Modelling the fragmentation kinetics of the heterogeneous lignin macromolecule during kraft pulping with stochastic graphs

•Gaining deeper insight in kraft pulping delignification.•Modelling the phenolic and non-phenolic βO4 bonds splitting during delignification.•Decoupling kinetics and solubility phenomena.•Discussion of further research direction related to mass transfer limitation. This article presents a novel concept for modelling the kinetics and related phenomena of the kraft pulping process on a macromolecule level with the initial objective to explicitly model and relate the breakage of phenolic and non-phenolic βO4 bonds to the observed three-stage delignification profile. The modelling frameworks consist of the building and the fragmentation of the lignin macromolecules. The macromolecules are modelled as stochastic graphs where monolignol object nodes are reassembled in a Monte Carlo approach into internal structures, which aggregate to a lignin macromolecule interconnected through chemical bonds represented by the edges of the graphs. The fractionation follows the splitting of βO4 ether bonds with different configurations resulting from functional groups attached to the monolignols, namely the phenolic and non-phenolic βO4 bonds with their respective stereochemistry. It is tested against a previously published model based on an extension of the established Purdue kinetic model and experimental data. The results align with the observed delignification trajectory during kraft pulping, and the hypothesis that βO4 bonds splitting is mainly responsible for the delignification. However, some discrepancies between the current model, the previous model and experimental data are presented. These differences are discussed in the context of recent experimental findings indicating that βO4 bonds splitting might not entirely be responsible for the delignification due to mass transfer/solubility effects limitations.