2009 Volume 1

In this work we propose a novel separation technique based upon the control of the threshold for motion of different classes of particles in yield stress fluids. The principle is demonstrated by observing the motion of particles under the influence of a centrifugal force in a weak gel. Here we develop calibration curves of the force required to initiate motion in a gel under numerous configurations of the particles. Demonstration separations of bidisperse suspensions are reported. Here we achieve complete separation of dilute suspensions based upon length, diameter, or density.

The objectives of this article are:

– First, to theoretically propose a unified concept: the net normal force per crossing point,

– Second, to experimentally undertake refining trials on a pilot disc refiner in order to compare all concepts for the refining intensity and to validate the chosen one.

We will begin by re-visiting the old concepts of the refining intensity, in the low consistency regime. After a theoretical proof based upon the physics of the phenomena, applied to beaters and industrial refiners, a unified concept of the refining intensity is proposed and strengthened: the net normal force per crossing point.

Then, experimentations are undertaken on a pilot refiner (single disc) in hydracycle (or batch) conditions. More precisely, the effects of the grinding codes and of the average crossing angle of the bars are analyzed in a set of 6 refining trials. For these experimentations, different engineering concepts of the refining intensity are compared (specific edge load Bs, specific surface load SSL, modified edge load MEL, net tangential force per crossing point and net normal force per crossing point). These refining intensities should allow to analysing the cutting kinetics of fibres.

All the chosen engineering concepts reach this goal more or less however the net normal force per crossing point is the best tool. Indeed, through the range of the data concerned, it revealed a clear monotonous evolution with the cutting kinetics on fibres. The more is the net normal force per crossing point, the more is the cutting effect on fibres.

Transport coefficients and correlations recently used to describe surfactant contribution to particle and water transport in a laboratory flotation column were used to simulate the impact of surfactant contamination on the flotation selectivity of industrial two-stage deinking lines. Simulation results showed that surfactants are slightly removed in the first flotation stage and are concentrated in the second one, where they induce a drop in ink flotation and in fibre entrainment. Subsequently, flotation units in the second stage displayed lower ink removal than in the first stage. In the presence of a constant water reject flow, the increase in surfactant contamination in the pulp stock gave a general decrease in the removal of suspended solids. Surfactant removal increased from 5 to 50%, however, this increase was not sufficient to prevent surfactant accumulation in the deinking line. Simulation results were compared with data collected in an industrial deinking line running in similar conditions and pulp composition, ink and surfactant removal obtained with low surfactant contamination were in line with experimental data.

Biochemical additives encompass materials added to the papermaking operation that are derived from biological origins. Other than starch, the majority of the biochemical additives currently used in the paper industry are enzymatic. Enzymes are protein structures that speed a particular chemical reaction. The enzymes are not consumed during the reaction and can be used repeatedly. The enzymes used in the paper industry typically target one of the four major components of wood: cellulose, hemicellulose, lignin or extractives. Enzymes have been used industrially to aid in bleaching, reduce pitch, enhance strength, alter pulp freeness, and aid in paper machine cleaning. This review focuses on the use of enzymes in the papermaking operation, but also addresses the use of enzymes in other areas of the pulp and paper mill. There has also been considerable work in the use of fungus for improving both mechanical and chemical pulping operations. This is considered a separate topic and is only briefly addressed in this review. The future of biochemical additives may extend well beyond the current use of enzymes and a few notes on potential application are given.

We consider here the behaviour of wood fibre suspension with fibre concentration above that of sedimentation in a pressure driven flow in a straight pipe with smooth walls. The flow behaviour can be roughly divided in two main regimes: the plug flow regime that occurs at low flow rates and the drag reduction regime that occurs at high flow rates. We utilized new experimental methods in order to gain more detailed understanding on the flow behaviour of wood fibre suspensions, and especially on the relevant physical phenomena inducing such behaviour. In addition to carrying out conventional loss experiment, the velocity profiles across the pipe were measured using pulsed ultrasound velocimetry (PUDV) techniques, and the thickness of the lubrication layer in fully developed flow was measured using a laser optical device. Based on our direct measurements, we were able to identify five different flow regimes in suspension flows. In addition, we refined the qualitative picture of these flows in relation to the forming of fibre plug and to the physical phenomena taking place in transition from one flow regime to another one.

