Runnability Problems Research Articles

The influence of different shapes and size distributions of coating pigments on packing and dewatering

Particle shape and particle-size distribution (PSD) are important factors for pigment packing and water retention in the pigment coating process, being closely associated with many runnability problems. Diverse experimental investigations on the packing and dewatering of pigment slurries have been made. However, theoretical approaches remain lacking. This paper presents a joint experimental and theoretical analysis of the influence of pigment shape and PSD on pigment packing and dewatering. The relative viscosities of very dilute pigment slurries were measured and used to determine the intrinsic viscosity, based on the Einstein equation. The deviation of pigment shape from spherical caused an increase in viscosity of the slurries. Acicular precipitated calcium carbonates gave lower shape factor values than platy clay particles, which indicated that the needle-like particles could rotate more easily around their long axes under shearing conditions. The packing of pigments, which determines porosity, permeability, light scattering, and mechanical properties, was examined. The packing volumes of nonspherical pigments were estimated based on their respective PSDs, using an algorithm developed for spherical particles, modified with a correction factor. This model is based on the close random-packing volume fraction ( $$\emptyset_{\text{p}}^{ 2}$$ ) of particles of uniform size being an independent variable, regardless of their shape. Therefore, it was valid as long as the particles always achieve the same close random-packing volume ( $$\emptyset_{\text{p}}^{ 1}$$ ) by sedimentation or tapping. Overall, the effect of the particle shape and PSD on the dewatering and filter-cake permeability (Kf) was analyzed. The dewatering rates were measured with a common pressure filtration method, and the permeability of the filter cakes was obtained by fitting the data to a filtration equation. The particle size and PSD were found to influence the permeability constant, but there was little correlation between the permeability and particle shape.

Papermaking: MD strain and bagginess due to calendering

One of the problems encountered in paper converting is caused by the occurrence of webs, which essentially is when the tension profile of the paper web is uneven. In an area with low tension the paper is longer, which results in bagginess. The baggy parts can not usually be stretched to even out the tension of the paper web in a converting machine, with the result that runnability problems are likely to occur. The aim of the work described in this thesis was to investigate three particular stages in papermaking, namely drying, calendering and storage, and rank them according to their propensity for inducing baggy webs. The focus was placed on investigating the effects of uneven moisture and grammage profiles on the machine-direction strain difference profile. The largest strain difference occurred when there were systematic thick streaks throughout a reel that formed ridges. Stress relaxation during storage then gave rise to a difference in strain of 0.14% when the ridge height was around 2-3 mm. Thickness variations due to variations in grammage is also a source of moisture variation. A difference in moisture of 5% in the calendering stage resulted in strain differences of about 0.05-0.08%. These strain differences resulted in creases being formed as early on as in the calender nip when differences in both grammage and moisture content were present. Most creases appeared when the moisture difference was 2-8%. The difference in grammage could be large without creases being formed when no differences in moisture content were present. A moisture difference of about 5-6% during drying resulted in a strain difference of 0.1% measured on isotropic samples. The moist area turned into a tight streak when the moisture difference appeared at moisture contents higher than 25%. At moisture contents lower than 20%, on the contrary, the moist area turned into a slack streak. The conclusion drawn is that papermakers should concentrate first and foremost on eliminating variations in grammage, especially if these are systematic. This would also eliminate some variations in moisture content, which would solve more problems.

