Hardwood Kraft Delignification

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 behaviour, indicating that accounting for alkali availability is critical to accurately describe delignification in chips.