Effect of Digester Relief on the Bleachability of Softwood Kraft Pulps from Modified Batch Processes, 2001 Pulping Conference Proceedings

Q. Dai, H. -M. Chang, H. Jameel--The objective of this study is to investigate the impact of the pretreatment on the extractive content of pulps. Due to the multistage treatments and black liquors recycling in batch extended delignification processes, it is very possible that the unsaponsible extractive compounds accumulate on the fiber surface and affect the bleachability of pulps. During the pretreatment stage, if a certain amount of vapor is released from the digester, some volatile neutral compounds may be removed with the vapor. A simplified extended delignification pulping process, with only one pretreatment stage and no post treatment, was used in this study to determine the effect pretreatment conditions on the bleachability and extractive content of the loblolly pine pulps. White liquors were used in the pretreatment and the digester relief process was included. Using the digester relief process significantly lowered the extractive content, especially the neutral extractive content of pulps. More than 10% of the total neutral wood extractive compounds could be removed from the system during the relief. The operational conditions of the digester relief, such as relief temperature and time, pretreatment alkalinity, affected the amount of extractives removed, the extractive distribution in the system and extractive content of pulps, and therefore affected the bleachability. More extractives were removed when pretreated with high temperature and low alkalinity. A kinetic study revealed that more extractives were removed during the first several minutes of releasing. Then, the rate of extractive removal leveled off. The effects of presteaming and white liquor profiling on the extractive distribution were also evaluated.