Heterogeneous kinetic model for the methylglucuronic and hexenuronic acids reactions during Kraft pulping of Eucalyptus globulus

A kinetic model considering simultaneously the removal of methylglucuronic acids (GlcA), the formation of hexenuronic acids (HexA), and their degradation/dissolution is proposed. In the model, the effective alkali concentration in the entrapped liquor was used instead of that in the bulk liquor, accounting for the heterogeneous nature of wood pulping. The results are very satisfactorily explained by assuming that GlcA are composed of two subgroups: the fast GlcA, which disappear early in the cook with a low activation energy (58 kJ/mol), and the slow GlcA, which are degraded/dissolved (E-a = 91 kJ/mol) or produce HexA, not being totally consumed. The degradation/dissolution reactions of both GlcA subgroups are second-order with respect to their content, while HexA formation (E-a = 92 kJ/mol) and degradation (E-a = 110 kJ/mol) are first-order with respect to slow GlcA and HexA contents, respectively. The effective alkali concentration influences the reactions that involve the slow GlcA and HexA, with a greater contribution to HexA degradation. The sulfidity is not relevant on any of these reactions. The model was experimentally validated with varying temperature, alkali, and sulfidity profiles and predicts reasonably well the GlcA and HexA contents during kraft pulping of Eucalyptus globulus.