First Principles Airlift Photobioreactor for the Energy Balance of Microalgae for Biofuels Applications
Open Access
- Author:
- Nugent, Lucas Alexander
- Area of Honors:
- Chemical Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Wayne Roger Curtis, Thesis Supervisor
Ali Borhan, Thesis Honors Advisor - Keywords:
- Microalgae
Biofuels
Photobioreactor
Photosynthetic Efficiency
Airlift
Energy Balance
Alginator
Chlorella vulgaris - Abstract:
- For better feedstock selection of microalgae towards commercial biofuels and co-products, direct comparison of microalgae species via photosynthetic efficiency (PE) is needed to maximize solar-to-biomass energy conversion in an economically viable manner. PE is defined herein as the percentage of chemical energy converted into biomass from light energy absorbed in the photosynthetically active radiation (PAR) range. In this work, a novel photobioreactor (PBR), dubbed the ‘Alginator’, was designed and built to enable evaluation of any microalgae’s PE under conditions that enable high-density, light-limited growth. A first principles approach was then taken to validate the Alginator’s suitability for such an analysis using the common biofuels candidate, Chlorella vulgaris. The Alginator was designed as a flat-panel airlift PBR. The flat-panel design was intended to support light limitation and provide straightforward calculation of PE by virtue of PAR flux measurements through a defined light path. The airlift design was intended to support high-density photoautotrophic growth through increased mass transfer and thorough mixing. These design elements were combined at bench-scale to promote ease of characterization and subsequent microalgal selection for commercial use. For the Alginator’s validation, the fluid dynamics of the system were first quantified using a dye tracer method for circulation time, fluid velocity, and flow turbulence. Next, the mass transfer of carbon dioxide into the reactor was characterized using a dynamic pH method to calculate the carbon dioxide mass transfer coefficient. Finally, C. vulgaris was grown in the Alginator towards providing both carbon mass and energy balances. Growth dynamics of C. vulgaris were characterized using optical density (OD) correlations. The combined carbon mass transfer and biomass growth measurements enabled quantification of growth conditions throughout the batch trial. From there, dissolved oxygen (DO) conditions throughout growth were analyzed via the stoichiometrically determined photosynthetic reaction. For the energy balance, spectral analysis characterized absorbed photon energy while the biomass’ total energy outputs were provided by the growth rate and bomb calorimetry measurements of energy density. Finally, the resultant maximum calculated PE was 22% with an average of 8.8% throughout exponential growth illuminated with a PAR flux of 119±3 μE/(m^2 s). By comparison, literature values that calculate growth efficiency using PAR flux give a theoretical maximum PE of 27-29% in microalgae and experimental 9.0% for average PE throughout exponential C. vulgaris growth. Compiled results from literature show large discrepancies in reported PE for the same microalgae species with different PBR types, light conditions, and energy balance assumptions. In addition to Alginator characterizations, this thesis also highlights the need for relevant reactor conditions for scale-up and for consistency in assumptions when evaluating microalgae as a biofuels production platform.