SI8911600A - PROCEDURE FOR PREPARING CELLULOSE SOLUTIONS - Google Patents

Abstract

For the preparation of cellulose solutions in tertiary amine oxides containing water from a suspension of cellulose in an aqueous solution of tertiary amine oxide by supplying heat at reduced pressure, the suspension, which has a viscosity between 50 and 15,000 Pas.sec, measured in the relative system, is spread in the form of a layer or film over a heating surface and transported until a homogeneous cellulose solution is formed, whereby the supply of the suspension and the withdrawn homogeneous solution is carried out continuously. These solutions can be prepared in an indirectly heated vessel, which is equipped with a stirring device and can be evacuated, and is designed as a cylindrical vessel (2) with a centrally mounted stirring shaft (7) and stirring blades (8) attached thereto, whereby the radial distance (13) from the stirring blades (8) to the inner wall (1) of the vessel (2) is a maximum of 20 mm, with an inlet (11) for the cellulose suspension provided in the upper part of the vessel and an outlet (12) for the homogeneous cellulose solution in the lower end (Fig. 1).

Description

Lenzing Aktiengesellschaft

Process for the preparation of cellulose solutions

MPK: C 08B

The invention relates to a process for the preparation of solutions of cellulose in tertiary amino acids containing water from a suspension of cellulose in aqueous solution of tertiary amine oxide, by supplying heat under reduced pressure, and to a device for carrying out the process.

Such a procedure is described in PCT Publication WO 83/04415 Thereafter, the cellulose is suspended in an aqueous solution of tertiary amine oxide containing up to 40% by weight of water and heated to between 90 and 120 ° C with stirring.

At the same time, they reduce the pressure to 80 to 150 mbar and so they drain the water until the pulp passes into solution. In this way, solutions with up to 15% by weight of cellulose, which can be passed through, can be prepared.

By forming these solutions in water, it obtains a foil, filament or cellulose-based mold, that is, objects that are now largely prepared by the viscous process. However, cellulose solutions in aqueous tertiary ammonia, which can be transported, have a decisive advantage over viscosity in terms of environmental portability. While tertiary amino acid can be recovered and reused during spinning, it is produced by the decomposition of viscose: HgS, COS, CS ^ and colloidal sulfur. However, the care of these substances can only be gotten rid of at great expense.

However, the above process with tertiary amino acids as solubility mediators has not been able to be implemented to date, as it has many disadvantages.

Thus, in the mixing boiler, due to the unfavorable surface-to-volume ratio of the liquid ^1, it is difficult to drain the water, leading to long holding times - in the range of 2 to 4 hours in the mixing boiler. During this time, a partial breakdown of the polymeric cellulose chain occurs, which is further facilitated by the elevated temperature. This partial degradation, however, again adversely affects certain properties of the end products after the spinning process, such as e.g. to their strength, elongation and loop strength. It is further known that, in particular, when heated above 130 ° C, strong coloration may occur due to the decomposition of the amino acid used. This decomposition may occur with some compounds, such as e.g. N-methyl-morpholine-N-oxide, even in explosive conditions with the rapid evolution of gas, such that the solutions present in the mixing boiler present a risk to safety due to their quantity.

- 3 In the case of the high-tech implementation of the process, therefore, when using mixing boilers, they should be adequately protected by high-pressure autoclaves, which, however, are not suitable for continuous operation for economic reasons. On the other hand, in the mixing boiler and without safety equipment, only a discontinuous mode of operation is possible, which makes the process flexibility insignificant, since it gives parameters such as e.g. temperature and degree of evaporation, only difficult to change. On top of this, due to the high viscosity of the cellulose solutions, much of the spinning mass is retained in the mixing boiler, which makes it difficult to clean the boiler and further worsen the economy.

Therefore, the invention seeks to eliminate these drawbacks and to make available a process for the preparation of cellulose solutions in tertiary amino acids containing water, which can be carried out continuously, whereby the thermal treatment of the suspension is substantially shorter in order to limit the thermal cellulose and tertiary amino acid loading to a minimum. In addition, the safety risk inherent in the state of the art should be avoided. Further, the invention is intended to provide a device for carrying out a process which has no drawbacks, which are in connection with the mixing boiler or high pressure autoclaves

This object of the invention is solved by transporting the suspension over a heating surface where it is spread in the form of a layer or. film until a homogeneous cellulose solution having a viscosity of between 50 and 15,000 Pas sec is formed, continuously delivering the suspension and draining the homogeneous solution.

Prevalence of cellulose suspension in the form of a layer or. The film leads to a large surface of the liquid over the heating surface, which facilitates the discharge of water. At the same time, it allows the suspension to be heated rapidly to the temperature required to prepare the solution. By means of transport through the heating surface, a constant stirring of the suspension is achieved, which further accelerates the thermal and material exchange.

