C.J.C. Derek

Comparison of harvesting methods for microalgae Chlorella sp. and its potential use as a biodiesel feedstock

Three methods for harvesting Chlorella sp. biomass were analysed in this paper – centrifugation, membrane microfiltration and coagulation: there was no significant difference between the total amount of biomass obtained by centrifugation and membrane microfiltration, i.e. 0.1174±0.0308 and 0.1145±0.0268 g, respectively. Almost the same total lipid content was obtained using both methods, i.e. 27.96±0.77 and 26.43±0.67% for centrifugation and microfiltration, respectively. However, harvesting by coagulation resulted in the lowest biomass and lipid content. Similar fatty acid profiles were obtained for all of the harvesting methods, indicating that the main components were palmitic acid (C16:0), oleic acid (C18:1) and linoleic acid (C18:2). However, the amounts of the individual fatty acids were higher for microfiltration than for centrifugation and coagulation; coagulation performed the most poorly in this regard by producing the smallest amount of fatty acids (41.61±6.49 mg/g dw). The harvesting method should also be selected based on the cost benefit and energy requirements. The membrane filtration method offers the advantages of currently decreasing capital costs, a high efficiency and low maintenance and energy requirements and is thus the most efficient method for microalgae harvesting.

Kinetic studies and thermodynamics of oil extraction and transesterification of Chlorella sp. for biodiesel production

In this work, a mixture of chloroform and methanol (1:1, v/v) was applied to oil extraction from Chlorella sp. at 30, 40, 50 and 60°C for 150 min extraction times. Kinetic studies revealed that the values of n and the rate constants were found to depend strongly on temperature. The activation energy was Ea=38.893 kJ/mol, and the activation thermodynamic parameters at 60°C were Δ S≠=−180.190 J/mol K, Δ H≠=36.124 kJ/mol and Δ G≠=96.128 kJ/mol. Both Δ H and Δ S yielded positive values, whereas Δ G was negative at 60°C, indicating that this process is endothermic, irreversible and spontaneous. The acidic transesterification process was also investigated by gas chromatographic analysis of the microalgae fatty acid methyl esters (biodiesel) at different temperatures and reaction times. The fatty acid profile indicated that the main components were palmitic, linoleic and linolenic acids. The concentration of linolenic acid increased and oleic acid decreased as the temperature increased. Two-hour transesterification is the best reaction time for biodiesel production because it produces the highest percentage of unsaturated fatty acids (74%). These results indicate the potential of Chlorella sp. to produce biodiesel of good quality.