CELLULOSE PULP

TECHNICAL FIELD

The invention relates to a device for preventing rewetting of pulp in an arrangement for washing and/or dewatering of cellulose pulp.

BACKGROUND

Pulp washing is a key operation in the pulping line. There are many different types of washers available, some of which are based on press washing and comprise means for pressing the pulp to remove liquid. After pressing, the pulp can, if suitable, be diluted to a desired consistency.

A well-known washing machine is a twin-roll press of the general type disclosed in US Patent 3,980,518, for example. It has two counter-rotating rolls with perforated outer surfaces. A web of pulp is formed on the respective rolls and transported in the direction of rotation in a vat partially surrounding the rolls, to the so-called press nip between the rolls. The liquid removed from the pulp, i.e. the filtrate, passes through the perforated roll surface in a radial inwards direction and is, by means of axial filtrate channels, led to the ends (or, alternatively, to one end) of the press roll, where it is output. Washing liquid may be supplied to the pulp web in the vat.

High levels of filtrate are common, especially in long press rolls. This leads to unwanted rewetting after the nip. After the nip, the pressure in the pulp decreases and a portion of the filtrate, which has not yet been drained out of the press rolls, returns to the pulp.

According to US Patent 5,470, 471 baffle plates are installed in drainage compartments, permanently or removably, to prevent rewetting of the pulp in a twin roll press.

WO 2010/101518 Al discloses an anti-rewet insert with a filtrate barrier strained against the outer wall of the filtrate channel by tightening screws. A plurality of springs are pivotally connected to the filtrate barrier to support it. WO 2010/138059 suggests a mechanism using a biasing member to obtain a reduced and extended condition, respectively, facilitating removal of the anti-rewet insert from the filtrate channel e.g. for cleaning. WO 2010/138056 discloses a mechanism of the latter type provided with a locking means to prevent axial dislocation.

Previous anti-rewet devices have rather complex designs, involving a number of different mechanical components acting together. This makes them comparatively difficult to produce and to handle. For example, devices relying on springs to hold a plate against the perforated roll require a large number of springs to work properly, and springs are difficult components since they need to be able to withstand the harsh environment in the filtrate channel and still present good spring properties. The anti-rewet device often becomes a little loose or wobbly.

Accordingly, there is a need for an improved anti-rewet device.

SUMMARY

An object of the invention is to provide an improved device for preventing rewetting in a roll press for cellulose pulp. This object is achieved in accordance with the appended claims.

The invention provides a new anti-rewet device which includes an anti-rewet beam, the filtrate barrier of which forms a closed shape together with a support member, leaning against a side wall of the filtrate channel, and one or more intermediate beam portions interconnecting the filtrate barrier and the support member. (The term "beam" here refers to an axially extending structure or member.) The anti-rewet device of the invention has a comparatively simple and straight-forward design, facilitating manufacturing, installation and handling of the device when installed in a roll press. The closed shape makes it possible to attach the anti-rewet beam only at the ends, which is a major advantage. The anti-rewet beam is preferably substantially self- supporting, the filtrate barrier being supported in its position towards the outer wall of the filtrate channel by the support member and the intermediate beam portion(s).

More specifically, a device is provided for preventing rewetting of pulp in an apparatus for washing and/or dewatering of cellulose pulp, which apparatus comprises two press rolls, arranged so as to, in operation, rotate in opposite directions and at least one having a permeable outer surface, inside of which a plurality of axially extending filtrate channels are arranged, each filtrate channel having an outer wall, an inner wall, a front side wall and a rear side wall, whereby pulp that in operation is input to the apparatus is transported on the permeable outer surface in the direction of rotation and pressed in a nip between the press rolls. The device for preventing rewetting is adapted to be installed in one of the filtrate channels and comprises an axially extending anti-rewet beam, the cross-section of which at least at parts of the length of the anti-rewet beam forms a closed shape. The anti-rewet beam in turn comprises an axially extending filtrate barrier adapted to be positioned towards the outer wall of the filtrate channel to prevent flow of liquid from the trailing part of the filtrate channel to the outer surface of the press roll, an axially extending support member adapted to be positioned towards the rear side wall of the filtrate channel, and at least one axially extending intermediate beam portion, interconnecting the filtrate barrier and the support member, so as to form the closed beam shape.

According to an advantageous embodiment, the anti-rewet beam is made such that fluid (filtrate) can flow inside it, e.g. provided with apertures. This means that a comparatively high flow of filtrate out from the filtrate channel can be maintained, thereby reducing the risk of rewetting from filtrate above the anti-rewet beam.

