EP2764158B1 - Controlling the formation of sheets in a paper production process - Google Patents

Description

The present invention relates to a method for controlling the sheet formation in a papermaking process, an apparatus for carrying out the method, and a papermaking machine having such a device.

WO 2006/134069 A1 describes a method and a device for the plasma treatment of a fiber suspension. This increases the strength of the sheet and achieves bleaching.

Foliation is an important process during papermaking. For this purpose, a suspension of paper fibers, auxiliaries and water, distributed as evenly as possible over the paper web width, a sheet forming device, e.g. a wire section of an industrial paper machine fed. For example, in long-sifting paper machines, the fiber suspension runs on an endlessly circulating sieve.

The quality of paper is determined inter alia by the so-called formation, i. the distribution of the fibers in the paper. Typically, due to the screen assembly and high screen speed in an industrial paper machine, the paper fibers in the paper are predominantly oriented along the screen direction so that the properties of the paper, e.g. Tear strength or elongation, differ significantly in the two main directions along and across the paper web. In order to achieve more similar properties of a paper web in the two main directions along and across the paper web, e.g. To obtain a higher tensile strength of the paper in the direction transverse to the paper web, it is necessary to align isotropic a larger part of the fibers than previously transverse to the screen direction or the fibers in general.

So far, an attempt is made by adjusting different process parameters, such as the volume velocity in the application of the suspension to the screen, the angle of incidence of the suspension on the screen and the screen speed, both the orientation of the fibers in the paper and the drainage on the sieve or in the so-called wet section to influence. However, the formation is determined by various parameters that can be influenced only conditionally by the conventional method of papermaking.

The object of the present invention is therefore to improve the formation of a paper web.

The object is achieved by a method for controlling the sheet formation in a papermaking process, wherein an electric field is generated, paper fibers present in an aqueous suspension are exposed to the electric field, and settling of the paper fibers into a fiber layer and / or an orientation of the paper fibers within the fiber layer is controlled by means of the electric field. The object is further achieved by a device for controlling sheet formation in a papermaking process, comprising at least one electric unit for generating an electric field, which is so exposed that paper fibers present in an aqueous suspension during papermaking are exposed to the electric field and settling the paper fibers in a fiber layer and / or an orientation of the paper fibers within the fiber layer by means of the electric field is controllable. The object is also achieved by a paper machine with a device according to one of claims 6 to 12.

The sheet formation takes place in the industrial paper production on the paper machine. The fiber suspension, which consists for the most part of water, is formed in the headbox to a thin, uniform as possible beam. This applies in Langsiebpapiermaschinen on an endless rotating sieve. As the jet from the head box hits the screen, the sheet formation of the paper begins and the properties of the paper sheet are formed and can be affected.

The theory of sheet formation describes the following sequence: As soon as the fiber suspension comes into contact with the screen, the paper fibers are moved towards the screen by their flow resistance. As a result, a fiber layer begins to form on the screen, which grows upwards. Depending on the freedom of movement of the fibers in the suspension, two different forms of sheet formation occur: filtration and thickening. The structure of most papers shows that the filtration prevails: During dewatering, a sharp transition between the already formed fiber layer on the screen, the so-called fiber mat, and the overlying suspension sets. The concentration of the substance in the liquid phase is approximately constant and the fibers are free to move relative to each other.

In the wire section, a very large part of the water runs off within a short time, typically within a few seconds, and the paper structure is created. In this case, suckers attached under the sieve and pulsating foils can contribute to the dewatering of the pulp. At the end of the screen, the paper web is transferred to a felt and enters the press section.

The electric field may be formed both as an electrostatic field and as an electrodynamic field comprising electric and magnetic fields. The term "electrical" can therefore refer to both electrical and magnetic fields.

The invention is based on the finding that paper fibers can be influenced by an electric field. Paper fibers consist essentially of cellulose, a dielectric material. Since the charge carriers are not freely movable in a dielectric, the dielectric can be polarized by an external electric field. In this case, electrical dipoles can be induced by displacement polarization. Interactions of such induced in the fibers Dipoles with an external electric field can be used to improve the sheet formation during papermaking or to shorten the time course.

Because the forces that can be generated on a fiber by a fiber-field electrical interaction are relatively small, the electrical interaction is preferably exploited as long as the fibers are in suspension and are relatively easily reoriented, i. during sheet formation, before the fibers are fixed in the fiber layer. A reorientation of the fibers, after the fibers are already fixed in the fiber layer, is not realistic by means of this electrical interaction under economic conditions.

