APPARATUS AND METHOD FOR PULP DRYING

Abstract: The invention relates to a pulp-drying apparatus, comprising a wet end, including at least a former section (2), which is supplied with pulp to be dried from a headbox (1) or the like, and a press assembly, comprising at least one press section (20, 30) through which a web (W) formed over the former section is adapted to travel. According to the invention, the pulp web (W) has its passage arranged as a closed transfer through the entire wet end. The invention relates also to a method for starting a pulp-drying machine and process, and to a method for increasing the dry content of pulp by means of a pulp-drying machine.

Description: Apparatus and method for pulp drying The present invention relates to an apparatus for pulp drying, said apparatus comprising a wet end, including at least a former section for supplying pulp to be dried thereto from a headbox or the like, and a press assembly constituted by at least one press section, a web formed within the former section being adapted to pass therethrough. The invention relates also to a method for starting up a pulp-drying or the like machine and process. Moreover, the invention relates to a method for increasing the dry content of pulp by means of a pulp-drying machine, said drying machine comprising at least a former section and a press section, in which method the dry content of pulp is increased through dewatering over the former section at least to about 13-20%, followed by increasing the dry content 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 principal type of the prior art wet end for pulp- drying machines, comprising a Fourdrinier-wire forming section 2, wherein the stock to be delivered by means of a headbox 1 onto a Fourdrinier wire 7 is first subjected to dewatering, the dry content downstream of 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 purpose, in addition to increasing the dry content, 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 standard press rolls. In this case, the lump breaker is linked with the run of a top felt 5 of a combi-press, which is not necessary. Downstream of the combi-press, the web has a dry content of roughly 40%, followed by advancing it 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 for a pulp-drying machine involve problematic aspects. Even though this type of solution has proved sufficient with presently available machines, such an arrangement shall not be capable of drying pulp with sufficient reliability at higher speeds. In the wet end, as the process is started up, the web is narrowed and it is expanded back to a machine width downstream of tail threading, said tail threading being a highly complicated process. The leading end of a web must be guided across several unsupported passages, i.e. the apparatus includes sections, over which the web or the tail is totally unsupported. Moreover, the lump breaker used in this solution is highly sensitive in terms of adjusting a nip pressure. If the linear 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. The susceptibility to web breaks in such a solution is further increased by a plurality of unsupported transfers at a relatively low dry content. One problem with a pulp-drying machine is the passage of web tail resulting from a possible break incident. Pulp web is heavy (here e.g. 1500 g/m 2 ), and, in the prior known solutions, such a tail may easily fold on itself. Such phenomenon is highly undesirable and, upon reaching the following press nip, it may cause structural damage, like break the press felt. Hence, it is an object of the invention to provide a pulp drying method, which seeks to eliminate among other things the above drawbacks. A further object of the invention is to provide an arrangement for effecting the web formation and wet pressing of pulp even at high speeds (> 200 m/min), and for reducing the tail threading time, and, furthermore, for making the web passage more stable, the wet end improving in terms of its runnability and the break risk being substantially reduced. In addition, by means of the invention the pulp web is essentially limited and/or supported both from below and above in terms of its movement over the press section, whereby the tail resulting from a possible break incident travels in a controlled manner through the presses without damaging the equipment. The present invention provides also a starting method for a pulp-drying machine and process, wherein the start-up of production is accomplished very rapidly and safely. In order to achieve the above objects, an apparatus of the invention is principally characterized by what is set forth in the characterizing section of claim 1. A method of the invention for starting a pulp-drying machine and process is principally characterized by what is set forth in the characterizing section of claim 9. A method of the invention for increasing the dry content of pulp by means of a pulp-drying machine is principally characterized by what is set forth in the characterizing section of claim 12. More specifically, the basic idea for one feature of the invention is that the passage of a pulp web is conducted as a supported transfer through the entire wet end, i.e. at least over such a machine section, in which the dewatering is effected in a substantially mechanical fashion. According to the method of the invention, the pulp web has its dry content increased at least to a value of more than about 40%, preferably to a value of more than about 50%, upstream of the first unsupported transfer. According to one preferred embodiment of the invention, the pulp-drying machine comprises a former section with its wire loop for forming a web from stock. According to the invention, the web is