According to another aspect of the invention, a roll press for washing and/or dewatering of cellulose pulp is provided, which comprises the device for preventing rewetting.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description and appended drawings, in which:

Fig. 1 is a schematic cross-sectional view illustrating a washing/dewatering apparatus with a device for preventing rewetting according to an exemplifying embodiment of the invention; Fig. 2 is a schematic cross-sectional view illustrating a device for preventing rewetting according to an exemplifying embodiment of the invention, the device being in a first radial position PI ;

Fig. 3 is a schematic cross-sectional view illustrating a device for preventing rewetting according to an exemplifying embodiment of the invention, the device being in a second radial position P2;

Figs. 4A and 4B are schematic longitudinal views illustrating an anti-rewet beam with apertures in accordance with exemplifying embodiments of the invention;

Fig. 5 is a schematic cross-sectional view illustrating a device for preventing rewetting according to another exemplifying embodiment of the invention; and

Figs. 6A and 6B are schematic longitudinal views illustrating a level-lowerer beam with apertures in accordance with exemplifying embodiments of the invention.

DETAILED DESCRIPTION

In the drawings, similar or corresponding elements are denoted by the same reference numbers

Fig. 1 illustrates an example apparatus 100 for washing and/or dewatering of cellulose pulp with an example device 10 for preventing rewetting according to the invention. A twin roll press 100 comprising two co-operating cylindrical press rolls/drums 2 is shown. The press rolls 2 are arranged to rotate in opposite directions R, R' during operation and each has a permeable outer surface 4, such as a perforated metal sheet. The press rolls 2 are partially enclosed by a vat 5 (also known as trough) formed by guide surfaces, arranged at a distance from the permeable outer surfaces 4 so as to partially enclose the press rolls 2 in the circumferential direction.

During operation, pulp enters a pulp distribution device 1 of the respective press roll 2. The input consistency of the pulp is typically in the range of 2-13%. A pulp web is formed on the permeable roll surface 4. The pulp web is transported, guided by the vat 5, in the direction of rotation R, R' to be pressed in a nip (also known as pinch) 3 where the distance between the press rolls 2 is smallest. The press rolls 2 comprise axial filtrate channels 6, which receive the filtrate that passes through the permeable roll surface 4, and typically transport it towards the ends of the press rolls 2, where it is output. Washing liquid may be supplied to the pulp web in the vat 5. The pulp is output by means of a discharge screw 7.

In Fig. 1, two press rolls 2, each provided with a device 10 for preventing rewetting, are arranged next to each other, with the rotation centers in the same horizontal plane. The invention is also suitable for washing/dewatering apparatuses where, for example, the rolls are differently arranged, or only one perforated roll is used.

In the simplified view of Fig. 1, there is only one device 10 for preventing rewetting for each press roll 2. Typically, there will be devices 10 for preventing rewetting installed in every filtrate channel 6 of the respective press rolls 2, or at least in a large subset of the filtrate channels 6.

The device 10 for preventing rewetting may be inserted as an upgrade of an existing press roll 2, or, alternatively, it may be built-in in the press roll 2 already during manufacturing.

The example device 10 for preventing rewetting will now be described with reference to Fig. 2, illustrating the device 10 installed in a filtrate channel 6 in operation mode (Position PI). An annular space inside of the permeable roll surface 4 is divided into axially extending (i.e. longitudinal) filtrate channels 6 by side walls 6c, 6d. Each filtrate channel has an outer wall 6a, an inner wall 6b, a front side wall 6c and a rear side wall 6d. The outer wall 6a is permeable, typically being part of the permeable outer surface 4 of the press roll 2. It should be noted that the terms "front and rear side wall" herein refers to a specific filtrate channel. One and the same side wall is thus the rear side wall 6d of the filtrate channel 6 above it and the front side wall 6c of the filtrate channel 6 below it, when the filtrate channel is rotating upwards as in Fig. The device 10 for preventing rewetting comprises an axially extending anti-rewet beam 12, which consists of a filtrate barrier 12a, positioned towards the outer wall 6a of the filtrate channel 6 to prevent flow of liquid from the trailing part of the filtrate channel 6 to outside the press roll 2, a support member 12b positioned towards the rear side wall 6d of the filtrate channel 6, and intermediate beam portions 12c, 12c', interconnecting the filtrate barrier 12a and the support member 12b.

