A simplified kinetic model for modern cooking of aspen chips

Abstract Kraft pulping kinetic models are an important component of any fundamental continuity based continuous digester model. These models can be used to further develop our understanding, or as a framework to support the development of real-world control, estimation and optimization strategies. Effective models are tailored to a specific species of wood and must be applicable for a wide range of expected cooking conditions. In this work a series of experiments were conducted on hardwood Aspen chips for 5 different cooking conditions. Each series of cooks were interrupted at different time intervals to capture the dynamic response of the cook. The key chips and liquor components were measured and reconciled at each interval. A dynamic model was then developed based on a simplification of a continuous digester model under batch conditions. This ensures continuity between the key assumptions governing both the batch kinetics model and the expanded continuous form. A kinetic model structure was adapted from the literature that quantifies all three accepted phases of lignin and cellulose degradation, i. e. the initial, bulk and residual phases as well as the effect of modern cooking practices such as intra-cook white liquor addition on the transition between the bulk and residual phases. Additionally, modifications were made to the kinetic model structure to reduce the overall number of states and to impose a floor limit on degradation, thereby reducing the overall complexity and computational burden. The model was then fit to the data using weighted least squares and simulation optimization techniques.