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Poster Presentation 3-16
Modeling Approaches to the Heterogeneous Dilute-acid Hydrolysis of Cellulose Microcrystallites Derived from Lignocellulose
Pär O. Pettersson1, Robert W. Torget2, Qian Xiang3, Y.Y. Lee3 and Guido Zacchi4
1Mid Sweden University 891 18 Örnsköldsvik, Sweden
2National Renewable Energy Laboratory Golden, CO, USA
3Auburn University, AL, USA
4Lund University, Sweden
Telephone: +46 660 578 36; Fax: +46 660 578 05; E-mail: par.pettersson@mh.se
It has been observed in bench-scale and pilot-scale experiments over the last decade that the reaction rate for dilute-acid catalyzed crystalline cellulose from yellow poplar is not only dependent on the chosen reactor configuration, but also the prehydrolysis history of the substrate prior to the hydrolysis of its cellulose. Although it has been widely accepted that glucose yields from crystalline cellulose should indeed vary according to whether or not the released glucose is washed out of the reaction zone (as in a percolation reactor) or held at reaction temperature, cellulose hydrolysis kinetic expressions have not changed significantly since Seaman’s proposed kinetics published in 1945. In an attempt to understand the different cellulose hydrolysis rates which cannot be explained by Seaman’s kinetics, we are developing a heterogeneous model, which, among other things, accounts for the transport of sugar from the surface to the bulk phase through some undefined boundary layer resistance. Although the total chemical and physical nature of this resistance remains to be determined, calculations have shown that this resistance is orders of magnitude larger than mere film diffusion resistances. Molecular dynamic simulations have demonstrated that under hydrolysis severities, a significant structured water barrier exists using pure cellulose, which could impede its observed hydrolysis kinetics. Further, this model suggests that the rate of cellulose hydrolysis is proportional to surface concentration of glucosidic units, and not its volumetric concentration. Additionally, the model allows for both the hydrolysis rate constant and the mass transfer rate constant to vary as a function of both conversion and extraneous compounds found both in the bulk phase and in the boundary layer. Although the proposed features of this model are quite simplistic, they can serve as a starting point for a discussion on various mechanisms.
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