NEW REFINER SEGMENTS TECHNOLOGY TO OPTIMIZE
FIBER QUALITY AND ENERGY CONSUMPTION OF REFINER MECHANICAL PULP

Author

Petteri Vuorio1 and Peter Bergquist2. Presented by Anders Hawén

Company and address

1 Metso Paper Service, Fiber Applications, PO.Box 125, FIN-37601 Valkeakoski, Finland
2 Metso Paper Service, Fiber Applications, Vretenvägen 6, SE-17154 Solna, Sweden

email

petteri.vuorio@metso.com

Keywords

low energy segments, refining, fibre quality, high pressure, chip refining, mechanical pulp

ABSTRACT

Metso Paper has focussed significant efforts on developing solutions for reducing energy consumption in refiner mechanical pulp production. One very successful tool to reduce energy consumption on the mill scale has been the development of low energy segments (LE™). Many mills today use this segment technology as standard practice. Low energy segments are also used to change the fiber length distribution of the pulp. With this segment technology, it is easy to reduce long fiber and shive content. Many mechanical pulp producers find a positive quality development when the coarse fiber fraction is refined very aggressively.

It is possible to save measurable amounts of the main refiner line energy consumption using low energy segment technology with very small impact on pulp quality. When striving for higher energy savings however, it is difficult to maintain the fiber length. Low energy segment development has now progressed to the point where pulps with fiber characteristics similar to those obtained with standard segments can be produced. The tool for achieving this has been to use high refining steam pressure in the primary stage refiner. This prevents fiber length reduction under the severe refining conditions which prevail when using aggressive low energy segments.

 A new segment innovation for optimising the defibration zone at the inlet of the chip refiner (DefiMax™), makes it possible to change fiber length distribution in the opposite direction to that achieved with aggressive low energy segments. With these new defibration segments, milder defibration of the chips takes place and the resulting pulp contains longer and stronger fibers.

This paper presents practical trial results with low energy segments from both the KCL pilot refiner and from mill scale refiners. The concept of employing higher refining pressures in conjunction with the use of low energy segments is presented for both pilot scale and mill application. Results from mill scale trials with the new innovative segments (DefiMax™) are also presented. These examples include information on all major refiner types to define the present status of this rapidly developing segment technology.

INTRODUCTION

During the last two decades refining processes and the pulp produced with the refiners has been studied very intensively. However in many presented papers the refiner itself has been treated like a macro scale black box. In these studies, the effect of the major operating or input parameters such as refining consistency, refining pressure and production rate have been studied with different refining processes. The refiner mechanical pulps have also been categorized by the type of refiner used to produce them and in many cases it has been possible to differentiate between double disc, single flat disc and conical disc pulps. At present, new refining processes employing high operating pressures and high refiner rotational speed are being compared to conventional refiner operation (1-2).

During recent years, intensive research concerning the TMP process has provided a better understanding on the flow phenomena occurring inside the refiner. A new theoretical model for a TMP-refiner has provided a view not only of the steam flow within the refiner but also of the pulp flow and energy consumption dissipation inside the refiner (3). This detailed information has been lacking from earlier theories which have been commonly used to describe the refining intensity (4). With a more precise knowledge on the mechanism prevailing in the disc gap, practical optimization of the refining process has been more and more focussed on phenomena occurring within the refiner and even within different sections of the disc gap.

The aim of this paper is to present the theories which are currently used in refiner segment development and to provide some examples from practical segment trials. The objective is to show examples from different refiners and to indicate how well the same segment design principles can be used with these different refiner types. This paper focuses on the development of primary stage refiner segments and the effect of  these on  pulp quality and specific energy consumption after the main line refiners. The refiner types studied in this paper include single flat disc, double disc and conical disc refiners. This paper presents two different approaches for optimizing the first stage refiner by segment design. In the first case, energy consumption in the refiners is reduced by decreasing the fiber residence time in the disc gap. As an example, the idea of employing higher operating temperatures together with shorter residence times is introduced. Refining temperature is not studied in depth in the paper and the selection of the higher pressure level and pressure differential over the refiner in the mill scale is only provided as an example. In the second case the defibration from chips to fibers is changed by increasing the residence time of the fibers in the inlet section of the refiner and by changing the segment geometry from the traditional to a new innovative design.

