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Journal articles
Optimization of energy efficiency and condensate production in evaporation plants for a modern softwood pulp mill, TAPPI Journal April 2026
ABSTRACT: To meet the need to further improve thermal efficiency and environmental performance of kraft pulp mills, new systems and techniques have been developed within the evaporation plant. This paper describes these novel approaches and how they were implemented in a project completed in 2018 for a new evaporator and condensate treatment system supplied by Valmet at the SCA Östrand market pulp mill in Sweden. This project was part of a stepwise upgrade of the complete mill to increase the production capacity of the mill from 430,000 to 900,000 air-dried metric tons/year (ADt/y). As part of this upgrade, the mill had the objectives to increase the energy efficiency of the pulp mill and to minimize the air emissions as much as possible, the effluent volume, and the water usage in the mill. The mill also wanted to have the disposal of the biosludge in the black liquor, and the production of tall oil from black liquor soap, liquid methanol, and turpentine. This required that the new evaporation and condensate treatment system be very closely integrated into the other process departments of the mill, including integration of the hot weak black liquor flash vapor from the digester directly into the evaporator train and the production of multiple streams of clean evaporation plant condensate at the correct temperature for the bleach plant. Heat and mass balance calculation values, which were found to do very well in predicting the effect on actual mill operation, are also presented in this paper.
Journal articles
Utilization of inline total dissolved solids measurement system in a pulp mill's brownstock washing line, TAPPI Journal January 2026
ABSTRACT: Efficient washing improves the recovery of sodium (Na) and sulfur (S) cooking chemicals, as well as wood-based dissolved organic material. It also reduces the additional consumption of chemicals in the subsequent bleaching stages. The operation of the brownstock washing of the pulp mill’s fiber line has a significant impact on the mill’s energy economy, material efficiency, and environmental emissions. Process refractometers can be used to measure the total dissolved solids (TDS) from both the washing liquor and the pulp suspension filtrate part directly from pipelines or pipe bypass loops. In addition, by measuring the three or four incoming/outgoing dissolved solids streams to the washers, together with consistencies and flow rate measurements, it is possible to build the real-time effectiveness calculation of the washer(s). In this work, an online efficiency calculation based on TDS measurements was built for a pressure filter after cooking and oxygen (O2) delignification. The washing performance was monitored over a longer period, and stepwise tests were conducted to find the optimal operating mode. The feed and washing consistencies, the washer’s torque, and the washing liquid distributions varied, and the washer’s efficiency values were monitored using realtime measurements. From the long-term trends of the efficiency calculation, process disturbances could be detected, and their causes could be found. Based on the results of the stepwise tests, an optimal operating model for the washer was found. By optimizing the washing consistency, the Y10 washing yield could be increased. More efficient washing with the same or even a smaller amount of washing liquid can relieve the operation of the evaporator, which is often a bottleneck in the mill. The study also found the effect of cooking-related carryover on the operation of the oxygen stage, as well as differences in the washability of softwood and hardwood.
Journal articles
Towards closed water systems in chemical pulp mills: Evaporation of acidic filtrate from ECF bleaching with high chloride content, TAPPI Journal January 2026
ABSTRACT: In modern bioproducts mills utilizing elemental chlorine-free (ECF) bleaching, the bleaching process is the primary effluent source. The pulp bleaching typically generates 10 m³/a.d. metric ton of acidic filtrate. Despite extensive studies on recycling methods, the acidic filtrate is still typically directed to wastewater treatment plants due to the challenges created by its volume and chloride content. Recently, the volume of acidic filtrate has significantly decreased to 5 m³/a.d. metric ton, reducing the capacity required for the recycling process closer to a feasible level. In this study, we investigated recycling of acidic filtrate by evaporation from the D0 stage of a D0-Eop-D1 bleaching sequence. In the mill, hydrochloric acid (HCl) was used for pH control instead of sulfuric acid (H2SO4) due to better control of precipitation. This arrangement substantially changes the composition of the acidic filtrate, increasing the chloride (Cl-) and decreasing the sulfate (SO4 2-) ionic content. For the above reasons, it is necessary to study the effect of evaporation on the quality of the resulting condensate and concentrate. The results provide new information on how to close water loops in a modern bioproducts mill with higher Cl- content as one option. The results show that the evaporation of the high-chlorine D0 filtrate produces a pure condensate with methanol as the main component. Only small amounts of Cl- were observed in the condensate. The majority of chlorine (Cl) compounds remains as dissolved compounds in the evaporated concentrate when the dry solids content of the concentrate is ~10%. The Cl compounds in the concentrate can be converted to sodium chloride (NaCl) by incineration.
Journal articles
Effect of pulp refining on thixotropy of cellulose fiber suspensions, TAPPI Journal September 2025
ABSTRACT: This paper presents results on the thixotropic behavior of low-consistency bleached eucalyptus fiber suspensions that had been mechanically treated. The pulps were refined in a PFI mill at different numbers of revolutions (0, 3000, and 6000) to study the effect of refining intensity on the viscosity time-response of fiber suspensions. In steady rheological tests, all the suspensions exhibited shear thinning behavior and no significant differences were observed after mechanical treatment. Stepwise experiments showed that viscosity continuously increased with time when shear rate was suddenly reduced. Additionally, a delay in time evolution of viscosity was observed as the pulp was more refined. A stretched exponential model was fitted to the experimental results of viscosity.
