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Wood and fibre mechanics related to the thermomechanical pulping process
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics. (FSCN – Fibre Science and Communication Network - Mekanisk massateknik)
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main objective of this thesis was to improve the understanding of some aspects on wood and fibre mechanics related to conditions in the thermomechanical pulping process. Another objective was to measure the power distribution between the rotating plates in a refiner.

 

The thesis comprises the following parts:

–A literature review aimed at describing fracture in wood and fibres as related to the thermomechanical pulping process

–An experimental study of fracture in wood under compression, at conditions similar to those in feeding of chips into preheaters and chip refiners

–An experimental study of the effect of impact velocity on the fracture of wood, related to conditions of fibre separation in the breaker bar zone in a chip refiner

–A micromechanical model of the deterioration of wood fibres, related to the development of fibre properties during the intense treatment in the small gap in the refining zone

–Measurements of the power distribution in a refiner.

 

The fracture in wood under compression was investigated by use of acoustic emission monitoring. The wood was compressed in both lateral and longitudinal directions to predict preferred modes of deformation in order to achieve desired irreversible changes in the wood structure. It was concluded that the most efficient compression direction in this respect is longitudinal. Preferable temperature at which the compression should be carried out and specific energy input needed in order to achieve substantial changes in the wood structure were also given.

 

The fibre separation step and specifically the effect of impact velocity on the fracture energy were studied by use of a falling weight impact tester. The fracture surfaces were also examined under a microscope. An increase in impact velocity resulted in an increase in fracture energy.

In the thermomechanical pulping process the fibres are subjected to lateral compression, tension and shear which causes the creation of microcracks in the fibre wall. This damage reduces the fibre wall stiffness. A simplified analytical model is presented for the prediction of the stiffness degradation due to the damage state in a wood fibre, loaded in uni-axial tension or shear. The model was based on an assumed displacement field together with the minimum total potential energy theorem. For the damage development an energy criterion was employed. The model was applied to calculate the relevant stiffness coefficients as a function of the damage state. The energy consumption in order to achieve a certain damage state in a softwood fibre by uniaxial tension or shear load was also calculated. The energy consumption was found to be dependent on the microfibril angle in the middle secondary wall, the loading case, the thicknesses of the fibre cell wall layers, and conditions such as moisture content and temperature. At conditions, prevailing at the entrance of the gap between the plates in a refiner and at relative high damage states, more energy was needed to create cracks at higher microfibril angles. The energy consumption was lower for earlywood compared to latewood fibres. For low microfibril angles, the energy consumption was lower for loading in shear compared to tension for both earlywood and latewood fibres. Material parameters, such as initial damage state and specific fracture energy, were determined by fitting of input parameters to experimental data.

Only a part of the electrical energy demand in the thermomechanical pulping process is considered to be effective in fibre separation and developing fibre properties. Therefore it is important to improve the understanding of how this energy is distributed along the refining zone.

Investigations have been carried out in a laboratory single-disc refiner. It was found that a new developed force sensor is an effective way of measuring the power distribution within the refining zone. The collected data show that the tangential force per area and consequently also the power per unit area increased with radial position.

The results in this thesis improve the understanding of the influence of some process parameters in thermomechanical pulping related wood and fibre mechanics such as loading rate, loading direction, moisture content and temperature to separate the fibres from the wood and to achieve desired irreversible changes in the fibre structure. Further, the thesis gives an insight of the spatial energy distribution in a refiner during thermomechanical pulping.

 

 

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2008. , p. 65
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 63
Keywords [en]
Acoustic emission, Chips, Compression tests, Defibration, Disc refiners, Energy consumption, Fibre structure, Force sensors, Fracture, Impact strength, Mathematical analysis, Moisture content, Picea abies, Refining, Stiffness degradation, Strains, Temperature, Thermomechanical pulping, Velocity
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-6725ISBN: 978-91-86073-15-2 (print)OAI: oai:DiVA.org:miun-6724DiVA, id: diva2:113576
Public defence
2008-11-21, O 102, SCA-salen, Holmgatan 10, Åkroken, Sundsvall, 10:00 (Swedish)
Opponent
Supervisors
Available from: 2008-10-24 Created: 2008-10-22 Last updated: 2009-06-08Bibliographically approved
List of papers
1. Effect of Temperature on Fracture of Spruce in Compression, Investigated by Use of Acoustic Emission Monitoring
Open this publication in new window or tab >>Effect of Temperature on Fracture of Spruce in Compression, Investigated by Use of Acoustic Emission Monitoring
2000 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 26, no 8, p. 294-299Article in journal (Refereed) Published
Abstract [en]

Acoustic Emission (AE) monitoring during compression of wood has been used to investigate the fracture history with specific emphasis on its dependence on temperature, moisture content, strain and loading direction. The wood was compressed in both the lateral and longitudinal directions in order to select preferred modes of deformation to achieve desired irreversible changes in the wood structure. The elastic modulus, the compressive strength and the cumulated number of AE events decreased with increasing temperature. It was concluded that the most efficient loading direction is longitudinal in order to introduce flaws in wood under compression and that a longitudinal compression of 24%, corresponding to a specific energy input of 3 kWh/ton, is needed in order to achieve substantial changes in the wood structure. The compression should be carried out at temperatures well below 120°C in order to introduce many failure sites.

