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Using a Berty-type CSTR reactor, Ji (2007. Kinetics and mechanism of oxygen delignification, Ph.D. thesis. The University of Maine) obtained oxygen delignification kinetics first order in lignin when neglecting the higher initial delignification phase. In the present study the same Berty/CSTR reactor with an improved temperature control was used to determine the oxygen delignification kinetics of Southern Pine kraft pulp, with the kinetics now corrected for lignin removed by oxygen-free alkaline leaching. This removes the initial high delignification rate peak so that the kinetics over the entire lignin range can be modelled as first order in "reactive" lignin, which is lignin corrected for a small amount of unreactive lignin. It suggests that softwood pulp oxygen delignification consists of two contributions; phenolic delignification and alkaline leaching. The initial alkaline leaching phase is mathematically described by semi-infinite alkaline diffusion of dissolved lignin trapped in the cell wall after pulp washing. For phenolic delignification, the reaction orders in alkali concentration and oxygen of the power law delignification equation are 0.3 and 0.44 respectively, while the activation energy is 63 kJ/mol. Taking into account the unreactive HexA, the small amount of unreactive lignin, and the lignin removed by alkaline leaching, the kappa number is well predicted by the new delignification kinetics. [ABSTRACT FROM AUTHOR]

Special oxygen delignification experiments were performed in a differentially operated continuous stirred tank reactor (CSTR). Because the dissolved oxygen and alkali concentrations in the reactor are constant, the rate of lignin removal may be determined from the dissolved lignin content in the outflow stream measured by UV absorption. The delignification kinetics were determined at different temperatures, oxygen pressures and caustic concentrations on softwood kraft pulps of different kappa numbers. The kinetics are first order in residual lignin content [hexenuronic acid (HexA) corrected], and follow a Langmuir-type behavior for adsorption of oxygen on the active aromatic lignin sites. The first order in residual lignin content implies that the active lignin sites are uniformly distributed and have the same reactivity. It is proposed that the active site is the 3 carbon of the aromatic ring, where oxygen reacts to form a hydroperoxide, with a p Ka of almost 2 units higher than that of phenolic lignin. The kinetics of phenolic delignification can be described by assuming that the reaction between adsorbed oxygen and carbon 3 of the aromatic ring is the rate determining step. Alternatively, the decomposition of the hydroperoxide anion is rate determining. Peeling delignification has been proposed as an additional delignification route by peeling of hemicelluloses which have lignin fragments covalently bound to them. It is shown that the impact of peeling delignification is relatively small and limited to the initial delignification phase. Finally, data are presented showing that radicals may provide an essential contribution to delignification by their removal of non-lignin and non-HexA oxidizable structures contributing to the kappa number. [ABSTRACT FROM AUTHOR]