Biodiesel from waste frying oils_Methods of production and purification

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Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Review
Biodiesel from waste frying oils: Methods of production and purification
Jhessica Marchini Fonsecaa, Joel Gustavo Telekenb, Vitor de Cinque Almeidaa, Camila da Silvac,⁎
a Departamento de Química, Universidade Estadual de Maringá (UEM), Av. Colombo, 5790, 87020-900 Maringá, PR, Brazil
bDepartamento de Engenharia e Ciências Exatas, Universidade Federal do Paraná (UFPR), R. Pioneiro 2153, Palotina, PR 85950-000, Brazil
c Departamento de Tecnologia, Universidade Estadual de Maringá (UEM), Av. Ângelo Moreira da Fonseca 180, Umuarama, PR 87506-370, Brazil
A R T I C L E I N F O
Keywords:
Waste frying oil
Biodiesel
Purification
Catalysis
Dry washing
Patent
A B S T R A C T
Waste frying oils (WFO) are a promising source in terms of feedstock for biodiesel production, and have a lower
cost than the typical refined vegetable oils. In addition, the use of WFO is an environmentally correct way to use
this residue; it also decreases the food versus fuel competition. However, the application of WFO has some
limitations, including their high free fatty acid and water content, which influences the final ester yield in the
base catalyzed reactions. This review article introduces a state-of-the-art use of WFO as feedstock for biodiesel
production. Here, catalytic and non-catalytic methods are reported and discussed, and the advantages and
disadvantages of using WFO are presented. Furthermore, techniques for the purification of biodiesel are dis-
cussed along with patents and potential future work available in terms of production and purification.
1. Introduction
Biodiesel is a biofuel derived from vegetable oils and/or animal fats.
It is non-toxic, biodegradable and produces less sulfur and hydro-
carbons, and can be used in diesel engines with minimal modifications
[1,2]. Biodiesel consists of a mixture of monoalkyl esters and long
chains of fatty acids derived from different types of oils and fats, ob-
tained mostly via transesterification with a lower alcohol in the pre-
sence of a catalyst [3]. Edible vegetable oils, such as soybean, sunflower
and palm oils, are the main feedstock used in the production of bio-
diesel. However, the high cost of these oils, which accounts for about
70% of the total value of biodiesel production, as well as the compe-
tition with food and soil degradation due to large planting scales, are
disadvantages for production and commercialization of biodiesel [4,5].
Waste frying oils (WFO) is considered a promising alternative in
biodiesel synthesis, due to their low cost and high availability. In ad-
dition, WFO use reduces competition with food demand. The cost of
WFO is two to three times lower than refined vegetable oils. Their use
would also reduce the costs of removal and treatment of this residue
[6–9]. On the other hand, WFO contains other compounds in addition
triacylglycerols, due to chemical reactions during the food cooking
process or raw food. These compounds include water, free fatty acids
(FFA), polar compounds and non-volatile compounds that mainly affect
homogeneous catalytic transesterification reactions [10,11].
In the production of biodiesel from WFO, it is necessary to point out
that some compounds formed during the cooking process are
unreactive, due to their greater polarity (polymers, dimers) than the
triglycerides. As a result, the percentage of FAME (fatty acid methyl
ester) are lower at the end of the reaction [12,13]. In addition, polar
compounds present in biodiesel negatively affect the efficiency of the
engines, which is potentially problematic in terms of fuel injection and
combustion [11,14,15].
The transesterification reaction to obtain biodiesel on an industrial
scale is usually carried out using a homogeneous catalysis with a strong
catalytic base. This reaction has advantages such as: lower reaction
time, higher conversion, and a relatively small amount of catalyst used,
when compared to other catalytic methods [16]. However, this homo-
geneous, basic catalysis is affected mainly by the presence of FFA and
water, which leads to the formation of soap and consequently reducing
the yield reaction. [17]. Acid-catalyzed reactions (sulfuric acid, hy-
drochloric acid), on the other hand, are not influenced by the presence
of FFA. However, they are sensitive to the presence of water, and as
such, their reactions are slower and require higher temperatures
[16,18].
