Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a method of controlling the
manufacture of mechanical pulp in a refining process where
cellulose-containing ma-terial in lumps, such as wood chips, is
refined. The chips prior to the refining can be treated with heat
and/or chemicals for manufacturing TMP (thermomechanical pulp) or
CTMP (chemi-thermomechanical pulp)~ The refining is carried out
in one or several steps by single- or double-disc reflners. These
refiners are provided with opposed refiner discs rotating relative
to one another. The discs are provided with disc segments compris-
ing bars and intermediate grooves. Opposed disc segments form a
gap where the material is refined during its passage outward.
The properties of the manufactured pulp are influenced,besides by the quality of the wood chips, by a great number of
system parameters. Among these can be mentioned the distance be-
tween the disc segments (gap), the load on the motor driving a
rotary refiner disc, the pressure by which the refiner discs are
pressed in a direction toward each other, the pressure at the feed-
in of the chips, the pressure in the housing enclosing the discs,
the supply of diluting water, the material flow through the refiner
(the production), the material concentration. Some of these
parameters are dependent on each other while other are substantial-
ly independent. For example, the motor load increases and so does
the pressure by which the discs are pressed toward each other when
the gap decreases.
It is impossible in practice to check and control all
parameters influencing the properties of the pulp. It was found,
however, that a desired pulp quality can be achieved with accept-
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571
ably high precision by controlling some especially important para-
meters, viz. the gap size, the material concentration and the
production.
A great problem is that the measuring of the system
parameters does not yield a direct measure of the pulp properties.
For being able to determine the properties of the pulp, such as
tensile strength, tearing resistance, dewatering capacity, shives
content, fibre length etc., it is, of course, necessary to analyze
the pulp and the paper made thereof. In a mill it takes normally
several hours to obtain the results of such analysis, and sampling
usually is carried out not more than 2-3 times per day. It is,
therefore, impossible to rapidly discover and compensate for such
variations in the pulp properties which are due to system para-
meters, which have not been determined, or where there is no simple
relation between the system parameter and the pulp properties.
One factor causing the relation between the measured
system parameters and pulp properties to change in operation is
the wear of the refiner disc segments. This implies that certain
pulp properties can deteriorate although the measured system para-
meters remain unchanged. This implies in practice, that the systemparameters must be adjusted on the basis of analysis results of a
pulp, which had been manufactured several hours earlier. This is,
of course, a great disadvantage.
The delay in obtaining the analysis results involves
substantial disadvantages also in connection with the testing of
and comparison between different refiner disc segments. It is
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desired, therefore, to be able during the refining process to
measure such system parameters, which render it possible to
predict the pulp properties with greater accuracy than it has been
heretofore possible.
The present invention offers a solution oE this problem.
The invention provides that the vibra-tions arising in the refiner
discs during the refining are utilized for calculating the pulp
properties.
According to the present invention there is provided a
method for controlling the properties of a mechanical pulp
produced in a refiner process comprising passing a cellulose-
containing material through a gap between a pair of opposed
reEiner discs having bars thereon and rotating relative to each
other, measuring the vibrations in at least one of the refiner
discs, calculating the total vibration energy within a
predetermined requency range associated with the at least one
refiner disc based upon the measurement of the vibrations, and
utilizing the calculated total vibration energy in combination
with at least one oE the following three process variables: rate
of production of the mechanical pulp, size of the gap, and
concentration oE the cellulose-containing material for controlling
the properties of the mechanical pulp produced in the refiner
process, the predetermined frequency range encompassing
frequencies associated with all bars on the refiner discs.
The process may be carried out in a single-disc refiner
and the vibrations may be measured in a disc segment located on a
stationary disc in the refiner.
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Conveniently the vibration energy may be utilized for
determining the condition of beating surfaces of the disc
segments.
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Eurthermore the vibration energy may be utilized for
evaluation and comparison between di:Eferent disc segment designs.
The invention is described in greater detail in the
following, with reference to embodiments and test results shown in
the accompanying drawing, in which
Figure 1 shows a frequency analysis of the measured
vibrations,
Figures 2 and 3 show the relationship between measured
and calculated tensile strength without and, respectively, with
utilization of the vibrations in the refiner discs.
