Comprehensive Kinetic Study of Delignification, Carbohydrate Degradation, Cellulose Chain Scissions, and Hexenuronic Acid Reactions during Kraft Pulping of Eucalyptus Globulus

Eucalyptus globulus represents one of the most suited fibre sources for the production of a wide range of high quality papers and high-purity dissolving pulps. The optimization of kraft pulping covering both continuous and batch processes demands kinetic models suitable for advanced control and offline optimization of the cooking operation. Quite recently, an improved kinetic model structure for softwood kraft cooking has been published, which considers varying alkali and temperature profiles during the cooking [1]. Unfortunately, most alkaline cooking models are based solely on softwood [2, 3]. This work contributes to an improved kinetic model for kraft cooking of Eucalyptus globulus suitable for advanced control of modern displacement cooking processes such as the continuous batch cooking process (CBC). After impregnation where the initial lignin has been removed, the bulk and residual delignification have been studied in a batch reactor as a function of the [OH], [HS], ionic strength, [Na], and temperature. The underlying kinetic expressions are similar to those used in the Andersson model also including a mechanism for determining the distribution of lignin and carbohydrate species 2 and 3 as a function of the cooking conditions. The kinetic equations were successfully validated against experimental data from a pilot plant digester applying conventional batch cooks. Furthermore, the formation of hexenuronic acid bound to the xylan backbone and its degradation / dissolution were described by a consecutive first-order reaction. In agreement to other studies, the temperature and the effective alkali charge revealed a strong influence on the hexenuronic acid content in the pulp while the effect of sulfidity proved to be insignificant. The role of ionic strength on the HexA content remained unclear. An increase in the ionic strength resulted in a higher maximum HexA content shifted to a higher kappa number (about 58), but with progressive delignification to a kappa number typical for unbleached E. globulus kraft pulp (about 15), the HexA content decreased to a level more than 20% lower as compared to pulps cooked at lower ionic strength.