Another type of catalysis uses heterogeneous catalysts, which can be
acidic and basic, enzymatic and recently ionic liquids. Heterogeneous
acidic and basic catalysts are advantageous due to theirs low costs,
recyclability, and the ability to simultaneously undergo esterification
and transesterification. However, the heterogeneous catalysis is dis-
advantageous because it has low concentration of active sites and
problems of diffusion limitations, reducing reaction rates [20,21]. En-
zyme-catalyzed reactions can promote esterification of FFA and
https://doi.org/10.1016/j.enconman.2019.01.061
Received 5 December 2018; Accepted 19 January 2019
⁎ Corresponding author.
E-mail address: camiladasilva.eq@gmail.com (C. da Silva).
Energy Conversion and Management 184 (2019) 205–218
0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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transesterification of triacylglycerols simultaneously. Despite the easy
separation of the catalyst products, the high cost and the deactivation of
the enzymes remain an obstacle [22,23]. Ionic liquids are considered a
new generation of catalysts, as they can act in the esterification and
transesterification of the oils and are easily separated. Although they
have good catalytic activity, the conditions necessary for those reac-
tions are considered dangerous due to the higher temperatures and
pressures [24,25].
Reactions under pressurized conditions without the presence of a
catalyst have been also reported in the literature. Among the ad-
vantages of these types of reactions are fast rates, decreases in mass
transfer limitations, no required purification processes and simulta-
neous esterification and transesterification. However, these reactions
require high molar ratios of alcohol to oil and thus have a high energy
cost [26–28].
The mixture of products of transesterification is composed of esters,
glycerol, alcohol, monoglycerides and diglycerides, depending on the
type of reaction and catalyst. The transesterification reaction is shown
in Fig. 1. The purity level of the esters has a great impact on the quality
of the biofuel. The process most commonly used in the removal of
impurities from biodiesel is known as wet washing [29,30]. The
drawback of this process is that large volumes of contaminated liquid
effluents are generated; three liters of water is consumed on average per
one liter of biodiesel produced [31,32]. As a way to reduce the pro-
duction of these effluents, adsorption stands out as fast and low-cost dry
washing method [33]. Several adsorbents composed of silica were de-
veloped for the purification of biodiesel. However, this method may
become impracticable due to the cost of the adsorbent [34,35]. In any
case, agro-industrial waste with silica in its structure can be used as a
substitute for commercial adsorbents [36,37].
This work presents a review on the potential of WFO as feedstock for
biodiesel production. The article explores the main chemical reactions
that occur during the cooking process, the main literature that describes
using WFO with different production methods in obtaining biodiesel,
the purification methods used and the published patentson production
and purification steps.
2. Biodiesel
Diesel engines are considered the most efficient in terms of internal
combustion and cost. They also emit relatively low levels of carbon
dioxide, but emit high levels of particulates and oxides of nitrogen
(NOx) [38]. As an alternative to diesel oil, biodiesel has been in-
vestigated by several researchers [39–43].
Biodiesel is a mixture of alkyl esters of long chain fatty acids derived
from different vegetable oils and animals fats [3]. Its main character-
istics are biodegradability, low sulfur content, no aromatic compounds,
a high flash point, characteristic lubrication, miscibility with petroleum
diesel in any mixing ratio, higher cetane number and higher oxygen
content (10 to 11 wt%) in relation to petrochemical diesel. However, to
improve the performance of biodiesel use, it is necessary to reduce its
NOx emission levels, improve its oxidative stability, cold flow proper-
ties and decrease kinematic viscosity values and density [44,45]. These
improvements are important, as the use of biodiesel has less of an en-
vironmental impact, reducing the dependence on fossil fuel resources
greenhouse gas emissions [46].
The esters of fatty acids present the corresponding fatty acid profile
of the feedstock from which it was obtained. Its major fatty acids are
Fig. 1. Transesterification reaction.
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
206
straight-chain, commonly with 16 to 18 carbon atoms. However, some
raw materials contain substantial amounts of other fatty acids. In ad-
dition to esters, the final biodiesel product includes other compounds
which are limited by quality standards. These quality standards include
standards D6751 of ASTM (American Society for Testing and
Materials), European Standard EN 14214 and the standards of other
countries [47]. Table 1 shows the main parameters evaluated in this
study and their respective limits in order to ensure the quality of bio-
diesel produced.
