf engem Raum abgebaut werden. Dadurch wird eine feinere Fibrillierung der Fibrets erreicht, wobei größere Viskositäten des Dopes verarbeitbar sind.
• Es steht ein größerer Scherraum als beim Stand der Technik zur Verfügung, in dem der Auf- und Abbau der Grundströmungsgeschwindigkeiten mehrmals erfolgt. Dadurch wird ebenfalls eine feinere Fibrillierung der Fibrets erzielt, das Teillastverhalten ist besser und die Feinheiten in der Wahl der Prozeßparameter werden größer.
• Im Scherraum gilt das Prinzip der Zwangsförderung, so daß ein Ausweichen von Teilströmungen in Zonen geringer Turbulenz nicht möglich ist.

The device according to the invention also has the following advantages:

• Stronger shear forces are generated because higher differences in the basic flow velocities are reduced in a small space. This results in a finer fibrillation of the fibrets, whereby larger viscosities of the dope can be processed.
• A larger shear space is available than in the prior art, in which the basic flow velocities are built up and down several times. This also results in a finer fibrillation of the fibrets, the part-load behavior is better and the finer points in the choice of process parameters become larger.
• The principle of forced delivery applies in the shear chamber, so that it is not possible to avoid partial flows in zones of low turbulence.

Exemplary embodiments are explained below with reference to the drawings.

Figur 1

eine schematische Darstellung der gesamten Anlage zur Herstellung von Fibrets,

Figur 2

einen vertikalen Schnitt durch eine Dispergiereinrichtung und

Figur 3

einen Schnitt längs der Linie III-III durch die in Figur 2 gezeigte Dispergiereinrichtung,

Figur 4

eine graphische Darstellung des Turbulenzgrades in Abhängigkeit vom Düsenabstand und

Figur 5

eine graphische Darstellung der Wasserkonzentration von der CA-Konzentration.

Show it:

Figure 1

a schematic representation of the entire plant for the production of fibrets,

Figure 2

a vertical section through a dispersing device and

Figure 3

3 shows a section along the line III-III through the dispersing device shown in FIG. 2,

Figure 4

a graphic representation of the degree of turbulence depending on the nozzle distance and

Figure 5

a graphical representation of the water concentration from the CA concentration.

FIG. 1 shows the plant for the production of fibrets. The raw material, for example cellulose diacetate, is fed via the feed line 1 to a dope preparation tank 2, to which solvent 31 is fed from the treatment plant 26 via the line 31. The dope is fed to a dispersing device 40 via a double line 3, where the precipitation is carried out. The precipitant is prepared in a precipitation bath 8 into which non-solvent, preferably water, is fed via line 27 from the processing plant 26. The precipitant used is introduced into the dispersing device 40 via a precipitation bath feed line 9, where the precipitant is combined with the dope. This will be explained in connection with FIG. 2.

The precipitation bath suspension with the fibrets produced is fed to the distillation plant 12 via the precipitation bath discharge line 13. Steam is fed to the distillation plant 12 via a steam feed line 15, and the separated solvent is first fed via a heat exchanger 16 to the treatment plant 26 via the solvent return line 17, where the treatment of the solvent takes place in such a way that it then returns to the dope preparation tank 2 or the precipitation bath preparation tank 8 can be used.

After the solvent has been separated off, the fibrets are fed to a high-pressure homogenizer 20 via the discharge line 19. From there, the fibrets go into a stacking tank 22 and further into a drum filter 24, where the fibrets are concentrated to the desired final concentration, the precipitation bath also being fed to the treatment plant 26 via the return line 25. The fibrets prepared in this way are fed via the fibret derivation 23 for further processing. From the treatment plant 26, the treated solvent, which can contain a certain proportion of non-solvent and the non-solvent, which in turn can contain a portion of solvent, is introduced into tanks 2 and 8 via lines 27 and 31, respectively.

