| Microalgae are a very promising biofuel feedstock that can potentially address the challenges of energy security, global warming, and environmental protection due to their fast growth rates, high lipid contents, and CO 2 biofixation capabilities. However, most microalgal cells are difficult to disrupt owing to their strong cell-wall structures. This research was focused on the use of ultrasound techniques to enable high-efficiency algal cell disruption for lipid extraction. The experiment investigations were conducted on the effects of ultrasound devices with different operating parameters on microalgal cell disruption and lipid extraction represented by the change of algal cell debris concentration, chlorophyll a fluorescence density (CAFD), Nile red stained lipid fluorescence density (NRSLFD) or cell sizes. Two algal species including Scenedesmus dimorphus and Nannochloropsis oculata were evaluated. The first objective was to understand the effect of operating parameters, including ultrasound amplitude, spraying pressure, nozzle orifice diameter, and initial cell concentration on microalgal cell disruption and lipid extraction in an ultrasonic nozzle spraying system (UNSS, 20 kHz). Optimal cell disruption was achieved at higher ultrasound amplitude, and moderate spraying pressure or nozzle orifice diameter depending on the algal strains and specific settings. Increasing initial algal cell concentration significantly reduced cell disruption efficiency. The second objective was to understand the effect of operating conditions of a continuous high-power ultrasonic flow system (UFS) on cell disruption of two algal strains. Increasing ultrasound intensity improved cell disruption efficiency of test algal strains owing to more severe cell disruption or de-clumping. Increasing sonicationprocessing time increased cell disruption. Cell recirculation was also found beneficial to cell disruption probably due to more uniform distribution of acoustic energy. The third objective was to evaluate the effectiveness of high-frequency focused ultrasound (HFFU, 3.2-MHz, 40- W) in microalgal cell disruption, when compared with a 100-W, low-frequency (20 kHz) non-focused ultrasound (LFNFU). It was found that HFFU was more effective and energy efficient in the disruption of microalgal cells than LFNFU. The combination of high and low frequency treatments was even more effective than single frequency treatment at the same processing time. (Abstract shortened by ProQuest.). |
