Device and method for producing a fibrous web Description
[0001] The invention relates to an apparatus for producing a fibrous web from a plurality of layers, comprising a multilayer headbox having an inner turbu- lence generator extending over the width of the multilayer headbox and at least two turbulence generators each directly adjacent to the inner turbulence generator, each extending over the width of the multilayer headbox, for form- ing at least three suspension layers, and with a nozzle adjoining the turbulence generators and having an upper nozzle wall and a lower nozzle wall, the noz- zle walls are arranged convergent to each other and form an outlet gap for the formation of a suspension jet and with two separating lamellas arranged in the nozzle for the separation of the inner suspension layer from the adjacent sus- pension layers on at least a part of the nozzle length, and further comprising supply systems for the supply of suspensions to the respective turbulence gen- erators, the feed system for the inner turbulence generator comprising a white water vat which is connected to at least one white water pump, a white water classifier, a first cross distributor via a white water line, the first cross distrib- — utor being connected to the inner turbulence generator for supplying white water to the inner suspension layer.
[0002] The document DE3112972 A1 discloses a multilayer headbox having three layers. The middle layer is fed with white water, while the adjacent outer layers are formed by fibrous suspensions. The layers are separated from each other in sections by means of flexible partition walls in a nozzle downstream of the turbulence generators. The white water is taken from a white water vessel and fed to the middle layer through a feed pump. — [0003] The unexamined German application DE102008000778 A1 describes a multilayer headbox having, for example, three layers, wherein a white water layer separates two adjacent layers each consisting of a fibrous suspension.
Auxiliary material metering devices are provided for metering retention agents, for example, into the white water layer. Metering can be carried out between the cross distributor and the turbulence generator.
[0004] Document EP2784214 BI shows a multilayer headbox, wherein one layer is fed with white water by a white water pump. In order to improve the quality of the paper, metering of ground fibres and auxiliary materials into the white water suspension is provided upstream of the white water pump. The white water pump is designed to improve the mixing of the auxiliary material — with the white water.
[0005] The disadvantage of the well-known apparatuses is that the individual layers of the fibrous web that is produced have too low and uneven paper strength, specifically gap strength, on the one hand, and poor covering quality — of the layers on the other.
[0006] It is therefore the object of the invention to improve the strength and uniformity of the produced fibrous web and the visual appearance.
[0007] The object is achieved by means of the features of claim 1. An appa- ratus for producing a fibrous web from a plurality of layers, comprising a multilayer headbox having an inner turbulence generator extending over the width of the multilayer headbox and at least two turbulence generators, each directly adjacent to the inner turbulence generator, each extending over the — width of the multilayer headbox, for forming at least three suspension layers and with a nozzle adjoining the turbulence generators and having an upper nozzle wall and with a lower nozzle wall, the nozzle walls are arranged con- vergent to each other and form an outlet gap for the formation of a suspension jet and with two separating lamellas arranged in the nozzle for the separation — of the inner suspension layer from the adjacent suspension layers on at least part of the nozzle length, and further comprising supply systems for the supply of suspensions to the respective turbulence generators, the feed system for the inner turbulence generator comprising a white water vat which is connected to at least one white water pump, a white water classifier, a first cross distrib- utor via a white water line, the first cross distributor being connected to the inner turbulence generator for supplying the inner suspension layer with white water. According to the invention, it is provided that downstream of the at least one white water pump, and preferably downstream of the white water classifier, an auxiliary material metering device is provided for generating a metering volume flow for metering an auxiliary material into the white water line, and in that the auxiliary material metering device is designed in such a — way that the metering volume flow can be controlled and/or regulated for set- ting the mixing guality between white water and metering volume flow.
[0008] The advantage of the solution lies in the uniform and controllable or regulable mixing between white water and auxiliary material for supplying — the inner suspension layer. This can be optimised for different headbox oper- ating situations, such as different volume flows and/or speeds of the suspen- sion jet. In contrast to this, with the known state of the art, in which the aux- iliary material is metered upstream of the white water pump, and especially in different operating situations, insufficient mixing of the auxiliary material can — occur. This can lead to strand formation of the auxiliary material in the white water. This inadequate mixing quality can lead to inhomogeneous separation of the two adjacent suspension layers. This, in turn, can lead to an inhomoge- neous strength distribution in the sheet plane, as well as inhomogeneous cov- erage of the two layers in the finished fibrous web, which is optically visible, especially in the case of large colour differences of the adjacent suspension layers. In addition, the effect of the auxiliary material can be reduced by the high shear forces occurring in the white water pump. This can be especially the case with long-chain polymers. — [0009] The inner suspension layer consists exclusively of white water and one or more auxiliary materials.
[0010] The white water contains few fibres and, depending on the type of paper that is produced, fines and fillers. The suspension layers adjacent to the inner suspension layer contain, in addition to fillers and fines, mainly fibrous materials, such as cellulose fibres, recycled fibres, etc., for the formation of the fibrous web.
