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Kinetic modeling of hardwood prehydrolysis. Part II. Xylan removal by dilute hydrochloric acid prehydrolysis

Knowledge of prehydrolysis kinetics has applicability to the design, development, and modeling of processes to separate wood into its basic chemical constituents (i.e., cellulose, hemicellulose, and lignin). The kinetics of xylan hemicellulose removal with 0.10 M HC1 at 120 C from quaking aspen, paper birch, American elm, red maple, and southern red oak was studied. The mathematical model developed in Part I to describe the kinetics of xylan removal by water prehydrolysis of these species could be used to model xylan removal with dilute hydrochloric acid. Xylan removal could thus be modeled as the sum of two parallel first-order reactions—one fast and one slow. However, unlike the case with water prehydrolysis where the rate constants for the fast (k f ) and slow (k s ) reaction processes could be correlated with each other, they could not be correlated for HC1 prehydrolysis. Instead the k f and k s values determined for each species clustered about average k f and k s values for all the species as a whole. A single set of parameters determined from a nonlinear least squares fit of the experimental prehydrolysis data for all the species as a whole to the model could be used to reasonably describe the course of xylan removal from all the species. The fact that one set of parameters could be used suggests that the same reactions are taking place on prehydrolysis and that the chemical structure and physical morphology of the xylan hemicellulose were essentially the same in the species studied and probably in all temperate hardwood species. The model thus provides a good approximation of xylan removal from any temperate hardwood with dilute hydrochloric acid at the reaction conditions studied.