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ISBN 978-951-42-8905-7 (Paperback)ISBN 978-951-42-8906-4 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)

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U N I V E R S I TAT I S O U L U E N S I SACTAC

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OULU 2008

C 305

Mirja Illikainen

MECHANISMS OFTHERMOMECHANICALPULP REFINING

FACULTY OF TECHNOLOGY,DEPARTMENT OF PROCESS AND ENVIRONMENTAL ENGINEERING,UNIVERSITY OF OULU

C 305

ACTA

Mirja Illikainen

A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 3 0 5

MIRJA ILLIKAINEN

MECHANISMS OFTHERMO-MECHANICALPULP REFINING

Academic dissertation to be presented, with the assent ofthe Faculty of Technology of the University of Oulu, forpublic defence in Auditorium TA105, Linnanmaa, onOctober 31st, 2008, at 12 noon

OULUN YLIOPISTO, OULU 2008

Copyright © 2008Acta Univ. Oul. C 305, 2008

Supervised byProfessor Jouko Niinimäki

Reviewed byDoctor Kari LuukkoProfessor Bruno Lönnberg

ISBN 978-951-42-8905-7 (Paperback)ISBN 978-951-42-8906-4 (PDF)http://herkules.oulu.fi/isbn9789514289064/ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)http://herkules.oulu.fi/issn03553213/

Cover designRaimo Ahonen

OULU UNIVERSITY PRESSOULU 2008

Illikainen, Mirja, Mechanisms of thermomechanical pulp refiningFaculty of Technology, Department of Process and Environmental Engineering, University ofOulu, P.O.Box 4300, FI-90014 University of Oulu, Finland Acta Univ. Oul. C 305, 2008Oulu, Finland

AbstractThe objective of this thesis was to obtain new information about mechanisms ofthermomechanical pulp refining in the inner area of a refiner disc gap by studying inter-fibrerefining and by calculating the distribution of energy consumption in the refiner disc gap.

The energy consumption of thermomechanical pulping process is very high althoughtheoretically a small amount of energy is needed to create new fibre surfaces. Mechanisms ofrefining have been widely studied in order to understand the high energy consumption of theprocess, however, phenomena in the inner area of disc gap has had less attention. It is likely thatthis important position is causing high energy consumption due to the high residence time of pulplocated there.

The power distribution as a function of the refiner disc gap was calculated in this work. Thecalculation was based on mass and energy balances, as well as temperature and consistencyprofiles determined by mill trials. The power distribution was found to be dependent on segmentgeometry and the refining stage. However, in the first stage refiner with standard refiner segments,a notable amount of power was consumed in the inner area of the disc gap.

Fibre-to-fibre refining is likely to be the most important mechanism in the inner area of discgap from the point of view of energy consumption. In this work the inter-fibre refining was studiedusing equipment for shear and compression. Fibre-to-fibre refining was found to be an effectiveway to refine fibres from coarse pulp to separated, fibrillated and peeled fibres if frictional forcesinside the compressed pulp were high enough. It was proposed that high energy of today’sthermomechanical pulping process could derive from too low frictional forces that heated pulp andevaporated water without any changes in fibre structure.

The method to calculate power distribution and results of fibre-to-fibre refining experimentsmay give ideas for developing today’s thermomechanical pulp refiners’ or for developing totallynew energy saving mechanical pulping processes.

Keywords: disruptive shear stress, energy balance, energy consumption, fibredevelopment, fibre-to-fibre refining, mass balance, power distribution, pulp padrefining, refining, thermomechanical pulping, TMP

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Acknowledgements

The research work reported in this thesis was carried out at the Fibre and Particle Engineering Laboratory, University of Oulu during the period 2003–2007. Financial support from the Graduate School of Chemical Engineering (GSCE), UPM-Kymmene and Metso Paper is gratefully acknowledged. I also appreciate the award of personal grants from the Walter Ahlström foundation, the Foundation of Technical Progress (TES), the Emil Aaltonen foundation, the Tauno Tönning foundation and the scholarship fund of Tyrnävä municipality.

I like to express my sincerest thanks to my supervisor Prof. Jouko Niinimäki for his guidance and encouragement during the work. Special thanks go to Dr. Esko Härkönen for advising and sharing his knowledge on mechanical pulping with me. I would also like to thank Dr. Mats Ullmar, Mr. Petteri Vuorio and Mr. Esa Viljakainen for many discussions, valuable comments and good advice during the work.

I am most grateful to the reviewers of this thesis, Dr. Kari Luukko from UPM-Kymmene and Prof. Bruno Lönnberg from Åbo Akademi. I would like to thank Mr. Mark Jackson for proof reading the English language of this manuscript.

All my colleagues and friends I have worked with during these years in the Fibre and Particle Engineering Laboratory are too acknowledged. Especially I like to thank Mr. Esa Anttila, Mr. Sami Ojala, my dear sister Elisa Karjalainen, Mr. Jarno Karvonen and Mr. Jani Österlund for their help in laboratory and experimental work.

Finally, I like to thank those nearest and dearest to me who have encouraged and supported me over the years.

Isä ja äiti, kiitos tuesta.

Sanna, Ansku ja Anna, kiitos rohkaisusta.

Mari, Helena, Pirjo ja Hanna, kiitos ystävyydestä.

Mikael, Joel, Enni, Joonas, Erika, Annika, Maija ja Iiro, kiitos kodista.

Oulu, September 2008 Mirja Illikainen

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List of original publications I Illikainen M, Härkönen E, Ullmar M & Niinimäki J (2006) Distribution of power

dissipation in a TMP refiner plate gap. Paperi ja Puu 88(5): 293–297. II Illikainen M, Härkönen E, Ullmar M & Niinimäki J (2007) Power consumption

distribution in a TMP refiner: comparison of the first and second stages. Tappi Journal 6(9): 18–23.

III Illikainen M, Härkönen E & Niinimäki J (2007) Power consumption and fibre development in a TMP refiner plate gap: Comparison of unidirectional and standard refiner segments. Proceedings of the 2007 International mechanical pulping conference, Minneapolis, USA.

IV Illikainen M, Härkönen E, Ullmar M & Niinimäki J (2008) Disruptive shear stress in spruce and pine TMP pulps. Paperi ja Puu 90(1): 47–52.

V Illikainen M & Niinimäki J (2007) Energy dissipation in a TMP refiner disc gap. Proceedings of The 6th Biennal Johan Gullichsen Colloquium, Espoo, Finland: 49–57.

Theoretical calculations of Papers I, II and III were done and reported by the present author. Experimental work presented in Papers III and IV were designed, analysed and reported by the present author. Additional authors participated in the designing and writing of papers by making valuable comments.

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Contents

Abstract Acknowledgements 5 List of original publications 7 Contents 9 1 Introduction 11

1.1 Background ..............................................................................................11 1.2 Problems to be studied ............................................................................ 12 1.3 The hypothesis ........................................................................................ 12 1.4 Initial assumptions .................................................................................. 12 1.5 Research environment............................................................................. 13 1.6 Outline of the thesis ................................................................................ 14

2 Refining mechanisms–present understanding 15 2.1 Flow phenomena inside the refiner ......................................................... 16

2.1.1 Pulp flow behaviour ..................................................................... 16 2.1.2 Pulp residence time in a disc gap.................................................. 18

2.2 Development of pulp and fibre properties in refining............................. 19 2.2.1 Separation of fibres....................................................................... 20 2.2.2 Development of pulp and fibre properties .................................... 21 2.2.3 Latency ......................................................................................... 22

2.3 Factors affecting fibre development........................................................ 23 2.3.1 Properties of wood raw material................................................... 23 2.3.2 Softening behaviour of wood ....................................................... 24 2.3.3 Specific energy consumption........................................................ 26 2.3.4 Intensity of refining ...................................................................... 27

2.4 Energy consumption of refining.............................................................. 28 2.4.1 Energy transfer and energy dissipation mechanisms .................... 29 2.4.2 Distribution of refining energy ..................................................... 31

3 Materials and methods 33 3.1 Equipment of shear and compression ..................................................... 33 3.2 Materials and methods used in experimental studies .............................. 35

3.2.1 Materials....................................................................................... 35 3.2.2 Compressibility of pulps............................................................... 36 3.2.3 Internal friction of pulps............................................................... 37 3.2.4 Pulp pad refining–development of fibre properties in

fibre-to-fibre refining ................................................................... 39

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3.3 Power consumption distribution in a refiner disc gap ............................. 41 3.3.1 Method.......................................................................................... 41 3.3.2 Mill trials ...................................................................................... 42

4 Results 45 4.1 Compressing and shearing behaviour of pulps........................................ 45 4.2 Power consumption distribution in a refiner disc gap ............................. 54

5 Discussion 57 5.1 Equipment of shear and compression and fibre-to-fibre refining............ 57 5.2 The method to calculate power consumption distribution ...................... 59 5.3 Mechanisms of refining – Why energy is dissipated?............................. 61

6 Conclusions 65 References 67 Original publications 73

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1 Introduction

1.1 Background

Thermomechanical pulp (TMP) is mainly used for mechanical printing papers such as newsprints as well as uncoated and coated magazine papers. The invention and development of thermomechanical pulping process in the 1970’s rapidly changed the structure of the world’s pulp and paper industry. Previously, printing papers were produced from a mixture of groundwood and kraft pulps. Thermomechanical pulping rapidly displaced traditional mechanical pulping, grinding, because similar paper properties could be achieved using fewer amounts of expensive reinforced kraft pulp.

Today, thermomechanical pulping process is the most dominate mechanical pulping process. The future of the process is, however, at stake. The continually rising price of electricity and high electrical energy consumption of the process have impaired the profitability of the process. TMP refining has been reported to be the most energy intensive unit process in the pulp and paper industry (Sabourin 2003).

The heart of the thermomechanical pulping process is refining. Although there are different refiner configurations used in the industry, depending on the manufacture and process requirements, the main operational principle of all of today’s refiners is the same: wood chips are fed between two parallel discs and at least one of those is rotating. The patterned refiner segments transfer the rotational energy into the pulp. Chips are broken down and due to compressive and shearing forces a wood material is developed suitable for papermaking.

