Proceeding Articles

Conditions for maximizing chemical and mechanical filler retention were studied through a combination of laboratory, semi-pilot and pilot scale experiments. In the first part of this work, we investigated the impact of particle size on the mechanical retention of particles in a fibre network using a modified laboratory hand sheet former. Quartz particles of well-de ned and narrow-size fractions were used to simulate pre-flocculated filler of different sizes. Five different size fractions were studied. The mechanical retention was found to increase linearly with both web fibre grammage for each quartz fraction and with particle size. These results were then validated through pilot- scale production trials where different filler floc sizes were created through pre-flocculation techniques. In the second part of this work, we studied the stability of pre-flocculated filler flocs through a set of semi-pilot scale ow loop trials. Trials were performed by pre-flocculating filler (PCC) with flocculating agents continuously and exposing the filler flocs to controlled levels of hydrodynamic shear created by ow through a partially closed gate valve. Changes in filler floc size were monitored continuously using Focused Beam Reflectance Measurements (FBRM). A clear reduction in the particle size was observed as the pressure drop increased. A major part of the floc degradation occurred at relatively low shear conditions while under the highest shear conditions, the pre-flocculated PCC floc size was reduced close to the unflocculated state. In the third part of this work, we investigated the effect of different forms of process related shear on retention polymer stability and its effect on chemical retention. A set of semi-pilot scale ow loop trials were performed to investigate the effect of elongational shear-strain and shear due to velocity differences created inside and outside a dosage nozzle respectively. We show that the effect of elongational strain created inside the dosage nozzle leads to significant retention polymer degradation while shear created outside the dosage nozzle due to velocity gradients has a smaller effect on polymer degradation. We investigate these results with a series of pilot scale production trials and show that high shear conditions created inside the dosage nozzle leads to significant reductions in chemical filler retention. However, pilot trials indicate that shear created outside the dosage nozzle can also have a significant effect on filler retention, although to a lesser extent.

Pulp screens remove contaminants from pulp suspensions and are critical to the production of high-quality paper products. Screen performance is determined by two internal components: a cylinder with apertures that pass acceptable fibres and block oversize contaminants, and a rotor that clears the apertures of any blockages. Capacity is an essential parameter of screen operation and a necessary consideration in evaluating changes made to enhance debris removal or reduce power consumption. The present study uses a pilot pulp screen to assess capacity limits, and a specialized laboratory screen with a-high-speed video camera to study what happens at a screen aperture.

What results is an understanding of some mechanisms related to the deposition and removal of fibres at an aperture where there is a time- varying ow bifurcation, and which is proposed herein as the essence of screen capacity.

In this study, we present a probabilistic approach for analysing runnability of a moving paper web with random defects. The paper web is modelled in the case of an open draw as an axially moving elastic plate that has an initial crack of random length. We derive a formula for the optimal velocity, at which the probability of fracture is limited. We study edge and central cracks perpendicular to machine direction (mode I cracks) and oblique central cracks (mixed mode). The crack length is modelled with the Weibull distribution. The effect of changing the value of distribution parameters and the probability of fracture on the optimal velocity is illustrated. It is found that the optimal velocity decreases when the expected value and variance of the crack length increase. The results also show the relation between the effect of edge and central cracks on the optimal velocity. The study is fundamental for rigorous analysis of the paper making process.

The refining impulse is here defined as the product of the normal loading force by the time. The refining impulse can be used in a beater for understanding and controlling the refining effects on fibres. The SR-degree and WRV evolutions depend only on the proposed refining impulse. For the shortening evolution of fibres, the normal loading force has to be introduced as a supplementary variable. By analogy with the variables controlling the pressing operation, namely press impulse and maximal applied pressure, the refining impulse alone, or complemented with the normal applied force have been experimentally shown to control the kinetics effects of refining on the fibrous suspension.

In recycled paper processes, stickies are at the origin of many production disturbances, such as machine breaks, defects in paper and converting problems. At the end of the recycling process, the most abundant and disturbing macro contaminants are fragments of pressure sensitive adhesives. These particles adhere to machines clothes, and clog the felts or even cause the break of the running paper web. The contamination is typically evaluated by measuring the total stickies concentration in the pulp after screening. However, industrial experience shows that it is difficult to correlate this stickies concentration with the occurrence of process disturbances. We suggest that only the amount of stickies that is effectively exposed at the surface of the sheet to the machine clothes is disturbing and is at the origin of runnability problems. In this work, we recall the definition of the stickies exposure, and use it to anticipate the effect of geometrical parameters on the fraction of stickies that are exposed at the surface of the sheet. Parameters such as stickies length and thickness, sheet thickness, or stickies orientation in the z-direction, are investigated. A new sensor is developed to characterise stickies in their 3 dimensions (without prior pressing), and discriminate them from other type of contaminants. Improvements compared to classical stickies measurements methods are discussed. The exposure of real stickies populations to machine clothes is measured in handsheets, and compared with results from the modelling.

