Thin film treatment apparatus

This invention relates to a thin film treatment apparatus suitable for the manufacture of solutions of cellulose in an aqueous organic solvent. It has particular, but not exclusive, reference to the manufacture of solutions of cellulose for use in the production of lyocell fibre.

The production of lyocell fibre is a well known concept. Essentially, cellulose in the form of pulp is mixed with water and an organic solvent for cellulose and a stabiliser. Under the action of heat and reduced pressure a part of the water is evaporated off to produce a solution of cellulose in the organic solvent, which additionally contains the remaining portion of the water. This solution is then passed to a spinning process whereby the solution is formed into shaped articles, typically filaments, the filaments are then treated so as to dissolve out the aqueous organic solvent, to precipitate out the cellulose and thus to form the cellulose shaped article.

Staple fibre is a product used on a large scale and the fibre production process has to be such that the fibre can be produced at a cost to make it economically viable against rival cellulosic fibres such as cotton or viscose fibres or even man made fibres such as polyester fibres.

This means that the economics of the process are such that for the manufacture of lyocell fibre, particularly lyocell staple fibre, the equipment used to produce the solution needs to be on a scale such that solution quantities of tens of thousands of tonnes of solution can be produced in a year.

The need to produce the fibre on a large scale has led to the adoption of a particular process of manufacturing the cellulose solution. In an alternative process the cellulose and the water and or organic solvent are mixed together in quantities such that direct dissolution is obtained. This is, however, very difficult to carry out on a large scale. In the other, now commercialised, process to which the present invention relates, the process involves the manufacture of a premix of cellulose, stabiliser, excess water and an organic solvent followed by heating and evaporation of the excess water so as to concentrate the organic solvent to permit the cellulose to be dissolved.

The most successful form of equipment which has been found to produce the solution by this commercialised process is a vertically orientated thin film treatment apparatus such as the equipment made by Buss AG and available under the trademark Filmtruder. Disclosed in EP 0 356 419 Bl is a thin film treatment apparatus of the type to which the present invention is applicable. In that disclosure the pre-mix is pumped into the thin film treatment apparatus

via an inlet and passes down through the thin film treatment apparatus partially under the action of gravity but mainly being forced down by transport blades on a central rotor. The internal surface of the thin film treatment apparatus is heated and a vacuum is applied to evaporate off excess water. This produces a solution of cellulose in aqueous solvent, which is pumped out of the thin film treatment apparatus via an exit. A preferred form of organic solvent is N-methyl morpholine N oxide, commonly abbreviated to NMMO.

The cellulose solution which is produced by the thin film treatment apparatus can be used to make many types of products. The principal product manufactured from this cellulose solution is cellulose fibre. However, many other cellulose materials can be produced such as films or sponges or tubes.

EP 0 660 743 discloses an improved process for producing cellulose solutions, using a thin film treatment apparatus. Especially, this document is concerned with the optimization of the process of EP 0 356 419 in terms of solution output and energy consumption.

A further modification of this known process is disclosed in WO 97/11973.

It has to be noted that whilst an increase in production capacity is a self-evident task for the engineer, particularly the lyocell process poses some severe limitations to the upscaling of the process. This is because the nature of the cellulose solution which is formed in the thin-film- treatment apparatus is exothermic. Hence, it is difficult to increase the production capacity of the lyocell process without at the same time facing the risk of exothermic reactions due to an increase in the temperature of the system.

It is an object of the present invention to yet further improve the use of a thin film treatment apparatus for the manufacture of cellulose solutions, especially on a commercial scale. Therefore, it is a further object of the present invention to provide an improved thin film treatment apparatus which can be used for the manufacture of cellulose solutions.

In a first aspect, this object is solved by a thin film treatment apparatus according to claim 1. Preferred embodiments of the thin film apparatus according to the present invention are disclosed in the dependent claims.

The thin film treatment apparatus according to the present invention comprises a rotor having at least one cylindrical section, at least one wiping blade being arranged on said cylindrical section, said wiping blade comprising at least two teeth, and is characterized in that the thickness (T) of the wiping blade or the teeth, respectively, is more than 5 mm.