THEORETICAL BACKGROUND

Theory and measurements concerning the residence time of the fiber in different sections of the disc gap clearly indicate that there is strong turbulence in the pulp flow prior to entering the refining zone. There is back flow of both fiber and steam in the refiner inlet and the pulp is rotating in a vortex motion. When feeding radioactive fiber samples into the disc gap from different radial positions, it was found that the residence time of the pulp is high in the inlet zone of the refiner but when the pulp enters the refining zone, it is transported very quickly through the refining gap into the housing. Theoretical calculations have made it possible to provide numerical values for the flow phenomena. In the published examples shown in figure 1, the volume fraction of the fiber is almost 60 % of the open volume in the section prior to the refining zone and compressive forces exerted on the fiber pad are very high in this area. These flow phenomena lead to the situation that a large portion of the refining power is transferred to the flow in the disc gap at the entrance to the refining zone. Simulation results for the volume fraction of the fiber and the power dissipation are presented in Figure 1 (3).

Figure 1a

Figure 1a

Figure 1b

Figure 1b

Figure 1.  Volume fraction of the fiber and power dissipation (kW) in the disc gap of the single disc refiner (3).

Quantitative visualisation of the disc gap of a production scale SD65 refiner using a multipulse stroboscope, endoscope optics, a specialized illustration system and a CCD camera provided measurements for the fiber flow in the stator side of the disc gap. These measurements support the theory presented above and indicate back flow of the fibers at all radial measurement points with the exception of the one located at the outermost measurement position beyond the peak pressure point. The fiber velocity was low at the inlet zone of the refiner and this zone was also full of pulp (5).

In our concept to save energy with low energy segments, the segment bars form a pumping angle at this very critical area in the disc gap. This type of geometry feeds pulp faster into the refining zone, thereby reducing the turbulence at the inlet section of the refiner and thus reducing the specific refining energy. The basic principle of these segments is presented in Figure 2. The effect of the segment geometry on the residence time in the disc gap has been measured with both standard and low energy segments (6). These measurements clearly show that with the latter, the fiber residence time in the disc gap is reduced to half that observed with standard segments and this reduction comes mainly from the inlet zone of the disc gap.

Figure 2

Figure 2.  Basic operating principle of the LETM Segments.

Shorter residence time of the fibers in the refiner in turn leads to narrower disc clearance. In some applications, this clearance might become so small that some fiber damage and fiber length reduction could occur. In some paper and board grades, shorter fiber length and a more severely refined long fiber fraction result in pulp quality improvement. In some grades, fiber length retention is important and in order to achieve this, increased pressure over the refiner resulting in higher temperature refining in conjunction with shorter residence time has been employed. Refiner pressure is a significant variable and has a strong impact on the fibers and the flow phenomena inside the refiner.

Much discussion has taken place recently concerning the effects of higher temperature on the fibers. It seems that at elevated temperatures, fibers do not lose their strength potential as easily as at normal pressure when refining is performed at extremely narrow disc clearances. When discussing refining pressure, it must be remembered that inside the refiner there may well be a high temperature maximum and compared to this, the pressures around the refiner are often lower. Increasing the pressure in the refiner feed increases the temperature in different sections of the refiner, thereby increasing the housing pressure and thus its effect on the fibers. High refining pressure has often led to reduced optical properties of the pulp. The pulp brightness however is always dependent on the time that the fibers are exposed to high temperature. Therefore, when increasing the temperature, the residence time of the fibers must be reduced to maintain the brightness. Thus, when residence time is reduced using low energy segments, the pressure in the refiner can be slightly increased without any brightness degradation.

Refining pressure and pressure differential over the refiner also have a strong influence on the flow phenomena in the disc gap. In the study with different housing and feed pressures, it was shown that higher housing pressure or increased pressure differential across the refiner increases the disc clearance (7). Higher housing pressure exerts a higher closing force on the back side of the rotor leading to reduced disc clearance. However, the flow phenomena can often change in such a way that the opening force exerted by the pressure in the disc gap could exceed the closing force.

