Biofuels in Perspective

5.5.2 Acid Catalysis
Process Technologies for Biodiesel Production 83
Acid catalysis offers the advantage of also esterifying free fatty acids contained in the fats
and oils and is therefore especially suited for the transesterification of highly acidic fatty
materials.
However, acid-catalyzed transesterifications are usually far slower than alkali-catalyzed
reactions and require higher temperatures and pressures as well as higher amounts of
alcohol. The typical reaction conditions for homogeneous acid-catalyzed methanolysis are
temperatures of up to 100 ◦ C and pressures of up to 5 bars in order keep the alcohol
liquid. 13 A further disadvantage of acid catalysis – probably prompted by the higher
reaction temperatures – is an increased formation of unwanted secondary products, such
as dialkylethers or glycerol ethers. 14
Because of the slow reaction rates and high temperatures needed for transesterification
acid catalysts are only used for esterification reactions. So for vegetable oils or animal fats
with an amount of free fatty acids over approx. 3 % two strategies are possible. The free fatty
acids can either be removed by alkaline treatment or can be esterified under acidic conditions
prior to the alkaline catalyzed transesterification reaction. This so-called pre-esterification
has the advantage that prior to the trans-esterification most of the free fatty acids are already
converted into FAME, so the overall yield is very high. If you have to remove the free fatty
acids prior to the trans-esterification, similar to the deacidification of vegetable oils during
refining, you don’t have to change the transesterification conditions; however, these fatty
acids are lost in the overall yield unless these fatty acid are esterified again in a separate step.
The cheapest and well-known catalyst for esterification reactions is concentrated sulphuric
acid. The main disadvantages of this catalyst are the possibility of the formation of
side products like dark colored oxidized or other decomposition products. As organic compound
also p-toluene sulphonic acid can be used; however, the high price of the compound
has prevented broader application. As heterogeneous catalyst also cationic ion exchange
resins can be used in continuous reaction columns; however, this approach has only been
used so far in pilot plants. The esterification of free fatty acids with methanol at increased
temperatures above the boiling point of methanol at ambient pressure was achieved by
introducing methanol into a preheated reaction mixture, 15 containing free fatty acids and
an acid catalyst. The reaction rates were significantly higher than under reflux temperature.
(See Table 5.3.)
5.5.3 Heterogeneous Catalysis
Whereas traditional homogeneous catalysis offers a series of advantages, its major disadvantage
is the fact that homogenous catalysts cannot be reused. Moreover, catalyst residues
Table 5.3 Overview of acidic catalysts
Type of catalyst Comments
Conc. Sulphuric acid Cheap, decomposition products, corrosion
p-Toluene-sulphonic acid High price, recycling necessary
Acidic ion exchange resins High price, continuous reaction possible, low stability