Mass Transport in Wood Disintegration: Implications for the Pulp and Paper Industry

Doktorsavhandling, 2023

Efficient delignification and fractionation processes are essential in the pulp and paper industry. Mass transport events in wood fractionation are of great importance, specifically in two main stages, namely, the mass transport of chemicals into wood and the mass transport of degraded lignin molecules out of cellulose confinements. It has been previously suggested that the mass transport of lignin molecules through fibers is the rate-determining step in kraft pulping. This thesis first investigates the effects of pore sizes, the alkalinity of the solution, the molecular weight of lignin, and specific ion effects on the mass transport of lignin through model cellulose membranes using diffusion cells. Furthermore, the adsorption of lignin on cellulose substrates in the presence of salts chosen from the Hofmeister series was studied using a quartz crystal microbalance with dissipation monitoring (QCM-D). It was observed that mass transport rates through the cellulose membrane were enhanced by an increase in pore size, alkalinity of the solution, and a decrease in lignin molecular weight. Higher alkalinity of the solution decreases the association between the lignin molecules, which increases the mass transport. QCM-D measurements showed that the adsorption of lignin on cellulose was increased in the presence of chaotropic anions. This behavior can be rationalized by the system's entropy gain, facilitated by the release of adsorbed ions and water molecules from the cellulose surface upon lignin adsorption. The thesis also explores the rate-determining step in the ionoSolv fractionation process. To achieve a more homogeneous fractionation, the effects of temperature, water content, and vacuum on the impregnation of wood by ionic liquids (ILs) were further studied. This thesis not only highlights the complexity of mass transport events in wood fractionation but, by comparing the concentration of lignin released from dry wood, IL-impregnated wood, and previously reacted wood samples over time, has also revealed that the mass transport of the IL into wood is the rate-determining step.