One property important for the quality of pulp is its
tensile strength. This applies especially to mechanical pulp in-
tended for papermaking.
By controlling and adjusting the three system parameters
gap size, material concentration and production, it is possible
with precision to maintain a desired pulp quality. Experiments car-
ried out on mill scale, however, have shown that the pulp quality
deteriorates with time due to wear of the refiner discs, without
the possibility of predicting this by control of the aforesaid
system parameters.
By measuring the high-frequency vibrations arising in the
refiner discs due to their relative rotation and their segment
design, it is possible to calculate the vibration energy over the
refiner disc segment. The frequency depending on the rotation speed
of the discs and the design of the disc segments can amount to
several thousands c.p.s. The measuring is carried out by means of
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an accelerometer attached to the disc, preferably to the rear side
of a segment. In a single-disc refiner the accelerometer is
attached on the stationary disc. It is also possible to attach
accelerometers to both discs in a single- or double-disc refiner,
in order to ohtain additional information on the vibrations of the
discs.
By including the vibration energy thus measured in the
calculation of the pulp properties, it was found, surprisingly,
that these properties can be predicted with higher precision. This
applies especially to the tensile strength properties of the pulp.
It was found possible, thus, to predict the reduction in tensile
strength caused by wear of the disc segments.
Simultaneously the vibration~energy also can be utilized
for determining the condition of the processing surfaces of the
segments. The vibration energy, furthermore, can be utilized for
comparing the efEiciency of disc segments of different types.
Example
In a single-disc refiner an accelerometer was mounted in
a hole drilled in the rear side of a disc segment in the stationary
disc. The segments were designed with three zones comprising bars
and grooves of different size.
The refining was carried out with pre-heated chips for
the manufacture of TMP. The system parameters and pulp properties
at two test runs were as follows:
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Test ITest II
Production (ton bone-dry pulp~24 h) 70 80
Material concentration ~%) 48 48
Gap size (mm) 0.46 0.38
Tensile index (Nm/g) 33 33
CSF (ml) 18~ 150
Tear index (mN m2/g) 8.5 7.5
Specific energy (kWh/ton pulp) 2150 2075
The signal from the accelerometer was simultaneously
measured and analysed~ The frequency range in question was 5 - 25
kc/s. In Figure 1 a frequency analysis of this signal is shown.
The signal can be divided into three different areas corresponding
to the three zones of the segments. In the inner zone comprising
the coarsest bars the frequencies 5.6 - 11.2 kc/s were noted. In
the central zone 11.2 - 17.6 kc/s, and in the outer zone comprising
the finest bars 17.6 - 25 kc/s were noted. The vibration energy
is represented by the surface beneath the frequency curve in Fig-
ure 1.
After 800 operation hours new measurements of the system
parameters and pulp properties were carried out. It was then
found, that most of the measured pulp properties agreed well with
the pulp properties which were calculated by means of measured
system parameters and results from previous tests. One exemption
was the tensiIe strength, of which the measured values were lower
than the calculated ones.
In Figure 2 the measured tensile index is shown as
a function of the tensile index which was calculated by means of
:
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measured values of production, gap siæe and material concentration.
It shows that there is a heavy systematic error. The fully drawn
line designates full agreement, and the dashed lines designate an
acceptable error range.
By including in the calculation of the pulp properties
the vibration energy obtained from the accelerometer signal, all
measured pulp properties could be predicted with high precision.
In Figure 3 the measured tensile index is shown as a function of
the calculated tensile index where the vibration energy has been
utilized together with the adjusted production, gap size and mate-
rial concentration. The agreement there lies within the error
range. No systematic errors could be stated.
The deterioration in the tensile strength of the pulp
can be explained by the wear of the disc segments. Heretofore it
has not been possible to find a controllable relation between the
tensile strength and the wear of the segments. The present inven-
tion, thus, offers such a control possibility. By measuring the
vibration energy according to the invention, the condition of the
disc segments can be determined, which also can be utilized for
determining the time when the segments have to be exchanged. The
invention can also be used for comparing different segment patterns
and materials.
The invention, of course, is not restricted to the em-
bodiments described, but can be varied within the scope of the
invention concept.
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