Modeling chlorine dioxide bleaching of chemical pulp (Aalto University)

ABSTRACT
 This doctoral thesis deals with the phenomenon-based modeling of pulp bleaching. Previous bleaching models typically utilize one or two empirical correlations to predict the kinetics in kappa number development. Empirical correlations are simple to develop, but their parameters are often tied to the validation system. A major benefit of physico-chemical phenomenon models is that they are valid regardless of the reaction environment. Furthermore, modeling the bleaching processes at molecular level provides a new way to examine the relative importance of various phenomena, the validity of theories and the bottle necks of bleaching applications.
The first part of the thesis introduces a model for the pulp suspension environment and describes the physico-chemical models in use (reaction kinetics, mass transfer, thermodynamics). The second part deals with inorganic oxychlorine reactions related to chlorine dioxide bleaching. The reaction kinetics and mechanism are documented for iron- mediated chlorite decomposition, chlorous acid self-decomposition, and for the reaction between hypochlorous acid and chlorous acid. The rate coefficient temperature dependency is reported for the two latter reactions. The reaction kinetic models were utilized in assessing the potential chlorate formation routes encountered in pulp bleaching. It was concluded that chlorous acid self-decomposition is unlikely to contribute to chlorate formation in bleaching applications. The iron-mediated chlorite decomposition and the reaction between hypochlorous acid and chlorous acid are expected to produce chlorate.
The last part introduces a model for chlorine dioxide delignification. Here the pulp suspension model was combined with a broad set of chemical reactions describing the ClO2bleaching chemistry. The incorporated reactions cover lignin oxidation, lignin chlorination, hexenuronic acid degradation, lignin dissolution, oxidation of extractives and the essential inorganic oxychlorine reactions. The model predictions were compared against experimental delignification results in order to validate the model and obtain the previously unknown reaction rate parameters. The predictions were generally in good agreement with xperimental results. Deviations related primarily to hypochlorous acid driven processes: organochlorine formation, hexenuronic acid degrading reactions, chlorite depletion or chlorate formation. The simulation results suggest that organochlorine formation is restricted by the amount of chlorination-susceptible substrates in pulp rather than the availability of chlorine. It was also concluded that, in addition to the fully inorganic reactions, chlorite is also likely to be consumed in reactions with pulp-related compounds, for instance with aldehydes.

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