Thermally
driven organic transformations can take place by conventional heating where
the reactants are slowly activated by an external heat source. Heat is
driven into the substance, passing first through the walls of the vessel
in order to reach the solvent and reactants. This is a slow and inefficient
method for transferring energy into the reacting system.
Alternatively,
microwave-accelerated heating can be employed where
microwaves couple directly
with the molecules of the entire reaction mixture, leading to a rapid rise
in temperature. Since the process is not limited by the thermal conductivity
of the vessel, the result is an instantaneous localized superheating of
any substance that will respond to either dipole rotation or ionic conduction
- the two fundamental mechanisms for transferring energy from
microwaves to the substance(s) being heated (Hayes, 2004).
In
the electromagnetic radiation spectrum, the microwave radiation region
is located between infrared radiation and radio waves.
Microwaves (MW)
have wavelengths of 1mm - 1m, corresponding to frequencies between 0.3
- 300 GHz. In general, in order to avoid interference, industrial and domestic
microwave apparatus are regulated to 12.2 cm, corresponding to a frequency
of 2.45 GHz, but other frequency allocations do exist (Mingos and Baghurst, 1991; Fini
and Breccia, 1999).
Microwaves, a non-ionizing radiation incapable
of breaking bonds, are a form of energy and not heat and are manifested
as heat through their interaction with the medium or materials wherein
they can be reflected (metals), transmitted (good insulators that will
not heat) or absorbed (decreasing the available
microwave energy and rapidly
heating the sample)
(Varma, 2001).
Microwaves, representing a non-ionizing
radiation, influence molecular motions such as ion migration or dipole
rotations,
but not altering the molecular structure.
A molecule possessing a dipole moment is sensitive to external electric
fields.
Therefore, when it is exposed to microwave
irradiation, the dipole will attempt to align with the applied electric
field by rotation. The applied field provides the energy for this rotation.
In the
microwave radiation region, the
frequency of the applied irradiation (2.45 GHz) is low enough so that the
dipoles will have time to respond to the alternating electric field and
therefore will rotate. However, the frequency is not high enough
for the rotation to follow the oscillating
field exactly generating a phase difference between the orientations of
the field
and that of the dipole. The continual reorientation
of the molecules results in friction giving rise to dielectric heating
(Lidstrom
et al., 2001).
If a molecule is charged, then the electric
field component of the
microwave irradiation will move the ions back and
forth through the sample while also colliding them into each other. This
movement again generates heat. The conductivity mechanism is a much stronger
interaction than the dipolar mechanism with regard to the heat generating
capacity
(Lidstrom
et al., 2001).
In the
Transesterification process, because
the mixture of
vegetable oil,
methanol and
potassium
hydroxide contains
both polar and ionic components, rapid heating is observed upon microwave
irradiation and because the energy interacts with the sample on a molecular
level, very efficient heating can be obtained.
To allow for a strict comparison between
microwave irradiation and conventional heating under similar conditions
(reaction medium, temperature and pressure), a monomode microwave reactor
should be used. This ensures wave focusing (reliable homogeneity in the
electric field) and accurate control of the temperature (using an optical
fiber or infrared detection) throughout the reaction (Perreux
and Loupy, 2001).
Reflux system has been developed in an
effort to use solvents in microwave assisted organic synthesis without
the risk of explosion.
Reflux systems are at minimal risk of explosion
because they are operating at atmospheric pressure and because flammable
vapors cannot be released into the microwave cavity.
Several examples of
microwave irradiated
TransEsterification methods have been reported using adapted domestic ovens
to use them as flow systems (Saiffudin
et al.,
2004) or batch laboratory ovens (Mazzocchia
et
al., 2004) but only moderate
conversions were obtained. A more recent
study used homogeneous catalysis, both in a batch and in a flow system
(Hernando
et
al., 2007).
Leadbeater and Stencel reported the use
of microwave heating as a fast, simple way to prepare biodiesel in a batch
mode (Leadbeater and Stencel, 2006).
This was followed by a continuous flow approach allowing for the reaction
to be run under
atmospheric conditions and performed at
flow rates of up to 7.2 L/min using a 4 L reaction vessel. (Barnard
et al., 2007).
In a study by Refaat
et
al. (2008b) the optimum parametric conditions obtained from
the conventional technique were applied using microwave irradiation
in order to compare both systems for the
production of biodiesel from
neat and
waste vegetable oils. The results showed that application
of radio frequency
microwave energy offers a fast, easy route to this
valuable
biofuel with advantages of enhancing the reaction rate and
improving the separation process.
From these results it was concluded that
using
microwave irradiation reduces the reaction time by 97 % and the separation
time by 94 %.
The methodology allowed for the use of
high free fatty acid content feedstock, including used cooking oil without
prior pretreatment processes. The authors also proved that
microwave-enhanced
biodiesel is not, at least, inferior to that produced by the conventional
technique.
A study was conducted by El Sheltawy and Refaat (2008) to compare three options for the production
of biodiesel from neat and waste vegetable
oil; the conventional base-catalyzed TransEsterification,
UltraSonication and
microwave-enhanced
TransEsterification. Despite the prominent
advantages the
UltraSonication and
microwave technologies offer compared
to the conventional base-catalyzed
TransEsterification, yet, these emerging
technologies need to be further investigated for possible Community scale for industrial
application.