It has surprisingly been found that by using blades and/or teeth, respectively, with a minimum thickness (T) of more than 5 mm, the production capacity of a thin film treatment apparatus may be significantly increased without at the same time exceeding the boundaries of the process in terms of process safety.

Under the thickness T, the thickness of the front face of the blades and/or the teeth, respectively, is to be understood. The front face is that face of the blade that is in contact with the material to be treated, and which distributes and transports the material over the inner surface of the thin film treatment apparatus.

The thickness T may be more than 11 mm, preferably 17 mm to 55 mm, most preferred 22 mm.

In a further embodiment of the thin film treatment apparatus according to the invenion, said teeth are being spaced apart from each other, thereby forming a gap, and the average ratio V between the length L of one tooth and the length (G) of the gap being located adjacent to the tooth is more than 2:1, respectively.

It has surprisingly been found that the production capacity of a thin film treatment apparatus may be significantly increased if the average ratio V between the length L of the teeth located on the blades on the thin film treatment apparatus and the length G of the gaps adjacent to the teeth is increased.

Particularly, it has been found that by using the thin film treatment apparatus according to the present invention, it is possible to increase production capacity but at the same time to keep the system parameters, such as the temperature of the system, at an acceptable level in terms of the process safety.

EP 0 660 743 Bl discloses that the teeth located on the blades (which may be in the form of strips with integrally formed teeth) may comprise 10% to 40% of the vertical length of the strip. This amounts to a ratio V of significantly lower than 2:1.

It is to be understood that the ratio V between the length L of the teeth and the length G of the respective adjacent gaps is not necessarily the same for all teeth and adjacent gaps, respectively, arranged on the apparatus according to the invention.

In one embodiment, the ratio V may be higher than 2: 1 for all the teeth arranged on the apparatus.

In other embodiments, in some zones of the apparatus, the ratio V may be lower than 2:1, whilst in other zones, the ratio V may be significantly higher. It is important, however, that the average value of the ratio V calculated from all the teeth located on the apparatus is higher than 2:1.

According to a preferred embodiment of the present invention, the average ratio V is more than 3:1, preferably 3.3:1. Again, the ratio V may be more than 3:1, preferably 3.3:1 for all the teeth arranged on the apparatus.

The length L of the teeth may range from 40 to 200 mm, respectively, and preferably from 90 to 110 mm.

In a further preferred embodiment of the thin film treatment apparatus according to the present invention, at least part of the teeth form an angle α towards the vertical direction which is more than 0°, wherein the mean angle α _m of all the teeth is less than 14°.

It is known that, especially in order to make cellulose solutions, at least part of the teeth being arranged on a thin film treatment apparatus may be inclined and, thus, form an angle α towards the vertical direction, which is more than 0°, in order to force the material being transported through the apparatus downward.

It has now surprisingly been found that if the mean angle α _m of all the teeth is less than 14°, the production capacity of a thin film treatment apparatus may be significantly increased without at the same time exceeding the boundaries of the process in terms of process safety.

By the term "mean angle", the average value of all the angles α which are formed by all the teeth towards the vertical direction.

In one embodiment, all teeth of the thin film treatment apparatus may be inclined and, thus, all angles α will be more than 0°.

In a further, preferred, embodiment, only a part of the teeth is inclined. This means that part of the teeth form an angle towards the vertical direction which is 0°, whilst the rest of the teeth form an angle towards the vertical direction which is more than 0°. The mean angle α _m is then, again, the average value of all the angles α. For example, if fifty percent of the teeth are not inclined (α = 0°) and the other fifty percent of the teeth are inclined towards the vertical direction by an angle of 26°, the mean angle α _m will be the average value, i.e. 13°.

Preferably, the mean angle α _m is less than 11°, preferably 10°.

The thin film treatment apparatus according to the present invention is especially suitable for the manufacture of a mouldable solution of cellulose in an aqueous tertiary amine oxide from a suspension of cellulose in said aqueous tertiary amine oxide.