3. Characteristics of waste cooking oil
Soybeans are one of the main feedstocks used as a substrate in the
production of biodiesel. They have a very specific crop and are pro-
duced in several regions of the world. In Brazil, for example, in the
2016/2017 harvest, approximately 3 million m3 of biodiesel were
produced from soybean oil, representing 77% of the total annual yield
[49]. However, biodiesel obtained from refined edible oils increases the
final cost of biodiesel, in addition to competition between food versus
fuel [4]. As such, oils that have already been used in frying have be-
come a promising source as a substrate for biodiesel production. Fast
food companies and industrial processing plants generate large quan-
tities of oils. The final amounts of these oils are constantly investigated;
in Brazil alone, the annual production of waste oils and fats reaches up
to 1.2 million tons [50]. In some cases, these oils are mixed in animal
feed, makeup and fertilizer. However the vast majority of waste oils are
dumped into sewage networks [51].
Frying is defined as the immersion of foods in oils or fats at high
temperatures (150 to 200 °C) in the presence of oxygen, moisture, pro-
oxidants and antioxidants of food [52,53]. The heating process leads to
the formation of various compounds, which vary according to the
composition of the oil and food [54,55]. During the frying process
several reactions may occur, including oxidation, hydrolysis, poly-
merization, isomerization and decomposition of the oil into several
volatile compounds [56].
The oxidation reaction occurs in the lipid, forming FFA, conjugated
dienes and trienes, peroxides and by-products such as alcohols, ketones
and aldehydes. The reaction rate of primary oxidation is influenced by
temperature, time of exposure, light, oxygen and oil type. Peroxide is
one of the main reactive byproducts of oxidation; it reaches a peak level
and gradually decreases, decomposing into other compounds, in-
dicating secondary oxidation [52]. Oils have high concentration of vi-
tamin E, an antioxidant compound, which is lost during the frying
process along with the oxidation of unsaturated fatty acids [57].
Another type of reaction that may occur is hydrolysis, which may
have either a negative or positive effect on the characteristics of the oil.
Water from food increases the acidity of the medium, but this water can
serve as a physical barrier forming a steam layer over the oil, reducing
the oxidation efficiency [58]. Furthermore, in hydrolysis, the formation
of monoacylglycerols, diacylglycerols and glycerol occurs [59].
Hydrolysis and oxidation are largely responsible for the formation
of polar compounds, including free fatty acids, monoacylglycerols,
diacylglycerols, polymers and oxidized compounds. Polar compounds
in frying oils are one of the main parameters for evaluating oil quality
and reliability, as they are degradation products of the fatty acids with a
maximum of 25 wt% of WFO composition [11,60].
Polymerization of oil occurs when it is heated repeatedly at elevated
temperatures. Together with the polymers, there is formation of non-
volatile polar compounds and triacylglycerol dimers. The amount of
these compounds that is formed varies according to the type of oil. The
increase in these compounds accelerates oil degradation, increases
viscosity, reduces heat transfer, produces foam during the frying pro-
cess and develops an undesirable color to the food. In addition, the
polymers may assist in the absorption of oil by food [61]. Various other
compounds are formed during frying, such as oxidized monomers of
triacylglycerol, sterol derivatives, nitrogen and heterocyclic compounds
containing sulfur and acrylamide [59]. The WFO can present a very
varied composition due to the cooking processes. Table 2 shows the
composition of WFO reported in the literature. The WFO have a very
varied composition with respect to water and FFA contents. The com-
position in fatty acids also varies considerably, since the types of oils
used for cooking are diverse (soybean, canola, corn). The saponification
values, although varying, are close to one another.
As already mentioned, WFO may contain various non-convertible
compounds in alkyl esters, whose maximum yield within the esters will
not reach 100%. Gonzalez et al. [62] reported that in order to achieve
maximum conversion, it was necessary to determine the “convert-
ibility” of the oil, which consists of a quantitative analysis that de-
termines the percentage of convertible and non-convertible compounds.
Table 1
Biodiesel specifications and their limits.
