Introduction
In recent decades, humanity has been concerned with the continued
instability of the international oil market. In addition, there has been
increasing concern about environmental issues, and this involves the
need for sustainable alternatives for the production of clean and
renewable energy. Among these technological alternatives, biofuels are
studied worldwide, and from this perspective, biodiesel plays a key role
in replacing petrodiesel because it is biodegradable and fully renewable
if it is made with bioethanol (Kucek et al., 2007; Aydin, 214).
Biodiesel is chemically defined as alkyl monoesters of long chain fatty
acids derived from renewable raw materials, such as vegetable oils,
animal fats and recycled cooking oil. The most well-known process is a
chemical reaction in which the triacylglycerides (TAGs) found in these
oily materials (e.g., soybean oil) combine with an alcohol (methanol or
ethanol) in the presence of an alkali catalyst (usually sodium
hydroxide, alkoxides) to produce alkyl (biodiesel) monoesters and
glycerin. As a co-product, glycerol has little or no fuel value, but its
various industrial applications are critical to supporting process
economics (Kucek et al., 2007; Ferrari et al., 205; Lôbo et al., 2009).
However, in the transesterification process some undesirable by-products
are generated which must be removed, such as unreacted tri, di and
monoacylglycerols; methanol; catalyst; soap; glycerin and water (Faccini
et al., 2011). The purification of biodiesel can occur in two ways: dry
or wet. The wet method requires water or solvent for the removal of the
by-products generated but produces large volumes of effluents due to the
need to use a lot of water in the process. However, purification with
dry adsorbents is an alternative method, as it uses chemical adsorbents
that act to remove by-products without generating liquid effluent
(Gomide, 1988).
In the last few decades, new natural and synthetic adsorbents have been
studied to purify oils of different origins as well as frying oil (oil
containing higher acidity and other contaminants that influence
biodiesel production) (Alves et al., 2016). Among the various adsorbents
used for the purification of biodiesel are some silicates and mixtures
of silicates with magnesium and aluminum oxides as well as various
silicates formed by the fusion of lime, magnesium and aluminum oxides
with diatomaceous earth (Faccini et al., 2011; Araujo et al., 2010;
Turan and Yalcuk, 2013).
Magnesol® is the most widely used commercial adsorbent
in the biodiesel purification process. This salt is the amorphous form
of hydrated magnesium silicate,
MgO.nSiO2.xH2O, on the surface of which
are active sites that adsorb the compounds based on their dielectric
constant and acidic and basic properties (Alves et al., 2016). In
biodiesel purification, it acts by removing free or even bound molecules
of glycerol, soap, potassium, sulfur, residual methanol and traces of
the catalyst. There is also evidence that the use of
Magnesol® increases the stability of biodiesel in the
oxidation process. The disadvantage of this is the high cost of the
product (Sundus et al., 2017).
In the literature, there are few studies on the reuse of different types
of adsorbents, thus this work intends to demonstrate the reusability of
Magnesol® and, consequently, the reduction of this
adsorbent as residue. For this, biodeisels from frying soybean oil
(possibility of using frying oil for energy purposes and reduction of
environmental impacts) and virgin were used to verify the efficacy in
both cases, as well as the cultivar soybeans because soy is responsible
for more than 90% of biodiesel production in Brazil and the USA.