Method for optimizing the refining energy during an operation of refining of a fiber composition

Abstract

The present invention concerns a method of optimization of the refining energy supplied by a refiner to a fiber composition during a refining operation, the refiner comprising at least two refining disks separated from each other by an adjustable gap. The invention also relates to a refining system adapted to the implementation of such a method.

Claims

1. A method of optimization of the refining energy supplied by a refiner to a fiber composition during a refining operation, the refiner comprising at least two refining disks separated from each other by an adjustable gap, said method comprising the following steps: a) setting an initial refining energy set point, b) measuring a vibration of the refiner, to obtain a corresponding vibration signal which depends on the gap, c) comparing at least one characteristic of the vibration signal with a determined maximum value and a determined minimum value so as to: c1) if the characteristic of the vibration signal is lower than the maximum value and higher than the minimum value, resume the method from step b), c2) if the characteristic of the vibration signal is higher than or equal to the maximum value, automatically decrease the initial refining energy set point down to a lower set point value, and automatically increase the gap so that the refining energy tends towards the lower set point value, and c3) if the characteristic of the vibration signal is lower than or equal to the minimum value, automatically increase the initial refining energy set point up to a higher set point value, and automatically decrease the gap so that the refining energy tends towards the higher set point value, wherein the lower set point of step c2) or the higher set point of step c3) is kept constant for a time interval of at least 5 seconds, whatever the vibration measured during said time interval, and wherein the characteristic of the vibration signal comprises an acceleration of the refiner.

2. A method according to claim 1, wherein the method is repeated at least once from step b) after the carrying out of step c2) or of step c3), the initial refining energy set point being replaced with the lower or higher set point, respectively.

3. A method according to claim 1, wherein the acceleration is measured by calculating an average in real time of a parameterizable number of acceleration values measured within a time interval in the range from 0.5 second to 5 seconds.

4. A method according to claim 3, wherein the parameterizable number of acceleration values is in the range from 10 to 500.

5. A method according to claim 1, wherein: the method is repeated at least once from step b) after the carrying out of step c2) or of step c3), the initial refining energy set point being replaced with the lower or upper set point, respectively; the characteristic of the vibration signal comprises an acceleration of the refiner; the acceleration is measured by calculating an average in real time of a parameterizable number of acceleration values measured within a time interval in the range from 0.5 second to 5 seconds; the parameterizable number of acceleration values is in the range from 10 to 500; the lower set point of step c2) or the higher set point of step c3) is kept constant for a time interval of at least 5 seconds, whatever the vibration measured during said time interval.

6. A method according to claim 1, wherein the lower set point of step c2) or the higher set point of step c3) is kept constant for a time interval of at least 10 seconds.

7. A method according to claim 1, wherein the lower set point of step c2) or the higher set point of step c3) is kept constant for a time interval of at least 20 seconds.

8. A method according to claim 1, wherein the acceleration is measured by calculating an average in real time of a parameterizable number of acceleration values measured within a time interval in the range from 1 second to 3 seconds.

9. A method according to claim 3, wherein the parameterizable number of acceleration values is in the range from 50 to 300.

10. A method according to claim 3, wherein the parameterizable number of acceleration values is in the range from 100 to 300.

11. A refining system for the optimization of the refining energy supplied by said refining system to a fiber composition during a refining operation, comprising: a refiner provided with at least two refining disks separated from each other by an adjustable gap, a vibration sensor configured to measure a vibration of the refiner, and to output a corresponding vibration signal which depends on the gap, a control system configured to receive the vibration signal of the vibration sensor, to compare at least one characteristic of the vibration signal with a determined maximum value or minimum value, and to control the refiner, according to the method of claim 1, wherein the vibration sensor comprises an accelerometer or a microphone, and the characteristic of the vibration signal comprises an acceleration of the refiner measured by said accelerometer or said microphone, wherein the control system is configured to keep constant the lower set point of step c2) or the higher set point of step c3) for a time interval of at least 5 seconds, whatever the vibration measured during said time interval.

12. A refining system according to claim 11, wherein the control system is configured to measure the acceleration by calculating the average in real time of a parameterizable number of acceleration values measured by the accelerometer or the microphone within a time interval in the range from 0.5 second to 5 seconds.

13. A refining system according to claim 12, wherein the parameterizable number of acceleration values is in the range from 10 to 500.

14. A refining system according to claim 11, wherein the control system is configured to keep constant the lower set point of step c2) or the higher set point of step c3) for a time interval of at least 10 seconds.

15. A refining system according to claim 11, wherein the control system is configured to keep constant the lower set point of step c2) or the higher set point of step c3) for a time interval of at least 20 seconds.

16. A refining system according to claim 11, wherein the control system is configured to measure the acceleration by calculating the average in real time of a parameterizable number of acceleration values measured by the accelerometer or the microphone within a time interval in the range from 1 second to 3 seconds.

