Birgitta Engberg

ABSTRACT By using the same selected input data in three different refining models the model predictions could be compared both in relation to each other and related to experimental data. Predictions delivered by the Miles and May model deviated most from the estimates based on measured values. The fluid dynamical model and especially the entropy model showed good agreement both when it came to describing the specific energy distribution and the local refining intensity distribution over the refiner radius. However, the entropy model had problems with predicting the mean fibre velocity close to the refiner inlet while the fluid dynamical model had a tendency to overestimate the fibre velocity when approaching the periphery. In conclusion, the fluid dynamical model and the entropy model could deliver realistic estimates. Still, the models need to be developed to be able to depict refining actions even better. To validate new or improved models, more research involving measurements from refiner gaps are needed. In a high consistency refiner fibre properties are changed to desired values in order to achieve a good papermaking pulp. High consistency refining also includes chip disintegration or defibration. The actions of a refiner can be characterised by the number of force impacts on the fibre and the severity of these impacts. The refining actions are in turn determined by both refining conditions and fibre properties. The effects of the actions result in fibres being cut, split, fibrillated etc. and thereby change the fibre properties. A refining model is here defined as a model that describes some of these phenomena. Today, there is a big uncertainty about how the fibre material is distributed in the refiner gap. However, as long as we have lower temperatures in the inlet and outlet of the refiner than we have in the plate gap, we will have a pressure peak somewhere in the refining zone. This pressure peak will then cause backflow steam. In a single disc refiner, fibre flow outwards on the rotor side and some of the fibres flow back on the stator side due to backflow steam, this according to Härkönen and Tienvieri (2001). In the region of the outward flowing steam, fibres also flow outwards on the stator side. Possibly, there is intensive turbulence in the fibre flow in the centre region before the plate gap narrows in the refining zone. This study is aiming to establish the state of the art of existing models that describe the refining actions. Models that can be verified by measurements in refining gaps, e.g. measurements of fibre distribution, load distribution, temperature profile, refiner gap width and the residence time were chosen.

ABSTRACT A new method for local strain measurement of soft materials like wood is proposed. Norway spruce samples were subjected to radial compression in an encapsulated split-Hopkinson device (ESHD). High speed photography was used at two magnifications for image based analysis. The strain estimation was made from high magnification images showing compression on local, fiber level for 1–2 growth rings and from low magnification images showing compression on sample level, for 5–8 growth rings. Strain gauges on the ESHD bars give stress and average strain for comparison. Image analysis based on PIV technique gives local and average strain propagation as a function of time. Wood is an inhomogeneous material and thus, local strain is a more proper measure of the response of the material. The high magnification captures differences between earlywood and latewood while the low magnification gives the strain distribution over the whole sample. Both magnifications are important in order to understand the response of the wood material to the sudden compression. A way to estimate the stress field was developed. The results showed similarity to the strain gauge measurement results.

Increased wood softening and refining intensity have earlier been utilized to improve refining efficiency in mechanical pulping. We have evaluated a combination of increased softening by low dose sulphite chip pretreatment and increased intensity by feeding segment design in a TMP line for production of high quality printing papers. Norway spruce wood chips were preheated, compressed in an Impressafiner and impregnated with water or sodium sulphite solutions (Na2SO3 charges 3.6 and 7.2 kg/t). Chips were refined in two parallel 68” double disc refiners using two different refining conditions: standard bidirectional segments at normal production rate (9 t/h) and feeding segments at increased production rate (11.1–12.1 t/h). The feeding segments enabled a 30 % increase in production rate. Refining with feeding segments at 12.1 t/h production rate combined with chip pretreatment with 3.6 kg/t sodium sulphite reduced the specific energy 360 kWh/t (19 %) compared to refining with standard segments and no pretreatment. Pulp properties were similar for the two configurations. The combination of feeding segments and chip pretreatment with water reduced the specific energy 180 kWh/t (9 %). Implementation of most of the technology presented has reduced the electrical energy use for the mill by approximately 80 GWh/year.

