WO1999066122A1

WO1999066122A1 - Apparatus and method for drying pulp

Info

Publication number: WO1999066122A1
Application number: PCT/FI1999/000490
Authority: WO
Inventors: Jorma Koskinen; Leo LUNDÉN
Original assignee: Valmet Corporation
Priority date: 1998-06-16
Filing date: 1999-06-07
Publication date: 1999-12-23
Also published as: FI981391A0; AU4620699A; FI109815B; FI981391A

Abstract

The invention relates to an apparatus and a method for drying pulp. The apparatus for drying pulp comprises at least a wire section (2), which receives the pulp to be dried from a head box (1) or the like, as well as a press assembly, comprising at least one press nip, the wire section's wet wire being adapted to pass therethrough. In the apparatus of the invention, the wire section has a wet wire (7) which is adapted to pass through a nip (PN1) of at least one long-nip press and the long-nip press is provided with means (5, 7) for achieving a two-way dewatering of the pulp web. In the method of the invention, the dry content of pulp is increased after dewatering the wire section by delivering the pulp web together with at least the wire section's wire (7) through a pressing element, in which pressing element the dry content of the pulp web is increased to a value of more than about 40 %, whereafter the pulp web is carried forward from the wire section to a next stage of increasing the dry content.

Description

Apparatus and method for drying pulp

The present invention relates to an apparatus for drying pulp, said apparatus comprising at least a wire section, which receives the pulp to be dried from a head box, as well as a press assembly, comprising at least one press nip, the wire section's wet wire being adapted to pass therethrough.

The invention relates also to a method for increasing the dry content of pulp by means of a pulp-drying machine, said drying machine comprising at least a wire section and a press section, by which method the dry content of pulp is increased through dewatering in the wire section at least to about 13-20%, the dry content being subsequently increased further by pressing the web.

The making of paper, with the exception of paper made completely from recycled fibers, is typically carried out by using pulp, i.e. a chemical stock, e.g. for reinforcing a web or providing it with desired qualities. Pulp is typically mixed in with e.g. a stock matter in a given desired amount. However, pulp is only used in paper making for a part of the entire amount of paper to be produced. The pulp can be received along pipelines from a pulp mill connected to the actual paper mill. However, a large portion of the produced pulp is used in paper mills far away from the pulp mill itself, i.e. the pulp is manufactured as so- called market pulp. Thus, naturally, it is desirable to dry the pulp to avoid carrying water between pulp mill and paper mill. This drying of pulp is performed by means of a pulp-drying machine.

Fig. 1 depicts schematically one prior art basic type for the wet end of pulp- drying machines, comprising a fourd nier-wire forming section 2, wherein the stock to be delivered by means of a head box 1 onto a fourdrinier wire 7 is first subjected to dewatering, the dry content following the wire section being typically about 20%. From the wire section a web W is carried to a press section, which includes e.g. a so-called combi-press, wherein the web travels in a supported manner through two press nips N2-N3. As shown in fig. 1 , the wire section can be provided with a separate nip N1 , a so-called lump breaker, whose main purpose is to break up pulp lumps which are formed too early and/or are excessively large, as suggested even by its name. Such a lump breaker, rolls 3, 16, is provided with normal press rolls. In this case, the lump breaker is linked with the top felt run of a combi-press, which is not necessary. Following the combi-press, the web is sufficiently dry (dry content more than about 40%) and, thus, durable to be advanced to a fourth press/separate press, e.g. to a SymBelt™-shoe press, rolls 18, 19, from whose press nip N4 the web is carried further to evaporation drying at a dry content of about 50%.

However, this type of wet end in a pulp-drying machine involves a number of problems. One drawback can be said to be a large number of rolls in the combi- press. In addition to press rolls, rolls 4, 15, 17, the combi-press requires a plurality of guide rolls for a felt 5, 8, as well as felt conditioners. Furthermore, the combi-press includes a centre roll 15 which is a suction roll, which means high investment costs.

Moreover, the lump breaker used in this mechanism is highly sensitive in terms of the adjustment of a nip pressure. If the line pressure is increased too much, the web becomes useless, in other words, its fabric structure breaks up and carrying the web forward in the machine becomes impossible.

Hence, an object of the invention is to introduce a pulp-drying method intended for minimizing the above drawbacks.

In the prior art solution, through the first nip N1 of a combi-press runs a wet wire 7 included in a wire section and it is known as such to provide the wire section with a nip constituted by two rolls for pressing the web W to a certain degree. However, the overall objectives of the process and the cooperation of the entire wet end/press section must be considered as a whole and, thus, there are certain requirements for a subsequently dried pulp web as it emerges from the wire/press section. The principal requirements are as follows: the web must be sufficiently dry to withstand a transfer into a drying cabinet, on the other hand, the drying must be performed so as not to cause structural damage to the pulp used as a raw stock.

