Utilization of the Lipid Fraction Biodiesel

Biodiesel has an interesting characteristic its fuel properties depend on the feedstock (Knothe, 2005). Unlike ethanol and hydrogen, biodiesel is not a pure substance but is a mixture of alkyl esters that result from the transesterification reactions between the triglycerides and an alkyl alcohol. The transesterification reactions proceed via the following general reaction. 

The formation of soap consumes catalyst and may cause problans downstream during the separation of the biodiesel from the glycerin phase. If the oil contains free fatty acids with concentrations above 1%, then the oil is usually first reacted with methanol and an acid catalyst to esterify the free fatty acids and form fatty acid methyl esters. The possibility that the saponification reaction may occur also necessitates that the oil be dry, thus requiring the dewatering steps described earlier in this chapter. Oil drying is another significant contributor to the energy consumption of the overall process. 

If the oil is of acceptable quality, the oil may be taken directly to the transesterification reactor where it is contacted with an excess of the chosen alcohol (usually methanol) and a base catalyst (usually NaOH or KOH). The transesterification reaction is conducted at about 60°C, which is a constraint set by the boiling point of methanol at atmospheric pressure. The oil is immiscible with the shorter-chain alcohols therefore, maintaining intimate contact between the two phases by providing adequate mixing is an important design and operating criterion for the reactor. It is also an important energy input that must be accounted for. 

Polyunsaturated, Other four or more double bonds 

Reproduced from Knothe (2011) with permission from the Royal Society of Chemistry. 

---