The choice of winder type for various web materials has long been a qualitative discussion. Web materials are vast and hence the range of web material properties is also vast. Valid but conflicting opinions for an optimal winder type have been developed from experience bases that represent this vast range of web materials. The purpose of this publication is to quantify how the internal stresses in wound rolls are affected by winder type and web material properties.

The aim of this research is to gain more understanding of the physics of the transportation of materials having viscoelastic characteristics, high transport speeds, a small thickness and a large surface area. This study introduces new models that take into account both material viscoelasticity and the fluid-structure interaction between the travel- ling material and the surrounding owing fluid. A web (continuum) travelling between two fixed supports is considered, modelling the web as a Kelvin–Voigt type viscoelastic panel and the air ow as a potential ow. Stability of the system is studied with the help of its eigenfrequencies (eigenvalues) for two different types of ow geometries. First, a ow inside an enclosure with a rectangular cross-direction, through which the panel is travelling, is added to the equations of out-of-plane motion of the panel with the help of added mass coefficients. Secondly, a free stream potential ow obstructed by the travelling panel is analyzed using the analytical solution for the aerodynamic reaction pressure. Some numerical examples are given for both models.

When the dry content during pulp drying reaches a level above 75% to 80% the free water, i.e. unbound water, has been evaporated. The remaining water is bonded to the surface due to physisorption, additional energy is necessary to overcome these bonding effects. This additional energy is called heat of sorption. At 80oC and a dry content of 95% for unbleached softwood kraft pulp the evaporation energy increases up to 2800 kJ per kg water compared to the latent heat of water of about 2300 kJ/kg at the same temperature.

Different measuring methods to determine the heat of sorption HS are described in the literature, the reported values for HS of pulp show large differences. The first aim of this work is to compare the results of different measurement methods using the same sample pulp. We investigated calculation of HS from sorption isotherms collected with a conventional climate chamber (ESC) and differential vapor sorption (DVS) analysis. Furthermore we applied direct measurement of the heat flux generated by sorption using differential scanning calorimetry (DSC), differential scanning calorimetry combined with thermogravimetric analysis (DSC/TGA) and reaction calorimetry (RC). All but one measurement method delivered consistent results in the range of HS=40 kJ/kg to 70 kJ/kg additional energy due to surface sorption. The advantages and disadvantages of the different measurement techniques are discussed.

In the second part of this work the impact of pulping and pulp treatment on the heat of sorption has been investigated. Refining and bleaching seems to have no impact. Pre drying reduces the overall heat of sorption by 15 %. The addition of inorganic fillers to paper reduces its heat of sorption due to a negligible HS of the filler.

In conclusion the total additional energy caused by sorption effects is less than 2% of the overall energy necessary to fully dry the pulp. Additionally the heat of sorption is nearly unaffected by pulping, bleaching, refining or pre-drying of the pulp. Therefor the heat of sorption HS is playing only a minor role for industrial applications of paper drying.

A rheometric method based on velocity pro ling simultaneously by optical coherence tomography and the ultrasound velocity profilometry was introduced and used in a preliminary study of the rheological and boundary layer ow properties of micro fibrillated cellulose. The two velocity pro ling methods appear adequate and complementary for rheological characterization of opaque complex fluids. The ultrasound method is useful in measuring the velocity profile in the interior parts of the tube, while the optical technique is capable of high-resolution measurement of the boundary layer ow close to the tube wall.

The preliminary results obtained for a 0.4% micro-fibrillated cellulose suspension show typical shear thinning behaviour in the interior part of the tube while the near wall behaviour shows existence of a slip layer of thickness ~200 m. Both the velocity profile measurement and the imaging mode data obtained by the optical coherence tomographic method indicate that the slip layer is related to a concentration gradient appearing near the tube wall. In a sublayer of thick- ness ~100 m, the fluid appears nearly Newtonian, and the viscosity value approaches that of pure water with decreasing distance from the wall.