guided, supported by a lower wire, to and through at least one press, said press preferably comprising, according to the invention, at least a first long-nip press, which is provided with at least one water- receiving fabric, in other words a felt, separated from the lower wire, and which includes an upper shoe roll with its felt loop (a first upper felt), as well as a lower, open-surface, water-receiving, preferably bored-through counter roll which, at the same time, functions as a drawing roll for the wire section. Downstream of this press, according to the invention, the web is guided, supported by the fabric, to a next press which preferably comprises, according to the invention, a second long- nip press which includes a preferably lower shoe roll with its felt loop (a first lower felt), as well as an upper counter roll with its felt loop. Between the presses, according to the invention, the web is carried at least over part of its passage between two fabrics. According to the invention, the section, across which the web is carried between the fabrics, involves the use of an arrangement provided with a so-called transfer vacuum box or the like for subjecting a desired fabric to a vacuum effect at least during the starting sequence of a drying process. Thus, according to the invention, the supported transfer of a pulp web is arranged preferably as a closed transfer between at least two successive presses, e.g. long-nip presses, included in a press assembly, such that the web travels the distance between the presses between two support fabrics, preferably between an upper felt and a lower felt. According to one preferred embodiment of the invention, the upper-felt guide roll and the lower-felt guide roll are positioned in such a way that the upper-felt guide roll is closer to the upper roll of a press associated therewith than the distance of the lower-felt guide roll from the lower roll of a second press associated therewith. Naturally, the felt loop includes other felt guiding rolls, as well. According to the invention, the starting of a pulp-drying machine and process comprises preferably as described hereinafter. Hence, the pulp-drying machine comprises at least a section, over which the dewatering is carried out essentially as so-called mechanical dewatering (for the sake of briefness, the term used hereinafter is wet end), whereby the starting sequence of a pulp-drying machine and/or process comprises running the wet end initially at a desired speed and supplying some diluent, preferably water, used in stock, as an input flow onto a wire section or the like from a headbox or the like, followed by increasing the consistency of the input flow, thus beginning to form a pulp web, whose grammage and thickness increase in a substantially continuous fashion, without discontinuities, and, from the moment of its formation, the forming web is carried through the entire wet end, i.e. that section of a pulp-drying machine, over which the dewatering is performed as essentially mechanical dewatering. Consequently, as the web is being transferred in machine width through the wet end, its grammage and thickness are initially substantially lower than in the production phase. However, the increase of pulp consistency from 0% to a desired consistency is preferably a relatively rapid process. Such a method is capable of starting production as soon as the web has desired qualities and, on the other hand, such a method eliminates completely a so-called tail threading sequence in the wet end, which requires that a cut narrow web strip be first threaded through a section of the machine. The method of the invention, wherein the web is guided through the entire wet end as soon as a fiber network, i.e. a web, is forming, is capable of avoiding e.g. the discontinuities for machine components encountered in a tail threading process performed with a web of standard thickness. One example of such is e.g. a situation, in which a shoe press has only a tail threaded therethrough, whereby e.g. a so-called belt will be subjected to major stresses as a rather narrow web travels therethrough exclusively along a certain line, this situation being particularly pronounced in the case of thick pulp. Thus, with the method of the invention, the process start-up is effected by guiding the web through the entire wet end as soon as it begins to form, the web traveling for the first time through the wet end in a condition thinner than a standard running condition, thus minimizing the risk of damaging the equipment. In this context, the term "machine width" is understood to refer either to a web having the same width as the web produced over a former section (i.e. untrimmed web) or to a trimmed web downstream of the wire section, i.e. nowhere within the boundaries of a wet end is the web narrowed for threading. The invention will now be described in more detail with reference made to the accompanying drawings, wherein: Fig. 1 shows one press assembly of 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 a second preferred embodiment, Fig. 4 shows a press assembly of the invention in yet another preferred embodiment. Fig. 1 shows schematically a pulp-drying machine of the prior art, comprising a headbox 1 for feeding pulp onto a Fourdrinier wire 7 included in a Fourdrinier- wire section 2, said wire extending around a reverse roll 3 present in 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. a forming board, foils, vacuum foil boxes, and vacuum boxes (not shown in fig. 1 , but schematically visible in fig. 3 designated by reference numeral 41 ), whereby the pulp carried on top of the Fourdrinier wire is dewatered such that the pulp web has a dry content of about 13-20% downstream of the Fourdrinier-wire