Reaction kinetics of softwood kraft delignification –General considerations and experimental data

Journal article, 2007

For a delignification reaction to occur, cooking chemicals must be transported to the reaction site. After fragmentation reactions in the lignin structure, lignin fragments need to be solubilised in order to diffuse out of the fibre wall and complete the complicated delignification reaction. Despite major research efforts, there are still several aspects of this complex process that are not fully understood. This study is part of a larger investigation aimed at increasing the knowledge of the reaction kinetics of a kraft cook. In the present paper, different aspects of reaction kinetics of delignification and approaches to the modelling of kraft delignification are discussed. In the experimental part, the effect of active chemical concentrations on the reaction kinetics of delignification of Scots pine (Pinus silvestris) was investigated at five temperature levels. Cooks with the addition of an industrial softwood black liquor and increased sodium ion concentration were also performed. To minimise the influence of mass transport on the reaction rates, wood meal was used as a substrate and all cooks were conducted at a high liquor-to-wood ratio (200: 1) to minimise the deviation from initial bulk concentrations. The rate of delignification in the kraft cook increased with increasing hydrosulphide ion and hydroxide ion concentrations. An increase in sodium ion concentration retarded the rate of delignification, while an increase in concentration of dissolved kraft lignin in the cooking liquor increased the delignification rate during the main part of the cook. These results qualitatively resemble many of the results found in the literature. Indications of the influence of covalent bonds between lignin and carbohydrates on the delignification rate are shown and discussed. Data obtained in this investigation cover a broad range of cooking conditions, and are suitable for modelling the reaction kinetics of delignification without the influence of mass transfer resistance.