A model is presented to describe the orientation and concentration state of semi-dilute, rigid fiber suspensions in a rectangular channel flow. A probability distribution function is used to describe the local orientation and concentration state of the suspension and evolves according to a Fokker-Plank type equation. Long range hydrodynamic fiber-fiber interactions are modeled using the approach outlined by Folgar and Tucker (J. Reinforced Plast. Comp. 3 98–119 1984). Near the channel walls, we apply the no-flux boundary conditions proposed by Schiek and Shaqfeh (J. Fluid Mech. 296, 271–324, 1995). Geometric constraints are used to couple the fibers’ rotary motion with its translational motion. This eliminates physically unrealistic orientation states in the near-wall region. A two-way coupling between the fiber orientation state and the momentum equations of the suspending fluid is considered. Experiments are performed to validate the numerical model by visualizing the motion of tracer fibers in an index-of-refraction matched suspension. The orientation distribution function is determined experimentally as a function of channel height. The results indicate that at distances less than one half fiber length from the channel walls, the model accurately predicts the available fiber orientation states and the distribution of fibers amongst these states. The model further predicts a sharp concentration gradient in this region.

A particle-level numerical model is used to simulate forming with a twin-wire former configuration. The development of the paper structure along the length of the former is observed to explain the effects of the dewatering elements on the paper structure at different jet-to-wire speed ratios, consistencies, and target basis weights. The simulations indicate that most of the structure development takes place in the initial part of forming (forming roll) and, in some instances, at the drop to atmospheric pressure after the forming roll. Dramatic effects on the through-thickness fibre orientation anisotropy are observed when the consistency is varied by changing the jet thickness, while changes in basis weight had less impact. The through-thickness concentration gradient was almost uniform throughout the forming process, except in the lower range of typical papermaking consistencies. This indicates that the dewatering mechanism is normally thickening, rather than filtration.

Single fibre flexibility is widely recognised as an important parameter in the papermaking process. A novel flow cell based method for its measurement was developed at the Institute for Paper, Pulp and Fibre Technology at Graz University of Technology. A special flow channel geometry is used to induce high shear forces in a laminar flow regime. The movement of single fibres passing the highly sheared region is recorded by means of a high speed image acquisition system. Based on the reactions of the fibres to the fluid forces a flexibility parameter is determined for each evaluated fibre.

The perceived value of paper products depends not only upon their performance but also upon their visual appeal. The optical properties of paper, including whiteness, brightness, opacity, and gloss, affect its visual perception and appeal. From a practical point of view, it is important to quantify these optical properties by means of reliable and repeatable measurement methods, and furthermore, to relate these measured values to the structure of paper and characteristics of its constituents. This would allow papermakers to design new products with improved quality and reduced cost. In recent years, significant progress has been made in terms of the fundamental understanding of light-paper interaction and its effect on paper’s appearance. The introduction of digital imaging technology has led to the emergence of a new category of optical testing methods and has provided fresh insights into the relationship between paper’s structure and its optical properties. These developments were complemented by advances in the theoretical treatment of light propagation in paper. In particular, wave scattering theories in random media are finding increasing applicability in gaining a better understanding of the optical properties of paper. In this document, a review of these advancements is presented.

The conventional way to evaluate dimensional stability, regardless of end-use purpose, is to measure the change in dimensions when the moisture content is changed by changing the relative humidity. Sorption of moisture from moist air is a relatively slow process and for the evaluation of printing papers this may not be the most appropriate method.

In the present work, data from conventional hygroexpansion measurements has been compared with data from hydroexpansion measurements, i.e. expansions caused by the sorption of liquid water, sprayed onto papers printed with a random speckle pattern, the expansion being monitored by electronic speckle photography.

Sheets made from different pulps, with different fines contents and different modifications were studied at different grammages and water-transfer levels. The effect of drying-mode, i.e. restrained drying or free drying, was also studied. It was concluded that sheets expand less with a given amount of adsorbed water when it is sorbed in liquid form rather than from moist air. Chemical treatments known to increase both the dry and the wet strength, e.g. polyelectrolyte multilayers and cross-linking through periodate oxidation, did not significantly improve the dimensional stability when the papers were exposed to liquid water.