Refining process is very complex: conditions inside the refiner are very harsh, flow phenomena inside the refiner are very complicated while two simultaneously phenomena occur, separation and development of fibres. Studying the mechanisms of refining is thus very challenging and until now it is not been clearly understood what happens in a refiner disc gap: what is the desired mechanism for efficient fibre development and how the energy is transformed into pulp? However, it has been proposed in literature that refining energy could be halved if the mechanisms of refining were known (Sundholm 1999a), because theoretical energy requirement for creating new fibre surfaces is small. The purpose of this work was to obtain new information on mechanisms of refining and energy consumption of refining in the inner area of the refiner disc gap. This

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was achieved by studying the power distribution in a refiner disc gap, as well as fibre-to-fibre interaction in refining.

1.2 Problems to be studied

In order to understand mechanisms of refining it is extremely important to identify and understand phenomena in the inner area of a disc gap. The residence time of pulp in the inner area of disc gap is high and therefore it is likely to be one important position that causes high energy consumption of refining. The role of this area for energy consumption of refining is not well known. It is also not known, if effective fibre development can occur in fibre-to-fibre contacts, which is very likely to be the most important mechanism in the inner area of disc gap.

1.3 The hypothesis

The aim of the thesis was to obtain new information about mechanisms of refining and high energy consumption of the process. To solve the above problems the following hypothesis were tested.

1. In the first stage refiner with standard refiner segments, the power consumption is significant in the inner refining zone in a disc gap. (Papers I, II and III).

2. The effective refining can be achieved in the fibre-to-fibre refining without any impacts due to bar crossing (Paper V). The effectiveness of pulp pad refining depends also on temperature and fines content of pulp (Paper IV).

1.4 Initial assumptions

The refining process and refiner type depends on mill and paper grade in question and there are several different possibilities to produce thermomechanical pulp. In this work the “standard” TMP line (Tienvieri et al. 1999) and the single disc refiners in the first and the second stages have been used in calculations. The proposals in literature about energy intensive inner refining zone inside the refiner and also about importance of fibre-to-fibre mechanisms have also been connected to the single disc refiners and so the results of experimental work can also be applied mainly to the single disc TMP refiners.

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In calculations some assumptions have been done. Mill trials and sampling were done during normal operational conditions and it was assumed that the measured values of consistency, temperature and the process variables well describe the common situation inside the refiner. The steam was assumed to be saturated in a disc gap. Water and fibres were assumed to be mixed and their temperature was assumed to be equal.

Conditions inside the TMP refiner are very harsh and thus it is very challenging to study refining mechanisms. To find a solution for this a piece of equipment for shear and compression measurements was developed for studying compression and shearing behaviour of pulp in a more controlled environment. Compared to a situation in a refiner disc gap, some simplifications have been made: in the experiments pulp was heated using steam, the compressive pressure directly affected the pulp and the rotational speed of the equipment was very low. Despite of these simplifications the study of compressing and shearing behaviour of pulps certainly widens the present knowledge on mechanisms inside the real refiner.

1.5 Research environment

The authors work about refining mechanisms consists of experimental and theoretical parts. In the experimental section behaviour of pulp under compressing and shearing forces was studied in the Fibre and Particle Laboratory at the University of Oulu using apparatus for shear and compression experiments. The raw material for experiments was produced at Metso Anjalankoski pilot plant. Analysis of pulps was mainly preformed using laboratory facilities at the University of Oulu and some at the Finnish Pulp and Paper Research Institute (KCL).

The theoretical section was based on calculations and mill trials. Pulp samples from the refiner disc gap were taken from the commercial SD-65 refiner first and second stage positions at UPM-Kymmene Kajaani mill. Pulp samples and process data collected during trials (used in Papers I and II ) were analysed at the University of Oulu. The remaining test trials (Paper III) were performed at UPM-Kymmene Kaipola mill in the first stage position using different refiner segment types. The process data was collected and pulp analysis was done by UPM-Kymmene.

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1.6 Outline of the thesis

The Chapter 1 of this thesis describes a short introduction to the study, the problem to be investigated, the hypothesis while presenting the research environment and an outline of the thesis. The Chapter 2 summarises previous published studies on TMP refining mechanisms while the Chapter 3 presents the materials and methods used in this study. The Chapter 4 summarises the main results achieved during the work. The interpretation and discussion, as well as proposals for further work are presented in Chapter 5. Finally, a conclusion is presented in Chapter 6.

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2 Refining mechanisms – present understanding

In refining, moist wood chips and dilution water are fed into the first stage refiner. The chips are broken down when they make contact with breaking bars of the rotor. Due to rotation of the disc and pattern of the refiner segment surface, rotational energy is transferred into the pulp by compression and frictional forces. The outcome of the refiner is a number of different kinds of wood particles: shives, coarse fibres, fibrillated and flexible fibres, as well as fines. The questions arising here are how these different fractions have been generated and how much energy is needed to create them.

The understanding of mechanisms of refining can be approached from various perspectives. The following chapters present an understanding of the refining phenomenon from three different approaches in order to get the best possible view on what happens inside the refiner: 1) what are the main flow phenomena’s occurring in a refiner disc gap based on visual and some theoretical methods, 2) how fibre and pulp properties are developed during refining and what factors are affecting fibre development and 3) energy consumption in a refiner disc gap: how and where it is consumed. The remaining approaches are not discussed here although the refining can be approached from the basis of computational fluid dynamics (Huhtanen 2004, Härkönen et al. 1997) and refining modelling (e.g. Berg & Karlström 2003, Corson 1973).

Flow phenomena, development of pulp properties, as well as energy consumption of refining vary significantly depending on the radial position in a refiner disc gap. Here, the disc gap is divided into three different zones, according to Fig. 1. The most inner part of the refining is referred as breaking zone (Zone I), the middle part of the refiner as inner refining zone (Zone II) and the narrow disc gap as outer refining zone (Zone III).

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Fig. 1. Refiner disc gap divided into sections.

2.1 Flow phenomena inside the refiner

Refining is a very complex process. Moist wood chips and dilution water are fed into the centre of the discs. Water is needed to soften the wood (see Chapter 2.3.2) and for protecting wood against burning. In a disc gap frictional forces cause the evaporation of huge amounts of water into steam which is flowing outwards and backwards. Flow direction and velocity of pulp depends on forces acting on pulp (centrifugal force, frictional forces, and force due to flowing steam) that are of different sizes in different positions inside the refiner. Pulp flow in the refiner has been studied using different visual methods while a theoretical study about fibre flows is presented (Chapter 2.1.1). The fibre residence time has also been studied by theoretical methods and by experimental residence time measurements (Chapter 2.1.2).

2.1.1 Pulp flow behaviour

Pulp flow patterns have been studied using high-speed photography in both atmospheric (Atack 1980, Atack et al. 1984, Stationwala 1992) and in pressurised (Atack et al. 1989) refiners. The main findings of these studies are presented as follows. Wood chips are shredded into coarse pulp immediately in the breaking zone of the refiner (Zone I, Fig. 1) (Atack 1980, Atack et al. 1984). In the breaking zone it has been shown that there is a considerable circulation of coarse pulp and shives (Atack et al. 1984, Atack et al. 1989). Backflow was observed to occur in the grooves of the stationary plate and forward along grooves of the

OUTER REFININGZONE

Rotor

Centre DischargeStator

BREAKING ZONE

INNER REFININGZONE

I II III

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rotational plate (Atack et al. 1984). In the breaking zone of the disc gap (zone I, Fig. 1) it has been reported that 70 to 80% of bar area is covered with pulp (Stationwala 1992). In the inner refining zone (zone II, Fig. 1), the coverage of pulp in the transition area between coarse and fine pattern was even higher. In the outer refining zone (zone III, Fig. 1) the amount of coverage was lowest.

In the outer refining zone fibre flow was observed to be radiating outwards (Atack 1980, Atack et al. 1984). Formed fibre flocs that moved tangentially, and changing their shape and being disrupted were also observed in the process (Stationwala 1992, Atack et al. 1989). The fibres and fibre flocs have also been seen to be trapped on the bar edges where they are subjected to the refining action (Atack 1980, Stationwala 1992, Atack et al. 1989).

Another research group (Alahautala et al. 1998) has also taken images inside the refiner using a CCD-camera, light stroboscope and endoscope optics to view the pulp more closely. The focus of their measurements was on pulp velocity, pulp orientation, pulp coverage and the presence of fibre flocs. The results of their studies showed that the back-flow of pulp in the breaking zone (zone I, Fig. 1) was very slow. In the grooves within the inner refining zone (zone II, Fig. 1) there was a pulp back-flow of 1 m/s while in the outer refining zone (zone III, Fig. 1) of the refiner the radial velocity of pulp was as high as 30 m/s. The orientation of fibres in the breaking zone was minor, but in the inner and outer refining zones the fibres were tangentially orientated. The breaking zone was seen to be full of pulp while in the inner refining zone average pulp coverage was below 60% and below 10% in the outer refining zone.

Härkönen et al. (1997) have presented a theoretical model of refiner which they have used to simulate the flow phenomena inside the refiner. Their computer model consists of a steam phase and a combined fibre and water phase whilst being based on mass, momentum and energy balance. The theoretical model has proposed to give an idea of velocities of steam and water as well as volume fraction of pulp in a disc gap. There is however some problems in the model in that several variables have to be determined by trial and error in connection with test calculations.

A calculated example using the model shows that volumetric density of pulp (volume fraction of pulp) is highest in the inner refining zone (zone II, Fig. 1) between the fine and coarse bar zones. This is analogous to visual studies of Stationwala et al. (1992). The strongest turbulence in the pulp flow was observed in the breaking zone of the refiner before it entered the plane section plate gap.

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Qualitative diagrams of pulp and steam flows in a refiner, as well as volume fraction of pulp in a refiner disc gap are presented in Figs. 2 and 3.

Fig. 2. Qualitative view of fibre and steam flows within the refiner (Härkönen et al. 2003).

Fig. 3. Qualitative view of volume fraction of pulp in a refiner disc gap (Härkönen et al. 2003).

2.1.2 Pulp residence time in a disc gap

In order to understand the mechanisms of refining the quantity of pulp in a refiner disc gap as a function of disc radius should be known, in addition to how rapidly the pulp is flowing at a given point. The residence time of pulp in different parts of the refiner disc gap is essentially important to know because it indicates how much pulp is available for refining action.

