METHOD FOR OPERATING A PULPER FOR PRODUCING A SUSPENSION, IN PARTICULAR A FIBROUS SUSPENSION

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of PCT application no. PCT/EP2023/077858, entitled “METHOD FOR OPERATING A PULPER FOR PRODUCING A SUSPENSION, IN PARTICULAR A FIBROUS SUSPENSION”, filed Oct. 9, 2023, which is incorporated herein by reference. PCT/EP2023/077858 claims priority to German patent application no. 10 2022 126 745.8, filed Oct. 13, 2022, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to screening devices.

2. Description of the Related Art

Due to digitalization, processes can be controlled and regulated more precisely. Increased computing capacities as well as Kl can thereby also be integrated. Voith already has a number of specific applications which are addressed below.

OnEfficiency.BreakProtect uses a unique Kl algorithm for automatic recognition of various causes for breaks in paper production. Thus, counter measures for each type of break can be defined early on, thereby ensuring a stable, cost-effective production.

OnEfficiency.DIP optimizes an existing DIP line toward constant DIP quality at minimum cost. The existing flotation is supplemented by additional actuators; and new sensors are installed for monitoring the quality parameters. The DIP quality fluctuations which are caused by incoming raw materials or production changes are reduced by dynamic adjustment of the losses during washing/flotation and by real time optimization of the bleaching chemicals dosage.

An application for a better refiner quality has been promoted by Valmet under the title “Fiber Furnish Control.”

Pulpers are used in particular for fiber extraction from wastepaper. Wastepaper and also water are fed to a pulper to produce a fibrous suspension. Wastepaper qualities can be subject to fluctuations. A process for optical detection of the wastepaper quality is described in patent application DE 10 2021 103 233.

Adjustment of the process parameters, such as addition of water is provided, subject to the detected properties of the wastepaper.

A pulper, operated intermittently, is first of all filled with water. Next, the wastepaper or the pulp are introduced and dissolved. The fibrous stock suspension is then pumped out and further processed. The pulping time thereby also depends on the qualities of the wastepaper supplied to the pulper.

In addition, there are continuously operating pulpers which are supplied continuously with wastepaper and water. The produced fibrous suspension then passes through a screen to an accepted stock discharge. In such pulpers, contaminants are removed from the pulper via a discharge provided above the screen plate. In conventional pulpers the stock density is approximately 3 to 6%.

Screening devices are used to clean fibrous stock suspensions having a high content of contaminants. A screening device is known for example, from EP 2104766 A1. This screening device includes an asymmetrical housing. A screen is located in the upper region of the housing. A rotor is assigned to the screen, and the screen is kept free of contamination by the rotor. In addition, the supplied suspension is caused to rotate. Accepted stock is discharged-relative to the vertical direction—from the top of the housing and above the suspension infeed. In the lower part of the housing there is a discharge opening for contaminants which is directed diagonally downward. Such screening devices are used in particular to process for example contaminated suspension coming from the pulper. This screening device is suitable in particular also for a fibrous stock suspension originating from a discontinuously operated pulper without passing through a screen, as previously described. The here-described screening device is operated cyclically. This means that the throughput cycle, any post-dissolving cycle, also known as “slushing,” the rinse cycle and the discharge cycle run cyclically.

During the throughput cycle, fibrous suspension is fed to the screening device, and the accepted stock discharge is opened.

The post-dissolving cycle is known as a cycle in which no more suspension is supplied, and the rinse cycle has not yet been started. Treatment of suspension present in the screening device occurs in the screening device.

During the rinse cycle no fibrous suspension is fed to the screening device, and rinsing water is introduced into the screening device. Thus, dissolved fibers can be flushed out of the screening device as accepted stock.

During the discharge cycle a contaminant discharge is opened, and contaminants are discharged as rejects from the screening device.

Due to fluctuations of the contaminant content in the fibrous stock suspension, high mechanical and/or process related stresses occur repeatedly. This results in frequent alarm messages. Due to the frequency of such alarm messages, these alarm messages are often confirmed without specific action being taken in regard to the process, and without the cause of the alarm message being investigated. Moreover, intervention is only possible to a limited extent because of the inertia of the system and is difficult due to the large number of influential factors.

