EP1775376B1 - Method and device for obtaining wood fibers from wood chips - Google Patents

Abstract

Making wood fibers from wood chips by feeding the chips through a tamping screw into a boiler, where they are exposed to high pressure and temperature, and discharging the chips under rapid expansion so that they separate into fibers, comprises measuring the conductivity of a plug of chips leaving the screw and using the result as a control parameter for controlling downstream processing of the chips and/or fibers. An independent claim is also included for apparatus for making wood fibers from wood chips, comprising a tamping screw (1) for feeding the wood chips into a boiler, where they are exposed to high pressure and temperature, and a discharge device for discharging the chips under rapid expansion so that they separate into fibers, comprises a sensor (11) for measuring the conductivity of a plug of chips leaving the screw.

Description

The invention relates to a method for recovering wood fibers from wood chips, the wood chips are fed with a plug screw a stove in which they are exposed to elevated pressure and elevated temperature, and wherein the wood chips discharged under rapid release of pressure from the digester and thereby dissolved in the wood fibers. Furthermore, the invention relates to a device for obtaining wood fibers from wood chips with a plug screw, a digester and a discharge device, wherein the plug screw feeds the wood chips to the digester, the wood chips are exposed in the digester elevated pressure and elevated temperature and wherein the discharge device the woodchips discharges from the cooker with rapid release of pressure so that they dissolve into the wood fibers.

STATE OF THE ART

Processes and devices of the type described above are used in the wood-based materials, pulp and paper industry (cf., for example US 5 262 003 A ). In the following, the description of the invention will be based on its application in the wood-based material industry. As a result, the scope of the invention, however, should be curtailed in any way.

In the production of wood-based panels in the form of so-called fiberboard, such as medium-density fiberboard (MDF), it is known to crush wood in wood chips, to water the wood chips in a storage tank and deduct with a plug screw from the bottom of the storage container and feed it to a digester. The feeding of the wood chips via a plug screw is required to the To promote wood chips against increased internal pressure of the digester into the digester into it while also sealing the digester against a pressure loss. The wood chips are compacted into a massive plug in the plug screw, with excess water being pressed out from the washing of the wood chips. In the digester, the wood chips are exposed to the increased pressure and elevated temperature to loosen the bond between the wood fibers. If the wood chips are conveyed out after a certain residence time in the digester by a discharge device from the digester, the pressure in the digester is suddenly relaxed, dismantle the wood chips into the individual wood fibers. These wood fibers are conveyed through a blown duct by a steam stream produced by the expansion in which they can already be glued with binder before they then enter a dryer in which they are adjusted in a stream of dry hot gas to a desired fiber moisture. The enriched by the excess water vapor gas stream is separated in a separator from the wood fibers, from which the wood fibers are then discharged for the formation of a fiber mat. If no gluing of the wood fibers took place in the blown duct, this should be carried out before or during the shaping of the fiber mat, which is then shaped at elevated temperature and usually also under elevated pressure to the desired fiberboard, whereby the binder hardens. In order to keep the wood fibers at the desired fiber moisture content, a measurement of the fiber moisture content at the wood fibers at the outlet of the separator is currently being carried out. If deviations between the desired fiber moisture and the actual fiber moisture are registered here, the setting of the dryer z. B. with respect to the temperature and / or the volume of the gas with which the fibers are dried modified. The modification is based on empirical values regarding the influence of the settings of the dryer on the fiber moisture content achieved thereby. In this type of feedback control of the dryer, in order to keep the fiber moisture constant, fluctuations in the fiber moisture of a few percentage points are to be observed, but they are not further reduced by fluctuating compositions of the woodchips. In principle, the highest possible, but not too high fiber moisture content of the wood fibers in the production of fiberboard is of interest. While excessively high fiber moisture leads to cracking of the fibreboard during hot pressing and therefore must be avoided, low fiber moisture results in fluctuating density profiles and other quality deficiencies in fiberboard. Due to a maximum high, but not too high fiber moisture on the one hand energy by less extensive drying of the Wood fibers and saved by a better thermal conductivity of the mats during hot pressing to the wood fiber board and thus on the other hand also achieved a maximum capacity utilization of a wood fiber board manufacturing plant. However, the greater the fluctuations in the fiber moisture of the wood fibers, the lower the fiber moisture must be adjusted on average in order to reliably prevent the problems associated with excessively high fiber moisture.

