In situ transesterification, better known as reactive extraction, has been developed with the purpose of simplifying the biodiesel production process by allowing extraction and transesterification to occur in a single step, in which oil-bearing seeds or algal biomass are in direct contact with the chemical solvent in the presence of a catalyst (acid or base). Through intensive research in recent years, the optimum conditions for in situ transesterification have become well established for different edible and nonedible oil feedstock, such as jatropha (Shuit et al., 2010), soybeans (Haas and Scott, 2007), and castor (Hincapie et al., 2011). However, the main constraint in commercializing this technology is the requirement of a high volume of chemical solvent, and the process is limited to homogeneous catalyst usage only.

In situ transesterification of algal biomass has been explored to attain high biodiesel con­version, including optimization of the alcohol-to-lipid molar ratio, reaction temperature, catalyst loading, and the effect of the use of a cosolvent, microwave, and ultrasonication. In a study performed by Ehimen et al. (2010), dried Chlorella biomass was subjected to in situ transesterification, attaining 90% of biodiesel yield at a reaction temperature of 60 °C, a methanol-to-lipid molar ratio of 315:1, a H2SO4 concentration of 0.04 mol, and a reaction time of 4 h.

To further reduce methanol consumption for the in situ transesterification, adding a cosolvent to the reaction mixture is suggested to increase the solubility of the algal lipids in methanol, creating a single phase reaction that could subsequently improve the reaction mass transfer rate. A yield of approximately 95% Chlorella pyrenoidosa biodiesel was attained when hexane was used as a cosolvent (hexane-to-lipid molar ratio of 76:1). The methanol-to — lipid molar ratio was significantly reduced to 165:1 and the total reaction time was shortened to 2 h at a reaction temperature of 90 °C and a catalyst loading of 0.5 M H2SO4. Nevertheless, the presence of water in the reaction media could impede the in situ transesterification and cause negligible biodiesel conversion (Ehimen et al., 2010). Thus, extensive drying of algal biomass is absolutely necessary to facilitate biodiesel conversion by avoiding the occurrence of any side reactions and to simplify the subsequent refining processes (Lam and Lee, 2012).

Other technologies that could further improve the reaction conditions for in situ transes­terification of algal biomass are microwave irradiation (Patil et al., 2011a; Patil et al., 2012), ultrasonication (Koberg et al., 2011), and supercritical alcohol (Levine et al., 2010; Patil et al., 2011b). However, these technologies are still far from commercialization due to safety — and health-related problems.