Biofilm-forming bacteria with varying tolerance to peracetic acid from a paper machine

Biofilms cause runnability problems in paper machines and are therefore controlled with biocides. Peracetic acid is usually effective in preventing bulky biofilms. This study investigated the microbiological status of a paper machine where low concentrations (≤ 15 ppm active ingredient) of peracetic acid had been used for several years. The paper machine contained a low amount of biofilms. Biofilm-forming bacteria from this environment were isolated and characterized by 16S rRNA gene sequencing, whole-cell fatty acid analysis, biochemical tests, and DNA fingerprinting. Seventy-five percent of the isolates were identified as members of the subclades Sphingomonas trueperi and S. aquatilis, and the others as species of the genera Burkholderia (B. cepacia complex), Methylobacterium, and Rhizobium. Although the isolation media were suitable for the common paper machine biofoulers Deinococcus, Meiothermus, and Pseudoxanthomonas, none of these were found, indicating that peracetic acid had prevented their growth. Spontaneous, irreversible loss of the ability to form biofilm was observed during subculturing of certain isolates of the subclade S. trueperi. The Sphingomonas isolates formed monoculture biofilms that tolerated peracetic acid at concentrations (10 ppm active ingredient) used for antifouling in paper machines. High pH and low conductivity of the process waters favored the peracetic acid tolerance of Sphingomonas sp. biofilms. This appears to be the first report on sphingomonads as biofilm formers in warm water using industries.

Paper Physics: Plastic strain of moisture streaks at different moisture contents

An uneven moisture profile in the paper web during drying can cause runnability problems due to slack areas in the paper web. This study aims at indentifying the moisture level at which it is most ...

Replacement of fresh water use by final effluent recovery in a highly optimized 100% recovered paper mill

A further closure of the water circuit in paper mills with a relative high optimization of their water network is limited by the increase of contamination in the water and runnability problems of the paper machine. Therefore, new strategies for saving water must be focussed on the treatment of final effluents of the paper mill, aiming to obtain high quality water that may replace fresh water use in some applications. An appropriate treatment train performed at pilot scale, consisting on a previous clarification stage followed by anaerobic and aerobic treatments, ultrafiltration, and reverse osmosis, made possible producing the highest water quality from the final effluent of the mill. Anaerobic pre-treatment showed very good performance assisting the aerobic stage on removing organics and sulphates, besides it produced enough biogas for being considered as cost-effective. Permeate recovery depended on the silica content of the paper mill effluent, and it was limited to a 50-60%. The reject of the membranes fully met the legislation requirements imposed to effluents arriving to municipal wastewater treatment plants.

Adsorption of surfactant-rich stickies onto mineral surfaces

"Stickies" are tacky species, present in recycled paper and coated broke, derived from coating formulations, adhesives etc. They impact negatively on paper quality and cause web runnability problems by deposit build-up. To sustain recycling, stickies are controlled by adsorbing them onto minerals added to the recycled stock. We report isotherms for a fatty acid ester defoamer and an acrylic acid ester copolymer adsorbing from colloidal suspension onto various talcs and modified calcium carbonates. We used commercial preparations of the fatty acid ester defoamer and acrylic acid ester copolymer to provide a simple analogue to the industrial process. The modified calcium carbonates are hydrophilic with anionic and cationic sites present. Adsorption isotherms for low surface area modified calcium carbonate conform to the Langmuir model, while those for high surface area modified calcium carbonate reflect a two stage process involving the formation of a monolayer over the mineral surface and subsequent partial aggregation. Talc platelets display hydrophilic edges and hydrophobic surfaces. Adsorption onto them appears to involve three stages; specifically, a hydrophilic interaction between hydrophilic groups on the molecules and the talc edges, followed by hydrophobic interactions between the molecules and the talc surfaces, and finally by formation of multilayers.

Modeling of bagginess due to storage of paper reels with ridges

Paper is subjected to stress relaxation during storage in reels. Ridges in the paper reel subject the paper in the ridge to a higher strain than the paper adjacent to the ridge. The strain difference can become a permanent strain difference, resulting in a baggy web after unwinding. Researchers used a geometrical model based on stress relaxation test results to determine when the bagginess is substantial enough to cause problems. The results showed that runnability problems will appear if the ridge is about 1–2 mm high. This applies for a reel with a diameter of 1.2–1.8 m stored for 1 week.