In the following, the present invention will be explained in more detail by way of the examples and figures.

Figure 1 is a schematic sectional view of a system in accordance with the present invention,

Figure 2 is an end view of the top of the rotor of the system shown in Figure 1,

Figure 3 is a more detailed view of a blade to rotor connection,

Figure 4 is a perspective view of the arrangement of Figure 3,

Figure 5 is a view of a part of the rotor of a thin film treatment apparatus according to a preferred embodiment of the present invention.

Figure 6 is a schematical top view illustrating an exemplary shape of a rotor blade of a thin film treatment apparatus according to a preferred embodiment of the present invention.

Figure 7 is a chart showing the influence of ratio V of the teeth of the blades on the specific output of a thin film treatment apparatus.

Figure 8 is a chart showing the influence of the mean angle α _m on the specific output of a thin film treatment apparatus.

Figure 9 is a chart showing the influence of the thickness T of the blades on the specific output of a thin film treatment apparatus.

Referring to Figure 1, this shows schematically a system for performing the process of forming a cellulose solution in a tertiary amine N-oxide.

A tertiary amine N-oxide, such as N-methylmorpholine Noxide, is fed into a container 1 along line 2. Feed lines are also provided for cellulose, 3, and water, 4. Typically, the premix formed by mixing these components contains typically 12% by weight of cellulose, 20% by weight of water and 68% by weight of N-methylmorpholine N-oxide. The three components are mixed in the container 1 by means of a paddle screw 5 rotated by an electric motor 6. The paddle screw agitates the mixture and passes the mixed components along a pipeline 7 to a thin-film treatment apparatus indicated generally by 8. The pipeline preferably has a diameter such that it is always full, otherwise a restriction may be provided in its outlet into the thin film treatment apparatus, so that material in the pipeline 7 is not exposed to the vacuum in the treatment apparatus 8.

The thin-film treatment apparatus 8 comprises a rotor within a cylindrical member 9 which is heated on the outside by means of a heating element 10. The heating element may be an electrical heating element, or an oil-filled element, or a complete steam-filled or hot- water- filled heating jacket. At the lower end of the cylindrical portion 9 there is a tapered portion 11 leading to a discharge line 12.

At the upper end of the cylindrical portion 9 there is a feed chamber 13 provided with an exit duct 14 through which evaporated vapours may be removed. Premixed material passes into the chamber 13 through the pipeline 7 and is distributed around the thin-film treatment apparatus by a distributor plate 15 on rotation of the rotor, indicated generally by 16.

The central shaft of the rotor 16 is rotated by means of an external electric motor 17.

The rotor 16 is provided with a series of blades 18 which are described in more detail below. In operation, a reduced pressure is applied through the duct 14, whereby, on heating of the premix by means of the heating element 10, the water is evaporated during operation of the thin-film treatment apparatus to reduce the water content of the premix as it is heated.

This continuous heating and evaporation results in a lowering of the water, i.e. the non- solvent, component in the premix to such an extent that the cellulose forms a true solution in the tertiary amine N-oxide.

In the lower portions of the thin-film treatment apparatus 8, therefore, there is formed a viscous solution which is pushed downwards by inclined blades 19 on a conical member 20 into the neck at the bottom of the tapered portion 11 of the thin-film treatment apparatus. By rotation of a screw member 21 the solution of cellulose in solvent is passed to a pump 22 driven by an electric motor 23. From there the solution is passed by means of suitable pipework 24 to a spin nozzle 25.

The. screw member 21 is rotated by means of an electric motor 26', and control of the electric motor 26' in conjunction with control of the electric motors 6 and 17 controls the flow of solution through the system.

Figures 2 to 4 show in more detail the structure of the rotor illustrated generally at 16 in Figure 1. It can be seen from Figures 2 and 3 that the rotor comprises a cylindrical central portion 26 having at its lower end a tapered conical portion. At its upper end the cylindrical portion has a terminating plate 28 to which the rotational shaft from motor 15 is connected.

