OPTIFEED - STABLE, AGILE AND INTEGRATED WET-END CONCEPT

Authors

Jussi Ahola 1, Petteri Soini1and Juha S. Kinnunen2

Companies and addresses

1 Metso Paper, P.O. Box 125, 37601 Valkeakoski, Finland
2 Metso Paper, P.O. Box 587, 40101 Jyväskylä, Finland

email

petteri.soini@metso.com

Keywords

wet-end stability, mixing, reduced volumes

ABSTRACT

Increased stock quality and efficiency of the stock preparation process are the main focuses of the OptiFeed concept. A conventional stock preparation system, including approach system, has some major drawbacks, such as uncontrollability of consistencies of separate furnishes and large dead volumes in tanks. Immediate problems with such systems include stability, such as consistency swings, and extended time required for grade changes.

Through applying the new process solutions, equipment and automation within the new concept, uniform stock feed parameters such as consistency and pressure of the furnish fed to the paper machine can be achieved. Concurrently, the process volumes can be reduced by optimizing chest dimensioning and automation. As a consequence, faster grade change time can be attained which also results in better efficiency. In addition, less space and power is required which decreases investment and operational costs.

In addition to several partial systems featuring new concept in operation, the first full-scope references have been successfully started up. These two new systems are discussed in this paper, including the first mill results and experiences from these rebuilds.

INTRODUCTION

The trend in new paper production has been towards bigger and faster machines. At the same time the requirements for higher efficiency have also increased, for these machines as well as smaller paper machines producing specialty papers. In addition to the impact on paper machines, these factors also affect stock preparation. Traditional stock preparation, including the approach system, features large process volumes and control for individual loops. As an immediate consequence stability problems, such as consistency swings and lengthened grade change times, occur.

The new concept was introduced in 1998 to meet these new requirements. The aim of the new system is to increase efficiency and quality of stock preparation system, resulting in better efficiency of the total paper-making line. The new wet-end concept contains several modifications and improvements compared with conventional systems. The process has undergone a simplification through a reduction in the amount of backwater loops, lower process volumes and re-connected sub-processes of the wet-end.

OptiFeed CONCEPT

The system consists of separate modules; stock line, broke handling, centrifugal cleaning, approach system and whitewater handling. Processes contained in these modules are tailored separately according to specific paper grade needs. For example, secondary dilution of wire water added between centrifugal cleaning and machine screening is used for fine paper and board grades, where headbox consistency is lower.

Figure 1

Figure 1.New developed stock preparation concept for a coated fine paper

Each paper-making process starts with the stock line. Some disturbances start here and they increase exponentially upstream. Accordingly, elimination of disturbances as early as possible is important for the overall process. Combined with a feed-forward principle, a new type of consistency control based on controlling both the flow and consistency rather than consistency alone is an important component of the new concept. The changes in basis weight and in proportioning are already applied in pumping of the stock component from the storage tower.

Both coated and uncoated broke are handled in a single broke line. Their ratio to the proportioning is controlled by the flow rates. Deflaking is applied for the coated grades. The screening is done with integrated, compact and energy efficient multistage screening that includes pre-screening and 2-3 screening stages, each of which can be separately optimized . Thickening is implemented in a separate loop based on consistency measurement.

The distinction from the conventional system is the absence of the both mixing and machine chest. Stock components are proportioned to the approach system separately. Mixing of stock components with deaerated wire water takes place in a high shear mixing pipe, LobeMix, just prior the first-stage cleaners. Centrifugal cleaners are operating in higher feed consistency than normally and with double accept coupling. As a benefit of this, the size of the centricleaner-plant will be smaller and more energy efficient.

Mixing of centrifugal cleaning accepts, recirculation flows and diluting wire water takes place in the second mixer. Due to the high mixing efficiency, the consistency profile prior to the fan pump is more uniform, resulting in less variation in paper profile.  Machine screening is done with a single screen, in which pulsation is reduced by the design of the rotor. The secondary screen can be arranged even without an intermediate chest and pump directly after the primary screen.

Deaeration in the new concept takes place earlier than in a conventional short-circulation process. Deaeration is arranged only for the wire water, which is then distributed to the dilution points. As a result, air does not disturb process pumping and the process remains cleaner. Water handling is arranged so that rich fraction of wire water remains in the approach system and in the stock line consistency controls. This reduces the solids overflowing into long circulation, which enables the use of a smaller save-all system.

M-real, Kyro PM 3, Kyröskoski, Finland

The first two approach system rebuilds featuring the new concept were started up during spring 2002. Both Machines were very much alike in terms of size, large production scale and requirements for high quality.

The first start-up took place at M-real Kyro Mill in Kyröskoski, Finland. PM 3 at the Kyro Mill is currently the largest wallpaper producer in the world, with a design speed of 1,000 m/min. Basis weight range in Kyro PM 3 produces several grades with a significant basis weight variation – usually between 80 g/m² 2 and 165g/m². The compact process volume and shortened grade change time provided by the new system were well suited to providing system stability into this rebuild project.

