UBC Theses and Dissertations
Understanding hemicellulose and silica removal from bamboo Yuan, Zhaoyang
In this work, the hydrothermal pretreatment under both acidic and alkaline conditions were conducted to study hemicellulose and silica removal from bamboo. In the first part of this work, evolution of proton concentration was examined during both auto- and dilute-acid hydrolysis of hemicellulose from green bamboo. An approximate mathematical model (toy model) to describe the proton concentration based upon conservation of mass and charge during deacetylation and ash neutralization coupled with a number of competing equilibria, was derived. This model was qualitatively compared to experiments where pH was measured as a function of time, temperature, and initial acid level. The toy model predicts the existence of a steady state proton concentration dictated by equilibrium constants, initial acetyl groups, and initial added acid. At room temperature, it was found that pH remains essentially constant both at low initial pH and autohydrolysis conditions. At elevated temperatures, one case of non-monotonic behaviour in which the pH initially increased, and then decreased at longer times, was found. As silica in bamboo creates processing problems, in the second part of this work, alkaline pretreatment of pure amorphous silica particles, bamboo powder and bamboo chips was carried out to study the underlying mechanism for silica and hemicellulose extraction. Response surface methodology was also used to optimize the treatment conditions that could completely extract silica and partially extract hemicellulose from bamboo chips prior to processing. Alkaline pretreatment resulted in significant improvement in the delignification of treated bamboo chips during subsequent kraft pulping, offering an option to reduce the effective alkali charge or the H-factor. The pre-extracted silica and hemicellulose in the liquor were recovered through a sequential procedure of CO2 and ethanol precipitation. Moreover, the feasibility of adopting alkaline pretreatment to the typical kraft pulping process was tested. Results demonstrated that all silica, and up to 50% of hemicellulose in raw feedstocks, could be extracted without degrading cellulose and lignin. Approximately 96% of extracted silica in the APEL could be recovered as a high purity (>99.8%) silica nanoparticles. These results demonstrated that the proposed modification may benefit kraft pulping and fit well into the proposed biorefinery concept.
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