The principal objective of a drying machine used for drying pulp is to bring the pulp to a desired dryness as cost-efficiently as possible, and it must be remembered that the material to be dried is pulp rather than a mixture of various fiber materials and additives (possibly just one component being pulp), which is the case in paper making. This involves e.g. the fact that mechanical dewatering of the stock should be as effective as possible, yet such that the actual pulp fibers do not suffer.

Differences between pulp drying and paper making include e.g. substantially different quadratic weights, which in the case of pulp are in the order of 600- 1000 g/m², while cardboard has a maximum quadratic weight of about 550 g/m² and for paper it is maxiamally about 150 g/m². In addition, the speed of a pulp- drying machine is typically less than 200 m/min, while the speeds of a paper making machine can be in the order of more than 1600 m/min. Moreover, in pulp-drying machines, the stock emerging from the head box has a temperature which in old machines is at least 55°C and in new ones typically more than 60°C, preferably about 70°C, while in paper making machines the respective temperatures are typically less than about 45°C. One of the most concrete differences is that paper board is always an end product which must meet requirements that are totally different from those set on dried pulp, which is a raw material. The requirements set on dried pulp concern primarily its handling during a drying process, requirements set by the handling of dried pulp, shipping, and pulping in a paper mill. The purpose of a pulp-drying machine is not to significantly influence the qualities of a pulp sheet itself. On the other hand, in the manufacture of paper and cardboard, the process is determined by qualities that the end product must have, such as smoothness, gloss, opacity, printing quality, etc.

The above indicates that the conditions in a pulp-drying machine and in paper machine are essentially different from each other and therefore the solutions implemented in a paper machine are not normally applicable as such to a pulp machine environment.

In order to achieve the above objectives, an apparatus of the invention for drying pulp, said apparatus comprising at least a wire section, which receives the pulp to be dried from a head box, as well as a press assembly, comprising at least one press nip, the fourdrinier wire section's wet wire being adapted to run therethrough, is principally characterized in that the wire section's wet wire is adapted to travel through a nip of at least one long-nip press, and that the long-nip press is provided with means for two-way dewatering.

A method of the invention for increasing the dry content of pulp by means of a pulp-drying machine, said drying machine comprising at least a wire section and a press section, by which method the dry content of pulp is increased through dewatering in the wire section at least to about 15-20%, the dry content of a pulp web being subsequently increased further by pressing the pulp web, is principally characterized in that the dry content of pulp is increased after dewatering in the wire section by delivering the pulp web together with at least the wire section's wire through a pressing element, in which pressing element the dry content of the pulp web is increased to a value of more than about 40%, whereafter the pulp web is carried forward from the wire section to a next stage of increasing the dry content.

A wire section of the invention for a pulp-drying machine, comprising at least a bottom wire loop, above which a pulp stock can be delivered for dewatering the stock, is principally characterized in that the bottom wire loop is adapted to travel through a long-nip press, constituted by a long-nip roll external of the wire loop and a counter-roll internal of the wire loop, together with a felt adapted to run around the long-nip roll, the dewatering of a pulp web being adapted to occur in said long-nip press in two directions.

According to one preferred embodiment of the invention, a method of the invention for increasing the dry content of pulp by means of a pulp-drying machine, the dry content of pulp being increased by said method through dewatering in a wire section, which is adapted to occur preferably at least partially in two directions, at least to about 15-20%, is principally characterized in that, after dewatering in the wire section, the dry content of pulp is increased by guiding the pulp web, together with the wire section's wet wire and an overhead felt, through a first long-nip press for dewatering the pulp web in two directions, in which long-nip press the dry content of pulp is increased in one nip to a value of more than about 40%, whereafter the pulp web is delivered even through a second long-nip press between the felts for dewatering the pulp web in two directions, in which second long-nip press the dry content of pulp is increased to a value which is preferably more than about 55%.

The invention will now be described with reference made to the accompanying drawings, in which:

fig. 1 shows one press assembly according to the prior art,

fig. 2 shows a press assembly of the invention in one preferred embodiment,

fig. 3 shows a press assembly of the invention in another preferred embodiment, and

fig. 4 shows a third solution of the invention.

