Here you will find doctoral theses from Resource Smart Processes.
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Dissertations:
Jenny Sjöström, KTH Royal Institute of Technology
Lignin release during oxygen delignification – kinetics, structure and potential
https://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-374258
Defense date: January 23, 2026
Oxygen delignification is a critical stage in modern kraft pulp production, enabling significant reductions in chlorine-based bleaching chemicals and environmental emissions while maintaining fiber quality. The process remains limited by challenges in efficiency and selectivity, governed jointly by chemical reactions and mass transport constraints. This thesis investigates the interplay between these mechanisms and explores the properties and valorisation potential of oxidized lignin (oxlignin) extracted from oxygen-stage wash liquors. Experimental results demonstrate that lignin removal during oxygen delignification is driven by a combination of rapid early-stage oxidative reactions and diffusion-controlled leaching. High oxygen pressure and sufficient alkalinity promote lignin depolymerization and oxidation, improving selectivity, while insufficient chemical conditions lead to lignin redeposition and cellulose degradation. Upstream factors such as brownstock washing efficiency and storage conditions significantly influenced lignin leaching and pulp quality, highlighting the importance of integrated process control. Oxlignin, isolated from industrial filtrates, differed markedly from conventional kraft lignin, exhibiting higher carboxylic acid content, improved water solubility, and a narrower molecular weight distribution. These properties suggest potential applications as dispersants or additives in biopolymer formulations. Ultrafiltration proved to be a viable approach for fractionating oxlignin. By connecting process optimization with resource valorisation, this work contributes to more sustainable kraft pulp production and supports the development of new lignin-based value streams in future biorefineries.
Eashwara Raju Senthilkumar, KTH Royal Institute of Technology
Efficient Washing of Papergrade Pulp in the Kraft Process
https://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-377289
Defense date: March 20, 2026
Brownstock washing is a critical unit operation in kraft pulping, responsible for removing dissolved lignin and other organic materials from the pulp prior to further delignification and bleaching. In the process, effective brownstock washing improves pulp quality and reduces chemical consumption, leading to better environmental and economic outcomes. A persistent challenge in modern kraft mills is lignin redeposition during washing, particularly under liquor recycling and process closure conditions. This work provides a mechanistic understanding of lignin removal, transport, and redeposition during brownstock washing of softwood kraft pulp. The work examines how physicochemical conditions, including liquor composition, ionic strength, pH, alkali addition, divalent cations, and storage conditions (storage time and temperature), significantly influence lignin diffusion, particularly in higher molecular weight lignin fractions. A combination of laboratory-scale washing experiments, filtrate analysis, pulp characterization, and lignin leaching studies was employed to isolate and examine these coupled mechanisms. The results demonstrate that lignin removal is frequently limited by diffusion from the fiber wall and that the stability of dissolved lignin is strongly influenced by ionic strength and ion-specific effects. Sulfate rich, high ionic strength liquors promote lignin aggregation and redeposition, whereas elevated pH and controlled alkali addition enhance lignin solubility and reduce redeposition within defined limits. Storage time and temperature were shown to significantly affect lignin diffusion, particularly for higher molecular weight lignin fractions. Overall, this work indicates that brownstock washing is governed by the interplay among diffusion-controlled transport, the thermodynamic stability of dissolved lignin, and surface-driven redeposition phenomena. The mechanistic insights gained provide a basis for optimizing washing strategies, reducing lignin carryover, and improving the sustainability and efficiency of kraft pulping operations.
Linus Kron, Chalmers
Hardwood Kraft Delignification – Fundamental Mechanisms at the Cell Wall Level
https://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-374258
Defense date: March 27, 2026
Kraft pulping is a globally significant industrial process whose complexity, stemming from the multi-scale heterogeneity of wood, means a complete understanding of its underlying physicochemical mechanisms remains elusive. This thesis presents research aimed at advancing the fundamental understanding of the kinetics of delignification-the primary step in kraft pulping-with a particular focus on the relative impact of reaction and transport mechanisms occurring at the cell wall level.
Delignification kinetics were investigated in laboratory-scale reactors by examining the effects of time, temperature, and cooking liquor composition on pulp and dissolved component properties. The work focused primarily on birch wood meal but also included comparisons with wood chip pulping and other Nordic hardwood species, namely alder, aspen, and beech. Based on the experimental findings, a novel delignification model incorporating both reaction and diffusion phenomena was developed.
Collectively, the studies highlight the multi-mechanistic nature of kraft delignification, indicating that different phenomena dominate the kinetics at different stages of the process. The results demonstrate that the sulphide dependency of the delignification rate diminishes part-way through the process, indicating the impact of additional reactions not previously considered to significantly influence the delignification rate. In contrast, the molecular size of dissolved lignin was observed to increase continuously throughout cooking, suggesting that solubility and diffusion effects significantly impacts the later stages of lignin removal. The developed model demonstrated that delignification kinetics in wood meal pulping can be accurately represented as primarily diffusion controlled. Finally, delignification of wood chips revealed additional heterogeneity at a larger length scale. An initial extension of the developed delignification model to the chip scale successfully captured this behavior, indicating that accounting for alkali availability is critical to accurately describe delignification in chips.
Keywords: Pulping, Model, Lignin, Wood meal, Black liquor, Transport, Reaction
Carolina Marion de Godoy, Chalmers
Kraft pulping of hardwoods: Investigating the impact of wood microstructure on impregnation and delignification rate
https://research.chalmers.se/en/publication/551284
Defense date: April 28, 2026
The experiments included wood chips of alder, aspen, beech, and birch, and their behavior was examined from two perspectives: changes in global and local chemical composition, and changes in wood microstructure. The latter was evaluated in-situ via synchrotron X-ray tomography. Additionally, a multiscale model describing the delignification of birch chips during kraft cooking was developed.
The results revealed that the concentration and distribution of alkali within the chips after impregnation have major impact on delignification and are strongly affected by wood chip porosity. Furthermore, structural analysis during impregnation showed that vessels provide the main path for liquor penetration and distribution among adjacent cells. No substantial changes in cell wall thickness due to alkaline swelling were observed.
When comparing hardwood species, lignin removal was significantly faster in aspen. Delignification uniformity also increased with chip porosity and was shown to improve when utilizing low cooking temperatures (e.g., 145 °C) or impregnation liquors with high alkali concentrations. Differences in ray cells among the hardwoods had no clear impact on local rates of lignin removal. In terms of microstructural changes, delignification led to increased chip porosity and a minor decrease in cell wall thickness.
Finally, the proposed modeling approach has potential to be used for investigating the defibration point of wood chips. According to it, lignin removal from the cell walls appears to be limited by reaction kinetics and diffusion of lignin fragments, whereas the overall delignification behavior at the chip scale is heavily influenced by the balance between alkali transport and consumption.