An overview of transesterification methods for producing biodiesel from waste vegetable oils

Compared to petroleum diesel, biodiesel is biodegradable and non-toxic. The use of edible vegetable oils for biodiesel production competes with the need for foodstuff. The continuously rising price of edible vegetable oils makes them uneconomical as a long-term biodiesel source. Waste vegetable oils are considered environmental pollution, despite being a potential raw material for biodiesel production, as well as cheap and readily available. Transesterification of vegetable oil is carried out to produce biodiesel in the presence of a catalyst. The catalyst can be homogeneous, heterogeneous, nanoparticles, or enzymatic. Homogeneous catalysts are considered more effective compared to their heterogeneous counterparts, because of reduced mass transfer limitations and high conversion. However, the challenges of separation and purification of biodiesel produced from homogeneously acid/base-catalyzed transesterification of vegetable oils has shifted attention to non-catalytic supercritical methanol/ethanol. It has been reported that at a reaction temperature of 350°C, with a methanol/oil molar ratio of 42, and in the short time of 400 s, 95% conversion is achievable under supercritical methanol. This study explores and presents the advantages and disadvantages of the biodiesel production processes in addition to the effect of the process variable, and also shows the key data set not presently available that would enhance commercialization and economics and improve biodiesel production.