AN APPARATUS FOR WASHING AND DEWATERING PULP, A SYSTEM FOR CONTROLLING SUCH AN APPARATUS, AND A METHOD FOR PROCESSING PULP IN SUCH AN APPARATUS

Abstract

A method for processing pulp in an apparatus for washing and dewatering pulp, and a system for controlling this apparatus, the apparatus comprising two rotatable press rolls (102, 104) having a permeable outer surface (106, 108), and a vat (114, 116, 118), the press rolls (102, 104) defining a press nip (112) between them. The processing is determined by a set of variable operating parameters, which are adjusted and/or maintained during operation in response to deviations of measured values of control parameters in relation to their respective predetermined threshold values. The variable operating parameters include rotation speed of the press rolls. The control parameters include vat pressure and/or linear load.

Claims

1. A method for processing pulp in an apparatus for washing and dewatering pulp comprising two rotatable press rolls (102, 104) having a permeable outer surface (106, 108), and a vat (114, 116, 118) in which the press rolls (102, 104) are installed, the press rolls (102, 104) defining a press nip (112) between them, in which press nip (112) the pulp is pressed, and the apparatus is arranged to feed the pulp in the direction of rotation of the press rolls through the press nip (112), and the processing of the pulp in the apparatus is determined by at least one variable operating parameter which is variable during operation, characterized by the steps of: including (301) the rotation speed of the press roll (102, 104) as variable operating parameter; including (302) vat pressure and/or linear load as control parameters, each being associated with a predetermined threshold value; measuring (304; 313; 402, 502) the control parameters; comparing (305; 314; 403, 404, 406, 408, 503, 505, 506, 508) measured control parameters and respective predetermined threshold value; and if a measured control parameter and the respective predetermined threshold value deviates from each other, adjusting and/or maintaining (306; 405, 407, 409, 504, 507, 509) the rotation speed during operation according to a first set of control process rules.

2. A method according to claim 1, wherein at least one of the press rolls (102, 104) is movable in relation to the other press roll (102, 104) to vary the press nip, further comprising: including (310) the distance between the press rolls (102, 104) as variable operating parameter; including (311) vat pressure and linear load as control parameters, each being associated with a predetermined threshold value; and if a measured value of a linear load and/or vat pressure and the respective predetermined threshold value deviates from each other, adjusting (315; 405, 407, 409, 504, 507, 509) the distance between the press rolls during operation according to a second set of control process rules, wherein said second set of control process rules are operated in synchronism with said first set of control process rules.

3. A method according to claim 2, characterized by including (320) the rotation speed of the press roll (102, 104) and distance between press rolls as variable operating parameters; including (321) vat pressure, linear load and torque as control parameters, each being associated with a predetermined threshold value; measuring (323) the control parameters; comparing (324) measured control parameters and respective predetermined threshold value; and if a measured control parameter and the respective predetermined threshold value deviates from each other, adjusting and/or maintaining (325) at least one operating parameter during operation according to a first set of control process rules.

4. A method according to claim 1, characterized by including (311) torque as control parameter, wherein said torque being associated with a predetermined threshold value.

5. A method according to claim 1, characterized by setting (303; 312; 401, 501) a desired maximum value of the respective control parameter as predetermined threshold value for the respective control parameter.

6. A method according to claim 1, characterized by, that said first set of control process rules includes, if a measured value of a control parameter and the respective predetermined threshold value corresponds to each other (406) and other measured control parameters are below their respective predetermined threshold value, maintaining (407) the rotation speed of the press roll (102, 104) at present level.

7. A method according to claim 1, characterized by, that said first set of control process rules includes, if the measured values of a control parameter and the respective predetermined threshold values corresponds to each other (406), maintaining (407) the rotation speed of the press roll (102) at present level.

8. A method according to claim 1, characterized by that said first set of control process rules includes, if at least one measured value of a control parameter is higher than respective predetermined threshold value (404), increasing (405) the rotation speed of the press roll (102, 104).

9. A method according to claim 1, characterized by that said first set of control process rules includes, if the measured values of all control parameters are below the respective predetermined threshold values (408), decreasing (409) the rotation speed of the press roll (102, 104).

10. A method according to claim 2, characterized by, that said second set of control process rules includes, if a measured value of vat pressure and linear load and a respective predetermined threshold value corresponds to each other (503), maintaining (504) the distance between the press rolls (102, 104) at present level.

