DEVICE AND METHOD FOR DILUTION OF CELLULOSE PULP

Abstract

A device and a method for dilution of cellulose pulp of a consistency of between 20% and 45%. The cellulose pulp is arranged to fall down in a pulp chute during an addition of dilution liquid. The dilution is performed to a consistency in the range of 6-18%, above a level of established diluted pulp in a bottom of a pulp chute. A central distribution device with nozzles for addition of dilution liquid to the falling pulp above the level of established diluted pulp in the bottom of the pulp chute is arranged in the center of the pulp chute. The nozzles of the distribution device are fed with dilution liquid via a pipe that is arranged at least partly through the pulp chute.

Description

The present invention relates to an improved device for dilution of cellulose pulp in the form of flocs and to a related method. The dilution is intended to be performed in order to dilute HC-cellulose pulp of an original consistency of about 20 to 45% (HC=High Consistency), the dilution being withheld until the formation of a MC-pulp, i.e. until the pulp reaches a consistency in the range of 6-18% (MC=Medium consistency).

BACKGROUND

In connection to the dilution of decomposed cellulose pulp of a high dryness, typically a consistency of above 20%, it is normal procedure to add dilution liquid whilst agitating by means of a dilution screw. This has also been the starting point in the Swedish patent SE 526292, which represents the prior art. Within this prior art dilution to the desired consistency is achieved by the addition of dilution liquid via a ring shaped supply chamber around a pulp chute.

A ring shaped supply chamber for the addition of dilution liquid has been used in the business for a long time and has for instance been utilized for the dilution of cellulose pulp after HC-Ozon reactors, i.e. bleaching reactors in which cellulose pulp of high consistency, i.e. above approximately 20%, is bleached with Ozon. A HC-Ozon reactor is conventionally a horizontal reactor with an interior transport and mixing screw, which normally is arranged directly after a shredding screw from a wash press, and in which the pulp falls down into a pulp chute after having been treated in the HC-Ozon reactor. In e.g. U.S. Pat. No. 5,174,861, the fundamentals of a horizontal. Ozon reactor is disclosed, in which the HC-pulp falls freely down into a dilution tank after the reactor.

It has been noticed that the use of a ring shaped supply chamber around a pulp chute that periferally injects dilution liquid towards the falling floes of pulp, only achieves a certain degree of penetration into the flow and that the maximum quantity of dilution liquid that may be added is limited.

In a plant with a capacity of 3000 ADT (Absolutely Dry Tons) of pulp a day, a pulp chute of a diameter in the range of 1.5 metre is utilized and conventionally no pulp chute for MC-pulp (Medium Consistency) with a diameter of less than 0.80 metres has been utilized. This means that a very good agitation of the pulp is required in order to achieve a satisfactory dilution by means of a ring shaped supply chamber.

SHORT DESCRIPTION OF THE INVENTION

The present invention relates to an improvement of the dilution with respect to a ring shaped supply chamber. An object of the invention is to overcome the problems of the prior art and to evenly distribute the dilution such that substantially the whole pulp volume will have the same consistency. This is achieved by means of a device according to claim 1.

According to a first aspect, the invention relates to a device for dilution of cellulose pulp of a consistency of between 20 and 45%, which device comprises a pulp chute through which cellulose pulp is arranged to fall down under the addition of dilution liquid, wherein the dilution is performed to a consistency in the range of 6-18%, and wherein the addition of dilution liquid is performed on the falling pulp above a level of established diluted pulp in the bottom of the pulp chute. Further, a central distribution device with a nozzle arranged in the centre of the pulp chute for the addition of dilution liquid to the falling pulp in the centre of the pulp chute and above the level of established diluted pulp in the bottom of the pulp chute, wherein the nozzle is fed with dilution liquid via a pipe that is arranged at least partly through the pulp chute.

According to a second aspect, the invention relates to a method of dilution of cellulose pulp with a consistency of between 20 and 45%, where dilution liquid is added to the cellulose pulp in a pulp chute, which dilution is performed to a consistency in the range of 6-18% as the cellulose pulp is in free fall above an established level of diluted cellulose pulp at the bottom of the pulp chute. The dilution liquid is added in the centre of the pulp chute.

In the device and the method according to the invention flocs of pulp are wetted throughout essentially the whole cross section of the pulp chute before the flocs hit the established pulp level in the chute. Thus, not only the flocs of pulp falling along the walls of the pulp chute will be wet, in which case the degree of wetting decreases with increasing distance from the walls of the pulp chute and is close to zero in the centre of the pulp chute. Instead, an even dilution, which is especially important with increasing capacity of the plants, and with the invention it is possible to reach a capacity of 4-5000 ADT pulp per day and plant.