Miles & May (1990) proposed a theoretical model to calculate pulp residence time in a refiner disc gap. They derived an equation for velocity of pulp in a refiner disc gap based on forces affecting pulp. Their model has, however, some

Chip flow

Steam flow

Stator

Rotor

Stator

Rotor

Radius

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assumptions which the model correctness has been questioned by e.g. Härkönen et al. (1997) and Sabourin et al. (2001). In addition there are many coefficients that are difficult to measure. The residence time model presented by Miles and May has later been shown (Ouellet et al. 1996, Härkönen et al. 1999, Murton & Duffy 2005) not to predict the measured residence time of pulp in a refiner disc gap.

The quantity of pulp has been observed to be at its highest in the inner parts of the refiner (Stationwala 1992) (zones I and II, Fig. 1) which was explained by recirculation of pulp in the inner parts of the refiner and restriction of the radial pulp flow caused by the fine plate pattern. This indicates the pulp’ residence time being highest in the inner parts of the refiner. The residence time of pulp inside the refiner has later been measured using a radioactive tracer in a commercial TMP refiner (Härkönen et al. 1999, Härkönen et al. 2003, Murton & Duffy 2005). In these studies the pulp velocity in the outer parts of the refiner was shown to be high compared to the inner parts. Härkönen et al. (1999) reported that residence time of pulp in the areas of breaker bar and centre segments was between 2.5 to 7 s (zones I and II, Fig. 1) while at the outer part of the plate gap it was approximately 0.5 s (zone III, Fig. 1). Murton & Duffy (2005) measured similar residence times: 0.5 s in the outer refining zone (zone III, Fig. 1) and 3–15 s within the refiner system (ribbon screw to blow valve). A number of reports have suggested the possibility of changing the pulp’s residence time by adujsting the segment geometry (Härkönen et al. 1999), by controlling the consistency and the pressure differential over the disc gap (Härkönen et al. 2003)

2.2 Development of pulp and fibre properties in refining

The main purpose of thermomechanical pulping process, as well as all other pulping processes, is to separate the fibres from the wood and to make them suitable for papermaking. Sundholm (1999b) has suggested that in an ideal mechanical pulping process the following phenomena should happen: fibres must be separated from wood matrix, fibre length must be retained, fibres must be delaminated, abundant fines must be generated by peeling off outer layers of the middle lamella, primary and secondary layers of fibre wall, and finally the surface of the remaining secondary wall must be fibrillated.

In the thermomechanical pulp refiner the separation of fibres and development of fibre properties occurs simultaneously but are considered separately in the following chapters. The latency of pulp is also discussed in a

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separate chapter due to its importance in the author’s experimental work on the development of pulp in fibre-to-fibre refining (Paper V).

2.2.1 Separation of fibres

It has been shown that wood chips are broken down immediately into coarse pulp in the breaking zone of the refiner (Atack et al. 1984). Further separation occurs when coarse pulp is affected by frictional forces in refiner disc gap so the shive content of pulp decreases as a function of radius of the disc gap (Atack et al. 1984, Härkönen et al. 2003).

The exact mechanism of fibre separation is not understood and this is highlighted by the very low amount of publications available about the initial breakdown of chips or by the separation of fibres. This is probably due to the complicated and simultaneous process of the fibre separation and development (Fundamental studies on disintegration of wood under different loading modes has been preformed by Koran 1968, Law & Koran 1981, Koran & Salmén 1985). In refining, separation of fibres occurs in the weakest part of the wood which is dependent on moisture, temperature and frequency (see Chapter 3.2.2), but pre-treatment methods (chemical, mechanical, biological) could also be possible factors. In refining, fibres are separated from each other through the primary wall – secondary wall interface or in the secondary wall (Gustafsson et al. 2003).

An initial separation step has been shown to be important factor for final pulp quality. Heikkurinen et al. (1993) have analysed pulps produced from using small amounts of energy (SEC 0.5 MWh/t) and they observed that conditions (temperature and rotational speed) in the initial breakdown process had a major effect on size and quality of undefiberised particles. Falk et al. (1987) tested single and double-disc refiners in the first and second stages and found that pulp properties were determined by the conditions in the first stage position. Rudie & Sabourin (2003) found that there are differences in wood breakdown rates between different types of wood (plantation and forest-grown loblolly pine wood). Different types of fibres have also been shown to behave differently in the defibration step. Murton et al. (2006) have concluded that in the fibre separation stage, thick-walled latewood type fibres are liberated first and more easily than large diameter, thin-walled earlywood fibres. They have also showed that with low specific energy consumption levels the shive content decreases rapidly while later in refining shive content reduces gradually. They proposed that individual liberated fibres prevent the refining energy to be applied directly to shives.

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2.2.2 Development of pulp and fibre properties

Careful studies have been made on the development of pulp and fibre properties during thermomechanical pulp refining. Development of fibre and pulp properties has been studied in the refiner disc gap, as a function of disc radius, or as a function of specific energy input (the term “more refined pulp” means pulp with higher specific energy input).

Development and understanding of pulp properties in a refiner disc gap has been studied by taking many samples from within this area. First attempts were preformed in the 1980’s using an atmospheric 1.9 MW refiner (Atack et al. 1984) and more recently Härkönen et al. studied pulp properties using the 10 MW pressurized SD-65-refiner (Härkönen et al. 2003). Their studies showed that pulp properties are developed in the breaking zone (I, Fig. 1), in the inner refining zone (II, Fig. 1), and in the outer refining zone (III, Fig. 1). According to these studies in the inner parts of the refiner there is no fibre cutting however defibration of shives and chips does occur. In the parallel disc gap the fibre length is reduced and fine material is created through the harsh refining.

The increasing specific energy consumption of refining changes pulp and fibre properties. Pulp fractional composition changes when energy input is increased: the amount of long fibre fraction decreases while the amount of fine material increases (Law 2000), shive content decreases (Karnis 1994), the average fibre length, as well as pulp freeness decreases (Corson 1989, Tienvieri et al. 1999).

Properties of different fibre fractions also change. The coarseness of long fibre fraction decreases (Stationwala et al. 1996, Law 2000, Karnis 1994) and fibres become more flexible (Corson 1989, Karnis 1994). Fibres are peeled, fibrillated (Moss & Heikkurinen 2003), collapsed and their cell wall thickness decreases (Corson & Ekstam 1994, Kure & Dahlqvist 1998, Jang et al. 1996, Jang et al. 2001). Internal properties of fibres also change, allowing the fibre cell wall to swell, and thus swelling increases when increasing the amount of refining (Moss & Heikkurinen 2003). In addition, properties of fine material changes too so that the specific surface area of fines increases when increasing the SEC (Stationwala et al. 1996). It has also been shown that by peeling the outer layers of fibres fines are generated (Heikkurinen & Hattula 1993, Luukko & Paulapuro 1999, Luukko & Nurminen 1999). In early stages of refining fines are generated from outer layers resulting in fines having good light scattering properties. Later in the process, properties of fines correspond to properties of inner fibre wall (S2

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layer). In addition to property alternations presented above, refining can cause different fibre damage like cracks in the fibre wall (Gregersen & Holmstad 2004).

The development of fibres has been shown to occur by the unravelling and peeling of outer layers of fibres (Forcags 1963, Karnis 1994, Stationwala et al. 1996, Heikkurinen & Hattula 1993) while, cutting of fibres is seen in refining. (Law 2005). A good example of a figure showing the mechanisms through which the development of refining occurs has been presented by Karnis (1994) (Fig. 4). Initial breakdown of chips to shives occurs by disintegration of particles, fibre development by disintegration and peeling mechanisms, the latter (peeling) being the dominating mechanism in refining.

Fig. 4. Separation and development of fibres in refining (Karnis 1994).

2.2.3 Latency

A special phenomenon occurring in thermomechanical pulp refining is the formation of latency which generates curled and twisted fibres. The most visible effect is the decrease of freeness during hot-disintegration (SCAN-M 10:77).

Beath et al. (1966) were the first to show that freeness of disc refined pulp decreased significantly during hot-disintegration. They understood that the refiner produces pulp with lower freeness but somehow freeness is increased in the process. They found part of pulp properties being effectively latent and thus it is why they referred to this property as latency. Beath et al. also proposed that latency is due to twisting and bending of pulp fibres at high temperature conditions inside the refiner. When pulp cools, fibres remain in their curled form.

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Jones (1966) published microscopic pictures on pulps before and after hot-disintegration. He showed that curled kinked and twisted mechanical pulp fibres can be straightened in hot-disintegration.

Removal of latency is extremely important in order to obtain reliable results in pulp testing. Hot-disintegration is the most common way to remove latency of mechanical pulps. Besides that, beating has been reported to be an additional suitable method (Beath et al. 1966). The removal of latency not only decreases freeness but increases a burst factor (Beath et al. 1966). Removal of latency has also been reported to decrease shive content and increase tensile strength (Mohlin 1980).

2.3 Factors affecting fibre development

In refining, there are many factors affecting fibre separation and refining result which are discussed in this chapter. Properties of wood raw material give a framework for final pulp properties. Temperature and moisture in refining are key factors affecting fibre separation and development due to softening and viscoelastic behaviour of wood material. Besides to these, refining results are strongly dependent on two important factors: how much and how the energy is transferred into pulp.

2.3.1 Properties of wood raw material

Wood species is the most important raw material parameter in thermomechanical pulp refining. Different wood species result in different final pulp properties and also differences in specific energy consumption. (e.g. Härkönen et al. 1989, Hatton & Johal 1996, Reme & Helle 2001). The most favourable raw material for thermomechanical pulping is spruce, especially Norway spruce that has favourable fibre properties, low extractive content and high initial brightness of wood (Varhimo & Tuovinen 1999). Most pine species consume much more energy compared to spruce species, and pulp properties are not as good.

Wood is an anisotropic material; its properties vary a lot between different species but also within a certain wood species themselves. The refining result is thus dependent on many other parameters, e.g. moisture content of wood, wood basic density and seasonal variations (earlywood and latewood fibres) (e.g. Fuglem et al. 2003, Reme et al. 1999, Mohlin, U.B. 1997, Tyrväinen 1997, Murton & Corson 1992, Corson 1991).

24

2.3.2 Softening behaviour of wood

The fracture of wood (separation and development of fibres) in mechanical stress occurs in the weakest parts of the wood structure which is defined by the moisture, temperature and strain rate. Wood consists mainly of three viscoelastic polymer materials, cellulose, hemicellulose and lignin, and their softening behaviour defines how wood material is broken down. The proportion of these polymers varies in fibre cell wall structure, lignin being the main component in the middle lamella (Fig. 5) (Panshin & De Zeeuw 1964).