To adjust the viscosity of the solution measured in the relative system and to influence the swelling of the cellulose in suspension, a diluent such as e.g. ethanol, to suspension.

Particularly good mixing is achieved if the layer extending over the heating surface has a maximum thickness of 20 mm, preferably 1.5 to 5 mm.

N-methylmorpholine-N-oxide is preferably used as a tertiary amino acid, preferably in an aqueous solution with 40% by weight of water.

A preferred embodiment of the process according to the invention is characterized in that the suspension is brought to a temperature between 50 and 150 ° C, preferably between 60 and 100 ° C ₇ and subjected to a pressure of 0.5 mbar to 1000 mbar, preferably 50 mbar to 150 mbar.

Particularly advantageous is the maintenance of the suspension for 1 minute to 60 minutes in contact with the heating surface. This period, on the one hand, is sufficient to prepare a homogeneous solution, and on the other hand, is so short that the decomposition of the tertiary amino acid and the breakdown of the pulp can be largely avoided.

- 5 A suitable device for carrying out the process of the invention, which can be evacuated, indirectly heated and equipped with a mixing device, is characterized by the fact that the container is designed as a cylinder with a centrally mounted mixing shaft and the mixing blades attached thereto, the radial distance from the mixing blades to the inner wall of the container is a maximum of 20 mm and an inlet for the pulp suspension and an outlet for a homogeneous pulp solution are provided at the upper end of the container.

A suitable embodiment of the device according to the invention has on the mixing shaft a distributor ring for spreading the cellulose suspension in the form of a layer or. film along the inner wall of the container.

In order to regulate the transport of the pulp suspension along the inner wall of the vessel, it has been shown to be advantageous that the mixing blades have an inclination angle to the axis of the mixing shaft, the size of which is possible

The process according to the invention, practicable with the device according to the invention, is extremely flexible with respect to changing operating parameters and has a significantly lower safety risk in comparison with the prior art, since we do not only heat large amounts of solvents at one time, but because of the layered spread over the heating surface. relatively insignificant amount.

The apparatus of the invention is further illustrated on the basis of Figures 1 and 2, showing

- 6 fig. 1 is a partial longitudinal cross-sectional view of the device of the invention and FIG. 2 is a cross-section along line II-II of FIG. 1 on an enlarged scale.

1 denotes the inner rings, preferably upright, of the standing rotary body, which, in the embodiment shown, is made almost in its entirety as a cylindrical vessel 2. The inner wall 1 is mostly surrounded by a heating jacket 3, with connectors 4 and 5 for the heating medium, wherein it uses connector 4 to supply the heating medium and connector 5 to discharge it.

In the container 2, there is a centrally mounted mixing shaft 7 driven by the motor 6, with mixing blades 8 connected thereto. Mixing blades 8, which in the embodiment shown are straight, extend radially to an axis, their plane having an inclined angle alpha to the axis 9 of the mixing shaft 7, the size of which can be safely adjusted. Above the mixing blades 8, a distribution ring 10 is mounted on the mixing shaft 7, which in the form of a layer extends the cellulose suspension introduced at the inlet 11 through the inner wall 1. The distribution ring 10 is thus located at the height of the inlet 11.

At the lower end, the container 2 narrows in the form of a truncated cone to the discharge 12 for a homogeneous cellulose solution. The mixing blades 8 have a constant radial distance 13 over the entire area of the container 2 to the inner wall 1 of the container 2, which is a maximum of 20 mm.

On the upper part of the container 2, above the plane of the distribution ring 10, an opening 14 is provided for the evacuation of the container 2 and the withdrawal of water vapor.

The operation of the device is as follows:.

The cellulose suspension - if desired in the preheated state - is continuously introduced through the inlet 11 into the container 2, which is under reduced pressure and therein is covered by the distribution ring 10, spread over the inner wall 1 and the mixing blades 8 transported along the indirectly heated of the inner wall 1, which is used as a heating surface, up to the discharge 12 at the lower end of the container 2. For indirect heating, heat transfer media such as water, oil or steam are suitable.

While transporting the cellulose slurry along the indirectly heated inner wall 1, the slurry is heated, evaporating water in a constant manner due to reduced pressure, so that the tertiary amino acid is concentrated until the cellulose passes into solution.

