Most current research on oil extraction is focused on microalgae to produce biodiesel from algal oil. The biodiesel from algal oil in itself is not significantly different from biodiesel produced from vegetable oils.

Dilution, microemulsification, pyrolysis, and transesterification are the four tech­niques applied to solve the problems encountered with high fuel viscosity. Of the four techniques, transesterification of oil into its corresponding fatty ester (biodiesel) is the most promising solution to the high viscosity problem. This is accomplished by mixing methanol with sodium hydroxide to make sodium methox — ide. This liquid is then mixed into vegetable oil. The entire mixture then settles and glycerin is left on the bottom while methyl esters, or biodiesel, is left on top. Biodiesel can be washed with soap and glycerin using a centrifuge and then filtered. Kinematic viscosities of the fatty acid methyl esters vary from 3.23 to 5.61 mm2/s (Knothe 2005). Methanol is preferred for transesterification because it is less expen­sive than ethanol (Graboski and McCormick 1998).

For production of biodiesel, a macroalga (Cladophora fracta) sample and a mi­croalga (Chlorella protothecoides) sample were used in one study (Demirbas 2009b). Proximate analysis data and higher heating values of algae samples are given in Ta­ble 6.1. As seen in Table 6.2, the higher heating value of Chlorella protothecoides (25.1 MJ/kg) is also higher than that of Cladophora fracta (21.1MJ/kg). Moisture content was determined by drying a 3- to 5-g sample at 378 K to constant weight (Demirbas 1999), ashing was carried out at 1,025 K for 2h (Demirbas 2001), and protein content was determined by the block digestion method and ether-extractable intramuscular fat content by solvent extraction (Boccard et al. 1981). Table 6.3 shows the average chemical composition of algae samples. The oil proportion from the lipid fractions of Chlorella protothecoides is considerable higher than that of Cladophora

fracta (Demirbas 2009b). Figure 6.2 shows the production of biodiesel from algae.

Oils were obtained by extracting algae with hexane in a Soxhlet extractor for 18 h. Transesteriflcation of algal oils was performed in a 100-mL cylinder using su­percritical methanol according to earlier methods (Kusdiana and Saka 2001; Demir — bas 2002). The fatty acids of the algal oils were fractionated into saturated, mo-

Figure 6.2 Production of biodiesel from algae

nounsaturated, polyunsaturated, and free forms by a preparative chromatographic thin layer on a glass plate coating with a 0.25-pm polyethanol succinate.

The fatty acid compositions of algal oils are given in Table 6.4. Fatty acids come in two varieties: saturated and unsaturated. Saturated fats come from animal prod­ucts such as meat and dairy. Most vegetable oils are unsaturated. The properties of the various individual fatty esters that comprise biodiesel determine the overall fuel properties of the biodiesel fuel. As seen in Table 6.4, the average polyunsat­urated fatty acids of Chlorella protothecoides (62.8%) are also higher than those of Cladophora fracta (50.9%). Algae generally produce a lot of polyunsaturates, which may present a stability problem since higher levels of polyunsaturated fatty acids tend to decrease the stability of biodiesel. However, polyunsaturates also have much lower melting points than monounsaturates or saturates; thus algal biodiesel should have much better cold weather properties than many other bio-oils (Demir — bas 2009b).

Xu et al. (2006) used Chlorella protothecoides (a microalga) for the production of biodiesel. Cells were harvested by centrifugation, washed with distilled water, and then freeze dried. The main chemical components of heterotrophic C. protothe — coides were measured as in a previous study (Miao et al. 2004). Microalgal oil was prepared by pulverization of heterotrophic cell powder in a mortar and extraction with n — hexane.

Biodiesel was obtained from heterotrophic microalgal oil by acidic transester — iflcation. Figure 6.3 shows the process flow schematic for biodiesel production (Xu et al. 2006). The optimum process combination was 100% catalyst quantity (based on oil weight) with 56:1 molar ratio of methanol to oil at a temperature of 303 K, which reduced product-specific gravity from an initial value of 0.912 to a fi­nal value of 0.864 in about 4 h of reaction time (Xu et al. 2006).

The technique of metabolic control through heterotrophic growth of C. protothe­coides was applied, and the heterotrophic C. protothecoides contained a crude lipid content of 55.2%. To increase the biomass and reduce the cost of algae, corn powder hydrolysate instead of glucose was used as an organic carbon source in heterotrophic culture medium in fermenters. The result showed that cell density significantly in­creased under heterotrophic conditions, and the highest cell concentration reached 15.5 g/L. A large amount of microalgal oil was efficiently extracted from the het­erotrophic cells using n-hexane and then transmuted into biodiesel by acidic trans­esterification (Xu et al. 2006).