With the invention it is possible to obtain a higher degree of homogen dilution of cellulose pulp in free fall, since more dilution liquid may be added, which furthermore may be more evenly distributed in the pulp. Also, it is possible to obtain a high degree of homogenously diluted cellulose pulp already in the free fall, i.e. without agitating the liquid. Another advantage is that it is possible to divide the dilution liquids such that a less aggressive dilution liquid may be added in the periphery as a complement to treat the walls of the pulp chute leniently, wherein a less costly material may be used for these, while more aggressive pH adjusting chemicals, such as acids or bleaching chemicals, are added in the centre of the chute and surrounded by a protecting curtain of less aggressive cellulose pulp, closer to the inner walls of the chute.

SHORT DESCRIPTION OF THE DRAWINGS

Below, an embodiment of the invention will be described in detail with reference to the accompanying figures, of which:

FIG. 1 schematically shows a comprehensive view of an embodiment of a dilution device according to the invention;

FIG. 2 shows a view from below of an embodiment of the invention;

FIG. 3 shows a cross sectional side view of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a position in the pulp line where decomposed cellulose pulp in the form of flocs is transported in a screw feeder 1 in which a feed screw 2 feeds flocs of cellulose pulp to a pulp chute 3. The screw feeder 1 may constitute the final part of a pulp shredder or transport screw in a bleaching reactor in which the cellulose pulp is bleached at a high consistency, above at least 20%.

In the upper part of the pulp chute 3 a dilution device 10 is arranged, which adds dilution liquid to the flow of flocs that falls freely down towards an established level of pulp 6 in the pulp chute 3. The dilution device 10 comprises a line 15 that is drawn to a central distribution device with nozzles 11 arranged in the centre of the pulp chute 3 in order to distribute dilution liquid to the cellulose pulp, as it falls freely in the centre of the pulp chute. Additionally, in the shown embodiment there is also a supply chamber 16, which is arranged for adding dilution liquid concentrically around the pulp chute, and which is provided with a plurality of nozzles 14 on the inner wall 3A of the chute 3 (see FIG. 2) for distributing dilution liquid to the flocs that falls freely.

The diluted pulp, which by the addition of dilution liquid has obtained a relatively homogenous concentration, forms an established level 6 of diluted pulp in the bottom of the chute 3. In connection to the bottom of the chute a MC-pump 4 is arranged, which in a conventional manner is provided with a fluidizer 5 in front of the pump wheel of the pump. The fluidizer 5 is at least partly arranged inside the chute 3 and provides a hefty agitation of the cellulose pulp in the bottom of the pulp chute, which is indicated with rotational arrows, and accomplishes the homogenisation of the concentration of the cellulose pulp. Normally, the fluidizer is integrated with the pump wheel of the pump, but it may also be separated from the pump wheel and be provided with an own driving means, and placed in immediate connection of the inlet to the pump wheel.

In FIG. 2, an embodiment of the dilution device 10 is shown in a view from below. Dilution liquid is added via the line 12, wherein the dilution liquid reaches a certain pressure inside a supply chamber 16 defined by an outer wall 10A and the inner wall 3A the pulp chute 3. A number of outlets 14 are arranged in the periphery of the pulp chute from the supply chamber 16 and directed inwards, towards the centre of the pulp chute. The pressure in the supply chamber 16 is such that the dilution liquid is sprayed into the pulp chute towards the falling pulp. The pressure control, the shape of the outlet and the quantity of outlets may vary in a number of different ways, which are evident to the skilled person. The object is however to yield maximal penetration into the flow of falling pulp flocs.

As is apparent from the figures, the central distribution device comprises at least one pipe 15 extending from the wall 3A of the pulp chute 3 inwards, towards the centre of the pulp chute. At the end of this pipe 15, at least one nozzle 11 is arranged, for the addition of at least a part of the dilution liquid. The nozzles 11 may have several outlets and may distribute the dilution liquid in form of a downwardly directed shower stream 11A with a substantially conical jet (see FIG. 3). However, the dilution liquid may also be added in a direction obliquely upwards or completely horizontally outwards towards the walls 3A in the pulp chute. An advantage of adding the dilution liquid obliquely upwards is that it thus is given more time for mixing with the falling pulp, whereby it of course mixes better. Another advantage is that the energy of impact from the dilution liquid hitting the pulp contributes in mixing the pulp, which leads to that the pulp mixing gets a more homogenous consistency. In order to further improve the distribution of dilution liquid to the falling cellulose pulp it is feasible to arrange several central nozzles 11, which are distributed over substantially the whole width of the pulp chute.