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
Toward environmental resilience in pulp and paper manufacturing: Water consumption and carbon dioxide emission reductions, TAPPI Journal September 2025
ABSTRACT: Pulp and paper manufacturing is a water- and energy-intensive industrial sector, necessitating improvement of its operational efficiency, as well as reduction of emissions to the maximum extent possible. This review focuses on the reduction of water consumption and carbon dioxide (CO2) emissions that originate in the pulp and paper industry (PPI). First, process simulation and optimization techniques used for water consumption reduction are reviewed. Then, techno-economic analyses of solvent-based CO2 capture from PPI are discussed. Additionally, key actions are proposed for enhancing water consumption reduction and CO2 capture in PPI.
Journal articles
Materials performance considerations in hydrothermal liquefaction conversion of biomass, TAPPI Journal June 2025
ABSTRACT: Hydrothermal liquefaction (HTL) is a promising thermochemical route developed to convert woody biomass and biowaste to biochemicals and bio-oils. However, the operating conditions are rather harsh to biorefinery structural metallic components. These conditions include alkaline catalysts such as potassium carbonate (K2CO3); hot, pressurized (sub-critical) water reaction; and medium and aggressive anions chlorine (Cl•) and hydrogen sulfide (H•) released from biomass feedstocks. Thus, selection of suitable structural alloys for biorefinery components involves striking a balance between mechanical properties, corrosion resistance, and cost. Alloys currently being considered for this application include ferritic-martensitic steels and austenitic stainless steels. From a corrosion perspective in hot pressurized water, the former typically exhibits higher stress corrosion cracking resistance, whereas the latter exhibits higher corrosion resistance. This study reviews cost-effective corrosion control strategies aimed at increasing the chromium (Cr) content for protective surface oxide formation, as screened by testing in simulated HTL alkaline water, to support materials selection and design. Corrosion control strategies include surface modification (increasing surface Cr content), alloying (increasing bulk Cr content), and stainless-steel type (ferritic vs. austenitic). Of the alloys considered (including those subjected to surface modification), ferritic stainless steels exhibit a promising balance between corrosion and stress corrosion cracking resistance, adding another family of candidate alloys for structural biorefinery component materials selection and design.
Journal articles
Magazine articles
Understanding the risks and rewards of using 50% vs. 10% strength peroxide in pulp bleach plants,TAPPI Journal December 2024
Authors: Alan W. Rudie and Peter W. Hart | TAPPI J. 17(11): 601(2018) - ABSTRACT: The use of 50% concentration and 10% concentration hydrogen peroxide were evaluated for chemical and mechanical pulp bleach plants at storage and at point of use. Several dangerous occurrences have been documented when the supply of 50% peroxide going into the pulping process was not stopped during a process failure. Startup conditions and leaking block valves during maintenance outages have also contributed to explosions. Although hazardous events have occurred, 50% peroxide can be stored safely with proper precautions and engineering controls. For point of use in a chemical bleach plant, it is recommended to dilute the peroxide to 10% prior to application, because risk does not outweigh the benefit. For point of use in a mechanical bleach plant, it is recommended to use 50% peroxide going into a bleach liquor mixing system that includes the other chemicals used to maintain the brightening reaction rate. When 50% peroxide is used, it is critical that proper engineering controls are used to mitigate any risks.
Journal articles
Magazine articles
Wheat straw as an alternative pulp fiber, TAPPI Journal December 2024
Author: Peter W. Hart | TAPPI J. 19(1): 41(2020) - ABSTRACT: The desire to market sustainable packaging materials has led to an interest in the use of various fiber types as a raw material. It has been suggested that the use of annual crops for partial replacement of wood fiber would result in more sustainable products. Several life cycle analyses (LCA) have been performed to evaluate these claims. These LCAs provided conflicting and contradictory results because of the local conditions and the specific pulping processes investigated. Selected LCAs are reviewed and the underlying reasons for these conflicting results are analyzed.
Journal articles
Magazine articles
Modeling the dynamics of evaporator wash cycles, TAPPI Journal July 2024
ABSTRACT: Kraft pulping is a process that utilizes white liquor, composed of sodium sulfide (Na2S) and sodium hydroxide (NaOH), for wood delignification and pulp production. This process involves washing the dissolved organics and spent chemicals from the pulp, resulting in the generation of black liquor. Prior to its use as fuel in the recovery boiler, the black liquor is concentrated in multiple-effect evaporators. During the evaporation process, the inorganic salts present in the liquor become supersaturated and undergo crystallization. Fluctuations in sodium, carbonate, sulfate, and oxalate can give rise to severe sodium salt scaling events, which significantly impact the thermal efficiency of the evaporators, and ultimately, pulp production. Dynamic modeling provides insights into fluctuations in liquor chemistry in the evaporators. The primary objective of this study was to employ dynamic modeling to evaluate the effects of wash liquor recovery from evaporator wash cycles. The dynamics associated with wash cycles encompass variations in the concentrations of salts and solids in the recovered wash liquor, changes in the flow rate of wash liquor recovery, and fluctuations in liquor volume within the liquor tanks. The dynamic model was developed using Matlab Simulink and applied to the evaporation plant of a pulp mill in South America. By utilizing one month of mill process data, the model enabled the evaluation of fluctuations in liquor chemistry due to evaporator wash cycles. The developed model has demonstrated the potential to estimate the concentration of key ions responsible for scaling and to contribute to enhancements in evaporator washing strategies.