What should the grindstone surface look like to produce pulp with least energy?

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Title: What should the grindstone surface look like to produce pulp with least energy?
Author: Salmi, Ari
Contributor: University of Helsinki, Faculty of Science, Department of Physics
VTT Technical Research Center of Finland
Publisher: Helsingin yliopisto
Date: 2012-05-25
Language: en
URI: http://urn.fi/URN:ISBN:978-952-10-7087-7
Thesis level: Doctoral dissertation (article-based)
Abstract: Globally, mechanical pulping consumes much energy (approximately 150 TWh of electric energy annually, approximately 27 Finnish nuclear reactors would be needed to produce this amount), and the paper consumption has been increasing despite the visions of a paper-free office. The research field has strived for energy efficiency for decades, and in recent years, innovations utilizing repetitive compressive loading have yielded significant energy savings. However, the fundamental physical reason for the energy efficiency of this technique is unknown it has been hypothesized that the compressive loading generates fatigue in the wood, resulting in an energy efficient process. In this thesis the physical question what is the significance of fatigue for mechanical pulping is broken down into five sub-questions: 1) can we model the repetitive compressive loading in a controlled manner (e.g. controllable frequency, amplitude of impacts and moisture content) and quantify the resulting decrease in stiffness (Paper I), i.e. the amount of fatigue? 2) What are the changes generated in the non-linear mechanical properties and where is the generated fatigue localized (Paper II)? 3) If the effects of fatigue in a well-controlled situation can be quantified, are the phenomena detected in the modelled process present in the actual industrial process (Paper III)? 4) What is the effect of fatigue on the energy consumption (Paper IV)? 5) How does the localization of the generated fatigue change in different wood geometries (radial and tangential, Paper V)? The proposed characterization techniques include (a) quantitative through-transmission ultrasonics for bulk wood (Paper I) and for depth profiling (Paper II, III and V). The latter allows determining stiffness tensor components in wood samples as a function of depth. (b) Micro-computed x-ray tomography which offers an image of the microstructure of the samples (Papers II, III and V); (c) An encapsulated split-Hopkinson device (ESHD, Paper II) combined with a high-speed photography system, enabling research of in situ mechanical phenomena occurring during fast compression; (d) thermoporosimetry, measuring changes in the nano-size pores in the wood cell walls induced by grinding (Paper III) and (e) quasi-static materials testing, measuring the changes in the tensile and compression behaviour in various wood directions both in the elastic and plastic regime (Paper IV). To answer the first sub-question, the EES grinding process was mimicked: a custom made device was used to generate cyclic unipolar compressional pulses at 500 Hz along the radial wood direction. These pulses were found to reduce the measured radial stiffness by up to 90%. A clear moisture dependence on the stiffness drop was discovered, lower moisture content yielding more fatigue. Above the fiber saturation point no change in the amount of fatigue as a function of moisture was discovered. Ultrasonic depth profiling results and X-ray tomography images answered the second and the fifth question: they revealed that the generated fatigue was concentrated to a layer near the sample surface in both radial and tangential wood geometries. This layer, 1-3 mm thick depending on the fatiguing parameters, comprised a zone with saturated fatigue and a transition zone extending from fatigued to intact wood. A theory based on elastic energy storage was formulated to explain this localization. ESHD experiments combined with the quasi-static tensile and compression tests revealed that at nanoscale, the fatigue is generated between the cellulose microfibrils and the surrounding lignin-hemicellulose matrix. To answer the third question, it was shown that the layer-like formation of fatigue is present in the actual industrial process, confirming a hypothesis originating from the 1960 s. After this, in order to answer the fourth question, experiments were done with a laboratory-scale grinder, which indicated that compared to non-fatigued samples the fatigued samples could be ground into similar quality pulp (Canadian Standard Freeness and fiber length) using less energy. This means that pre-fatiguing, revealed as a reduced radial stiffness, induced by repeated cyclic compression, reduces the energy consumption of the grinding process. This improvement was hypothesized to result from the previously detected, induced micro-scale damage into the cellulose lignin-hemicellulose matrix boundary, which makes it easier to liberate the fibers. The amount of energy used for pre-fatiguing was not included in the energy balance since we aimed to show whether pre-fatiguing wood reduces the energy consumption rather than to engineer an industrial process. The main conclusion is that inducing pre-fatigue can reduce the energy consumption of the wood grinding process. This could be done with a grinding surface comprising of two zones: a pre-fatiguing zone and a shearing zone.Paperin valmistamiseen tarvittavan massan tuottaminen mekaanisin keinoin (hionta, jauhaminen) kuluttaa paljon energiaa, ja niinpä mekaanisen massantuotannon tutkimus on jo pitkään pyrkinyt pienentämään kyseisten prosessien energiankulutusta. Väitöskirjatyössäni lähestyn tätä ongelmaa hiontaprosessin kannalta, ja pyrin vastaamaan kysymykseen: Millainen hionnassa käytettävän hiomakiven pinnan pitäisi olla, jotta prosessi olisi energiatehokas? Lähtökohtani oli 2000-luvun alussa julkistettu keksintö, jossa todettiin, että syklinen puristaminen yhdistettynä perinteiseen hiomapintaan tuottaa massaa energiatehokkaasti. Tutkimuksessani selvitin ne puun ominaisuudet, jotka ovat hionnan kannalta merkittäviä ja määritin ne fysikaaliset ilmiöt, joilla voisi olla merkitystä energiatehokkuuden kannalta, ja tutkin niiden vaikutusta puun kuiduttamiseen. Väitöskirjatyössäni kehitin ultraäänimenetelmän, jolla voi määrittää näytteen mekaaniset ominaisuudet syvyyden funktiona. Lisäksi käytin röntgenmikrotomografiaa (jolla määritin rakenteelliset muutokset), Split-Hopkinsonin laitetta (jolla tutkin epälineaarisia ominaisuuksia), termoporosimetriaa (jolla määritin huokosten koon) sekä mekaanista jännitys- ja puristusmittaria (jolla määritin mekaaniset ominaisuudet jännityksessä). Tutkimuksessa määritin syklisen kuormituksen vaikutuksen puun mekaanisiin ominaisuuksiin. Tärkein tulokseni oli, että syklisessä puristuksessa puuhun syntyy mekaanisesti hajonnut pintakerros. Tällaisen kerroksen synnyttäminen hionnan yhteydessä näyttää olevan mahdollinen tapa vähentää energian kulutusta prosessoinnissa. Lopulta esitin käsitykseni siitä, miksi vyöhykkeinen hiomakivipinta voisi vähentää hionnan energiakulutusta.
Subject: fysiikka
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