In refining conditions wood is saturated with water. In water-saturated conditions at 20 ºC cellulose and hemicellulose are already softened (Back & Salmén 1982). Thus lignin softening is a critical factor from the fibre separation point of view. Lignin is a viscoelastic material and at low temperatures (below the glass transition) it is stiff and glassy. In transition region the stiffness decreases and at high temperatures lignin behaves like a rubbery material (Irvine 1984). In refining, if lignin is too stiff the fracture occurs throughout more soften fibre wall and the refining results in broken fibres and high fines content (Salmén et al. 1999). If lignin is too soft refining produces pulp covered in a layer of lignin and it is impossible to refiner any further (Atack 1972).

Softening of lignin is dependent on strain rate or strain frequency. At low strain rates, lignin readily softens at about 70 ºC (Salmén et al. 1984) while the softening temperature increases at higher strain rates. It has been reported that refining conditions between 120–135 ºC is needed for the lignin to become soft (Atack 1972) but it has also be reported that the softening temperature range is much wider, from 100 to 170 ºC. The optimal refining occurs in lignin softening transition zone where lignin is not glass-like anymore but not yet become a total rubbery material. In optimal thermomechanical pulp refining, fibres are separated from each other through a primary wall or through a primary wall – secondary wall interface (Gustafsson et al. 2003), as illustrated schematically in Fig. 6.

25

Fig. 5. Distribution of the principal chemical constitutes within the various layers of the cell wall in conifers (Panshin & de Zeeuw 1964).

26

Fig. 6. Breakdown of the wood matrix as a function of refining temperature. RMP 20–95ºC, TMP 110–150ºC, MDF 170–190ºC (Franzén 1986).

2.3.3 Specific energy consumption

How much energy is put into pulp is defined as specific energy consumption (SEC). In refining, it is determined as energy per unit mass of pulp, generally calculated from refining power and production rate of pulp (Eq. 1). The unit of SEC normally used is MWh/t. The more energy is put into pulp the more refined the pulp is and greater the changes in fibre and pulp properties are (see Chapter 3.2.2) However, with the same SEC totally different kinds of pulps can be produced while the other important factor in refining is how the energy is put into pulp, commonly referred as a “intensity of refining”

PSECm

=&

, (1)

where P is the power (MW), m& is the mass flow rate (t/h).

27

2.3.4 Intensity of refining

The concept of refining intensity describes how rapidly energy is put into pulp. It has a theoretical background, presented in the following paragraph. The theoretical concept of refining intensity has, however, some limits and it is thus more appropriate to use refining intensity as a qualitative term to describe if the refining is harsh or gentle.

The concept of refining intensity was adopted in thermomechanical pulp refining by Miles and May in the 1990’s (Miles & May 1990, Miles 1990). They proposed that the important factor affecting refining phenomenon is a number of impacts the pulp is receiving during refining. This is caused by refiner bar crossing and how much energy is transferred into pulp during a particular bar crossing. They derived an equation for radial velocity of pulp in a refiner disc gap, based on forces acting on pulp. This equation was then used to calculate pulp residence time in a refiner disc gap, and consequently for calculating the number of impacts pulp is receiving in a disc gap. Refining intensity was then determined according to Eq. 2 and it was proposed to be an important factor affecting the refining result.

Een

= , (2)

where e is the refining intensity (MWh), E is the refining energy (MWh), n is the number of impacts.

The concept of refining intensity was rapidly adopted into research of thermomechanical pulp refining. The effect of theoretically calculated refining intensity on pulp properties and energy consumption has been studied in several papers. The effect of refiner speed (e. g. Miles & Karnis 1991, Miles & Omholt 2003), as well as effects of plate pattern (Murton 1998, Murton et al. 2002), consistency (Alami et al. 1997) and production rate (Murton & Corson 1997) on refining intensity, energy consumption and pulp properties has been studied.

According to studies presented above, the increasing refining intensity decreases energy consumption of refining and the result, in general, is pulp with lower amounts of long fibres, shorter average fibre length, more fines, lower strength properties and better optical properties. In some publications it has been

28

shown that a reduction of energy consumption can be achieved with high intensity refining without any significant changes in pulp properties (Kure et al. 1999 Sabourin et al. 2001, Sabourin et al. 2003).

The theoretical determination of refining intensity is not exactly correct. In the model of Miles and May there are several factors that are difficult to determine. Pulp residence time measurements (presented in Chapter 1.1.2) have also shown that the theoretical residence time of pulp (determining the refining intensity) do not correspond to those determined experimentally. The calculated refining intensity has also been shown to have a limit when directional plate patterns were used (Kure et al. 1999). This indicates that pulp residence time, more than a number of refiner bar impacts, affects pulp quality. The comparison of high intensity (RTS: high speed, temperature, low residence time) and normal TMP processes, Sabourin et al. (2001) concluded that theoretically determined refining intensity does not correspond to the quality of pulp: in their experiments similar pulp properties were obtained using low and high refining intensities.

Because there are limitations in theoretical refining intensity, the concept should be rather used as a qualitative term: to describe if the refining is harsh or gentle. High intensity refining can be understood as a harsh refining, or more rapid refining (with lower residence time) compared to gentle refining. Harsh refining reduces fibre length, creates fine material and reduces long fibre fraction whereas gentle refining defibrates fibres without fibre cutting.

2.4 Energy consumption of refining

TMP refiner has been shown to work ineffectively from the fibre development point of view (Law 2004) and the energy saving potential of the process has been reported to be higher than in any other unit process in pulp and paper industry (Münster et al. 2003). Uhmeier and Salmén (1996) have surveyed that the theoretical energy needed to produce new fibre surfaces has been estimated from less than 1% to 30% of the total energy consumption of pulp. They reviewed the energy needed to develop fibres suitable for papermaking and proposed it to be much higher compared to the separation energy. They also proposed that a large amount of energy is wasted due to other losses which do not contribute to transforming the wood chips into a useful pulp (Uhmeier & Salmén 1996).

Reasons for high energy consumption of TMP process are not known although some proposals have been put forward. The present understanding and opinions about energy transfer and energy dissipation mechanisms are presented

29

in Chapter 2.4.1. Chapter 2.4.2 presents studies on energy distribution in a refiner disc gap.

2.4.1 Energy transfer and energy dissipation mechanisms

The purpose of refining segment surface is to transfer the rotational energy of the refiner disc into the pulp. The contacts between segment surface and pulp are thus extremely important. For energy transfer and fibre development the other interaction, fibre-to-fibre interaction, is important as well.

A generally accepted theory about refining mechanism is that fibres are subjected to repeated compressions and decompressions caused by bar crossing and development of fibres occurs by process of wood fatigue. Salmén et al. (1985) has performed wood fatigue tests observing similar structural changes of fibres in wood matrix and those occuring for long fibre fraction in refining. Since then the refining has often been described as a fatigue phenomenon (e.g., Kurdin 1998, Salmén et al. 1997, Uhmeier & Salmén 1996, Wild et al. 1999). The number of refiner bar impacts is believed to be the critical factor affecting the development of fibres and energy consumed in refining (Miles & May 1990) while the concept of refining intensity (energy per impact) has been widely used to describe the refining phenomenon (See Chapter 3.3.4). Uhmeier and Salmén (1996) have proposed that a reason for high energy consumption of mechanical pulping could be the large number of large radial compressions. They studied repeated large radial compressions of heated spruce to determine the most efficient way to achieve the desired fibre changes. They suggested that a small number of large compressions efficiently increased the collapsibility of fibres while a large number of large compressions were observed to be an inefficient way to cause fibre changes.

The purpose of refiner segment surface can be seen, however, also as a frictional surface that transfer the energy inside the compressed pulp pad where refining occurs by shearing of pulp. Härkönen et al. (2003) proposed that shear stress mechanisms occurrs in a disc gap. In this model the segment surface works as a frictional surface transferring refining energy (or not) inside the pulp. Thus energy transfer occurs by frictional forces inside the pulp or in the surface between pulp and segment surface. Fig. 7 presents different possible shear stress mechanisms. Continuous shear deformation in a fibre bed (Case 1), or at the segment surface (Case 2) creates power consuming mechanisms in a disc gap. Case 3 has been proposed to describe the situation in reality.

30

Fig. 7. Shear stress mechanisms in a refiner disc gap (Härkönen 2003).

According to Kurdin (1986) a fibre mat is needed in the refiner disc gap. He reported that optimum refining is accomplished in fibre-to-fibre action as opposed to plate-to fibre action. Härkönen et al. (2003) proposed that in the inner parts of the refiner (Zones I and II in Fig. 1), the power is mainly consumed in fibre-to-fibre refining, by shearing of compressed pulp (Case 1 in Fig. 7) while in the outer parts contacts between segment surface and pulp became more important. Murton & Duffy (2005) have also proposed the mechanism in which inter-fibre contacts are important. They proposed that a increased in refining energy input requires a reduction in the plate gap causing a “throttling effect” which causes pulp volume to decrease (pulp network density to increase) and increase inter-fibre contacts and friction between them. It has also been reported that inter-fibre contacts are harmful in refining. Law (2000) proposed that formation of fibre aggregates in the refiner disc gap reduces the efficiency of refining: aggregation shells could work as a shield protecting the inner component from the shearing action of refiner bars.

Although the fibre-to-fibre interaction is believed to be important for energy consumption during refining, there are no publications available that clearly show what role of this mechanism is for energy consumption and the result of the refining process. In this work shearing of compressed pulp pad was studied and the development of pulp properties in pulp pad refining was clarified.

Tangential speed of rotor

Rotor

Rotor bars

Plate gap

Stator bars

Stator

Case 1Shear deformation in fibre bed, no gliding of fibre against segment surfaces

Case 2No shear deformationin fibre bed, hitting of fibre against segment bars

Case 3Real situation in plate gap,combination of cases 1 and 2

31

2.4.2 Distribution of refining energy

Due to the very intricate nature of the refining process and complex flows inside the refiner (Chapter 2.1 presents main fibre flows) it is essential to know in which part of the disc gap energy is consumed in order to understand the high energy consumption of the process. Miles and May (1990) assumed that energy of refining is consumed mainly in the refining zone, energy consumption in the inner parts being minor. Härkönen et al. (2001) have reported, based on their studies, that energy consumption before the temperature maximum inside the refiner is as large as the energy consumption in the outer parts of the refiner. A qualitative picture about energy distribution in a refiner disc gap is presented in Fig. 8. Measurements of shear forces in a refiner disc gap, in a laboratory refiner by Gradin et al. (1997), and in a commercial refiner by Backlund et al. (2003), have proposed energy consumption to increase along the disc radius. Backlund et al. (2003) noticed more scattered forces in the inner parts of the refiner and proposed that refining mechanism is different there compared to outer parts of the refiner.

