Multi-phase thermodynamic modelling of pulp suspensions: Application to a papermaking process

During the past decade, multi-phase thermodynamics has been applied to pulp suspensions in order to support calcium chemistry control in neutral papermaking. During this period, multi-phase modelling was initially applied in the development of pulp acidification and pH buffering concepts, and unit operation models have been used to quantify the doses of acidification chemicals and buffering aids. The inclusion of multi-phase chemistry calculation in conventional mass and energy process simulation has enabled the monitoring of the chemistry of large process integrates. The experiences and case examples of multi-phase modelling are reviewed in the present paper. In the present study, the pH and calcium chemistry of a large process integrate were simulated in conditions that had been previously found to cause problems in paper production. The difficulties were thought to be due to increased dithionite dosing, which had been used to obtain the desired brightness of the paper product. The modelled integrate consisted of two paper machine lines with mechanical and deinked pulp as feed stocks. Simulation of the production of the high brightness grades revealed that a relatively high quantity of solid calcium carbonate is lost under these production conditions. The results improved our understanding of bleaching practices in neutral papermaking.