Development of a continuous flow ultrasonic harvesting system for microalgae
Date
2014
Authors
Hincapié Gómez, Esteban, author
Marchese, Anthony J., advisor
Willson, Bryan D., committee member
Dasi, Lakshmi Prasad, committee member
Peers, Graham, committee member
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Abstract
Microalgae have vast potential as a sustainable source of biofuel. However, numerous technoeconomic analyses have indicated that microalgae harvesting represents a critical bottleneck in the microalgae value chain in terms of energy requirements, capital cost and operating cost. This dissertation presents an approach that uses a combination of acoustophoretic, fluid mechanical, and gravitational forces toward the development of a continuous flow microalgae harvesting system. Ultrasonic Standing Waves have been widely reported in the literature as an approach to manipulate particles in a fluid, a phenomena known as acoustophoresis. These waves exert an acoustic force that agglomerate the cells in the wave nodes or antinodes and the force is directly proportional to the cell acoustic contrast factor. Ultrasonic microalgae harvesting is a promising low cost and low energy approach. However, a better understanding of the acoustic properties of microalgae is essential for the development of this technology. Accordingly, a major component of this work focused on accurately quantifying the acoustic contrast factor of microalgae cells of Nannochloropsis oculata, Nannochloropsis gaditana, Phaeodactylum tricornutum and Chlamydomonas reinhardtii by measuring the average cell density and speed of sound using a vibrating tube densitometer. The results indicate a linear correlation of density and speed of sound as a function of cell concentration. Using this correlation, non-scattering volume average relationships were used to compute density and speed of sound for the average algal cell. The acoustic contrast factor was estimated to be between 0.04 - 0.06 for microalgae cells in their corresponding growth media. Second, particle tracking velocimetry was used to determine the magnitude of the acoustophoretic force. In these studies, in addition to microalgae cells, polyamide seeding particles were used as a surrogate. The results obtained conclude that the maximum acoustophoretic forces are approximately 5 pN for Chlamydomonas reinhardtii cells and the results also show that there is change in the acoustic contrast factor from positive to negative with lipid accumulation. This dissertation also presents a novel device for the acoustic harvesting of microalgae. The design is based on using the acoustophoretic force, acoustic transparent materials and inclined settling (Boycott effect). A filtration efficiency of 70% ± 5% and a concentration factor of 11.6 ± 2.2 were achieved at a flow rate of 25 mL • min-1 and an energy consumption of 3.6 ± 0.9 kWh • m-3. The effects of the applied power, flow rate, inlet cell concentration and inclination were explored. It was found that the filtration efficiency of the device is proportional to the power applied. However, the filtration efficiency experienced a plateau at a 100 W • L-1 of power density applied. The filtration efficiency also increased with increasing inlet cell concentration and was inversely proportional to the throughput of the device as measured flow rate. It was also found that the optimum settling angle for maximum concentration factor occurred at an angle of 50° ± 5°. At these optimum conditions, the device had higher filtration efficiency in comparison to other similar devices reported in the previous literature.
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Subject
harvesting
dewatering
microalgae
acoustic contrast factor
acoustophoresis