11. A method according to claim 2, characterized by that said second set of control process rules includes, if a measured value of vat pressure or linear load and a respective predetermined threshold value deviates from each other (505, 506, 508), adjusting (507, 509) the distance between the press rolls (102, 104).

12. A method according to claim 11, characterized by that said second set of control process rules includes, if a measured value of linear load and the respective predetermined threshold value deviates from each other and the predetermined threshold value and the measured value of the vat pressure corresponds to each other (508), decreasing (509) the distance between the press rolls (102, 104).

13. A method according to claim 12, characterized by that said second set of control process rules includes, if a measured value of linear load is below the predetermined threshold value and the respective predetermined threshold value and the measured value of the vat pressure corresponds to the respective predetermined threshold value (508), decreasing (509) the distance between the press rolls (102, 104).

14. A method according to claim 11, characterized by that said second set of control process rules includes, if a measured value of vat pressure and the respective predetermined threshold value deviates from each other and the linear load corresponds to each other (506), increasing (507) the distance between the press rolls (102, 104).

15. A method according to claim 14, characterized by that said second set of control process rules includes, if a measured value of vat pressure is below the respective predetermined threshold value and the measured linear load corresponds to the respective predetermined threshold value (506), increasing (507) the distance between the press rolls (102, 104).

16. A method according to claim 1, wherein said control parameters may include pressure of the pulp distribution device and/or the throttle gap width, which refers to the opening gap of a throttle arranged close to or in the outlet of the pulp distribution device.

17. A system for controlling an apparatus for washing and dewatering pulp comprising two rotatable press rolls (102, 104) having a permeable outer surface (106, 108), and a vat (114, 116, 118) in which the press rolls (102, 104) are installed, the press rolls (102, 104) defining a press nip (112) between them, in which press nip (112) the pulp is pressed, and the apparatus is arranged to feed the pulp in the direction of rotation of the press rolls through the press nip (112), and the processing of the pulp in the apparatus is determined by at least one variable operating parameters which are variable during operation, characterized in that the rotation speed of the press roll (102, 104) is included as variable operating parameter, and in that the system comprises measuring means (170, 175) for measuring a value of control parameters, wherein vat pressure and/or linear load are included as control parameters; an analysing device (182) arranged to compare a measured value of a control parameter and a respective predetermined threshold value in order to identify deviations between them, and adjustments means (214) for adjusting and/or maintaining the rotation speed during operation if deviations is identified during operation according to a first set of control process rules.

18. A system according to claim 17 characterized in that the system further comprises at least one of the press rolls (102, 104) being movable in relation to the other press roll (102, 104) to vary the press nip, wherein the distance between the press rolls (102, 104) is included as variable operating parameter, wherein vat pressure and linear load are included as control parameters, and wherein adjustments means (202) is arranged for adjusting the distance between the press rolls during operation according to a second set of control process rules, wherein said second set of control process rules are operated in synchronism with said first set of control process rules, if a linear load and/or vat pressure and a respective threshold value deviates from each other.

19. A system according to claim 17, characterized in that torque is included as control parameter, wherein said torque being associated with a predetermined threshold value, and further comprising measuring means (174) for measuring the torque of the press rolls (102, 104)

20. A system according to claim 17, characterized in that the system further comprises setting means (160) for setting at least one threshold value for the respective control parameter as the predetermined threshold value, wherein vat pressure, linear load, and torque are included as control parameters,

21. A system according to claim 17, characterized in that a controller (166) is arranged to, according to said first set of control process rules, instruct the adjustments means to, if a measured value of a control parameter and a respective threshold value corresponds to each other (406), maintain (407) the rotation speed of the press roll (102, 104) at present level.

22. A system according to claim 18, characterized in that the adjustment means includes at least one hydraulic driving device for urging the movable press roll away from the other or towards each other.

23. A system according to claim 17, characterized in that the controller is arranged to, according to said first set of control process rules, instruct the adjustments means to, if a measured value of a control parameter and a respective predetermined threshold value corresponds to each other (406) for all control parameters, maintain the distance between the press rolls (102, 104) and the rotation speed of the press roll (102, 104) at present level.

24. A system according to claim 18, characterized in that the controller is arranged to, according to said second set of control process rules, instruct the adjustments means to, if a measured value of vat pressure and linear load and a respective predetermined threshold value corresponds to each other (503), to maintain the distance between the press rolls (102, 104) and the rotation speed of the press roll (102, 104) at present level.