Properties EN 14214 ASTM - D6751
Composition of Biodiesel C12-C22 C12-C22
Esters Content greater than96.5% (m/m) –
Density at 15 °C 860–900 kgm−3 –
Viscosity at 40 °C 3.5–5.0mm2 s−1 1.9–6.0mm2 s−1
Flash Point ≥ 101 °C ≥ 130 °C
Sulphur Content ≤ 10mg kg−1 ≤ 50mg kg−1
Carbon Residue ≤ 0.3% (m/m) ≤0.05% (m/m)
Cetane Number ≥ 51 ≥ 47
Sulfated Ash ≤ 0.02% (m/m) ≤ 0.02% (m/m)
Water Content ≤ 500mg kg−1 ≤ 0.05% (v/v)
Corrosion – 3 h
Oxidative Stability 110 °C ≥ 4 h ≥ 3 h
Acid Value ≤ 0.50mg KOH g−1 ≤ 0.50mg KOH g−1
Iodine Value 130 g I2/100 g –
Methanol Content ≤ 0.02% (m/m) –
Monoacylglycerols Content ≤ 0.8% (m/m) –
Diacylglycerols Content ≤ 0.2% (m/m) –
Triacylglycerols Content ≤ 0.2% (m/m) –
Free Glycerin ≤ 0.02% (m/m) ≤ 0.20% (m/m)
Total Glycerin ≤ 0.25% (m/m) ≤ 0.25% (m/m)
Pour Point – −15 to −16 °C
Phosphorus Amount ≤ 4mg kg−1 ≤ 0.001% (m/m)
Cloud Point – −3 to −12 °C
Adapted from Ambat et al. [48].
Table 2
Variability of WFO composition.
Reference Composition
FFA (%) Water content (mg kg−1) C16:0 (%) C18:0 (%) C18:1 (%) C18:2 (%) C18:3 (%) Saponification value (mg KOH g−1)
Eze et al. [64] 1.53 1153 6.1 1.8 64.2 19.4 8.4 NI
Mansir et al. [5] 5.5 6000 60.1 10.8 27.2 1.14 NI 187.2
Tacias-Pascacio et al. [65] 1.05 400 17.82 5.75 40.98 28.77 4.51 198.54
Gharat and Rathod [66] 2.402 NI 9.08 6.82 30.6 53.5 NI 208
Gonzalez et al. [62] 1.5 6200 11.6 3.9 25.5 51.9 4.8NI
Yahya et al. [67] 1.88 2000 34.80 7.90 53.30 4.00 NI 194.40
NI – Not informed.
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
207
The authors reported that the WFO employed in the work had a con-
vertibility of 92.1%, so the substrate contained 7.9% of compounds that
would not produce esters at the end of the reaction. Abdala et al. [63]
also determined the convertibility of WFO, with 93.1% and 6.9% of
convertible and non-convertible compounds, respectively.
4. Methods of preparation of biodiesel
Several methods can be used to produce biodiesel, including micro-
emulsions, pyrolysis and transesterification. Transesterification (alco-
hololysis) is the conventional method, employed on an industrial scale,
in which the obtained biodiesel can be pure or in a mixture. The re-
action can be carried out in various ways, catalyzed or not. Each of
these methods and their advantages and disadvantages [19] are sum-
marized in Table 3.
The following sections report the main studies in recent literature,
using the methods described in Table 3 for the production of biodiesel
using WFO as feedstock.
4.1. Homogeneous catalysis
4.1.1. Alkali
The base-catalyzed transesterification reaction takes place in short
(60min) reaction times, and uses sodium hydroxide and potassium
(NaOH and KOH, respectively) as catalysts. These reagents have high
activity, affordability and availability. NaOH is preferentially used on
an industrial scale because of its low molecular weight. However, KOH
is beneficial in the mixture after the reaction, because it can be reacted
with phosphoric acid in the neutralization step, forming potassium
phosphate, which can be applied as fertilizer. It is worth mentioning
that the efficiency of this type of catalysis is directly related to the
concentration of impurity within the feedstock used in biodiesel pro-
duction [80]. The method is the one commonly used industrially, being
estimated an energy consumption of 2326 kW for 10,000 tons/year.
Although it is the most widely used method, basic catalysis has dis-
advantages, such as the generation of large amounts of contaminated
liquid effluent. More specifically, glycerol, the byproduct of the reac-
tion, is not pure and so it is not possible to reuse the catalyst. Sodium
methoxide is also used as a catalyst in the transesterification reactions,
in which the reaction is completed faster than NaOH and KOH [81,82].
