Influence of wash liquid properties on the efficiency of pulp washing

In a laboratory washing cell, the mechanism of the displacement washing of unbleached kraft pulp was investigated. Wash liquids with various pH values varying in the range from 2 to 11 were used to displace black liquor from the pulp bed. Using the step function input change method, the washing breakthrough curves obtained experimentally were described by the axially dispersed plug flow model. To characterize the displacement washing process, the wash yield, as well as the flow model parameters were evaluated. The effect of the superficial wash liquid velocity and of the mobility of displacing and displaced liquids upon the axial dispersion coefficient was discussed. A correlation between the wash yield, Peclet number, and pH of wash liquid was derived as well.