Fig. 8. A qualitative picture about power consumption distribution presented by Härkönen et al. (2001).

As presented above there are different, even contradictory estimations about the proposed energy distribution. In this work a new method to calculate the energy consumption distribution is offered and energy consumption distribution in a refiner disc gap is calculated.

Stator

RotorP/Δr

Radius

32

33

3 Materials and methods

The authors study about refining mechanisms has been experimental and theoretical. In experimental work the internal frictional properties of compressed pulps were studied and development of pulp properties in fibre-to-fibre refining, or pulp pad refining, was clarified. In theoretical work the methods to calculate power consumption distribution in the refiner disc was presented. In this chapter equipment of shear and compression used in experimental work is presented (3.1), as well as materials and methods (3.2) used in experimental work. Theoretical methods are also introduced as well as mill trials in which these calculations are based on (3.3).

3.1 Equipment of shear and compression

Equipment of shear and compression (ESCO) was developed during this work in the Fibre and Particle Engineering Laboratory. The ESCO was used to study compressibility of TMP pulps, internal frictional properties of compressed pulps, friction between pulp and different surfaces, as well as development of pulps under compressing and shearing forces (pulp pad refining). ESCO is presented in Fig. 9.

Fig. 9. Equipment of shear and compression (ESCO).

34

Main parts of the equipment consist of a cylindrical vessel of 152 mm diameter, a pneumatic press and a rotating bottom of the vessel. The device is connected to a steam line and high pressure steam is used to heat up the pulp. A computer is used for operating the equipment and for collecting and storing all the measured data: temperatures inside the cylinder, piston position, compressive force and torque. The main operational principles of the equipment are presented in Fig. 10 into five operational principles. 1. Pulp is placed into the cylinder, and the base and top of the cylinder are bolted in position. 2. The pulp is heated using steam flowing through the pulp pad and is also present through the experiment in order to keep temperature at its target value. The temperature of the steam is controlled using a pressure-reducing valve. Temperatures up to 180 degrees are possible. There is also a valve for condensed water on the lower side of the vessel. 3. The pulp is compressed using the piston and the pneumatic press, during which the upper and lower sides of the cylinder are connected for pressure balancing. 4. In the shearing test the pulp is first compressed and then the bottom plate is rotated so that the compressed pulp pad is disrupted. 5. In refining tests the pulp is first compressed and then the bottom is rotated continuously for a desired time. The motor is connected to the axis of the bottom plate and is used for rotating the bottom. The torque required for disruption is measured. Rotational speeds from 5 to 84 rpm can be used.

Fig. 10. Operational principles of the ESCO.

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

1. 2. 3.

4. 5.

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

Pulp in

Steam

1. 2. 3.

4. 5.

35

The equipment can be assembled using different surfaces on the piston and bottom plates. Smooth surfaces are used to study the compressibility of pulps, while the bottom of the cylinder and the piston are fitted with needles for studying shearing properties. Bar type segments can be used when studying frictional properties between segment surface and pulp. Needle plates and bar type segments are presented in Fig. 11.

Fig. 11. Needle plates (left) and bar type segment plates (right).

3.2 Materials and methods used in experimental studies

3.2.1 Materials

Thermomechanical pulps with different freeness values produced from Norway spruce (Picea abies) chips and from both sawmill and thinned Scots pine (Pinus sylvestris) chips were refined for experiments using a 20-inch pilot refiner in Metso Anjalankoski pilot plant. Freeness levels and specific energy consumptions are presented in Table 1. Table 1 also summarises materials used in different experiments (compressibility, internal friction of pulp, fibre-to-segment friction, pulp pad refining) and papers in which details of different experiments are presented.

36

Table 2. A list of thermomechanical pulps used in experimental work. Spruce is Norway spruce (Picea abies), pine is Scots pine (Pinus sylvestris).

Experimental Wood

species

Refining

stages

SEC

(MWh/t)

Total SEC

(MWh/t)

CSF

(ml) Compressibility

(Paper IV)

Internal

friction

(Paper IV)

Fibre-to-

segment

friction

(Paper V)

Pulp pad

refining

(Paper V)

spruce 1 0.61 0.61 747.5 x x

spruce 2 0.57 1.76 299 x x x

spruce 2 1.42 2.61 118 x x

spruce 2 2.14 3.33 65

sawmill

pine

1 0.81 0.81 723 x x x

sawmill

pine

2 0.51 1.86 340 x

sawmill

pine

1 1.36 1.36 480 x x

sawmill

pine

2 1.48 2.83 156 x x

sawmill

pine

2 2.32 3.67 54 x

thinning

pine

1 0.79 0.79 719 x

thinning

pine

2 0.76 1.92 298 x x

thinning

pine

2 1.90 3.06 97.5 x

3.2.2 Compressibility of pulps

In compressibility tests 100 g of absolute dry pulp was loaded into the cylinder at a consistency level of about 30% and heated with steam at temperatures of 120, 150 and 170°C for 15 minutes or soaked with water and left at room temperature of 15°C. Pulps were compressed using compressive pressures of 1, 2, 4, 6 and 8 bars. It has been reported that the average fibre force (force needed to balance the sum of axial thrust and steam pressure round the rotor). in a refiner disc gap is about 0.5 bars (Härkönen 1993) and because fibre pressure is certainly not distributed evenly in a disc gap, higher pressures were used in this study.

37

Eq. 3 incorporates the definition of the volume fraction of pulp. The density of the fibre cell wall is used to calculate the volume of solid fibre material, which is then compared with the total volume of pulp

fibrematerial fibre wall

total totalvolume volume

mV ρε

V V= = , (3)

where ε is the volume fraction of pulp, V is the volume [m3], m is the mass of pulp [kg] and ρ is the density of fibre wall [kg/m3].

3.2.3 Internal friction of pulps

Internal frictional properties of compressed pulp pad were studied as a disruptive shear stress. The disruptive shear stress was measured as a function of compressive pressure or volume fraction of pulp at different temperatures. Two test points were also measured using pulp from which the fine fraction (P200) had been removed by Bauer McNett fractionation. The amount of pulp in the experiments was from 75 to 130 g of absolute dry pulp, depending on the pressure used. The amount of pulp was chosen so that the distance between the needles was about 5 mm in all experiments, as calculated using the results of the compressibility tests. A heating time of 15 minutes was also used in the shearing experiments. After heating, the pulp was compressed at a certain pressure, the compressed pulp pad was then disrupted and the torque needed for this was measured. The rotational speed of the bottom of the cylinder was 8.4 min-1.

The compressed pulp pad was disrupted by rotating the bottom of the vessel. It was assumed that pulp pad behaves like an elastic material before disruption; therefore Hooke’s law (Beer et al. 1992) was used to describe its behaviour. The compressed pulp pad was considered as a rigid circular shaft. The shear stress in the rigid shaft varies linearly with the distance from the axis of the shaft (Fig. 12). The disruption starts when the highest value of shear stress is achieved. The maximum shear stress in a pulp pad as a function of torque is presented in Eq. 4 (Beer et al. 1992) and is referred here as disruptive shear stress. After disruption

38

shearing surfaces slide against each other and shear stress is then more likely to be uniformly distributed.

Fig. 12. Shear stress in the rigid shaft.

maxmax

cT rJ

τ ⋅= , (4)

where

maxτ is the disruptive shear stress [N/m2],

maxT is the torque [Nm], r is the radius of the cylinder [m] amd J is the polar moment of inertia [m4],

4

2crπ ⋅

.

Friction between pulp and different surfaces was studied using various types of surfaces on the piston and bottom plates: smooth surface, needle surface and bar type segment surface, according to Fig. 13. Pine TMP pulp with freeness value of 720 ml was compressed between the piston and bottom plates, the torque needed to disrupt the compressed pulp pad was measured and the disruptive shear stress was calculated using Eq. 4. Experiments were done using compressive pressures of 1, 2, 3 and 4 bars at 120 ºC.

τ τMax

r

τ τMax

r

39

Fig. 13. Different surfaces used in experiments.

3.2.4 Pulp pad refining – development of fibre properties in fibre-to-fibre refining

For the pulp pad refining experiments the ESCO was assembled using needle plates (Fig. 11) on the bottom and piston plates. 220 g of absolute dry pulp (pilot mill refined the first stage pulp with freeness value of 750 ml) was loaded and then the bottom and top of the cylinder were bolted. The pulp was heated for 15 minutes using steam at a temperature of 150ºC, after which the pulp was compressed using a certain pressure of the pneumatic cylinder. After compression the position of the piston plate was locked and the motor connected to the bottom of the cylinder was started. The bottom plate was rotated continuously for different lengths of time. Steam was present during the experiment and it worked not only as a heater but also as a cooler when steam was generated due to friction during the refining experiment. The torque, distance between needles and compressive pressure were measured. After the experiment the cylinder was opened, the piston was elevated and pulp was collected from between the tops of the needles for laboratory analysis (Fig. 14). Coarse, non-refined pulp was observed in the needles, while the pulp between the tops of the needles was visibly refined. An additional curl-experiment was performed using pulp with a freeness value of 120 ml to clarify the effect of shearing treatment on the freeness value of the pulp.

Smooth surface Needle plates Bar type segmentsSmooth surface

Smooth surface Needle plates Bar type segmentsSmooth surface

40

Fig. 14. Pulp was collected from between the tops of the needles after the experiments.

Laboratory analyses

Pulp collected from between the tops of the needles after the experiment was weighed, its dry content measured and the following analyses carried out after hot disintegration (SCAN-M 10:77): Bauer McNett (BMN) classification (SCAN-M 6:69), Canadian standard freeness, CSF (SCAN-C21:65) and fibre length and curl index using the FiberLab-analyser. Scanning electron microscopic (SEM) pictures were taken of various BMN fractions, and the so-called “band” microscopic method (Heinemann 2006) was used to study the peeling of the outer layers of the fibres. Laboratory sheets were made from a pulp mixture (pulps from three different refining experiments were mixed together) and tensile strength analysis (SCAN-P38) was done.