25. A system according to claim 17, characterized in that the controller is arranged to, according to said first set of control process rules, instruct the adjustments means to, if a measured value is higher than the predetermined threshold value (404), instruct the adjustment means to increase the rotation speed of the press roll (102, 104)

26. A system according to claim 17, characterized in that the controller is arranged to, according to said first set of control process rules, instruct the adjustments means to, if a measured value of a control parameter and a respective predetermined threshold value deviates from each other and the measured values are below the respective predetermined threshold values (408), decrease the rotation speed of the press roll (102, 104).

27. A system according to claim 18, characterized in that the controller is arranged to, according to said second set of control process rules, instruct the adjustments means to, if a measured value of vat pressure or linear load and a respective predetermined threshold value deviates from each other (505, 506, 508), adjust the distance between the press rolls (102, 104).

28. A system according to claim 27, characterized in that the controller is arranged to, according to said second set of control process rules, instruct the adjustments means to, if a measured value of linear load and the respective predetermined threshold value deviates from each other and the predetermined threshold value and the measured value of the vat pressure corresponds to each other (508), decrease the distance between the press rolls (102, 104).

29. A system according to claim 28, characterized in that the controller is arranged to, according to said second set of control process rules, instruct the adjustments means to, if a measured value of linear load is below the predetermined threshold value and the measured value of the vat pressure and the respective threshold value corresponds to each other (508), decrease the distance between the press rolls (102, 104).

30. A system according to claim 27, characterized in that the controller is arranged to, according to said second set of control process rules, instruct the adjustments means to, if a measured value of vat pressure and the respective predetermined threshold value deviates from each other and the linear load corresponds to each other (506), increase the distance between the press rolls (102, 104) to reduce the difference to be below the predetermined level.

31. A system according to claim 30, characterized in that the controller is arranged to, according to said second set of control process rules, instruct the adjustments means to, if a measured value of vat pressure is below the predetermined threshold value and the linear load and the respective predetermined threshold value corresponds to each other (506), increase the distance between the press rolls (102, 104) to reduce the difference to be below the predetermined level.

32. A system according to claim 17, wherein said control parameters may include pressure of the pulp distribution device and/or the throttle gap width.

33. An apparatus for washing and dewatering pulp comprising two rotatable press rolls (102, 104) having a permeable outer surface (106, 108), and a vat (114, 116, 118) in which the press rolls (102, 104) are installed, the press rolls (102, 104) defining a press nip (112) between them, in which press nip (112) the pulp is pressed, and the apparatus is arranged to feed the pulp in the direction of rotation of the press rolls through the press nip (112), and the processing of the pulp in the apparatus is determined by a set of variable operating parameters which are variable during operation, and the apparatus comprises a system for controlling the apparatus, characterized in that the system comprises the features which are mentioned in claim 17.

34. A computer program product comprising instructions which, when a program is executed by a computer causes the computer to carry out a method according to claim 1.

35. Computer-implemented method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which:

[0069] FIG. 1 is a schematic view of an embodiment of the apparatus for washing and dewatering pulp provided with an embodiment of the system for controlling it according to present invention,

[0070] FIG. 2 is a schematic view illustrating a part of the embodiment of the apparatus according to the present invention,

[0071] FIG. 3a is a schematic flow schedule illustrating aspects of the method according to the present invention,

[0072] FIG. 3b is a schematic flow schedule illustrating further aspects of the method according to the present invention,

[0073] FIG. 3c is a schematic flow schedule illustrating further aspects of the method according to the present invention,

[0074] FIG. 4 is a schematic flow schedule illustrating further aspects of the method according to the present invention,

[0075] FIG. 5 is a schematic flow schedule illustrating further aspects of the method according to the present invention, and

[0076] FIG. 6 is a simplified and schematic diagram shown an example of how control parameters may vary over time depending on adjustment of operating parameters under certain operating conditions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0077] FIG. 1 schematically shows an embodiment of the apparatus for washing and dewatering cellulose-containing pulp according to the present invention provided with an embodiment of the system according to the present invention schematically illustrated with blocks. Also in WO 2014/109693, an apparatus for washing and/or dewatering of pulp in which the present invention can be used is described, which hereby is incorporated with reference.

