Bio-diesel is commonly produced by the transesterification of the vegetable oil or animal fat feedstock. There are several methods for carrying out this transesterification reaction including the common batch process, supercritical processes, ultrasonic methods, and even microwave methods.
Chemically, transesterified bio-diesel comprises a mix of mono-alkyl esters of long chain fatty acids. The most common form uses methanol (converted to sodium methoxide) to produce methyl esters as it is the cheapest alcohol available, though ethanol can be used to produce an ethyl ester bio-diesel and higher alcohols such as isopropanol and butanol have also been used. Using alcohols of higher molecular weights improves the cold flow properties of the resulting ester, at the cost of a less efficient transesterification reaction.
A lipid transesterification production process is used to convert the base oil to the desired esters. Any Free fatty acids (FFAs) in the base oil are either converted to soap and removed from the process, or they are esterified (yielding more bio-diesel) using an acidic catalyst. After this processing, unlike straight vegetable oil, bio-diesel has combustion properties very similar to those of petroleum diesel, and can replace it in most current uses.
A by-product of the transesterification process is the production of glycerol. For every 1 tonne of bio-diesel that is manufactured, 100 kg of glycerol are produced.
Usually this crude glycerol has to be purified, typically by performing vacuum distillation. This is rather energy intensive. The refined glycerol (98%+ purity) can then be utilised directly, or converted into other products.
There are number of bio-diesel producers from as many raw materials in India. However, due to falling diesel prices and higher cost of production of good quality bio-diesel the conversion from petrodiesel to bio-diesel is slow.