17. A refining system according to claim 12, wherein the parameterizable number of acceleration values is in the range from 50 to 300.

18. A refining system according to claim 12, wherein the parameterizable number of acceleration values is in the range from 100 to 300.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics of the invention will occur upon reading of the following description given as an illustrative and non-limiting example, in relation with the following accompanying drawings:

(2) FIG. 1 is a graph according to the state of the art, which illustrates the variation of the specific energy and of the gap of a pair of disks of a refiner according to the number of refining cycles.

(3) FIG. 2 shows a flowchart which illustrates the different steps of the method of optimization of the refining energy according to the invention.

(4) FIG. 3 shows a signal of measurement of the acceleration of the refiner during a refining operation.

(5) FIG. 4 shows a plurality of graphs A to F according to the invention, which illustrate the variation of the refining energy according to the number of refining cycles, for a plurality of operations of refining of different fibrous compositions.

DETAILED DESCRIPTION

(6) The invention concerns a method of optimization of the refining energy supplied by a refiner to a fiber composition during a refining operation, as well as a refining system adapted to the implementation of such a method.

(7) The refining system comprises a refiner.

(8) The refiner is provided with at least two refining disks separated from each other by an adjustable gap, This means that the disks are mobile with respect to each other, and may be drawn towards or away from each other. Generally, in a pair of disks, a single one of the disks may be mobile (rotor) while the other remains fixed (stator). The center of each disk is on the same axis as the shaft of the refiner. One of the disks is generally located on an opening part of the refiner i.e. a door, which allows an easy rearranging or changing of the disks.

(9) It is possible to provide several pairs of disks, arranged in series or in parallel, according to the desired type of refining. Those skilled in the art will know how to optimize the number of disks and their arrangement in order to obtain the desired refining performance.

(10) The refining system also comprises at least one vibration sensor, configured to measure a vibration of the refiner, and to output a vibration signal of the refiner.

(11) For this purpose, one or a plurality of vibration sensors may be used, to make the vibration measurement more accurate. The sensor(s) may be arranged at different locations of the refiner, for example, on the refiner body and/or on the refiner motor. According to a particular embodiment, the refiner does not comprise any vibration sensor on the refining discs.

(12) According to another particular embodiment, the refiner does not comprise any vibration sensor on the refiner door. In general, a sensor on the refiner door allows detecting vibrations that are parallel to the refiner shaft. According to this embodiment, the vibration sensor is not along the shaft.

(13) According to a preferred embodiment, the refiner comprises a vibration sensor on its main body. The vibration sensor is preferably located such as it allows detecting vibrations that are perpendicular to the shaft of the refiner.

(14) Preferably, at least two vibration sensors are used, a first sensor of which is positioned on the refiner body and a second sensor of which is positioned on the refiner motor.

(15) According to a preferred embodiment, the vibration sensor comprises an accelerometer (or a microphone), configured to measure an acceleration of the refiner and output an electric signal characteristic of said acceleration of the refiner.

(16) Alternatively, the vibration sensor may comprise a microphone, configured to measure a sound vibration resulting from the vibration of the refiner, and to output an electric signal characteristic of said sound vibration.

(17) The refining system also comprises a control system, configured to control at least one of the elements of the refiner, including at least one of the refining disks.

(18) The control system is configured to receive as an input the vibration signal of the vibration sensor, to compare at least one characteristic of the vibration signal with a determined maximum value or minimum value, and to control one at least of the disks to accordingly modify the gap as explained in further detail in the rest of the present text.

(19) The refining system preferably comprises at least one calculation device, configured to calculate a refining energy, that is, a specific energy. The calculation device is connected to a plurality of sensors, from which it receives the data used for the calculation of the specific energy, for example, the flow rate of the fibrous composition, the rotation speed of the motor and of the disks, or also the gap.

(20) The calculation system may calculate the specific energy in real time, that is, continuously, or discretely at determined times.

(21) The calculation device may be integrated to the control system, or distinct therefrom.

(22) The refining system also comprises a memory where data relative to the operation of the refining system can be stored. These data particularly comprise one or a plurality of specific energy set points. They may be recorded by the manufacturer at the manufacturing of the system and/or by an operator before or during a refining operation.

(23) The memory may be integrated to the control system, or distinct therefrom.

(24) The method of optimization of the refining energy according to the invention will now be described in reference with FIGS. 2, 3, and 4. This method is based on the implementation of the refining system which has just been described.

(25) It is started by starting the refining system, which is fed with a flow of fibrous composition.

(26) Preferably, the refining energy is measured from as soon as the beginning of the refining and all along the refining operation. This measurement is performed by the calculation device.