section. After the Fourdrinier-wire section, a web W is guided to a first press nip N1 of a combi-press, which is a so-called lump breaker, already described in more detail, and which is constituted by rolls 3 and 16, and thence further to nips N2 and N3, the centre roll thereof comprising e.g. a suction roll 15 and the counter rolls thereof comprising e.g. smooth rolls 4 and 17, respectively. Around the rolls 15 and 16 and the necessary guide rolls extends an upper felt 5, and around the roll 17 and various guide rolls extends a lower 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 extends an upper felt 6 and, respectively, a lower felt 9. From the nip N4 the web is typically delivered to evaporation drying. Downstream of the nip N4, the web has a dry content which is typically about 50%. In figs. 2-4, the same or equivalent components are designated by the same reference numerals. Fig. 2 shows schematically an apparatus of the invention for forming a web W. The apparatus comprises a headbox 1 for supplying pulp, in accordance with the invention, into a converging wedge-shaped space defined between two endless wire loops, such as an upper wire 29 and a lower wire 28. The wedge-shaped space is constructed by adapting the wires 28, 29 to travel around spaced-apart reverse rolls 25a, 25b, set in the upstream end of the space, and thereafter in a converging fashion for defining a wedge-shaped space. In engagement with the inner surface (the side facing away from pulp flow) of the wires are mounted chamber units 40, including a plurality of dewatering chambers 40a. While being carried through the space, the pulp is dewatered in such a way that, downstream of a former section 2, the dry content is about 18%, preferably about 20%. Thereafter, more water is removed according to the invention, while the web W remains supported between the two wires, by compressing the assembly between two rolls 45, 46. This is followed by guiding the upper wire 29 and the lower wire 28 away from each other, and the web W is advanced further according to the invention on the top surface of the lower wire 28, the web having a dry content of roughly 28%. Thus, according to the invention, the web travels fully supported. If necessary, the lower wire 28 has its bottom surface provided with a so-called vacuum transfer box 44 for a secure passage of the web along with the lower wire. The web W, supported by the top surface of the lower wire 28, is carried further through a nip PN1 of a press 20, said press 20 comprising a first long-nip press provided with an upper felt 23, i.e. the web W travels with its top against the felt 23 and its bottom against the lower wire 28 in the long nip PN1 between rolls 21 and 22. In this process, the web W is dried such that, downstream of the long nip PN1 , it has a dry content of roughly 40%. Drying of the web W is continued according to the invention in such a way that the web is first carried, supported by the upper felt 23 on its bottom surface, to a next press 30, comprising, according to the invention, preferably a second long-nip press provided with a lower shoe press 32 with its felt loop 34 (first lower felt), as well as with an upper counter roll 31 with its felt loop, wherein a felt 33 extends around guide rolls 36. According to the invention, the first upper felt 23 travels from the first press 20 to an upper-felt guide roll 24a, from which it is guided upwards for a so-called return run (by way of guide rolls 24b-24f), while the first lower felt 34 of the second press returns from a so-called lower return run (guide rolls 35e, 35d, 35c, 35b) to a lower-felt guide roll 35a, whereby the first lower felt 34 is guided to travel to the second press 30. According to the invention, the web W is initially guided, supported by the first upper felt 23 on its bottom surface, a certain given distance, then between the first upper felt 23 and the first lower felt 34 a certain given distance, and then on the top surface of the first lower felt 34 to the second press 30, and further therethrough. As the web W is traveling on the bottom surface of the upper felt 23, its passage can be secured by vacuum boxes or the like in the inner run of the felt (not shown in the figure). According to the invention, that section, over which the web is carried between the first upper felt 23 and the first lower felt 34, has the first lower felt preferably provided with a so-called transfer vacuum box 42 or the like for subjecting the first lower felt 34 to a vacuum effect at least during the machine start-up sequence. According to the invention, the upper felt 23 has its first guide roll 24a and the lower felt 33 has its last guide roll 35a positioned in such a way that said guide roll 24a of the upper felt 23 is closer to the upper roll 21 of the first long-nip press 20 than the distance that separates the last guide roll 35a of the lower felt 33 from the lower roll 32 of the second long-nip press 30. The above-described method of the invention is capable of forming the web W in a pulp-drying machine efficiently and reliably. This solution simplifies the web forming section and provides a reliable assembly for pulp drying, as the web W is comprehensively supported until its dry content is even more than 50%, whereby it has a sufficient strength to withstand without breaking a first unsupported transfer to the next process phase, even at high production rates. In a second preferred embodiment of the invention depicted in fig. 3, the combi- press of fig. 1 constituted by two nips N2-N3 has been replaced with a long-nip press 20, comprising an extra-wire long nip roll 21 and an intra-wire drawing roll 22, constituting therebetween a long nip PN1. Pulp web is typically very thick