Reactive extraction of microalgae for biodiesel production

Abstract

Fatty acid methyl ester (FAME “biodiesel”) is a renewable transport fuel that can be produced from waste/refined oil, pre-extracted oil from oilseeds or microalgae. The most common method converts extracted oil from biomass to FAME through transesterification using acidified or alkalised methanol. Alternatively, FAME can be made by contacting the oil-bearing biomass directly with an alcohol containing a catalyst. This approach is potentially a cost-effective alternative way of making algal FAME due to its elimination of the solvent extraction step and its higher water tolerance. This study reports reactive extraction of Nannochloropsis occulata and Chlorella vulgaris for FAME production using NaOH, H2SO4, zirconium dodecyl sulphate (“ZDS”) or H2SO4/SDS (a surfactant) as catalysts. It is possible to produce FAME using all of them. A relationship was found between FAME yield, catalyst concentration, methanol to oil molar ratio, moisture content or algal cell wall chemistry. NaOH is the most effective catalyst, producing high FAME yields (96 %) in relatively short reaction times (10 min), at 925:1 methanol to oil molar ratio and 0.5N NaOH. This was achieved despite high levels of free fatty acid (6 % lipid) in Chlorella vulgaris. A numerical model derived by Eze et al. (2014) fitted with experimental data from this study shows that other side reactions including FAME and triglyceride saponification, free fatty acid neutralisation occur alongside the desired FAME synthesis in a NaOH-catalysed reactive extraction. Regardless of the catalysts used, methanol to oil molar ratios in the range 600:1-1277:1 caused 5-30 wt %/(wt dry algae) moisture tolerance: significantly greater than the 0.5 wt % oil moisture required in conventional transesterifications. Both the phosphorus mass balance and conversion of the isolated algal phospholipids into FAME revealed that pre-soaking pre-treatment solubilises the phospholipid bilayer to some degree, and iii [Abstract continued] contributes to an increased FAME yield in Nannochloropsis occulata (98.4 %) and Chlorella vulgaris (93.4 %). Residual protein loss in Chlorella vulgaris and Nannochloropsis occulata were respectively 6.5 and 10 %. The carbohydrate content was significantly reduced by 71 % in Chlorella vulgaris and 65 % in Nannochloropsis occulata.