Biofuels in Perspective

156 Biofuels
Raw oil
(pressing, extraction)
Chemical refining Physical refining
Degumming Phospholipids
Gums
Neutralisation
Soaps
Degumming
Bleaching
Fatty acids
Soaps,
color pigments, metals
Bleaching
Deodorisation Fatty acids, Deodorisation
volatiles, artifacts
Figure 9.1 Chemical and physical refining of vegetable oils.
Refined oils have been the subject of an intensive additional pre-treatment in order to
remove unwanted components. Crude lipids can be refined via two variations (Figure 9.1).
The first process is chemical refining which involves a degumming step using water, phosphoric
acid or citric acid in order to remove phospholipids and other gums. Neutralization
with NaOH and KOH is transforming the free fatty acids into a soapstock which is removed
by centrifugation followed by a bleaching process using an adsorbent (in most cases silicates)
in order to remove high molecular colored materials (e.g. carotenoids, chlorophyll),
artifacts (e.g. pesticide residue’s, PCBs), traces of soaps, and metals (Fe, Cu). The last step
involves a deodorization by heating the oil at 230–240 ◦ C for 30–40 minutes in the presence
of 1–2 % steam at 1–2 mm vacuum. During this step volatile compounds originating
from the oxidation of the fatty acids (low molecular aldehydes, ketones, alcohols, acids)
are removed as a deodorizer distillate next to the remaining acids and eventual contaminants
(pesticide residues, PCB’s, dioxins etc.). For some oils winterization (crystallization
of high melting products such as waxes) is necessary in order to avoid the formation of
deposits.
The second process, physical refining, does not include the neutralization of the FFA by
base, in order to avoid the production of soapstock and the additional environmental cost
for water purification. The degumming and bleaching steps are identical as in the chemical
refining but in the deodorization step the free fatty acids are stripped off next to the other
contaminants at a temperature of 240–260 ◦ C for 40–60 minutes.
Vegetable oils and animal fats which have been refined using this sequence of treatments,
can easily be converted in a two-step or continuous transesterification resulting in a biodiesel
which fulfills the EU and US standards on condition that the suitable technology has been
used. O’Brien et al. (2000) wrote an extensive overview of the refining of vegetable oils.
It will be advisable to use a similar technology for the treatment of waste oil as an initial
step before transesterification, but this additional cost will make the use of waste oil less
attractive. In addition, used oils contain more contaminants due to side reactions which
occurred during the use of these oils.