Effect of fibres on fluidized bed expansion parameters

Better ways to protect water resources and the environment must be implemented in the papermaking industry to help Canada achieve its strategic target in maintaining a healthy environment and ecosystems. Closing down paper machine whitewater systems is the only way to reach that goal. However, under such conditions, a build-up of dissolved and colloidal substances in whitewater occurs. This results in lower paper quality and paper machine runnability problems. A fluidized bed reactor with modified solid sorbents is proposed to remove contaminants from paper machine whitewater prior to recycling. However, the whitewater fibre content significantly modifies bed expansion creating a major impact on the terminal particle velocity and the Richardson and Zaki expansion coefficient. Fibre flocculation was identified as a key characteristic that must be included in the classical equations describing fluidization. A modification for Newton's terminal velocity law and a new model for the expansion coefficient have been developed.

SIMULATION OF A RAPID NIP PRESSURE STRIKE AND ITS EFFECT ON PRESS FELT SAMPLES

Abstract Special technical textiles are used in papermaking to support, transfer, and dewater the paper web. These textiles, paper machine cloths, have many essential functions connected to their position on the paper or board machine. Mechanical wet pressing uses press felts, whose porosity and resiliency are important for effective dewatering. Water is squeezed out by two overlapping paper machine rolls, which form a nip. After squeezing, the porous felt should be void and return to its original thickness to ensure efficient dewatering. Friction forces are also present at the nip, since abrasive interfaces occur between the cloths and the rolls by pressure, heat, and movement. Thus in time, the harsh papermaking process wears out the press felt, deteriorating its quality. At high machine speed, defects in press felts, rolls, or other parts of the nip environment can cause runnability problems such as noisy run, that is, vibration in the pressing section, decreasing paper quality and output capacity. This study sought for a new way to simulate the ambiguous nip phenomenon on laboratory scale and to find out a way to predict this noisy run problem. A dynamic test method, the Hopkinson Split Bar, was used to define the ease with which strike energy passed through from the upper roll to the lower roll and the damping of strikes by new and worn felt samples. In our study, the elasticity of the press felt was strongly linked with the ageing time. Decreased elasticity lets a nip impulse more easily through the press felt.

In press conditions and their effect on runnability and convertability in digital package printing

A major advantage of digital printing is that printing can be performed on demand, but like all printing techniques it suffers from specific runnability problems. This study reports on an investigation of problems during printing and how to improve the runnability, with the aim of pointing out the most critical factors. Test printing has been performed on three different paperboard types in digital print units, based on both dry and liquid toner electrophotographic technology. The results show that the three main problems during printing were feeding in sheet-fed print units, cutting in web-fed print units, and increasing curl after printing. Feeding problems in sheet-fed print units were mainly explained by curl and varying format size of the paperboard, but also by exceeding thicknesses limits for the press. Curl after printing principally occurred due to high temperature loads caused by, for example, fusing and conditioning. Copyright © 2005 John Wiley & Sons, Ltd.

A torque-based analysis of the reverse roll coating process

In a laboratory film coater operated at industrial speed, the torque exerted by the metering rod was used to investigate the hydrodynamics in the metering nip flow. CFD simulations were also performed taking into account the deformation of the roll cover to depict the elastohydrodynamics of the metering nip flow. From the experiments, it was found that the torque increases with transfer speed, load and viscosity. When runnability problems such as spitting were observed, the torque signal decreased. Torque measurements could also predict excessive friction between rod and roll, which may damage the elastic cover. The numerical simulations showed trends similar to the experiments. Regions of positive and negative torque values demonstrated how the fluid interacts with the rod, a phenomenon apparently related to the occurrence of secondary flows in the metering nip. Finally, the numerical results also showed that the minimum nip gap becomes constant at high loads.