In other words, the filtrate barrier 12a is adapted to bear or lean against the outer wall 6a of the filtrate channel 6 when the device 10 for preventing rewetting is in operation (position PI in Fig. 2) to prevent flow of liquid from the trailing part of the filtrate channel 6 to outside the press roll 2. The support member 12b is adapted to bear or lean against the rear side wall 6d of the filtrate channel 6.

The intermediate beam portions 12c, 12c' interconnect the filtrate barrier 12a and the support member 12b so as to form a substantially closed shape in cross-section. "Closed shape" means that the cross section of the anti-rewet beam 12 is hollow, not solid. It defines a substantially closed area as seen in the cross-sectional view of Fig. 2. "Cross-section" here refers to the section formed by a plane cutting through the anti-rewet beam 12 at an angle to its length axis.

It should be understood that the anti-rewet beam 12 does not have to form a closed shape around its circumference along its entire length. For example, there may be holes, slots or the like interrupting the closed shape. However, the cross-section of the anti-rewet beam 12 should always form a closed shape at least at parts of the length of the anti-rewet beam 12. Advantageously, the cross-section of the anti-rewet beam 12 forms a closed shape at most of the length of the anti-rewet beam 12, i.e. the total length of the portions of the anti-rewet beam 12 that form a closed shape is more than half the length of the anti-rewet beam 12.

In Fig. 2, there is one intermediate beam portion 12c interconnecting the leading end of the filtrate barrier 12a and the inner end of the support member 12b, and one intermediate beam portion 12c' interconnecting the trailing end of the filtrate barrier 12a and the outer end of the support member 12b. (The terms "leading" and "trailing" here refer to the direction of rotation R. "Inner" and "outer" refer to a cross sectional view of the press roll 2, "inner" being closer to the rotation axis of the press roll 2.)

Still referring to Fig. 2, the anti-rewet beam 12 may with advantage be substantially self- supporting, the filtrate barrier 12a being supported in its position towards the outer wall 6a of the filtrate channel 6 by the support member 12b and the intermediate beam portions 12c, 12c'.

The anti-rewet beam 12 may with advantage consist of a metal sheet frame (or plate frame). The metal sheet should be selected so as to provide a comparatively stiff or rigid beam structure.

According to one advantageous embodiment, the filtrate barrier 12a of the anti-rewet beam 12 extends to at least one third of the height of the outer wall 6a of the filtrate channel 6. According to another advantageous embodiment, the cross section of the anti-rewet beam 12 encloses an area covering at least one fifth of the inner cross-sectional area of the filtrate channel 6.

As illustrated in Figs. 2 and 3, the device 10 for preventing rewetting preferably comprises means 14 for attachment of the end parts of the support member 12b to the rear side wall 6d of the filtrate channel 6. The term "end parts" refers to the parts of the support member 12b at or close to the respective ends of the press roll 2 as seen in the longitudinal (axial) direction. The structure of the anti-rewet beam 12 of the invention with its closed shape makes it possible to attach it only at the ends, which is a major advantage when it comes to installation and handling of the anti-rewet device 10. For very long press rolls 2, it may however be suitable to arrange a support also between the end parts, e.g. at the middle of the support member 12b to prevent deflection.

The attachment means 14 may be adapted for radial movement of the anti-rewet beam 12 between a first position PI, where the filtrate barrier 12a bears against the outer wall 6a of the filtrate channel 6 (Fig. 2), and a second position P2, where the filtrate barrier 12a is at a distance from the outer wall 6a (Fig. 3) so as to enable cleaning of the permeable outer surface 4 of the press roll 2 with the device 10 for preventing rewetting still in the filtrate channel 6. For example, a bolt (or screw) and nut attachment can be used, locking the support member 12b at positions PI and P2 through holes or slots in the support member 12b and holes or slots in the rear side wall 6d. In one advantageous embodiment, the attachment means 14 is arranged so that the anti-rewet beam 12 may slide radially to and from the outer side wall 6a upon loosening the bolt and nut attachment. It is a major advantage that cleaning of the press roll 2 can be performed without removing the anti-rewet device 10 from the filtrate channel 6. The cleaning requires comparatively few and simple steps and becomes faster than with conventional anti- rewet solutions.