What is needed in the art is to improve a method for operation of a screening device. In particular, occurrence of overloads should be avoided. In particular, frequency of alarm messages should be reduced without overloading the system as a result.

SUMMARY OF THE INVENTION

The invention relates to a method for operating a screening device for cleaning a fibrous suspension and also relates to an arrangement with a screening device for implementation of the method.

The present invention provides a method for operating a screening device for cleaning a fibrous suspension. The screening device has a rotor arranged in a housing of the screening device and a screen arranged in the housing. Fibrous suspension and rinsing water can be fed to the screening device. The portion of the fibrous suspension passing through the screen is discharged via an accepted stock discharge. Inflow into the screening device and/or a cycle time duration is regulated or controlled subject to the power consumption of the rotor drive. Adjustment and control can therein pertain to a power consumption of the rotor in a previous cycle. This allows the inertia of the system to be taken into account on the one hand and to ensure continuous adjustment or control during the operation of the screening device on the other.

In one advantageous embodiment, it is provided that the cycle time duration of at least one cycle of a subsequent cycle sequence is regulated depending on the power consumption of the rotor drive. A cycle sequence includes at least one throughput cycle, rinse cycle and discharge cycle. Optionally, a post-dissolving cycle can be conducted between the throughput cycle and the rinse cycle. A post-dissolving cycle can be provided in particular depending on the discharge of accepted stock during the throughput cycle.

One design variation of the method provides that, if the power consumption of the rotor drive exceeds a predetermined limit value—at least at the start of a subsequent rinse cycle—the inflow of rinsing water is reduced compared to a maximum possible inflow of rinsing water. Overloading the rotor drive can thereby be prevented. As a result, frequently occurring alarm messages can be prevented. Especially at the start of the rinse cycle, the admission flow of rinsing water can cause a further increase in the power consumption of the rotor of the screening device. It has become evident that especially a moderate addition of rinsing water—in particular at the start of the rinse cycle—can have significant influence over the maximum power consumption occurring in one cycle sequence of the rotor.

One design variation of the method for operating a screening device provides that in addition or alternatively a cycle time duration of the throughput cycle and/or the rinse cycle is regulated, depending on the flow of accepted stock, and/or that an optional post-dissolving cycle is provided. The duration of the post-dissolving cycle can also be regulated depending on the flow rate of accepted stock, especially during the throughput cycle or the post-dissolving cycle. Alternatively, the post-dissolving cycle can be implemented with a predetermined duration if the post-dissolving cycle is activated.

One design variation of the method for operating a screening device provides that an infeed pressure and/or a differential pressure is considered for the pressure of the accepted stock discharge when regulating a subsequent cycle time. Thus, inference can be made in regard to the suspension and its quality, present in the screening device. Depending on the suspension quality and quantity present in the screening device, this can be taken into consideration when controlling the screening device.

One design variation of the method for operating a screening device provides that system-specific limit values are read in or are manually entered or stored during start-up. As a result, these values can be reverted to. Thus, provision may for example be provided to revert to this stored data in the event of conflicting measured data. If this data is used, the device is operated in basic mode. This is not a dynamically adaptive operation. When applying this basic operation, no adaptation of the stored system-specific limit values occurs. However, an adaptive adjustment of the stored limit values may be provided during the dynamic operation.

One design variation of the method for operating a screening device provides that a maximum system-specific accepted stock flow will be/is stored. This ensures that malfunctions can be recognized and can be rectified by way of the stored procedures.

One design variation of the method for operating a screening device provides that a factory default and/or a basic setting is stored in the control unit, whereby operation without adaptive control is possible with the factory default and with the basic setting. However, the efficiency of the device compared to operation with adaptive control is lower.

One design variation of the method for operating a screening device provides that if the power consumption of the rotor drive is outside a stored power range, in particular in the throughput cycle, operation is continued on the basis of data from a basic setting or factory default until the recorded operating data is again within the stored standard range.

In one design example of the method for operating a screening device it is provided that if an operating parameter is outside a stored standard range, the method is continued by reverting to a predetermined number of cycles and the operating parameters used for these cycles are adaptively adjusted. This is intended to ensure that the system does not revert to basic operation without adaptive control every time there is a deviation from the standard range. However, if no operation can be attained in the stored standard ranges within the predetermined number of cycles, a switch to basic operation occurs. This can prevent permanent or excessive overloading of the device.

