Modelling of chemical pulp bleaching

Conventional pulp bleaching processes consist of the following unit operations: chemical mixing, bleaching reactions (retention towers) and pulp washing. The quality and profitability of chemical pulp bleaching depends on how well one is able to utilise the physico-chemical phenomena (chemical reactions, fluid dynamics, thermodynamics, mass and heat transfer) that determine the performance of these unit operations. This study introduces a predictive pulp bleaching model utilizing physical phenomenon models and chemical reaction kinetics. Our approach provides means to compare the relative importance, benefits, and restrictions of each phenomenon in various pulp bleaching operations. The model also provides a possibility to examine hypotheses concerning the bleaching reaction routes and the role of fast-lived intermediates. The applied phenomenon theories and the parameters required for their utilization are introduced. Examples dealing with the importance of the buffering properties of pulps in chlorine dioxide delignification, and mass transfer limitations between the fibre wall and the bulk suspension are given. The kinetics of hexenuronic acid hydrolysis is used as an example of how the applicability of experimental reaction kinetic information of pulp bleaching reactions can be widened.