In Figure 2, the dispersing device 40 is shown in vertical section. The precipitant is fed into the interior of the dispersing device 40 via the curved feed line 9. The dope feed line 3 is located within the feed line 9 and is enclosed by it, so that both the dope and the precipitant can be introduced centrally into the dispersing device 40. The double line 3 branches inside the housing 41 and merges into the nozzles 46 and 47, which are arranged in a star shape together with the nozzles 48 and 49, which is shown in FIG. The nozzles extend outward in the radial direction and end at a short distance in front of the innermost ring gear 50, which is part of the rotor 44. This rotor 44 is driven by a drive shaft 65 which extends downward from the housing 41 and is driven by a motor (not shown). Sealing to the housing 41 takes place via a mechanical seal 64. The rotor 44, which has a base plate 10, comprises, in addition to the first ring gear 50, a further ring gear (third ring gear) 52 spaced apart therefrom. Between the two ring gears 50 and 52 there is the second ring gear 51, which belongs to the stator 43. The stator 43, which has an annular base plate 11, is arranged above the nozzles 46-49, so that the second ring gear 51 extends downward and is fastened to the housing 41. A further stator 45, which has the outer or fourth ring gear 53, is arranged below the rotor.

Since the precipitation bath feed line 9 is sealed against the stator 43, the precipitant is fed centrally and flows around the nozzles 46 to 49. The dope which is fed through the double line 3 exits the nozzles 46 to 49 in the radial direction and into the shear field one, which extends through the ring gears 50-53 into the outer region 14. Dope and precipitant will be first gripped by the ring gear 50 and accelerated. Due to the radial direction of flow of dope and precipitant, which is superimposed on the cross-flow, the dope or the fibrets being formed leave the zone 60 in the radial direction through the gaps 54 between the teeth 55 of the first ring gear 50, whereby dope and precipitant are delayed . The liquid then passes into a further zone 61 between the ring gear 50 and the ring gear 51, where the liquid is accelerated again. This ring gear 51, since it belongs to the stator, is stationary. On the further route, the fibrets successively reach the further zones 62 and 63 between the ring gears 51 and 52 or 52 and 53, where accelerations and decelerations alternate. After the fourth ring gear 53 has also been left, the fibrets are discharged together with the precipitation bath in the form of a suspension via the discharge pipe 13. Fibrets were produced with this device and are described in the examples below.

FIG. 4 shows schematically the course of the average degree of turbulence of the dope / precipitant mixture as a function of the distance from the nozzle according to the prior art (US 4,047,862 - curve I) and according to the invention (curve II). A minimum degree of turbulence, which is characterized by straight line III, must be exceeded for the production of fibrets. Below curve III, the turbulence is too low for the precipitation, so that the desired fineness of the fibrets is not achieved.

Curve I describes the degree of turbulence between the rotating nozzles and the fixed wall, which are only 0.1875 inch (= 4.7625 mm) apart, but ultimately less than this distance is available for fibrin production. The degree of turbulence required for fibrin production is only reached shortly behind the nozzles, and then drops exponentially, so that only a relatively short distance is available for fibrin production.

In contrast to this, the shear field according to the invention extends in a dispersing device according to FIGS. 2 and 3 over approximately 14 mm (distance from ring gear 50 to ring gear 53), the minimum degree of turbulence required for fibrin production being exceeded and remaining constant after exiting the nozzle. The curve II drops steeply only when the outer region (see FIG. 2) 14 is reached. While working according to US 4,047,863 with circumferential speeds of about 30 m / sec, the rotational speed of the rotor of the dispersing device is 41 m / sec in this example.

Example 1:

480 g of a cellulose diacetate (CA) from Eastman Chemical Company (type CA 398-3) are dissolved in 3840 g of acetone and 480 g of water. The resulting dope thus contains 10 Ma% CA, 10 Ma% water and 80 Ma% acetone. The ratio of acetone to water is 8. The dope is passed into the precipitation bath at ambient temperature with a mass flow of 3 kg / min through a four-jet nozzle, each with a diameter of 5 mm and the ends of which are 0.10 mm from the inner rotor headed. Water is also used as the precipitation medium at ambient temperature and enters the precipitation chamber with a mass flow of 34.5 kg / min. The mass flows of precipitation bath and dope are in a ratio of 11.5 to 1. The dispersing machine is equipped with four sprockets of the "fine" specification (see Table 1). The precipitation takes place at a speed of 12,000 min ^-1 . After the precipitation, CA fibrets are present at a concentration of 0.8% by mass. Acetone is 6.5% by weight. The values mentioned and the values for the following tests are compared in Table 2. The acetone is removed by open distillation at ambient pressure. After the acetone has been removed, the fibrets are homogenized in one stage using a high-pressure homogenizer from APV GAULIN, type LAB 60, at a pressure of 150 bar. The fibrets are concentrated for storage with a nutsche. A filter cake with a solids concentration of 8.6% by mass is formed.

