Kinetic Modeling of Hardwood Prehydrolysis. Part III. Water and Dilute Acetic Acid Prehydrolysis of Southern Red OAK

Abstract

The hemicelluloses in wood are more readily hydrolyzed than is cellulose. Because it is advantageous to process the hemicellulose sugars separately from the glucose obtained from the cellulose, most processes for utilizing wood as a source of chemicals and liquid fuels include a prehydrolysis step to remove the hemicellulose prior to the main hydrolysis of the cellulose to glucose. Kinetic data are required to model the reactions that occur during prehydrolysis so that optimum conditions and product mixes can be predicted. Two promising prehydrolysis methods, the Iotech steam explosion process and the Stake process, are based on water prehydrolysis (autohydrolysis). The kinetics of water and of dilute (5%) acetic acid prehydrolysis of southern red oak wood over the temperature range of 170 to 240 C were investigated. Kinetic parameters were determined that permitted modeling not only of xylan removal from the wood but also of the occurrence of xylan oligosaccharides, free xylose, furfural, and further degradation products in the prehydrolyzate. At lower temperatures (approximately 170 to 200 C), xylan removal could be modeled as the sum of two parallel reactions (one for an easily hydrolyzed portion and one for a more resistant portion of xylan) using the equation derived in Part I. At the highest temperature studied (236.9 C), the removal of xylan from the wood was best modeled as a single reaction with a small fraction of the xylan being essentially nonreactive. The occurrence of xylan oligosaccharides, xylose, furfural, and further degradation products in the prehydrolyzate was modeled as consecutive, irreversible pseudo first-order reactions. A timelag associated with the de-polymerization of the xylan oligosaccharides to xylose was accounted for in the model by allowing the apparent rate constant for the formation of xylose to increase exponentially with time to a maximum value. Increasing the temperature decreased the time required for the overall reactions to occur, increased the portion of xylan removed from the wood, and increased the yield of total anhydroxylose units (xylose + xylan oligosaccharides) that were recovered in the prehydrolyzate. Prehydrolysis with dilute acetic acid does not greatly affect the maximum yields of products in the prehydrolyzate over those observed with water prehydrolysis; however, the time to maximum yield decreased. The data presented in this report indicate that, at higher temperatures, water or dilute acetic acid prehydrolysis gives yields comparable to those for dilute sulfuric acid prehydrolysis at 170 C recently reported in the literature. Preliminary results with lignin isolated from the water and acetic acid prehydrolysis residues confirm recent reports that lignins of this type are useful as phenol substitutes in phenol-formaldehyde adhesives.