In FIG. 2 is in the detail *! shows how the cellulose suspension is processed in container 2. The mixing shaft 7 is shown, together with the mixing blades 8, the inner wall 1 and the heating jacket 3, assuming the direction of rotation of the mixing shaft 7 clockwise and indicated by the arrow 7 ' . Distribution in the form of a layer or. the thickness of the cellulose suspension layer is ensured by a radial distance of 13 mixing blades 8 from the heated inner wall 1. On the mixing blades, tracer waves of the cellulose suspension shown schematically in FIG. 2 are formed on the mixing blades. In these tracer waves, as indicated in FIG. 2, the cellulose particles are rotated, and this motion is also transmitted to the suspension layer spread over the inner wall 1. In this way,

- 8 Continuous reversal of the layers as well as intensive mixing of the suspension is ensured, significantly facilitating the thermal and material exchange. _z

It is essential for the continuous carrying out of the process according to the invention that the recovered steam is discharged counter-current to the transport of the suspension. In addition, it is important to allow a sufficiently large space 15 for the steam to be rapidly drawn off for steam, which is given when the ratio of the length to the diameter of the cylindrical portion of the vessel 2 is between 4 and 8.

The invention allows the preparation of cellulose solutions with up to 30 wt. % cellulose.

The invention will be further explained by the following examples.

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EXAMPLE 1:

A suspension of pre-hydrolyzed sulphate cellulose (polymerization rate of about 1400) in an aqueous solution of N-methylmorpholine-N-oxide with a water content of 40% by weight was heated to 70 ° C and continuously introduced in an amount of 90 kg / h through the inlet 11 of the device of the invention . The content of the pre-hydrolyzed sulphate cellulose in the suspension was chosen to give a final cellulose concentration of 10% by weight after evaporation of the excess water.

Mixing shaft 7 was driven at a speed of 450 min ~ \, with a thickness of layer spread over the inner wall 1, 15 mm. The indirectly heated inner wall 1 had a surface of 0.5 m ² and such a heat transfer oil application that a mean temperature difference of 83 ° C was obtained to suit the heating of the suspension (in counter current with the heat transfer oil). A pressure of 100 mbar was established in space 15 of the evil soparo.

At exit 12, 72 kg of homogeneous cellulose solution could be obtained per hour, which corresponds to a suspension residence time in the apparatus of the invention of 3 minutes. The solution was allowed to be degassed. Its viscosity was 1500 Pas.sec (measured in a relative system. Microscopic examination of the solution revealed that no undissolved particles of cellulose were present in the solution.

The resulting steam was counter-flowed at 70 ° C and then condensed, with a distillate flow rate of 29 kg per hour.

EXAMPLE 2:

A suspension of ground pre-hydrolyzed sulfate cellulose (polymerization rate of about 1400) in an aqueous solution of N-methyl-morpholine-N-oxide with a water content of 40% by weight was heated to 80 ° C and continuously introduced in an amount of 90 kg / h through the inlet 11 of the device according to the invention. The content of pre-hydrolyzed sulphate cellulose was chosen so that after evaporation of the excess water a final concentration of cellulose of 15% by weight was obtained. The 7 rr agitator shaft was driven by a number of turns of 450 rain ^- ^ with a layer thickness extending over the inner wall 1, 1.5 mm. The indirectly heated inner wall 1 had a surface area of 0.5 m ² and had a heat transfer oil application such that a mean temperature difference of 112 ° C was obtained to suit the heating of the suspension (in counter current to the heat transfer oil). A pressure of 150 mbar was established in the evaporator compartment 15.

At Exit 12, 64 kg of homogeneous solution, which was formed in a degassed form, was obtained per hour. This mass flow corresponded to a holding time of 4 minutes.

The solution was obtained as a high-viscous mass (11,000 Pas. Sec., Measured in a relative system), and no undissolved cellulose particles could be detected under the microscope. The solution was fed directly to the spinning mill and spun into cellulose fibers. The resulting vapor was withdrawn counter-flow at 80 ° C and then condensed, with a distillate flow rate of 26 kg per hour.

Claims (5)

PATENT APPLICATIONS A process for the preparation of solutions of cellulose in water-containing tertiary amine oxides from a suspension of cellulose in aqueous tertiary amino solution by supplying heat under reduced pressure, characterized in that it is transported in the form of a layer or. The suspension is spread over the heating surface until a homogeneous cellulose solution having a viscosity of between 50 and 15,000 Pas sec is formed, continuous delivery of the suspension and withdrawal of the homogeneous solution. Method according to claim 1, characterized in that it has a thickness of maximum 20 mm, preferably 1.5 to 5 mm, over the heating surface. 3- Process according to claim 1 or 2, characterized in that N-methyl-morpholine-N-oxide, preferably in aqueous solution with 40% by weight of water, is used as a tertiary amine. A method according to one or more of claims 1 to 3, characterized in that the suspension is brought to a temperature between 50 and 150 ° C, preferably between 60 and 100 ° C, and subjected to a pressure of 0.5 mbar to 1000 mbar, preferably 50 mbar to 150 mbar. A method according to one or more of claims 1 to 4, characterized in that the duration of the transport while the suspension is in contact with the heating surface is 1 minute to 60 minutes.