This invention relates to 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 or 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 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 evaporator such as the equipment made by Buss AG and available under the trademark Filmtruder. Disclosed in EP0356419-A is a thin film evaporator of the type to which the present invention is applicable. In that disclosure the pre-mix is pumped into the thin film evaporator via an inlet and passes down through the thin film evaporator 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 evaporator 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 evaporator 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 evaporator 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.

One of the characteristics of the thin film evaporator, when used to produce a cellulose solution is that the combination of the heating jacket and the non transparent nature of the premix means that it is not possible to see inside the thin film evaporator during use.

For most purposes the solution of cellulose produced by the thin film evaporator needs to be a clear amber colour solution with all of the cellulose fibres from the premix dissolved in the aqueous NMMO. However, in the event that a poor solution of cellulose is produced - such as a solution containing un-dissolved cellulose - it can be extremely difficult to determine what has caused the production of incompletely dissolved cellulose.

In accordance with the present invention, it has now been discovered that a thin film evaporator such as is disclosed in EPO356419-A may be operated in a manner such as to increase the reliability of the thin film evaporator to produce a solution with almost all of the cellulose dissolved in the aqueous NMMO.

By the present invention there is provided a method of manufacturing a solution of cellulose in an aqueous organic solvent which comprises the steps of:- (i) forming a premix containing cellulose, water and an organic solvent for cellulose,

(ii) swelling the cellulose in the premix with the water and/or the solvent, (iii) pumping with a pump the premix through a supply line into a vertically orientated thin film evaporator having a centrally located rotor adapted to wipe the premix over the interior of the thin film evaporator and to transport it downwards through the evaporator,

(iv) applying heat to the walls of the thin film evaporator and reducing the internal pressure within the thin film evaporator to below atmospheric pressure, so as, (v) to evaporate some of the water to permit the cellulose to go into solution in the aqueous organic solvent, and (vi) withdrawing the solution of cellulose in the solvent from the bottom of the thin film evaporator characterised in that the premix is forced by the pump into the thin film evaporator in a controlled manner and is not substantially pulled into the thin film evaporator

in an uncontrolled manner by the pressure difference between the supply line for the premix and the interior of the thin film evaporator.

One method of controlling the pumping of the premix into the thin film evaporator so that it is not substantially pulled in by the low pressure within the thin film evaporator is to have a constriction in the premix feed line close to the entrance to the thin film evaporator. This constriction can be a permanently arranged constriction - such as a narrowing of the premix feed pipe. Alternatively it can be a variable constriction, such as a valve, which can be throttled down particularly during the start up phase of the thin film evaporator and then opened up once normal running conditions have been stabilised.

Alternatively, the pump itself can be placed very close to the inlet so that there is not a volume of premix which can be sucked into the thin film evaporator. If the pump is a long way - for example 2 metres or more - from the inlet to the thin film evaporator a volume of premix can build up in the pipe so that it can be sucked in, particularly during the start up phase.

The thin film evaporator may have an internal heated surface area in the range of 5-45m2.

By way of example, embodiments of the present invention will now be described with reference to the accompanying drawings, of which

Figure 1 is schematic view of a cross section of a thin film evaporator in accordance with the disclosure of EPO356419-A.

Figure 2 is a schematic cross section of an inlet pipe with one form of constriction. Figure 3 is a schematic cross sectional view of an inlet pipe with a second form of constriction, and

Figure 4 is a schematic cross sectional view of an inlet pipe with a third form of constriction.

Figure 1 of the drawings taken from EP0356419 and Figures 2-4, show alternative methods of controlling the flow of premix into the inlet to the thin film evaporator.

The commercial process for forming a solution of cellulose from an aqueous NMMO solution is known and is outlined here by way of background. Essentially a premix of water, cellulose and NMMO together with a stabiliser such as propyl gallate is prepared and permitted to stand. This enables the cellulose to become swollen with the aqueous NMMO. Typically such as solution would comprise a mixture of 60% NMMO and 40% water mixed in with cellulose and a small quantity of propyl gallate such that the cellulose in the final concentration is approximately 13% by weight after evaporation of excess water. Such a suspension is pumped into the interior of a thin film evaporator, shown generally in Figure 1 , through an inlet 11. A central shaft 7 which is rotated by a electric motor 6 carries on it a spreader disk 10 located axially opposite inlet 11. Such a spreader disk forces the premix downwards. The premix moves downwards under, the action of gravity and also by being pushed by the transporter blades 8 mounted on the shaft 7. These blades are inclined at an angle alpha to the longitudinal axis of the shaft and the motor 6 rotates in a direction such that the blades 8 push the premix down as they engage with the premix. Under the action of the blades and gravity, the premix passes from a position opposite the entrance port 11 down through the thin film evaporator.

A heated jacket 2 surrounds the cylindrical thin film evaporator and a heating fluid passes through the thin film evaporator from the inlet 4 to the outlet 5 via a passage way 3. This results in the inside wall of the thin film evaporator

being heated by the fluid medium passing through the heating jacket. Typically, steam at 1300C is used as the heating medium.

The inside of the thin film evaporator is maintained at a low internal pressure typically 100 millibars by pumping out the gaseous contents of the thin film evaporator through the exit aperture 14 with a suitable vacuum pump.

The combination of the heating via the jacket 2 and the low pressure results in evaporation of the water in the NMMO mix. This evaporation results in a gradual concentration of the NMMO and as the NMMO becomes more concentrated it reaches a point where it is able to dissolve the cellulose. This process continues down the length of the thin film evaporator so as to form a clear amber coloured cellulose solution which is pumped out of the bottom of the thin film evaporator through the exit 12. The whole assembly is symmetrical about the centre line 9 apart from the entrance and exit pipes such as 11 , 14, 4 and 5. Also located within the bottom of the thin film evaporator is a supporting cone 14 which supports the bottom of the rotor. The blades 8 are separated from the inside wall of the thin film evaporator by a small gap indicated at 13.

