Multiscale Modeling of Hydrothermal Pretreatment: From Hemicellulose Hydrolysis to Biomass Size Optimization

In the general process for lignocellulosic bioconversion to ethanol, pretreatment has been viewed as the main cause of low process yield. Consequently, it is believed that obtaining a better understanding of the hydrothermal pretreatment method would pave the way for overall process optimization. This work focuses on developing a model for a pretreatment process that considers both the chemical and physical natures of the process. The chemical aspect of the process mainly involves the hydrolysis of hemicellulose to monomeric sugars. This paper considers all xylooligomers, with a degree of polymerization up to 30 as soluble, and that the bond breakage is a function of the position in the hemicellulose chain. Also, all of the bonds with the same position undergo breakage at the same time. The physical aspect of the process involves reducing the size of the feedstock as well as heating the feedstock to a desired temperature. For this aspect, we have developed a model to estimate the energy requirements for size reduction and proposed a method to find the optimum chip size for pretreatment. Finally, we performed two sets of sensitivity analysis: first, to compare the dynamic importance of xylooligomer size evolution versus xylose decomposition and, second, to compare the relative importance of kinetic parameters versus the length of particle on the overall process yield. Sensitivity analysis revealed that, at the beginning of the process, the chemical reaction is more important than diffusion; however, as the reaction proceeds, diffusion becomes the determinant factor. It was also shown that the solid-phase reaction rate, xylooligomer size evolution, and xylose decomposition are all determinant factors; therefore, any model for hemicellulose hydrolysis should take all three of these factors into account.