Local transport processes may be strongly affected by the presence of dispersed particles.
This presentation reports on an experimental study on local transport interactions in pulp suspensions by using mixing-sensitive chemical reactions. It is shown that there is a significant reduction in the amount of local energy dissipation, with the reduction mainly dependent on dispersed phase volume concentration, (Cv) particle asymmetry (L/d), as well as the characteristic particle size in relation to the scale of energy containing eddies.
Paper 317b: Diffusion of Moisture in Liquid Packaging Board
Moisture transport governs the performance of liquid packaging board to large extents.
In this paper, we present results from our experimental program to determine the moisture diffusivity in liquid packaging boards. The diffusivity of moisture was measured in all the principal directions of paper. In-plane diffusivity is measured and compared with transverse values of the diffusivity.
A comparison of the diffusion properties in-plane and in the transverse direction with the flow properties i.e. permeability of paper is also presented. A theoretical formulation to describe unsteady diffusion processes in such materials is also shown.
Paper 317c: Porosity Change and Plastic Strain Profiles in Paper
Calendering: Computer-Aided Theory and Confocal Laser Scanning
Microscopy
Microstructure of the fiber network of paper changes during soft-nip calendering. The purpose of this study is to correlate the structural changes with the macroscopic elasto-viscoplastic response of the paper sheet. Alteration of thickness and porosity, and plastic strain are modeled with computer-aided theory and compared with not only paper thickness measurements but also visualizations of the fiber network before and after calendering.
Finite, compressible elasto-viscoplasticity is used to describe the paper deformation in the deformable nip. A modified Drucker - Prager yield criterion, represented by a yield surface in stress space, separates regimes of finite, compressible elastic deformation from regimes with post-yield finite viscous or viscoelastic response. The deformation of the soft calendering roll is modelled by a constitutive relation of rubber-like material. The governing equations of mass and momentum conservation are solved with the Galerkin / Finite Element Method to predict the caliper reduction, the extent of elastic and viscoplastic regimes, and the stress distribution in the paper.
Handsheets made of Aspen pulp were labeled with a fluorescent dye. Subsequently, the alterations in the fiber structure were visualized by confocal laser scanning microscope (CLSM) before and after calendering. Thresholding methods were employed to measure the porosity change and plastic strain induced by the process. Confocal images are unambiguous when the focal plane is less than about 10 mm from a surface of the 60 g/m2 grammage handsheets that were tested. Visualization of the curvature change, compaction and breakage of fibers revealed how the deformation mechanisms varied with depth into the paper under different loading, temperature, and speed of processing.
Paper 317d: Investigation and Improvements of the Modified Water
Retention Value Test
The Modified Water Retention Value test (MWRV) is under development at the IPST. The targeted goal is to provide a robust and relatively quick, yet simple and sensitive laboratory procedure for characterizing pulp properties in a useful way.
It is well-known that the conventional Water Retention Value (WRV) correlates with press dewatering better than conventional freeness tests. However, the standardized WRV test does not provide a material property, rather the results are "procedure dependent" - especially dependent on sample basis weight. Our approach strives to provide more fundamental information about material characteristics, which also combines with robustness of the test procedure - the results must not depend strongly on small variations in the measurement routine.
During centrifugation, the sample remains in a stratified condition when equilibrium is attained. On one hand the saturation gradient will correspond to the relationship between capillary pressure and saturation level, on the other hand there may be compressibility effects causing stratification as indicated by porosity.
One way to deal with the first stratification effect, variation of saturation level, is to replace the low capillary pressure and high saturation region of the sample with a "dummy" porous medium, so that all of the actual sample would ideally be at nearly irreducible saturation. In this work, the determination of properties of porous support plates is shown, using an uncommon variation of the centrifugation technique, and the relationship of properties of these plates to measurements with pulp samples is inspected and discussed - some practical guidelines for selection of suitable porous support disks have emerged.
The variation of centrifugation technique is based on using a stack of platelike samples, centrifuged until equilibrium is reached, and immediately separated after centrifugation to detect local average saturation levels for each of the plates. It is shown that, for the type of incompressible plates inspected, the Brooks-Corey type approximation matches our data reasonably well. Three parameters are identified by a non-linear least-squares fit, when this model is used.
Calculations for inspecting equilibration times, during centrifugation and prior to separation of the plates, are presented.