New segment innovation in chip refining using DefiMax TM Segments is based on the opposite approach. With a new type of design in the inlet zone, it is possible to increase the fiber residence time in this section and thus change the mechanism of chip defibration. The effect of this type of geometry on the residence time has also been measured in production scale (6). When traditional segment bars located at the inlet of the chip refiner are changed to circular pins, the fiber characteristics are also changed. With these new defibration segments, milder defibration of the chips takes place and the resulting pulp contains longer and stronger fibers. In many cases however, the energy consumption required to achieve a certain freeness level is higher. When fibers are retained for a longer time in the inlet of the refiner, they form a higher volume fraction there and this stabilizes the refiner against disturbances in the chip feed. With these new type of refiner segments, power variations have been smaller in many cases. A basic picture of a DefiMax TM Segment  is presented in Figure 3.

Figure 3aFigure 3b

Figure 3.  Basic pictures of DefiMaxTM Segments.

SHORTER RESIDENCE TIME IN DIFFERENT REFINER PROCESSES AS A TOOL FOR REDUCING ENERGY CONSUMPTION

In CD type refiners, the flat zone is normally operated with wide disc clearance. Compared to single flat disc refiners which normally operate at a disc clearance of 0.5 mm measured by the TDC (True Disc Clearance) sensor, the disc clearance for the flat zone of the CD refiner can be twice this amount. Furthermore, the energy consumption in the conical disc refiners is often somewhat higher than that of the standard flat single disc type refiner. In CD refiners, the low energy segments appear to reduce only that portion of the energy which is normally wasted in turbulence and rotation in the inlet zone of the refiner. There are a number of practical examples where the energy consumption of the CD refiner line has been reduced without any change in pulp quality by using low energy segments in the first stage flat zone. Table 1 shows results from the CD76 refiner line at the NSI Skogn mill at Norway when the flat zone segments of the first stage refiner were changed from standard to low energy segments. The energy consumption and pulp quality data are taken at the same production level using standard and low energy segments. The refining conditions, energy consumption and pulp quality data refer to the same TMP line operating at the same production rate for the standard and low energy segments and the pulp quality measurements were measured by the NSI Skogn mill. The pulp quality profiles are interpolated to the freeness level of 150ml from a large number of data points. The two trial periods were, of course, separated in time, during which small differences in chip quality could have occurred.

Table 1

Similar results to those above have been observed on a pilot plant scale under well controlled conditions. Trials carried out on the RGP44 single flat disc refiner at KCL (Finnish Pulp and Paper Research Institute) have shown that it is possible to decrease the refining energy by 20 % relative to refining with standard segments to a final freeness level of 135 ml. These savings did not have any negative impact on pulp quality with the exception of the tear index which in some cases could be slightly reduced. On a positive note, sheet density and optical properties were slightly improved. In these trials, the pulp was refined twice through the refiner to simulate two stage refining operation. Therefore, this trial would correspond to the case where there are trial segments in both refining stages, reducing the fiber residence time compared to a standard two stage refining line. Results with the low energy segments compared to the standard segments from the KCL trials are presented in Figure 4. In these trials, the pressure in the first stage refiner was 300 kPa with both segment types and in the second stage refining the pressure was 150 kPa in the trials with the low energy segments. For the case of standard refining, the results refer to refining pressures of 100 kPa or 300 kPa in the subsequent stages. However, these small changes in the refining temperature in the subsequent refining stages did not have any noticeable effect on the refining results. Pulp quality was tested at KCL. Results are presented also in a Table 3 and are interpolated to a freeness level of 175ml.

Figure 4

Figure 4aFigure 4b

Figure 4cFigure 4d

Figure 4.  Results from the KCL RGP 44 single disc refiner with LE TM and standard segments. The refining pressure of each stage is mentioned in brackets (8).

With production scale refiners, the task of achieving energy savings by reducing the residence time of the fibers by segment geometry optimization is more difficult. As mentioned earlier, these refiners operate at narrow disc clearances even with standard segments because the production rate in almost all cases is as high as possible. The energy consumption of these refiners is also quite low. When the residence time is then reduced, this leads to narrower disc clearance and in many cases, it is easy to reach a limit where the pulp quality starts to change. This change is not only negative. Generally the fiber length of the pulp is reduced but on the other hand the shive content is also decreased. The tensile and bonding strength do not change but the tear strength is typically lower. This quality change however can lead to slightly improved optical properties of the pulp. There are a number of paper and board grades where this kind of quality improvement would be viewed as positive. Shorter and more severely refined fibers result in good formation of the fiber network and there are no problems with shives or coarse fibers.