Specific energy consumption

Specific energy consumption (SEC) during the experiment was estimated using Eq. 5.

2Pt T t T ntSECm m m

ω π= = = , (5)

where P is the power [W], t is the time [s], T is the torque [Nm] and ω is the angular velocity [1/s],

PULP FOR ANALYSISPULP FOR ANALYSIS

41

n is the rotational speed [1/s], m is the mass of the pulp between the needles [kg].

3.3 Power consumption distribution in a refiner disc gap

3.3.1 Method

The calculation of the power consumption distribution is based on mass and energy balances, and also on mill trials during which consistency and temperature profiles inside the refiner are determined, as well as the production rate of the refiner and the actual motor power. Refiner disc gap is divided into sections according to sampling points (Fig. 15) and power consumption which is calculated separately in different sections. The mass balance (Eq. 6, 7, and 8) is used to calculate the mass flow through the sections. Enthalpy values of water, fibres and steam, mass flows and temperature profile are used to calculate the power consumption in different sections by energy balance (Eq. 9). Steam, fibres and water are assumed to have equal temperatures while steam is also assumed to be saturated in a disc gap. Boundary of the calculation is either the stagnation point where steam velocity is assumed to be zero or the feed of the refiner where amount of blow back steam is calculated. The consistency of pulp in the feed of the refiner was not determined in connection to mill trials. It was either calculated or assumed.

Fig. 15. Refiner disc gap divided into sections according to sampling points.

Rotor

Centre Discharge

350 608 669 728 797 825 mm

Stator

T2 T3 T4 T5 T6T1c1

c2 c3 c4 c5 c6

SECTION 1 2 3 4 5r

42

i i 1m m +=& & , (6)

i

i

(100 )i icW mc−= ⋅& & , (7)

i 1 i i i 1S S F+ − += +& & & , (8)

where

im& is the pulp flow at point i [kg/s],

iW& is the water flow at point i [kg/s],

iC is the consistency [%],

iS& is the steam flow [kg/s],

i i 1F − +& is the mass flow between steam and water phases [kg/s].

i - i 1 m(i 1) (i 1) mi i

(i 1) iW(i 1) Wi

(i 1) iS(i 1) Si

P H m H m

H W H W

H S H S ,

+ + +

++

++

= ⋅ − ⋅

+ ⋅ − ⋅

+ ⋅ − ⋅

& &

& &

& &

& &

& &

& &

(9)

where

i - i 1P + is the power consumption between points i and i + 1 [kW],

miH & is the enthalpy of pulp at point i [kJ/kg],

WiH & is the enthalpy of water at point i [kJ/kg],

SiH & is the enthalpy of steam at point i [kJ/kg].

The total power consumption ( totP ) is the sum of partial power consumption of sections, as presented in Eq. 10.

tot i - i 1P P +=∑ . (10)

3.3.2 Mill trials

Calculations were based on three series of mill trials presented below:

– Papers I and II: Mill trials during 2005 at UPM-Kymmene Kajaani mill. The SD-65 refiner, at the first stage position was used in trials. The sampling was repeated five times to eliminate process variations. Temperature profile was not measured. Production rate was 3.0 kg/h, actual motor power 10.5 MW.

– Paper II: Mill trials during 2006 at UPM-Kymmene Kajaani mill. The SD-65 refiner, at the second stage position was used in trial. The sampling was repeated five times to eliminate process variations. Temperature profile was not measured. Production rate was 3.0 kg/h, actual motor power 6.3 MW.

43

– Paper III: Mill trials between 1994–1995 at UPM-Kymmene Kaipola mill. The SD-65 type single disc refiner, at the first stage position was used in the trials: in one test trial standard refiner segments were used whereas in the other trial the low energy (LE) segments were used (Fig. 16). Temperature was measured using PT100-sensors. Temperature profiles are presented in Fig. 17. For LE-segments the motor power was 9.2 MW, production rate was 2.3 kg/s. For standard segments the power was 9.7 MW, production rate was 2.43 kg/s. The SEC was the same in both trials. Sampling was preformed only once and pulp properties were measured.

Fig. 16. Standard (left) and LE (right) segments.

Fig. 17. Temperature profile in a disc gap of the first stage refiner equipped with standard and LE-segments.

44

45

4 Results

4.1 Compressing and shearing behaviour of pulps

Pulps were more compressible at high temperatures (120–170 °C) compared to room temperature (Fig. 18) due to softening of wood. No differences were observed between higher temperatures indicating that fibres were already totally softened at 120 °C. Internal friction of the compressed pulp pad was observed to be affected by temperature (Fig. 19). The lower the temperature became, higher the disruptive shear stress was. Differences between coarse (720 ml, 540 ml) and fine pulps (160 ml) were also observed: the internal friction was lower in pulps with a lower freeness value which indicated that fines work as a kind of lubricant in shearing. The highest disruptive shear stress was achieved when the fine material was removed by Bauer McNett fractionation, as can be seen in Fig. 20.

Fig. 18. Compressibility of Scots pine (Pinus sylvestris) thinning TMP pulp (CSF 720 ml) at different temperatures.

0

0,1

0,2

0,3

0,4

0,5

0 2 4 6 8 10

P [bar]

ε

20 °C120 °C150 °C170 °C

46

Fig. 19. Disruptive shear stress as a function of the volume fraction of pulp at different temperatures. Norway spruce (Picea abies), CSF 300 ml.

Fig. 20. Disruptive shear stress as a function of volume fraction of pulp. The effect of freeness and fines. Scots pine (Pinus sylvestris).

The effect of segment surface on frictional properties, e.g., disruptive shear stress of compressed pulps, was measured using different segment surfaces in the ESCO. The total disruption was achieved only if needle plates were used in equipment, as

0,00E+00

5,00E+04

1,00E+05

1,50E+05

2,00E+05

2,50E+05

3,00E+05

3,50E+05

4,00E+05

0 0,1 0,2 0,3 0,4 0,5ε

Dis

rupt

ive

shea

r stre

ss [N

/m2]

sawmill pine 156 ml

sawmill pine 340 ml

sawmill pine 480 ml

sawmill pine 723 ml

sawmill pine - fines removed

0,00E+00

5,00E+04

1,00E+05

1,50E+05

2,00E+05

2,50E+05

3,00E+05

0 0,1 0,2 0,3 0,4 0,5 0,6

ε

Dis

rupt

ive

shea

r stre

ss [N

/m2]

120 °C140 °C150 °C160 °C

47

can be seen in Fig. 21. Using a smooth surface the lowest disruptive shear stress was measured. Using bar type surfaces the disruptive shear stress was between values obtained for smooth and needle surfaces.

Fig. 21. Disruptive shear stress as a function of compressive pressure. Different segment surfaces were used.

In pulp pad refining experiments, the development of pulp and fibre properties as a function of specific energy consumption was studied. The main results are collected in Figs. 22–27. The pulp properties were compared to mill refined pulp in the first and second stages and to pilot refined TMP pulp.

As can be seen in Fig. 22 the proportion of the R14 fibre fraction decreased from its initial value of 50% to around 12% when a total energy input of 2.4 MWh/t was achieved. The proportion of the R28 fraction did not change as a function of specific energy input while proportions of the R48, R200 and P200 fraction increased with energy input. The R14 fraction was at a much lower level than that of the reference pulps using the same specific energy input, while proportion of the R48 fraction was higher for ESCO pulps. In the cases of other fibre fractions negligible differences were observed.

The fibre length is presented in Fig. 23 where circle points refer to the reference mill pulp. Fibre length decreased in ESCO refining as energy input increased but was at a lower level compared to mill pulp. When comparing fibre length to a pilot refined pulp (raw material was the same as in pulp pad refining experiments) the fibre length was at the same level. The freeness (Fig. 24) decreased as a function of the specific energy input but it remained at a very high level compared to mill TMP pulps. In the same figure there is a curl index of pulp

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

4.5E+05

5.0E+05

0 1 2 3 4 5 6 7Compressive pressure [bar]

Dis

rupt

ice

shea

r stre

ss [N

/m2]

smooth

segments

needles

48

pad refined pulps. Pulp pad refining produces highly curled fibres which explain the measured high freeness values. An additional curl-experiment was performed to clarify the effect of pulp pad refining on the freeness of pulp. Thermomechanical pulp with an initial freeness value of 120 ml was refined using a specific energy input of 0.57 MWh/t. The freeness and curl index of the pulp before and after this experiment are also presented in Fig. 24. The curl index increased from 8% to 21% and the freeness from 120 ml to around 330 ml.

Fig. 22. Proportions of BMN fractions of pulp pad refined pulps and the first and second stage mill pulps.

0

5

10

15

20

25

30

35

40

45

50

0 0.5 1 1.5 2 2.5 3

SEC [MWh/t]

Mas

s pr

opor

tion

of B

MN

frac

tion

[%]

R 14R 28R 48R 200P 200

1st stage

2nd stage

1st stage

2nd stage

49

0

5

10

15

20

25

0.4 0.9 1.4 1.9 2.4 2.9

SEC [MWh/t]

Cur

l ind

ex [%

]

0

100

200

300

400

500

600

700

800

0 0.5 1 1.5 2 2.5 3SEC [MWh/t]

Free

ness

[%]

2nd

1st Curl experiment

Curl experiment

Fig. 23. Fibre length as a function of specific energy input. Pulp pad refined pulps ♦, mill refined pulps ο and pilot refined pulp .

Fig. 24. Curl index and freeness as a function of SEC. Pulp pad refined pulps (initial freeness of 750 ml) ♦, pulp pad refined pulp (curl experiment) and mill refined pulps ο.

Microscopic pictures of the fibre fractions (R14, R28, R48 and R200) of coarse pulp used in experiments, as well as commercial first stage pulp and ESCO-refined (pulp pad refined) pulps are presented in Figs. 25 and 26. Compared to the microscopic pictures taken before refining, it can be seen that pulp pad refining effectively develop fibres. ESCO refining produced highly fibrillated R28 and R48 fractions compared with second stage pulp, but the R200 fraction seems to have been more fibrillated in the case of the commercial second-stage pulp. Peeling of fibres was measured using a microscopic “band” method. The proportion of the inner fibre wall exposed, as presented in Fig. 27, was greater at

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5 3

SEC [MWh/t]

Fibr

e le

ngth

[mm

]

1st2nd

pilot refined

50

higher specific energy inputs and pulp pad refining produced almost totally peeled fibres. In the same figure three points were measured for the pilot refined TMP pulps. Fibres of pilot refined pulps were less peeled compared to fibres refined in pulp pad refining.

