In-situ observation and quantification of microalgae downstream processing on a microfluidic platform
Keywords: Astaxanthin
Bioenergy
Hydrothermal liquefaction
Microalgae
Microfluidics
Sustainable energy

Producing biofuels and bioproducts from microalgae is a promising path for low-carbon energy and products. Microalgal biomass is an attractive feedstock for the generation of carbon neutral biofuels and high-value bioproducts because of the high growth rate and lipid content of many microalgae species. Understanding the downstream processing of converting microalgal biomass to valuable products is a critical step in the biofuel industry. In this thesis, a novel microfluidic platform capable of precise control of processing parameters and providing optical access to reactions at high temperature and pressure was developed and applied to observe and quantify the biomass-to-bioproducts conversions in three distinct studies. First, for bioenergy application, hydrothermal liquefaction of microalgae was performed on this microfluidic platform monitored using fluorescence microscopy. A strong shift in the fluorescence signature from the algal slurry at 675 nm (chlorophyll peak) to a post-HTL stream at 510 nm is observed for reaction temperatures at 260°C, 280°C, 300°C and 320°C (P = 12 MPa), and occurs over a timescale on the order of 10 min. Biocrude formation and separation from the aqueous phase into immiscible droplets is directly observed and occurs over the same timescale. Second, many algal bioproduct efforts currently focus on high-value products such as astaxanthin due to the much-improved economics over producing fuels. Hydrothermal disruption of the cell wall for astaxanthin extraction from wet biomass using high temperature and pressure was demonstrated and studied using this microfluidic platform. Hydrothermal disruption at a temperature of 200 °C was shown to be highly effective, resulting in near-complete astaxanthin extraction from wet biomass - a significant improvement over traditional methods. Third, supercritical CO2 has relatively low critical temperature and pressure (31.1 °C and 7.4 MPa) is considered a greener solvent for bioactive compounds extraction. Supercritical CO2 extractions of astaxanthin with and without co-solvents (ethanol and olive oil) were performed on the microfluidic platform to study the extraction mechanism in each case. Astaxanthin extraction using ScCO2 achieved 92% recovery at 55 °C and 8 MPa applied over 15 hours. With the addition of co-solvents, ethanol and olive oil, the timescales of extraction process are reduced significantly from 15 hours to a few minutes, representing the fastest complete astaxanthin extraction at such low pressures. The direct observation of these complex reaction processes was made possible for the first time here, allowing visual characterization, fluorescence spectroscopy, and quantitative imaging of the conversion at the single-cell scale during all stages. This level of insight has simply not been possible with previous conventional reactors. Although batch reactors have advantages in, for instance, quantifying yields requiring large volumes of products, microfluidic reactors have advantages with respect to process control and visualization at cellular level - providing high resolution, real-time data on complex reactions. The innovative platform and results presented in this thesis provide new insight in the challenging area of biomass-to-bioproduct conversion, and provide insight that can inform larger scale operations.