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Understanding and modelling softwood hemicellulose hydrolysis and its adsorption to pulp fibres

University of British Columbia

Pulp and paper facilities are transforming into innovative biorefineries producing chemicals and materials in order to enhance competitiveness and environmental performance. Hemicellulose fractionation and its integration with the pulp mill are crucial to successful biorefinery processes. Fundamental insight is needed to support technology growth. Hemicellulose oligomers were extracted by hydrolysis of softwood chip fines using hot water; conditions were optimized for high yield and high molar mass. Comprehensive characterization of hydrolysate and hydrolyzed solids is reported. A two-dimensional calibration method enabled measurement of oligomer molar mass and concentration simultaneously by size exclusion chromatography. A population balance model describing evolution of hemicellulose molar mass during hydrolysis was posed. The model describes the full evolution of oligomers from initial softwood solubilization, depolymerization to ever-smaller molecules until final generation of degradation products. A maximum yield and corresponding treatment condition for a specific molar mass could be predicted. Likely modes of hemicellulose bond breaking within the wood matrix and bulk solution are proposed and physical insights are explained. This work provides fundamental insights into the relative reactivity of hemicellulose intermediates to facilitate future conversion technologies. Two hemicellulose hydrolysis integration methods to kraft pulping are proposed. First, adsorption of locust bean gum (LBG, model compound) to Northern Bleached Softwood Kraft (NBSK) pulp was shown to improve paper tensile and burst strength and lower refining time by strengthening inter-fibre bonding. LBG adsorption to NBSK pulp fibre is dependent on electrostatic forces, and high salt addition at low pH facilitates adsorption. The adsorption followed pseudo-second-order kinetics and the Langmuir adsorption isotherms, indicating a reversible, monolayer, homogenous adsorption to a finite number of sites on the fibre surface with chemisorption as the rate determining step. Second, mild hydrolysis was combined with kraft pulping as pre-treatment to produce hemicellulose oligomer and kraft pulp. Particle size effects on hydrolysis and subsequent kraft pulping were assessed. Kraft pulping of pre-hydrolyzed softwood chips enhances delignification, reduces fibre yield and chemical consumption, producing fibres with decreased fibre dimension but increased water retention value. The advantages and disadvantages of pre-hydrolysis integration to kraft pulping and associated challenges and future recommendations are discussed.

Text

2020-01-27

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/73394

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/