Flow- guiding anti-rewet beam

A general problem when inserting an anti-rewet device in the filtrate channel is that the flow of filtrate out from the filtrate channel is reduced. If the filtrate level is above the anti-rewet beam, it may still rewet the pulp outside the filtrate channel to some extent. Therefore, the anti-rewet beam 12 of the invention is preferably made such that fluid (filtrate) can flow inside it and typically has open ends, for output of filtrate. It can with advantage be provided with apertures, e.g. as illustrated in Figs. 4A and 4B (views corresponding to A-A of Fig. 2). In these examples, the intermediate beam portion 12c interconnecting the leading part of the filtrate barrier 12a and the inner part of the support member 12b is provided with apertures 16, 16' such that filtrate can flow within the anti-rewet beam 12. The apertures 16, 16' can e.g. be openings, holes, orifices, slits, and/or perforations. The apertures 16, 16' result in an improved flow of filtrate out from the filtrate channel 6. Only a small portion of the cross sectional area in the filtrate channel 6 from which rewetting is not a problem is lost, i.e. occupied by the anti- rewet beam 12. See also the anti-rewet beam 12 with dots in Fig. 5, illustrating fluid (filtrate) inside the beam.

According to some advantageous embodiments, the apertures 16, 16' are concentrated on one or more areas of the intermediate beam portion 12c, so as to control the flow through the intermediate beam portion 12c and achieve a desired flow pattern through the anti-rewet beam 12. This means that there are more apertures 16, 16' in this area and that the apertures are unevenly distributed over the anti-rewet beam 12. It has been observed that, in the filtrate channel 6, the filtrate level sinks significantly faster closer to the ends than in the middle (as seen in the longitudinal, i.e. axial, direction). The filtrate level in the middle of the filtrate channel 6 is sinking slowly and there is a risk of rewetting due to filtrate rising above the anti-rewet beam 12. To reduce or minimize this problem the apertures 16 can be substantially centered at the middle part of the intermediate beam portion 12c, see Fig. 4A. The area exposed to dry pulp is hereby quicker reduced.

In another example embodiment, a subset of the apertures 16' are arranged at a higher level than the other apertures 16, such that the filtrate level in the filtrate channel 6 is leveled out as seen in the longitudinal direction. By placing the apertures 16' relatively high, the filtrate flow is controlled such that filtrate will flow into the anti-rewet beam 12 there only when the filtrate level is above a certain level. Possibly, filtrate will even flow out from the anti-rewet beam 12 where the filtrate level is low. The anti-rewet beam 12 then functions as a filtrate distributor, distributing the filtrate within the anti-rewet beam 12 so that the filtrate level will be below the upper edge of the anti-rewet beam in the entire filtrate channel 6 as fast as possible. See Fig. 4B, where the apertures 16' are placed higher than the apertures 16 at the middle part.

Additional beam - level lowerer

As described above, sometimes the filtrate level in the filtrate channel 6 is not sinking as fast as desirable and sometimes the filtrate level is sinking at different rates in different parts of the filtrate channel 6. According to one advantageous embodiment of the invention, which will now be described with reference to Fig. 5, it is suggested to arrange an additional beam, herein referred to as level-lowerer beam 20, together with the anti-rewet beam 12 in the filtrate channel 6. This embodiment requires that there is fluid connection between adjacent filtrate channels 6 by means of openings 8 in the side walls 6c, 6d of the filtrate channels 6. The level- lowerer beam 20 is an axially extending beam arranged at least partly in front of the anti-rewet beam 12 and adapted for fluid communication, through the front side wall 6c, with an adjacent ("upper") one of the filtrate channels 6. This means that, in the area around the nip, filtrate can flow downwards from one filtrate channel 6, via one or more openings 8 in the rear side wall 6d, and into the level-lowerer beam 20 of the filtrate channel 6 below it. By means of the level-lowerer beam 20, a considerable part of the filtrate flow of a filtrate channel 6 is transferred to the filtrate channel 6 below it. In other words, a filtrate channel 6 can utilize a space belonging to the filtrate channel 6 below it. This is illustrated by dots in Fig. 5. The area of the filtrate channel 6 above the anti-rewet beam 12 is considerably decreased and the level of filtrate in the filtrate channel 6 will much faster sink below the upper edge of the anti-rewet beam 12.

As illustrated in Fig. 5, the fluid communication between the filtrate channel 6 in which the level-lowerer beam 20 is arranged and the adjacent filtrate channel 6 can be provided by at least one aperture 22 in a beam portion 20a of the level-lowerer beam 20 adjacent to the front side wall 6c. The aperture(s) 22 correspond to or are smaller than the opening(s) 8 in the front side wall 6c. As illustrated in Fig. 6A (view corresponding to A' - A' in Fig. 5), the apertures 22 may with advantage be concentrated on one or more areas of the beam portion 20a, so as to control the flow and achieve a desired flow pattern through the level-lowerer beam 20. The illustrated example has apertures 22 more densely arranged at the middle of the level-lowerer beam 20 to lower the filtrate level faster there.

