Transesterification turns vegetable oil or animal fat into biodiesel by replacing the glycerol backbone of each fat molecule with three methanol-derived groups, using a little lye (sodium or potassium hydroxide) as the catalyst; glycerin is the byproduct. It works with dry, low-acid oil and fails with water or free fatty acids, which turn the lye into soap, and it involves methanol and lye, both seriously hazardous.
Part of Biodiesel Production
Transesterification is the reaction of a fat or oil with an alcohol and a catalyst that produces fatty acid esters (biodiesel) and glycerol.
Why Oil Needs Changing Before It Is Diesel
Vegetable oil and animal fat are triglycerides: one glycerol molecule carrying three long fatty-acid chains. They are far too thick (viscous) to spray and burn well in a diesel engine. Transesterification detaches the three chains from the glycerol and caps each with a methyl group from methanol, producing three smaller, thinner molecules of fatty acid methyl ester (FAME), which is biodiesel, and one of glycerol, which sinks out as glycerin.
The reaction runs both ways, so the parent uses methanol in excess to push it towards biodiesel: 200 ml of methanol per litre of oil. The exact balance is worked on Triglyceride + Methanol → FAME + Glycerin.
The Three Inputs And What Each Does
- Oil or fat: the feedstock (Feedstock Sources). It must be dry and low in free fatty acid.
- Methanol: the alcohol that supplies the methyl groups. Toxic, flammable, absorbed through skin.
- Lye catalyst: sodium hydroxide (3.5 g per litre of clean oil) or potassium hydroxide (Lye Catalyst). It is dissolved in the methanol first, making methoxide, and speeds the reaction without being used up, except where it is lost to free fatty acids (Free Fatty Acid Problem) or water.
Outputs: about one litre of biodiesel from a litre of oil, and roughly 1-1.5 litres of glycerin from every 10 litres of biodiesel (parent), the glycerin layer also carrying much of the leftover methanol and lye.
Why Water And Acid Ruin A Batch
Lye reacts faster with free fatty acid or with water-driven soap formation than it catalyses the main reaction. The batch then does not separate cleanly, or sets as a soapy gel. That is why the oil is dried and titrated before it is used (Oil Preparation & Titration).
Methanol, lye and methoxide
Methanol is poisonous, absorbed through intact skin, vaporises readily and blinds permanently in small amounts; lye burns skin and eyes; dissolved together as methoxide they are caustic, toxic and flammable. Work outdoors or in strong ventilation, with eye protection and gloves, never near a flame, and never measure methanol with bare hands (Methanol Hazards, Protective Gear).
What’s Next
- Learn next: Production Process, the whole batch step by step
- Combine with: Saponification, the soap reaction that competes with this one