The use of an electric field can reduce the tendency of the fibers to flocculate or form lumps. This can generally improve the formation of the paper. Another advantage of the targeted influencing of the fiber behavior by the use of an electric field is the fact that the water content of the fiber suspension can be reduced without this leading to an undesirable flocculation or lumping. A paper web can thus be produced with a smaller proportion of water in the suspension than conventional. Thus, less water is subsequently removed from the suspension in papermaking, and thus the energy required for drying the wet paper web is lower.

On the other hand, by accelerating the deposition of the fibers into a fiber layer, the sheet formation can be shortened in time, which enables higher machine speeds in the paper machine. In addition, further properties of the paper web formed from the paper fibers can be adjusted in a targeted manner by changing the magnetic field over time.

Advantageous embodiments and modifications of the invention will become apparent from the dependent claims. In this case, the inventive method can also be developed according to the dependent device claims, and vice versa.

According to a preferred embodiment of the invention, the paper fibers are exposed to the electric field while they are in the area of a headbox of the suspension on a sieve and / or in the region of a wire section of a paper machine. In these areas of the paper machine, the paper fibers are still in aqueous suspension. The fibers are free to move relative to each other in these areas of the paper machine, and the forces on the fiber that can be generated by an electrical interaction between a fiber and an external electric field, although relatively small, are sufficient to control the orientation of a fiber To change fiber or to accelerate the fiber in a desired direction, eg towards a forming fiber layer.

According to a preferred embodiment of the invention, the electric field is generated repeatedly. It is particularly preferred that the electric field is pulsed. The duration of a pulse, ie a period during which the electric field is continuously formed, is preferably to be chosen such that the electric field in a fiber can induce an electric dipole by displacement polarization, but the electric field is degraded again before it becomes one Discharge over the water of the fiber suspension can come. Preferably, a pulse of the pulsed electric field has a duration of less than 10 μs. Since during the short duration of a single pulse, the majority of the fibers can only be deflected by a relatively small angle in a desired direction, for example across the wire direction, against the resistance of the surrounding water, the fibers are subjected to a series of successive pulses of electrical energy Field applied, ie the electric field is pulsed. The number of pulses is preferably chosen so that a fiber, which is originally aligned in the wire direction, at the end of the pulse sequence is aligned transversely to the wire direction.

Alternatively or additionally, several positions may be provided in the transport direction of the fibers, at which an electric field is generated, e.g. a plurality of devices arranged one behind the other to form an electric field.

According to a preferred embodiment of the invention, the field strength of the electric field is in a range of 1 to 10 kV / cm. Such a field strength can still be achieved economically with conductor tracks which are arranged in the region of the fiber suspension.

According to a preferred embodiment of the invention, the electric field is an electrostatic field. In electrostatics only static charges are considered. Without currents, there is no magnetic field, the electrostatic field is therefore not only stationary, so fixed in time, but also rotation-free, so has a potential.

According to a preferred embodiment of the invention, the electrical unit comprises at least one electrical conductor, which is connectable to an electrical voltage source. It is possible that after connecting the conductor track to the voltage source between two points of the conductor track there is an electrical voltage, and thus an electric field exists, which causes a force on charge carriers in the conductor track. If the charge carriers are freely movable, the voltage causes the charge carriers to be set in motion and an electrical current begins to flow in the printed conductor in accordance with Ohm's law. In this case, forms an electric field around the current-carrying conductor.

It is also possible that the conductor track consists of two mutually insulated portions, so that after connecting the conductor to the voltage source no current can flow through the conductor. When a DC voltage is applied to this trace, an electrostatic field is formed. If the insulation resistance between the two voltage-carrying sections is high, the electrical equivalent circuit of the track is an ideal open capacitor with a parallel high resistance.

According to a preferred embodiment of the invention, the at least one electrical conductor can be arranged above and / or below a screen of a paper machine. The fiber suspension is located after the headbox on a sieve of a paper machine. Therefore, an arrangement of the at least one electrical trace above and / or below the screen allows the electric field to be formed in the immediate vicinity of the fibers. In this way, a high field strength in the vicinity of the fibers and thus an effective reorientation or displacement of the fibers can be achieved.