In a method for operating a line having at least one first cyclically operated screening device and a second cyclically operated screening device arranged in parallel and a pulper, it is provided that the cycles of the least two screening devices are coordinated with each other for utilization of peripheral equipment of the line, such as a downstream sorting drum. Adaptation is implemented by extending at least one of the rinse cycles from the currently ongoing sequences and/or by shortening at least one of the throughput cycles. Thus, peripheral devices can be used to optimal capacity and do not have to be provided separately for each screening device. For example, a pump for supplying fibrous suspension can be accessed jointly. It can also be possible to supply rinsing water to two screening devices by way of just one pump.

One design variation of the method for operating a line with a pulper and at least one screening device provides that fibrous suspension is returned to the pulper. It is provided that the consistency and volume of the fibrous suspension that is returned to the pulper is considered for achieving a constant stock density in the pulper. In particular, depending on the consistency and inflow of the fibrous suspension returned to the pulper, control of the supply of dilution water may be provided. In this way, fluctuations in the stock density of the fibrous suspension leaving the pulper can be minimized. In particular, a predetermined stock density range can thus be maintained more precisely, which means that subsequent processing and cleaning of the fibrous suspension can be better coordinated with the fiber consistency. For example, a predetermined stock density consistency of +/−0.2% absolute percent can be maintained.

A design is provided with a pulper and at least one screening device, wherein a control valve is arranged in the supply of rinsing water to the screening device. The control valve is intended for a controlled supply of rinsing water into the screening device. Thus, the supply of rinsing water can be regulated, particularly during the rinse cycle.

By way of this regulated rinsing water supply, overload of the rotary drive can be prevented, or at least reduced. The control valve facilitates a continuous increase in rinsing water supply.

The supply of fibrous suspension is interrupted in the rinse cycle and the discharge cycle. Rinsing water is fed to the screening device in a targeted manner. The rinsing water initially flushes out an increased amount of fibers. It turned out that this phase specifically is highly susceptible for overloads. Not only does the required drive power for the fibrous suspension with enriched contaminants have an effect on the rotor, but so does the provided rinsing water inflow. Although the rinsing water serves to dilute the fibrous suspension present in the screening device, and it could be assumed that fibers can now flow more easily through the screen, it has been shown that surprisingly overload situations occur precisely then. By reducing the initial rinsing water inflow, an overload situation can be avoided. Additional pressurization of the rotor by way of rinsing water can be reduced. Accepted stock can drain off through the screen, even if it may be partially covered by contaminants. This can reduce the peak load on the rotor drive on the one hand and can also reduce the total load on the drive in the rinse cycle.

The duration of the rinse cycle can be adaptively and predictively extended, and the amount of rinsing water per rinse cycle—optionally with +/−10% deviation from a predetermined amount of rinsing water—can nevertheless be kept constant in the rinse cycle. As a result, a good yield of fibrous material can be achieved by adapting the rinse cycle. Volumes are stated in cubic meters. If reference is made to a flow, such as an inflow, throughflow or outflow, volumes per time which are often stated in liters per minute are meant.

In a discharge cycle following the flushing cycle, rinsing water flows in with a predetermined maximum inflow. This flushes the suspension remaining in the screening device with concentrated contaminants out of the screening device. The accepted stock discharge is closed.

One design variation provides that the duration of the discharge cycle is adjusted subject to the power consumption of the rotor. Thus, an improved discharge of contaminants can be achieved. On the other hand, the discharge cycle should only last as long as noticeable contaminants are being discharged. For example, if the power consumption of the rotor drops below a predetermined level, this can be used as an indication that the screening device has been sufficiently cleaned of contaminants.

In some applications it has proven to be advantageous to vary the duration of the cycles in a predetermined time frame depending on at least the parameter of the power consumption of the rotor. This can prevent pausing in one operating step in the event of incorrect operating data.

With a more effective discharge of contaminants, the subsequent cycle can be more effective. Thus, the drive power of the rotor is reduced because contaminants do not remain in the screening device needlessly long. Moreover, there is the positive effect that the contaminants are exposed to the rotor for a shorter period of time, thus preventing or at least reducing pulverizing by the rotor.

