Biofuels in Perspective

160 Biofuels
In many cases animal fats and palm oil do not contain sufficient amounts of phospholipids
requiring degumming and can be used without further treatment with water and/or acid.
Crude and waste lipids containing high amounts of gums and phosphorous compounds
should be treated by a degumming step involving washing the lipids with a solution of
phosphoric and/or citric acid solution at 80 ◦ C for 30 minutes. If necessary, adsorption on
silica (Tonsil, Magnesol) can further reduce the gum content to acceptable levels in order
to obtain a biodiesel with a P-content of less than 10 ppm.
During the degumming anorganic contaminants can be reduced, especially the concentration
of alkali and earth alkali metals and the sulphated ash.
Application of the bleaching step resulting in removal of metals (Fe, Cu), colored
materials, polar polymers, and soaps lowers the sulfur content (especially a problem in
crude chicken fat).
9.3.2.4 Removal of Free Fatty Acids
It has been generally accepted that for producing biodiesel from crude and waste oils by
transesterification with KOH in methanol, a free acid content up to 3 % does not affect the
process negatively (Ahn et al., 1995). If higher amounts of free fatty acids are present, the
reaction of the alkaline catalyst with the acids gives rise to soap formation which leads to
loss of catalyst and in most cases to lower yields of esters due to the poor separation of the
glycerol layer and the wash water layer in the presence of higher amounts of soap.
Free fatty acids can be removed from the feedstock by a variety of methods. As already
mentioned steam injection results in a decrease of FFA in waste oils from 6.3 % to 4.3 %
(Supple et al., 2002). Free fatty acids can also be removed by chemical neutralization
with KOH or NaOH with formation of soapstock which is separated from the lipids by
centrifugation. These separations are not always easy to perform and are resulting in lower
yields. The soapstock can be used for the production of free fatty acids which in turn can
be converted into biodiesel by acid esterification (see Sections 9.3.2.8 and 9.3.2.9).
An alternative removal of free fatty acids (Figure 9.2) is by extraction with the glycerol
layer which has been separated from the ester layer during the alkali-based transesterification,
since the glycerol layer contains a considerable amount of the alkaline catalyst in
methanol. Mixing of this glycerol layer with the raw material containing free fatty acid
results in neutralization and soap formation and the free fatty acids initially present are
removed with the glycerol layer, which is separated from the oil. The separated oil layer
can be used directly in the transesterification step (Harten, 2006).
On a laboratory scale, waste oils can be purified (reduction from 10.6 % FFA to 0.23 %)
by passing through a column containing 50 % magnesium silicate and 50 % basic aluminium
oxide (Ki-Teak et al., 2002).
Deacidification can be carried out by stripping off the free fatty acids in a simplified
deodorizer at 200–240 ◦ C and 1–4 mbar for 40 minutes in the presence of 1–2 % steam.
During this vacuum evaporation the free fatty acid content can be lowered from 20 % to
0.5 % in animal fat. Simultaneously oxidation products and partly sterols and tocopherols
are removed as well as monoglycerides. In addition colored materials are destructed and
contaminants (pesticide residues, polyaromatic hydrocarbons, PCBs dioxins etc) are discarded.
However, cis-trans isomerization and dimer formation can occur which can give
rise to higher viscosities and higher CP and CFPP (O’Brien et al., 2000).