(27) The flow of fibrous composition as well as the rotation speed of the motor and of the disks of the refiner are preferably kept constant all along the refining operation. Apart from obvious convenience and industrial constraint reasons, this enables to vary a small number of parameters, and thus to better control the variation of the specific energy over time.

(28) At a step a), an initial refining energy (or initial specific energy) set point, noted Cs1, is set.

(29) A step b) comprises measuring a vibration of the refiner, to obtain a corresponding vibration signal. The vibration signal depends on the gap, in that it is all the stronger as the disks are close to each other. In practice, this measurement variation is performed by the vibration sensor, which receives as an input the refiner vibrations, and outputs a corresponding vibration signal. The vibration signal is then transmitted to the control system.

(30) The vibration signal can then be processed by a system for processing the signal provided for this purpose. The signal for example is filtered over a given frequency interval.

(31) The control system receives as an input the vibration signal. At a step c), it compares at least one characteristic Cq of said vibration signal with a determined maximum value V.sub.max or minimum value V.sub.min.

(32) In the flowchart of FIG. 2, these conditions are noted: Cq V.sub.max? and CqV.sub.min?.

(33) In practice, the maximum and minimum values V.sub.max and V.sub.min depend on the settings of the refining system. They are thus empirically determined by the operator during different refining trials, and recorded by the latter in the memory of the refining system. The operator can modify them before or during a refining operation.

(34) At the end of the comparison, if the characteristic Cq of the vibration signal is lower than maximum value V.sub.max, or higher than minimum value V.sub.min, then the previous condition is not fulfilled (N). The method is then repeated from measurement step b). This alternative is called c1) or c3) in the flowchart of FIG. 2.

(35) Conversely, if the characteristic Cq of the vibration signal is higher than or equal to maximum value V.sub.max, or lower than or equal to minimum value V.sub.min, then the previous condition is fulfilled (O).

(36) Condition CqV.sub.max fulfilled means that the refiner has started resonating and that the disks are very close to each other. The control system then automatically decreases the specific energy set point Cs1 down to a lower set point value Cs2. For the specific energy to tend to the lower set point value and then to stabilize around this value, the control system automatically controls the disks to draw them apart, thus increasing gap Ef. Thereby, the refiner leaves the resonance state. This alternative is called c2) in the flowchart of FIG. 2.

(37) Incidentally, steps c) and c2) enable to avoid for the refiner disks to collide, and this, whatever the energy set point in force. This implies that it is possible to select a very high initial energy set point to maximize the efficiency of the refining, without fearing an incident caused by a collision of the disks.

(38) Condition CqV.sub.min fulfilled means that the vibration is very low, which suggests that the disks are too distant from each other to provide the energy sufficient to efficiently refine the fibers. The control system then automatically increases the specific energy set point Cs1 up to a higher set point value Cs2. For the specific energy to tend to the higher set point value and then to stabilize around this value, the control system automatically controls the disks to bring them closer to each other, thus decreasing gap Ef. This alternative is called c4) in the flowchart of FIG. 2.

(39) According to a preferred embodiment, the characteristic Cq of the vibration signal comprises an acceleration of the refiner. Such an acceleration is usually expressed in g, where 1g is equivalent to approximately 9.82 m/s.sup.2. Then, comparison c) as well as alternatives c2) and c4) are carried out with an acceleration value, by comparison of said acceleration value with maximum and minimum acceleration values V.sub.max and V.sub.min.

(40) The acceleration used may be a rms. value of the amplitude of the vibration signal filtered over a determined frequency range. The determined frequency range may for example be in the range from 4 kHz to 10 KHz.

(41) The acceleration is preferably measured by calculating the average in real time of a parameterizable number of acceleration values measured within a given time interval. It is then spoken of a mobile or sliding average. This means that the average is permanently modified by the continuous taking into account of new acceleration values and the rejection of old acceleration values, which are used for its calculation.

(42) The time interval within which the acceleration average is calculated is advantageously in the range from 0.5 second to 5 seconds, preferably from 1 second to 3 seconds.

(43) Further, the parameterizable number of acceleration values is advantageously in the range from 10 to 500, preferably from 50 to 300, more preferably from 100 to 300.

(44) At the end of step c2) or of step c4), the method is preferably repeated at least once from step b) to optimize the specific energy all along the refining operation. In this case, the new specific energy set point Cs2 is likely to be automatically modified into a lower or higher set point Cs3, according to the result of comparison step c), and so on along the successive iterations of the method.

(45) Preferably, the lower or higher set point Cs2 resulting from step c2) or from step c4), respectively, is kept constant for a time interval of at least 5 seconds, preferably for at least 10 seconds, more preferably for at least 20 seconds (advantageously less than 60 minutes), whatever the vibration measured during said time interval. This enables to properly stabilize the system energy at the new set point value, and to avoid any energy loss.

