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Oxygen delignification as a pretreatment for the enzymatic hydrolysis of lignocellulosic material Charles, Nichola

Abstract

With the increasing importance of environmental conservation and the inevitable depletion of the world's fossil fuel supplies, it is necessary to develop renewable and clean sources of energy. Bioconversion, the process of converting plant biomass to fuel grade ethanol, is one process that addresses these concerns. However, despite promising results, the development of a more feasible pretreatment process to be used upstream of enzymatic hydrolysis is required. Oxygen delignification has long been successfully employed in the pulp and paper industry to remove the lignin from pulp samples. Since the general aim of a pretreatment process is to disrupt the close relationship between lignin and cellulose, a study was taken to evaluate the effectiveness of oxygen delignification in this regard. Softwood Kraft pulp samples were used as a model substrate to determine the potential effectiveness of oxygen delignification. In the initial phase of the study, hemlock pulp samples were taken upstream (kappa number of 34) and downstream (kappa number of 16) of the oxygen delignification unit in a commercial pulp mill. When subjected to enzymatic hydrolysis under standard conditions, the initial rate of sugar saccharification and the final yield of sugars improved by 62 - 82% and 76 - 80%, respectively, for the oxygen delignified sample. These results were sufficiently promising to carry out a more detailed laboratory-scale study. In an effort to optimize the delignification conditions, lab-scale tests were carried out in a 750- mL reactor over a range of temperature, oxygen partial pressure, retention time and caustic loading based on a factorial experimental design, measuring the kappa number and pulp viscosity as response parameters. The dependence of the kappa number and pulp viscosity on the oxygen delignification conditions were found to follow a linear model: [Kappa = 49.73-0.19T- 3.09S - 0.051 - 0.050; Viscosity (mPa.s) = 55.91 - 0.03t - 0.19T-4.06S-0.020] Where t = retention time (minutes), S = caustic loading (% NaOH per gram oven-dry weight of pulp), T = reaction temperature (°C), O = oxygen partial pressure (psig). The laboratory-delignified samples were subsequently hydrolyzed and analyzed for their susceptibility to hydrolysis. Generally, as the kappa number of the pulp decreased, the final yield of sugars and initial rate of sugar saccharification increased. However it was apparent that there is a point when hydrolysis is maximized and as the lignin content is further reduced (below 1.5%), the rate and extent of hydrolysis decrease. In all cases, the laboratory-delignified pulps were easier to hydrolyze than the original substrate, and based on these positive results, the effectiveness of oxygen delignification was evaluated using a waste lignocellulosic material. Rejects from the post digestion screens in a sulfite mill (22 - 26% lignin) were chosen for this purpose. A portion of the rejects was mechanically refined and both refined and unrefined samples were subjected to oxygen delignification under a standard set of conditions. The particle size appeared to have an effect on the kappa number after the oxygen delignification since a 20% and 25% decrease was recorded for the unrefined and refined samples respectively. The final yield of sugars increased by 21% when the samples were only refined (not oxygen delignified), and increased by 19% and 58% for the unrefined and refined oxygen-delignified samples respectively, further evidence of the importance of particle size to enzymatic hydrolysis.

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