Furthermore, soap formation may occur depending of the feedstock
used [83,84]. Oils after frying have a high content of FFA and water,
making the production of biodiesel by the homogeneous alkaline cat-
alysis method unfeasible. These byproducts promote the formation of
soap, which reduces the yield of the reaction and hinders the process of
separation and purification [17].
Çayli and Küsefoǧlu [85] carried out the WFO transesterification of
WFO, which had an acidity of 6.5 mg KOH g−1 and 0.7 wt% water. That
study compared the process in two stages, using NaOH as a catalyst in
two reactions at room temperature. In addition, the authors evaluated
the effect of the presence of water and suspended particles in the final
yield. They concluded that the removal of water and particulates raises
the final ester content and that the two-step process was more profit-
able (94%) than only one step (73%). Chhetri et al. [86] evaluated the
percentage of catalyst (NaOH) and the time in the transesterification of
WFO. The catalyst concentration varied from 0.4 to 1.2%, and the use
of 0.8 wt% NaOH produced 94.5% of esters in 20min of reaction. The
researchers also evaluated the main parameters according to the ASTM.
The main operating conditions (methanol to oil molar ratio, amount of
NaOH catalyst, reaction time and temperature) in biodiesel production
were investigated by Meng et al. [87]. In that study, the frying oil had
an acidity of 7.25mg KOH g−1. The optimal condition was found to be
a methanol to oil molar ratio of 9:1, with 1 wt% of catalyst (based on oil
mass), at 50 °C for 90min. However, the ratio of 6:1 is more appropriate
to the process, so the final yield was 89.8% in esters. In another study,
Phan and Phan [88] used KOH as a catalyst in the transesterification of
different WFO, in which the acidity varied between 0.67 and 3.64mg
KOH g−1, obtaining ester yields between 88 and 90% with using molar
ratios of 7:1–8: 1 (methanol to oil), temperature between 30 and 50 °C
and 0.75 wt% catalyst (based in the oil mass). Bautista et al. [89] stu-
died the effects of biodiesel synthesis parameters, including tempera-
ture, catalyst concentration and FFA content in the oil. They found that
catalyst concentration is the most important factor and temperature is
the least important factor influencing the yield.
Hingu et al. [90] investigated the use of sonochemical reactors in
the synthesis of biodiesel from WFO, in which the oil had an initial
acidity of 2.805mg KOH g−1. Different operating parameters were
evaluated, such as the methanol to oil molar ratio, catalyst concentra-
tion, temperature, ultrasound power and pulse on the oil conversion.
The best ester yield (89.5%) was obtained using a methanol to oil molar
ratio of 6:1, 1 wt% catalyst, 45 °C and 200W potency for 40min. Thanh
et al. [91] used a two-step process to produce biodiesel from WFO
(initial acidity of1.07mg KOH g−1 and 0.015 wt% water) in an ultra-
sonic reactor. The main operational variables were evaluated and the
catalyst used was KOH. At the end of the second step, there was ob-
tained 99% of esters. Chen et al. [92] evaluated the WFO biodiesel
synthesis that had an acidity of less than 2mg KOH g−1, with two
different catalysts (NaOH and sodium methoxide). In that study, the
effects of time, molar ratio and microwave power were evaluated,
finding that sodium methoxide was the best catalyst, yielding close to a
98% in esters.
Maddikeri et al. [93] carried out an interesterification of WFO with
methyl acetate and KOH. They reported an initial acidity of 4.3 mg KOH
g−1 and a maximum ester yield of 90% using a concentration of 1 wt%
Table 3
Advantages and disadvantages of the methods used in the synthesis of biodiesel.