[0078] The apparatus comprises a first rotatable press roll 102 and a second rotatable press roll 104, each press roll 102, 104 having a permeable outer surface 106, 108 which is perforated, i.e. provided with apertures, whereby the outer surface 106, 108 is permeable to filtrate pressed out of the pulp. The shape of the apertures is normally circular, but any shape is possible. The press rolls 102, 104 comprise a number of filtrate channels 110 radially inwards of the outer surface 106, 108 to lead evacuated filtrate away. The two press rolls 102, 104 defines a press/roll nip 112 between them, in which press nip 112 the pulp is pressed, and are arranged to rotate in opposite directions, the left press roll 102 being arranged to rotate in counter clockwise direction and the right press roll 104 being arranged to rotate in clockwise direction. The apparatus is arranged to feed the pulp in the direction of rotation of the press rolls 102, 104 through the press nip 112. The axes of rotation of the press rolls 102, 104 being in substantially the same horizontal plane, and the apparatus is arranged to feed the pulp through the press nip 112 in a substantially vertical direction from below upwards. One of the press roll 102, 104, herein the first press roll 102, is laterally movable in relation to the other press roll 104 to vary the press nip 112 under the operation of the apparatus. In other embodiments of the press, both press rolls 102, 104 are laterally movable to vary the press nip 112 under the operation of the apparatus. The processing of the pulp in the apparatus is determined by a set of variable operating parameters which are variable during operation.

[0079] The apparatus comprises a casing which includes a vat 114, 116, 118 in which the press rolls 102, 104 are installed, the vat 114, 116, 118 partly enclosing the outer surface 106, 108 of each press roll 102, 104, whereby a gap 124, 125 for each press roll 102, 104 is defined, limited by the vat 114, 116, 118 and the outer surface 106, 108 of the respective press roll 102, 104. The vat 114, 116, 118 can be pressurized and comprises a first side vat segment 114 which partly encloses the outer surface 106 of the first press roll 102, and a second side vat segment 116 which partly encloses the outer surface 108 of the second press roll 104.

[0080] Said casing comprises a first casing member 115 which extends between the ends of the first press roll 102 and to which the first side vat segment 114 is mounted, and a second casing member 117 which extends between the ends of the second press roll 104 and to which the second side vat segment 116 is mounted. The first casing members 115 is pivotable about a first axle 121 and the second casing members 117 is pivotable about a second axle 123, whereby the casing members 115, 117, together with their side vat segments 114, 116, are movable between a closed position and an opened position for providing access to the press rolls 102, 104. Further, the vat 114, 116, 118 comprises a central vat segment 118 partly enclosing the outer surface 106, 108 of the press rolls 102, 104 between the press nip 112 and the side vat segments 114, 116.

[0081] The apparatus comprises a first pulp distribution device 126 for distributing pulp on the first press roll 102 and a second pulp distribution device 128 for distributing pulp on the second press roll 104. The pulp distribution devices 126, 128 are arranged to distribute pulp to the gap 124 along the whole length of each press roll 102, 104. The pulp distributed on the outer surface 106, 108 of the press roll 102, 104 forms a mat on the press rolls 102, 104. Herein, the pulp distribution device 126, 128 is in the form of a pulp distribution screw. However, other kinds of pulp distribution devices are possible. The pulp distribution device 126, 128 can also comprise several separate pulp distribution means distributed one after the other along the longitudinal extension of the press roll 102, 104. Each pulp distribution device 126, 128 is connectable to a pulp supplying system via connection means 140, 142, and the supplying system supplies pulp to each pulp distribution device 126, 128.

[0082] The apparatus also includes a pulp transport screw (not shown), which for example can be in the form of a pulp disintegrating screw or a shredder screw, towards which the pulp which has been pressed in the press nip 112 is conveyed. The pulp transport screw extends in parallel to the longitudinal axes of the press rolls 102, 104, and is arranged to disintegrate the pulp and transport the pulp axially away from the press for further processing.

[0083] The system of the present invention is adapted to control the apparatus by controlling at least one variable operating parameter which is variable during operation and determine the processing of the pulp in the apparatus. According to preferred embodiments of the present invention, the operating parameters may include: [0084] the rotation speed of the press rolls 102, 104, which is used in the first control loop;
In certain embodiments, the control of the apparatus is based on the first control loop. Further, the operating parameters may also include: [0085] the distance between the press rolls 102, 104, which corresponds to the cross-section of the press-nip 112, which is used in the second control loop;
In embodiments where the first and second control loop is run in synchronism, both operating parameters are hence used. In addition, further variable operating parameters can also be included such as: [0086] pressure in the pulp distribution device 126, 128. [0087] Throttle gap width, which refers to the opening gap of a throttle arranged close to or in the outlet of the pulp distribution device. This is, for example, described in the international patent application WO 2014109693.

