Towards complete impregnation of wood chips with aqueous solutions. Part 4. Effects of front-end modifications in displacement batch kraft pulping.


The front-end of kraft cooking, i.e. steaming, impregnation and heating, controls the cooking performance in terms of mass transfer, delignification and cooking uniformity. Therefore, it is of vital importance to understand the process mechanisms and optimise the front-end process conditions. This article discusses the effect of applying chip presteaming and higher pressure at the front-end of kraft displacement batch pulping on the cooking performance, bleachability and paper making potential of the pulp. By applying chip presteaming and higher pressure it is possible to significantly reduce the amount of rejects after the cook and to lower the kappa number at constant cooking conditions. The better delignification uniformity and efficiency can be explained by the more thorough penetration of liquor into the wood chips achieved with the front-end modifications. According to the results, chip presteaming and higher pressure do not have a significant effect on the bleachability and papermaking potential of pulp. However, better pulp uniformity and the resulting lower pulp rejects content allows higher production of accept pulp and less fibre losses. The lower residual lignin content of the cooked pulp can be used as a trade-off for higher cooking throughput at the same chemical charge and recovery load or reduced bleaching chemical consumption due to lower pulp kappa number.