The relationship between the impactor velocity and the amount of strain localization in a single impact compression of cellular solids is known. However, few studies report on the effects of repeated high frequency compression. We therefore studied the mechanical behavior of Norway spruce, a cellular viscoelastic material, before, during, and after cyclic high frequency, high strain rate, compression. A custom made device applied 5000-20 000 unipolar (constrained compression and free relaxation) fatigue cycles with a 0.75 mm peak-to-peak amplitude at 500 Hz frequency. The consequences of this treatment were quantified by pitch-catch ultrasonic measurements and by dynamic material testing using an encapsulated Split-Hopkinson device that incorporated a high-speed camera. The ultrasonic measurements quantified a stiffness modulus drop and revealed the presence of a fatigued low modulus layer near the impacting surface. Such a localized plastic deformation is not predicted by classical mechanics. We introduce a simple model that explains several changes in the mechanical properties caused by fatiguing. The high speed images indicated pronounced strain localization in the weakest (thinnest walls) parts of the earlywood layers, and revealed strain propagation as a function of time. We present a hypothesis explaining why there is a fatigued layer formed in a piece of wood that has sustained cyclic compression and free relaxation.

Abstract Increased wood softening and refining intensity have earlier been utilized to improve refining efficiency in mechanical pulping. We have evaluated a combination of increased softening by low dose sulphite chip pretreatment and increased intensity by feeding segment design in a TMP line for production of high quality printing papers. Norway spruce wood chips were preheated, compressed in an Impressafiner and impregnated with water or sodium sulphite solutions (Na2SO3 charges 3.6 and 7.2 kg/t). Chips were refined in two parallel 68” double disc refiners using two different refining conditions: standard bidirectional segments at normal production rate (9 t/h) and feeding segments at increased production rate (11.1–12.1 t/h). The feeding segments enabled a 30 % increase in production rate. Refining with feeding segments at 12.1 t/h production rate combined with chip pretreatment with 3.6 kg/t sodium sulphite reduced the specific energy 360 kWh/t (19 %) compared to refining with standard segments and no pretreatment. Pulp properties were similar for the two configurations. The combination of feeding segments and chip pretreatment with water reduced the specific energy 180 kWh/t (9 %). Implementation of most of the technology presented has reduced the electrical energy use for the mill by approximately 80 GWh/year.

The aim of this investigation was to find new approaches to evaluate the performance of a full sized two-zoned low-consistency refiner i.e. a refiner with two stators and one rotor in between. Data from a paper mill producing TMP from Norway spruce was used in order to find a possible way to calculate the power split between the two zones. An assumption of equal amount of fibres captured between overlapping bars was found successful in order to develop equations for the power split. The equations predicted equal power in both zones at equal disc gaps. The power was found to correlate approximately linearly with the disc gap. The power split was essential to know for calculating refining intensity expressed as specific edge load and forces on fibres in the two zones. The reduction in fibre length was about 5% at 0.17 mm disc gap corresponding to 0.03 N force on fibres and 0.7 J/m specific edge load. Disc gap, force on fibres and specific edge load was found to predict the fibre shortening with approximately equal sufficiency upon changes in power and flow rate through the refiner.

The mechanical properties of Norway spruce were studied and a compression model for mechanical pulping was developed. The split-Hopkinson pressure bar technique was combined with high-speed photography to analyse local radial compression. Data analysis focussed on the differences between mechanical properties of earlywood and latewood. Measurements were conducted at both room temperature and 135 °C. The effect of pre-fatigue treatment was also studied. A simple material model was defined linearly in parts and fitted to the measurement data to quantify the differences. New results were found on the differences in inelastic behaviour of earlywood and latewood at large deformations. In addition, other results were in line with previously published results.

