|Institution:||San Diego State University|
|Full text PDF:||http://hdl.handle.net/10211.3/138263|
Algal biofuels have gained increased attention over the past decade due to its potential for substituting fossil fuels and sequestering carbon dioxide in the atmosphere. One of the major obstacles for producing biofuels from microalgae is extracting intracellular lipids, which requires penetration of solvents into the cell wall and membrane. Lipid extraction and the algae concentration processes combined account for the majority of the energy input required to make algal biofuels. Improvements in both of these steps are necessary for making algal biofuels production a net-positive energy process. The goal of this study was to improve the energy efficiency of lipid extraction from microalgae by either decreasing the amount of drying necessary for lipid extraction, or increasing the amount of lipids extracted via pretreatment methods. To achieve the goal, the following objectives were completed: (1) the effects of biomass concentration on solvent extraction yields with chloroform and n-hexane was investigated, (2) the efficiencies of chloroform and n-hexane as an extracting solvent were examined, and (3) the impact of pretreatment of microalgae with ultrasonication, microwaves, and electroporation on extraction yields was investigated. The microalgae Chlorella vulgaris (C. vulgaris) was grown in the laboratory in batch bioreactors. The microalgae was concentrated to different biomass concentrations and the lipids were extracted using two solvent systems: chloroform/methanol/water and n-hexane/ methanol/water. For the chloroform/methanol/water solvent system, the highest total lipid yield of 0.248 g per g of dry C. vulgaris was achieved at algal biomass concentration of about 15% on weight basis. On the other hand, the total lipid yield of 0.139 g per g of dry C. vulgaris was obtained at about 24% algal biomass concentration for the n-hexane/ methanol/water solvent system. Extraction of lipids with n-hexane was 76% of the yield of the extraction with chloroform. Electroporation, ultrasonication, and microwaves were studied for their potential pretreatment methods to increase lipid extraction from C. vulgaris. The yield for lipid extraction increased from 0.246 to 0.311 g per dry g of C. vulgaris when the cells were pretreated with ultrasonication, which is equivalent to a 26.4% increase. Pretreatment with microwaves resulted in a lipid yield of 0.317 g per dry g of C. vulgaris, which is a 28.9% increase. Electroporation resulted in a lipid yield of 0.259 g per dry g of C. vulgaris, which is a low increase of 5.3%, but electroporation was the most efficient in terms of energy requirements. It was also found that pretreatment of the algae does have the potential to replace polar solvents in lipid extraction for cell disruption, however improvements need to be made in the process in order to gain the same yield as a combined chloroform/methanol extraction.