Tensile strength was measured for mixed pulp that consisted of pulps refined with higher specific energy consumptions (circled points in Fig. 28). The result was a tensile index of 26 kNm/kg which was poor compared to the first (28 kNm/kg) and second (46 kNm/kg) stage pulps. The reason lies behind the high curl-index because high curl has been reported to affect the strength of pulp. On the other hand, pulp pad refined pulps contain some untreated coarse pulp and shives from amongst the plate needles that confuse strength measurements.

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Fig. 25. Scanning electron microscopic (SEM) pictures of BMN +14 and +28 fractions of coarse TMP pulp with freeness of CSF 750 ml (before pulp pad refining), of mill pulp (second stage, 2.2 MWh/t) and of pulp pad refined pulp.

R 14 CSF 750 ml

R 14 pulp pad refined

R 14 2nd stage

R 28 CSF 750 ml

R 28 2nd stage

R 28 pulp pad refined

52

Fig. 26. Microscopic pictures of BMN +48 and +200 fractions of coarse TMP pulp with freeness of CSF 750 ml (before pulp pad refining), of mill pulp (second stage, 2.2 MWh/t) and of pulp pad refined pulp (~2.2 MWh/t).

R 200 CSF 750 mlR 48 CSF 750 ml

R 48 2nd stage R 200 2nd stage

R 200 pulp pad refinedR 48 pulp pad refined

53

Fig. 27. Proportion of exposed inner fibre wall as a function of specific energy input of pulp pad refined pulps and pilot refined TMP pulps (published by permission of Sabine Heinemann, KCL).

Fig. 28. Pulps from three different refining experiments were mixed together and strength properties were measured from this pulp mixture. Circled points present the mixed pulps.

90

91

92

93

94

95

96

97

98

99

100

0 0.5 1 1.5 2 2.5 3

SEC [MWh/t]

Prop

ortio

n of

exp

osed

inne

r fib

re w

all [

%]

0

100

200

300

400

500

600

700

800

0 0,5 1 1,5 2 2,5 3

SEC [MWh/t]

Free

ness

[%]

2nd

1st

54

4.2 Power consumption distribution in a refiner disc gap

Measured consistency profiles for first stage and second stage refiners are presented in Fig. 29 (Papers I and II). Fig. 30 presents consistency profiles for standard and LE-segments (Paper III). In both figures circular points are measured values. During calculations it was assumed that consistency increases monotonously in a refiner disc gap. Due to the curved shape of measured profiles different monotonously increasing consistency profiles were used as highlighted in Figs. 29 and 30 by grey areas.

Fig. 29. Measured consistency profiles and grey areas used in calculations for first stage refiner (left) and second stage refiner (right), presented in Papers I and II.

Fig. 30. Measured consistency profiles and grey areas used in calculations for first stage refiner with standard (left) and LE-segments (right), presented in Paper III.

The calculated results are presented in Figs. 31 and 32. The cumulative power distribution in the first and second stage refiners are presented in Fig. 31. In the

Radius [mm] Radius [mm]

Con

sist

ency

[%]

Con

sist

ency

[%]

55

first stage refiner more power is consumed in the inner parts of the refiner while in the second stage refiner the power is consumed mainly in the outer parts. When LE-segments were used in the first stage refiner, the power distribution had a similar shape to the second stage refiner, as can be seen in Fig. 32.

Fig. 31. Cumulative power consumption in the disc gap of first and second stage refiners.

Fig. 32. Cumulative power consumption in the disc gap of the first stage refiner. LE- and standard refiner segments were used.

Development of pulp properties also depends on the segment geometry used. Freeness as a function of disc radius for standard and LE-segments is presented in

1st stage

2nd stage

1st stage

2nd stage

Standard

LE

Standard

LE

Standard

LE

56

Fig. 33. Freeness decreased more rapidly in outer parts of the refiner equipped with LE-segment compared to a standard refiner. After first stage refining pulp properties were measured and are presented in Table 2. The specific energy consumption was the same in both trials but pulp properties varied: LE-segments produced shorter fibres and lower freeness.

Fig. 33. Freeness as a function of disc radius for LE- and standard refiner segments.

Table 3. Pulp properties measured after the first stage refiner for Standard and LE-segments.

Standard segments LE-segments

CSF [ml] 505 370

Fibre length [mm] 1.95 1.65

BMN +14 fraction [%] 43 34.1

BMN +28 fraction [%] 10.5 11.4

BMN +100 fraction [%] 23.6 31.1

BMN +200 fraction [%] 3 4.8

BMN -200 fraction [%] 19.9 18.6

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5 Discussion

The aim of the work was to obtain new information about refining mechanisms by studying the fibre-to-fibre interaction by equipment of shear and compression, and by calculating energy distribution based on mill trials and theoretical calculations. In Chapter 5.1 the applicability of equipment for studying refining phenomenon and results of shearing experiments are discussed. In Chapter 5.2 the theoretical method to calculate the power distribution and its limitations are considered. In the Chapter 5.3 the achieved results are pulled together and based on these results the refining mechanisms are discussed. Plausible reasons for high energy consumption of refining process are also proposed.

5.1 Equipment of shear and compression and fibre-to-fibre refining

In equipment used for shear and compression pulp was heated using steam, compressed between two plates and then the bottom of the vessel was rotated. The behaviour of pulp under compressing and shearing forces was measured as a volume fraction of pulp (Eq. 3) and as a disruptive shear stress (Eq. 4). The aim was to obtain new information about refining mechanisms, however, compared to the laboratory equipment, the situation is much more complex in the real refiner.

In the refiner, the rotational speed is very high as fibre and steam flows are very complex while volume fraction of pulp in the refiner disc gap changes with time and location. The pulp is also in motion in the refiner disc gap and thus the disruptive shear stress does not describe the state within a refiner. Heating of pulp in the equipment of shear and compression was achieved using steam flowing through the pulp pad and the heating time was so high that pulp was totally heated and softened. Inside the refiner heating of pulp occurs by frictional forces (compressing and shearing forces) and also by means of generated steam. The separation of fibres is strongly dependent on how wood softens and in this respect the situation within the equipment of shear and compression does not correspond to the situation in the real refiner. The volume fraction of pulp and disruptive shear stress are, however, exactly determined magnitudes that can be used to describe the behaviour of pulp under compressing and shearing forces. These forces affect pulp in the refiner disc gap and it is thus valuable to understand pulp behaviour under these forces even more so in a simplified environment.

Results of shearing experiments showed that friction inside the compressed pulp is dependent on temperature, freeness and fine material. The lower the

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temperature the higher the friction inside the pulp was. Removal of fines also decreased the friction inside the pulp which indicates that fine material works as a kind of lubricant in shearing. The role of friction inside the refiner is not clearly understood. However, the energy consumption of refining has been reported to decrease using refiner plates from which steam and fine material were removed through perforated plates (Johansson & Richardson 2005). In these plates the refining zone temperature was much lower compared to standard plates and also the amount of fines in the refining zone was decreased. The reason for more efficient refining is probably due to the increased friction between fibres because of the removal of fines, as well as a reduction in temperature decreases the friction inside the pulp.

The decreased friction in the refiner can affect refining, however, also in the desired way. Efficient refining has been reported by using high temperature refining (Höglund et al. 1997) and the friction in this case is lower compared to a standard TMP refining. To achieve the desired refining effect (or load), the pulp has to be compressed more than refining at lower temperatures. The result is a smaller disc clearance at higher temperatures and thus harsher refining conditions. The high temperature probably enables the harsh refining without undesirable fibre damage.

The development of fibres under compressing and shearing forces was also studied. Experiments were done using a temperature of 150 °C and compressive pressures of 1 and 3 bars. Conditions in the pulp pad refining experiments were not optimised at all. The results of the experiments showed that pulp pad refining separates the fibres from each other. In fibre-to-fibre refining the outer layers of fibres had effectively been peeled off (Fig. 27) while the fibres themselves were highly fibrillated (Figs. 25 and 26).

A proportion of coarse fibre fraction (R14) decreased to a very low level when energy input was increased, and a proportion of the middle fibre fractions and fines increased too (Fig. 22). Comparisons with commercial first and second-stage pulps showed that the amount of coarse fibre fraction (R14) was much lower in the case of ESCO pulp at the same specific energy consumption while the amount of the R48 fraction was much higher. Other fibre fractions did not differ much from those in the commercial TMP pulps. The “band” analysis (Fig. 27) performed to study the peeling of the outer layers of the fibres showed that a proportion of the inner fibre wall that was exposed increased with specific energy consumption, and pulp pad refining produced a pulp with more peeled fibres compared to pilot refined pulp. Microscopic photographs also showed highly

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fibrillated fibres (Figs. 25 and 26). The fines content of pulp refined using the ESCO device was slightly lower than that of the commercial pulps and the microscopic picture showed a less fibrillated R200 fraction. Pulp pad refining causes the fibres to fibrillate but seems not to separate the fibrils from the fibre surface.

The freeness of pulp when refined in a pulp pad was very high (Fig. 24), much higher than that of a commercial TMP pulp, even though other fibre properties were fairly similar. The reason lies in the very high curl index of the former, as presented in Fig. 24, due to the fact that latency of pulp reduces its filtration resistance (Chapter 2.2.3). To verify this result, a pulp pad refining experiment was performed using pulp with an initial freeness value of 120 ml, whereupon the freeness increased to a much higher level (330 ml) during ESCO refining. At the same time, the curl index increased from its initial level of 10% to around 21%. High curl index after pulp pad refining may derive from the experimental arrangement – pulp was cooled off in a curled state in the ESCO-equipment and did not re-straighten in hot-disintegration. Otherwise, inside the refiner, fibre-to-segment contacts in the outer refining zone (Zone III in Fig. 1) probably straighten fibres. Due to high curl index of pulp the strength properties of pulp pad refined pulps were very much lower compared to the commercial pulps.

Pulp pad refining produced pulp with generally similar papermaking properties to commercial TMP pulps. It effectively fibrillated and peeled the fibres, and coarse fibre fraction decreased rapidly to a very low level. It seems that contacts between fibres are also important for fibre development inside a commercial TMP refiner.

