Kinetics of Delignification during Magnesium Bisulfite Pulping, 1999 Pulping Conference Proceedings

Jianghua Li & Guojun Kang

Dr. Jack McKenzie Limerick Pulp and Paper Research and Education Centre

University of New Brunswick

Adriaan R. P. van Heiningen

Department of Chemical Engineering

University of Maine

The kinetics of delignification during magnesium bisulfite pulping are modeled as the sum of three modes of delignification: cleavage of a-ether bonds, cleavage of b-ether bonds, and homolytic cleavage of the free phenolic b-ether bonds. The reaction constants were determined by a non-linear fit of the kinetic model to magnesium bisulfite pulping delignification data. The kinetic data were obtained by pulping wood shavings with an average thickness of about 0.3 mm at a constant bisulfite concentration and constant pH throughout each cook. The range of the variables were: cooking temperature 155-175°C, pH 3.2-4.0 and total SO2 concentration 0.2-0.94 mol/L. A close fit between the model predictions and experimental data was obtained. The activation energies obtained are 157, 147 and 99 kJ/mol for the three modes of delignification respectively, showing that delignification via the homolysis reaction has a lower temperature dependency than the other two degradation pathways. Delignification by homolysis becomes significant at the highest pH and lowest temperature. However, in general, the relative rates of delignification of the three pathways are comparable at normal bisulfite pulping conditions.