Bed Efficiency and Volumetric Mass Transfer Coefficient as Mass Transport Parameters of Displacement Pulp Washing

Abstract

The long-fibred spruce sulphite and kraft pulps, as well as short-fibred soda pulp cooked from rapeseed straw were subjected to displacement washing under laboratory conditions. Pure water was used as wash liquid, the conductivity of which was described by the permeability. The unmovable pulp bed was characterised by its average effective flow porosity, specific resistance, and equivalent pore diameter. The displacement washing process was divided into two periods. The first period, when spent liquor was displaced from the inter-fibre pores, was described by bed efficiency. The transport of solute in the second period, when leaching prevails, was characterised by the volumetric mass transfer coefficient. The results obtained showed that the bed efficiency increases with increasing Péclet number, however, it decreases with increasing dimensionless dispersion length. The bed efficiency obtained for long-fibred spruce pulps was found to be greater compared to that for short-fibred soda rapeseed pulp. The average volumetric mass transfer coefficient determined for all pulp types had an increasing trend with increasing superficial wash water velocity.