Esterification of the microalgal FFAs in the optimal
conditions The optimal conditions for the esterification of FFAs
from UVO were: 25C, 1.5:1 methanol/FFA molar ratio, stirring at
200 rpm, IOT ¼ 0.1 g Novozym 435 h/g FFAs (for example 0.1 g
Novozym 435 and 4 h for 4 g of FFAs) and absence of molecular
sieves. The reaction velocity can be increased and the reaction time
reduced using higher lipase amounts. These conditions were used
for the esterification of FFAs from the microalga N. gaditana. The ED
attained was 92.6 0.5%, similar to that obtained with FFAs from
UVO in these conditions (92.3%). This yield is similar to those obtained
by other authors who transform FFAs into biodiesel using
other catalysts. For example, Hayyan et al. (21) reduced the FFA
content of sludge palm oil by sulphuric acid catalysed
esterification. This FFA content was reduced from 23.2% to less
than 2% (conversion of around 91.4%) using 0.75 wt% sulphuric
acid, an 8:1 molar ratio of methanol to oil (2.7:1 expressed as
methanol/FFA, as opposed to 1.5:1 in the present work), 60C
(25C in this work) and 60 min for a similar conversion to that
attained in this work. This comparison shows that enzymatic
catalysis consumes less energy than acid catalysis and can even
give higher reaction velocities if higher lipase amounts are used.
Krohn et al. (28) produced algal biodiesel from oils with high FFA
content from several microalgae species using supercritical
methanol and porous titania microspheres in a fixed bed reactor
to catalyse the simultaneous conversion of triacylglycerols and
FFAs to biodiesel; a maximum conversion of 85% was attained
and this method requires very high pressures (2250 psi) and
temperatures (340C).
The microalgal biodiesel obtained from the esterification reaction
was 78.6 1.3 wt% pure, which is similar to the purity of FFAs
extracted from microalgal biomass (73.5 2.6 wt%) following the
procedure shown in Fig. 1. This result highlights the fact that the
enzymatic conversion of FFAs to methyl esters does not reduce
biodiesel purity.