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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.

Feedstock

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

Transesterification process

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. 

Reaction equation of the transesterification process
Source: NREL 2004

 

Products

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.

FNR 2012. * Median values are used for simplification. Please refer to the standards for ranges. **Directive 2009/28/EC, total for cultivation, processing, transport and distribution
Comparison of fuel properties of FAME and conventional diesel
FNR 2012. * Median values are used for simplification. Please refer to the standards for ranges. **Directive 2009/28/EC, total for cultivation, processing, transport and Distribution

 

Relevant standards/regulations defining the specification of biodiesel are EN14214, ASTM D 6751 and EN590.

 

Source:

Subcontractor Report – Biodiesel Production Technology, Etitor; NREL - National Renewable Energy Laboratory, July 2004 (NREL/SR-510-36244).

  • Transesterification to biodiesel
  • Hydrotreatment to HVO
  • Ethanol fermentation
  • Anaerobic digestion to biogas
  • Small-scale combustion
  • Large-scale combustion
  • Biomass co-firing

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