Modeling of Oxygen Delignified Wheat Straw Enzymatic Hydrolysis as a Function of Hydrolysis Time, Enzyme Concentration, and Lignin Content

Abstract

Enzymatic hydrolysis is one of the most expensive operations of producing lignocellulosic ethanol, primarily due to high enzyme costs. Enzyme loadings must be reduced, and a well-developed kinetic model that can be easily implemented in process simulation software would greatly assist in determining optimum processing conditions. Oxygen-delignified wheat straw with different lignin contents was subjected to enzymatic hydrolysis at two different enzyme loadings for 72 h. Glucan conversion increased with increasing enzyme loading, decreasing lignin content, and decreasing solids concentration. By measuring total protein concentration and predicting the Novozyme 188 protein concentration, it was possible to calculate the cellulase protein concentration as a function of time. This work is the first report of a mass-based kinetic model capable of predicting glucose production during enzymatic hydrolysis of oxygen-delignified wheat straw, at different cellulases loadings (20 and 40 filter paper units/g glucan), lignin contents (5 and 9 wt%), and solids concentrations (5 to 10 wt% dry basis). The presented hydrolysis model includes a novel lignin factor to describe the amount of cellulases irreversibly adsorbed on lignin. The lignin factor also links glucose production during enzymatic hydrolysis to pretreatment severity. Mass transfer limitations present at 10 wt% solids were accounted for using a diffusion factor. Due to the model's simple solution and use of only five parameters, it can be easily implemented in process simulations.