and, therefore, a long action time of compression according to the invention is particularly preferred, especially at the dry content proposed by the invention, i.e. at the stage, in which the pulp web already has its fiber network sufficiently bonded, yet the web still contains plenty of water removable by pressing. From the long nip PN1 the pulp web is carried, according to the invention, to a next press 30, preferably a long-nip press, constituted by rolls 31 , 32, whereof the lower roll 32 is preferably a long nip roll, the web W traveling through two successive long nips PN1 and PN2 prior to being delivered to evaporation-drying equipment or some other further treatment. Downstream of the second long nip PN2, the web has a dry content of more than about 50%, preferably more than about 55%, and most advantageously about 60%. According to the invention, the handling of the entire wet end of a web is performed throughout the passage in a fully supported manner. In fig. 3, the web is hence initially carried downstream of the press PN1 supported by a first upper felt 23, on its bottom surface, to the next, second press 30, preferably comprising, according to the invention, a second long-nip press provided with a lower shoe roll 32 with its felt loop 34 (first lower felt), as well as with an upper counter roll 31 with its felt loop 33. According to the invention, the first felt 23 runs from the first press 20 to a first upper-felt guide roll 24a, from which it is guided upwards for a so-called return run while the first lower felt 34 of the second press 30 returns from a so-called lower return run to the last lower-felt guide roll 35a used for guiding the first lower felt to advance to the second press. According to the invention, the web W is carried first supported by the first upper felt 23 on the bottom surface thereof for a given distance, then between the first upper felt 23 and the first lower felt 34 for a given distance, and thereafter on the top surface of the first lower felt 34 to the second press and further therethrough. According to the invention, as shown in fig. 3, the upper felt 33 of the last press 30 is fitted with devices 37, 38 for releasing the web W from the surface of the felt 33. Such devices may be needed in the start-up of a machine as the presently thin web may strive to tag along with the upper felt, which is undesirable. Such devices include preferably a roll 37, fitted with a doctor 38 which is engageable with the surface of the felt 33, whereby the forming web W accompanies instead the smoother roll surface and is guided on top of the lower felt. Alternatively, it is also possible to use doctors 38 directly on the felt 33, as shown in fig. 2. In the respective position, it is also possible to use for example air blasting, which is also shown in fig. 2. The described methods can also be used concurrently. Fig. 4 depicts yet another possibility of implementing a supported transfer of the invention, as well as the start-up of a machine. However, this is slightly more inconvenient implementation than those depicted in figs. 2 and 3. In this case, a closed transfer between the long-nip presses 20, 30 has been accomplished by means of the lower wire 28 of the wire section and a second upper felt 33 associated with the downstream long-nip press 30. The upper felt extends around guide rolls 47a-47f. The upper felt 33 is preferably provided with a transfer vacuum box 43 or the like for subjecting, if necessary, the second upper felt 34 to a vacuum effect. The invention has been described above in light of examples essential with regard to the invention, but such examples are by no means intended to limit the scope of protection of this application, and it is obvious that the features of the invention can also be applied to other corresponding processes, whenever this is desirable.

Claims: Claims 1. A pulp-drying apparatus, comprising a wet end, including at least a former section (2) with its wire loops, which is supplied with pulp to be dried from a headbox (1 ) or the like, and a press assembly, comprising at least one press section (20, 30) with its felt loops (23, 24), through which a web (W) formed over the former section is adapted to travel, characterized in that the pulp web (W) has its passage arranged as a closed transfer through the entire wet end. 2. An apparatus as set forth in claim 1 , characterized in that the press assembly includes a first press (20) provided with a first upper felt (23), the pulp web (W) being passable through said press between a wet wire (28) included in the former section (2) and said first upper felt (23). 3. An apparatus as set forth in claim 2, characterized in that the apparatus further includes a second press (30), the pulp web (W) being passable therethrough in a supported fashion between a first lower felt (34) and a second upper felt (33) associated with the second press (30). 4. An apparatus as set forth in claim 3, characterized in that the first press (20) comprises a first long-nip press provided with an upper shoe roll (21) and a first upper felt (23) associated therewith, as well as with a lower counter roll (22), which functions at the same time as a drawing roll for the wet wire (28) included in the former section (2), and that the second press (30) comprises a second long-nip press provided with a lower shoe roll (32) and a first lower felt (34) associated therewith, as well as with an upper counter roll (31 ) and a second upper felt (33) associated therewith. 