It has proven to be advantageous that the increase in the inflow of rinsing water in the rinse cycle is adaptive subject to the current load only as long as the maximum power consumption is below a predetermined maximum upper limit value. On exceeding this upper limit value, an overload situation of the rotor drive occurs. By not supplying the maximum amount of rinsing water, overload situations of the rotor drive can be prevented, their occurrence at least reduced, and the overload minimized.

For example, the inflow of rinsing water in the rinse cycle can be reduced during the first 5 seconds, at most during the first 20 seconds, from a predetermined limit value, compared to the maximum inflow of rinsing water. This limit value is below the maximum upper limit value. This is to prevent the maximum upper limit value from being exceeded.

It has proven to be advantageous that, with a reduced inflow of rinsing water, the inflow is reduced to a minimum of 20% of the maximum inflow of rinsing water in the rinse cycle. In an advantageous operating variant, the rinsing water volume remains as predetermined. This extends the time of the rinse cycle and thus a constant amount of rinsing water is used for flushing out fibers. As a result, fibers can still be flushed out and overloading of the rotor drive can be countered.

In one design variation it can be provided that rinsing is conducted adaptively predictively from the current contaminant load based on the drive power of the rotor until a predetermined lower limit value of the power consumption is likely to be reached. Thus, it can be ensured that a large portion of the contaminants have been flushed out. This again provides a good starting position and thereby filling capacity of the screening device for subsequent cleaning of fibrous suspension in the throughput cycle and subsequent cycles.

Adaptive in this case means adjusting. It is an adjustment based on the detected operating parameters.

A predictive setpoint adjustment and its control is regarded as predictive. In particular, the previous cycles 1 to 50 can be used.

It may be provided that, depending on the power consumption at the end of the discharge cycle above a lower limit value, the length of the subsequent or the following newly starting throughput cycle of fibrous suspension furnish will be shortened. The drive power of the rotor at the end of the discharge cycle provides an indication of remaining contaminants. This contamination can be a result of a high contaminant load in the fibrous suspension fed to the screening device. However, a high contaminant load may also have accumulated in the screening device due to a temporally long previous cycle.

The possibility of reacting dynamically to the contaminant load can increase the efficiency of the screening device. If there is a low contaminant load in the fibrous suspension supplied in the throughput cycle, throughput cycles can be extended without risking overloading the rotor drive.

On the other hand, the duration of the throughput cycles can be shortened if the contaminant load is high. This can be recognized by the speed of an increase in the drive power of the rotor. The contaminants are then discharged more frequently, countering an overload of the rotor drive. This has a positive effect on the service life of the screening device.

If provision is made that the accepted stock is again fed to the pulper, more frequent discharge of contaminants in the screening device can also achieve that a contaminant load in the pulper is reduced more quickly. This results in less comminution in the pulper. A consequence of comminution would be that the crushed contaminants would manifest themselves as increased load in subsequent cleaning process stages and could possibly negatively affect the final accepted stock quality.

In one variation of the method, it is provided that, depending on the power consumption at the end of the discharge cycle, the amount and also the inflow of fibrous suspension that is supplied is reduced. Therein the new contaminant furnish per time interval can be reduced, as well as the total amount of fibrous suspension supplied during this cycle and the associated contaminants in the screening device at the end of the throughput time. Overload situations can thereby be countered especially effectively.

According to the present invention, the arrangement with a screening device is characterized in that, the arrangement includes a control valve in the rinsing water supply for adjustment of the inflow of rinsing water. This makes it possible to dynamically control the inflow of rinsing water. The drive power of the rotor drive can be recorded by a control system. In order to avoid overload situations, it is now possible by way of the control valve to reduce the inflow of the rinsing water, depending on the drive power of the rotor. For this purpose, characteristic curves can be stored in the control system or can also be taught.

One design variation wherein a pulper is arranged upstream from the screening device provides that the control system reduces the supply of material into the pulper after a predetermined number of cycles with a power consumption of the rotor of the screening device that exceeds a limit value stored in the control system. This counteracts an overload in fiber preparation. The speed of preparation from wastepaper into an accepted stock can thus be adapted to the quality of the wastepaper that is fed into the pulper.

A computer program product can be used to retrofit an existing line with a pulper and at least one downstream cyclically operated screening device for carrying out the method according to one of the previously described inventive methods. The efficiency of old systems can therewith be increased.

