WO1991010904A1

WO1991010904A1 - Acoustic emission monitoring of wood chip refiners

Info

Publication number: WO1991010904A1
Application number: PCT/CA1990/000015
Authority: WO
Inventors: Guy Albert Dumont; Adrian Paul Wade; Oliver Lee
Original assignee: Pulp And Paper Research Institute Of Canada
Priority date: 1990-01-11
Filing date: 1990-01-11
Publication date: 1991-07-25

Abstract

By measuring the energy and frequency of acoustic emission from wood chips and fibers being broken down into mechanical pulp between opposing refiner discs, one is able to determine the physical parameters of the refined pulp, observe if rot is present, observe the degree of sulphonation and determine species of wood being refined. The apparatus comprises an acoustical transducer positioned to produce a signal representative of acoustical energy and frequency of wood chips between refiner discs, a filter for filtering out frequencies of the signal below about 25 kHz and a display device to display acoustical energy and frequency from the signal.

Description

ACOUSTIC EMISSION MONITORING OF WOOD CHIP REFINERS

Background of the Invention

The present invention relates to the refining of wood chips into mechanical pulp and more specifically to an improved method of monitoring the refining of wood chips in a refiner.

In the past several years, mechanical pulping has evolved to the point that it now provides an alternative to chemical pulping. The attractiveness of mechanical pulping lies in its reduced production costs, increased yield and reduced environmental impact. Thermomechanical (TMP) and chemithermomechanical (CTMP) pulps have both been developed to the point where the paperroaking properties of the pulp produced are approaching those obtained by the kraft method. At the heart of the mechanical pulping system is the chip refiner that breaks down wood chips into fibers and develops the paper making properties of the fibers by beating them between two grooved discs that rotate relative to each other. High speed photographic studies have shown that wood chips are disintegrated before entering the refining zone_* The released fibers then form networks aligned across the surface of the refiner plate bars. While moving outward along the bars, these networks are beaten by passing bars, and every so often disintegrate to reconstitute elsewhere. The passing bars apply pressure pulses to the fibers, causing fractures in the fiber wall and partial fiber collapse. This increases fiber flexibility and the amount of fine material and so develops the papermaking properties of the fibers. The need for consistent production of pulp with predictable characteristics, together with the increasing size of refiners and the elimination of chemical pulp in newsprint furnishes, imposes the need for strict control of the refining process. One obstacle towards improved control is the current lack of sensors for monitoring the conditions inside the refiner and in particular for assessing the action of the refining discs on the wood chips and fibers between the discs.

Various attempts to monitor chip refiners have been developed. One such method utilizes an accelerometer positioned on a refiner disc which measures the vibrational frequencies of the discs. The signal from the accelerometer is converted to units of vibration energy and is used in conjunction with one or more process measurements to control and adjust the mechanical pulping process. This method is disclosed in PCT publication No. W086/06770, published November 20, 1986. All previous work done in this field monitors frequencies below 25 kHz. These are associated with pressure pulsations due to the groove and bar patterns on the refiner discs or plates. Several variables affect the characteristics of pulp produced by refiners. These include the space or gap between the plates or discs. A variation in this distance has a marked effect on the fibers. Feed rate of the chips and dilution water flow rate to the refiner affect the residence time in the refiner. The rotational speed of the discs is another factor to consider. In some refiners, one disc is stationary and the other disc rotates relative to the first disc. In other refiners the discs may counter-rotate. For different refiners, the plates may vary in pattern, taper and construction material. Other operating parameters include inlet temperature, pressure differential between presteaming vessel and refiner housing, refining pressure and refining temperature. Many of these parameters can be monitored and often the values measured bear no direct relationship to the pulp quality. In the case of CTMP or CMP, the fibers have been softened by a chemical treatment prior to the refining step so as to modify the mechanical properties of the resulting pulp.

There is a requirement to know the wood being processed, whether it be a mixture of wood species or a single wood species. There is also a requirement to identify the presence of rot in the wood. These factors influence the optimum operating conditions for the refiner. In the past, such measurements have not been readily available, and thus the scope for process optimization has been limited. At the present time, samples of pulp from a refiner are monitored off-line in a test laboratory in order to determine pulp quality and consistency. Results from these tests are generally available on an hourly basis. During this time process operating conditions may have changed, and thus process control decisions made on the basis of the prior information are out of date. This is especially so when relatively rapid fluctuations in pulp quality exist between infrequent analyses. The laboratory testing undertaken requires highly skilled personnel and even so there exists considerable variability between results obtained by different testers. Freeness is the principal laboratory test used in control of the process. However, on-line freeness tests have been unsatisfactory and have led to adoption of on-line drainage sensors. In addition further on-line tests such as shive content, consistency and fiber length distribution have been proposed. An improved on-line method for predicting the end pulp quality during the refining process is needed to address the shortcomings of the present off-line empirical laboratory tests and current on-line control systems.