5.2 The method to calculate power consumption distribution

The method to calculate power consumption distribution was based on mass and energy balances and also on mill trials in which consistency profile and temperature profile, as well as production rate was measured. For calculations, the following assumptions and simplifications were made and all the possible inaccuracies in the calculation were derived from the following sources:

– Steam was assumed to be saturated in a disc gap. Fibre saturation point (the point at which all water is bounded in the cell walls) generally falls between a moisture content of 25 to 30% (Panshin & De Zeeuw 1964) which means that

60

up to 70% consistency, there is still free water. Thus the assumption of saturated steam is justified in the refining consistency range.

– Enthalpy of fibres was assumed to be in accordance to an equation determined for wood. There are no available enthalpy values determined for spruce TMP pulps. The enthalpy of wood is used to calculate the energy needed to heat pulp inside the refiner. This energy is very low compared to the total refining energy and if there is a slight inaccuracy in the enthalpy values of pulp its effect on the result of the calculation is insignificant.

– Fibres, water and steam were assumed to have equal temperatures. The temperature in a disc gap is measured by a PT-100 temperature sensor. The steam, water and fibre temperatures are not measured separately and their local temperatures can be different from those measured by the sensor. The effect of a temperature profile on the result of power consumption calculation is not, however, the critical factor. Changing the maximum temperature by 10% means the total power consumption changes less than 1% (Paper I).

– No steam flow was assumed to be over the point of maximum temperature. In refining there are steam flows in both directions (blow-back steam and forward steam). It is generally assumed that there is no steam flow over the point of maximum temperature (e.g., Miles & May 1990) and also in the calculations presented here. In the Paper I the assumption was not applied, but the results were comparable to the results of Papers II and III. The assumption thus is not a critical factor for calculated results.

– Consistency profile was assumed to be in accordance to measured values, the feed consistency was assumed to be at a certain level. Sampling from the refiner plate gap is very challenging (sampling process is presented in Papers I, II and III) and the reliability of the measured consistency profiles can be questioned. The shape of the consistency profile has been proposed due to the dilution water not being totally mixed with the pulp (Paper II). It was assumed that water can mix easily with pulp in the second stage refiner compared to chips in the first stage refiner. An acceptable reason for the strange shape found in the consistency profile can be due to back-flow of fibres in the stator side. The drier pulp from the outer parts of the refiner is flowing back in the stator side and the sample is thus not a representative sample of the whole pulp.

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In order to take account of the uncertainties connected to the consistency profile calculations were done using different monotonously increasing consistency profiles or grey areas in Figs. 29 and 30.

Results of calculations showed that power distribution depends on segment geometry used in a refiner, as well as the refiner stage. In the first stage refiner more power was consumed in the inner parts of the refiner compared to the second stage refiner. The breaking of chips occurs in the inner area of the first stage refiner and there is an intensive mixing and backflow of fibres. In the second stage refiner mechanisms are not similar because the feed of the second stage refiner is pulp instead of chips. Due to the different mechanisms the power distribution is also different.

When using unidirectional LE-segments the mixing and pulp residence time in the inner parts (Härkönen et al. 1999) as well as energy consumption in this area are reduced, compared to standard refiner segments. Differences in the pulp properties with these different segment types were, however, noteworthy. The role of the inner refining zone and its high energy consumption seems to be important for final pulp quality as well.

5.3 Mechanisms of refining – Why energy is dissipated?

It is possible to distinguish three zones (Fig. 1) inside the refiner. The role of these zones on energy consumption of refining, separation and development of fibres seems to be different. There is a chip breaking zone at the centre of the refiner where chips are broken down rapidly into coarse pulp. In the inner parts of the refiner the disc gap is high and quantity of pulp has been shown to be high too. Consequently inter-fibre contacts play an important role. In the outer parts of the refiner the segment area is higher, disc gap narrower and the coverage of pulp is lower compared to inner parts. Thus contacts between segment surface and pulp become more important in the outer parts of the refiner.

In this work inter-fibre refining or pulp pad refining was studied. The equipment of shear and compression aims to explain the phenomena mainly located in the inner refining zone of the refiner. The fibre-to-segment contacts were not considered in this work. However, based on pulp pad refining experiments it can be assumed that the function of the outer refining zone is to separate fibrils from fibre surfaces, and possibly straighten curled fibres.

The results of this study (Papers I, II and III) showed that with standard refiner segments power consumption was considerable in the inner area of disc

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gap (zones I and II, Fig. 1) where inter-fibre contacts play an important role. In this work inter-fibre refining or pulp pad refining was studied in Papers III and IV.

Pulp pad refining (Paper V) produced pulp with generally similar fibre properties to commercial TMP pulp (Chapter 5.1), even though conditions in experiments were not optimised. The coarse pre-refined pulp (720 ml) used as an initial material in pulp pad refining experiments had been produced in a pilot refiner with a specific energy consumption of 0.6 MWh/t. When this material was submitted to pulp pad refining at a specific energy level of 0.6 MWh/t with a total specific energy of 1.2 MWh/t, which is comparable to the first stage in a normal mill, fibre properties were also comparable. Energy required to separate fibres has been reported to be insignificant compared to energy needed to fibrillate fibres (Uhmeier and Salmén 1996). However, energy needed to break chips into shives in pilot refining was equal to energy required to develop shives into fibrillated fibres in pulp pad refining. The cause of high energy consumption of today’s TMP process probably derives from the phenomena in the inner parts of the refiner disc gap where refining appears not to occur in the most efficient way.

The factors affecting the pulp pad refining are temperature, compressive force and friction between pulp and segment surface. Temperature is the key factor affecting fibre separation, and it has to be high enough to soften fibres and to prevent fibre cutting. The compressive force affects the internal friction in the pulp: the higher the compressive force, a more compressed state the pulp is and higher is the frictional force inside the pulp pad. At the moment it is not known what optimum force inside the pulp pad is required to achieve the most effective fibre development. Too low a compressive force will mainly cause heating of the pulp by frictional forces without any changes in fibre structure, while too high a compressive force may cause undesirable damage to fibres. In the inner parts of the refiner, before the narrow disc gap the compressive force acting on the pulp is probably too low to achieve effective refining between fibres. The existence of a too low compressive force results in not enough friction between the fibres, so that the pulp is heated without any changes in fibre structure. This is most probably the basic cause of high energy consumption in the operation of current refining processes.

On the other hand, heating of pulp is extremely important and the role of inner refining zone is probably to heat wood material appropriately in order to separate fibres in a optimal way. Too low a temperature results in broken fibres and a similar effect can be observed if the residence time of pulp in the inner parts of the refiner is too low. Power distribution calculations showed decreased power

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in the inner parts of the refiner but also poorer pulp properties. In one situation too high a temperature affects lignin covered fibres that can not be refined any further. However, the energy needed to heat fibres is minimal compared to refining energy while the heating of fibres could certainly be preformed more effectively.

The results of pulp pad refining experiments showed that repeated compression and relaxation actions due to grooved discs are not required in order to further develop fibres that are suitable for papermaking; even if it has been mentioned to be an important mechanism inside a refiner. The function of a refiner segment surface is to cause a high tangential frictional force that such energy is efficiently transferred into the pulp pad. If friction is too low there will be no pulp pad refining at all, but energy consumption will occur at the gliding interface between pulp and segment surfaces. Results of the shearing experiments with smooth surfaces and bar type segments (Fig. 13) showed that even with the use of real segments it did not give a perfect energy transfer inside the pulp but a gliding interface between pulp and the segment surface occured. This suggests that the tangential frictional force between segment surface and the pulp in a commercial TMP refiner is not high enough to transfer energy entirely into the pulp pad but that some gliding takes place between pulp and the segment surface. Some energy inside the refiner is probably dissipated due to the frictional force between segment surface and the pulp being too low.

To attain an effective pulp pad refining inside the refiner a highly compressed pad is required. This calls for large amounts of pulp in the disc gap and the segment surface should be such that energy is very effectively transferred into the pulp. The pulp should be in such a compressed state that its shearing causes high enough frictional forces inside the pulp pad to cause efficient development of fibres and not only to heat them.

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65

6 Conclusions

Results presented in this thesis provide new information on refining mechanisms and especially on the phenomena in the inner refining zone. Based on studies done in Papers I, II and III, hypothesis one has be stated to be true. In the first stage refiner with standard refiner segments, the power consumption seems to be considerable in the inner area of disc gap where contacts between fibres are important. Based on studies presented in Papers IV and V, hypothesis two has also been stated to be true. Effective refining can be achieved in fibre-to-fibre refining without any impacts due to bar crossing if friction between segment surface and pulp is high enough and if pulp is in such a compressed state that pulp pad refining can occur. The high energy consumption of thermomechanical pulping and ineffectiveness of the refining process probably derives from having a too low frictional force that heats pulp and evaporates water without any changes in the fibre structure.

The method to calculate power distribution in a refiner disc gap gives a new tool for improving the present refining process and developing new energy saving refiner segments. Results of efficiency of the fibre-to-fibre refining may also give ideas for developing today’s refiners – how to achieve the effective pulp pad refining when effectiveness of pulp pad refining depends on, e.g., temperature and freeness of pulp (Paper IV). Otherwise, the results of this study may give ideas for developing totally new energy saving mechanical pulping processes.

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Original publications I Illikainen M, Härkönen E, Ullmar M & Niinimäki J (2006) Distribution of power

dissipation in a TMP refiner plate gap. Paperi ja Puu 88(5): 293–297. II Illikainen M, Härkönen E, Ullmar M & Niinimäki J (2007) Power consumption

distribution in a TMP refiner: comparison of the first and second stages. Tappi Journal 6(9): 18–23.

III Illikainen M, Härkönen E & Niinimäki J (2007) Power consumption and fibre development in a TMP refiner plate gap: Comparison of unidirectional and standard refiner segments. Proceedings of the 2007 International mechanical pulping conference, Minneapolis, USA.

IV Illikainen M, Härkönen E, Ullmar M & Niinimäki J (2008) Disruptive shear stress in spruce and pine TMP pulps. Paperi ja Puu 90(1): 47–52.

V Illikainen M & Niinimäki J (2007) Energy dissipation in a TMP refiner disc gap. Proceedings of The 6th Biennal Johan Gullichsen Colloquium, Espoo, Finland: 49–57.

Reprinted with permission from Paperi ja Puu (I and IV), Technical Association of the Pulp and Paper Industry (II and III) and Finnish Paper Engineers’ Association (V).

Original publications are not included in the electronic version of the dissertation.

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