Numerical simulation of pulp flow in rotating and non-rotating grooves is carried out to investigate the effect of pulp rheological properties and groove geometry on the rotational motion of the pulp flow. The eucalyptus pulp suspension is considered as a working fluid in the present study whose apparent viscosity correlation is available from the experimental measurements reported in the literature. The simulations are carried out with OpenFoam for different values of pulp material, fiber concentrations, and groove cross-section. Helicity is introduced to measure the turnover rate of pulp flow in the groove due to the importance of such motion on the final properties of the pulp flow. A measurement of helicity magnitude and its distribution along the groove revealed that a change in the pulp material would significantly affect the flow structures within the groove. Further investigation on the effects of fiber concentration, c, showed that this parameter does not have a significant effect on the averaged helicity magnitude for c = 2.0 and 2.5, whereas the helicity distribution over the groove cross-section changes clearly for c = 1.5. The results showed that the helicity level is negligible for almost half of the cavity cross-section in the non-rotating groove simulations, which can be considered as a shortcoming of the original geometry of the groove. Therefore, a smaller cross-section for the groove is considered through which an enhancement in the helicity magnitude is observed.

Abstract Large eddy simulation (LES) of turbulent flow of concentrated fiber suspension or pulp is carried out to investigate the flow and turbulence structures in a channel. The simulations are carried out for the turbulent flow of Eucalyptus pulp suspension using OpenFOAM for three fiber concentrations (c=1.5, 2.0 and 2.5) and six different Reynolds numbers (6 ≤Res≤ 16600). It is observed that the variation in flow regime is similar in the two lower fiber concentrations while the flow regime is highly affected by fiber concentration for c=2.5. Visualization of vortical structures for different Reynolds numbers and fiber concentrations are used to investigate different flow regimes. Variation of apparent viscosity with Reynolds number and fiber concentration is also presented to show its effect on the turbulent properties of fiber suspension flow. It is shown that the deviation of turbulent velocity profile from that of a Newtonian flow increases with an increase in Reynolds number and fiber concentration. Also, the extend of buffer layer increases at higher Re. Using the calculated turbulent velocity profile, the values of constant in logarithmic velocity profile is proposed for fiber suspension. Finally, a discussion is presented on the variation of turbulent intensities and Reynolds stress with Reynolds number and fiber concentration.

Fatigue is essential in mechanical pulping when producing flexible paper fibers energy efficiently from stiff wood fibers. Fatigue propagation depends mainly on strain variation. The objective of the study is to determine how compressive strain is distributed between earlywood and latewood. Uneven distribution of local strain variation is likely to cause different amount of fatigue for earlywood and latewood and thus affects the usability of these fibers in the paper products. To characterize the material's response to mechanical defibration at high strain rates a unique testing equipment was utilized. The ESHD (Encapsulated Split-Hopkinson Device) at Mid Sweden University allows generation of such high strain rate pulses. Combining this with high speed photography at 50 000 fps offered the possibility of quantitative strain field mapping by image analysis. Since both wood temperature and moisture content affect strain behavior, moisture content was in this study kept constant at 45 %. Testing temperature was varied between 20°C and 135°C. Additionally, four levels of compressive fatigue treatment in the radial direction, prior to the high strain rate tests, were used to investigate how different parts of the annual rings were changed due to the fatigue treatment. In the measurements, it was noted that the strain was concentrated in the softer earlywood layers and there were unexpectedly little strain in the latewood layers. The temperature and the fatigue level clearly affected the magnitude of the strains. Higher temperature and fatigue level increased the strain difference between earlywood and latewood in the sample.