Acoustic emission is the release of ultrasonic energy by a system disturbed from its state of equilibrium and is measured both in terms of energy units (which may be arbitrary units) and in terms of frequencies. Acoustic emission is routinely used for testing composite materials, plastics and metals. Frequencies are generally above 50 kHz, although frequencies below this threshold may be measured.

It is generally found that frequencies below this figure result in extraneous vibrations being included. We have found the acoustic emission from wood chips and fibers is distinct from the vibration of the refiner plates or discs.

Plate and disc vibrations are of lower frequencies than acoustic emissions from chips and fibers. By measuring the emissions from the wood chips and fibers, we have found one is able to have a direct indication of the wood material being processed rather than of the process equipment alone.

Aims of the Invention

It is an aim of the present invention to measure the patterns of the energy and frequency of the acoustic emission from the wood chips and fibers being processed between the discs. It is another aim to utilize the acoustic emission to monitor the refining action on-line and predict pulp quality without significant delay, thus allowing improved control. Yet another aim of the present invention is to utilize acoustic profiles from the wood chips and fibers to identify the wood species being refined. Furthermore, it is an aim of the present invention to utilize the acoustic emission from the wood chips and fibers to assist in predicting on-line pulp quality.

It is a further aim to detect and measure the degree of chemical pretreatment in the chip supply to the refiner using acoustic emission. The acoustic emission may be used to compare a measured emission against a predetermined standard, and then adjust one or more of the process parameters to control the process and produce pulp to the predetermined standard.

Summary of the Invention

The present invention provides in a method of refining wood chips into mechanical pulp, wherein the wood chips are broken down into fibers and the fibers are beaten between opposing refiner discs that rotate relative to each other, the improvement comprising the steps of measuring energy and frequency of acoustic emission from the wood chips and fibers between the discs, and utilizing the measurement of energy and frequency of the acoustic emission to determine the physical parameters of the refined mechanical pulp.

In another embodiment the energy and/or the frequency of the acoustic emission is adjusted by varying at least one process parameter, such as the plate gap, to control the quality of the mechanical pulp. In a further embodiment the measured energy and frequency of the acoustic emission pattern is compared with a predetermined pattern representing required physical parameters of the refined mechanical pulp, and then varying at least one of the process parameters to bring the measured pattern into conformity with the predetermined pattern, and so produce the required pulp quality.

In a still further embodiment there is provided in a method of refining wood chips into mechanical pulp wherein the wood chips are broken down into fibers and the fibers are beaten between opposing refiner discs that rotate relative to each other, the improvement comprising the steps of measuring a pattern of the energy and frequency of acoustic emission of the wood chips and fibers between the discs, and comparing the measured energy and frequency of the acoustic emission pattern with a series of predetermined emission patterns representing different species to determine species of wood chips being refined. In another embodiment, the predetermined emission pattern represents rot in the wood chips, and comparison determines the presence of rot in the wood chips. In yet a further embodiment, the predetermined emission pattern represents chemical treatment of the wood chips prior to refining, and comparison determines whether the chemical treatment of the wood chips is in accordance with the predetermined pattern.

In yet another embodiment there is provided an apparatus for measuring acoustic emission energy and frequency of wood chips and fibers being refined into mechanical pulp between opposing refiner discs that rotate relative to each other, comprising an acoustical transducer positioned to produce a signal representative of acoustical energy and frequency from wood chips and fibers between the discs, means to filter out frequencies from the signal below about 25 kHz, and means to display acoustical energy and frequency of wood chips and fibers from the signal.

One more embodiment provides an apparatus for determining species of wood chips in a feed stream comprising means to deviate a portion of the feed stream containing wood chips from the remaining feed stream, a wood chip refiner for the portion of the feed stream deviated from the remaining feed stream, the refiner having opposing refiner discs that rotate relative to each other, an acoustical transducer positioned to produce a signal representative of acoustic energy and frequency from the wood chips and fibers between the discs, means to filter out frequencies from the signal below about 25kHz, and means to compare the signal against predetermined signals representing different species to determine wood species of the wood chips in the feed stream.

