Impregnation and delignification during kraft pulping of hardwood chips: characterization using in-situ X-ray tomography

Abstract In this paper, in-situ X-ray microtomography was used to analyze liquor penetration/impregnation and delignification of wood chips during kraft pulping, allowing microstructural changes to be assessed over time. The study was conducted with sapwood of three hardwood species (alder, aspen and birch), using a reactor designed to provide liquor circulation and temperature control. Each wood sample was digested at 141 °C for four hours and, throughout this time, fifteen 3D images of the central portion of the samples were acquired. The images were segmented and used to measure lumen size, cell wall thickness and wood chip porosity. The results confirmed that vessels offered the preferred path for liquor penetration in the hardwoods. Moreover, liquor penetration from ray cells to adjacent fibers was shown to be a less efficient path for impregnation. Regarding delignification, fiber separation began first in aspen and last in alder, and the complete separation took between 1 and 1.5 h to occur in the center of the samples. The porosity of the chips increased continuously after liquor penetration, whereas cell wall thickness decreased more substantially during fiber separation, especially in aspen, but remained relatively stable afterwards. Furthermore, the position of the fibers in relation to vessels and rays did not impact the rate of delignification significantly. Overall, this work shows that in-situ tomography can be a valuable technique to move forward research on wood impregnation and on topochemistry of lignin removal during pulping.