Liquid CO2 and CO2-Expanded Methanol for Lipid Extraction From Microalgae
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Interest in developing biofuels, which are biodegradable, clean burning and renewable energy sources alternative to petroleum-based fuels, has increased in recent years. Microalgae are considered to be the most promising alternative energy resources and have potential to replace non-renewable fossil fuels. However, the economical extraction of lipids from microalgae for the production of biofuel is limited in part by the energy costs related to solvent use, as well as the potentially harmful effects of traditional solvents. This study investigated the use of CO2-expanded methanol and liquid CO2 as greener alternatives to traditional solvents to extract lipids from dried or wet biomass of microalgae. Unlike extractions using conventional solvents, these new methods require little to no volatile, flammable, or chlorinated organic solvents. CO2-expanded methanol (cxMeOH) consists of pressurized gaseous CO2 dissolved in methanol. When compressed CO2 dissolves in methanol, the solvent expands in volume and decreases in polarity, reducing the amount of organic solvent needed and using much lower pressures than those required for supercritical CO2. The decreased polarity of methanol in cxMeOH increased the selectivity of the solvent towards the extraction of biodiesel-desirable neutral lipids (NLs) and free fatty acids (FFAs). The selectivity was up to 82% for those desired lipids when cxMeOH was used, compared to about 50% when a conventional solvent was used. Moreover, the use of cxMeOH reduces the volume of organic solvent needed for the extraction by up to five times. The use of liquid CO2 (lCO2) presents many advantages of sustainability, safety and selectivity over the use of conventional solvents because this technique requires no flammable, highly volatile, or chlorinated solvents. The lCO2 with its low polarity has high affinity for non-polar lipids. The lCO2 was used to extract lipids from dried as well as the wet biomass of microalgae. Using lCO2, it is possible to extract up to 10% neutral lipids relative to the mass of dry algae. Also, the lCO2 extracts were up to 96% biodiesel desirable lipids with high content of oleic acid compared to only about 50% desirable lipids in conventional solvent extracts.