Additional advantageous features of the present invention are explained with reference to design examples and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic representation of a system according to the invention;

FIG. 2 is a schematic representation of a system according to the invention with continuously operated pulper;

FIG. 3 is a schematic representation of a system according to the invention with two screening devices arranged in parallel;

FIG. 4 is a schematic representation of a control system;

FIG. 5 is a schematic representation of measurement operating curves:

• • a) Drive power, pump
  • b) Accepted stock discharge
  • c) Drive power, rotor of screening device 50
  • d) Volume of supplied dilution-/washing water.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one embodiment of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1, 2 and 3 present examples of arrangements 1 for the production of a fibrous suspension. The problem with fiber extraction from wastepaper consists in removing as much of the contaminants as possible. However, removal of fibers associated with the removal of contaminants should be kept to a minimum.

Paper stock—also referred to as wastepaper and shown here as bales—is fed to a pulper 20 via a conveyor 11. In pulper 20, for example, wastepaper as the paper stock is mixed with water and converted into a fibrous suspension. For this purpose, dilution water is fed to pulper 20 via a controllable water supply 10 and infeed 15. In addition, pulper 20 is further supplied with a suspension classified as accepted stock from a screening device 50 and post-screening device 70 downstream of pulper 20. Screening device 50 and post-screening device 70 will be discussed in detail later.

In its bottom region, pulper 20 is equipped with a screen 29 through which part of the suspension can pass into the annular space. A rotor 25 is arranged above screen 29 by way of which the paper stock/water mixture is caused to rotate on the one hand and on the other hand keeps screen 29 free. Rotor 25 is driven by drive 27. The fibrous suspension which has passed through screen 25 is discharged via an accepted stock discharge 21. The portion of suspension produced in pulper 20 which is discharged via contaminant discharge 23 is directed into a storage tank 30 which is equipped with a contaminant outlet 31. Coarse heavy dirt contained in the introduced fibrous suspension can be separated in storage tank 30. Ideally, the suspension is diluted to 3.5% in collection tank 30 for more effective separation of heavy particles. The coarse heavy dirt sinks down into collection tank 30 and is discharged directly through an automatically controlled trash discharge with a contaminant outlet 31. Additional dilution water may be added to the trash discharge to improve fiber backwashing.

An outlet 33 for suspension is provided in the upper region of the storage tank. Fibrous suspension is removed in this upper region of storage tank 30. By way of pump 40, said fibrous suspension is fed via a line and infeed 52 to screening device 50 with housing 51 for further treatment. Pump 40 is a special pump for contaminant-loaded fibrous suspension. Pump 40 can be used to convey the contaminated suspension from the pulper into screening device 50 in a predominantly blockage-free and reliable manner. Pump 40 is designed to have a large nozzle diameter and a non-clog impeller for clog-free pumping of fibrous suspensions with a high proportion of contaminants. In particular, the impeller can be optimized for particularly high resistance by way of hard-facing. Drive 45 with frequency converter facilitates optimized pump speed for discontinuous pump operation. Thus, the volume flow—also referred to as flow—conveyed by pump 40 can be regulated, optionally infinitely variably regulated.

The accepted stock from the storage tank enters downstream screening device 50 for further processing. This is a cyclically operated screening device. A screen 57 is provided in the housing. A rotor 58 is arranged prior to screen 57. Rotor 58 causes the fibrous suspension introduced into housing 51 of the screening device to rotate prior to screen 57. The rotor is driven by a drive 59. A portion of the supplied fibrous suspension passes through screen 57 and is discharged via an accepted stock discharge 67 from the screening device. In FIG. 5 the power consumption of drive 59 of the rotor over time is plotted. Moreover, the power consumption of drive 59 of the rotor over time is also potted in FIG. 5.

There is a cycle for the supply of fibrous suspension that is also referred to as the throughput cycle. In throughput cycle 117, fibrous suspension is fed to screening device 50. Rotor 58 causes the suspension to rotate. Rotor 58 is arranged prior to screen 57. Dissolved fibers pass through screen 57. A differential pressure is applied, which conveys the outflow of dissolved fibers through screen 57. Accepted stock discharge 67 for the discharge of the accepted stock with switching valve 69 (see FIG. 4) is open. Contaminant discharge 66 with switching valve 56 (see FIG. 4) is closed. As a result, contaminants are concentrated in the fibrous suspension in screening device 50 before screen 57.

