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Advanced real-time digital microscopy of foaming processes, TAPPI Journal January 2023
ABSTRACT: The properties of aqueous foams play a major role in foam forming and foam coating. Inline real-time foam measurements provide highly desired opportunities for optimization and control of foaming processes. This paper presents inline digital microscopy measurements of aqueous foams in foaming processes. It presents methods for providing detailed information on foam quality parameters, such as foam density and foam homogeneity in real time from the process. In addition, this study evaluates the performance of transillumination and front-light illumination in imaging of foams. The tests show very good results for the transillumination approach. Limitations of the image-based optical technique are discussed, and the precision of bubble size distribution measurement is assessed with a certificated reference substance. The measured foam densities are compared against the reference foam densities in the range 100•300 g/L, providing a linear correlation with R2 value of 0.99. In the case of heterogenous foams with a wide bubble size distribution, the bubble size-dependent dimensionless depth of field must be taken into account to obtain accurate estimates of foam density. Bubble-scale foam homogeneity is described by the standard deviation of bubble size distribution in foam.
Journal articles
Magazine articles
Evaporation of process water from recycled containerboard mills, TAPPI Journal July 2023
ABSTRACT: The reduction of the specific effluent discharge volumes of paper mills leads to concentrated process waters that are difficult to treat. Evaporation is an effective water reclamation technology; however, its feasibility largely depends on the fouling behavior of the calcium rich process water. A pilot plant study was conducted to investigate fouling of an evaporator processing the production water from a recycled containerboard mill. The evaporator was operated continuously for five weeks at an evaporation temperature of 55°C and a differential temperature of 5°C, and with a recovery rate of approximately 90%. The calcium ion concentration of the circulating liquor exceeded 7,000 mg/L with a pH of 6. Despite the high fouling potential of the circulating liquor, the heat transfer coefficient did not decline over the investigated trial. The absence of deposits on large areas of the heating surfaces demonstrate that the process water does not generally form deposits under the conditions that were investigated. Calcium sulfate deposits were only found in areas where there was inadequate coverage of liquid over the heating surfaces.The findings show that evaporators can be used to effectively close the water system of recycled containerboard mills without fouling impacting the energy efficiency.
Journal articles
Magazine articles
Pareto-based design of experiments for identifying and comparing optimum sealing parameters of heat sealing applications in packaging machines, TAPPI Journal June 2023
ABSTRACT: Sealing is one of the most important process steps in industrial packaging, because the sealed seam is the most sensitive section of a package in terms of quality. For this reason, a major focus in flexible packaging is the sealing process, and among this, heat sealing is the most frequently used technology. In detail, applications of heat sealing processes are confronted with four conflicting objectives: increasing seam quality, reducing dwell time, reducing sealing temperature, and increasing process robustness towards varying conditions. Typical problems, such as identification of the optimum process parameters or selection of the most appropriate packaging film, are subject to these conflicting objectives.This paper presents a recently published design of experiments for characterizing and comparing heat sealing properties of packaging films based on a multi-objective optimization algorithm. The approach provides easy-to-read charts showing all optimum sealing parameters with regard to the four essential objectives of heat sealing: seam quality, dwell time, sealing temperature, and process robustness. Three case studies show exemplary applications of the new approach: 1) analyzing transport damages of beverage powder packages; 2) identifying and comparing optimum sealing parameter of a standard, mono-material, and fiber based packaging film regarding tightness and visual properties of the produced sachets; and 3) analyzing the effect of additional aluminum layers on sealing characteristics regarding hot-tack.The new design of experiments may provide the basis of a standard test method for the identification of optimal sealing parameters in the heat sealing processes.
Journal articles
Magazine articles
Recovery boiler back-end heat recovery, TAPPI Journal March 2023
ABSTRACT: Sustainability and efficient use of resources are becoming increasingly important aspects in the operation of all industries. Recently, some biomass-fired boilers have been equipped with increasingly complex condensing back-end heat recovery solutions, sometimes also using heat pumps to upgrade the low-grade heat. In kraft recovery boilers, however, scrubbers are still mainly for gas cleaning, with only simple heat recovery solutions. In this paper, we use process simulation software to study the potential to improve the power generation and energy efficiency by applying condensing back-end heat recovery on a recovery boiler. Different configurations are considered, including heat pumps. Potential streams to serve as heat sinks are considered and evaluated. Lowering the recovery boiler flue gas temperature to approximately 65°C significantly decreases the flue gas losses. The heat can be recovered as hot water, which is used to partially replace low-pressure (LP) steam, making more steam available for the condensing steam turbine portion for increased power generation. The results indicate that in a simple condensing plant, some 1%•4% additional electricity could be generated. In a Nordic mill that provides district heating, even more additional electricity generation, up to 6%, could be achieved. Provided the availability of sufficient low-temperature heat sinks to use the recovered heat, as well as sufficient condensing turbine swallowing capacity to utilize the LP steam, the use of scrubbing and possibly upgrading the heat using heat pumps appears potentially useful.
