Kinetics of carbohydrate degradation duringoxygen bleaching: Part1-A review

摘要

Carbohydrates are degraded during oxygen delignification by either peeling or chain cleavagernreactions. Studies on model compounds indicate that the peeling reaction occurs at the end of thernpolysaccharide chain where there is a carbonyl group (C=O). Stopping reactions, which are thernoxidation of these end groups, convert them to stable and unreactive sites. The peeling reaction,rnwhich causes yield loss, is not significant unless the random chain cleavage reaction is excessive.rnEach chain breakage creates two new chain ends, one of which is a reducing end group, and canrnactivate a new chain of peeling reactions. These cleavage reactions are related to the presence ofrnhydroxyl radicals (OH?) which are generated via thermal disintegration or transition metal inducedrnhomolytic decay of hydroperoxides.rnThe rate of the carbohydrate degradation depends on oxygen pressure, temperature and alkalirnconcentration. Although some strong correlations have been established for different types of pulpsrnthere are significant differences in the formulation of the kinetic models for the process. There is anrnapparent lack of understanding of the heterogenous nature of the process.