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INDUSTRIAL REFINING PROCESS VERSUS THEORY
Finally while loading the refiner the frequency of power measurements must be high enough (an oscilloscope is required) to accurately determine the no-load power as described by the figure below.
After only 2 minutes of refining (14 to 21°SR or 730 to 580 CSF) the measurement of the no-load power went from 12.6 to 12.3 KW, which may seem very small. This difference is already big enough to create some false conclusions. Actually, this is the selected result from several measurements of the virgin pulp and after 2 minutes of refining with the same pulp grade.
The incertitude is +/- 0.5 KW on the virgin pulp. The incertitude gap is lower with hardwood or with refined softwood. The no-load power is usually measured at the loading time that is the same as the virgin pulp.
For a well determined pulp composition, refined through a given refiner with determined plate patterns under the same conditions of flow and pressure, the no-load power only depends on the dynamic pulp viscosity. In the case of industrial refiners in series, the no-load power is increasingly lower from the first to the last refiner under the same state of plate wearing provided that the above conditions are confirmed. In case of a pilot refiner working in hydra cycle mode, the no-load power decreases in relation to the refining time.
When the effective power (and not the applied power) is perfectly controlled and maintained constant throughout each trial, the net specific energy can be accurately calculated versus the refining time.
Now the question is: under all these prerequisites, could we rely on the results given from each trial? Yes, as long as we do not compare trials to each other. To compare refining trials under different effective powers or with different plate patterns, the formulation of physics or mathematical models is again required. These models can only be built up from correct parameters that have a true physical meaning. It has been proven   that the classical concept of specific edge load SEL must be proscribed. The true physical concepts such as the reference specific edge load SEL0 and g* must be carried.
From the fundamental expression:
we can establish:
in which the factors λ , δ and γ >1 are the parameters to be determined from the refining trials for a well determined pulp composition with λ (0) = 0.
If the Canadian Standard Freeness is preferred to the Shopper-Riegler, the conversion formula will be then applied:
where u and v stand for the specific parameters of the pulp grade and the white water.
To reach an acceptable accuracy, at least three trials should be carried out under different effective powers that must be properly selected to cover a wide range of reference specific edge loads. From the relation (5) the fastest freeness development (CSF or °SR) can be determined for a well determined pulp composition being refined through a well determined refiner with well determined plate patterns.
Actually, from the relation (5) under a well determined level of net specific energy Ej we can write:
that gives :
and consequently, the reference specific edge load to be applied to each industrial refiner in series to get minimal energy consumption is given by the relation:
in which Ej is the net cumulated specific energy (KWH/T)
From the relation (6), it follows that the function d (Ej) is continuously increasing which means that the reference specific edge load must imperatively decrease with the freeness development. In hydra cycle mode, the refiner must be progressively unloaded versus the refining time. With industrial refiners in series the reference specific edge load must imperatively decrease from the first to the last refiner according to the law 1/ d (Ej). Unless the function d (Ej) is well know, any attempt to decrease the reference specific edge load from the first to the last refiner could bring some disappointments. Furthermore, this law only works if the refiners are proven to work perfectly and are properly sized versus the flow, which is unfortunately not very common at industrial level.
It is easy to understand that during the refining process the fibres must first be delaminated before any fibrillation and hydration treatment. The required shearing actions to delaminate the fibres greatly depend on the wood species and the morphological parameters of the fibres. Once delaminated, the fibres become more fragile and consequently should be treated increasingly gentler, to avoid a harsh cutting effect and extra energy consumption. This way of operating will not only save energy but will also develop better strength characteristics.
Nowadays, environmental protection and energy saving are new challenges to deal with. The above described method is quite simple to put into practice, provided that the study is carried out with great care and rigor such as described in this chapter. With a good pilot refiner, the function ( ) j d E can be accurately calculated and transposed to any industrial refining unit, after the refiners have been properly diagnosed and the characteristics of the white water have been taken into consideration. In so doing, the financial aspects are also very important.
Lower energy consumption (50% and sometimes much more), less expensive chemical additives, better stability, higher runnability and less maintenance are usually the results from this technique, called "bijective diagram technique".
Let us consider the case of bleach kraft softwood from southern European pines, prepared in clear water at 20°C and refined under optimal hydraulic conditions, through correct disc refiners with pseudo-sectorial plate configurations characterized by a grinding code 3-3-4 and g* =24o.
At the beginning of the refining process (the first refiner in the line), the pulp must be refined under 1.63 Ws/m to reach an optimal development of the delamination process without damaging the fibres. When the freeness reaches 25°S R (about 500 CSF) the fibres are more fragile and the reference specific edge load must be reduced to 1.4 Ws/m (second refiner in the line for example). Under 40 °SR or 310 CSF the fibres are still slightly more fragile, and as a result the reference specific edge load must be decreased again to now reach 1.3 Ws/m (third refiner in the line, for example). The delamination process should be completely achieved (possibly in the second refiner in the line), so that the effect of the pulp dynamic viscosity should predominate from which gap reduction arises under the same effective power. To maintain the same gap or, better still, to optimize the gap versus the fiber fragility to prevent a harsh cutting effect while maintaining a fast freeness development, the reference specific edge load must be controlled in keeping with the function 1/ d (Ej).