The Stora-Enso Fors mill in Sweden uses CTMP (Chemi Thermomechanical Pulp) in the packaging board. The board users typically set very high requirements on the printability of the product and the Fors mill has worked intensively to meet these demands. With low energy segments, it has been possible to change the fiber length distribution. Figure 5 shows the fiber length distribution after the single stage RSA1300 double disc refiner equipped with both types of segments. The energy consumption and pulp quality data from the same trials are presented in Table 2. All the results are averages from the entire refiner segment life time. Pulp quality was tested at the Stora-Enso Fors mill laboratory.

Figure 5

Figure 5.  Fiber length distribution after the single stage RSA1300 double disc refiner equipped with standard and low energy segments.

Table 2

HIGH REFINING TEMPERATURE AND SHORTER RESIDENCE TIME IN THE PRIMARY STAGE REFINER

As indicated earlier, fiber length reduction could be considered to be a negative quality development in some paper grades and the refining pressure should be increased to avoid this. First of all, in the same trial series that was run with low energy segments at KCL, a parallel trial was included using a pressure of 500 kPa in the primary refiner instead of 300 kPa. The subsequent stages were refined using the low energy segments and a refining pressure of 150 kPa but at otherwise unchanged conditions. The higher refining pressure increased the energy consumption back to the level required by the standard segments. The fiber length and strength properties were improved above the level achieved by the standard segments. The brightness of the pulp was reduced by 1.5 points at the higher pressure. Table 3 summarizes the results from this trial. The pulp quality was measured at KCL.

Table 3

From the KCL trials, it is evident that when refining at the same pressure with standard type segments and segments which result in shorter fiber residence time, there is a tendency for the fiber length to be reduced somewhat and for the brightness to be improved. In this example, the pressure of 500 kPa was clearly excessive because the energy consumption increased to the level of the standard segments and brightness was lost. However, at some pressure level between the two investigated, it would be possible to identify an interesting balance between energy savings and pulp quality characteristics.

This idea has already been tested on the SD65 refiner line in the UPM-Kymmene Kajaani mill in Finland. Low energy segments were used in the first refining stage and the refiner was operated at increased pressure. This trial was run under well controlled conditions. The standard segments were about 24 hours old when the samples were taken after which the first stage refiner segments were changed. After 24 hours of operation, additional samples were taken. During this trial time, there was no change in the chip quality. Production level and other operating conditions of the line were kept constant during this period. The second stage refiner was operated at the same specific energy consumption throughout the trial to ensure that the effect of the first stage refiner changes could be seen in the main line pulp quality. In the Kajaani mill, the refining pressure in the primary stage refiner is normally 130 kPa in the inlet and 350 kPa in the refiner housing. During the trial, the inlet pressure was elevated to 250 kPa and the housing pressure was 750 kPa. This high housing pressure was reduced to the normal level of 350 kPa directly after the refiner with the blow valve. In general, higher refining pressure did not cause any problems with line runnability. In Figure 6, power spectra and housing pressure are shown for the primary stage refiner during sampling according to both modes of operation.

Figure 6a

Figure 6b

Figure 6.  Power spectra and housing pressure for the primary stage refiner during sampling according to both modes of operation.

The difficult aspect in this type of short residence time and high pressure refining is that the operating window of the refiner becomes very small. Changes in the primary stage refiner power consumption affect the final line freeness level significantly. When the freeness after the primary stage refiner is high, the energy savings after the line are very modest and when the freeness level is low, the pulp quality is reduced. When the line is run with optimal power split between the first and the second stage, the energy savings on the line amount to about 10 %. Figure 7 shows the relationship between the specific energy consumption and freeness during the trial period. With standard segments, decreasing the freeness after the first stage from 650 ml to 400 ml requires an increase in refiner specific energy from 850 kWh/BDMT to 1050 kWh/BDMT. With high pressure operation, this same freeness change could be achieved by increasing the specific energy from 750 kWh/BDMT to 850 kWh/BDMT. This narrower operating window sets high requirements on line control and stability. The Kajaani mill has succeeded to operate at high pressure and produce stable pulp quality for lengthy periods at a freeness level of 150 ml. This is achieved because the operators have given support to the development and have been able to control the line extremely well. The segment life has been at the normal level and as the segments age, the refining line becomes easier to operate.

