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Fulltext I DIVA Thesis for the degree of Doctor of Technology, Sundsvall 2008 WOOD AND FIBRE MECHANICS RELATED TO THE THERMOMECHANICAL PULPING PROCESS Jan-Erik Berg Supervisors: Per Engstrand Per A. Gradin FSCN - Fibre Science and Communication Network Department of Natural Sciences, Engineering and Mathematics Mid Sweden University, SE-851 70 Sundsvall, Sweden ISSN 1652-893X, Mid Sweden University Doctoral Thesis 63 ISBN 978-91-86073-15-2 FSCN Fibre Science and Communication Network - ett skogsindustriellt forskningsprogram vid Mittuniversitetet Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall framläggs till offentlig granskning för avläggande av teknologie doktorsexamen fredag, 21 november, 2008, klockan 10.00 i sal O 102 (SCA-salen), Mittuniversitetet Sundsvall. Seminariet kommer att hållas på svenska. WOOD AND FIBRE MECHANICS RELATED TO THE THERMOMECHANICAL PULPING PROCESS Jan-Erik Berg © Jan-Erik Berg, 2008 FSCN - Fibre Science and Communication Network Department of Natural Sciences, Engineering and Mathematics Mid Sweden University, SE-851 70 Sundsvall Sweden Telephone: +46 (0)771-975 000 Printed by Kopieringen Mittuniversitetet, Sundsvall, Sweden, 2008 i WOOD AND FIBRE MECHANICS RELATED TO THE THERMOMECHANICAL PULPING PROCESS Jan-Erik Berg FSCN - Fibre Science and Communication Network Department of Natural Sciences, Engineering and Mathematics Mid Sweden University, SE-851 70 Sundsvall, Sweden ISSN 1652-893X, Mid Sweden University Doctoral Thesis 63 ISBN 978-91-86073-15-2 ABSTRACT 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. ii 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. iii Keywords: 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 iv SAMMANFATTNING Det huvudsakliga syftet med föreliggande avhandling var att öka förståelsen för några aspekter av ved- och fibermekanik i samband med betingelser som råder vid framställning av mekanisk massa. Ytterligare ett syfte var att mäta den radiella effektfördelningen mellan de roterande skivorna i en raffinör. Avhandlingen består av följande delar: – En litteratursammanfattning över brott i ved och fibrer i samband med framställning av mekanisk massa – En experimentell undersökning av brott i veden vid kompression vid betingelser som råder vid inmatning av flis till förvärmare och raffinörer – En experimentell undersökning av slaghastighetens inverkan på brott i veden vid betingelser som råder vid defibrering i den inledande grovmönstrade malzonen – En mikromekanisk modell av vedfiberns nedbrytning i samband med att fiberegenskaper utvecklas i den intensiva bearbetningen i den smala spalten mellan skivorna i den avslutande raffineringszonen – Mätning av den radiella effektfördelningen mellan de roterande skivorna i en raffinör. Brott i ved under kompression undersöktes med hjälp av akustisk emmisionsteknik. Veden komprimerades både lateralt och longitudinellt för att förutsäga vilka belastningsfall som är att föredra för att ge vedstrukturen irreversibla förändringar. Slutsatsen var att longitudinell kompression var den mest effektiva i denna bemärkelse. Vidare angavs vid vilken temperatur kompressionen bör utföras samt den specifika energiförbrukning som behövs för att uppnå betydande förändringar i vedstrukturen. Fiberseparationsfasen och speciellt brottenergins beroende av slaghastigheten studerades med hjälp av en fallhammare och brottytorna studerades i mikroskop. Slutsatsen var att ökad slaghastighet resulterade i ökad brottenergi. Vid framställning av mekanisk massa utsätts fibrerna för kompression tvärs fiberriktningen, drag och skjuvning vilket leder till mikrosprickor i fiberväggen och därigenom reduceras fiberväggens styvhet. En förenklad analytisk modell presenteras som förutsäger styvhetsreduktionen pga. skadetillståndet i en vedfiber som belastas enaxligt i drag eller skjuvning. Modellen baserades på en lämplig förskjutningsansats tillsammans med satsen om potentiella energins minimum. Ett energikriterium användes för att beräkna skadans utveckling. Genom att tillämpa modellen på ett specifikt exempel kunde relevanta styvhetskoefficienter beräknas som funktion av skadetillståndet. v Energiförbrukningen för att uppnå ett visst skadetillstånd visade sig vara beroende av mikrofibrillvinkeln i den mellersta sekundärväggen, belastningsfall, tjocklekarna på fiberväggens skikt och betingelser som fukthalt och temperatur. Vid betingelser liknade de som råder vid ingången av spalten mellan skivorna i en raffinör var energiförbrukningen mestadels lägre för vårvedsfibrer i jämförelse med sommarvedsfibrer. Materialparametrar som initialt skadetillstånd och specifik brottenergi bestämdes genom anpassning av indata till experimentella data. Endast en del av elenergibehovet vid termomekanisk massaframställning betraktas som effektiv för fiberseparation och utvecklande av fiberegenskaper. Det är därför viktigt att öka förståelsen hur denna energi fördelas längs raffineringszonen. Försök har utförts i en skivraffinör
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