Kraft pulping of model wood chips: local impact of process conditions on hardwood delignification and xylan retention

Local evolution of delignification and xylan removal inside wood chips was investigated throughout the initial stages of kraft cooking. Model chips of birch sapwood were pulped at 145, 155 and 165 °C, utilizing white liquors with hydroxide content ranging from 0.25 to 0.55 mol/kg. The composition of different sections in each cooked sample was then determined. Xylan was isolated from selected samples and analyzed using size exclusion chromatography and HSQC NMR. Most changes in concentration and structure of residual xylan occurred early in the process (<45 min). Furthermore, xylan samples isolated from the tissue of different cooked chips had similar average molecular weights, indicating that temperature and alkali content had little impact over the extent of reactions affecting residual xylan. In contrast, xylan dissolution was significantly dependent on pulping conditions, increasing with hydroxide concentration. The lignin profile inside the cooked chips also varied with alkali content and temperature, and it was shown to be more uniform when applying low cooking temperatures (145 °C). Finally, increased delignification and xylan removal were detected close to the transverse surfaces of chips (likely due to the fast mass transport in vessels/lumen), implying that anatomical features of wood can have a significant impact on pulping.