This thesis focuses on the electric energy efficiency of single stage double disc refining for production of printing grade mechanical pulp from Norway spruce wood chips. The thesis is based on the hypothesis, that more energy efficiency refining can be attained by balanced increases of wood softening and refining intensity. Five mill scale trials were performed where wood softening and refining intensity was varied by applying or changing the following process parameters and variables:Chip pretreatment/impregnation with waterLow dosages of sodium sulfite (Na2SO3) added to impregnationTemperature and retention time in the atmospheric preheater binRefining temperature (housing pressure)Feeding segment design combined with increased production rateBy combining suitable increases in wood softening and refining intensity, it was possible to reduce the specific electric energy consumption in refining by 15% (~290 kWh per bone dry ton (bdt)) while preserving important pulp properties within ±5%, compared to the standard double disc refining process. This was done by combining chip impregnation, using an addition of 0.36% (on bone dry basis) sodium sulfite, with a new feeding segment design which enabled 25% higher production rate.When using the new feeding segment design at an increased production rate at unchanged wood softening, it led to reduced fiber length and increased sheet light scattering coefficient at certain tensile index, compared with the standard segment design at normal production rate. This is consistent with the effects normally seen when the refining intensity is increasing. The specific electric energy consumption was 8% lower at a tensile index of 43.5 Nm/g (on Rapid Kothen laboratory sheets) compared to refining at lower intensity using the standard segment design at normal production rate.Mechanical chip pretreatment with subsequent water impregnation showed a reduction in specific electric energy consumption of 6% (~120 kWh/bdt). When chip impregnation was applied in a later trial with a milder chip compression, it led to increased wood softening seen as better preserved fiber length and reduced light scattering coefficient. This resulted in a reduction in tensile index at certain specific electric energy consumption when applied with the standard refining condition but to an increase in tensile index when applied with refining at higher intensity using the feeding segment design at higher production rate.An addition of 1.2% sodium sulfite during impregnation led to a sulfonate content of pulps of ~0.28% (as Na2SO3 equivalents, including post sulfonation) and an average increase in tensile index of about 8.3 Nm/g, when compared to unsulfonated pulps at certain specific electric energy consumption. The increase in tensile index correlated with increased delamination and internal fibrillation of fibers (measured by Simon’s staining), which indicate that the increase in tensile index for sulfonated pulps was a result of improved fiber flexibility and collapsibility. The reduction in disc gap at certain specific electric energy consumption in refining due to an increased wood softening after sulfonation may explain the increase in delamination and internal fibrillation for sulfonated pulps. The smaller disc gap probably led to a more intense refining, i.e. loading at higher deformation rates due to a higher degree of deformation in bar crossings.Different temperatures (80 vs. 97°C) and retention times (6 vs. 9 min.) in the atmospheric preheater bin were studied. This showed that the lower temperature and shorter retention time was beneficial for the tensile strength and light scattering of pulp when applying low dosage sodium sulfite pretreatment. This was most likely a result of too high degree of wood softening prior to defibration in the breaker bar zone when combining low dosage sodium sulfite pretreatment with the higher preheating bin temperature at longer retention time.Different refining temperatures (4.6 and 6.4 bar(g) refiner housing pressure) were evaluated both without and with low additions (0.6% and 1.2%) of sodium sulfite. Raising the refining temperature increased tensile index by 3.2 Nm/g and the addition of 1.2% sodium sulfite by 8.6 Nm/g. The combined increase (~12 Nm/g) was similar to the effect of increasing the specific electric energy consumption by 380 kWh/bdt, when comparing pulps at equal tensile index. However, the pulps produced with increased refining temperature and sodium sulfite addition had lower light scattering coefficient at certain tensile index. The combination of increased refining temperature and addition of 0.6% sodium sulfite was interesting and resulted in pulp with higher tensile index, light scattering coefficient and brightness together with lower shives content at certain specific electric energy consumption, compared with pulp produced at the lower refining temperature without addition of sodium sulfite.Finally, an implementation of the…

