Displacement washing of kraft pulp with various consistency

Abstract

The paper deals with the displacement washing of kraft softwood and hardwood pulps under various consistency of the pulp fibre bed. The results obtained revealed the differences between both types of kraft pulp with various morphological properties of fibres. The washing efficiency decreased with decreasing Péclet number for both kraft pulps, but, due to lower longitudinal dispersion of the wash liquid, the greater washing efficiency was achieved for short-fibred hardwood pulp. On the other hand, the long-fibred softwood pulp manifested lower hydraulic resistance of the pulp fibre bed. The change in pulp consistency and similarly the volumetric mass transfer coefficient, characterising the rate of leaching of alkali lignin from pulp fibres, did not have a significant effect on the washing efficiency. However, the mass transfer coefficient decreased with increasing specific resistance of the pulp bed. With increasing bed consistency of both softwood and hardwood pulps, as the amount of black liquor in the inter-fibre pores decreased, the difference between the space time characterising the holding time of wash liquid and the mean residence time of alkali lignin increased.