OBJECT OF THE INVENTION

The invention has for its object to provide a method and apparatus of the type described above such that the fiber moisture content of the wood fibers obtained within narrow limits can be kept constant to date, the average fiber moisture content in the production of wood fiber boards without the risk of too high To be able to increase fiber moisture significantly.

SOLUTION

This object is achieved by a method having the features of claim 1 and by a device having the features of claim 6. Preferred embodiments of the new method are described in the subclaims 2 to 5. The dependent claims 7 to 10 relate to preferred embodiments of the new device.

DESCRIPTION OF THE INVENTION

In the new method, the conductivity of the wood chips in the region of a plug from the wood chips at the outlet of the screw plug is measured and used as a control parameter for controlling the further processing of wood chips and / or wood fibers. The conductivity of the wood chips represents a measure of the fiber moisture of the wood fibers, which are still related to the individual wood chips. Surprisingly, this measure of the fiber moisture content of the wood fibers in the wood chips in the plug from the wood chips at the outlet of the plug screw can be obtained with sufficient accuracy for subsequent use as a control parameter in the control of further processing of wood chips and / or wood fibers it's a relevant statement in terms of the This is especially true when the plug screw in the region of the graft applies a substantially constant pressure on the woodchips or when measured by the plug screw in the region of the graft on the wood chips measured pressure and at the Use of the measured conductivity of the wood chips as a measure of their fiber moisture in terms of calibration is taken into account. At a certain pressure in the region of the graft, the fiber moisture can be determined relatively accurately from the conductivity of the wood chips in the graft.

In fact, the new process provides meaningful information about the wood fibers in the wood chips, which determine the fiber moisture content of the wood chips despite their extensive treatment until after the woodchips have been dissolved into the individual wood fibers. By determining this crucial starting point for the fiber moisture of the wood fiber obtained very early in the new process, it can be used as a control parameter for all subsequent treatments of the woodchips and the wood fibers in order ultimately to achieve a constant fiber moisture content. In the new method, a feedforward control is realized, with which the fiber moisture content can be kept within narrower limits with fluctuating input parameters than with the feedback control system known from the prior art.

Of course, in the new method, the measured conductivity of the wood chips in the area of a plug from the wood chips can also be used in the sense of a feedback control for the control of preceding steps of the treatment of wood chips. The special feature of the present invention, however, is the feedforward control of the further processing of wood chips and / or wood fibers.

Specifically, the conductivity of the wood chips can be used in the new method of controlling a dryer, with which the fiber moisture content of the wood fibers is set, for example, to a desired level for the production of fiberboard.

In this case, the dryer can continue to be controlled in response to a measured at its output fiber moisture content of the wood fibers, as was previously the case exclusively in the prior art.

It is understood that when controlling the dryer, the running time of the wood chips and the wood fibers obtained from them from the screw conveyor to the dryer must be taken into account.

When the woodchips are watered in the digester before being fed into the digester, excess water in the stuffing press is squeezed from the wood chips before the conductivity of the woodchips is measured. The conductivity of the wood chips does not depend on previously added free water, but only on the fiber moisture content of wood chips forming wood chips.

This is true at least as long as the plug screw is able to completely squeeze out the excess water. This ability of the screw conveyor, in turn, goes hand in hand with its ability to seal the cooker by the plug from the woodchips with respect to the pressure prevailing in it. Thus, the sealing of the digester by the plug screw in the new method can be monitored by observing the long-term history of the conductivity of the wood chips. If there is a sustained increase in conductivity, this indicates no longer completely depressed portions of the excess water and thus wear of the screw conveyor. If, in addition to the conductivity, the pressure applied by the plug screw in the region of the plug to the wood chips is also measured in order to calibrate the measured conductivity with regard to the fiber moisture corresponding thereto, it is of course possible to monitor the sealing function of the screw plug directly by following this pressure.

The conductivity of wood chips can be measured in the new method by directly contacting the plug from the woodchips with measuring electrodes. However, an indirect conductivity measurement is also possible in which the inductive or capacitive properties of the plug, which depend on the conductivity of the wood chips, are recorded. For example, the plug can be part of a dielectric of a capacitor arrangement or part of the core of a coil arrangement whose alternating current resistance changes with each change in the conductivity of the woodchips.