An alternative (not shown) to the beam portion 20a with apertures 22, is that the fluid communication between the filtrate channel 6 in which the level-lowerer beam 20 is arranged and the adjacent filtrate channel 6 is provided by the opening(s) 8 in the front side wall 6c, and the level-lowerer beam 20 is arranged to connect with and protrude from the front side wall 6c. The level-lowerer beam 20 may then protrude directly from the front side wall 6c. In this embodiment, the upper wall of the level-lowerer beam 20, i.e. beam portion 20a, or at least a major part thereof, can be left out.

The cross section of the level-lowerer beam 20 preferably forms a closed shape in itself (when there is a beam portion 20a adjacent to the front side wall 6c) or together with the front side wall 6c (when the level-lowerer beam 20 is arranged to protrude directly from the front side wall 6c), except for the apertures 22 or openings 8 which allow fluid to enter from the adjacent filtrate channel 6. In other words, there should be no interruptions in the axially extending beam portions protruding from the front side wall 6c. This means that filtrate is not allowed to enter the level- lowerer beam 20 from "below", but only from the filtrate channel 6 preceding the filtrate channel 6 in which the level-lowerer beam 20 is arranged.

If there are holes distributed lengthwise over the level-lowerer beam 20, the filtrate flow in the level-lowerer beam 20 will increase considerably towards its ends. Therefore, according to one advantageous embodiment illustrated schematically in Fig. 6B, the level-lowerer beam 20 has cross section increasing towards its end(s), where the flow is output. Still referring to Fig. 6B, it may also be preferred to design the level-lowerer beam 20 with a downward slope towards its end(s). This is to increase the flow out from the level-lowerer beam 20 and thereby lower the level of filtrate in the filtrate channel 6 faster.

According to one advantageous embodiment, the level-lowerer beam 20 is adapted to be installed in front of the anti-rewet beam 12, such that radial movement of the anti-rewet beam 12 is possible. (The term "in front of refers to the rotation direction.) The lowermost edge of the level-lowerer beam 20 is then arranged above the uppermost edge of the anti-rewet beam 12, such that the anti-rewet beam 12 can be moved radially without being obstructed by the level- lowerer beam 20. This facilitates cleaning of the permeable outer surface 4.

According to one advantageous embodiment, the shape of the level-lowerer beam 20 partially corresponds to the shape of the anti-rewet beam 12 such that there is a substantially parallel flow passage 26 between the anti-rewet beam 12 and the level-lowerer beam 20. In other words, the cross sectional area transverse to the filtrate flow is substantially constant in the flow passage 26. Referring to Fig. 5, the portion of the level-lowerer beam 20 facing intermediate beam portion 12c of the anti-rewet beam 12 has a corresponding shape and there is no throttling of the flow of filtrate between the two beams 12, 20.

According to one advantageous embodiment (not shown) the level-lowerer beam 20 is interconnected with the anti-rewet beam 12 so as to form one integrated structure. The level- lowerer beam 20 can then iunction so as to iiirther support the anti-rewet beam 12, adding to the favorable self-supporting feature of the anti-rewet device 10. Although the invention has been described with reference to specific illustrated embodiments, it is emphasized that it also covers equivalents to the disclosed features, as well as changes and variants obvious to a man skilled in the art, and the scope of the invention is only limited by the appended claims.

In particular, it should be understood that the shape of the anti-rewet beam 12 can be varied within the scope of the invention. It may for example have only one intermediate beam portion 12c arranged to interconnect the leading part of the filtrate barrier 12a and the inner part of the support member 12b, the trailing part of the filtrate barrier 12a being directly connected with the outer part of the support member 12b. It is likewise possible that the anti-rewet beam comprises two or more intermediate beam portions 12c together interconnecting the leading part of the filtrate barrier 12a and the inner part of the support member 12b, and/or two or more intermediate beam portions 12c' interconnecting the trailing part of the filtrate barrier 12a and the outer part of the support member 12b.

Also the shape of the level-lowerer beam 20 can be varied within the scope of the invention. In the example of Fig. 5, the level-lowerer beam 20 has a substantially pentagonal shape. In another embodiment (not shown) the level-lowerer beam 20 resembles a quadrangle. The skilled person realizes that various modifications are possible.