Transesterification to biodiesel
The transesterification of vegetable oils, animal fats or waste cooking oils is the process behind conventional biodiesel. In the transesterification process a glyceride reacts with an alcohol (typically methanol or ethanol) in the presence of a catalyst forming fatty acid alkyl esters and an alcohol.
The feedstock for transesterification can be any fatty acids from vegetable or animal origin, or used cooking oils (UCO). Typically used vegetable oils originate from rapeseed, sunflower, soy and oil palms.
Depending on the origin of the oils and fats some pretreatment is necessary before processing.
- In any case water is removed as it causes the triglycerides to hydrolyze during base-catalyzed transesterification, producing soapstock instead of biodiesel
- Virgin oils are refined, but not to food grade level
- In some cases the removal of phospholipids and other plant matter is done by degumming
- Recycled oils as UCO are purged from impurities such as dirt or charred Food
The transesterification process is a reversible reaction and carried out by mixing the reactants – fatty acids, alcohol and catalyst. A strong base or a strong acid can be used as a catalyst. At the industrial scale, mostly sodium or potassium methanolate is used. The end products of the transesterification process are raw biodiesel and raw glycerol. In a further process these raw products undergo a cleaning step. In case of using methanol as alcohol FAME (fatty acid methyl ester) biodiesel is produced. The purified glycerol can be used in the food and cosmetic industries, as well as in the oleochemical industry. The glycerol can also be used as a substrate for anaerobic digestion.
The physical characteristics of fatty acid esters are very close to those of fossil diesel fuels; the high viscosity that pure vegetable oils display is reduced by the transesterification process. The exact properties of the finished biodiesel depend on the raw material. Biodiesel is non-toxic and biodegradable.
Some properties of biodiesel are different from those of fossil diesel and for correct low temperature behaviour and for slowing down oxidation processes biodiesel requires a different set of additives than fossil diesel. Impurities, such as metals, in FAME must be limited for use as a motor fuel.
Relevant standards/regulations defining the specification of biodiesel are EN14214, ASTM D 6751 and EN590.
Subcontractor Report – Biodiesel Production Technology, Etitor; NREL - National Renewable Energy Laboratory, July 2004 (NREL/SR-510-36244).