[0088] The system may comprise an operator unit 160 for setting at least one threshold value for a specific control parameter, the specific control parameter being included in a group of control parameters. The unit 160 is only schematically illustrated in FIG. 1 and may include a input means 162 via which the control parameters is set, and display means 164 for displaying the set control parameter. In embodiments, the setting means may be an operator unit, a stationary computer, a laptop, a mobile unit or any other device communicating with the system and enabling an operator to setting and adjusting threshold values.

[0089] According to preferred embodiments of the present invention, the following control parameters are included in said group: [0090] the torque of the press rolls 102, 104r; [0091] the linear load which acts on the pulp in the press nip 112 and is affected by the torque of the press rolls 102, 104; and/or [0092] the vat pressure, i.e. the pressure within the vat, (and/or pressure at inlet of vat).
Further control parameters can also be included in the group, such as: [0093] outlet consistency; [0094] the flow rate of the pulp into the pulp distribution device 126, 128; and [0095] the ratio between the rotation speed of the press roll 102, 104 and the flow rate of the pulp into the pulp distribution device 126, 128.
The system includes measuring means for measuring the specific control parameter, and the measuring means include means 168 for measuring the outlet pulp consistency, means 170 for measuring the vat pressure, means 172 for measuring the distance between the press rolls 102, 104, means 174 for measuring the torque of the press rolls 102, 104, means 175 for measuring the linear load, means 178 for measuring the rotation speed of the press roll 102, 104, means 180 for measuring the flow rate of the pulp into the pulp distribution device 126, 128, and means for 182 for measuring or calculating the above-mentioned ratio. Above-mentioned means for measuring are in the form of suitable sensors or equipment. The means 170 for measuring the vat pressure can be installed at various places in the vat, for example at lowest region/-s in the vat to measure the vat pressure there. The linear load can for example be measured by providing the means 175 for measuring the linear load in connection to the bearings of the press rolls 102, 104, and determine the linear load from the measured load on the bearings.

[0096] The system includes adjustment means for adjusting one or several of the variable operating parameters of the set during operation. The adjustment means include means 214 for adjusting the rotation speed of the press rolls 102, 104 and means for adjusting the distance between the press rolls 102, 104, which means include hydraulic driving devices 202 for urging the laterally movable press roll 104 away from and towards the other press roll 102, and for providing and keeping a specific distance between the press rolls 102, 104 (see FIG. 2). However, other means for adjusting the distance between the press rolls 102, 104 are possible, such as means based on electro-mechanics.

[0097] The adjustment means can be adapted to adjust only one variable operating parameter, or be adapted to adjust two or more variable operating parameters. When adjusting two or more variable operating parameters, the adjustment means can be adapted to adjust several operating parameters simultaneously, or adapted to adjust one operating parameter at a time, and thereafter another.

[0098] The adjustment means, the measuring means, the setting means and the display means are connected to a controller or control device 166 adapted to control or instruct the adjustment means and adapted to retrieve data from the measuring means and the setting means. The control device 166 comprises a processor 167 and storing means 169 for storing data. The control device 166 includes means 182 for performing calculations or an analysing device, for example, said ratio from the measurement of the means 180 for measuring the flow rate of the pulp from the pulp distribution device 126, 128 and the means 178 for measuring the rotation speed of the press roll 102, 104.

[0099] FIG. 2 schematically illustrates an embodiment of the apparatus in which the present invention can be implemented, showing the pivotally mounting of the press rolls at the first end of the apparatus. The shaft 204 of the first press roll 102 and the shaft 206 of the second laterally movable press roll 104 are supported by bearings housed in bearing housings 208, 210. The bearing housing 208 of the first press roll 102 is fixedly attached to the casing 212 of the apparatus, and the bearing housing 210 of the laterally movable press roll 104 is movably attached to the casing 212, whereby the second press roll 104 is movable in relation to casing 212 and laterally movable in relation the other press roll 102. The hydraulic driving device 202 is positioned between the shafts 204, 206 of the press rolls 102, 104 and is connected to the bearing housing 208, 210 of each press roll 102, 104. A corresponding hydraulic driving device is mounted at the second end of the apparatus. The hydraulic driving devices 202 are adapted to urge the laterally movable press roll 104 away from the other press roll 102 and towards the other press roll 102 for providing a specific distance between the press rolls 102, 104 and for keeping this distance. The means 174 for measuring the linear load is in the form of a load cell 201 connected to the hydraulic driving device 202.

