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Empirical kinetic modeling of oxygen delignification pretreatment of wheat straw Li, Sam King Ho


With diminishing supplies of oil reserves and surging oil prices, research on renewable and sustainable energy has significantly increased. Biofuels have shown their potential in replacing traditional fossil fuels, such as gasoline. Second generation biofuels that use nonfood lignocellulosic biomass to produce bioethanol have been identified as one of these renewable sources. Oxygen delignification has been identified as an effective pretreatment method for agricultural waste, such as wheat straw, to increase the enzymatic hydrolysis yield. The purpose of this study was to develop a kinetic model for the delignification of wheat straw. An experimental design was planned to enable the development of an empirical model of the reaction kinetics for oxygen delignification of wheat straw. This was accomplished by studying the effects of substrate loading (2-4% w/w), reaction temperature (90-130°C) and caustic loading (5-15% w/w). From the experiments, an empirical model that can predict the lignin content of wheat straw after oxygen delignification pretreatment based on reaction temperature, caustic loading and lignin content was developed: (d[L])/dt= -k_L 〖[L]〗^(a₁) (k'([L]-[L₀])+[〖OH^-₀]〗)^(a₂) Where: kL = kinetic constant of lignin k’ = constant for the relationship between kinetic constant of lignin and hydroxide ion L = concentration of lignin in substrate L0 = concentration of initial lignin substrate OH0- = concentration of hydroxide ion a1 = reaction order for lignin a2 = reaction order for hydroxide ion The pretreated substrate was analyzed and showed increased sugar concentration and sugar yield when subjected to enzymatic hydrolysis at 20 FPU/g glucan. It was also found that caustic loading would become saturated when it was above 10-12% w/w. Out of all the operating parameters, caustic loading had the greatest effect on lignin solubilization, carbohydrate recovery and sugar yield. An economic analysis on the oxygen delignification pretreatment process was performed with Aspen Plus and Aspen Economic Analyzer. Using sugar produced as a basis, it was found that the pretreatment cost was 26.20 ¢/lb sugar. A sensitivity analysis was also performed on the cost of biomass, caustic (NaOH), and enzyme. It was concluded that the cost of enzyme had the most significant effect on the cost of pretreatment.

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