If the conductivity is determined as for a single direct measurement via measuring electrodes only for a local area of the circumference of the plug, it is preferred that the Conductivity of the wood chips is measured in several peripheral regions of the graft from the wood chips. Even with a constant fiber moisture content of the wood fibers in the wood chips, it is not to be expected that the fiber moisture of interest can be measured as a constant value which is not subject to fluctuations. Rather, the value measured in each case is also dependent on the electrical contacting of the woodchips and structural inhomogeneities of the graft from the woodchips. It therefore makes sense to obtain as many conductivity measurements as possible for the amount of wood chips each forming a graft. The same applies if this is additionally carried out with regard to the measurement of the pressure in the graft.

The device according to the invention, with which the new method can be implemented, has at the outlet of the plug screw a conductivity measuring device for measuring the conductivity of the wood chips in the region of a plug from the wood chips. In this case, if the conductivity measuring device is provided for a direct measurement of the conductivity of the wood chips, a housing of the screw plug having at least one transverse bore, which receives a sealed relative to the housing measuring electrode sealingly. The measuring electrode contacts the plug on the inner circumference of the housing. Any abrasion of the housing affects equally on the measuring electrode. The measuring arrangement thus remains always the same despite an unavoidable abrasion on the inner circumference of the housing. The conductivity measuring device can measure the conductivity between the already mentioned measuring electrode and the housing and / or at least one further measuring electrode. It is important to ensure that the current flowing through the measuring arrangement at a certain voltage actually depends essentially on the conductivity of the wood chips.

Preference is given to conductivity measuring devices in the new device, which have a plurality of at least three measuring electrodes distributed over the circumference of the screwing plug in order to also detect the homogeneity of the distribution of the fiber moisture over the respective plug from the woodchips.

For an additional measurement of a pressure applied by the plug screw in the region of the plug on the wood chips, a pressure measuring device may be provided which has at least one, preferably a plurality of pressure sensors distributed over the circumference thereof at the outlet of the screw plug.

Advantageous developments of the invention will become apparent from the claims, the description and the drawings

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1

zeigt einen Querschnitt durch eine Stopfschnecke als zentrales Element eines Flussdiagramms zum Gewinnen von Holzfasern aus Holzhackschnitzeln.

Fig. 2

zeigt die Leitfähigkeitsmesseinrichtung an einer Stopfschnecke in einer gegenüber Fig. 1 abgewandelten Ausführungsform.

Fig. 3

zeigt eine weitere Ausführungsform der Leitfähigkeitsmesseinrichtung; und

Fig. 4

zeigt ein Ablaufdiagramm zur Gewinnung von Holzfasern aus Holzhackschnitzeln für die Herstellung von Holzfaserplatten.

In the following the invention will be explained and described in more detail with reference to the accompanying drawings with reference to individual embodiments.

Fig. 1

shows a cross section through a plug screw as a central element of a flow chart for obtaining wood fibers from wood chips.

Fig. 2

shows the conductivity measuring device on a plug screw in a opposite Fig. 1 modified embodiment.

Fig. 3

shows a further embodiment of the conductivity measuring device; and

Fig. 4

shows a flow diagram for the production of wood fibers from wood chips for the production of wood fiber boards.

DESCRIPTION OF THE FIGURES

In Fig. 1 a stuffing screw 1 is shown in cross section. In a housing 2 of the screw conveyor 1, a screw conveyor 3 runs around an axis 4, so that a screw spiral 5 of the screw conveyor 3 woodchips 6, which are supplied to the plug screw 2 at its inlet 7 and are not shown here individually, promotes to the right and compressed into a compact plug. For the drive of the screw conveyor 3 is an in Fig. 1 only schematically reproduced drive motor 8 is provided. The plug from the wood chips 6 serves primarily to seal a cooker 9, the wood chips 6 are fed. In the cooker 9 prevails not only an elevated temperature, but also an increased pressure, which should not escape via the plug screw 2 and against the wood chips 6 must be pressed into the stove 9. In this case, at the entrance of the digester 9 usually a counter-pressure body not shown here for the wood chips 6 is provided, which ensures that the desired plug from the wood chips 6 in the plug screw 2 builds up. When building up the graft from the wood chips, excess water with which the woodchips 6 can be watered in a storage container 10 is pressed, so that only the water bound in the wood fibers of the woodchips 6 is present in the region of the graft. This water content, ie the moisture content of the wood chips forming the wood chips 6, is measured by a conductivity measuring device 11. Concretely, the conductivity meter 11 measures the conductivity of the plug of the woodchips 6 between two electrodes 12 and 13 contacting the plug on the inner periphery 17 of the housing 2 at two spaced apart points. The greater the fiber moisture content, the greater the conductivity recorded between the electrodes 12 and 13 by the conductivity measuring device 11. The measured conductivity can be used as a control parameter 14 not only for the cooker 9 as shown here but also for all subsequent devices for further processing Furthermore, a long-term observation of the conductivity of the wood chips 6 in the region of the plug can be used to determine to what extent, despite an inevitable wear of the plug screw 1 both on its housing 2 as well as their screw conveyor 3 is still a complete Abpressung the excess and not bound in the wood fibers water from the wood chips. If this pressure is no longer guaranteed, it is time to replace at least the screw conveyor 3, because then the seal of the stove 9 is no longer ensured.

