Over the past decade, there has been a revival in algal research and attempts at large scale cultivation for bioenergy production. Among various types of microalgae culturing systems, Open Raceway Ponds (ORP) are considered as an economic system for large-scale microalgae cultivation. In order to improve the algal growth and productivities in ORPs, it is very important to understand the effects of design parameters and operating conditions on mixing and light distribution patterns. The goal of this dissertation was to develop computational tools and experimental techniques to assess key variables that affect algal growth and productivity, and to improve microalgal cultivation in ORPs. The effects of major parameters on growth, were investigated and the optimum C. vulgaris growth condition was determined at 52 W/m2, 24°C, and pH of 7.4, using Response Surface Methodology. The C. vulgaris grown in swine wastewater with 102 mg/L nitrogen and 76 mg/L phosphorus at the optimum environmental condition achieved the average growth rate of 0.16 g/L/day, compared to 0.19 g/L/day for its growth in the modified Bold's medium with 100 mg/L nitrogen and 53 mg/L phosphorus, at the same condition. Results indicated that at NC weather conditions, C. vulgaris grown in swine wastewater in a pond with 0.3 m medium depth, can reach a biomass and lipid productivity of 80 and 20 tons/hectare/year, respectively, at the harvesting cell density of 0.1 g/L. However, the algal productivity decreased significantly with the increase of harvesting cell density. A specific growth rate model of C. vulgaris was generated as a function of light intensity, temperature and pH. A Computational Fluid Dynamics (CFD) model was developed to simulate the multiphase flow in ORPs to investigate the effects of operational conditions on biomass concentration and light intensity distribution. Operating large scale ORPs at 0.2 m/s inlet velocity resulted in a significant decrease in dead zone areas in comparison with 0.1 m/s. However, further increase in velocity to 0.3 m/s did not make significant changes. CFD models were then integrated with the growth kinetic model to simulate the dynamic growth of C. vulgaris in ORPs. The predicted algal growth and productivity well agreed with those measured values. The predicted average algal productivities for the 3-week cultivation of C. vulgaris in the lab-scale ORPs were 7.34, 7.4, and 7.46 g/m2/day for medium depths of 0.20, 0.25, and 0.30 m, respectively, which well agreed with the measured values of 6.78, 7.23 and 7.39 g/ m2/day for medium depths 0.20, 0.25, and 0.30 m, respectively. Simulations were conducted to study different harvesting methods. The average algal productivity for the 3-week cultivation in the ORP with 0.2 m depth by harvesting 50% algae at the target 0.2 g/L cell density was 10.5 g/m2/day, which was 54.7% higher than 6.78 g/ m2/day for the 3-week cultivation under the same condition without harvesting. The average algal productivity decreased with the increase of harvesting cell density.
|Commitee:||Bikdash, Marwan, KC, Dukka, Shahbazi, Abolghasem, Yuan, Wenqiao|
|School:||North Carolina Agricultural and Technical State University|
|Department:||Computational Science and Engineering|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 78/04(E), Dissertation Abstracts International|
|Subjects:||Alternative Energy, Chemical engineering, Environmental engineering|
|Keywords:||Computational fluid dynamics, Harvesting cell density, Light transfer, Microalgae growh, Multiphase flow, Open raceway ponds|
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