Course: BFM 202 Biodiesel Production

Lecture

Transesterification Process
Credits
BFM 202 Biodiesel Production Unit 3 Lecture has been adapted from "ENGY 131 Unit 3 Transestrification Made Simple" by Southeastern Illinois College.

ENGY 131 Biodiesel Production course content by Southeastern Illinois College is licensed under a Creative Commons Attribution 3.0 Unported License. Except where indicated otherwise.

Transesterification Process

Transesterification is a reversible reaction performed to lower the viscosity of vegetable oils for use in diesel engines. Although there are a number of methods that can be used in this conversion, transesterification is the current best practice. It is referred to as alkali-catalyzed transesterification and operates under mild production conditions with minimal side reactions and reasonable reaction times. The downside is that the original feedstock contains water and free fatty acids (FFA).
Triglyceride Transesterification Reaction Schematic

“Figure 1. Basic Transesterification Process,” image by NREL from Biodiesel Handling and Use Guide Fourth Edition, U.S. Department of Energy

FFA causes soap to form during production and separation, thus inhibiting the reaction. Because of the water and FFA, the oils must be highly refined which adds pretreatment steps requiring time and money. Maximum biodiesel yields are dependent upon quality feedstock that has very low water content and an FFA of less than 0.5%.

Other options that enhance transesterification include adding cosolvents such as tetrahydrofuran (THF) or methyl tert-butyl (MTBE) to accelerate the reaction rates. Both are hazardous chemicals with MTBE  outlawed from use. Ultrasonic irradiation and microwaves can be used to accelerate tranesterification by introducing energy into the chemical process.

Chemically, transesterification involves transforming one ester into a different ester by reacting alcohol with oil to release three "ester chains" from the glycerin backbone of each triglyceride molecule. Methanol and ethanol can be used, however, methanol is less expensive and is the primary alcohol used.

Typical catalysts are sodium hydroxide (caustic soda), sodium methoxide (sodium methylate), or potassium hydroxide (potash).

Biodiesel has one-eighth the viscosity of the original oil, the feedstock, and is an oxygenated vegetable-based fuel with almost 10% oxygen by weight.

Transesterification Process for Biodiesel

Click on this video by StartMakingBiodiesel.com for an excellent visual of the molecular level of the transesterification process.