This variant is in the range of the prior art with regard to the concentrations, only the precipitation was carried out with the dispersing machine according to the invention and the filtration after the precipitation was dispensed with.

Example 2:

Tabelle 2 Daten für Ausführungsbeispiele Parameter Beispiel 1 2 3 4 5 6 7 8 9 10 Dopezusammensetzung in g CA 480 480 480 480 480 480 480 480 480 480 Aceton 3840 3840 3840 3840 2400 1680 2400 2400 2400 1680 Wasser 480 480 480 480 550 510 550 550 550 840 Dopezusammensetzung in % CA 10 10 10 10 14 18 14 14 14 16 Aceton 80 80 80 80 70 63 70 70 70 56 Wasser 10 10 10 10 16 19 16 16 16 28 Verhältnis Aceton : CA 8 8 8 8 5 3,5 5 5 5 3,5 Fällbadzusammensetzung in % Wasser 100 100 100 100 100 100 95 90 85 95 Aceton 0 0 0 0 0 0 5 10 15 5 Massenströme in kg/min Dope 3,0 3,0 3,00 3,0 3,0 3,0 3,0 3,0 3,0 3,0 Fällbad 34,5 22,0 15,75 12,0 21,7 29,7 21,9 21,6 21,7 29,1 Massenstromverhältnis Fällbad zu Dope 11,5 7,3 5,25 4,0 7,2 9,9 7,3 7,2 7,2 9,7 Suspensionzusammensetzung in % CA 0,8 1,2 1,6 2,0 1,7 1,65 1,7 1,7 1,7 1,5 Aceton 6,4 9,6 12,8 16,0 8,5 5,80 12,8 17,3 21,7 9,8 Wasser 92,8 89,2 85,6 82,0 89,8 92,55 85,5 81,0 76,6 88,7 The experiment was carried out analogously to experiment 1, but the precipitation bath mass flow was reduced to 22 kg / min. There was a ratio of the mass flows of precipitation bath to dope of 7.3 and a concentration of the fibrets after precipitation of 1.2 mass% (at 9.6 mass% acetone). High-pressure homogenization was dispensed with.

Table 2 Data for exemplary embodiments parameter example 1 2nd 3rd 4th 5 6 7 8th 9 10th Dope composition in g CA. 480 480 480 480 480 480 480 480 480 480 acetone 3840 3840 3840 3840 2400 1680 2400 2400 2400 1680 water 480 480 480 480 550 510 550 550 550 840 Dope composition in% CA. 10th 10th 10th 10th 14 18th 14 14 14 16 acetone 80 80 80 80 70 63 70 70 70 56 water 10th 10th 10th 10th 16 19th 16 16 16 28 relationship Acetone: CA. 8th 8th 8th 8th 5 3.5 5 5 5 3.5 Precipitation bath composition in% water 100 100 100 100 100 100 95 90 85 95 acetone 0 0 0 0 0 0 5 10th 15 5 Mass flows in kg / min Dope 3.0 3.0 3.00 3.0 3.0 3.0 3.0 3.0 3.0 3.0 Felling bath 34.5 22.0 15.75 12.0 21.7 29.7 21.9 21.6 21.7 29.1 Mass flow ratio precipitation bath to dope 11.5 7.3 5.25 4.0 7.2 9.9 7.3 7.2 7.2 9.7 Suspension composition in% CA. 0.8 1.2 1.6 2.0 1.7 1.65 1.7 1.7 1.7 1.5 acetone 6.4 9.6 12.8 16.0 8.5 5.80 12.8 17.3 21.7 9.8 water 92.8 89.2 85.6 82.0 89.8 92.55 85.5 81.0 76.6 88.7

Example 3:

The experiment was carried out analogously to experiment 1, except that the precipitation bath mass flow was reduced to 15.75 kg / min. There was a ratio of the mass flows of precipitation bath to dope of 5.25 and a concentration of the fibrets after precipitation of 1.6 Ma% (at 12.8 Ma% acetone). High-pressure homogenization was dispensed with.

