(258a) Mechanistic Modeling of Simultaneous Enzymatic Saccharification of Cellulose and Xylan Using Continuous Distribution Kinetics

Cost effective production of biofuels and biochemicals often involves enzymatic saccharification, which has been the subject of intense research and development. In this work, we develop a mechanistically based kinetic model for the enzymatic hydrolysis of biomass composed of cellulose, xylan, and lignin. The prototypical biomass particle in our model has a cellulosic core that is partially covered by xylan and lignin that impede the access of glucanase enzymes to the cellulosic core. Our model hence incorporates both glucanase and xylanase enzymatic saccharification of surface xylan and the cellulosic core. For degradation of insoluble cellulose chains we consider the actions of endoglucanase (endo-β-1,4-glucanase,) and exoglucanase (exo-β-1,4-cellobiohydrolase). Likewise for insoluble xylan we consider the actions of endoxylanase (endo-β-1,4-xylanase) and exoxylanase (xylan β-1,4-xylosidase), the latter enzyme hydrolyzing soluble xylan oligomers as well. The enzyme system also includes beta-glucosidase for the hydrolysis of soluble glucan oligosaccharides. The kinetics for the depolymerization of cellulose and xylan chains are described using population-balance equations, which are capable of describing the evolution of the distribution of chain sizes during hydrolysis, without solving equations for all chemical species present in the reacting mixture.  This approach affords computationally efficient simulations, enabling the use of the model for process design and optimization. In combination with experiments, the kinetic model is used to study the transformations of the cellulose and xylan chain-size distributions during hydrolysis by an enzyme cocktail of all the enzyme species.