( The second stratification effect - porosity or compression level gradient - can be counteracted by using a top load or a low basis weight sample. As the compression response of pulp diminishes with base loading level, a top load will ensure that "autocompression" by the sample's own weight will have a minimal effect - autocompression is similarly avoided by low overall mass. Pending availability of data, this effect is also discussed based on experimental data. )
Paper 317e: Visualization of Flexographic and Offset Ink Adsorption Processes at Bubble Surfaces
This paper presents results from a technique that images and quantitatively measures adsorption rates for ink removal at bubble surfaces for systematic study of froth flotation and dissolved-air flotation processes important to recycling newsprint fibers. Two facilities were designed and constructed for visualization of ink particles in flows around bubbles: the bubble suspending facility where bubbles are suspended in a carefully controlled down flow of water in a 2-in clear pipe and the stationary bubble facility where bubbles are held at the tip of a needle in a clear tank. A bubble injection system creates bubbles with consistent but variable sizes ranging in diameters from 0.2 to 1.9 mm. An imaging system was developed to obtain high resolution and high speed (500 fps) digital images and video. Experiments have been conducted with offset inks and flexographic inks dispersed in model ink solutions that allow control of the solution chemistry while the water is in contact with the air bubble.
Time series visualizations and images are presented for flexo and offset ink attachment and interaction at the surfaces of bubbles in sodium oleate and calcium chloride solutions. Flexo ink and bubble surface interactions imaged for stationary bubbles in the tank facility show that, as expected, there is no observable ink adsorption. Image sequences show the offset ink adsorbing to bubbles when the pH is above 9.5. Some offset ink particles and agglomerates adsorbed to the bubble surface, while others were observed to pass by the bubble without adsorbing. Results from imaging in bubble suspending facility show the adsorption of the ink on bubbles in a flowing field. An image processing method is described for quantifying the size and number of adsorbed ink particles. Studies are presented to analyze effectiveness of different chemistries, inks, and bubble sizes to characterize models for design and operation of dissolved air flotation and flotation deinking processes.
In papermaking, the dewatering of the pulp is a crucial processing step in which water is forced out of the pulp and drained through a press fabric, consisting of several layers of felt needled onto a baseweave. In order to improve processability, the relations governing liquid flow in the fibrous material need to be known. Vital for this flow is the constant of proportionality between the driving pressure gradient and the flow rate, i.e. the permeability. This work focuses on the anisotropic permeability of felt, in our case a material consisting of long, crimped PA6 fibres, loosely needled together. The in-plane permeabilities of the felt is measured as a function of fibre volume fraction, fibre diameter and fibre orientation. The data compares surprisingly well with conventional permeability models, but those only apply for flow along or perpendicular to aligned fibre arrays. To model the influence of fibre orientation distribution, a micromechanical approach based on an anisotropic drag law is proposed. The drag law accounts for hydrodynamic interactions with other fibres in a mean-field sense. The model requires a second-order orientation tensor, which is obtained experimentally from a frequency plot of a Fourier transformed image of a felt. For simplicity, the fibre orientation distribution is assumed to be planar.
Paper 317g: Transverse and in-plane Pore Structure Characterization
of Paper
The structure of paper and board plays a critical role not only during the manufacturing process but also during end use applications. In this paper, image analysis technique is used to characterize the microstructure of paper samples. The images of paper samples were obtained using a non-destructive technique "X-Ray micro computed tomography" (X-ray CT). Three different thresholding methods are used to convert the X-Ray images into bi-level images. Porosity, specific surface area and pore size distribution are then calculated using image analysis and compared with those obtained from traditional techniques such as mercury intrusion porosimetry, caliper and basis weight. Pore structure as calculated from the in-plane and transverse directions have been shown to very different. This may help partly explain the different liquid and vapor transport properties in the in-plane and transverse directions.
Paper 317h: Water and Air Permeability of Wet Sheets
The water and air permeabilites of wet sheets as functions of sheet porosity and solids were experimentally studied. sheets as functions of sheet porosity and solids were experimentally studied. The purpose was to identify the basic trends and possible correlations between water and air permeability.
Air permeability of wet sheet was found to be not unique and not monotonic function of porosity. Air permeability may significantly vary at the same porosity depending on compressive history of the sheet. Water permeability was unique and monotonic function of porosity. The sheets at different initial solids had similar water permeabilities at equal porosities. The results obtained have practical implications for optimal design of pressing and drying section of paper machine.
Experimental results for water and air permeability measurements were translated into non-dimensional form. In general, a discrepancy between non-dimensional water and air permeability was observed. However, there were regions of the correlation which may indicate similarity of porous structure for water and air flow.
Paper 317i: Ultrafiltration Treatment of Bleach Plant Filtrates
Ultrafiltration of alkaline bleach plant filtrate has proven a feasible way to reduce the environmental impact of kraft pulp mills. A good removal of hard-to-biodegrade organic substances combined with a low retention of chloride ions make the process very attractive. Besides reducing the load on the biological treatment plant the ultrafiltration improves the possibility to degrade the remaining organic matter. The concentrated stream from the membrane filtration is recycled to the brown side. The potentially negative impact in form of corrosion is minimized by the small amount of recycled chloride ions. In a project at IPST, the ultrafiltration process has been studied through pilot-scale trials at a pulp mill.