Example 4:

The experiment was carried out analogously to experiment 1, except that the precipitation bath mass flow was reduced to 12 kg / min. There was a ratio of the mass flows of precipitation bath to dope of 4.0 and a concentration of the fibrets after precipitation of 2.0 mass% (at 16.0 mass% acetone). High-pressure homogenization was dispensed with.

Example 5:

The experiment was carried out analogously to experiment 1, except that a mass ratio of acetone to CA of 5 was set. With a mass flow ratio of precipitation bath to dope of 7.2 there are 1.7 Ma% fibrets with 8.5 Ma% acetone after the precipitation.

Example 6:

The experiment was carried out analogously to experiment 1, except that a mass ratio of acetone to CA of 3.5 was set. With a mass flow ratio of precipitation bath to dope of 9.9, 1.65% by mass of fibrets with 5.80% by weight of acetone are present after the precipitation.

Example 7:

The experiment was carried out analogously to experiment 5, except that 5% by mass of acetone was present in the precipitation bath in this case. After the precipitation, this resulted in an acetone content of 12.8% by mass. High-pressure homogenization was not used.

Example 8:

The experiment was carried out analogously to experiment 5, only 10% by mass of acetone was contained in the precipitation bath. After the precipitation, this resulted in an acetone content of 17.3% by mass. High-pressure homogenization was not used.

Example 9:

The experiment was carried out analogously to experiment 5, only 15% by mass of acetone was contained in the precipitation bath. After the precipitation, this resulted in an acetone content of 21.7% by mass. High-pressure homogenization was not used.

Example 10:

Experiment 10 was carried out analogously to experiment 6, except that water of up to 28% by mass was additionally added to the dope. 5% by mass of acetone were present in the precipitation bath. After precipitation, the fibrets had a concentration of 1.5% by mass with an acetone concentration of 9.8% by mass.

Example 11:

The preparation was carried out analogously to experiment 5, the "coarse" toothed wheel geometries being used instead of the "fine" toothed ring geometries.

Example 12:

The production was carried out analogously to experiment 5, with instead of the "fine" gear rim geometries, the rotor rings with the "coarse" geometry and the stator gear rings with the "fine" geometry.

Each of the samples had specific surfaces above 20 m ² / g and individual fiber dimensions, preferably below 1 µm. However, further parameters such as homogeneity, accessibility of the fiber networks for the flow etc. are decisive for the filtration.

25 Ma%

CA-Fibrets

35 Ma%

mikrokristalline Zellulose mit einem Modalwert der Partikelgrößenverteilung bei 28 µm

30 Ma %

Langfaserzellstoff, ungemahlen

10 Ma%

Langfaserzellstoff, gemahlen auf 80°SR.

The quality of the fibrets produced was therefore assessed using a filter layer. The following composition was selected for the filter layer:

25%

CA fibrets

35%

microcrystalline cellulose with a modal particle size distribution at 28 µm

30%

Long fiber pulp, unground

10%

Long fiber pulp, ground to 80 ° SR.

Based on the total solids, 0.4% by weight of an epichlorohydrin resin was added to adjust the wet strength. The layers had a basis weight of 1350 g / m ² .

The layers were tested with a test suspension of 0.5% by weight of raw raw cane sugar in water. The test area was 100 cm ² . The throughput was determined after 30 min at a differential pressure of 1 bar. After a filtration period of 15 minutes, the turbidity measurement was sampled to determine the separation effect. The initial turbidity for all layers in the test series carried out in parallel was approximately 2.40 TE / F. Each of the filter layers produced with the described fibrets was tested in three runs. Table 3 shows the mean values. Table 3 Results of the filtration tests Layer with fibrets Example no .: Throughput in 30 min liters Turbidity of the TE / F sample 1 12.1 0.24 2nd 13.5 0.28 3rd 13.8 0.29 4th 14.2 0.32 5 14.4 0.30 6 15.2 0.36 7 14.1 0.29 8th 13.7 0.26 9 13.3 0.24 10th 15.7 0.39 11 14.1 0.33 12th 14.3 0.29

Differences can be seen between the layers. However, these are not so clear that the fibrets would have to be rejected according to one of the exemplary embodiments with regard to their use in filter layers. Usually, a slightly higher turbidity is associated with a correspondingly higher throughput. As a relation for the assessment of the filtration behavior Throughput and separation efficiency is used, this behavior should not be assessed negatively. It is also possible to change the filtration behavior by changing the formulation of the filter layer.