An alternative or complement to such a distribution of the nozzles 11 in the centre of the pulp chute 3 is to, as in the shown embodiment, also arrange peripheral outlets 14. In FIGS. 1 and 3, the nozzle 11 of the central distribution device is arranged below the peripheral outlets 14 of the supply chamber 16 in the wall 3A of the pulp chute. In this way the addition of dilution liquid is directed towards the upper side of the pipe 11 of the central distribution device, partly in order to keep this free from flocs that otherwise easily may gather thereon. Therefore, as is shown in FIG. 3, one of the jets 14B from the peripheral outlets 14 may directed in parallel with the upper side of the pipe 11. The pipe may also be drawn into the pulp chute in the way that is shown in FIG. 3, i.e. obliquely downwards such that the flocs falling from above towards the pipe shall fall of more easily. To a skilled person it is obvious that the pipe 15 also may be drawn in several other fashions. It is of course important that the pipe does not binder the falling the pulp too much, a certain contact may however be positive since it may function as an agitation of the pulp, which may contribute to better mixing and homogenisation of it.

In one embodiment of the invention the feeding pipe 15 is provided with a separate feeding, wherein the outlets or nozzles 11 of the central distribution devices may be fed with a different dilution liquid from another source than the one that is being fed to the peripheral nozzles 14. Thus, it is possible to add a pH-regulating agent, bleaching chemicals or other additives centrally in the pulp chute, whereby the falling flocs of pulp that has been totally or partly diluted with a less aggressive dilution liquid shields off the centrally added dilution liquid such that it does not reach the inner wall 3A of the pulp chute. The design of the supply chamber 16 and the upper parts of the pulp chute 3 may hence be designed in a simpler and cheaper material, where only the pipe 15 and nozzle 11 of the central distribution device requires materials that are resistant to a concentrated dilution liquid.

Large quantities of dilution liquid are added to the flow of freely falling pulp flocs and may be followed by air or gas in the flow, wherein an active deaeration may be needed in order to vent redundant volumes accompanying the flow. The pulp chute may therefore be designed with a deaeration canal 13 that is connected to the pulp chute above the supply chamber 10. The deaerated gases may either be released to the surrounding atmosphere or to a decomposition system depending on whether the gases are environmental harmful or not.

Claims

1. A device for dilution of cellulose pulp of a consistency of between 20% and 45%, comprising:

a pulp chute through which cellulose pulp is arranged to fall down,

means for adding the dilution liquid on the falling pulp above a level of established diluted pulp at a bottom of the pulp chute to dilute the consistency to a range of 6-18%,

a central distribution device with a nozzle arranged in a center of the pulp chute adapted to add dilution liquid to the falling pulp in the center of the pulp chute and above the level of established diluted pulp at the bottom of the pulp chute, and

means for feeding the nozzle with dilution liquid via a pipe that is arranged at least partly through the pulp chute.

2. The device according to claim 1, wherein the central distribution device has several nozzles that are evenly distributed over a width of the pulp chute.

3. Device The device according to claim 1 wherein a peripheral distribution device with peripheral nozzles is arranged for adding dilution liquid from an inner wall of the pulp chute.

4. The device according to claim 3, wherein the central distribution device is arranged below the peripheral nozzles of the peripheral distribution device.

5. The device according to claim 4 wherein the device further comprises means for separating the feeding to the nozzles of the central distribution device from the feeding to the peripheral nozzles of the peripheral distribution device, and means for feeding dilution liquid from different sources to the peripheral nozzles of the peripheral device and the nozzle of the central distribution device.

6. The device according to claim 5 wherein a deaeration canal is arranged in connection to the pulp chute above the nozzle of the central distribution device and the peripheral nozzles of the peripheral device.

7. A method for dilution of cellulose pulp having a consistency of between 20% and 45%, comprising:

adding a dilution liquid to the cellulose pulp in a pulp chute, which dilution is performed to a consistency in a range of 6-18% as the cellulose pulp is in free fall above an established level of diluted cellulose pulp at a bottom of the pulp chute, and

adding dilution liquid in a center of the pulp chute.

8. The method of claim 7 wherein the dilution liquid is added from several nozzles that are substantially evenly distributed over a width of the pulp chute.

9. The method of claim 7 wherein dilution liquid also is added from an inner wall of the pulp chute.

10. The method of claim 9 wherein different dilution liquids are added in the center and a periphery of the pulp chute, wherein the dilution liquid added in the center of the pulp chute is more concentrated than the dilution liquid added in the periphery of the pulp chute.

11. The method of claim 7 wherein at least 25% of the dilution liquid is added in the center of the pulp chute.