Shear-induced particle migration in a short-dwell coater

This paper aims at simulating the flow of a pigment suspension in a short-dwell coater (SDTA), a common device used for the production of value-added paper. The process consists of depositing onto a paper web a thin coating layer, the uniformity and the quality of which depend on the short-dwell coater geometry and the coating color rheology. In order to understand the possible interactions between the hydrodynamics in the pond and the rheological behavior of the suspension, this study focuses on a phenomenon known as shear-induced particle migration, which may result in an non-homogeneous suspension. For this purpose, a numerical methodology based on a shear-induced particle migration model (Sim) and the finite element method is presented and validated against analytical results and experimental data. This numerical approach constitutes the first step towards a better understanding of the hydrodynamic mechanisms that govern particle migration in a short-dwell paper coater. In this work, a parametric study of the suspension flow behavior is carried out via the introduction of a new dimensionless number that characterizes the importance of the shear-induced migration with respect to particle convection. The proposed model is used to generate particle distribution templates. It is shown that the particle size and Reynolds number are preponderant factors that affect the extent at which migration occurs in the short-dwell coater. In particular, it is shown that increasing the Reynolds number amplifies the migration phenomena and the formation of high concentration zones within a ring located in a vortex just before the nip. Such a concentration overshoot may then be dragged into the nip, resulting in runnability problems such as web breaks or coating color spitting.

EV-Mist Removal Wire Cleaning System

There has been a remarkable increase in the PM speed from approx. 800m/min in the 1970's to the current speeds of over 1, 700 m/min. Several factors have contributed to this development which began when the fourdriniers changed to fourdriniers with top formers and were followed by gap former machines.Because of this revolutionary development in paper machine speed there are more and more demands put on the paper machine runnability. One special area where runnability problems are having serious results is the wire section. This article concentrates on highlighting the mist as a cause of wire section runnability and paper quality problems and how these problems are eliminated by the Mist Removal System developed by EV Group Oy.

High shear rheology of paper coating colors – more than just viscosity

AbstractCapillary viscometry is used to characterize viscosity, entrance pressure loss and apparent wall slip of paper coating colors at high shear rates. Special emphasis is laid on the dependence of these phenomena on solids content in order to account for changes in the rheology due to the dewatering of the color during the coating process. Coating colors with substantially different runnability have been investigated. Differences in apparent wall slip and high shear viscoelasticity (manifesting itself in extremely high entrance pressure losses) are observed at increased concentration, even if these phenomena do not show up at the initial solids content. Poor runnability is observed when viscosity, entrance pressure loss and wall slip increase strongly with increasing solids content. But all rheological features change simultaneously with the coating color recipe and it is not possible to separate out the contribution of the particular rheological features on the runnability of the coating colors or to correlate the runnability to a single rheological prorameter. Future work will have to focus on a numerical analysis of the blade coating process taking into account all the rheological features described here. First simulations including slip at the color/blade interface indicate that wall slip may cause severe runnability problems, at least when the apparent slip velocity exceeds the web velocity.

A new pigment disperser for high solids paper coating colors

AbstractIn paper coating make‐down, a poor dispersion of pigments in coating colors may lead to coater runnability problems. This is manifested by coat defects at the paper surface, excessive coater blade wear, and an over‐consumption of pigments. For this reason, high shear impellers operated at high speed are used, yielding a high energy consumption. A new method to disperse high solids mix at low energy consumption is presented. This method based on the use of a rod impeller scraping the fluid surface in the vessel, is tested on a slurry composed of calcined and delaminated clays up to a solids content of 66.7 wt.% without dispersing aids.

Practical Pitch Control for Efficient Papermaking

Japanese papermakers are one with papermakers around the world in their concern over the quality, productivity and runnability problems associated with pitch. This article discusses types of pitch control currently in practice, with their advantages and disadvantages, and introduces a novel approach to retaining pitch with the sheet and removing it from the system via the most efficient method. Particle size optimization of the pitch is also critical, as it could be the difference between a microscopic spot or a macroscopic sheet defect.