The measuring arrangement of the conductivity measuring device 11 according to Fig. 1 in which two electrodes 12 and 13 sealingly penetrate the housing 2 in the radial direction, it is necessary that at least the associated portion 15 of the housing is made of electrically non-conductive material or is coated electrically insulating on its inner circumference 17, because otherwise the measuring distance between the Electrodes 12 and 13 would be short-circuited by the housing 2 itself. Fig. 2 outlines another training a measuring distance, which as well as the measuring distance in Fig. 1 remains the same at occurring wear of the housing 2, but does not require a portion 15 of the housing 12 of electrically non-conductive material. In this measuring arrangement is used as a counter electrode to the electrode 12, the electrically conductive housing 2, wherein the electrode 12 is electrically insulated from the housing 2 by a cylinder jacket insulation 16, which together with the electrode 12 a radially extending to the axis 4 transverse bore 30 in the housing 2 seals.

In Fig. 3 is sketched for another embodiment of the new device, that a plurality of electrodes 12, which are distributed over the circumference of the housing 2 of the screw conveyor 1, may be connected to the conductivity measuring device 11. In this case, the portion 15 of the housing 2, in which the electrodes 12 are arranged, consist of electrically non-conductive material or be provided on its inner circumference 17 with an insulating coating, or for each of the electrodes 12, the measuring arrangement according to Fig. 2 be provided. In any case, with the plurality of electrodes 12, it is possible to measure the conductivity of the woodchips in the graft forming in the section 15 of the housing 2 at several points simultaneously or in succession to provide more information about the fiber moisture of the wood fibers in the woodchips win, for example, to capture the homogeneity of the distribution of fiber moisture. In addition, in the measuring arrangement according to Fig. 3 individual electrodes 12 fail, without the device loses its overall function. The conductivity measuring device 11 can scan the individual electrodes 12 or the respective electrodes 12 and / or the associated counterelectrodes, which may be other electrodes 12, in parallel or else sequentially. In addition to the electrodes 12, the conductivity measuring device 11 is according to FIG Fig. 3 also connected to pressure sensors 31. The pressure sensors measure a pressure exerted by the screw conveyor 1 on the wood chips in the region of the electrodes to calibrate the measurement of their fiber moisture based on their conductivity to this pressure. The relationship between the electrical conductivity of woodchips and their fiber moisture is pressure dependent. With a determination of the fiber moisture content from the conductivity Therefore, without consideration of the associated pressure, errors are associated which remain small only at a largely constant pressure of the wood chips in the area of the electrodes 12. On the other hand, taking into account the associated pressure, it is possible to deduce from the measured conductivity very precisely the actual fiber moisture content of the wood fibers in the woodchips.

