Microalgae Lysis through Hydrodynamic Cavitation

Hydrodynamic cavitation occurs when vapor bubbles form and subsequently collapse due to localized pressure drops in a flowing fluid. This phenomenon holds significant potential for applications such as microalgae cell disruption, as the collapse of bubbles releases high-energy shockwaves capable of rupturing robust cell walls. The primary objective of this thesis was to investigate whether hydrodynamic cavitation could be effectively exploited for lysing Galdieria sulphuraria microalgae. The research focused on the hydrodynamic characterization necessary to optimize microalgae cell disruption. Experiments were conducted using a custom-designed stainless-steel hydraulic circuit, incorporating interchangeable orifice plates with diameters of 2 mm, 3 mm, 4.5 mm, and 6 mm. Systematic characterization involved measuring upstream and downstream pressures, flow rates, and utilizing high-speed imaging techniques to qualitatively evaluate bubble dynamics. This comprehensive approach enabled the analysis of how changes in orifice dimensions influenced cavitation number and bubble characteristics, which directly impact the effectiveness of microalgae cell disruption. Initial manual cavitation tests were conducted using a sealed glass container to visually observe bubble formation and collapse across varying algae-water mixtures. High-speed imaging was employed throughout to perform qualitative analyses of bubble sizes and cavitation intensities under different experimental conditions. Experimental findings revealed that the 2 mm orifice diameter generated lower cavitation numbers due to higher fluid velocities and greater pressure differentials. Despite these intensified cavitation conditions, the resulting vapor bubbles were smaller, providing insufficient energy upon collapse to effectively disrupt microalgae cells. Conversely, larger orifice diameters produced significantly larger vapor bubbles with greater collapse energy. However, these larger orifices were unable to achieve sufficiently low cavitation numbers due to limited fluid velocities. Thus, no effective cell disruption occurred at the smallest orifice diameter of 2 mm despite its intense cavitation regime. Based on these results, it is concluded that further advancements are necessary in reactor design to simultaneously achieve low cavitation numbers and large bubble implosions. Accordingly, an improved cavitation reactor configuration, coupled with a newly proposed pumping system designed to achieve higher cavitation intensities, is suggested for future work.