Brief Description of the Drawings

In drawings which illustrate embodiments of the invention.

Figure 1 is a block diagram illustrating the method of monitoring according to the present invention. Figure 2 is a layout of equipment according to one embodiment set up for carrying out the process of the present invention.

Figure 3 is an isometric diagrammatic view showing a transducer mounted on a refiner housing.

Figures 4 and 5 are patterns of the acoustic energy against frequency for white spruce and jack pine achieved from monitoring acoustic emission.

Figure 6 is a pattern depicting the effect of sulphonation on the frequency pattern of the acoustic emission for jack pine.

Figure 7 is a pattern depicting the effect of rot on the frequency pattern of the acoustic emission for white spruce.

Figure 8 is a graph showing the variation of the power spectrum of the acoustic emission of black spruce for different plate gaps.

Figure 9 is a graph showing the acoustic energy for jack pine against different plate gaps, for different feed rates.

Figure 10 is a three dimensional figure depicting freeness as a function of specific energy and acoustic energy for jack pine and lodge pole pine.

Figure 11 is a graph of freeness against acoustic energy for jack pine.

Figure 12 is a graph of fiber length against acoustic energy for jack pine.

Figure 13 is a graph of freeness against energy input for pine.

Detailed Description of the Invention

The refiner used for tests was a refining pilot plant manufactured by Sunds Defibrator Inc. The refiner motor was rated at 100 hp and rotated one of the 30 cm diameter disc plates at 3536 rpm, the other disc plate being stationary. Chips were presteamed in the hopper for 7 to 10 minutes and further heated in a preheater for 10 minutes at 133*C and 157 kPa. The pressure differential between the prestearoer holding tank and refiner housing was set at about 14 kPa. Refining was conducted with a refiner plate temperature of 133 to 137"C while a target pressure of about 171 kPa was maintained. Dilution water was added at a constant rate of 0.025 L/s. In one embodiment a broad band acoustic transducer was attached on the casing behind the fixed refiner plate in a way that provided efficient acoustic coupling. Figure 1 illustrates a block diagram and Figure 2 illustrates one specific agreement. The acoustic emission signals from the sensor are filtered and amplified, the filtering removes frequencies below about 25 kHz and so avoids the vibration frequencies of the plates or discs. In the present invention it is necessary to obtain the acoustic emissions from the wood chips and fibers between the plates but not to measure the vibration of the plates. The A.C. signal is amplified, filtered and displayed either on an oscilloscope or using a digitizer card and microcomputer, both of which digitize signals at high frequencies. In the first case signals may be displayed on an oscilloscope screen or on a video display of an associated computer. In the second case, the microcomputer screen alone is used. Where only amplitude information is required, a chart recorder connected to the D.C. output from the amplifier may be used, and the signal fed to a computer for processing.

Figure 3 illustrates a refiner housing 10 with a stationary refiner plate 12 therein. A transducer 14 supported by a steel clamp 16 is acoustically coupled via an extension piece 18 to a bolt 20 which is in ontact with the stationary refiner plate 12. The transducer was a broadband piezo-electric acoustic transducer (Bruel and Kjaer, type 8312) with a built-in preamplifier. Efficient acoustic coupling at the interfaces between the transducer 14, the extension piece 18 and the bolt 20 was provided by a high vacuum grease silicone lubricant. The transducer 14 was connected to a conditioning amplifier (Brueland Kjaer, type 2316) having selectable gain (0 to 60db in single db increments) and a filter. A digital storage oscilloscope

(Tektronix, model 2430) with 100M samples per second digitizing rate was run at a sampling rate of 5 MHz for wave form digitization and capture. Signals were transferred as 1024 point records with 8 bit resolution to a 12 MHz IBM compatible PC-AT computer via an IEEE-488 interface (National Instruments) for waveform storage and processing. Data transfer routines allowed signa -triggered and time-based signal acquisition. Power spectral densities were obtained by applying a fast Fourier transform to the digital time signal. An average power spectrum was obtained for each data set by averaging the power spectra of individual signals. RMS energy levels were calculated and plotted against plate gap and feed rate.