Journal articles
Magazine articles
Non-process elements in the recovery cycle of six Finnish kraft pulp mills, TAPPI Journal March 2023
ABSTRACT: In this work, the aim was to study the distribution and accumulation of the non-process elements (NPEs) in the recovery cycle of Finnish pulp mills and look at whether the geographical location (North vs. South) correlates with the current Finnish NPE levels. In addition, a comparison to older similar Finnish measurements was made with an attempt to analyze the reasons behind differences in the most typical non-process elements, aluminum (Al), silicon (Si), calcium (Ca), phosphorus (P), magnesium (Mg), manganese (Mn), chlorine (Cl), and potassium (K), taking into account the main elements in the white liquor, sodium (Na) and sulfur (S). The extensive laboratory results gained in this study are from seven sampling points at six pulp mills and present analytical data of metal concentrations. The data obtained presents an update to previous NPE studies. The levels found did not statistically differ between North and South Finland. The NPE levels, apart from phosphorus, found in Finnish pulp mills today have not changed considerably compared to the levels in earlier investigations in the 1990s. In the newest data, the phosphorus concentration was consistently higher in the as fired black liquor, electrostatic precipitator (ESP) ash, lime mud, and green liquor than in the previous results. In addition, the levels of Al, Si, Ca, P, and Mg in recovery boiler ESP ash were consistently higher compared to the older results. As the mills start to close their systems more, a stronger accumulation of NPEs can be expected, increasing the likelihood of more operational problems in the process. Further understanding of where the NPEs accumulate and how they can be most effectively removed will be valuable knowledge in the future.
Journal articles
ABSTRACT: Laboratory experiments with bleached kraft furnish were carried out to quantify the effects of major differences in electrical conductivity of papermaking process water (due to the addition of sodium sulfate) on the performance of various paperm
ABSTRACT: Laboratory experiments with bleached kraft furnish were carried out to quantify the effects of major differences in electrical conductivity of papermaking process water (due to the addition of sodium sulfate) on the performance of various papermaking additives. Batches of refined pulp were prepared with conductivity levels of 168 (tap water), 1000, and 10,000 ìS/cm. The absolute values of the calculated zeta potential, in various cases, were shown to decrease with increasing logarithm of electrical conductivity. The performance of retention aid systems, including cationic polyacrylamide (cPAM), were not adversely affected by increased salinity, even up to an electrical conductivity level of 10,000 ìS/cm. In fact, treatment involving sequential addition of cPAM and colloidal silica showed superior retention of mineral filler at the highest conductivity level. Likewise, combinations of papermaking additives that promote the dewatering of paper continued to perform well in furnish prepared with increasing salinity. The ability of various chemical systems to induce flocculation among papermaking fibers decreased moderately at the highest level of salinity considered.
Journal articles
ABSTRACT: Water hardness, which can be defined as the concentration of calcium and magnesium ions, is known to vary greatly depending on geographical locations. Laboratory tests were carried out to evaluate effects of large differences in water hardness o
ABSTRACT: Water hardness, which can be defined as the concentration of calcium and magnesium ions, is known to vary greatly depending on geographical locations. Laboratory tests were carried out to evaluate effects of large differences in water hardness on the performance of certain wet-end additives to the paper machine process. Tests were carried out at hardness levels of 25, 125, and 225 ppm (as calcium carbonate equivalents). Increased water hardness was found to have a generally negative effect on the performance of a cationic acrylamide- type retention aid, although the extent of performance loss depended on experimental details. Likewise, rates of dewatering in systems containing cationic retention aid were adversely affected by increasing hardness, though the effects were not statistically significant in all cases considered. The tendency of bridge-forming flocculants (cationic retention aid or sequential addition of a cationic additive and then anionic retention aid) fell slightly with increasing water hardness.