It is highly recommended to avoid using hit or miss techniques to approach the above function , unless the refining unit has been properly diagnosed and clear key points about the pulp composition have been investigated. The above characteristic greatly depends on the pulp temperature, pulp consistency and the state of white water .
3. INCONSISTENCIES AND ABERRATIONS
The purpose of this chapter is to prove how easy it is to obtain inconsistencies or to produce contradictions, simply from lack of rigor. Consider a pilot single disc refiner 12" that is supposed to work perfectly with correct plate patterns under optimal hydraulic conditions. Assume that the laboratory staff are wonderfully efficient and reliable, and work under strict and accurate procedures. As a consequence, the freeness, weighted fibre length and hand sheet paper characteristics from each pulp sample are correct. Finally, assume that the sensors deliver values without any relative error. Under these dreamlike conditions, who could suspect that something wrong could happen in the line? And yet, as mentioned above, the measurement of the no-load power cannot be perfect.
Consider 35 g/l softwood pulp grade consistency being refined through the refiner. With pseudo-sectorial configuration plates (grinding code 3-3-4 °SR 9 and g* = 24°), the no-load power from this 12" refiner (1500 RPM) is 13.2 KW. This value has been determined accurately under the conditions depicted in the above chapter. With a perfect oscilloscope, without considering the problems of turbulence, flocculation and refiner gap control, the 20 no-load power measurements (repeated from 20 refining trials with the same pulp grade, T°c, flow and consistency ) range from 13.5 to 14.5 KW as depicted in the figure below.
At industrial level with a 22" double-disc refiner, the no-load power measured on one refiner under the same above mentioned conditions through an oscilloscopic method integrated in the DIATRONIC (control and auto-diagnostic system produced by MATECH-EUROPE) for one week is given by the diagram below.
We can notice that accurate measurements on a refining pilot unit or on an industrial refiner show the same dispersion, but not entirely for the same reasons. Actually, the flow at the inlet of a suitable pilot refiner is laminar, so the power measurement is very sensitive to the rate of flocculation of the pulp. At industrial level, the dynamic pulp viscosity is not as stable from the variation of the white water characteristics. Much higher fluctuations can even arise when the floating rotor does not stick to its central position.
Coming back to the pilot refining unit, the four refining trials under 32 KW, 24 KW, 20 KW and finally 18 KW (applied or total power) are carried out. The relevant effective powers are given in the table here below.
Also take into consideration the no-load power drifting versus the dynamic
pulp viscosity, which means that the above mentioned effective powers are decreasing unless under control that is never implemented with classical pilot refining units.
The following table depicts what would happen in terms of effective powers and their relevant relative errors if, for example, 12 kg of dry pulp is refined for 20 minutes and a pulp sample is taken every 4 minutes.
Note that the decreasing law of the no-load power is not the same for each trial but for simplification, the values have been averaged. In practice, the refining times for each trial are calculated to get similar distributions of specific energy that gives birth to conditioned matrixes. In so doing, we reach maximum accuracy.
Coming back to the example, this simplification can be dealt with due to the fact that the no-load power is classically always measured at the beginning of each refining trial. At this step, it is possible to have an error of 27% under an applied power of 18 KW. The lower the applied power, the higher the relative error. Some laboratories did carry out similar trials under 16 KW to study low SEL refining process, from which a relative error of 47% is quite realistic. Other laboratories are working with smaller single disc refiners ( 9 ") that can easily deliver erroneous results (more than 50% of relative error only from the no-load power considerations).
The determination of the relevant specific edge loads is also affected by the error from the no -load powers. The table below gives the reference specific edge loads for each trial.
The lack of homogeneity of any pilot refining unit even under sophisticated controls must be also taken into consideration. This is a second source of error usually ignored by most paper laboratories. How can we control the gradient of freeness in the tank? How can we manage to get the freeness to drop linearly from the top to the bottom of the tank at any moment during the trial?
To approach this situation, the velocity of the impeller and the recirculation flow should be controlled versus the pulp dynamic viscosity. In practice, a perfect homogeneity cannot be obtained. Without the above mentioned controls, the pulp sample could be very far from its expected representation. Under optimal control, a drifting error of +/-5% is already a reliable situation. Furthermore, the virgin pulp will not easily mix with the "one pass refined pulp" and the mixing process cannot start before the "one pass refined pulp" comes back to the tank. Actually, this phenomenon cannot be observed because the pulp is already running through the refining unit during the loading operation.
