Glycerolysis of free fatty acids: A review

Lesser-polluting and carbon-efficient systems are the new standards of energy, globally. Biodiesel is an excellent bioenergy source due to its industrial feasibility and engine adaptability. However, biodiesel resource inventory still lacks feedstock flexibility on a commercial scale and mostly relies on vegetable oils for production. This creates a problem regarding allotment of resources for biodiesel production, at the cost of the food supply. An alternative is to consider the low-grade non-edible sources of oil, which contain high free fatty acids such as the crude vegetable oil refining rejects and waste cooking oil. To make it worst, the surplus crude glycerol glut is caused by the increased biodiesel production. Crude glycerol is refined for numerous other purposes such as for chemical synthesis. However, re-routing the crude glycerol in-situ for the pre-treatment of low-grade feedstock is suggested as one of the ways in increasing biodiesel profitability and sustainability. This review is focused on the use of glycerolysis reaction as a method of reducing free fatty acids content. A broad spectrum of different catalysts such as enzymatic, homogeneous and heterogeneous, as well as hybrid systems, was reviewed and discussed. Based on the findings, suggestions regarding the development of the glycerolysis reaction were made. In comparison to the homogeneous catalyst, more yield at the increased residence time is obtained for the heterogeneous catalyst. However, leaching, thermal degradation and swelling are the known problems of the heterogeneous catalysts, while product separation is a challenge with homogeneous catalysts. Development of homogenous-based catalysts is suggested as a focus for future studies in glycerolysis reactions.