The chip-refining stage in the production of high yield pulps as TMP and CTMP determines a large part of the optical and mechanical characteristics of the pulp. Softening of the raw material influences where the fiber walls fracture sites will be located.In the thermomechanical pulping (TMP) processes, refining is performed at temperatures close to the lignin softening temperature, which normally leads to fractures located in the in the primary wall and outer parts of the secondary wall. When wood material is chemically treated before chip-refining the position of the fracture is altered due to that the softening and swelling properties of the fiber walls are changed. In the chemi-thermomechanical (CTMP) process most of the fractures are, therefore, located in the middle lamella or in the primary wall due to introduction of charged groups in the lignin which facilitates the fiber separation. At alkaline pH phenolic lignin structures are sulphonated, at lower pH non-phenolic structures in the lignin are also sulphonated. In the mechanical pulping processes the wood material is subjected to both low strain rates (e.g. plug screw treatment) and high strain rates (refining). Since wood is a viscoelastic material it behaves differently at different strain rates.In this study, sulphonation has been carried out using different sulphite concentrations and pH-levels and we have thereby influenced the sulphonation degree as well as where in the lignin the sulphonation takes place.We used a hydraulic testing machine for low strain-rate testing and a Split-Hopkinson pressure bar device for high strain-rate testing to categorize pretreatments according to their material softening effect and the energy needed for fiber separation. This gives us increased fundamental knowledge of how the mechanical properties of wood are affected by the sulphonation in order to develop new/improved pretreatments.

In this study the rheological properties of both native and fatigued wood was quantified under conditions relevant to mechanical pulping. This work was part of a Nordic cooperation project on "Basic phenomena in mechanical pulping. The new knowledge is intended to give us guidance on process development in mechanical pulping. The recent EES-concept [1] is assumed to offer low specific energy consumption mostly because of wood fatigue. A modulated loading device (MLD) developed at KCL was here used to fatigue wood samples. The samples were then evaluated by impulsive loading experiments carried out in an encapsulated Split-Hopkinson device (ESHD) developed at Mid Sweden University. Impulsive loadings, which extended to the non-elastic deformation region, were performed at four temperatures: 20, 65, 100 and 135°C. A higher testing temperature softened the wood samples to a high extent as could be expected. The results show that the reference samples (that was not subjected to fatigue treatment) had the highest stiffness in the investigation. The stress-strain curves of the fatigued samples were less steep, which indicate a softer more loosened material. It was evident that the fatigue treatment had loosened or weakened the wood material considerably and the differences were most pronounced at the two lowest testing temperatures.

Radial compression behaviour of spruce wood was here studied in a steam environment at high strain rate and elevated temperatures. The difference in compression characteristics between earlywood and latewood was the main focus of the study. Effects of mechanical and chemical pre-treatments were also investigated. Mechanical pre-treatment affected the E- modulus of the samples more than the chemical pre- treatments. Also elevated temperature softened the wood. No significant compression of latewood could be registered with the methods tested.