For purposes of experimentation five wood species were used for pulping. White spruce was obtained from the Prince George region of British Columbia. Hemlock was taken from the Haney region of British Columbia. Black spruce was obtained from the Sherbrooke region of Quebec, jack pine from La Tuque, Quebec, and lodgepole pine from the interior of

British Columbia. No sign of rot was present in any of the samples. In order to study rot, a white spruce sample with rot was obtained from the Caribou region of British Columbia.

All wood samples were chipped to a face size of approximately

2.5 sq cms and placed in cold storage until used.

Typical power spectrums for white spruce and jack pine are shown in Figures 4 and 5. It was found that these patterns apply to these particular wood species. Different patterns were also found for Hemlock, black spruce and lodge pole pine. These patterns are signatures which allow wood species to be identified.

When wood chips are chemically treated, the fibers soften and high-frequency components of the acoustic emission generally drop in intensity. The effect of sulphonation on the power spectrum obtained for jack pine is shown on Figure 6. When compared to non-sulphonated jack pine, the effect of sulphonation is to significantly reduce the signal at frequencies above 200 kHz, and in particular the peak around 250 kHz.

The occurrence of rot implies softer fibers, and the presence of very fine, easily disintegrated material. Thus rot results in weakened high-frequency acoustic emission. The effect of rot on the power spectrum of white spruce is shown in Figure 7. It is seen that rot virtually suppresses all frequencies above 250 kHz. Various refiner operating variables affect the characteristics of the acoustic emission. For instance. Figure 8 shows that varying the plate gap changes the amplitude of the various peaks, without changing the fundamental frequency distribution for that species.

Figure 9 gives the RMS acoustic energy against plate gap in the refiner for jack pine for different feed rates (FR). Like the specific energy, the acoustic energy generally increases with decreasing plate gap. However, as seen on the graph, this is not always the case. Inside the refiner, it is believed that fibers have to form networks to be beaten and cut when impacted by passing bars. As the specific energy increases, more fines are being produced. Because of its lower length-over-diameter ratio, this fine material has a greatly diminished ability to form networks. This material then goes through the refiner without further beating action. This results in a significant reduction in acoustic energy. This theory does not form part of the invention, but the end result is a reduction in acoustic energy.

Figure 10 shows that pulp of 100 to 150 freeness is produced when both specific and acoustic energies are high. However, if for the same high specific energy, the acoustic energy is low, this appears to be indicative of intense cutting having occurred, as demonstrated by the resulting low freeness of about 30. The specific energy only tells how much energy is put into the pulp. Combining it with the use of acoustic energy indicates how this energy is distributed between beating and cutting. Thus, if the acoustic energy and the specific energy are compared to predetermined patterns characteristic of the desired fiber development, the refiner variables can be adjusted on-line to produce the desired pulp quality.

Figures 11 and 12 illustrate the pulp characteristics as compared to the acoustic energy for jack pine. A negative correlation is observed for freeness and fiber length to the calculated acoustic energy. Fiber length correlates well with the acoustic energy, except for the point that corresponds to extensive cutting. This point, that corresponds to high specific energy and low acoustic energy is easily identifiable. On the other hand, and seen on Figure 13, on a plot of freeness vs. specific energy, this point cannot be distinguished from the others. This is because specific energy alone cannot distinguish between beating and cutting.

As well as feed rate and plate gap, residence time within the refiner and relative rotational speed between the discs are other parameters. The results indicate that there is a correlation between the acoustic emissions and the pulp properties. Furthermore, it is seen that these can be controlled by varying process parameters. As illustrated in Figures 4 and 5, definite particular patterns are apparent for different wood species thus enabling operators to be aware of the presence of particular wood species. In some mills rapid variation in species is a major cause of pulp quality variability. As illustrated on Figure 6, the magnitude of acoustic energy in certain frequencies may be used to measure the degree of sulphonation in a CTMP and CMP process. Acoustic emission may also be used to identify rot in the wood raw material during refining. Rot in wood is indicated by the absence of higher acoustic frequencies normally associated with that species as shown in Figure 7.

Various changes may be made to the embodiments, and examples disclosed herein without departing from the scope of the present invention which is limited only by the following claims.

Claims

The embodiments of the present invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of refining wood chips into mechanical pulp wherein the wood chips are broken down into fibers, and the fibers are beaten between opposing refiner discs that rotate relative to each other, the improvement comprising the steps of:

measuring energy and frequency of acoustic emission from the wood chips and fibers between the discs, and

utilizing the measurement of the energy and frequency of the acoustic emission to determine the physical parameters of the refined mechanical pulp.