Method Advantages Disadvantages
Homogeneous Alkaline High yield, low cost and fast reaction rate [16] Neutralization of FFA, generation of wastewater, difficult catalyst
recovery and purification of products required [68]
Homogeneous Acid High yield (under certain conditions), conversion of FFA to biodiesel, low
cost and medium reaction rate [69]
Generation of wastewater, corrosion of equipment, difficult catalyst
recovery and purification of products required [48,70]
Heterogeneous Alkaline High yield, medium cost, fast reaction rate, reusability and can be used in a
continuous process [71]
High energy requirement, tedious catalyst preparation, catalyst leaching
and low surface area [72]
Heterogeneous Acid High yield, medium cost, fast reaction rate, reusability, conversion of FFA to
biodiesel and can be used in continuous process [73]
Low concentration of active sites, high cost of the catalyst feedstock,
tedious catalyst preparation and catalyst leaching [74]
Enzyme High yield, conversion of FFA to biodiesel, low energy requirements, high
product purity, reusability and can be used in a continuous process [75]
Inhibition by alcohols and high cost [76]
Ionic Liquid Simultaneous esterification and transesterification, possibility of several
functional groups and water tolerant [24]
Difficult recovery and recycling methods, glycerol can accumulate on
the surface and decrease catalytic activity and high temperatures
required for the reaction [77]
Noncatalytic No catalyst, high reaction rate, tolerant to the presence of FFA and water,
simultaneous esterification and transesterification and fewer number of
processing steps [78]
High temperature and pressure are required, high cost of equipment and
high energy consumption [79]
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
208
catalyst(by weight of oil), with a methyl acetate to oil molar ratio of
12:1 at 40 °C. The transesterification of WFO (FFA and water contents
were 0.24mg KOH g−1 and 1.21 wt%, respectively) using NaOH as a
catalyst was investigated by Rabu et al. [94]. The reactions were con-
ducted at 60 °C with stirring at 400 rpm, and varying the molar ratio,
time and catalyst concentration, obtaining a 95% in esters under the
best operating conditions. Banerjee et al. [95] evaluated the effect of
the methanol to oil molar ratio on the esters yield, ultimately synthe-
sizing biodiesel from the WFO. The other variables were kept fixed at
55 °C for 90min and NaOH was used as catalyst. When the molar ratio
was 15:1, the yield reached 94% of esters. In another study, Luu et al.
[96] studied the effect of the addition of a co-solvent (acetone) on
biodiesel synthesis. The FFA content was 0.92% and the water content
was 1231.3 mg g -1. Temperature, time, amount of catalyst and molar
ratio were also assessed. The highest yield (98%) was obtained under
the conditions of 1 wt% of KOH (catalyst), 20 wt% acetone, methanol to
oil molar ratio of 5:1, 40 °C and a reaction time of 30min.
Bilgin et al. [97] investigated the biodiesel production of WFO with
ethanol and NaOH as catalyst, aiming to obtain a product with the
lowest possible kinematic viscosity. The lowest kinematic value was
found when using a 1.25% catalyst concentration, at 70 °C for 120min,
and ethanol to oil molar ratio of 12:1. Furthermore, an experimental
design was applied by El-Gendy et al. [98] to verify the interaction and
potential of the main operational parameters used to obtain the highest
yield in esters. In that study, the FFA content of the WFO was 1.04mg
KOH g−1. The optimum conditions for higher ester (∼99%) were me-
thanol to oil molar ratio of 7.54:1, 0.875wt% of KOH, 52.7 °C, a re-
action time of 1.17 h with constant stirring at 266 rpm.
4.1.2. Acid
The acid catalysts usually investigated in homogeneous reactions
are generally sulfuric and hydrochloric acid. Reactions with acid cata-
lysts are not influenced by the amount of FFA in oils. Instead, the acids
are capable of simultaneously esterifying the FFA and transesterifying
the triacylglycerols. However, this type of catalysis requires high molar
ratios of alcohol, high catalyst concentrations and long reaction times,
as the reaction is corrosive and up to 4000 times slower than that
catalyzed by a base [99,100,101].
Reactions using a homogeneous acid are generally applied as a pre-
treatment to acidic oils, such as WFO, in order to reduce the acidity of
the medium and apply a basic transesterification. Liu et al. [102] used
WFO (acidity of 68.2 mg KOH g−1) for biodiesel production and a pre-
treatment step was carried out using an experimental design with radio
frequency heating, evaluating the conditions of time, catalyst dose and
methanol to oil weight ratio on acid-catalyzed. The condition of
greatest reduction of acidity was in 8min, 3 wt% H2SO4 and methanol
to oil of 0.8:1, obtaining an oil with 1.64mg KOH g−1 of acidity.