5. An apparatus as set forth in claim 1 , characterized in that it comprises at least two presses (20, 30), the first upper felt (23) associated with the first press thereof being adapted to travel from the first press (20) to an upper-felt guide roll (24a), from which it is adapted to travel further upwards for a so-called return run, while the first lower felt (34) associated with the second press (30) being adapted to return from a so-called lower return run to a lower-felt guide roll (35a), whereby the first lower felt (34) is guidable to proceed to the second press (30), whereby the web (W) can be first carried, supported by the first upper felt (23) on the bottom surface thereof, for a given distance, then between the first upper felt (23) and the first lower felt (34) for a given distance, and thereafter on the top surface of the first lower felt (34) to and through the press (30). 6. An apparatus as set forth in claim 1 , characterized in that it comprises at least two presses (20, 30), at least one of which is provided with an upper felt loop (23; 33) and the other with at least a lower felt loop (34), the web (W) being adapted to travel between the presses (20, 30) for a given distance between the upper felt (23) of one press and the lower felt (34) of the other press. 7. An apparatus as set forth in claim 6, characterized in that the upper felt (23) has its first guide roll (24a) and the lower felt (34) has its last guide roll (35a) positioned in such a way that the distance of the first upper-felt guide roll (24a) from an upper roll (21) of the press (20) associated therewith is shorter than the distance of the last lower-felt guide roll (35a) from a lower roll (32) of the press (30) associated therewith. 8. An apparatus as set forth in claim 1 , characterized in that it comprises at least two presses (20, 30), the first of said presses being passed through by a first upper felt (23) and by a wet wire (28) included in the former section (2), said wet wire extending below a second upper felt (33) included in the second press (30), the web (W) being guidable through the first press (20) and thence forward initially between the first upper felt (23) and the wet wire (28), then between the second upper felt (33) and the wet wire (28), and further between a first lower felt (34) and the second upper felt (33) associated with the second press (30) to and through the second press (30). 9. A method for starting a pulp-drying or the like machine, comprising a former section (2) and a press section (20, 30) and the like with its felt loops (23, 34) for starting a process, characterized in that pulp feeding onto the former section (2) included in the wet end of a pulp-drying or the like machine is started from a headbox (1 ) or the like, and that a web (W) forming over the former section (2) is carried in a single run through the entire wet end. 10. A method as set forth in claim 9, characterized in that the wet end of a pulp machine is first run at a desired speed and a diluent, preferably water, used in stock is delivered as an input flow onto a wire section (28) or the like from the headbox (1) or the like, whereby the consistency of the input flow is increased, thus initiating the formation of the pulp web (W), having a grammage and thickness which increase in a continuous fashion, without discontinuities, and the forming web being carried, preferably from the moment of its formation, through the entire wet end. 11. A method as set forth in claim 9, characterized in that, as the web (W) is transferred in machine width through the wet end, its grammage and thickness are initially substantially lower than at the production stage. 12. A method for increasing the dry content of pulp by means of a wet end in a pulp-drying machine, said wet end comprising at least a former section (2) and a press section (20, 30) with its felt loops (23, 34), in which method the dry content of pulp is increased by dewatering in the former section (2) to at least about 13- 20%, followed by increasing the dry content further by pressing the web with the press section, characterized in that a pulp web (W) is transferred in a supported fashion through the entire wet end, and that the method comprises increasing the dry content of the pulp web to a value of at least more than about 40% upstream of the first unsupported transfer. 13. A method for increasing the dry content of pulp by means of a pulp-drying machine, said drying machine comprising at least a former section (2) and a press section (20, 30), in which method the dry content of pulp is increased by dewatering in the former section (2) to at least about 13-20%, followed by increasing the dry content further by pressing the web, characterized in that the dry content of pulp is increased in the method after dewatering in the former section (2) by carrying the pulp web (W), along with a wet wire (28) of the former section (2) and an upper felt (23), through a first long-nip press (20) while removing water from the pulp web in two directions, the dry content of the pulp web (W) being increased in said first long-nip press (20) to a value of more than about 40%, whereafter the pulp web (W) is carried in a supported fashion to a second long-nip press (30) and, between felts (33, 34), through the second long- nip press (30) while removing water from the pulp web (W) in two directions, the dry content of the pulp web (W) being preferably increased in said second long- nip press (30) to a value of more than about 50%.

Inventors: KOSKINEN JORMA, KURKI JARMO, PAKKALA ILKKA, SAVIA RISTO, TAEHTI HARRI, TAMMEKAS TIMO, KOSKINEN, JORMA, KURKI, JARMO, PAKKALA, ILKKA, SAVIA, RISTO, TAEHTI, HARRI, TAMMEKAS, TIMO
Applicants: KOSKINEN JORMA, KURKI JARMO, PAKKALA ILKKA, SAVIA RISTO, TAEHTI HARRI, TAMMEKAS TIMO, VALMET CORP, KOSKINEN, JORMA, KURKI, JARMO, PAKKALA, ILKKA, SAVIA, RISTO, TAEHTI, HARRI, TAMMEKAS, TIMO, VALMET CORPORATION
Application year: 1999
Main classification: D21C9
IPC codes: D21F9