2. The method according to Claim 1 wherein frequencies below about 25 kHz are filtered out.

3. In a method of refining wood chips into mechanical pulp wherein the wood chips are broken down into fibers, and the fibers are beaten between opposing refiner discs that rotate relative to each other, the improvement comprising the steps of: measuring energy and frequency of acoustic emission from the wood chips and fibers between the discs, and

adjusting the energy and frequency of the acoustic emission by variation of at least one process parameter to control the quality of the mechanical pulp.

4. The method according to Claim 3 wherein the variation of at least one process parameter is selected from the group consisting of: spacing between the opposing refiner discs, absolute rotational speed of the discs, relative rotational speed between the discs, feed rate of wood chips to the opposing refiner discs, residence time for the wood chips and fibers between the discs and dilution water flow to the opposing refiner discs.

5. The method according to Claim 4 wherein the frequencies below about 25 kHz are filtered out.

6. In a method of refining wood chips into mechanical pulp wherein the wood chips are broken down into fibers, and the fibers are beaten between opposing refiner discs that rotate relative to each other, the improvement comprising the steps of:

measuring a pattern of energy and frequency of acoustic emission from the wood chips and fibers between the discs. comparing the measured energy and frequency of the acoustic emission pattern with a predetermined pattern representing required physical parameters of the refined mechanical pulp, and

varying at least one process parameter to adjust the measured pattern into conformity with the predetermined pattern.

7. The method according to Claim 6 wherein the variation of at least one process parameter is selected from the group consisting of spacing between the opposing refiner discs, absolute rotational speed of the discs, relative rotational speed between the discs, feed rate of wood chips to the opposing refiner discs, residence time for the wood chips and fibers between the discs and dilution water flow to the opposing refiner discs.

8. The method according to Claim 7 wherein the frequencies below about 25 kHz are filtered out.

9. In a method of refining wood chips into mechanical pulp wherein the wood chips are broken down into fibers, and the fibers are beaten between opposing refiner discs that rotate relative to each other, the improvement comprising the steps of: measuring a pattern of energy and frequency for acoustic emission from the wood chips and fibers between the discs, and

comparing the measured energy and frequency of the acoustic emission pattern with a series of predetermined emission patterns representing different specieβ to determine species of wood chips being refined.

10. The method according to Claim 9 wherein frequencies below about 25 kHz are filtered out.

11. The method according to Claim 9 wherein the measured energy and frequency of the acoustic emission pattern is further compared with a predetermined pattern representing chemical treatment of wood chips prior to refining, to determine if the chemical treatment of the wood chips is in accordance with the predetermined pattern.

12. The method according to claim 9 wherein the measured energy and frequency of the acoustic emission pattern is further compared with a predetermined pattern representing rot in the wood chips to determine the presence of rot in the wood chipβ.

13. An apparatus for measuring acoustic emission energy and frequency of wood chips and fibers being refined into mechanical pulp between opposing refiner discs that rotate relative to each other, comprising:

an acoustical transducer positioned to produce a signal representative of acoustical energy and frequency from wood chips and fibers between the discs,

means to filter out frequencies from the signal below about 25 kHz, and

means to display acoustical energy and frequency of wood chips and fibers from the signal.

14. The apparatus according to Claim 13 including means to compare the signal against predetermined signals representing different species of wood to determine wood species being refined.

15. The apparatus according to Claim 13 including means to amplify the signal.

16. The apparatus according to Claim 13 wherein one of the refiner discs is stationary and the transducer is attached to the back of the stationary disc.

17. The apparatus according to claim 13 wherein one of the refiner discs is stationary and the transducer is attached on the outside of the casing.

18. The apparatus according to Claim 13 wherein the refiner discs are housed in a casing and the transducer is attached on the outside of the casing.

19. The apparatus according to Claim 13 wherein the display means is a digital display.

20. An apparatus for determining species of wood chips in a feed stream comprising:

means to deviate a portion of the feed stream containing wood chips from the remaining feed stream,

a wood chip refiner for the portion of the feed stream deviated from the remaining feed stream, the refiner having opposing refiner discs that rotate relative to each other,

an acoustical transducer positioned to produce a signal representative of acoustic energy and frequency from the wood chips and fibers between the discs,

means to filter out frequency from the signal below about 25 kHz, and

means to compare the signal against predetermined signals representing different species of wood to determine species of the wood chips in the feed stream.