Course: BFM 202 Biodiesel Production

Lecture

Biodiesel Chemistry
Credits
BFM 202 Biodiesel Production Unit 1 Reading has been adapted from "ENGY 131 Unit 1 Commentary: What is biodiesel?" 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.

Biodiesel Chemistry

In layman's terms, diesel fuel and vegetable oil are similar since they
both have carbon and hydrogen atoms bound together in big molecules.

This gives them a high energy density and makes them good fuels. They
are different in that vegetable oil is thicker than diesel fuel. Older Indirect
Diesel Injection engines used mechanical fuel Injection Pumps and could
push this dense fuel into the combustion chambers. As diesel engines
became more fuel efficient, they became less tolerant of high viscosity
diesel fuels.

The difference in viscosity is due to the way the atoms are arranged in the
molecules.  Diesel fuel is one long chain of carbon atoms studded with
hydrogen atoms. Vegetable oil has three of those long chains stuck onto
a glycerin backbone. Biodiesel chemistry is about breaking those long
carbon chains off that glycerin backbone with the result being closer to
diesel fuel than vegetable oil.

The chemical process used is to separate the long chain of carbon atoms
off the glycerin backbone is called transesterification. It just means
removing the long chain of carbon atoms from the glycerin and placing
them onto another alcohol, like methanol .

In order to get the carbon atoms to move from the glycerin to the
methanol, something has to 'make' that happen. A catalyst causes this
chemical reaction from vegetable oil to biodiesel and returns to its
original state at the end of the reaction.

The process to make biodiesel involves dissolving the catalyst, either
NaOH or KOH, into methanol and mixing it with vegetable oil heated to
about 50°C. Most of the reaction takes place quickly, but mixing is
continued in order to gain the maximum conversion from vegetable oil to
biodiesel. Once mixing is complete, glycerin settles to the bottom and
biodiesel separates to the top.

Fats and Oils are Lipids

Depending up their chemical structures, room temperature lipids can be a
solid (fat) or liquid (oil).

Biodiesel is a mixture of mono alkyl esters of long chain free fatty
acids, or FFA.

Fats have a higher viscosity and melting points than oils. Therefore,
biodiesel derived from saturated fats have higher gel and cloud points
than biodiesel made from unsaturated oils. As a result, vegetable oils
with lower FFA levels are favorable for commercial biodiesel fuel.

Yellow and brown grease contain 15% and 33% FFA but are a common
feedstock because of their low cost and availability.

In commercial processes, highly-refined vegetable oils, primarily
consisting of triglycerides are typically used as feedstocks. These oils
are transesterified with low molecular weight alcohols such as methanol
and ethanol, using alkali catalysts such as sodium hydroxide or
potassium hydroxide.

When virgin oils are used, it is estimated that their cost accounts for
80% of the biodiesel production. Economical feedstocks such as waste
fats and oils contain low to moderate amounts of free fatty acids (FFA),
moisture, and impurities. Producing biodiesel from these low quality
oils is challenging due to undesirable side reactions between FFAs and
water. Pretreatment stages are used to reduce acid concentrations and
water content to allowable threshold limits before being processed by
standard biodiesel production methods.