Figure 7

Figure 7.  Relationship between the specific energy consumption and freeness during the trial period.

Pulp quality data after the second stage refiner are presented in Figure 8. In these diagrams, the effect of the first stage narrow operating window with high pressure is clearly evident. At the line freeness level of 190 ml, high pressure operation was still producing pulp with good strength potential but when the freeness was at the level of 130ml, pulp strengths were reduced. Pulp quality was tested in the UPM-Kymmene Research laboratory in Valkeakoski.

Figure 8a

Figure 8b

Figure 8c

Figure 8d

Figure 8e

Figure 8f

Figure 8g

Figure 8h

Figure 8.  Pulp quality data after the second stage refiner for standard and high pressure operation.

At a freeness level above 150ml, operating the primary refiner at high pressure resulted in a pulp exhibiting the same fiber length but significantly lower shive content compared to normal operation. There were no large differences in the Bauer McNett classification. The strength values of the high pressure pulp were at least as good as those achieved under standard refining conditions. Both light scattering coefficient and brightness were somewhat lower with the high pressure operation. However, when the pulp was over-refined in the high pressure primary stage refiner, final main line pulp quality was reduced.

Figure 9 shows the quality characteristics of  handsheets produced from the R28 long fiber fraction and Figure 10 the sedimentation volume of the P200 fines fraction of the samples as functions of the final main line freeness. At final freeness levels above 150 ml, similar trends were observed for density with both modes of operation but high pressure refining resulted in improved bonding potential of the long fiber fraction as evidenced by the tensile and tear indices. The quality of the fines fraction however, as measured by the sedimentation volume, appeared to be decreased somewhat by operation at high pressure. Figure 11 shows the specific filtration resistance of the P28/R200 middle fraction of the samples as a function of the main line freeness. Also in this quality measurement, good results can be seen at high freeness levels with high pressure but with low freeness levels this measurement is lower than that measured with standard segments.

Figure 9a

Figure 9b

Figure 9c

Figure 9d

Figure 9.  Quality characteristics of  handsheets produced from the R28 long fiber fraction as a function of the final main line freeness.

Figure 10

Figure 10. Sedimentation volume of the P200 fines fraction as a function of the final main line freeness.

Figure 11

Figure 11. Specific filtration resistance of the P48/R200 middle fraction as a function of the final main line freeness.

CHANGING THE SEGMENT GEOMETRY IN THE DEFIBRATION STAGE

New segment innovation at the inlet of the primary stage refiner was evaluated on the SD65 refiner line at the UPM-Kymmene Jamsa River mill in Finland. In this case, only the center segments of the first stage refiner were changed to this new type. Results from two parallel lines having the same production rate are shown in Table 4. The pulp quality was tested at the mill. The energy consumption of the refining line was increased with the new segments but the strength potential of the pulp was also improved.

Table 4

When a similar change was made in the production scale 65 inch single stage double disc refiner in the Stora-Enso Kvarnsveden mill, the result was even more pronounced. Changing the breaker bars to this new type stabilized the refiner and it was possible to reach a freeness of 75 ml in single stage refining at a production rate of 156 BDMT/day. When comparing the effect of the breaker bar design in the double disc refiner at the more normal freeness level of 150 ml, the effect of the breaker bar change was similar to that observed for the case of the single disc refiner. Table 5 summarizes the trial results from the Kvarnsveden mill. Pulp quality was tested in the mill laboratory and chip quality, production rate, refining consistency and operating pressure levels were maintained constant during the trial period.

Table 5

Generally, it is said that the characteristics of the pulp are established in the primary stage refiner and that it is difficult to modify the fiber length distribution in subsequent refining stages. When defibration is made in such a way that pulps exhibit better strength characteristics and where fibers are less damaged after the primary stage refiner, it is possible to see improvement also in the final pulp quality. Even if the energy consumption is increased in the refining operation, the improved strength potential of the mechanical pulp might result in savings from the papermaking operation by furnish optimization for certain grades of paper.

