Advanced biodiesel production technologies: novel developments

Biodiesel, i.e. a mixture of monoalkyl esters of long chain fatty acids derived from renewable biological sources such as vegetable oils has in recent years emerged as an alternative fuel for transportation sector. The conventional method of producing biodiesel is through homogeneous catalytic transesterification; however, increased production costs associated with downstream purification steps have led to the development of more cost-effective and environmental friendly technologies. These advanced production technologies involve heterogeneous or enzymatic catalysts to produce biodiesel, as well as no catalysts in supercritical conditions. Heterogeneous catalytic systems can ease the separation of biodiesel from the reaction mixture along with the possibility of catalyst recovery, potentially leading to lower production costs; enzymatic catalysts give the same advantages, but transesterification can be carried out in milder conditions and with a wider range of feedstocks. Biodiesel synthesis in supercritical conditions composes another alternative to conventional methods due to higher reaction rates, shorter reaction times, and simpler biodiesel separation steps. Nevertheless, mass transfer limitations caused by diffusion problems between phases represent an hindrance for future establishment of these technologies, calling for the development of novel methods to intensify the process. These process intensification technologies include ultrasound irradiation, microwave heating, use of co-solvents, and membrane reactors. The main focus of this review is to discuss recent advances as regards to biodiesel production technologies, devoting a special attention to the use of novel catalysts, diversified feedstocks, besides an analysis of main operational parameters of transesterification processes.