EP2350383A1

EP2350383A1 - Apparatus and method for dewatering cellulose pulp with improved dewatering efficiency

Info

Publication number: EP2350383A1
Application number: EP09823903A
Authority: EP
Inventors: Rickard Andersson; Patrik Pettersson; Jörgen Lundberg
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2008-10-27
Filing date: 2009-10-23
Publication date: 2011-08-03
Also published as: EP2350383A4; CN102197175A; SE533737C2; CA2737310A1; SE0802285A2; WO2010050883A1; SE0802285A1

Abstract

The invention relates to an apparatus (1) for dewatering cellulose pulp, which apparatus includes two press rolls (2), arranged next to each other, each press roll (2) being provided inside guide surfaces (3), such that a pulp passage (4) is formed between each press roll (2) and the corresponding guide surface (3), wherein an inlet (5) for transferring pulp or the like to said pulp passage (4) is arranged at a first point at each press roll, and wherein the press rolls (2) are arranged to rotate in opposite directions (D1, resp. D2) so as to convey the pulp in a first operational direction (F1) towards a nip (6) that is formed between them at a second point, close to where the guide surfaces (3) intersect, each press roll (2) having a perforated outer surface (7) for withdrawing filtrate from said pulp or the like. At their point of intersection, the guide surfaces (3) include an outlet (8') for withdrawing filtrate that is rejected from the nip (6), said outlet (8') being arranged substantially opposite of the nip (6) and directed such that the filtrate removed through said outlet (8') is conveyed from the pulp passage through a clearance (9') that is formed in a space between the guide surfaces (3). The invention also relates to a corresponding method.

Description

Apparatus and method for dewatering cellulose pulp with improved dewatering efficiency

FIELD OF THE INVENTION

The present invention relates to washing and dewatering of cellulose pulp and in particular to a wash /dewatering apparatus with one, or preferably two co-operating, cylindrical press rolls.

BACKGROUND

Pulp washing is a key operation in the chemical pulping line. There are many different types of washing and dewatering apparatuses available, some of which are based on washing by pressing the pulp such that fluid is removed.

A well-known type of wash press has two co-operating cylindrical press rolls, arranged in the same horizontal plane next to each other with parallel axis of rotation. The outer surface of each press roll is perforated such that liquid may be extracted from the cellulose pulp that is conveyed in the restricted space between the press roll and a surrounding vat, through said perforated surface. During operation, when the press rolls rotate, a pulp web is formed on the perforated surface of each roll. A pinch or nip is formed between the press rolls at the point where the distance between them is the smallest, and the press rolls are arranged to rotate in opposite directions so as to transport the respective pulp webs towards said nip, such that the pulp webs merge shortly before the nip and are subsequently jointly pressed in the nip.

Thus, the pulp webs are continuously dewatered throughout the whole pulp passage, wherein a final dewatering is achieved in the nip, where the pulp web is pressed to a desired degree.

The fluid removed from the pulp, i.e. the filtrate, passes through the perforated roll surface in a radial inward direction and can for example be transported to the axial ends of the press rolls by means of axial filtrate channels. There is normally a filtrate tank arranged in connection to the wash press to collect the filtrate that is removed from the washing in the press. Washing liquid is supplied to the wash press in order to wash the pulp and displace the fluid in the pulp. The washing principle is thus a combination of displacement, dewatering and pressing.

Conventionally, the vat has been arranged to converge towards the nip in order to build up a certain increasing pressure. Nevertheless, a wash press with a non-converging vat geometry is disclosed in FI 102 977 B. The objective of this wash press is to convey pulp towards the nip under a relatively low pressure, in order to avoid plugging and other problems related to high pressures. This is achieved by the use of a vat that, in contrast to conventional apparatuses, is arranged at a constant distance from the press roll towards the nip. In FI 102 977 B, no regard is taken to the pressure profile that is formed inside the pulp passage.

