Reactions: Transesterification and Esterification
The diagram below shows the conversion of plant oil or fat into biodiesel of fatty acid esters. Converting oil or fat into biodiesel involves two processes, regardless of the type of catalyst used: transesterification and esterification. When the feedstock is triglyceride/oil/fat, the process is call a transesterification reaction. When the feedstock is fatty acid, the process is called an esterification reaction. Biodiesel is also commonly referred to as fatty acid methyl ester (FAME) since the most common alcohol reactant is methanol.
In a chemical (alkaline or base) process, free fatty acid (FFA), which is naturally present in plant oil or animal fat, has to be separated from the triglyceride component because it can react with the alkaline catalyst to produce soap. Aside from consuming the catalyst, soap mixed with water, glycerol and oil creates an emulsion, which makes it difficult to separate the product (biodiesel) from the byproduct (soap, glycerin) downstream. Furthermore, in either an alkaline or an acid approach, neutralization of the catalyst will entail the use of water washes, creating therefore wastewater.
If free fatty acid is present in a high amount, two processes -acid and alkaline- are required. The free acid is first converted followed by the oil. Producers can opt to proceed without the acid-catalyzed process but the free acid component would nonetheless have to be separated and thrown out, increasing the feedstock cost relative to product yield.
Biodiesel is used worldwide and has become increasingly popular. Currently, the greatest concern in biodiesel production is feedstock availability and utilization of the crude glycerol component (caused by soap formation from chemical processes). The annual consumption of biodiesel was 33.5 billion liters in 2016. The trend shows a 14% increase in demand by 2020.