The rotor central portion 26 is essentially a hollow cylinder having integrally projecting from it a series of, e.g., six parallel blade roots 29, 30 etc. These blade roots extend the length of the central portion 26 of the rotor.

The blade roots are welded to and form an integral portion of the central region of the rotor.

Bolted to the roots such as root 30 are a series of plates 31,32,33,34,35 and 36 which form the actual blades of the thin-film treatment apparatus. As is shown more clearly in Figure 3, a blade plate 38 is bolted to a blade root 39 by means of a conventional bolt 40. As can be seen in Figure 4, a blade plate 41 has a series of teeth members 42,43,44 and 45 which extend to the far edges of the blade plate 41 and the blade plate 41 is secured to a blade root 46 by means of bolts 47,48 and 49.

The blade teeth 42 to 45 may be angled to push the viscous premix and viscous solution downwards through the thin-film treatment apparatus. Because the rotor 16 of the thin-film treatment apparatus 8 is arranged in a vertical disposition, the action of the inclined blades cooperates with the action of gravity to enhance movement of the premix and solution downwards through the treatment apparatus.

Figure 5 schematically exemplifies a preferred embodiment of the present invention by reference to two adjacent blades 41, 35 (which are not depicted) on which teeth 42, 43, 44 and teeth 42', 43' and 44', respectively, are arranged.

According to the embodiment depicted in Figure 5, part of the teeth are oriented vertically (i.e. teeth 42, 43 and 44) whilst another part of the teeth (teeth 42', 43' and 44') are inclined towards the vertical direction, thereby forming an angle α of more than 0°.

Each tooth has a length L (depicted in Figure 5 with regard to teeth 42 and 42', respectively). For the purposes of the present invention, with regard to a tooth that is inclined, the length L refers to the length of the projection of the tooth on the plumb line (as shown in Figure 5 with regard to tooth 42').

Adjacent teeth, e.g. teeth 42 and 43, are spaced apart from each other, therefore forming a gap G. The ratio of length L of the tooth to the length G of the gap is ratio V. According to the preferred embodiment of the present invention, the average ratio V for all the teeth and adjacent gaps located on the cylindrical section of the rotor should be more than 2:1, respectively.

Furthermore, according to the preferred embodiment of the present invention, the mean angle α _m which is the average value of the angles α of all teeth (including those teeth which are not inclined), should be less than 11°.

Figure 6 illustrates schematically the shape of an embodiment of tooth 42. The tooth 42 has a front face 42a which is in contact with the material to be treated, and which distributes and transports the material over the inner surface of the cylindrical portion 9. Front face 42a runs essentially parallel to the inner surface of cylindrical portion 9.

According to the preferred embodiment of the present invention, the front face 42a of the teeth exhibits a thickness T, which should be more than 5 mm.

Examples

The production capacity of a thin film treatment apparatus is especially limited by two factors:

1) Maximum circumferential speed of the blades

2) Maximum temperature within the apparatus (measured at the jacket).

In an exemplary thin film treatment apparatus, factor 1) (i.e. the maximum circumferential speed) may have a value of around 7 m/s.

The maximum temperature (factor 2)) should not exceed 160° for risk of exothermic reactions.

The product of these two values, which will be referred to as the maximum "F-factor" in the following, defines the boundary for an increase in production capacity. I.e. it is not possible to increase production capacity above this value without either reaching technical limitations or risking exothermic reactions.

With regard to the present examples, the product of factors 1) and 2), i.e. the maximum "F- factor", is 112O ⁰ C m/s.

Example 1:

A thin film treatment apparatus having a rotor with 8 blades (thickness T = Il mm) was employed for this example. The apparatus was used to process a cellulose suspension in aqueous NMMO into a cellulose solution with 13% cellulose.

In a comparison example, the length of the teeth arranged on the blades was 62 mm, respectively. The length of the gaps adjacent the teeth was 31 mm, respectively. I.e. the average ratio V was 2:1.

In the example according to the invention, the length of the teeth was 102 mm, whilst the length of the gaps was kept constant at 31 mm. I.e. the average ratio V was 3.3.