The fibrets of sample 9 are positive. The addition of acetone to the precipitation medium results in better fibrillation. In contrast, the geometry of the sprockets used in the examined area has no significant influence on the quality of the fibrets.

FIG. 5 shows the dependence of the maximum possible water content on the concentration of the cellulose acetate in the dope (both in% by mass) at a temperature of 20 ° C. The acetone content is obtained by subtracting the respective CA and water concentrations from 100% by mass.

In addition to the concentration at the precipitation point (curve I), the maximum possible concentrations of water according to the invention (curve II) and according to the prior art (US 5,071,599 and US 5,175,376) - curve III - are also plotted. According to the prior art, between 5 and 15% by mass of cellulose acetate can be dissolved in the dope. The cellulose acetate is dissolved in a solution that can contain a maximum of 20 mass% water. The information for the water thus relates to the two-substance system acetone + water, while the information according to the invention always applies to the three-substance system acetone + water + Ca. With 5% by mass of cellulose acetate, only 20% of 95%, i.e. 19 Ma% water can be included.

The diagram shows that the greatest difference to the prior art is at 5% by mass of cellulose acetate and the smallest at 15% by mass of cellulose acetate. 5% by mass of cellulose acetate: Falling point concentration: 5.0 Ma% cellulose acetate 39.7 mass% water 55.3 mass% acetone Maximum water content according to the invention 5.0 Ma% cellulose acetate 37.7 mass% water 57.3 mass% acetone State-of-the-art maximum water content 5.0 Ma% cellulose acetate 19.0 mass% water 76.0 mass% acetone Difference in water content compared to the prior art 18.7 Ma% (based on dope) 15% by mass of cellulose acetate: Falling point concentration: 15.0% by mass of cellulose acetate 25.5 mass% water 59.5 mass% acetone Maximum water content according to the invention 15.0% by mass of cellulose acetate 23.5 mass% water 61.5 mass% acetone State-of-the-art maximum water content 15.0% by mass of cellulose acetate 17.0% water 68.0 mass% acetone Difference in water content compared to the prior art 6.5% by mass (based on dope)

Figure 5 clearly shows the advantages of the invention, which consist in the fact that the cellulose acetate portion can be chosen much larger (see expanded area according to the invention) and that the water portion can be significantly higher, which, as described above, has decisive advantages in the Process management (elimination of the filtration stage) and the process costs (preparation with less effort).

Reference numerals

1

Supply line for cellulose derivative

2nd

Dope preparation tank

3rd

Double line

8th

Precipitation batch tank

9

Precipitation bath supply

10th

Base plate

11

ring-shaped base plate

12th

Distillation plant

13

Precipitation bath drain

14

Outdoor area

15

Steam supply

16

Heat exchanger

17th

Solvent return line

19th

Discharge line

20th

High pressure homogenizer

22

Stacking tank

23

Fibretal drainage

24th

Drum filter

25th

Return line

26

Processing plant

27

management

31

management

40

Dispersing device

41

casing

42

inner space

43

inner stator

44

rotor

45

outer stator

46

jet

47

jet

48

jet

49

jet

50

first ring gear

51

second ring gear

52

third ring gear

53

fourth ring gear

54

gap

55

tooth

60

first zone

61

second zone

62

third zone

63

fourth zone

64

Mechanical seal

65

drive shaft

Claims (29)

