A dynamic model for calculating pH inside kraft pulp fibers

One of the main control parameters in the kraft pulping process is pH. The pH inside kraft pulp fibers can be shown theoretically to differ considerab ly from that in the free process liquor surrounding the fibers. The steady- state distribution of ionic species between the free liquor and the immobil e liquor inside the fibers, and therefore also the pH, is strongly affected by active chemical groups in the kraft pulp fibers. A dynamic mass transfe r model was constructed to simulate the concentrations of four mobile ions (H+, OH-, Na+, SO42-) in the free process liquor surrounding the fibers and in the immobile liquor inside the pulp fibers. The model takes into accoun t the mass transfer resistance between the two liquor phases, the different mobilities of the mobile ions, the effect of carboxylic and phenolic group s in the pulp, and the sorption of sodium onto the pulp. The dynamic pH beh avior is shown by calculating examples of acid addition and displacement wa shing operations in kraft pulp bleaching. These examples indicate that the pulping process cannot be well understood without taking into account the d ynamic behavior of concentrations in different liquor volumes in a pulp-wat er suspension. The difference between the pH inside the pulp fibers and the pH in the free liquor surrounding the fibers should be considered far more often when discussing the chemical modification of kraft pulp fibers.