(46) Optionally, step c) (that is, sub-steps c2) and c4) may be followed by a step d) according to which the measurement device performs a measurement of the average length of the fibers of the fibrous composition. It may be a number, weight, or length average, according to what suits the operator.

(47) If the average length L of the fibers is greater than a minimum length Lmin, the refining continues. If, on the contrary, said average length of the fibers is smaller than or equal to minimum length Lmin, the refining is stopped.

(48) FIG. 3 illustrates a graph showing a vibration signal measured by the sensor and transmitted by the latter to the control system. The characteristic of the vibration signal which is measured and then compared here is an acceleration, noted Ac (expressed in g), which is plotted in ordinates.

(49) The left-hand signal (I) relates to a first reactor, for which a maximum acceleration value is set to 5 g (1 g=9.80665 m/s.sup.2). The calculation device determines in real time the average of the acceleration based on the amplitude of the signal. When this average is greater than or equal to maximum value 5 g, the control system lowers the set point value down to a new lower set point value (for example, 4.5 g), and sends a control order to the disks to draw them away from each other, thus increasing the gap, and then decreasing the specific energy. The control system adjusts the gap so that the specific energy tends towards, or even reaches, the new lower set point value. The specific energy is thus controlled in top-down fashion.

(50) The right-hand signal (2) relates to a second reactor, for which a maximum acceleration value is set to 3 g. Similarly to the left-hand reactor, when the measured average acceleration is higher than or equal to maximum value 3 g, the control system lowers the set point value down to a new lower set point value, and controls the disks to increase the gap.

(51) It is also possible to provide for value 3 g to represent a minimum acceleration value for the left-hand reactor (1), independently from or in combination with maximum value 5 g. In this case, when the measured average acceleration is smaller than or equal to minimum value 3 g, the control system increases the set point value to a new higher set point value (for example, 3.5 g) and sends a control order to the disks to bring them closer to each other, thus decreasing the gap, and then increasing the specific energy. The control system adjusts the gap so that the specific energy tends to, or even reaches, the new higher set point value. The specific energy is thus controlled in bottom-up fashion.

(52) Respectively, it is of course possible to provide for value 5 g to represent a maximum acceleration value for the right-hand reactor (2), independently from or in combination with minimum value 3 g. The operating principle is the same as the foregoing.

(53) When both a minimum value 3 g and a maximum value 5 g are set, the measured average acceleration is compared with each of these two limiting values, and the specific energy is increased or decreased according to its value. The specific energy is thus controlled from in top-down and in bottom-up fashion.

(54) FIG. 4 shows 8 graphs bearing references (A), (B), (C), (D), (E), (F), (G), and (H), which illustrate the variation of specific energy E according to the number of passes Np in the refiner, for different fibrous compositions.

(55) What is important to notice is that these graphs all have a relatively similar profile, that is, a staged energy decrease, along the refining of the composition. However, the number of stages, their duration, and their respective energy differs from one graph to the other, and thus from one fibrous composition to the other.

(56) For example, graph (A) comprises 8 stages, to be compared with 7 stages only for graph (B). Similarly, graph (G) comprises 9 stages, to be compared with 7 stages only for graph (H).

(57) Further, and still as an example, the 23-kWh/t stage of graph (A) lasts for approximately 1 pass, to be compared with approximately 3 passes for that of graph (B). Similarly, the 17-kWh/t stage of graph (G) lasts for approximately 1 pass, to be compared with approximately 5 passes for that of graph (H). The 17-kWh/t stage is, besides, the last stage of graph (H) since the fibers have then reached their minimum size, conversely to graph (G) which further comprises the 14-kWh/t and then 11-kWh/t stages, necessary to complete the refining.

(58) This can be explained as follows. The compositions of graphs (A) to (H) differ by their physico-chemical properties, particularly by the nature of the fibers, their relative quantity in the composition, their length distribution, or also by the consistency of the composition, and by the presence and the nature of fillers incorporated in the composition.

(59) These property differences have an impact on the rheology of the compositions, that is, on their flow properties according to the strain applied during the refining.

(60) Further, the more the refining advances, the more the fibers are shortened and sheared, so that the previous properties are modified, as well as the rheology.

(61) Accordingly, the variation of the specific energy measured over time differs from one composition to the other, thus resulting in a regulation of said energy by the control system according to the invention, itself also different. In practice, the variation profile of the gap during the refining varies according to the composition to be refined.

(62) The method of the invention takes into account the variation of the physico-chemical properties of the composition, and automatically adjusts, accordingly, the specific energy at closest to the optimal energy.

(63) Thus, the method of the invention automatically adjusts and optimizes the specific energy according to the physico-chemical properties of the composition to be refined, by using the vibration of the refiner as a measurement element.