Charoenchaitrakool and Thienmethangkoon [103] used a WFO for
biodiesel production that contained 1 wt% FFA and 0.1 wt% water.
Sulfuric acid was used as a catalyst in the pre-treatment of oil, where
the best FFA reduction condition was methanol to oil molar ratio of
6.1:1, 0.68 wt% sulfuric acid, at 51 °C with a reaction time of 60min. A
WFO (acidity of 17.41mg KOH g−1)was used by Patil et al. [104] and
the highest yields (∼90% in esters) were found using 0.5 wt% of sul-
furic acid and methanol to oil molar ratio of 6:1.
4.2. Heterogeneous catalysis
4.2.1. Alkali
Heterogeneous catalysts must have chemical stability, reusability,
high activity at ambient temperatures and affordability. Basic hetero-
geneous catalysts are generally composed of metal oxides, alkaline
zeolites and clays, and are applied in reactions that use feedstock of
high purity and low content of FFA and water. These catalysts generally
have more active sites than heterogeneous acid catalysts. The main
disadvantage of these catalysts is associated with the leaching of the
active sites after reactions, thus reducing their catalytic activity
[105,106].
Oxides of TiO2-MgO were employed as catalysts by Wen et al. [107]
in the transesterification of the WFO, where the initial acidity was
3.6 mg KOH g−1 and the water content was 1.9 wt%. The main op-
erational variables (methanol to oil molar ratio, percentage of catalyst
and temperature) were evaluated in this study. The highest ester yield
(92.3%) was achieved by using 10 wt% of catalyst (based on oil mass)
and methanol to oil molar ratio of 50:1 at 160 °C. The study also
showed that the catalyst can be reused up to four times. Several ma-
terials have been used as precursor material, such as snail shells that
were calcined and applied as a basic catalyst in the production of bio-
diesel from WFO. This catalyst contained an acidity of 1.948mg KOH
g−1, as verified in Birla et al. al. [108]. In that study, operating con-
ditions were varied in order to achieve higher yields, reaching∼87% of
esters under optimized conditions. In another study, Boey et al. [109]
obtained calcium oxide (CaO) from calcined shells and boilers ash from
agricultural waste was used as a catalyst in the transesterification of
WFO. The WFO initially had 0.86mg KOH g−1 acidity and 0.35 wt%
water. The mixture of 3 wt% of the ashes with CaO decreased the re-
action time from 3 h to 0.5 h, converting 99% of the FFA. Mixed oxides
of Ca and Zr were prepared by Dehkordi and Ghasemi [110] in various
methanol to oil molar ratios applied in the reaction of WFO with me-
thanol. The initial acidity and water concentrations were 0.98mg KOH
g−1 and 0.124 wt%, respectively. Under appropriate conditions (65 °C,
catalyst load of 10 wt%, methanol to oil molar ratio of 30:1 and 2 h of
reaction), 92.1% of esters were obtained.
Yahya et al. [67] used a titanium calcium catalyst to produce bio-
diesel from WFO. The oil had an acidity of 3.75mg KOH g−1 and
0.20 wt% of water. The effect of time, temperature, catalytic load (re-
lative to oil mass) and methanol to oil molar ratio were evaluated. The
highest ester yield (80%) was achieved using 0.2 wt% catalyst and
methanol to oil molar ratio of 3:1 for 1 h at 65 °C. Potassium im-
pregnated zinc oxide was used as a catalyst by Yadav et al. [111] in the
reaction of WFO (acidity of 1.01mg KOH g−1) with methanol. The
highest ester yield (98%) was achieved at optimized reaction condi-
tions, at catalyst loading of 2.5 wt%, methanol to oil molar ratio of
18:1, 65 °C and 50min. Jung et al. [112] used different biochars to
catalyze the transesterification of WFO, in which the oil had an acidity
of 2.85mg KOH g−1 and 0.108 wt% of water. The temperature effect
was evaluated and yield in esters greater than 95% were obtained.
Catarino et al. [113] employed various materials rich in calcium (do-
mestic wastes and collected from the beach) as a catalyst to produce
biodiesel from WFO (acidity of 2.18mg KOH g−1). The reaction was
conducted using 5 wt% catalyst (relative to oil mass), methanol to oil
molar ratio of 12:1 for 2.5 h and at reflux temperature of methanol. The
yield of the reaction was 65% of esters and the results showed relative
stability of the catalysts.