For the purposes of this example, the term "length" refers to the actual face length of the teeth. For those teeth in the apparatus used that were inclined, the length of their projection on the plumb line was only insignificantly different from the actual face length and, hence, did not alter the average ratio V.

By means of adjusting circumferential speed and/or the temperature of the heating jacket of the thin film treatment apparatus various operating points were set. These operating points correlated with a certain specific output (in kg/d fibre - based on a moisture content of 11% - produced per m ² of surface area of the apparatus; this refers to the amount of fibre which can be spun from the cellulose solution produced by the apparatus).

The F-factor which resulted from the process conditions (circumferential speed and temperature) which were necessary in order to achieve a certain output was measured.

The results of the experiments are summarized in the following tables:

It can be seen from the above and from figure 7 that the use of the thin film treatment apparatus with a higher ratio V (V=3.3:l) enabled production with a specific output of 1750 kg/d.m ² at a much lower F-factor (891.9) as compared with an apparatus where ratio V is only 2:1 (F-factor 983.8).

With regard to figure 7, it can furthermore be seen that if one extrapolates the essentially linear curves obtained when plotting the F-factor against specific output, the maximum output that can be obtained - without exceeding the maximum F-factor of 1120°C m/s - is 2390 kg/d.m ² using a thin-film treatment apparatus having a ratio V of 3.3 : 1 as compared with 1940 kg/d.m ² with a thin-film treatment apparatus where V is only 2:1.

Example 2:

The experimental setup was the same as with regard to example 1 with the following exceptions:

In the comparison example, the length of the teeth arranged on the blades was 62 mm, respectively. The length of the gaps adjacent the teeth was 31 mm, respectively. The mean angle α _m (towards the vertical direction) of all the teeth was 14°. The teeth had a thickness T of 11 mm.

In the example according to the invention, the same apparatus was employed, however, the mean angle α _m of all the teeth was 10°.

Again, the F-factor which resulted from the process conditions (circumferential speed and temperature) which were necessary in order to achieve a certain output was measured.

The results of the experiments are summarized in the following tables:

It can be seen from the above and from figure 8, that the use of the thin film treatment apparatus with a mean angle α _m towards the vertical direction of 10° enabled production with a specific output of 1250 kg/d.m ² at a much lower F-factor (702.1) as compared with an apparatus where the mean angle α _m is 14° (F-factor 747.5).

With regard to the figure 8, it can furthermore be seen that if one extrapolates the essentially linear curves obtained when plotting the F-factor against specific output, the maximum output that can be obtained - without exceeding the maximum F-factor of 1120°C m/s - is 2550 kg/d.m ² using a thin-film treatment apparatus having a mean angle α _m of 10° as compared with 2110 kg/d.m ² with a thin-film treatment apparatus where α _m is 14°.

Example 3:

The experimental setup was essentially the same as in example 1 and example 2, however, a thin film treatment apparatus having a rotor with only 4 blades was employed for this example.

In the comparison example, the length of the teeth arranged on the blades was 62 mm, respectively. The length of the gaps adjacent the teeth was 31 mm, respectively. The teeth had a thickness T of 11 mm.

In the example according to the invention, the same apparatus was employed, however, the teeth had a thickness T of 22 mm.

The results of the experiments are summarized in the following tables:

It can be seen from the above and from figure 9 that the use of the thin film treatment apparatus with a thickness T of the teeth of 22 mm enabled production with a specific output of 1250 kg/d.m ² at a much lower F-factor (905.4/921.6) as compared with an apparatus where thickness T was 11 mm (F-factor 924.5/1063.8).

With regard to figure 9, it can furthermore be seen that if one extrapolates the curves obtained when plotting the F-factor against specific output, the maximum output that can be obtained -

without exceeding the maximum F-factor of 1120°C m/s - is 1510 kg/d.m ² using a thin-film treatment apparatus with a thickness T of the teeth of 22 mm as compared with 1300 kg/d.m ² with a thin-film treatment apparatus where T was 11 mm.