The pulp-drying machine shown schematically in fig. 1 includes a head box 1 for supplying stock onto a fourdrinier wire included in a fourdrinier wire section 2, said wire 7 running around a reversing roll 3 at the downstream end of the wire section, and around a drawing roll 4 as well as a number of guide rolls. The fourdrinier wire section 2 includes typically e.g. foil strips, foil vacuum boxes, and suction boxes, not shown in the figure and used for eliminating water from a stock carried on top of the fourdrinier wire, such that the pulp web will have a dry content of about 13-20% downstream of the fourdrinier wire section. Following the fourdrinier wire section, a web W is delivered to a first press nip N1 of a combi-press, which is a so-called lump breaker, described earlier and constituted by rolls 3 and 16, and thence further to nips N2 and N3, whose centre roll comprises a suction roll 15 and counter-rolls comprise e.g. smooth rolls 4 and 17, respectively. Around the rolls 15 and 16 and necessary guide rolls runs a top felt 5 and around the roll 17 and various guide rolls a bottom felt 8. From the combi-press nip N3 the web W is carried to a nip N4 constituted by rolls 18 and 19, around which runs a top felt 6 and, respectively, a bottom felt 9. From the nip N4 the web is carried to evaporation drying. Downstream of the nip N4 the web has a dry content which is typically about 50%, and in presently available machines the maximum dry content is about 53%.

In one preferred embodiment of the invention as shown in fig. 2, the combi- press of fig. 1 constituted by two nips N2-N3 has been replaced with a long-nip press, comprising a long-nip roll 10 external of the wire 7 and a drawing roll 4 internal of the wire, with a long nip PN1 formed therebetween. According to the invention, the length of such a long nip is selected such that the dwell time of a pulp web is preferably more than 40 ms, more preferably more than 50 ms. It can be roughly said that, in a solution of the invention, the dwell time is about quintuple with respect to the dwell time achieved by a combi-press. The pulp web is typically very thick and, thus, a long active press time of the invention is particularly beneficial, especially at a dry content suggested by the invention, i.e. by the time the fabric of a pulp web is already sufficiently bonded, yet the web contains plenty of water to be pressed out. With the help of this solution, the dry content of the pulp web W can be increased with a single nip press from a value of about 20% downstream of the fourdrinier wire to a value of more than about 40%, which matches the dry content of a pulp web downstream of the prior art combi-press of fig. 1 after two separate nips N2-N3. From the long nip PN1 the pulp web is drawn into a further nip press constituted by rolls 11 and 12, the bottom roll 11 of which is preferably a long nip roll, said web W running through two consecutive long nips PN1 and PN2 prior to being guided to a drying apparatus 14. Following the second long nip PN2, the web has a dry content which is more than about 50%, preferably more than about 55%, and most preferably more than about 60%.

The embodiment of fig. 3 differs from that of fig. 2 in that the wire section has its wire 7 adapted to run around a reversing roll 3 and a drawing roll 4 as in the prior art solution of fig. 1 , the first long nip PN1 being formed between a long nip roll 10 and the reversing roll 3. In addition, the overhead felt loops 5 and 6 depicted in the solution of fig. 2 are in fig. 3 combined as a single felt loop 13, extending around the long nip roll 10 of the first long nip and a counter-roll 12 of the second long nip, as well as around necessary guide rolls.

In the solution of the invention shown in fig. 4, the pulp stock is delivered from a head box 1 onto a wire section 2 into a narrowing space between two wire loops, wherein the increase of a dry content in the pulp web occurs essentially immediately in two directions. The area constituted by two wires is preferably provided with a so-called lump breaker N1 \ and then the bottom wire is provided with a fourdrinier vacuum box assembly. Thereafter, the pulp web, supported by the bottom wire, is carried according to the invention along with at least the wire section's wet wire through a long nip press PN1. This is followed by advancing the pulp web, supported by the wire, by means of the top felt of a second long nip press PN2 through the second long nip press between the felts while dewatering the pulp web in two directions, the dry content of the pulp web being increased in said second long nip press to a value of more than about 55%.

The solution of fig. 4 can be used e.g. for reducing substantially the machine length, whereby, for example in connection with machine renovations, it will be possible to increase the size of an evaporation drying cabinet in view of increasing production over the entire line.

In the embodiment of figs. 2-4 upstream of the long nip presses, and preferably upstream of the second long nip press, it is possible to arrange, if necessary, separate heating means for further increasing the dry content of a pulp web. Such heating means can be adapted to heat either directly the pulp web or a felt coming to contact with the pulp web. In addition, the embodiments of figs. 2 and 3 can be supplemented, if necessary, with dewatering means mounted above the wire section for an enhanced increase of the dry content of a pulp web.

Instead of a smooth counter-roll it is conceivable to use, e.g. a suction roll, a grooved roll or a through-drilled jacketed roll as a counter-roll of the long-nip press. When a through-drilled jacketed roll is used as a counter-roll the area of the bores of the through-drilled roll is preferably at least about 20 % of the total area of the roll's jacket in order to achieve sufficient open area for a pulp-drying machine's dewatering capability.