According to a preferred embodiment of the invention, the at least one electrical conductor track is designed in the form of a meander or a comb. It is preferred if the conductor track comprises a number of conductors, wherein the conductors of one group each lie between the conductors of the other group, and the conductors of the two groups are each connected at opposite ends to a common conductor. Thus, each group has the shape of a comb, with the teeth of both combs intertwined. A similar shape can be achieved with a meandering track. If there is a potential difference between the groups of conductors which is generated by a voltage source, then a strong electrostatic field forms, which can hold or attract a fiber.

It is also possible that the at least one electrical conductor track is designed in the form of a grid. Thus, when applying a voltage can also train the above-described electrostatic field. It is preferred if the grid is a two-pole meander. Preferably, the at least one electrical conductor track comprises insulated conductor track sections. Due to the insulation, these conductor track sections, between which an electrical voltage is applied, are guided one above the other, i. intersect without electric current flowing between them.

According to a preferred embodiment of the invention, the grid is designed for attachment to a wire of a paper machine. This makes it possible to form the electric field in the immediate vicinity of the fibers. In this way, a high field strength in the vicinity of the fibers and thus an effective reorientation or displacement of the fibers can be achieved. By attaching directly to the screen, the grid can be easily formed with the same length and width as the screen. Thus, fibers can be applied to any position of the screen with an electric field.

Fig. 1

kammförmige Leiterbahnen in Draufsicht;

Fig. 2

eine mäanderförmige Leiterbahn in Draufsicht;

Fig. 3

einen Teil einer Papiermaschine im Schnitt;

Fig. 4

eine Papiermaschine in Ansicht; und

Fig. 5

einen Stoffauflauf und eine Siebpartie einer Papiermaschine in Ansicht.

The above-described characteristics, features, and advantages of this invention, as well as the manner in which they will be achieved, will become clearer and more clearly understood in connection with the following description of the embodiments, which will be described in detail in conjunction with the drawings. It shows in each case schematically and not to scale

Fig. 1

comb-shaped conductor tracks in plan view;

Fig. 2

a meandering conductor in plan view;

Fig. 3

a part of a paper machine in section;

Fig. 4

a paper machine in view; and

Fig. 5

a headbox and a wire section of a paper machine in view.

Fig. 1 shows in plan view a device 1 for controlling the sheet formation in a papermaking process, comprising two comb-shaped conductor tracks 4a, 4b. In each case, a first end of the conductor tracks 4a, 4b is connected to a different pole of an electrical voltage source 5, while the respective other end of the conductor tracks 4a, 4b is exposed. The conductor tracks 4a, 4b are arranged below a sieve 6 (dashed lines) of a paper machine. The conductor tracks 4a, 4b can also be arranged above the screen 6. The voltage source 5 supplies a voltage which leads in the conductor tracks 4a, 4b to generate an electric field. In this case, the electric field is formed so that paper fibers in an aqueous suspension, which is located on the screen 6, the electric field are exposed. By the electric field settling of the paper fibers in a fiber layer and / or alignment of the paper fibers within the fiber layer is controllable.

Fig. 2 shows in plan view a device 1 for controlling the sheet formation in a papermaking process, comprising a meandering conductor 4, the ends of which are connected to the poles of an electrical voltage source 5. The conductor track 4 has the form of a grid 8. The conductor track 4 is arranged below a sieve 6 (dashed lines) of a paper machine. The voltage source 5 supplies a voltage which leads in the conductor track 4 to generate an electric field. In this case, the electric field is formed so that paper fibers in an aqueous suspension, which is located on the screen 6, the electric field are exposed. By the electric field settling of the paper fibers in a fiber layer and / or alignment of the paper fibers within the fiber layer is controllable.

It is possible that the grid 8 with the meandering strip conductors is an electrostatically chargeable grid, which forms an electrostatic field which accelerates the formation of the fiber layer in the suspension in the wire section.

Fig. 3 shows in section a headbox 20 and a wire section 10 of a paper machine. In the headbox 20, the fiber suspension passes through a screening table 11 on an endlessly rotating sieve 6. Then the suspension passes in Sieblaufrichtung 21 the following components of the wire section 10: Register roller 12, Nasssaugkasten 13, single foils 14, Nasssaugkasten 13, Egoutteur 19, suction boxes 15th In the area of a wire suction roll 17, the paper web which has meanwhile been formed from the fiber suspension is lifted off the wire 6 and runs into a press section of the paper machine. About a Siebantriebswalze 18 and a breast roll 16, the empty wire to the forming table 11 is transported back.

