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Non-destructive solvent extraction of lipids from the freshwater microalgae Botryococcus braunii

Jackson, Brent (2019) Non-destructive solvent extraction of lipids from the freshwater microalgae Botryococcus braunii. PhD thesis, Murdoch University.

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As the human population and expectations of quality of life continue to increase, with it so does the demand for energy. Until recently, much of this energy has been obtained through the combustion of fossil fuels. As these natural carbon sinks are consumed for energy and transportation, carbon which was once sequestered in the earth’s crust is being released into the atmosphere at an unsustainable rate. Current CO2 levels are the highest on record which has resulted in a decrease in ocean pH having devastating effects on marine ecology. Further high CO2 levels combined with elevated levels of other greenhouse gases (i.e. CH4, NO2) has been proven to increase the absorption and reflection of surface radiated infrared light in the atmosphere, resulting in increased global temperatures, generating devastating weather patterns which are only expected to continue to grow in both intensity and frequency. The human race now seeks renewable energy sources which can be carried out sustainably with significantly reduced net carbon emissions. Solar, wind, hydro, thermal and nuclear are all examples of renewable energies which are now gaining increased interest as we try to curb our reliance on fossil fuels. Biofuel is one potential source of energy which if harnessed can help in achieving a sustainable future. Plants are able to harness light from the sun and convert it through photosynthesis into carbon-rich lipids which can then through various technologies be converted into fuel sources and or other oil-based products. Biomass for biofuels can be obtained from four main sources; 1. Food crops such as corn and soy; 2. Forestry waste and other high lipid content terrestrial crops which are grown specifically for fuel production; 3. Algal biomass generated from unaltered species found in nature; and 4. Algal biomass generated from algae with altered genomes for increased carbon capturing capacity, biomass productivity, and lipid accumulation. For the past few decades microalgae have been gaining interest for their high solar radiation conversion efficiencies, high growth rates and the ability to be grown in relatively low-quality, high salinity water in locations which would otherwise be unsuitable for terrestrial plant production.

There are several issues which make the production of fuel from microalgae uneconomical, one of these being high energy requirements in the harvesting and dewatering of biomass. Botryococcus braunii is a green microalga with the unique ability to synthesize and store lipids externally. This species of microalga grows in colonies of cells surrounded by an extracellular matrix (ECM) of hydrocarbons. Not only are these lipids accessible without the loss of cellular integrity, but B. braunii can synthesize and replace extracted hydrocarbons for future extractions. Botryococcus braunii is a relatively slow-growing species of microalgae which makes conventional extraction difficult, the re-use of cells means the culture does not need to replace cellular components, only extracted lipids. This allows for a higher density culture post-extraction for increased lipid productivities. The nutrients associated with cellular growth are also retained which significantly reduces the nitrogen and phosphorus requirements and ensures the sequestered carbon is utilised in extracellular lipid production rather than cellular structure. Furthermore, solvent extraction from B. braunii can be carried out in-situ, significantly reducing the harvesting and dewatering requirements of the process.

The purpose of this study was to determine from the literature which of the four known races of B. braunii (A, B, S and L) is likely to be best suited to a repetitive extraction process, which solvent should be used for the extraction process, and which method of extraction is likely to result in the highest extraction efficiencies while retaining cellular integrity for future extractions. The selected strain was then tested for its tolerance to hydrodynamic shear while exposed to the extracting solvent. The optimal culture/solvent ratio and mixing intensity were then used at different initial culture densities and culture recovery periods to find the optimal sustainable lipid productivity for the strain tested. Using this data, a basic mass balance of the extraction process was carried out and scaled up for an indication of the potential savings in culture nutrient and area requirements compared to conventional solvent extraction found in the literature.

B Race was chosen for this study due to its ability to accumulate high lipid content, of a suitable nature and purity for biofuel production. Initially, heptane was chosen as the extraction solvent for its high extraction efficiency, low cost, low boiling point and biocompatibility with B. braunii. From initial solvent extraction runs it was evident that B. braunii was unaffected by the levels of hydrodynamic shear tested in the absence of solvent. Once heptane was added to the culture a drop in culture quantum yield was observed. After four hours of exposure to shear rates of up to 415 s-1 with solvent present no irreparable damage to B. braunii was observed. At this rate, the culture quantum yield and lipid content were able to recover in three days at an initial density of 1.62 g L-1. This was repeated three times with the culture recovering to a healthy state at the end of each recovery phase, achieving total lipid and botryococcene productivities respectively of 71.3 and 29.9 mg L-1 d-1. The same conditions were tested with dodecane, a solvent with lower toxicity, which proved ineffective with a maximum total lipid productivity of 4.06 mg L-1 d-1. For comparison purposes column extraction, a low shear high solvent contact method of extraction, was then tested with both dodecane and heptane. This proved ineffective with low botryococcene productivities respectively of 6.4 mg L-1 d-1 and 1.1 mg L-1 d-1, which is likely due to a lack of applied shear stress on the culture, an important step in the disruption of colonies and the release of ECM contents. Based on the productivities achieved, high shear solvent extraction with heptane was selected as having the highest potential for scale-up. Using the data obtained from completed repetitive extractions and growth data available in the literature a mass balance around the culture recovery and solvent extraction was carried out and scaled to open raceway pond conditions.

The completed mass balance showed the potential for total lipid and botryococcene productivities respectively of 41.8 and 104.4 mg L-1 d-1, significantly higher than that reported in current literature (< 16 mg L-1 d-1). The uniquely high lipid content of B. braunii (B Race) combined with the retaining of cellular biomass post extraction results in exceptionally low nutrient requirements for the non-destructive extraction process (approximately one third when compared to destructive extraction from Dunaliella spp.). Further, the high lipid productivities of this process result in projected pond areal requirements of less than half of that expected from a conventional 3rd generation biofuel process. As cultivation and extraction technologies continue to advance resulting in increased recovery rates from improved cultivation conditions and more efficiency extraction techniques it is likely the operating and capital costs of this process will in time reduce for more affordable 3rd generation biofuel. This combined with continued increase in fossil fuel prices may one day be enough to tip the scales favouring biofuels for a sustainable future.

Item Type: Thesis (PhD)
Murdoch Affiliation: Engineering and Energy
Supervisor(s): Bahri, Parisa and Moheimani, Navid
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