Figure 2

Figure 2. Kyro PM 3 concept after rebuild

The delivery for the rebuild from Metso Paper consisted of the approach system, new dilution headbox, MB hybrid forming unit and coating section as well as a new automation system with controls. Figure 2 shows the approach system concept carried out for the Kyro PM 3. Deaeration with deaerator is used only for the wire water and air-free water is then distributed into the dilution points. The system does not include wire pit. Mixing in the approach system takes place with two LobeMix units. The rebuild also included a new centricleaner plant that is connected in a double accept configuration in which accepts from both first and second stages are forwarded. Screening for both stock and dilution water is carried out with new screens. Basis weight is controlled with IQWeightMD that features compensation for consistency drifts.

The Kyro PM 3 rebuild started up in March 2002. Stability and air-removal benefits are clearly seen and verified by process analysis. Long-term analysis of cleanliness and deaeration is continuing, but already now the improvements on cleanliness, retention levels and runnability indicate good result. As a result of the good deaeration, the total air content in the headbox header is less than 0.30% and free air cannot be found at that position at all.

Figure 3

Figure 3 Variance component analysis (VCA) of basis weight stability from 90 g/m² paper sheet before and after the rebuild (1-sigma).

Figure 3 shows a comparison of the variance component analysis (VCA) stability results before the rebuild, with and without MB forming unit. A clear improvement in CD-values is achieved by new approach system and headbox, However, this is further improved with an MB forming unit. Improved VCA MD values are reflecting stability of the approach system mixing. Residual VCA variation is slightly enhanced by the improved stock mixing due to LobeMix. However, the main improvement here is achieved with a MB unit. Machine direction basis weight stability is compared in figure 4 with present situation. Again, the improvement is due mainly to the new approach system concept.

Figure 4

Figure 4. MD basis weight stability and comparison with different wet-end concept (basis weight 90 g/m²).

Kanzan PM 6, Düren, Germany

Almost simultaneously with Kyro unit, the second start-up with new concept took place at Kanzan Spezialpapiere GmbH (a member of Oji group). The mill is producing special papers (thermal and ink-jet papers) with one paper machine. The company's strategic strengths are its independence and flexibility of its production, with more than 100 different grades.

In order to enhance its strategic strengths, the aim of the rebuild of Kanzan PM 6 was to further exploit flexibility and increase production up to 800m/min. The rebuild contract, including stock preparation and approach system, was awarded to Metso Paper. The rebuild in paper machine included a new dilution headbox, Symformer MB hybrid forming unit, press section rebuild with shoe press and some modification to the drying section.

Figure 5

Figure 5. Kanzan PM 6 concept after the rebuild

For a process with as many different grade changes as this, the stability, fast grade change potential and compact process are important factors. Special attention was focused on consistency and basis weight control during the Kanzan PM 6 rebuild. Accurate consistency control was applied for the broke line combined with new multistage broke screening. Broke is proportioned to the approach system without mixing and machine chests. The rebuild in the approach system consists of basis weight control for both broke and machine stock. Deaeration from the wire water is done without wire pit with Flume, from which the water flows are divided to the dilution point. Mixing and dilution of the stock is carried out with LobeMix. The centricleaner plant is connected to the double accept instead of a conventional cascade. Machine screening was carried out without intermediate chest and pump using the existing machine screen but with new screens in secondary stage and the dilution line.

Figure 6

Figure 6 Variance component analysis (VCA) of basis weight stability from 57 g/m² paper sheet before and after the rebuild (1-sigma).

The start-up of the system took place on April 2002. The new production speed was achieved for 57 g/m² main grade with top quality figures.  The study of the advantages of new system and benefits is under way, but the initial results already confirm the remarkable improvement in quality.  It is evident that benefits can be reported in the investment and operational side as well as in terms of quality gains.

SUMMARY

Optifeed was developed to meet the challenges of high-speed paper machines and dilution headboxes. However, benefits from the new concept can also be achieved in smaller process rebuilds. 

Deaeration with the Flume wire water tray has proven to be a reliable concept for low-speed paper machines, where total deaeration is not required. In cases, where complete wire water deaeration is required, the deaeration tank is utilized. This concept for wire-water deaeration reduced the total air content to virtually zero in the case of the Kyro mill. 

From the paper-quality point of view, the most important factor of the approach system is the mixing of both stock and water and stock with chemicals. Therefore, in the new concept, we have paid special attention to mixing by introducing the new mixers, the LobeMix for stock mixing and the RetaMix for retention aid mixing. This creates options also for minor, partial process upgrading through the advanced mixing technology, for example with new mixer for rebuilds of traditional wire pit configurations to enhance the stability of the existing process.  

Recent start-ups have proven the potential for advantages of the new in cases of smaller paper machines, partial rebuilds or where improved paper properties and production cost-savings are desired.

REFERENCES

Kokko, T., Hietaniemi, M., Ahola, J., Huhtelin, T., Lautala, P. Development of paper machine wet end using simulation.

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