(490k) Utilization of Static Mixers in the Oil Transesterification Reaction for Biodiesel Production

Biodiesel is a low-emission, highly degradable
fuel, obtained by transesterification of a vegetable oil or an animal fat
with an alcohol, usually methanol. Because oil and alcohol form two separate
phases, mixing is crucial in the biodiesel synthesis reaction, in order
to provide a fine dispersion of alcohol in oil, and therefore to favour
mass-transfer phenomena. Most transesterification studies focused on mechanically-agitated
systems. Conversely, only a few works tested the utilization of static
mixers (SM) for the generation of the alcohol/oil dispersion, despite the
fact that SM generally require less power input than mechanical agitation
at equivalent performance; besides, thanks to the absence of moving parts,
SM are characterized by minimum maintenance requirements.

In this study, the utilization of Sulzer Chemtech 15 mm SMV^®
static mixers for the KOH-catalysed transesterification of sunflower oil
was studied by means of batch tests conducted in an experimental ring equipped
with a 22 L reactor. Oil and methanol (containing the dissolved KOH) were
initially loaded into two tanks, whose headspace was connected to a line
of compressed gas. The two streams were mixed in a tee connection and then
fed to the reactor by gas displacement. The methanol:oil molar ratio was
set to 6, the temperature to 60 °C, and the KOH concentration to 0.8%
of the initial oil mass. The effect of SM number (0-5) and superficial
velocity (0.4-2.5 m/s) was investigated; the results were compared to those
of analogous tests performed in batch conditions with only mechanical agitation,
at different rotational speeds.

The test conducted with one single SM at a 1.3 m/s superficial velocity
(Re = 1490) resulted in a profile of sunflower oil conversion versus time
equivalent to that obtained in the best-performing tests with mechanical
agitation, characterized by the presence of two Rushton turbines operated
at a rotational speed of at least 400 rpm (Remixing >= 2870). In these
tests, the equilibrium conversion was equal to 93-96%, and the time for
the attainment of a conversion equal to 90% of the equilibrium value (t₉₀)
was equal to about 2.5 minutes.

The tests conducted, with 1 SM, at superficial velocities higher than
1.3 m/s provided the same reaction profile as the test at 1.3 m/s. This
observation, in agreement with an evaluation of the reaction and transport
characteristic times, indicates that the test at 1.3 m/s was affected by
a negligible mass transfer limitations (kinetically controlled condition).
Conversely, a test conducted with 1 SM at a superficial velocity of 0.4
m/s resulted to be characterized by a significant mass-transfer resistance.

In the SM tests conducted at 1.3 m/s, the utilization of more than
one SM did not lead to any significant increase of the reaction rate.

In order to evaluate the actual contribution of the SM section in the
generation of the methanol/oil dispersion, a further group of tests was
conducted, with the same system for loading oil and methanol, in the absence
of SM. The test conducted at a velocity in the empty SM section equal to
1.3 m/s (corresponding to 2.9 m/s in the piping) led to a 65% increase
of t₉₀. This result indicates on the one hand that the single
SM provides a relevant contribution to the generation of the dispersion,
but on the other that contribution of the simple ?T? junction of the loading
system is not negligible.

Lastly, the mixing energy required for the generation of the methanol/oil
dispersion with SM and with mechanical agitation was evaluated. In the
SM test at 1.3 m/s, the specific energy requirement evaluated at an oil
conversion equal to 90% of the equilibrium value (e₉₀) resulted
equal to 15 J/kg_biodiesel. In the batch tests with mechanical
agitation with two Rushton turbines, in order to attain a similar energetic
performance it was necessary to reduce the rotational speed to 250 rpm,
which led to a 15-20 % increase of t₉₀. These results show that,
with both mechanical agitation and static mixing, the mixing energy for
biodiesel production can be reduced to very low values by means of a careful
evaluation of the optimal rotational speed or SM superficial velocity.
On the whole, this study suggest that static mixing can be effectively
applied to oil transesterification processes for biodiesel production.