It has now been discovered that the quality of the solution formed by the thin film evaporator can be improved by placing a restriction for the premix feed close to the inlet point 11. Examples of such restrictions are illustrated in Figures 2 to 4.

As can be seen in these figures, the thin film evaporator used for forming the cellulose solution can be any form of thin film evaporator and the operation of the thin film evaporator is at the discretion of the user of the invention.

In practice, installations involving thin film evaporator are usually large complex structures in which there will be a considerable amount of piping

connecting the various inlets and outlets such as the inlet 11. Premix will be prepared relatively close to the thin film evaporator but because of the size of the apparatus and because of the number of components required, there is often a pipe of many metres length between the premix feed system and the entrance to the thin film evaporator. The distance between the pump and the entrance point can be several metres long. Premix has to be pumped as it will not flow, having the consistency of a pasty mix of cellulose in a mixture of water and amine oxide. A suitable pump for pumping the premix is a piston pump or a Netzch Nemo Mono pump. These pumps are such that they not only pump in the premix but either also meter the premix in to the thin film evaporator for proper control of the dissolution process or pump it in in a controlled manner.

The pump such as pump 20 in Figure 2 is connected via suitable pipe 21 to the inlet point 11. Shown in Figure 2 is a throttle valve 22. Prior to the making of the present invention the throttle valve 22 was not provided and the pump 20 was directly connected to the inlet 11 via the pipe 21 which could be of indeterminate length - and was frequently several metres long.

Occasionally, during operation of the thin film evaporator it has been found that some of the cellulose was carried through in an un-dissolved state so that the solution emerging from the exit 12 of the thin film evaporator had a high level of un-dissolved cellulose. Inevitably there will always be some small quantity of un- dissolved cellulose but this can easily be removed by filtering. The greater the amount of cellulose present in the solution the more filtration is required and therefore the more often filters have to be changed. It will be clear, therefore, that the purer the solution produced by the thin film evaporator the better.

It is has now been found that by putting a constriction 22 into the pipe 21 enhanced quality solution can be prepared by the thin film evaporator. It is believed that the reason for this is that the constriction 22 prevents the low

pressure within the thin film evaporator sucking a slug of premix in to the thin film evaporator, overloading the distribution ring and the top transport blades and resulting in a transport of the slug through the system to give a partially un- dissolved amount of cellulose coming out in the solution 12. It has been particularly found that during start-up of the process there is liable to be a slug of cellulose sucked into the centre of the thin film evaporator and therefore a variable constriction valve has been found to be the preferred way of constricting the pipeline 21. It has also been found that after the thin film evaporator has run into a steady state, it can sometimes be possible to open the constrictor valve 22 thereby reducing the load on the pump 20 and therefore the pumping costs.

Although the variable constrictor valve 22 is the preferred form of apparatus to carry out the process of the present invention, it has also been found that a beneficial affect can be obtained by providing a constriction such as 32 in the pipeline 21 between the pump 20 and the inlet 11. The constriction can be an aperture plate with a suitable sized orifice. The orifice can be circular or elliptical or rectangular. The orifice can be combined with a valve so that the constriction can be increased during start up.

An alternative way of obtaining the desired result of the present invention is to provide the pump 20 closely adjacent to the entrance 11 to the thin film evaporator. The problem with this however is that it provides a severe restraint on the possible layout of the thin film evaporator and premix pumping assembly and because the thin film evaporator is a large heated assembly it can be difficult to provide equipment very close to it. This form of the invention is only therefore available if the plant layout permits.

The constriction also assists in the prevention of dewatering of the premix as it enters the thin film evaporator. This can cause plugs of material which block the feed line. Additionally, the constriction also assists in the prevention of air

entering the feed line as a result of pressure differences between the interior of the thin film evaporator and the premix in the feed line.

We Claim:

1. A method of manufacturing a solution of cellulose in an aqueous organic solvent which comprises the steps of:- (i) forming a premix containing cellulose, water and an organic solvent for cellulose,

(ii) swelling the cellulose in the premix with the water and/or the solvent, (iii) pumping with a pump the premix through a supply line into a vertically orientated thin film evaporator having a centrally located rotor adapted to wipe the premix over the interior of the thin film evaporator and to transport it downwards through the evaporator,

(iv) applying heat to the walls of the thin film evaporator and reducing the internal pressure within the thin film evaporator to below atmospheric pressure, so as, (v) to evaporate some of the water to permit the cellulose to go into solution in the aqueous organic solvent, and (vi) withdrawing the solution of cellulose in the solvent from the bottom of the thin film evaporator characterised in that the premix is forced by the pump into the thin film evaporator in a controlled manner and is not substantially pulled into the thin film evaporator in an uncontrolled manner by the pressure difference between the supply line for the premix and the interior of the thin film evaporator.

2. A method as claimed in Claim 1 in which the control is affected by a constriction in the premix feed line close to the entrance to the thin film evaporator.

3. A method as claimed in Claim 2 in which the constriction is a fixed constriction.

4. A method as claimed in Claim 2 in which the constriction is a variable constriction.

5. A method as claimed in Claim 1 in which the feed pump is located closely adjacent to the thin film evaporator.

6. A method as claimed in any of the preceding claims in which the thin film evaporator has an internal heated surface area in the range 5 to 45m2.

7. A method as claimed in Claim 4 in which the variable constrictor is used to constrict the flow only on start-up of the process.