Course: BFM 202 Biodiesel Production

Review Lecture

Glycerol
Credits
BFM 202 Biodiesel Production Unit 11 Lecture 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.

Unit 11 Glycerol

Glycerol is a backbone chemical structure and the foundation of mono-,
di-, and triglycerides with free fatty acids (FFA) that are attached as
chains of carbon and hydrogen atoms with a carboxylic acid group at one end.

The glycerol by-product is only about 50% glycerol and also contains
methanol, residual catalyst, and soap. With this makeup, it's disposal
must be treated as hazardous waste. In glycerol refinement, acid may be
used to split the soaps into salts and FFA which rise to the top and can
be removed and recycled. Some salts may precipitate out. Potassium
hydroxide can be used as a catalyst with phosphoric acid to neutralize
with a final product of potassium phosphate used in fertilizers.

The glycerol should now be at about an 85% concentration and ready to be
sold for glycerol refining. Here the purity is improved to 99.5-99.7%
via a vacuum distillation or ion exchange process.

Methanol collected from the biodiesel and glycerine streams holds water
and must be treated through a distillation column, then returned to the
process. If ethanol or isopropanol were used as a catalyst, a molecular
sleeve is used to remove water.

Used cooking oil contains 2-7% FFAs. Animal fat contains 5-30% FFA. Trap
grease can have up to 100% FFA. If an alkali (base) catalyst is added,
the free fatty acids form soap and water.

Alkali catalysts can be used with FFAs up to 5% but additional catalyst
is required to offset that which transforms to soap. The soap is removed
with the glycerol or water wash. When FFA levels are above 5%, soap will
inhibit the separation of the methyl esters and glycerol while
increasing the emulsion formation during the wash. An acid catalyst such
as sulfuric acid would be added to convert the FFA to methyl esters.

The addition of sulfuric acid can also be done as a pretreatment to
convert FFAs to methyl esters.

Pretreatments are necessary for palm oil which can contain more than 50% FFA

It has also been suggested that acid-catalyzed esterification can be
used to convert low-grade by-products such as soapstock to biodiesel.
This process uses a large amount of alcohol which must be recovered and
adds costs.

HYSYS is a commercial software package that simulates biodiesel
production and provides data as to the cost of variations in feedstock
and processes.

ASTM D 6751-02 is the standard that specifies the acceptable total
glycerol that may remain in fuel.

Total glycerol is the sum of the free and bound glycerol.

Bound glycerol is equal to the glycerol portion of the residual mono-,
di-, and triglycerides.

The amount of residual methanol, catalyst, soap, and glycerol is
controlled by limits on the fuel's flashpoint, ash level, and free
glycerol. When these limits are met, the biodiesel can be used in most
modern engines without modifications while maintaining the engine's
durability and reliability.