EFFECT OF HEMICELLULOSE CONTENT IN KRAFT BROWNSTOCK ON OXYGEN DELIGNIFICATION

Abstract

Kraft pulping experiments were performed on mixed Northeastern hardwood chips to investigate the effect of hemicellulose content on the response of brownstock pulps to medium consistency oxygen delignification. High levels of anthraquinone were used to produce pulps with high pulp yield which coincided with high hemicellulose content brought about by elevated levels of xylan. Response variables were found to depend upon hemicellulose content in the brownstock. The kappa number versus time data fit a classical power law model, indicative of lignin fragmentation. There was a systematic increase in the oxygen delignification rate and in the Schoon rate constant as the hemicellulose content decreased in the digester. The pulp selectivity increased with increasing pulp hemicellulose content until a maximum was reached. Chemical analysis of the pulps indicated that the increase in pulp yield was caused primarily by an increase in the xylan polymer as additional anthraquinone was added in the digester. In the oxygen reactor, the loss in pulp yield resulted from a reduction in the lignin as well as a loss of carbohydrates, primarily from loss of the xylan polymer. The observed results support the hypothesis that oxygen delignification is limited by reduced accessibility due to the presence of increased amounts of xylan.