Cultivation of nannochloropsis sp. for biodiesel production via microwave irradiation direct transesterification using ionic liquid

Abstract

Nannochloropsis sp. has been recognized as a potential source of biomass feedstock for biodiesel due to their abilities to grow rapidly, high oil and lipid content. However major challenges faced are the expensive downstream processes such as dewatering and drying; utilisation of large volumes of solvent; and tedious extraction processes. The novelty of this study is the production of biodiesel from wet microalgae biomass via microwave irradiation direct transesterification (DT) process with ionic liquid as co-solvent system. Nannochloropsis sp. was cultivated aseptically for 9 days at different light intensities (50, 100 and 200 µmolm-2 s-1) and photoperiod cycles (24:0, 18:06 and 12:12 h light:dark) in a 5 L photobioreactor at 23 oC using F/2 and Walne?s medium for maximum growth and lipid content. Microwave irradiation was used in the extraction of lipids from microalgae and also the transesterification and the results were compared with that obtained via conventional waterbath heating. The effect of microwave irradiation on microalgae cell morphology was then identified. The effects of parameters such as different types of solvent and microwave irradiation time on biodiesel yield were also investigated. A DT process was performed taking into account the important factors such as reaction temperature, reaction time, ratio of methanol to algae and ratio of ionic liquid to methanol. Results revealed that the maximum growth and lipid content were obtained under 100 µmolm-2s-1 light intensity and photo period cycles (18:06 h) after 7 days cultivation using Walne's medium; achieving maximum cell concentration of 12.5 x 107 cell.ml-1, which corresponded to the growth rate of 0.453 d-1 and lipid content of 38.31 %. Microwave accelerates the disruption of microalgae cell walls resulting in rapid release of oil. The use of IL2 as co-solvents was found to be most effective in disrupting microalgae cell. It was seen that the cell disruption achieved a maximum of 99.73 % after 15 min of DT process. The mixture of methanol-IL2 extracted the lipid from microalgal biomass, concurrently transesterified the extracted lipids to produce biodiesel under microwave irradiation. The maximum (36.79 %) biodiesel yield was obtained in 15 min when IL2 was used as co-solvent. The biodiesel production via DT under microwave irradiation was optimized using Central Composite Design (CCD). The main effects considered were reaction time (5 – 25 min); wet algae to methanol ratio (1:4 – 1:12) and methanol:IL2 ratio (1:0.5 - 1:1); and the response variables measured was biodiesel yield. Statistical analysis revealed optimum biodiesel yield (40.9 %) was achieved at 15 min of reaction time, algae:methanol ratio of 1:4 and methanol:IL2 ratio of 1:0.5 (v/v). The major composition of fatty acid methyl esters (FAMEs) were oleic acid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:1n9c) and linoleic acid (C18:2n6c). In conclusion DT process using combination of methanol-IL2 coupled with microwave irradiation is a very attractive novel method to produce maximum biodiesel yield.