UBC Theses and Dissertations
Lignin model compound approach to modelling oxygen delignification reaction kinetics Lee, Michael
The oxygen delignification stage is implemented in modern kraft pulp mills to cut the cost of producing bleached bright pulp and reduce emissions from the bleaching process. While several oxygen delignification kinetic models are presented in the literature, most models are derived from and limited to, specific pulp blends. In this work, an oxygen delignification kinetic model was developed based on lignin model compound chemistry found in the literature to create a universal kinetic model applicable to a range of pulp blends. The kinetic model splits the pulp into three reactive lignin groups (fast, slow and non-reacting) each with their own kinetic constants. Along with the starting kappa number, the proposed testing protocol measures kappa number from oxygen delignification experiments at 90°C for three hours (to determine non-reacting lignin fraction) and at 50°C for five minutes (to determine the fast lignin fraction). From the three kappa measurements, the fraction of each lignin group is determined and combined with their respective kinetic constants to create the overall kinetic model. Coupling separately developed mass transfer governing equations for the pulp suspension with the kinetic model, the overall oxygen delignification stage model was developed. The oxygen delignification model was compared with experiments performed both in the laboratory and in an industrial pulp mill. Laboratory oxygen delignification experiments on four pulp blends (three softwood and one hardwood) showed good agreement between experimental data and simulation values indicating the proposed testing protocol and kinetic constants are able to model the oxygen delignification reaction. In particular, oxygen delignification reaction result at 50°C for five minutes in conjunction with delignification result at 90°C for three hours was able to determine the split between fast, slow, and non-reacting lignin groups. Model simulations using the determined lignin fraction split was able to model experimental data especially well for 90°C experiments that mimics the conditions experienced in an industrial setting. Experiments at a pulp mill showed agreement within one kappa number between measured values and model simulations signifying the applicability of the proposed overall oxygen delignification model to simulate an industrial process.
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