Fig. 4 outlines the production of glued wood fibers 18 for the production 19 of wood fiber boards. For this purpose, the woodchips 6 from the cooker 9 according to Fig. 1 discharged with a discharge device 20, in which the pressure in the interior of the digester 9 is abruptly reduced, so that the woodchips 6 dissolve into individual wood fibers. These wood fibers pass through a blowing passage 21 in a dryer 22. Previously, they are sprayed within the blowing passage 21 with binder 23. In the dryer 22, the moisture of the wood fibers is adjusted by a stream of hot dry gas. The hot dry gas is prepared in a burner 24 by burning fuel gas 25. The control of the burner 24 is indicated here by a valve 26 which adjusts the amount of hot gas 25 supplied to the burner 24. The adjustment of the valve 26 takes place as a function of the control parameter 14, which is derived from the conductivity of the woodchip chips determined by the conductivity measuring device 11. In this case, for the control of the burner 24, the running time of the wood fibers is taken into account by the graft from the woodchips, at which the conductivity is measured, until they enter the dryer 22. From the dryer 22, the wood fibers pass into a separator 27, where they are separated from the gas flow of the dryer 22 and the excess moisture absorbed by this. At the output of the separator 27, the fiber moisture content of the glued wood fibers 18 is measured with a measuring device 28, and from this a further control parameter 29 is determined, which is also used to control the dryer 22. By the feedforward control of the burner 22 with the aid of the determined by the conductivity measuring device 11 fiber moisture in the wood chips 6 allows in Fig. 4 sketched method for obtaining wood fibers 18 from wood chips 6 total to keep the fiber moisture of the wood fibers 18 in a very small window. Thus, the fiber moisture can be increased to the limit at which there is a risk of a gap, to save energy and fully exploit the capacity of a wood fiber board manufacturing plant.

LIST OF REFERENCE NUMBERS

1

stuffing screw

2

casing

3

Auger

4

axis

5

screw spiral

6

wood chips

7

inlet

8th

drive motor

9

Stove

10

storage container

11

Conductivity measuring device

12

electrode

13

electrode

14

control parameters

15

section

16

insulation

17

inner circumference

18

wood fibers

19

Production of wood fiber boards

20

discharge

21

blow line

22

dryer

23

binder

24

burner

25

heating gas

26

Valve

27

separators

28

measuring device

29

control parameters

30

cross hole

31

pressure sensor

Claims (10)

1. A method of obtaining wood fibres from wood chips, wherein the wood chips are supplied to a digester, in which they are subjected to increased pressure and increased temperature, by means of a feed screw, and wherein the wood chips are discharged from the digester while quickly removing the pressure, and are thus digested in the wood fibres,
   characterized in that an electric conductivity of the wood chips (6) is measured in the area of a plug of the wood chips (6) at the outlet of the feed screw (1), and is used as a control parameter (14) for controlling the further processing of the wood chips (6) and/or the wood fibres (18).
2. The method of claim 1, characterized in that a pressure applied by the feed screw (1) in the area of the plug onto the wood chips (6) is measured and considered in using the conductivity of the wood chip (6) for controlling the further processing of the wood chips (6) and/or of the wood fibres (18).
3. The method of claim 1 or 2, characterized in that the conductivity of the wood chips (6) is used for controlling a dryer (22) by which the fibre moisture content of the wood fibres (18) is adjusted.
4. The method of any of the claims 1 to 3, characterized in that the wood chips (6) are watered prior to being supplied to the digester (9), and that excess water is pressed from the wood chips in the feed screw (1) prior to measuring the conductivity of the wood chips (6).
5. The method of any of the claims 1 to 4, characterized in that the conductivity of the wood chips (6) is measured in a plurality of circumferential areas of the plug.
6. An apparatus for obtaining wood fibres from wood chips, the apparatus comprising a feed screw, a digester and a discharge unit, wherein the feed screw supplies the wood chips to the digester, wherein the wood chips are subjected to increased pressure and increased temperature in the digester, and wherein the discharge unit discharges the wood chips with a quick relief of the pressure out of the digester so that they are digested into the wood fibres,
   characterized in that a conductivity measuring unit (11) for measuring an electric conductivity of the wood chips (6) in the area of a plug of the wood chips (6) is provided at the outlet of the feed screw (1).
7. The apparatus of claim 6, characterized in that a housing (2) of the feed screw (1) comprises at least one cross-hole which, in a sealed way, accepts a measurement electrode of the conductivity measuring unit (11), which is isolated with regard to the housing (2).
8. The apparatus according to claim 7, characterized in that the conductivity measuring unit (11) measures the conductivity between the electrode (12) and the housing (2) and/or at least one further electrode (12, 13).
9. The apparatus according to claim 7 or 8, characterized in that the conductivity measuring unit (11) comprises a plurality of at least three electrodes (12) over the circumference of the feed screw (1).
10. The apparatus according to any of the claims 6 to 9, characterized in that a pressure measuring unit (31) for measuring a pressure, which is applied by the feed screw onto the wood chips (6) in the area of the plug, is provided at the outlet of the feed screw (1).

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