Towards complete impregnation of wood chips with aqueous solutions. Part 5: improving uniformity of kraft displacement batch pulping.


Pulping uniformity is critical for improving the performances of cooking and downstream operations such as bleaching and papermaking. In spite of certain success of recent modifications of batch and continuous kraft cooking systems, there is still a lot of room for improving the contemporary cooking systems in order to address problem of heterogeneous pulping. Incomplete penetration and inadequate diffusion of chemicals into wood chips remain the primary reasons for delignification non-uniformity at the scale of a single chip. These factors can be minimized by improved quality of wood chips and optimised cooking conditions. This paper examines the effect of modifying the front-end conditions of kraft displacement batch cooking on pulping uniformity. Efficient steaming of the chips and application of higher-pressure profile at the front-end of the displacement batch cooking resulted in reductions in amount of rejects and kappa number of the bulk pulp as well as improved uniformity of delignification. In addition, the lower the kappa number level and its variability within the single heartwood and sapwood pine chips were achieved. Application of both modifications simultaneously seemed to provide the biggest effect.