SUMMARY

An object of the invention is to improve the efficiency of the dewatering in a washing or dewatering apparatus. This is achieved in a first aspect by an apparatus for dewatering cellulose pulp according to claim 1 , and in a second aspect by a method of dewatering cellulose pulp.

In recent tests it has been discovered that the increasing pressure towards the nip is almost independent of the vat geometry, such that a pressure gradient is built up irrespective of the vat geometry. Thus, especially in a vat with a non converging geometry, this pressure gradient produces a counter flow of filtrate based on the filtrate that is rejected from nip.

The invention is based on this discovery, and the presence of a counter flow opposite the direction of rotation of each press roll is in this invention used to enhance the dewatering efficiency of the apparatus.

According to a first aspect, the invention relates to an apparatus for dewatering cellulose pulp, which apparatus includes two press rolls, arranged next to each other, each press roll being provided inside a guide surface, such that a pulp passage is formed between each press roll and the corresponding guide surface, wherein an inlet for transferring pulp or the like to said pulp passage is arranged at a first point at each press roll, and wherein the press rolls are arranged to rotate in opposite directions so as to convey the pulp in a first operational direction towards a nip that is formed between them at a second point, close to where the guide surfaces face each other, each press roll having a perforated outer surface for withdrawing filtrate from said pulp or the like. At the point where they face each other, the guide surfaces include an outlet for withdrawing filtrate that is rejected from the nip, said outlet being arranged substantially opposite of the nip and directed such that the filtrate removed through said outlet is conveyed from the pulp passage through a clearance that is formed in a space between the guide surfaces.

An advantage of this apparatus is that it presents a possibility to withdraw the counter flow of filtrate that generally is formed near the nip in a direction opposite the main flow of the pulp in the pulp passage. Thus, the efficiency of the de watering is enhanced.

In a first embodiment the outlet comprises outlet openings arranged between the two guide surfaces. Also, the outlet may comprise a plate with outlet openings arranged at even intervals in said plate, such that strips are formed between said openings.

Such a placement of the outlet is advantageous, since only the flow rejected from the nip is directed to pass through said outlet, wherein the main pulp flow, flowing towards the nip is not affected.

In another embodiment the outlet comprises outlet openings in the form of perforations through the guide surfaces on both sides of a tip that is formed by the guide surfaces at their point of intersection. This is advantageous since it allows for an enhanced dewatering with a minimum of alterations of an existing apparatus. Preferably the outlet openings are arranged such through the guide surfaces that they are directed away from the nip substantially opposite the operational direction of the pulp. Such an arrangement is advantageous, since it opens towards the flow rejected from the nip such that the main pulp flow, flowing towards the nip is not affected.

The apparatus may involve a line for conveying the removed filtrate from the outlet and re-circulating the removed filtrate to the pulp passage. With such an arrangement the filtrate may be re-circulated and the loss of fibres is minimised.

Preferably the line is connected to the inlet of the pulp passage for recirculation of the removed filtrate to said inlet.

According to one embodiment of the apparatus the outlet openings are dimensioned so as to withdraw liquid from the pulp passage, such that a minimum amount of fibres are withdrawn. Preferably, the outlet openings have a maximum diameter of 10 mm.

In another embodiment of the apparatus the outlet openings are dimensioned so as to let through some free pulp fibres with the filtrate that is being withdrawn from the pulp passage. This may be advantageous as it diminishes the risk of plugging the outlet openings. Preferably, the outlet openings have a diameter of between 10 and 200 mm.

According to a second aspect, the invention relates to a method of dewatering pulp or the like, comprising the steps of feeding pulp or the like to an inlet of a pulp passage formed between a first press roll and a surrounding first guide surface; conveying the pulp through the pulp passage by rotating the press roll towards a nip formed between the press roll and second similar press roll surrounded by a similar second guide surface which intersects with the first guide surface close to the nip; dewatering the pulp through perforations in the outer surface of each press roll; pressing the pulp at the nip between the two press rolls. Filtrate is withdrawn through an outlet arranged through the guide surfaces, close to their intersection near the nip.

