MODELING THE EFFECT OF FIBER PROPERTIES AND OPERATING-CONDITIONS ON DIFFUSION WASHING OF DIFFERENT EUCALYPT KRAFT FIBERS

Diffusion washing of unbleached kraft pulp is dependent on pulp type, process conditions and fibre dimensions. The importance of process con ditions and fibre dimensions to washing is explored with a mathematica l model. The assumptions made in the model are critically examined and the effects of variation in some of these tested. The model is based on Fick's law and simulates the diffusion of lignin from the fibre wal l under various washing conditions. The model predicts the effects of fibre dimensions, lignin concentration and profile on the rate of wash ing, The model predicts that with typical fibre dimensions for eucalyp t kraft fibres the rate of washing depends on fibre wall thickness but not on fibre diameter and length. Simulations with the model suggest that the washing of lignin from kraft pulps is highly sensitive to lig nin concentration profile in the fibre wall and concentrations of lign in in the lumen and wash liquor.