Executive SummaryAs energy prices continue to rise long-term it is very important to come up with suggestions toefficiency-improving solutions based on modifications of the existing refining technology withoutlarge investments. There are several suggestions to relatively large modifications of processsolutions, in design of refiner plate patterns, chip pre-treatment and chip feed strategies to existingrefiners, but these suggestions are often expensive and difficult to implement as the knowledge ofthe mechanisms prevalent in the refiner gap is still insufficient.To help solving this problem FSCN and CIT initiated the research project “Filling the Gap” togetherwith the companies Dametric, Holmen, Metso Paper, Norske Skog, Poyry, SCA and Stora Enso cofinancedby the Swedish authorities Vinnova and the Swedish Energy Agency. The research projectwas designed with the intension to show how to improve the electric energy efficiency of chiprefining by means of utilizing fundamental knowledge of wood material properties relevant for chiprefining in relation to refining hypotheses and in combination with output variables from new andimproved refining zone measurement methods as; exact gap distance, temperature-, force- and fibrematerial radial distributions combined with the traditional out/in-put variables normally used. Thepotential of the above mentioned ideas as well as the specific goal of this project was to show how toreach 25% efficiency improvement in existing refiners and at the same time reduce refiner causedstops by >50% and plate wear also by >50%.The data produced within the project was utilized in two ways:1. To optimize refining conditions in a static way, i.e. optimization of conditions to maximizeenergy efficiency to reach the functional fibre properties aimed for.2. To maximize process stability and minimize quality variations at the functional fibreproperties aimed for.The general conclusion from the project is that we can show that there are great opportunities toimprove electric energy efficiency in refining according to the goal by means of using the abovementioned measurement techniques. More specifically the full-scale trials performed during theperiod 2010 – beginning of 2013 showed the possibility to improve the electric energy efficiency by25% at similar functional properties of the pulp, i.e. a reduction in electricity consumption by 20%. Inorder to implement similar strategies in other TMP or CTMP lines it will just as in this case benecessary to use the same measurement system and evaluation techniques together with verythorough and statistically well controlled pulp/fibre evaluation techniques. It would of course beinteresting to implement the same techniques on as many other production lines as possible withinthe participating companies, but it must be emphasized that the procedure is very demanding. Eachproduction line needs to perform a corresponding detailed process analysis as the one performed inthe mill case study of this research project. Furthermore it would also be necessary to utilize therefiner gap measurement techniques, especially the combination of temperature profile and gapdistance measurements, in a modern but still simple process control system making it easier for theoperator to continuously run the process in a more energy efficient mode. Implementation of thetechniques evaluated in pilot scale within this research project, i.e. fibre distribution and force3distribution measurements, would of course have potential to further improve the process efficiencyas well as improve the fibre property level.

A mill scale trial was performed where chips were mechanically pretreated and impregnated with sodium sulfite (<7.2 g/kg). Pretreated chips were refined in two parallel double disc refiners (RGP68DD) using two different conditions: Turbine™ segment at higher production rate (higher intensity) and Low Shive™ segment at normal production rate (lower intensity). By combining the Turbine segments with chip pretreatment using a sodium sulphite charge of 3.6 g/kg it was possible to reduce the specific energy consumption by 15%, while maintaining pulp properties, compared with the lower intensity refining without pretreatment.

To reach maximum effect of different pretreatments we need to know how wood properties can be changed and how this can be related to both refining conditions and pulp characteristics. To understand how the material properties were affected, sulphonated wood samples were tested using several new testing techniques. The data was correlated to pulp properties of batch refined chips to learn more how the initial defibration mechanisms and pulp properties were affected by the pretreatments.

Softwood pulp flow in rotating and non-rotating grooves is numerically simulated in the present study to investigate the fluid flow and the forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp with various consistencies is available from the measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness, angular velocity and radial positions from which the pressure coefficient and shear forces values are obtained. It is found that the shear forces within the gap increase linearly with the angular velocity for all fiber consistencies investigated and in both grooves. Also, this behavior can be successfully predicted by modeling the gap flow as a Couette flow in a two-dimensional channel. Meanwhile, a more detailed analysis of the flow kinetic energy close...

The concentration of fibers and fillers in the pulp suspension is an important parameter in the monitoring process. This paper proposes a versatile optical measurement system to estimate the concentration of a solids mixture in water. The geometry used in a multi-spectrophotometer (MSM) enables the controlled observation of transmission, and forward scattering light from the suspension in the UV-visible spectral range. We have developed the new fibers mixing system which gives a homogenous distribution of the fines and fillers making it possible to increase the reproducibility and accuracy of the measurement. The data analysis is based on the Beer-Lambert law and CIELAB color space equations. The results show that the proposed method is accurate for measuring the fines and filler concentrations in multicomponent suspensions.