To increase the processing of this suspension in screening device 50, the supply of fibrous suspension can be interrupted. Switching valve 53 (see FIG. 4) is provided for this purpose. The suspension in screening device 50 is processed by the movement in screening device 50. Valve 56 of contaminant discharge 66 is still closed and the accepted stock discharge is open. The movement of the suspension can loosen fiber clumps present in the suspension. The dissolved fibers can pass through the screen and flow off via accepted stock discharge 67. This cycle is also referred to as post-dissolving cycle 119.

Post-dissolving cycle 119 or throughput cycle 117 is followed by a rinse cycle 127. A supply of rinsing water is provided in rinse cycle 127. The rinsing water is provided by the water supply 10. The inflow into screening device 50 occurs via control valve 65. The inflow into screening device is implemented via a control valve 65. The inflow is adjustable by way of control valve 65. Depending on power consumption 162 of the rotor, inflow 152 of rinsing water can be regulated. The inflow of rinsing water is initially throttled, depending on the drive power of rotor 58 of screening device 50 before rinse cycle 127. Overloading of the rotor drive is thereby prevented or at least reduced.

Control of the supplied rinsing water inflow could also occur via an adjustable drive of pump 60 for the rinsing water admission. During this cycle, fibrous suspension is increasingly flushed out via an accepted stock discharge 67.

Rinse cycle 127 is followed by discharge cycle 137. The suspension with the concentrated contaminants is discharged from housing 51 of screening device 50. For this purpose, contaminant discharge 66, valve 56, is opened. The throughput cycle and rinse cycle can overlap or can merely be executed in series. The length of the respective cycle can be changed by a control unit 100 and can thereby be adapted to the respective level of contamination or paper quality of the wastepaper. The fraction of accepted stock extracted by screening device 50 is returned back to pulper 20 via supply 17. The fraction of contaminants extracted in screening device 50 is fed to a post-screening device 70.

Additional washing out of fibers still present in the contaminants is provided in post-screening device 70. Here, post-screening device 70 includes a screen drum and a dilution water supply 75. Due to rotation of the drum the introduced fraction is caused to rotate and also to move axially. Fibers pass through openings in the drums together with rinsing water to an accepted stock discharge 73 and are fed via infeed 19 from post-screening device 70 to pulper 20. The portion that does not pass through the screen is discharged as contaminants via a contaminant discharge 76. The inflow of rinsing water can also be regulated at post-screening device 70. In particular, the amount of rinsing water can be regulated depending on the drive power of the drum in order to avoid overloading the drum drive.

In arrangement 1 shown in FIG. 2, fibrous suspension produced in pulper 20 is fed directly cyclically to a downstream screening device 50. The accepted stock of screening device 50 is fed to a large storage tank 68 and the contaminants that are separated from screening device 50 are directed via a pump 40 to a storage tank 30 which is equipped with a contaminant outlet 31. Via an outlet 33, suspension can be continuously fed from storage tank 30 to a post-screening device 70 via infeed 71. This post-screening device 70 is consistent with post-screening device 70 described in connection with FIG. 1. Here, the accepted stock extracted by post-screening device 70 is fed to pulper 20 via accepted stock discharge 73. The portion that does not remain as accepted stock in post-screening device 70 is discharged via a contaminant discharge 76.

FIG. 3 presents an additional variation of a system for fiber preparation. This system differs from the system shown in FIG. 1 in that two screening devices 50 are arranged in parallel relative to each other. The operating principle of screening devices 50 and 250 does not differ from the operating principle of screening device 50 described in FIG. 1. The second screening device also has a housing 251, an infeed for suspension 252, a drive 259, an infeed for rinsing water 261, a contaminant discharge 266 and an accepted stock discharge 267. The cycles of the two screening devices are coordinated with each other for more efficient operation. Only one pump 40 is provided by way of which fibrous suspension is fed to first screening device 50 or second screening device 250. Thus, a common supply of rinsing water is also provided. The contaminants from the screening devices are fed to a post-screening device 70.