Claims

1 . An apparatus for drying pulp, said apparatus comprising at least a wire section (2), which receives the pulp to be dried from a head box (1 ) or the like, as well as a press assembly, comprising at least one press nip, the wire section having a wet wire being adapted to pass therethrough, characterized in that the wire section's wet wire (7) is adapted to pass through a nip (PN1) of at least one long-nip press, and that the long-nip press is provided with means (5, 7) for achieving a two-way dewatering of the pulp web.

2. An apparatus as set forth in claim 1 , characterized in that said long-nip press comprises a long-nip roll (10) external of the wire and a counter-roll (3; 4) internal of the wire.

3. An apparatus as set forth in claim 1 , characterized in that the arrangement further includes at least one further nip press, wherein the dewatering is adapted to occur in two directions.

4. An apparatus as set forth in claim 3, characterized in that the further nip press (11 , 12) is also a long-nip press.

5. An apparatus as set forth in claim 2, characterized in that the counter-roll of a long-nip press at the end of the wire section comprises the wire section drawing roll (4) or reversing roll (3).

6. An apparatus as set forth in any of claims 1 -5, characterized in that the counter-roll (3, 4) of the long-nip press comprises a suction roll.

7. An apparatus as set forth in any of claims 1 -5, characterized in that the counter-roll (3, 4) of the long-nip press comprises a through-drilled jacketed roll.

8. An apparatus as set forth in claim 7, characterized in that the area of the bores of the through-drilled roll is at least about 20 % of the total area of the roll's jacket.

9. An apparatus as set forth in any of claims 1 -5, characterized in that the counter-roll (3, 4) of the long-nip press comprises a grooved roll.

10. An apparatus as set forth in any of claims 3-9, characterized in that between the long-nip press and the further press of the wire section are arranged heating means for increasing the dry content of a pulp web (W).

11. An apparatus as set forth in any of claims 1 -10, characterized in that the wire section comprises a narrowing gap constituted by at least two wire loops, wherein the increase of the dry content of a pulp web occurs essentially in two directions.

12. A method for increasing the dry content of pulp by means of a pulp-drying machine, said drying machine comprising at least a wire section and a press section, in which method the dry content of pulp is increased through dewatering in the wire section at least to about 13-20%, the dry content of a pulp web being subsequently increased further by pressing the pulp web, characterized in that the dry content of pulp is increased after dewatering in the wire section by delivering the pulp web together with at least the wire section's wire through a pressing element, in which pressing element the dry content of the pulp web is increased to a value of more than about 40%, whereafter the pulp web is carried forward from the wire section to a next stage of increasing the dry content.

13. A method as set forth in claim 12, characterized in that the pressing element in the method comprises a long-nip press.

14. A method as set forth in claim 12 or 13, characterized in that the method further involves the use of a second pressing element, whereby the dry content is increased to a value of more than about 50%, preferably to a value of more than about 55%, most preferably to a value of more than about 60%.

15. A method as set forth in claim 14, characterized in that the second pressing element in the method comprises a long-nip press.

16. A method as set forth in claim 13 or 15, characterized in that the dwell time of a pulp web in the long-nip press is preferably more than 40 ms, most preferably more than 50 ms.

17. A method as set forth in claim 12 or 14, characterized in that, after dewatering in the wire section, the dry content of pulp is increased by guiding the pulp web, together with the wire section's wet wire and an overhead felt, through a first long-nip press for dewatering the pulp web in two directions, in which long-nip press the dry content of pulp is increased to a value of more than about 40%, whereafter the pulp web is delivered even through a second long-nip press between the felts for dewatering the pulp web in two directions, in which second long-nip press the dry content of pulp is increased to a value which is preferably more than about 50%.

18. A wire section for a pulp-drying machine, comprising at least a bottom wire loop, above which a pulp stock can be delivered for dewatering the stock, characterized in that the bottom wire loop (7) is adapted to travel through a long-nip press, constituted by a long-nip roll (10) external of the wire loop and a counter-roll (3, 4) internal of the wire loop, together with a felt (5; 13) adapted to run around the long-nip roll (10), the dewatering of a pulp web being adapted to occur in said long-nip press essentially in two directions.

19. A wire section for a pulp-drying machine as set forth in claim 18, characterized in that, above the bottom wire loop (7), it comprises a top wire loop, said wire loops constituting a narrowing gap into which the pulp stock is directly fed from a head box, and said narrowing gap being provided with dewatering elements within both the top and the bottom wire loop for two-way dewatering.