Keywords
Acoustic emission, Compression tests, Fracture, Moisture content, Picea abies, Stiffness degradation, Strains, Temperature, Velocity
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-1642 (URN)000088770000006 ()2-s2.0-0034244712 (Scopus ID)247 (Local ID)247 (Archive number)247 (OAI)
Available from: 2008-09-30 Created: 2008-10-21 Last updated: 2017-12-13Bibliographically approved
2. Effect of impact velocity on the fracture of wood as related to the mechanical pulping process
Open this publication in new window or tab >>Effect of impact velocity on the fracture of wood as related to the mechanical pulping process
2001 (English)In: Wood Science and Technology, ISSN 0043-7719, E-ISSN 1432-5225, Vol. 35, no 4, p. 343-351Article in journal (Refereed) Published
Abstract [en]

The fibre separation step in refining is crucial for energy consumption in subsequent refining where the pulp properties are developed. The size reduction of chips during refining is dependent on refining intensity and chip strength. Factors affecting these two parameters are discussed in a literature review. The impact strength of chips and the break down of chips to separate fibres are also discussed. Specifically the effect of impact velocity on the fracture of wood has been studied by use of a falling weight impact tester. Samples were prepared from a freshly cut log of Norway spruce, Picea abies, and the impact strength was measured using an instrumented falling weight impact tester. An increase in impact velocity from about 2.7 to 4.8 m/s resulted in an increase in impact strength of about 50%.

 

Keywords
Energy consumption, Force sensors, Fracture, Impact strength, Picea abies, Velocity
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-1643 (URN)10.1007/s002260100100 (DOI)000170441500005 ()2-s2.0-0035423016 (Scopus ID)250 (Local ID)250 (Archive number)250 (OAI)
Available from: 2008-12-04 Created: 2008-10-21 Last updated: 2017-12-13Bibliographically approved
3. A Micromechanical Model of the Deterioration of a Wood Fibre
Open this publication in new window or tab >>A Micromechanical Model of the Deterioration of a Wood Fibre
1999 (English)In: Journal of Pulp and Paper Science, ISSN 0826-6220, Vol. 25, no 2, p. 66-71Article in journal (Refereed) Published
Abstract [en]

A simple analytical model is presented in this paper for the prediction of the stiffness degradation and the damage state in a wood fibre, loaded in uniaxial tension or shear. The model is based on an assumed displacement field together with the minimum total potential energy theorem. For the damage development, an energy criterion is employed. The model is applied to a specific example and the relevant stiffness coefficients are calculated as a function of the damage state. The damage development as a function of the applied loads is also given. The results from a specific example considered indicate that a tensile load affects the stiffness degradation to a larger extent than does a shear load.

Keywords
Energy consumption, Fibre structure, Fracture, Mathematical analysis, Moisture content, Picea abies, Refining, Stiffness degradation, Strains, Temperature, Thermomechanical pulping
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-1636 (URN)000079295000007 ()197 (Local ID)197 (Archive number)197 (OAI)
Available from: 2008-09-30 Created: 2008-10-21 Last updated: 2009-06-08Bibliographically approved
4. On the energy consumption for crack development in fibre wall in disc refining - A micromechanical approach
Open this publication in new window or tab >>On the energy consumption for crack development in fibre wall in disc refining - A micromechanical approach
2009 (English)In: Holzforschung, ISSN 0018-3830, E-ISSN 1437-434X, Vol. 63, no 2, p. 204-210Article in journal (Refereed) Published
Abstract [en]

An analytical model has been applied to calculate the acquired strain energy density in order to achieve a certain damage state in a softwood fibre by uniaxial tension or shear load. The energy density was found to be dependent on the microfibril angle in the middle secondary wall, the loading case, the thicknesses of the fibre cell wall layers, and conditions such as moisture content and temperature. At conditions, prevailing at the entrance of the gap between the plates in a refiner and at relative high damage states, more energy is needed to create cracks at higher microfibril angles. The energy density was lower for earlywood compared to latewood fibres. For low microfibril angles, the energy density was lower for loading in shear compared to tension for both earlywood and latewood fibres. Material parameters, such as initial damage state and specific fracture energy, were determined by fitting of input parameters to experimental data.

Keywords
Energy consumption, Fracture, Numerical methods, Stiffness degradation, Strain, Stress, Wood fibre
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-6720 (URN)10.1515/HF.2009.040 (DOI)000263932200011 ()2-s2.0-59449097726 (Scopus ID)
Available from: 2008-12-08 Created: 2008-10-20 Last updated: 2017-12-14Bibliographically approved
5. Measurement of the Power Distribution in a Single Disc Refiner
Open this publication in new window or tab >>Measurement of the Power Distribution in a Single Disc Refiner
1999 (English)In: Journal of Pulp and Paper Science, ISSN 0826-6220, Vol. 25, no 11, p. 384-387Article in journal (Refereed) Published
Abstract [en]

A vital parameter in the production of mechanical pulp through refining is the energy consumption. Until recently, the pulping industry has focused on the total energy consumed, without a full understanding of how this energy is distributed in the refiners. In this paper two methods are suggested by which it is possible to estimate the energy supplied at an arbitrary point along the disc radius on refiner segments. The methods are based on instrumented bar with strain gauges. In the first method, a bar is slotted so that the sensor can be viewed as consisting of a number of cantilever beams separated by slots. In the second method, the sensor consists of one continuous bar along the disc radius. The sensors are calibrated such that influence coefficients are obtained for the slotted case and an influence function is obtained for the continuous one. The tangential forces are obtained directly in the first method, while in the second they are given by an integral equation which can be solved numerically. From the knowledge of the tangential force the power distribution can be calculated. Experimental results from the use of the slotted sensor are also given.

Keywords
Disc refiners, Energy consumption, Force sensors, Mathematical analysis, Picea abies, Thermomechanical pulping
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:miun:diva-1641 (URN)000083844000003 ()245 (Local ID)245 (Archive number)245 (OAI)
Available from: 2008-09-30 Created: 2008-10-21Bibliographically approved

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