The paper fibers in the suspension are exposed to an electric field while they are in the area of the headbox 20 and / or in the area of the wire section 10. It is possible that the apparatus 1 for controlling sheet formation in a papermaking process comprises two tracks 4 arranged above and below the screen 6, for example in the area of the single foils 14, as in FIG Fig. 3 specified.

Fig. 4 shows in view a paper machine 7, in the left part of which a headbox 20 and a wire section 10 are located. By arranging a device for controlling the formation of sheets in a papermaking process in the designated area 10 + 20, ie in the area of the headbox 20 / the wire section 10, an optimized alignment of the pulp fibers by utilizing electrostatic charging forces or electromagnetic fields in the area of the headbox 20 / the wire section 10 can be realized.

Fig. 5 shows in view a headbox 20 and a wire section 10 of a paper machine. In the area of the headbox 20 is a first electrical unit 2 for generating a electric field, in the area of the register rollers 12 of the wire section 10, a second and a third electrical unit 2 for generating an electric field are successively arranged in the wire direction. The electrical units 2 are bar-shaped transversely to the wire direction and serve to generate an electro-static or electro-magnetic field. In Sieblaufrichtung 21, ie in the transport direction of the fibers, thus a total of three positions are provided, where an electric field is generated. By repeatedly applying a portion of the fibers of the fiber suspension to electric fields, reorientation of the fibers in any desired direction can be achieved.

In the wire section 10, the predominant orientation of the paper fibers 3 in the forming fiber layer is indicated schematically over the length of a section of the fiber suspension 9 or paper web. It can be seen that the orientation of the fibers 3, as viewed in the direction of wire winding direction 21, alternates between an alignment along and an alignment transversely to the screen direction 21. The orientation of the fibers 3 along the wire-width direction 21 is the original orientation of the fibers 3 as determined by the conditions of the pulp buffing on the wire moving in the wire-winding direction 21. The orientation of the fibers 3 transversely to the screen direction 21 is the orientation of the fibers 3 as provided by the sheet forming control apparatus comprising the three electrical units 2.

Multiple and / or combined installation of sheet formation control devices in a papermaking process can achieve integrated process optimization through electrostatic / magnetic interference of the orientation of pulp fibers.

Although the invention has been further illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims (13)

1. Method for controlling the formation of sheets in a paper production process, wherein an electric field is generated, paper fibres present in a watery suspension (9) are exposed to the electric field, and a settling of the paper fibres (3) in a fibre layer and/or an alignment of the paper fibres within the fibre layer is controlled by means of the electric field, wherein the paper fibres (3) are exposed to the electric field, while they are located in the region of a headbox (20) of the suspension (9) on a sieve (6) and in the region of a moving screen (10) of a paper machine (7).
2. Method according to claim 1, characterised in that the electric field is generated repeatedly, in particular pulsed.
3. Method according to claim 2, characterised in that a pulse of the electric field has a duration of less than 10 µs.
4. Method according to claim 1, characterised in that the electric field is an electrostatic field.
5. Method according to one of the preceding claims, characterised in that the field strength of the electric field lies in the range of 1 to 10 kV/cm.
6. Apparatus (1) for controlling the formation of sheets in a paper production process, including at least one electric unit (2) for generating an electric field, which is embodied such that during paper production, paper fibres (3) present in a watery suspension (9) are exposed to the electric field and a settling of the paper fibres (2) in a fibre layer and/or an alignment of the paper fibres (3) within the fibre layer can be controlled by means of the electric field, wherein the paper fibres (3) are exposed to the electric field, while they are located in the region of a regulator (20) of the suspension (9) on a sieve (6) and in the region of a moving screen (10) of a paper machine (7).
7. Apparatus (1) according to claim 6, characterised in that the electric unit (2) includes at least one electric conductor path (4), which can be connected to an electric voltage source (5).
8. Apparatus (1) according to claim 6 or 7, characterised in that the at least one electric conductor path (4) can be arranged above and/or below a sieve (6) of a paper machine (7).
9. Apparatus (1) according to one of claims 6 to 8, characterised in that the at least one electric conductor path (4) is embodied in the form of a meander or a comb.
10. Apparatus (1) according to one of claims 6 to 9, characterised in that the at least one electric conductor path (4) is embodied in the form of a grid (8).
11. Apparatus (1) according to claim 10, characterised in that the grid (8) is a two-pole meander.
12. Apparatus (1) according to one of claims 10 and 11, characterised in that the grid (8) is embodied to be fastened to a sieve (6) of a paper machine (7).
13. Paper machine (7) having an apparatus (1) according to one of claims 6 to 12.

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