Journal articles
Assessing lignin content in Nordic hardwood and softwood species using models based on near-infrared (NIR) spectral data and partial least squares regression (PLSR), TAPPI Journal September 2025
ABSTRACT: Continuous kraft cooking digesters face challenges affecting product quality, making it valuable to improve control through advanced techniques like near-infrared (NIR) spectroscopy, model predictive control, and machine learning models. The primary goal of this study was to use NIR spectra to predict the amount of lignin in hardwood and softwood samples. This study investigated the correlation of NIR derivative spectra with the amounts of lignin relative to other constituents, namely cellulose, hemicellulose, and water, in wood chip samples of varying chip sizes and shapes from six Nordic wood species. It employed partial least squares regression (PLSR) on the NIR data to construct a model that predicted the lignin fraction and the relative fraction of acid-soluble lignin. When trained on a group of five wood species, the model achieved a satisfactory predictive ability, striking a balance between a wide range of lignin content and a consistent chemical environment. The accuracy increased further when the model was restricted only to spruce and pine, reflecting the benefits of a more homogenous dataset. Additionally, the optimal number of latent variables was identified as two, indicating that three distinct chemical components — cellulose, lignin and water — can be effectively differentiated using NIR.
Journal articles
Magazine articles
Numerical analysis of the impact of rotor and screen hole plate design on the performance of a vertical pulper, TAPPI Journal April 2025
ABSTRACT: The dissolving of mechanical pulp is one of the most important process steps in stock preparation, since pulping occurs at the very beginning of the papermaking process. Efficient mixing of the pulp in a short amount of time is essential to achieve high furnish volume flow rates. The design of the rotor, as well as the pulper vat and inserts, significantly affects the overall performance of the pulper, such as mixing efficiency and power demand. Using advanced numerical methods such as computational fluid dynamics (CFD) can accelerate the development process. The CFD simulations allow for detailed analysis of flow phenomena, making it possible to study a real-size machine numerically. This approach is particularly advantageous because it can reduce the need for timeconsuming and costly experiments associated with scaling up test rigs. In this study, we compared two different rotor designs utilized in a vertical pulper and evaluated the numerical results with experimental data. Rotor A is designed for low turbulence and low power demand, while rotor B is designed for high turbulence with high power demand. The CFD results showed good agreement with the experimental measurements. We investigated how the rotor design influences the free fluid surface and the mixing efficiency. Our study also highlights the differences in results depending on whether water or furnish is simulated, which exhibit Newtonian or, respectively, non-Newtonian fluid behavior. Additionally, a detailed numerical investigation of various screen hole plate designs revealed that the newly developed hole design significantly reduces pressure loss compared to a standard drilled hole. This outcome was consistent for both types of fluids investigated: water and furnish.
Journal articles
Reinforcing folding boxboard ply stock with refined pulp and its effect on dewatering potential, TAPPI Journal February 2025
ABSTRACT: The folding boxboard (FBB) filler ply typically contains a significant amount of mechanical pulp such as bleached thermomechanical pulp (BTMP), bleached chemi-thermomechanical pulp (BCTMP), and chemi-thermomechanical pulp (CTMP), etc. It is usually reinforced with either refined broke from the same paper machine line and converting process or by utilizing traditional bleached kraft pulp (BKP). In response to the drive for improved/increased ply bond (to avoid undesired delamination), increased bulk, reduced basis weight, and minimized energy consumption, papermakers have experimented with various options and strategies. In between the common approaches, choices have been made between reducing the refining of the mechanical pulp, increasing the broke refining (more frequently practiced on the production scale), or increasing the BKP refining (a potentially superior choice). This study focused on a simplified approach to assess the impact of three reinforcement pulps with different refining levels on the dewatering of the filler ply. The reinforcement pulp was added to the core stock for the FBB filler ply — a mechanical pulp, BCTMP, with a drainability of 25 °SR. The proportions of the reinforcement pulp, hardwood bleached kraft pulp (HWBKP), were 20% of 30-35 °SR, 10% of HWBKP 50-55 °SR, and 5% of HWBKP 70-75 °SR. The intention behind using varying percentages of reinforcement pulp was to attain a controlled internal bond while enhancing bulk through increased mechanical pulp content. The dewatering potential of the stock mixtures was assessed at three vacuum levels — 4, 10, and 25 kPa — that can be found in progressive stages within production- scale forming sections. Our goal was to find an optimal reinforcement strategy for filler ply that would minimize the use of the reinforcement pulp, give better strength, retain bulk, and lower basis weight. The success of this strategy was verified with an actual FBB machine.