To get a minimal drifting process, the volume of pulp running through the main pipe should be very low compared to the effective volume of the pulp in the tank. Some paper laboratories use very small tanks, which shortens the refining trial time but it also worsens the problem of lack of homogeneity. The freeness development appears to be slower than it should be and the drift can reach 10% (under optimal conditions) during the first run, after which homogeneity is achieved. To solve this problem, implementation of sophisticated controls and corrections through a reliable attached software (first order models) is a prerequisite. The diagram here after depicts a classical situation of the CSF development (blue characteristic) compared to the true development (red characteristic) after correction from the attached software.
Now, we can go on with the determination of relative errors relevant to the calculation of the levels of net specific energy for each pulp sample from our four trials. One must also presume the freeness and the slowness measured by our wonderful laboratory staff as being completely reliable.
The range has been calculated from the above information taking into consideration the initial drift of 10% under the first level of energy E1. At first glance, the errors do not seem to be high enough to bring about confusion. Let us examine the classical fundamental characteristics °SR(SEL O, E) and CSF(SELO, E) plotted here after from the reliable values and analyzed by the attached software called FIBROLOGIC partly described here above. Let us consider both characteristics.
From the diagrams, we can see that the reference specific edge load must decrease from 2.1 to 1.7 Ws/m within the range [13 ; 40] °SR or [750 ; 320] CSF. Now, if we enter into the attached FIBROLOGIC program, the lowest level of specific energy obtained from inaccurate no-load power measurement and lack of corrections from the heterogeneity of the pilot refining unit, we get the diagrams here below:
Although the freeness and slowness characteristics have been corrected by the relation (5), the first contradiction appears. The refiners must be loaded from 1.2 to 1.4 Ws/m during the refining process, which is completely absurd. However, note that only the errors from the no-load power and the lack of homogeneity in the refining pilot were taken into consideration. In practice, there are plenty of further sources of error. If we consider the characteristics before being corrected through the relation (5), the results are in complete contradiction with the physical reality of the refining process as illustrated by the next figure.
The working points move leftwards (lower SELO) and downwards (lower E). Further, the lower the reference specific edge load, the greater the drift of the working points. As a consequence, the °SR appears to be overestimated (or CSF underestimated). This also means that some asymptotic limit conditions could not be met. For example:
If one takes into consideration the drift of the no-load power, the above wrong shaped characteristic °SR (SEL O) could even turn its maximums into minimums, as shown in the figure below, which is nonsense.
A dreamlike pilot refining unit under the care of a wonderful laboratory staff is not the only one prerequisite to avoid contradictions and inconsistencies. Further important parameters govern the correct achievement of a refining study. Namely, it is imperative that the refining pilot unit be controlled by the effective power throughout each trial and that the results are diagnosed through ad hoc attached software. This will link the required corrections dictated by the sampling operations to the lack of perfect homogeneity of any pilot refining unit. To compare plate patterns, the operation is still more complex because some specific parameters far beyond the frame of this paper are also involved in the process.
At an industrial level, any attempt to compare refining units or to analyze results can also easily lead to misunderstandings and contradictions. Actually, we estimate that 70% of industrial refiners are not working properly and these extra parameters (floating rotor, film mat breaking, partial plugging, bad hydraulic conditions, compacting, soaping, parallelism, cavitations, balancing and so on) must also be considered.
It is easy to understand why most papermakers consider the refining process as a mysterious topic. In general, most theories or optimization techniques are not trusted. This is probably the main reason why most industrial refining units are far from optimal. Stock preparation is really the weak point in most paper mills, and unfortunately paper is produced under high levels of energy consumption with plenty of expensive chemical additives to compensate the poor fibre development from the refining process. It is high time that this philosophy is changed to not only preserve our environmental conditions of life but to also obtain a fruitful financial feedback. As we all know, protection of the environment costs a lot of money. How extraordinary would it be to propose saving the planet and money at the same time, by simply spending some time and thought on the stock preparation.
When a pilot refining unit is accurately controlled and the results are analyzed and corrected through adequate attached software and when an industrial refining unit is also controlled under the same conditions, reliable and consistent results are obtained. From this, it is possible to optimize the stock preparation unit and to anticipate most problems. In so doing, it appears that the pulp composition is constantly under physical developments of its dynamic viscosity and fibre morphology that are modifying the pumping characteristic of the refiner, its gap clearance, its shearing actions on the fibres and the film mat stability.
This means that without adequate control against the state of the pulp composition, a huge potential of characteristic development is simply lost and energy consumption can consequently rises very sharply. From 102 industrial diagnostics (that represents 482 refiners) carried out in papermills all over the world, the loss is on average $12 to $25 US per ton of paper produced. Under some circumstances, with the help of a DIATRONIC, the savings could exceed $50 US per ton of paper produced.
Many thanks to Doctor Denis Curtil from the French Paper School (University of Grenoble), for his fruitful collaboration in the conception of the DIATRONIC and the control of the pilot refining units.
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