In one embodiment of the method the withdrawn filtrate is returned to the pulp passage, and possibly the filtrate is inserted at the pulp inlet.

In order to wash the pulp in the pulp passage wash liquid may be inserted at at least one point along the pulp passage.

SHORT DESCRIPTION OF THE DRAWINGS

Below, specific embodiments of the invention will be described with reference to the accompanying drawings, of which:

Fig. 1 schematically shows a conventional apparatus for dewatering cellulose pulp;

Fig. 2 shows the flow of pulp and filtrate, according to a discovery on which the invention is based;

Fig. 3 schematically shows a first embodiment of the apparatus according to the invention for dewatering cellulose pulp;

Fig. 4 schematically shows the apparatus in fig. 3, seen from above;

Fig. 5 schematically shows a second embodiment of the apparatus according to the invention; and

Fig. 6 schematically shows a third embodiment of the apparatus according to the invention.

DETAILED DESCRIPTION OF SHOWN EMBODIMENTS

Fig. 1 schematically shows a conventional apparatus for dewatering cellulose pulp. The apparatus includes two press rolls 2, arranged next to each other. Each press roll is provided inside a guide surface 3, such that a pulp passage 4 is formed between each press roll 2 and the corresponding guide surface 3. An inlet 5 for transferring pulp or the like to said pulp passages 4 is arranged at a first point in connection to each press roll 2.

The press rolls 2 are arranged to rotate in opposite directions Dl and D2, respectively so as to convey the pulp in an operational direction Fl towards a nip 6 that is formed between them at a second point, close to where the guide surfaces 3 intersect. The term operational direction is intended to denote the direction in which the pulp is conveyed during operation, i.e. as long as there is pulp in the pulp passage and as long as the press rolls are rotating in their intended directions D l and D2. In order to dewater the pulp in the pulp passage 4, each press roll 2 has a perforated outer surface 7, through which filtrate is withdrawn from said pulp or the like.

Recently it has been discovered that a counter flow F2 opposite the direction of rotation Dl and D2 of each press roll, and thus opposite the operational direction Fl of the pulp in the pulp passage, is created by the filtrate that is rejected from the nip 6. These flows Fl and F2 are demonstrated in fig. 2.

In figures 3-6, three different embodiments of the invention are shown. In order to facilitate comparison with prior art, the reference numerals used in figures 1 and 2, are reused in figures 3-6 to denote same or similar details.

In figure 3, a first embodiment of the apparatus 1 according the invention is shown, which in addition to the details of the conventional apparatus shown in figure 1 , includes some additional details. Firstly, an outlet 8, for withdrawing the flow F2 of rejected filtrate from the nip 6 is arranged substantially opposite of the nip 6. Thus, in the illustrated embodiment, since the pulp is conveyed upwards towards and through the nip 6, the outlet 8 is arranged to allow the filtrate to exit downwards.

The outlet 8 is formed through or between the guide surfaces 3, where they face each other and possibly intersect, and is directed such that filtrate removed through said outlet 8 is conveyed away from the pulp passage 4 via a clearance 9 that is formed between the guide surfaces 3. A line 10 is arranged to lead the filtrate from the outlet 8, through the clearance 9 and on to subsequent processing steps. Possibly, the filtrate may be conducted to a tank or, as will be described below with reference to fig. 5, it may be re- circulated to the pulp passage 4. Preferably, the line 10 includes at least one valve 1 1 in order to control the exit flow. Also, the line may include a pump (not shown) for pumping the filtrate to subsequent processing steps. However, due to the relatively high pressure near the nip 6 in the pulp passage 4, a pump may be omitted, since the pressure of the rejected filtrate may in itself very well be enough to conduct the filtrate to the desired location.