Increased wood softening and refining intensity have earlier been utilized to improve refining efficiency in mechanical pulping. We have evaluated a combination of increased softening by low dose sulphite chip pretreatment and increased intensity by feeding segment design in a TMP line for production of high quality printing papers. Norway spruce wood chips were preheated, compressed in an Impressafiner and impregnated with water or sodium sulphite solutions (Na2SO3 charges 3.6 and 7.2 kg/t). Chips were refined in two parallel 68” double disc refiners using two different refining conditions: standard bidirectional segments at normal production rate (9 t/h) and feeding segments at increased production rate (11.1–12.1 t/h). The feeding segments enabled a 30 % increase in production rate. Refining with feeding segments at 12.1 t/h production rate combined with chip pretreatment with 3.6 kg/t sodium sulphite reduced the specific energy 360 kWh/t (19 %) compared to refining with standard...

When producing thermomechanical pulps (TMP), wood chips and fiber material are loaded mechanically in a disc-refiner to separate the fibers and to make them flexible. In the process, much of the energy supplied is transferred to the fiber material through cyclic compression, shear and friction processes. Therefore, compression and friction characteristics are needed in order to gain a better grasp of the forces acting during refining. To this end, in this thesis, the compressive and frictional behaviors of wood were investigated under simulated chip refining conditions (i.e., hot saturated steam, high strain rate compression, and high sliding speed). Two new, custom-designed, experimental setups were developed and used. The equipment used for compression testing was based on the split Hopkinson pressure bar (SHPB) technique and the friction tester was a pin-on-disc type of tribotester (wear rig). Both pieces of equipment allow a testing environment of hot saturated steam. In the woo...

A dynamic elastoplastic compression model of Norway spruce for virtual computer optimization of mechanical pulping processes was developed. The empirical wood behaviour was fitted to a Voigt-Kelvin material model, which is based on quasi static compression and high strain rate compression tests (QSCT and HSRT, respectively) of wood at room temperature and at high temperature (80–100°C). The effect of wood fatigue was also included in the model. Wood compression stress-strain curves have an initial linear elastic region, a plateau region and a densification region. The latter was not reached in the HSRT. Earlywood (EW) and latewood (LW) contributions were considered separately. In the radial direction, the wood structure is layered and can well be modelled by serially loaded layers. The EW model was a two part linear model and the LW was modelled by a linear model, both with a strain rate dependent term. The model corresponds well to the measured values and this is the first compression model for EW and LW that is based on experiments under conditions close to those used in mechanical pulping.

Fatigue is essential in mechanical pulping when producing flexible paper fibers energy efficiently from stiff wood fibers. Fatigue propagation depends mainly on strain variation. The objective of the study is to determine how compressive strain is distributed between earlywood and latewood. Uneven distribution of local strain variation is likely to cause different amount of fatigue for earlywood and latewood and thus affects the usability of these fibers in the paper products. To characterize the material's response to mechanical defibration at high strain rates a unique testing equipment was utilized. The ESHD (Encapsulated Split-Hopkinson Device) at Mid Sweden University allows generation of such high strain rate pulses. Combining this with high speed photography at 50 000 fps offered the possibility of quantitative strain field mapping by image analysis. Since both wood temperature and moisture content affect strain behavior, moisture content was in this study kept constant a...

This thesis focuses on the electric energy efficiency of single stage double disc refining for production of printing grade mechanical pulp from Norway spruce wood chips. The thesis is based on the hypothesis, that more energy efficiency refining can be attained by balanced increases of wood softening and refining intensity. Five mill scale trials were performed where wood softening and refining intensity was varied by applying or changing the following process parameters and variables:Chip pretreatment/impregnation with waterLow dosages of sodium sulfite (Na2SO3) added to impregnationTemperature and retention time in the atmospheric preheater binRefining temperature (housing pressure)Feeding segment design combined with increased production rateBy combining suitable increases in wood softening and refining intensity, it was possible to reduce the specific electric energy consumption in refining by 15% (~290 kWh per bone dry ton (bdt)) while preserving important pulp properties with...