FIG. 4 shows one version of a control unit 100 for a screening device 50 in more detail. On the one hand, control unit 100 controls motor 45 of pump 40. A pump control unit 140 is provided for this purpose. Pump control unit 140 issues a setpoint value and an actual value of the drive frequency of the pump and a drive power 144. The objective is to ensure a maximum flow in the throughput cycle without overloading the drive at changing boundary conditions such as admission pressure and contaminant content. The supply of suspension to screening device 50 is hereby controlled via pump 40. A control unit 150 is provided for water pump 60 for the supply of rinsing water and the inflow of rinsing water. Via this control unit, valve 65 provided in water supply 61 can be controlled. Valve 65 is an adjustable valve. Valve 65 can be used to regulate or control the inflow of supplied rinsing water. Control unit 150 of pump 60 of the rinsing water includes a setpoint of a delivery rate and a basic value of the delivery rate of water pump 60. In addition, the current drive power is recorded.

For control of pump 60 of the rinsing water and control of the rinsing water flow and the volume in the respective cycle—in particular during the rinse cycle—values of drive power 117 of drive 59 of rotor 58 of screening device 50 are also considered. FIGS. 4 and 5 describe the control scheme in more detail depending on the drive power of rotor 58.

Control unit 100 can be used to dynamically adjust the duration of individual cycles and the volume of suspension and rinsing water delivered per time.

Control unit 110 records throughput cycle 117. The throughput cycle is the period of time in which the fibrous suspension is fed to screening device 50. A basic value of a time duration for a throughput cycle is specified, and a setpoint value of a time duration of a throughput cycle is adaptively-predictably specified. An adaptive-predictive specification is a predictive presetting that adapts depending on parameters. An adaptive-predictive control can be based on stored characteristic curves, or it can be provided by an adaptive learning control system. If initially no values or characteristic curves are available, then initial teaching is required. Subsequently, a self-learning adjustment of basic values as well as target values can be provided.

Different basic values/starting values can also be stored in the control system for predetermined grades of wastepaper. The duration of the cycles and inflows of rinsing water and suspension are regulated by control unit 100. A determined setpoint value is provided by the control unit for the individual cycles. Thereby, the power consumption of rotor drive 58 of the screening device is considered, in particular. It may be provided that the duration of the throughput cycle is regulated depending on the minimum power consumption of the rotor in the rinse cycle. It may furthermore be provided that the quality and/or also the volume of the discharged accepted stock are taken into consideration. An adjustment of a throughput cycle in the range of 15 to 150 seconds, optionally in the range of 60 to 120 seconds has proven to be a suitable adjustment range. In an equivalent arrangement, a duration of 50 seconds was specified for the throughput cycle in static operation. Here, the increase in efficiency through dynamic operation is evident. If the measured values are inconsistent, the values from static operation can be used initially to continue operation.

Control unit 120 is provided for control of rinse cycle 127. In rinse cycle 127, rinsing water is introduced into the screening device with the objective of flushing or washing out fibers from the contaminant-enriched suspension prior to the screen. Fibrous suspension is no longer supplied by pump 40. Switching valve 53 is or will be closed. The control unit regulates rinse cycle 127. An actual rinse time is designated, and a basic value of a rinse time is stored. In addition to the duration of the rinse cycle, the flow of supplied rinsing water is also controlled depending on the consumed or detected power consumption of drive 59 of rotor 58. Control unit 160 is provided for the rotor drive. The rinse cycle can also be referred to as wash cycle. In this cycle, fibers from the suspension present in the screening device are increasingly washed out by way of the supplied rinsing water. No suspension is fed to screening device 50.

In order to regulate the inflow of rinsing water, a control unit 150 is assigned to the drive of pump 60. A maximum value 154 is provided for the rinsing water in the control unit of the pump. An inflow of rinsing water can be increased from an initially low value to the maximum value over the course of a cycle, as shown in FIG. 5. A basic value is stored for the duration of rinse cycle 127 and discharge cycle 137. A duration of 15 seconds to 50 seconds has proven to be suitable as a dynamic range for rinse cycle 127. In comparison, a duration of 15 seconds is provided for this in a static operation. A good yield of fibers can be achieved by extending this time frame.