SUMMARY

Intensive research work on the refining process has resulted in more precise theories for describing steam and pulp flow phenomena within the refiner. By using these theories, it has been possible to identify clear guidelines for segment development based on the analysis of different sections of the disc gap. The LETM and DefiMaxTM segments presented in this paper give logically similar results from flat single disc, double disc and conical disc refiners in different locations. Conical disc refiners operate with wide disc clearance in the flat zone compared to flat disc refiners and the reduction in the fiber residence time in this zone provides energy savings without any reduction in pulp quality. Savings of up to 20% have been achieved from a well-controlled pilot scale refiner line by changing the segments. When trying to achieve such high energy savings from production scale flat disc refining systems, the reduced fiber residence time often leads to such narrow disc clearances that changes can be observed at the fiber level. Generally the fiber length of the pulp is reduced but on the other hand the shive content is also clearly lower. The density and bonding strength characteristics do not change but the tear strength is typically lower. This quality change however can lead to slightly improved optical properties of the finished pulp.

There are a number of paper and board grades where this kind of quality development would be positive. If the reduction in fiber length and consequently in tear strength is considered negative, this quality change can be prevented by increasing the refiner operating pressure. The combination of the short residence time in the refiner achieved by using a combination of low energy segments and higher refiner pressures provides an interesting possibility to achieve energy savings at maintained pulp quality. This type of refining however takes place within a narrower operating window and sets higher requirements on refiner stability and control.

With the new type of segment design in the inlet zone, it is possible to increase the fiber residence time in this section of the refiner and thus modify the chip defibration mechanism. When traditional segment bars are changed at the inlet of the chip refiner to these circular pins, the fiber characteristics are changed to provide a pulp with higher fiber length and better strength properties than is the case with standard segments but in this case, the refining energy is also somewhat higher.

ACKNOWLEDGEMENTS

The authors would like to thank the personnel from KCL as well as from the NSI Skogn, UPM -Kymmene Jamsa River, Stora-Enso Fors, UPM-Kymmene Kajaani and Stora-Enso Kvarnsveden mills for their valuable contributions to this paper. They would also like to thank Mr. Nils Virving of Metso Paper for his creative work with segment development , Mr Lennart Rohden and Mr Urpo Sulander of Metso Paper for their work to tune the segments at the mills, Dr. Esko Härkönen of UPM-Kymmene for detailed discussions on pulp and steam flow phenomena in the refiner disc gap and Mr. Michael Jackson of Michael Jackson Consultants Inc. for his help in preparing the manuscript. 

REFERENCES

1. Höglund H., Bäck R., Falk B., Jackson M.: ThermopulpTM – a new energy efficient ,mechanical pulping  process. International Mechanical Pulping Conference. Ottawa, Canada, 12-15 June 1995, pp. 213-225.

2. Sabourin M. Xu E. Musselman R.: Evaluation of refiner disc speed, plate design and consistency on high  pressure / low residence (RTS)TMP pulps. International Mechanical Pulping Conference. Stockholm, Sweden, 9-13 June 1997, pp. 241-249.

3. Härkönen E., Ruottu S., Ruottu A., Johansson O.: A theoretical model for a TMP-refiner. International  Mechanical Pulping Conference. Stockholm, Sweden, 9-13 June 1997, pp. 95-102.

4. Miles K.B., May W.D.:The flow of pulp in chip refiners. Journal of Pulp and Paper Science, 16(1990):2, 63-71.

5. Alahautala T., Vattulainen J., Hernberg R.: Quantitative visualisation of pulp refining in a production line  refiner. International Mechanical Pulping Conference. Houston, TX, USA, 24-26 May 1999, pp. 87-95.

6. Härkönen E., Huusari E., Raviala P.: Residence time of fiber in a single disc refiner. International Mechanical Pulping Conference. Houston, TX, USA, 24-26 May 1999, pp. 77-86.

7. Jackson,M. and Akerlund, G.: Preheating and refiner housing pressure affects the quality of TMP and CTMP.  Tappi 67(1),1984, p54-58.

8. Nurminen I.: Oy Keskuslaboratorio-Centrallaboratorio Ab, Internal report (1999).

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