SFGP 2007 - Modelling Kraft Cooking of Wood Species Mixtures

Abstract

A batch kraft cooking simulator has been developed with the purpose of simulating the cook of wood chip mixtures. Surveyed equations are modelling diffusion and simultaneous reactions inside a planar chip and external transfers in the outer liquor phase. Three reaction phases (initial, bulk and residual) are described by apparent first-order kinetics, affected by temperature (Arrhenius law). Cooking is controlled by reagent charges, heat-up time and plateau temperature and time. In past works, kraft modelling had been designed for batch cooks of mono-species chips. In this work, simulation was upgraded to allow the cook of chip mixtures: either mixed-size chips, according to a thickness distribution law, or mixtures of different wood species. In the latter case, initial composition, morphological characteristics and chemical kinetics are proper for each wood category. Simulation results are average parameters such as lignin content (also given as kappa number), pulp yield and residual alkali as well as chip profiles (lignin, polysaccharides and alkali), from which homogeneity indexes are calculated. The effects of thickness distribution law and oversized chips in the population are first studied. Oversized chips cook much less when they are in few numbers in a large population of small chips. This affects mean cooking results and extent of lignin distribution within chips. Studying chip mixtures from different woods focused first on the effect of morphological parameters such as wood porosity which effected delignification rates and apparent activation energies. Porosity acts on the inner-outer liquor and alkali partition, also affected by diffusion. Morphological characteristics were compared to chemical effects by varying chemical reaction activation energies. It was found that a difference of 20% on the activation energies of two woods cooking together acted dramatically on cooking results. As a consequence, lignin content and pulp yield within chips at the end of the cook were widely distributed. Finally, cooking results are more affected by chemical differences than by morphological differences, but both effects cumulate and contribute to spread lignin and polysaccharide content distributions in the final pulp.