Biofuels in Perspective

Table 5.6 Comparison between classical and supercritical methanolysis
Process Technologies for Biodiesel Production 87
Base catalyzed methanolysis SCM method
Catalyst Alkali hydroxides, alcoholates none
Methanol amount Slight excess High excess
Reaction temperature [ ◦ C] 20–60 250–300
Reaction pressure [MPa] 0.1 10–25
Reaction time 30–120 7–15
Free fatty acids soaps FAME, water
Purification of glycerol Salt formation No salts, possible condensation
products (methyl ethers)
Energy consumption low high
5.7 Alternative Approaches
Classical alkaline catalyzed transesterification reactions are multiphase reactions. In the
beginning methanol is not soluble in the vegetable oil, but during increased formation of
fatty acid methyl esters the reaction mixture becomes homogenous until the formation
of glycerol begins, which again is insoluble in the methyl ester phase and separates at
the bottom. This fact facilitates the completeness of the reaction by separation of the end
product out of the reaction mixture. However, also the catalyst is removed together with
the glycerol. In order to overcome the mass transfer limitations the use of the co-solvent
Tetrahydrofuran (THF, Oxolane) was suggested. 32 However, for a technical application the
solvent has to be evaporated after the reaction which consumes quite a lot of energy.
A novel biodiesel-like material was developed by reacting soybean oil with dimethyl
carbonate, which avoided the co-production of glycerol. 33 The main difference between
this new route and classical biodiesel production is the presence of fatty acid glycerol
carbonate monoesters in addition to FAME. The presence of these compounds influence
both fuel and flow properties. Also the microwave assisted alcoholysis has been reported,
however, the evidence of significant improvement over classical routes is still missing, also
an industrial scale application does not seem to be realistic. 34
For complete conversion of oil directly from oil-containing seeds or other materials
the so-called in-situ transesterification can be used. That means that the lower alcohol
serves both as an extracting agent for the oils and the reagent for alcoholysis. In-situ
transesterification offers a series of advantages. First, hexane is no longer necessary as a
solvent in oil recovery. Second, the whole oil seed is subjected to the transesterification
process, so that losses due to incomplete oil production are minimized. Finally, the esterified
lipids tend to be easier to recover from the solid residue than native oils due to their
decreased viscosities. 35 The in situ production of FAME with the use of supercritical fluids
or microwaves also has been suggested. 36,37
5.8 Overview of Process Technologies
In the oleo chemical industry the production of fatty acid methyl esters has a long tradition,
because these products are an important intermediate for the further production of
fatty alcohols and fatty alcohol ethoxylates. These production units mainly use sodium