A long-term goal of the pulping industry is to optimize process parameters for efficiently removing degraded and soluble lignin during the fiber line processes such as kraft pulping, brownstock washing, and bleaching. This study investigates how pulp storage affects the efficiency of brownstock washing and oxygen delignification. Three pulp groups were rinsed with warm and cold water at 40 °C and 5 °C, respectively, and then stored under varying conditions (1 day, 1 week at temperatures of 5 °C and 60 °C. Our findings indicate that after one week of storage at 60 °C, more lignin was extracted, highlighting the influence of storage temperature and time on Kappa reduction (lignin removal) during storage. Additionally, larger lignin fragments were removed with increased storage temperature and time, suggesting that degraded lignin molecules trapped within the fibers can leach out during storage and be subsequently removed in washing. The different storage conditions had only a slight effect on oxygen delignification performance. We conclude that storage conditions, particularly temperature and time, significantly impact lignin removal efficiency and can enhance the pulp washing process. This study also provides valuable insights into lignin mass transfer during storage, offering guidance for industrial applications. The study also revealed that pulp quality after oxygen delignification is influenced by pH and lignin agglomeration and retention in the fibers during preceding washing and storage operations, emphasizing the need for careful control of the latter conditions to minimize cellulose degradation.

The role of sodium sulfate was considered relative to pulp washing liquors and its impact on the reattachment of lignin to pulp fibers during the brownstock washing process. The dissolution of lignin during washing and its potential redeposition onto the pulp fibers is influenced by various factors. Three distinct types of pulp-unbleached, bleached, and cotton linters-were used to explore these effects. The washing experiments were conducted using industrial wash liquor and were repeated further with the addition of sodium sulfate. The resulting products of the washing process, including the liquor discharge and the washed pulp, were thoroughly evaluated. Analytical techniques, such as UV measurements of lignin content in the liquor discharge and characterization of the pulp, were employed to assess the outcomes. The findings reveal that the addition of sodium sulfate to the washing liquor resulted in an increase in its conductivity and ionic strength. Moreover, it was observed that lignin reattachment to pulp fibers was noticeably greater when washing was performed with sodium sulfate addition. Among the pulps studied, unbleached kraft pulp exhibited the highest degree of lignin reattachment, followed by bleached kraft pulp, with cotton linters showing the least.

A computational fluid dynamics (CFD) study of the parameter sensitivity of a wood chips model was performed on an industrial impregnation vessel, which is the first step in a continuous cooking system. The solid and liquid phases were both treated as continua and it was found that the continuum model for the solid wood chips phase could capture the previously observed oscillating formation of arches in the contracting part of the vessel, which will occur at different levels of volume fraction depending on the material constants. The parameters that were examined are the solid pressure, permeability, viscosity, and wall friction. It was found that all the parameters strongly affect the distribution of the wood chips in the vessel as well as the oscillation effects, hence also the flow field which is important to accurately predict in order to ensure optimal performance of the impregnation vessel. Thus, correct material data for these types of simulations are crucial to the outcome and should be chosen for the appropriate situation and bio-material.