[0011] On the one hand, the inner suspension layer forms a separating layer between the adjacent suspension layers in order to minimize mixing before and during sheet formation in the forming zone, and on the other hand, said inner suspension layer serves to bring auxiliary materials, such as starch, be- tween the two suspension layers adjacent to the inner layer and to increase the strengths, in particular the cohesion of the adjacent layers of the finished fi- brous web. The auxiliary material can include multiple different types of aux- iliary material.
[0012] The auxiliary materials may be selected individually or in combina- tions from the following group: starch, in particular cationic starch, in order to increase strength; polymers, in particular long-chain polymers as retention agents; retention agent systems comprising multiple components; wet strength agent.
[0013] In an advantageous embodiment, an auxiliary material predilution line with the auxiliary material metering device is provided for diluting the auxil- iary material with white water. The auxiliary material predilution line is pref- erably connected to the white water line for the inner turbulence generator in the area downstream of a white water classifier. In order to adjust the volume flow, a valve may be provided in the auxiliary material predilution line. By prediluting the auxiliary material, the mixing of the auxiliary material with the white water can be improved, depending on the type and effectiveness of — the auxiliary material.
[0014] In addition, the auxiliary material predilution line may be directly or indirectly connected to the white water vat.
[0015] In a practical embodiment, the multilayer headbox has a dilution water system with a second traverse distributor for sectional controllable and/or reg- 5 ulable metering of dilution water over the width of the multilayer headbox into at least one suspension layer for adjusting the transverse profile of the basic weight of the produced fibrous web. The dilution water is preferably metered into at least one suspension layer that is adjacent to the inner suspen- sion layer. The transverse profile of the basic weight of the produced fibrous — web can be adjusted by means of local, i.e. sectional, dilution of the corre- sponding suspension. The invention can have a particularly advantageous ef- fect in this exemplary embodiment. The good mixing of the auxiliary material with the white water of the inner suspension layer allows greater scope for adjusting the transverse profile of the basic weight by means of local dilution. In the case of strand formation, on the other hand, there is a risk that both effects overlap and thus disturb the quality of the fibrous web.
[0016] In a further development, the white water line is connected to the sec- ond traverse distributor of the dilution water system downstream of the white — water pump and downstream of the white water classifier via a dilution water line.
[0017] According to a further preferred practical embodiment, the upper noz- zle wall and preferably the lower nozzle wall of the nozzle have an orifice. The advantage is an almost symmetrical final geometry of the nozzle, through which the suspensions flow without detrimental deflection.
[0018] In a practical design, the nozzle is free of further lamellas. This means that only the separating lamellas are provided.
[0019] In a possible further development, the nozzle has at least one interme- diate lamella.
[0020] It may also be advantageous if at least one intermediate lamella is of the same length or longer than the shortest separating lamella. This results in an advantageous layer structure of the fibrous web.
[0021] In a possible practical embodiment, at least one of the two separating lamellas can extend through the outlet gap.
[0022] It is also conceivable that at least one of the two separating lamellas ends inside the nozzle.
[0023] Furthermore, it is also possible that the multilayer headbox is followed by a forming zone with at least one endlessly circulating dewatering screen for receiving the suspension jet and with a dewatering element, the dewatering element having a lining contacting the dewatering screen, and dewatering openings are formed in the lining for discharging the accumulating white wa- ter, and in that the dewatering openings are designed and arranged in such a way that the pressure pulses acting on the suspension are minimized. This prevents uneven mixing of the inner suspension layer with the adjacent sus- — pension layers.
[0024] Here it is advantageous if the areas, i.e. the cross-sectional areas of the dewatering openings in the running direction of the endlessly circulating de- watering screen, are zonally different and increasing. In a first zone in the running direction, the cross-sectional area is therefore smaller than in a second zone that adjoins the first zone in the running direction of the circulating de- watering screen. This has the advantage that the initial fibrous mat that forms on the dewatering screen is formed gently and in an undisturbed manner.
[0025] Advantageously, the dewatering element is sucked by a suction device. The suction device can be a vacuum system or a suction siphon. The suction allows the dewatering intensity to be adapted to the operating situation.
[0026] The object is also achieved by means of a method according to claim 15 for producing a fibrous web from a plurality of layers by means of an ap- paratus. The apparatus comprises a multilayer headbox with an inner turbu- lence generator extending over the width of the multilayer headbox and at least two turbulence generators directly adjacent to the inner turbulence gen- erator, each extending over the width of the multilayer headbox, for forming at least three suspension layers, and with a nozzle adjoining the turbulence generators and having an upper nozzle wall and a lower nozzle wall, the noz- zle walls are arranged convergent to each other and form an outlet gap for the formation of a suspension jet and with two separating lamellas arranged in the nozzle for the separation of the inner suspension layer from the adjacent sus- pension layers on at least a part of the nozzle length, and further comprising supply systems for the supply of suspensions to the respective turbulence gen- — erators, wherein in the feed system for the inner turbulence generator, white water is fed via a white water line from a white water vat via at least one white water pump, via a white water classifier, to a first cross distributor, and the inner turbulence generator is supplied with white water through the first cross distributor, characterized in that downstream of the at least one white water pump, and preferably downstream of the white water classifier, an auxiliary material metering device is provided for generating a metering volume flow for metering an auxiliary material into the white water line, and in that the auxiliary material metering device is de- signed in such a way that the
[0027] metering volume flow can be controlled and/or regulated in order to set the mixing guality between white water and metering volume flow.
