Basics of Biodiesel Production

Biodiesel is an alternative fuel for diesel engines that is gaining attention in the United States
after reaching a considerable level of success in Europe. Its primary advantages are that it is one
of the most renewable fuels currently available and it is also non-toxic and biodegradable. It can
also be used directly in most diesel engines without requiring extensive engine modifications.
The purpose of this book is to describe and explain the processes and issues involved in
producing this new fuel.

The most cursory look at the literature relating to biodiesel will soon reveal the following
relationship for prediction of biodiesel from fats and oils.

100 lbs of oil + 10 lbs of methanol   →  100 lbs of biodiesel + 10 lbs of glycerol  

This equation is a simplified form of the following transesterfication reaction.

where R1, R2, and R3 are long chains of carbons and hydrogen atoms, sometimes called fatty acid
chains. There are five types of chains that are common in soybean oil and animal fats (others are
present in small amounts):

These chains are designated by two numbers separated by a colon. The first number designates
the number of carbon atoms in the chain and the second number designates the number of double
bonds. Note that the number of carbon atoms includes the carbon that is double bonded to the
oxygen atom at one end of the fatty acid (called the carboxylic carbon). This is the end that the
methanol attaches to when methyl esters are produced. Table 1 shows the percentages of each
fatty acid chain present in common oils and fats.

For simplicity, consider an oil such as soybean oil to consist of pure triolein. Triolein is a
triglyceride in which all three fatty acid chains are oleic acid. This is near the actual number of
carbons and hydrogens and gives a molecular weight that is near the value for soybean oil. If
triolein is reacted with methanol, the reaction will be that shown in Figure 2. Note that weights
for each of the compounds in the reaction are given. These are based on the fact that one
molecule of triolein reacts with 3 molecules of methanol to produce 3 molecules of methyl
oleate, the biodiesel product, and one mole of glycerol. Chemists typically multiply all the terms
of this equation by a large number that corresponds to the number of molecules in a quantity
equal to the molecular weight of the substance. This quantity is called a mole of the substance.
To calculate the molecular weight of triolein, we count the number of carbons in the molecule
(57) and multiply

to be continoued..


source by J. Van Gerpen, B. Shanks, and R. Pruszko, D. Clements and G. Knothe on book Biodiesel Production Technology