Biodiesel From Conventional Feedstocks - Biotechnology in China: Biofuels and Bioenergy - page 60

3.3 Lipase-Mediated Process
In recent years, enzymatic approaches for biodiesel production have received
much attention, since they have many advantages over chemical methods:
moderate reaction conditions, low alcohol to oil ratio, easy product recovery, and
environmentally friendly. In addition, FFAs contained in waste oils and fats can
be simultaneously completely converted to alkyl esters. In terms of the forms
of biocatalyst, enzyme-mediated biodiesel production can be classified into
immobilized lipase, whole cell catalyst, and liquid lipase-mediated alcoholysis,
respectively. The related R&D as well as the progress of industrialization is
discussed here.
3.3.1 Immobilized Lipase-Mediated Transformation
A solvent-free system was first proposed for lipase-mediated methanolysis for
biodiesel production. In such a system, it was demonstrated that methanol has poor
solubility in oil feedstocks and too much methanol existing as drops in the system
has an inhibitory effect on lipase activity [ 1 , 2 ]. Stepwise methanol addition was
therefore recommended for enzyme-mediated biodiesel production in the solvent-
free system. In addition, glycerol, a by-product of the process, is very hydrophilic
and insoluble in the oils, and is easily adsorbed onto the surface of the immobilized
lipase, leading to a negative effect on lipase activity and operational stability.
Several methods have been proposed to eliminate this phenomenon, such as
adding silica gel to the reaction system to absorb glycerol or periodically washing
the lipase with some organic solvents to remove glycerol. In practice, these
methods do reduce the negative effect caused by glycerol to some extent, but make
the system more complicated, especially for large-scale continuous biodiesel
production [ 23 , 24 ].
With the aim of reducing the aforementioned negative effects caused by
methanol and glycerol, some researchers tried carrying out the enzymatic reaction
for biodiesel production in organic solvent systems. According to the traditional
principle of non-aqueous enzymology, enzymes show higher activity in relatively
hydrophobic organic solvents with higher logP ([3), such as n-hexane and
petroleum ether. Based on this knowledge, hydrophobic organic solvents have
been extensively explored as the reaction medium for immobilized lipase-cata-
lyzed biodiesel production. In reality, with such relatively hydrophobic organic
solvents as the reaction media, methanol and glycerol have poor solubility in these
systems. Therefore the negative effects caused by methanol and glycerol cannot be
completely eliminated, and lipase still exhibits poor operational stability.
Tsinghua University developed a novel process with a relatively hydrophilic
organic solvent, tert-butanol, as the reaction medium for lipase-mediated alco-
holysis for biodiesel production [ 2 ]. Both methanol and the by-product glycerol
are soluble in tert-butanol medium and the whole system is homogeneous, and the