[0100] FIG. 3a-3c illustrates aspects of the method according to the present invention by way of schematic flow schedules, which methods are applicable to the apparatus shown in FIG. 1 and FIG. 2, for example, but can also be applied to other apparatus for washing and dewatering cellulose-containing pulp comprising fixed rotatable press rolls (the first control loop shown in FIG. 3a), and apparatuses comprising two movable press rolls.

[0101] In FIG. 3a, the first control loop is generally shown. First, rotation speed of the press rolls is included as variable operating parameter, at 301. The torque and linear load and/or vat pressure is included as control parameters, at 302, and an operator may set a threshold value, e.g. a maximum value for each of them, at 303. During operation of the apparatus, the specific control parameters are measured, at 304. At 305, measured values of control parameters and respective threshold value are compared to identify deviations between them. In practice, the measured value and the respective set threshold value is compared to identify whether the measured value is below or above the respective threshold value (taking into account any tolerance interval surrounding the threshold value). If no, i.e. if there is a deviation between the measured value and respective threshold value differs, one or several variable operating parameters of said set is/are adjusted according to a first set of control rules, at 306, to bring or align the control parameter closer to the respective threshold values for that control parameter. If yes, i.e. if the measured values and respective threshold values corresponds to each other, the process returns to step 304 and the operating parameters are maintained at present values or levels. During the running of the process, a user can manually adapt the setting of the control parameters.

[0102] In FIG. 3b, an embodiment of the first and second control loops operating in synchronism is generally shown. At 310, the distance between the press rolls and the rotation speed of the press rolls are included as operating parameters. At 311, the vat pressure and linear load are included as control parameters. At 312 operator sets a threshold value, e.g. a maximum value for each of the control parameters. During operation of the apparatus, the specific control parameters are measured, at 313. At 314, measured values of control parameters and respective threshold value are compared to identify deviations. In practice, the measured value and the respective set threshold value is compared to identify whether the measured value is below or above the respective threshold value (taking into account any tolerance interval surrounding the threshold value). If no, i.e. if there is a deviation between the measured value and respective threshold value differs, one or several variable operating parameters of said set is/are adjusted according to first and second set of control rules, at 315, to bring or align the control parameter closer to the respective threshold values for that control parameter. If yes, i.e. if the measured values and respective threshold values corresponds to each other, the process returns to step 313 and the operating parameters are maintained at present values or levels. During the running of the process, a user can manually adapt the setting of the control parameters.

[0103] In FIG. 3c, a further embodiment of the first and second control loops operating in synchronism is generally shown. At 320, the distance between the press rolls and the rotation speed of the press rolls are included as operating parameters. At 321, the vat pressure, linear load and torque are included as control parameters. At 322 operator sets a threshold value, e.g. a maximum value for each of the control parameters. During operation of the apparatus, the specific control parameters are measured, at 323. At 324, measured values of control parameters and respective threshold value are compared to identify deviations. In practice, the measured value and the respective set threshold value is compared to identify whether the measured value is below or above the respective threshold value (taking into account any tolerance interval surrounding the threshold value). If no, i.e. if there is a deviation between the measured value and respective threshold value differs, one or several variable operating parameters of said set is/are adjusted according to first and second set of control rules, at 325, to bring or align the control parameter closer to the respective threshold values for that control parameter. If yes, i.e. if the measured values and respective threshold values corresponds to each other, the process returns to step 323 and the operating parameters are maintained at present values or levels. During the running of the process, a user can manually adapt the setting of the control parameters.