Figure 4 shows a view from above of the outlet shown in figure 3. From this view it is evident that the outlet 8 in this embodiment comprises outlet openings 12 in the form of oval holes. These oval holes are uniformly formed in a plate 13 that connects the two guide surfaces 3, since the tip that is normally formed at the intersection of such guide surfaces has been cut off. On the plate 13 , the outlet openings 12 are separated from each other by connecting strips 14 or flanges, which unite the two guide surfaces 3. In figure 4, the openings are oval, and the spacing between the strips 14 is about 100 mm, and typically the diameter may be between 10 and 200 mm. It is however also feasible to arrange the openings as one central slit or several slits extending through part or the whole width of the guide surfaces. Such a slit may for mechanical reasons include strips that bridge the guide surfaces 3 to strengthen the construction.

In figure 5, a second embodiment of the apparatus 1 according to the invention is shown. In this embodiment, the line 10 is arranged to conduct the filtrate that has been removed through the outlet 8 back to pulp passage 4, up streams of the nip 6, either to the inlet 5 where it may be mixed with pulp that it is fed to the pulp passage, or further down the pulp passage. Preferably though, the filtrate is re-circulated to the first part of the pulp passage 4, in order not to negatively affect the optional ongoing washing of the pulp. This second embodiment may be combined with the first embodiment shown in figure 3 and 4, or with the third embodiment, which is described below.

In figure 6, a third embodiment is shown. The same reference numerals are used to indicate details in figure 6 that are similar to details of previous embodiments. However, for this third embodiment an apostrophe (') is used to indicate details that differ slightly in design and/ or function from the details of the first embodiment. For instance, in the third embodiment there is an outlet, however, this outlet 8' differs from the outlet 8 of the first embodiment.

The outlet 8' of the third embodiment does not include a horizontal perforated plate, instead part of the guide surfaces 3 have been perforated. Thus, the guide surfaces intersect at a point just below the nip 6 to form a tip 15 that points towards the nip 6. The main difference of the third embodiment with respect to the first embodiment is that the outlet openings 12' in the third embodiment are dimensioned so as to let out mainly liquid from the pulp passage, such that a minimum amount of the fibers are withdrawn.

On the contrary, in the first embodiment shown in figures 3-4, the outlet openings 12 are dimensioned so as to also allow a certain amount of fibers to exit from the pulp passage through them. Thus, it is of greater interest to re- circulate the filtrate to the first part of the pulp passage in the first embodiment, in order not to lose valuable fibers.

The outlet openings 12', in the third embodiment may preferably be located opposite the nip as directional openings opposite the upwardly main pulp flow direction Fl . Thus, the outlet openings 12' may be constituted of vertical holes or slits, through which at least part of the counter flow F2 of rejected filtrate may exit the pulp passage downwards. An easier manner to design the outlet openings 12' is, of course, to arrange radial bore holes through the guide surfaces 3, orthogonally with respect to the surface of the respective guide surface 3. However, vertical slits are advantageous since they (A) allow the flow rejected from the nip 6 to flow straight down through the guide surfaces, and (B) since they minimize the risk of letting out the upwardly going flow Fl , which may include a high density of pulp.

Preferably, the outlet openings 12' of this third embodiment have a maximum diameter of 10 mm, such that a minimum amount of pulp fibres is allowed to exit the from the pulp passage along with filtrate.

The outlet 8' comprises a clearance 9' between the guide surfaces 3, which clearance involves a chamber that is connected to a line 10 for removal of the withdrawn filtrate. In order to increase the withdrawal of filtrate a sub- pressure may be provided in the line 10.

The aim of the invention is mainly to increase the dewatering of a dewatering apparatus. However, dewatering is an important step the washing procedure in a washing apparatus. Thus, the apparatus may very well include one or several inlets for adding wash liquid to the pulp passage 4, wherein the apparatus would constitute a washing apparatus with increased dewatering capacity.