The rinse cycle is followed by a discharge cycle 137. Discharge cycle 137 is regulated by control unit 130 for the discharge cycle. Control valve 65 is open during discharge cycle 137. Valve 56 for the discharge of contaminants is or will be open and valve 69 for the accepted stock discharge is closed. The supply of fibrous suspension through valve 53 is prevented. In discharge cycle 137, a maximum inflow of rinsing water is fed to the screening device in order to achieve effective flushing out of the suspension enriched with impurities. In dynamic operation, time spans in the range of 15 to 35 seconds are stored on the control unit. In static operation 35 seconds are stored as predetermined constant time. Since the contaminants are transported out of the screening device during discharge cycle 137, this cycle is often also referred to as reject cycle.

If the drive power of rotor 58 does not drop below a predetermined lower limit value during discharge cycle 137 and also starting throughput cycle 117—arrow in FIG. 5—then the control unit shortens throughput cycle 117 in the next newly starting throughput cycle 117. In the next cycle, the following discharge cycle and thus the duration for a reject discharge is extended. Due to the inertia of screening device 50, immediate intervention in an already started throughput cycle 117 is not yet possible with the currently available screening devices 50. However, if drive power 162 of rotor 58 drops below a predetermined limit value, an alarm is issued.

System (1) can be provided with a pulper (20) and at least one screening device (50, 250) and a control unit (100) to carry out the method according to the present invention, wherein control valve (65) is arranged in the infeed of rinsing water to screening device (50, 250), wherein control valve (65) is optionally a control valve (65) which can be actuated at least depending on a detected power consumption (162) of rotor drive (59, 259).

FIG. 5 shows examples of temporal progressions. The inflow of fibrous suspension 112 and the outflow of accepted stock 114, power consumption 162 of the drive of rotor 58 and the inflow of rinsing water 152 are shown in relation to each other and over time. From the temporal progression of the inflow of rinsing water, a reduced inflow 158 of rinsing water can be seen. In discharge cycle 137 the maximum inflow of rinsing water is supplied. Cycles such as throughput cycle 117, post-dissolving cycle 119, rinse cycle 127 and discharge cycle 137 are entered.

This process makes it possible to dynamically adapt the yield of fibers and the volume of purified suspension to the quality of the supplied suspension. The duration of rinse cycle 127 and the volume of rinsing water added can be dynamically adjusted.

Component Reference Listing

1	System for producing a fibrous suspension
10	Water supply
11	Conveyor
13	Wastepaper bales
15	Infeed dilution water
17	Infeed accepted stock from screening device
19	Infeed accepted stock from post-screening device
20	Pulper
21	Accepted stock discharge, pulper
23	Contaminant discharge, pulper
25	Rotor, pulper
27	Drive, pulper
29	Screen, pulper
30	Storage tank
31	Outlet, contaminant storage tank
33	Outlet, suspension, storage tank
40	pump
45	Pump drive
50	Screening device
51	Housing
52	Infeed-screening device
53	Switching valve suspension supply
56	Switching valve contaminant discharge
57	Screen (screening device)
58	Rotor (screening device)
59	Drive—screening device
60	Pump for rinsing water
61	Water supply
65	Control valve
66	Contaminant discharge
67	Accepted stock discharge
68	Large storage tank—accepted stock screening device
69	Switching valve, accepted stock discharge, screening
	device
70	Post screening device
71	Infeed, post-screening
73	Accepted stock discharge
75	Dilution water supply
76	Contaminant discharge
100	Control unit
110	Throughput cycle—control
112	Inflow, suspension
114	Outflow, accepted stock
117	Throughput cycle
119	Post-dissolving cycle
120	Rinse cycle—control
127	Rinse cycle, wash cycle
130	Control unit, discharge cycle control
137	Discharge cycle, reject cycle
140	Pump control unit (fibrous suspension)
150	Control unit, rinsing water supply (flow and volume)
151	2nd screening device
152	Infeed rinsing water
154	Rinsing water inflow, maximum
158	Reduced rinsing water volume
160	Control unit, rotor drive
162	Power consumption, rotor drive
250	2nd screening device
251	Housing, 2nd screening device
252	Infeed, 2nd screening device
259	Drive, 2nd screening device
261	Rinsing water infeed
266	Contaminant discharge
267	Accepted stock discharge

While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.