[0104] Below, a number of exemplary control process rules (which however not are exhaustive) are described: [0105] According to the first set of control rules, if a measured value of a control parameter is at a respective threshold value (i.e. the values corresponds to each other or a difference between the values is acceptable), at 406, the rotation speed of the press roll 102, 104 is maintained, at 407, at present level. [0106] According to the first set of control rules, if a measured value of a control parameter is at respective threshold value for all control parameters (the values corresponds to each other or a difference between the respective values is acceptable), at 406, 503, the distance between the press rolls 102, 104 and the rotation speed of the press roll 102, 104 are maintained, at 407, 504, at present level. [0107] According to the first and second set of control rules, If a measured value of vat pressure and linear load is at a respective threshold value (the values corresponds to each other or a difference is acceptable), at 406, 503, the distance between the press rolls 102, 104 and the rotation speed of the press roll 102, 104 are maintained, at 407, 504, at a present level. [0108] According to the first set of control rules, if a measured value of a control parameter differs from a respective threshold value (the values deviates from each other or a difference is not acceptable) and the measured value exceeds the threshold value, at 404, the rotation speed of the press roll 102, 104 is increased, at 405, to reduce the difference. [0109] According to the first set of control rules, if a measured value of a control parameter differs or deviates from a respective threshold value and the measured values are below the respective threshold values, at 408, the rotation speed of the press roll 102, 104 is reduced, at 409, to reduce the difference. [0110] According to the second set of control rules, If a measured value of vat pressure or linear load differs or deviates from a respective threshold value, at 505, 506, 508, the distance between the press rolls 102, 104 is adjusted, at 507, 509, to reduce the difference. [0111] According to the second set of control rules, if a measured value of linear load differs from the threshold value and the vat pressure is at the threshold value (the values corresponds to each other or a difference between measure value and threshold value is acceptable), at 508, the distance between the press rolls 102, 104 is decreased, at 509, to reduce the difference between the linear load and the threshold value. [0112] According to the second set of control rules, if a measured value of vat pressure differs from the threshold value and the linear load is at the threshold value (the values deviates from each other or a difference between the measured value and threshold value is acceptable), at 506, the distance between the press rolls 102, 104 is increased, at 507, to reduce the difference between vat pressure and the threshold value

[0113] With reference now to FIGS. 4-6, further embodiments of the present invention will be discussed.

[0114] In FIG. 4, the first control loop for controlling the rotation speed of the press rolls based on torque, linear load and vat pressure is shown. In embodiments of the present invention, the control loop for the rotation speed of the press rolls is based on at least one or a combination of any one of torque, linear load and vat pressure.

[0115] In FIG. 5, the second control loop for controlling the distance between the press rolls based on linear load and vat pressure is shown. According to the present invention, the control loops for the rotation speed of the press rolls and the distance between the press rolls are combined and thereby the washing efficiency, capacity and stability of operation of the apparatus can be optimized. However, it is possible to operate the two control loops independently of each other. For example, in certain embodiments, only the first control loop based on rotation speed is run. This embodiment is for example used in system where the press nip is not adjustable.

[0116] In FIG. 6, a simplified and schematic example operating scenario with the two control loops combined is illustrated.

[0117] Turning first to FIG. 4, the maximum values for the control parameters linear load, torque and vat pressure are set, at 401, and the current values for the respective control parameters are measured at 402. If one of the measured values exceeds the maximum value or a predetermined highest level in relation to the maximum value (for example, the measured value may be allowed to be within a range about the maximum value), at 403, the rotation speed will be increased at 404. At the same time, as shown in FIG. 5, the distance between the rolls will be adjusted only if it is the linear load or the vat pressure that deviates from the maximum value, see 505-509. In particular, if the linear load is at the maximum level or within the set tolerance and the vat pressure is not, at step 505 and 506, the distance between the press rolls is increased, at 507. On the other hand, if the vat pressure is at the maximum level or within the set tolerance and the linear load is not, at step 505 and 508, the distance between the press rolls is decreased, at 509.

[0118] Returning ow to FIG. 4, if one of the control parameters is at the maximum level, but not above, at step 406, the rotation speed of the press rolls is maintained at 407. At the same time, in the control loop for the distance between the rolls, the distance between the press rolls will be adjusted if the linear load or the vat pressure is not at their respective maximum value as shown at 505-509, as described above. If all threshold values are below their maximum values, the rotation speed of the press rolls will be reduced at 405. At the same time, the distance between the press rolls will be increased or decreased depending on whether the linear load deviates from its maximum value, at 506, 507, or the vat pressure deviates from its maximum value, at 508, 509.

[0119] This is also illustrated in FIG. 6, a simplified and schematic example where values of the control parameters, vat pressure, torque and linear load, are shown and how adjustments of the operating parameters, distance between the press rolls and the rotation speed of the press rolls, affects the control parameters over time.

[0120] Even though the present invention has been described above using exemplifying embodiments thereof, alterations, modifications, and combinations thereof, as understood by those skilled in the art, may be made without departing from the scope of the invention as defined in the accompanying claims.