Above, three embodiments of the invention have been described. These may combined in all feasible combinations, such that e.g. the outlet openings may be dimensioned either to let through filtrate with some pulp fibers, or to only let through filtrate with as little pulp fibers as possible, regardless of the general design of the outlet. Thus, the first embodiment may be provided with relatively small holes, and the third embodiment may be provided with relatively large holes.

The invention is however not limited to these embodiments. The scope of the invention is defined by the appended claims.

Claims

1. An apparatus (1) for dewatering cellulose pulp, which apparatus includes two press rolls (2), arranged next to each other, each press roll (2) being provided inside a guide surface (3), such that a pulp passage (4) is formed between each press roll (2) and the corresponding guide surface (3), wherein an inlet (5) for transferring pulp or the like to said pulp passage (4) is arranged at a first point at each press roll, and wherein the press rolls (2) are arranged to rotate in opposite directions (Dl , resp. D2) so as to convey the pulp in a first operational direction (Fl) towards a nip (6) that is formed between them at a second point, close to where the guide surfaces (3) face each other, each press roll (2) having a perforated outer surface (7) for withdrawing filtrate from said pulp or the like; the guide surfaces (3), at the point where they face each other, including an outlet (8 ^s) for withdrawing filtrate that is rejected from the nip (6), said outlet (8^*) being arranged substantially opposite of the nip (6) and directed such that the filtrate removed through said outlet (8') is conveyed from the pulp passage through a clearance (9') that is formed in a space between the guide surfaces (3), characterised in that the guide surfaces (3) intersect at the point where they face each other to form a tip (15), wherein the outlet (8') comprises outlet openings (12⁷) in the form of perforations through the guide surfaces (3) on both sides of said tip (15).

2. Apparatus (1) according to claim 1 , characterised in that the outlet openings (12') are arranged such through the guide surfaces (3) that they are directed away from the nip (6) substantially opposite the operational direction (Fl) of the pulp.

3. Apparatus (1) according to any of the preceding claims, characterised in that it involves a line (10) for conveying the removed filtrate from the outlet (8') and re-circulating the removed filtrate to the pulp passage (4).

4. Apparatus (1) according to claim 3, characterised in that the line (10) is connected to the inlet (5) of the pulp passage (4) for re-circulation of the removed filtrate to said inlet (5).

5. Apparatus (1) according to any of the preceding claims characterised in that the outlet openings (12, 12^s) are dimensioned so as to withdraw liquid from the pulp passage (4), such that a minimum amount of fibers are withdrawn.

6. Apparatus (1) according to claim 5, characterised in that the outlet openings (12⁵) have a maximum diameter of 10 mm.

7. Apparatus (1) according to any of the claims 1-4, characterised in that the outlet openings (12') are dimensioned so as to let through some free pulp fibres with the filtrate that is being withdrawn from the pulp passage (4).

8. Apparatus (1) according to claim 7, characterised in that the outlet openings (12') have a diameter of between 10 and 200 mm.

9. A method of dewatering pulp or the like, comprising the steps of: feeding pulp or the like to an inlet (5) of a pulp passage (4) formed between a first press roll (2) and a surrounding first guide surface (3); conveying the pulp through the pulp passage (4) by rotating the press roll (2) towards a nip (6) formed between the press roll (2) and second similar press roll (2) surrounded by a similar second guide surface (3) which faces the first guide surface (3) at a point close to the nip (6); dewatering the pulp through perforations in the outer surface (7) of each press roll (2); pressing the pulp at the nip (6) between the two press rolls (2); characterised by the step of withdrawing filtrate through an outlet (8⁵J arranged through the guide surfaces (3), close to their intersection near the nip (6).

10. Method according to claim 9, characterised by returning the withdrawn filtrate to the pulp passage (4).

11. Method according to claim 10, characterised by inserting the returned withdrawn filtrate at the pulp inlet (5) .

12. Method according to any of the claims 9 to 11, characterised by the step of inserting wash liquid at at least one point along the pulp passage (4), for washing the pulp in the pulp passage (4).