Radial compression behaviour of spruce wood was here studied in a steam environment at high strain rate and elevated temperatures. The difference in compression characteristics between earlywood and latewood was the main focus of the study. Effects of mechanical and chemical pre-treatments were also investigated. Mechanical pre-treatment affected the E- modulus of the samples more than the chemical pre- treatments. Also elevated temperature softened the wood. No significant compression of latewood could be registered with the methods tested.

In this study the rheological properties of both native and fatigued wood was quantified under conditions relevant to mechanical pulping. This work was part of a Nordic cooperation project on "Basic phenomena in mechanical pulping. The new knowledge is intended to give us guidance on process development in mechanical pulping. The recent EES-concept [1] is assumed to offer low specific energy consumption mostly because of wood fatigue. A modulated loading device (MLD) developed at KCL was here used to fatigue wood samples. The samples were then evaluated by impulsive loading experiments carried out in an encapsulated Split-Hopkinson device (ESHD) developed at Mid Sweden University. Impulsive loadings, which extended to the non-elastic deformation region, were performed at four temperatures: 20, 65, 100 and 135°C. A higher testing temperature softened the wood samples to a high extent as could be expected. The results show that the reference samples (that was not subjected to fat...

A mill scale trial was performed where chips were mechanically pretreated and impregnated with sodium sulfite (<7.2 g/kg). Pretreated chips were refined in two parallel double disc refiners (RGP68DD) using two different conditions: Turbine™ segment at higher production rate (higher intensity) and Low Shive™ segment at normal production rate (lower intensity). By combining the Turbine segments with chip pretreatment using a sodium sulphite charge of 3.6 g/kg it was possible to reduce the specific energy consumption by 15%, while maintaining pulp properties, compared with the lower intensity refining without pretreatment.

Executive SummaryAs energy prices continue to rise long-term it is very important to come up with suggestions toefficiency-improving solutions based on modifications of the existing refining technology withoutlarge investments. There are several suggestions to relatively large modifications of processsolutions, in design of refiner plate patterns, chip pre-treatment and chip feed strategies to existingrefiners, but these suggestions are often expensive and difficult to implement as the knowledge ofthe mechanisms prevalent in the refiner gap is still insufficient.To help solving this problem FSCN and CIT initiated the research project “Filling the Gap” togetherwith the companies Dametric, Holmen, Metso Paper, Norske Skog, Poyry, SCA and Stora Enso cofinancedby the Swedish authorities Vinnova and the Swedish Energy Agency. The research projectwas designed with the intension to show how to improve the electric energy efficiency of chiprefining by means of utilizing fundamental knowledg...

The chip-refining stage in the production of high yield pulps as TMP and CTMP determines a large part of the optical and mechanical characteristics of the pulp. Softening of the raw material influences where the fiber walls fracture sites will be located.In the thermomechanical pulping (TMP) processes, refining is performed at temperatures close to the lignin softening temperature, which normally leads to fractures located in the in the primary wall and outer parts of the secondary wall. When wood material is chemically treated before chip-refining the position of the fracture is altered due to that the softening and swelling properties of the fiber walls are changed. In the chemi-thermomechanical (CTMP) process most of the fractures are, therefore, located in the middle lamella or in the primary wall due to introduction of charged groups in the lignin which facilitates the fiber separation. At alkaline pH phenolic lignin structures are sulphonated, at lower pH non-phenolic structur...

By using the same selected input data in three different refining models the model predictions could be compared both in relation to each other and related to experimental data. Predictions delivered by the Miles and May model deviated most from the estimates based on measured values. The fluid dynamical model and especially the entropy model showed good agreement both when it came to describing the specific energy distribution and the local refining intensity distribution over the refining radius. However, the entropy model had problems with predicting the mean fibre velocity close to the refiner inlet while the fluid dynamical model had a tendency to overestimate the fibre velocity when approaching the periphery. In conclusion, the modelscould deliver realistic estimates. Still, the models need to be developed to be able to depict refining actions even better. To validate new or improved models, more research involving measurements from refiner gaps are needed.