[0028] Further features and advantages of the invention are shown in the fol- lowing description of preferred exemplary embodiments with reference to the drawing.
[0029] In the drawings Figure 1 shows a first embodiment of an apparatus according to the invention in simplified representation; Figure 2 shows a second embodiment of an apparatus according to the invention in simplified representation; Figure 3 shows a third embodiment of an apparatus according to the invention in simplified representation;
[0030] Figure 1 shows an apparatus 1 for the production of a fibre web from three layers for a paper machine. The multilayer headbox 2 has an inner tur- bulence generator 7.8 that extends over the width of the multilayer headbox 2 and two turbulence generators 7.10, 7.11 that are directly adjacent to the inner turbulence generator 7.8, each turbulence generator extending over the width of the multilayer headbox for forming three suspension layers. The turbulence generators 7.8, 7.10, 7.11 comprise flow channels, which in this example are formed by pipes, which are arranged next to each other in rows over the width. In lieu of round pipes, the flow channels can also be designed as pipes having a rectangular flow cross-section.
[0031] The inner turbulence generator comprises one pipe row, while the ad- jacent turbulence generators 7.10, 7.11 in each case comprise two pipe rows. A nozzle 4 adjoins the turbulence generators 7.8,7.10 7.11 and has an upper nozzle wall 4.1 and a lower nozzle wall 4.2, the nozzle walls 4.1, 4.2 are ar- ranged convergent to each other and form an outlet gap for the formation of a suspension jet. In the nozzle 4, the two separating fins 5 are arranged on at least part of the nozzle length for the separation of the inner suspension layer from the adjacent suspension layers. The separating lamellas 5 are located between the inner turbulence generator 7.8 and the adjacent turbulence gen- erators 7.10, 7.11 and start in the area at the outlet of the respective suspension from the turbulence generators 7.8, 7.10, 7.11. The separating lamellas 5 are flexible and can be pivotally attached. Rigid connection is also possible. At the end of the upper nozzle wall 4.1 there is an orifice 3.1 in the form of a rectangular profile in order to limit the gap width. Accordingly, the lower nozzle wall 4.2 also has an orifice 3.2 at the end. However, in contrast to the upper orifice 3.1, the orifice 3.2 is moulded to the lower nozzle wall 4.1. How- ever, it is also possible to have an orifice design similar to the design on the upper nozzle wall 4.1. This almost symmetrical end geometry of the nozzle 4 allows the suspensions to flow without adverse deflections. In this example, in addition to the separating lamellas 5, optional intermediate lamellas 6 are provided, which are shown in dashed lines. The intermediate lamellas 6 are arranged in each case between the pipe rows of the turbulence generators 7.10,
7.11 that are adjacent to the inner turbulence generator 7.8 and that begin in the area at the outlet of the respective suspension from the turbulence gener- ators 7.10, 7.11. The intermediate lamellas 6 are flexible and can be pivotally attached. Rigid connection is also possible. In this example, the separating lamellas 5 and the intermediate lamellas 6 are of the same length. In the op- eration of a paper machine, a suspension flows through each turbulence gen- erator 7.8, 7.10, 7.11, wherein corresponding suspension layers are formed.
[0032] The turbulence generators 7.8, 7.10, 7.11 are preceded by feed systems for the supply of suspensions. While the turbulence generators 7.10, 7.11 that are adjacent to the inner turbulence generator 7.8 are supplied with fibrous suspensions, white water is supplied to the inner turbulence generator 7.8 as a suspension. The inner suspension layer comprises exclusively white water and one or more auxiliary materials. The white water contains few fibres and, depending on the type of paper produced, fines and fillers. The suspension layers adjacent to the inner suspension layer contain, in addition to fillers and fines, mainly fibrous materials, such as cellulose fibres, recycled fibres, etc, — for the formation of the fibrous web.
[0033] The feed system for the internal turbulence generator 7.8 comprises a white water vat 8 which is connected to at least one white water pump 8.1, a white water classifier 8.2, a first cross distributor 8.6 for the middle layer via a white water pipe 8.31. The first cross-distributor 8.6 is in turn connected to the inner turbulence generator 7.8 for supplying the inner suspension layer with white water via the distribution lines 8.3. An optional booster pump is provided downstream of the white water classifier 8.