Process for the production of fibrets, in which a dope containing a cellulose derivative, such as a cellulose ester or a cellulose ether, and a solvent suitable therefor is introduced into a precipitant and the suspension formed from dope and precipitant is alternately accelerated and decelerated in the shear field at least once , wherein the dope is precipitated and the fibrets are formed, and in which the fibrets, the solvent and the precipitant are subsequently separated, characterized in that that the precipitation is carried out in a dispersing device according to the rotor / stator principle. A method according to claim 1, characterized in that the suspension is alternately accelerated and decelerated at least twice. A method according to claim 2, characterized in that the suspension is alternately subjected to a radial and a transverse flow. Method according to one of claims 1 to 3, characterized in that the dope is introduced into the precipitant through stationary nozzles, means for generating a flow are moved past their outlet opening. Method according to one of claims 1 to 4, characterized in that for the addition of the dope cellulose diacetate, cellulose triacetate, Cellulose acetate butyrate, cellulose acetate propionate, benzyl cellulose or ethyl cellulose or mixtures of these materials can be used. A method according to claim 5, characterized in that a cellulose diacetate with an acetyl value between 54 and 56% is used. Method according to one of claims 1 to 6, characterized in that the proportion of the cellulose derivative in the dope is 3-20%. Method according to one of claims 1 to 7, characterized in that the ratio of solvent to cellulose derivative in the dope is adjusted to 2.8 - 4.4. Process according to one of claims 1 to 8, characterized in that acetone, acetic acid, methyl acetate, methyl ethyl ketone, 1,4-dioxane, acetaldehyde, ethyl acetate, tetrahydrofuran or methyl isopropyl ketone or mixtures of these solvents are used as solvents . Method according to one of claims 1 to 9, characterized in that a non-solvent is additionally added to the dope. A method according to claim 10, characterized in that water, ethanol or methanol is added as the non-solvent. Method according to one of claims 10 or 11, characterized in that the non-solvent fraction is at least 2% below the non-solvent fraction (precipitation point fraction) at which precipitation begins for the cellulose derivative / solvent mixture used. A method according to claim 12, characterized in that the non-solvent fraction is 2 - 20% below the precipitation point fraction. Method according to one of claims 10 to 13, characterized in that the non-solvent portion in the dope is up to 40%. Method according to one of claims 1 to 14, characterized in that basic flow velocities of up to 100 m / sec are generated in the precipitation bath. Method according to one of claims 1 to 15, characterized in that the volume flow of the precipitant is adjusted so that the fibret content in the precipitation bath is between 1 and 2.5% by mass. Method according to one of claims 1 to 16, characterized in that the volume flows of the precipitation bath and dope are adjusted to a ratio between 10: 1 and 2.5: 1. Method according to one of claims 1 to 17, characterized in that the precipitation bath contains up to 25% by weight of solvent after the precipitation. Method according to one of claims 1 to 18, characterized in that the proportion of solvent in the precipitation bath after precipitation is 15-25% by mass. Method according to one of claims 1 to 19, characterized in that after the solvent removal, the fibrets are homogenized without prior filtration. Device for producing fibrets from cellulose derivatives with a device for preparing a dope from cellulose derivatives and solvent, with at least one nozzle for introducing the dope into a precipitant and with a device for separating the fibrets from solvent and precipitant, characterized in that that the at least one nozzle (46-49) is arranged in a dispersing device (40) having a rotor (44) and stator (43, 45), which has at least two toothed rings (50-53), of which at least one toothed ring (50) Part of the rotor (44) of the dispersing device (40) and that the dispersing device (40) has a feed line (9) for the precipitant. Apparatus according to claim 21, characterized in that the nozzle (46-49) is arranged in the interior of the dispersing device (40) and is aligned radially outward on the first ring gear (50). Device according to claim 21 or 22, characterized in that the first ring gear (50) is part of the rotor (44) of the dispersing device (40). Device according to one of claims 21 to 23, characterized in that the distance between the nozzle and ring gear is 0.01 - 5 mm. Device according to claim 24, characterized in that the distance between the nozzle and the toothed ring is 0.01-1 mm. Device according to one of claims 21 to 25, characterized in that the nozzle diameter is 5-10 mm. Device according to one of claims 21 to 26, characterized in that at least two nozzles (46, 47, 48, 49) are arranged symmetrically within the first ring gear (50). Device according to one of claims 21 to 27, characterized in that the precipitant feed line (9) for the precipitant surrounds the double line (3). Device according to one of claims 21 to 28, characterized in that the at least one nozzle (46-49) is arranged outside the toothed rings (50-53) of the dispersing device (40) and is aligned radially inward on the outermost toothed ring (53).

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