Temperature dependence of residual phase delignification during kraft pulping of softwood

The kinetics of delignification in kraft pulping has been studied to determine the activation energies of the bulk and residual phase delignification processes. A model for delignification in cooks with high liquid-to-wood ratios (constant composition cooks) has been extended to include cooks with changing concentration profiles and significant amounts of dissolved lignin (normal cooks). The delignification in the kraft process can be successfully described by three parallel first order reactions. Using this model, the activation energies of the bulk and residual phase delignifications have been determined, and have been found to be 136 kJ/mol and 152-156 kJ/mol (determined from constant composition cooks as well as normal cooks) respectively. Although the finding that the activation energy of the residual phase delignification is higher than that of the bulk phase delignification is contrary to previous results, this seems reasonable since lignin solubilisation is more difficult during the residual phase delignification than during the bulk phase delignification. Moreover, because dissolved lignin is present, the delignification rate in the bulk phase is 20% higher in the normal cooks than in the constant composition cooks while the delignification rate in the residual phase is 30% lower for the normal cooks. The fact that modelling the delignification in kraft cooking using three types of native lignin works so well indicates that the residual phase lignin is in fact native i.e, exists in the wood and is not the product of unfavourable reactions during the bulk phase.