4.2.2. Acid
Heterogeneous acid catalyses are performed by inorganic, poly-
meric materials and sulfonated carbons. Solid acid heterogeneous cat-
alysts have the advantage over basic ones because they are less sensitive
to the presence of FFA and can be applied in reactions where feedstock
are of lower quality. The heterogeneous acid catalysts can be applied as
a pre-treatment to reduce the FFA content, or at higher temperatures,
simultaneously esterify the FFA and transesterify the triacylglycerols
without soap formation, reducing purification steps and forming the
purest possible glycerol. However, due to the lower activity of the acid
catalyst, higher reaction temperatures are required, increasing theprocess energy consumption. Another disadvantage is associated with
leaching of the catalyst, as already mentioned [105,114,115].
In one study, Cao et al. [116] employed a heteropolyacid as a cat-
alyst in the transesterification reaction of WFO. They found that there
was a 15.65% FFA content and 0.1 wt% water content. Both the con-
centration of the catalyst and the reaction time were evaluated; in the
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
209
best conditions, the authors obtained 87% of esters and the catalyst
showed stability for at least 5 cycles of reuse. Several solid acid cata-
lysts were used by Jacobson et al. [117] to produce biodiesel from WFO
(FFA content of 15%). According to that study, zinc stearate im-
mobilized on silica gel was the best catalyst, yielding 98% of esters
under optimal conditions. Carbohydrate-derived catalysts were in-
vestigated as catalysts in the synthesis of biodiesel from WFO by Lou
et al [118]. The catalysts were carbonized and subsequently sulfonated.
When simultaneous esterification and transesterification were per-
formed, the starch-based catalyst showed a higher yield in esters (92%).
The authors even reported the same yield after 50 cycles of catalyst
reuse. In another study, Ozbay et al. [119] used acidic ion exchange
resins in the esterification of WFO, with 0.47% of FFA. The increase in
temperature (50 to 60 °C) and the percentage of catalyst (1 to 2 wt%)
increased the conversion values. The higher FFA conversion (45.7%)
was obtained for the higher surface area catalyst and the larger average
pore diameter.
To catalyze the transesterification reaction of WFO (with a FFA
content of 15%), Komintarachat and Chuepeng [120] applied various
catalysts supported with ammonium metatungstate. Effects of tem-
perature, time, methanol to oil molar ratio and catalyst to oil were
investigated. Under best conditions, a 97.5% in esters was obtained
using an alumina carrier. The SO42-/ZrO2 superacid catalyst was used to
transesterify the WFO, which had a previously reported acidity of
81.25mg KOH g−1 [121]. The authors obtained a ester yield of 93.6%
when using a methanol to oil molar ratio of 9:1, 3 wt% catalyst, after
4 h at 120 °C. Furthermore, Lam et al. [122] studied a superacid sulfate
tin oxide (SO42-/SnO2) catalyst and the bimetallic effect with other
oxides in order to improve the catalytic effect in the transesterification
of WFO (initial acidity of 5mg KOH g−1 and 0.162 wt% of water). The
effect of the main reaction parameters were investigated. A yield of
92.3% in esters at 150 °C, 3% SO42-/SnO2-SiO2 catalyst, methanol to oil
molar ratio of 15:1 for 3 h was obtained. In another study, Li et al. [123]
used Zn1.2H0.6PW12O40 nanotubes that had Lewis and Brønsted acid
sites to stimulate the transesterification of WFO. In that study, the oil
contained 1 wt% water and acidity of 53.8mg KOH g−1. The zinc
catalyst showed a high tolerance towards the presence of water, good
catalytic performance and reusability of up to 5 times without loss of
activity or efficiency and with yield above 95% in esters.
Corro et al. [124] carried out a two-step process in which the WFO
contained 11.69% of FFA. First, the esterification of the fatty acids was
carried out with methanol catalyzed with SiO2. The experimental
variables were methanol to oil molar ratio of 30:1, temperature be-
tween 40 and 70 °C, catalyst mass ranging from 2 to 8% (oil mass ratio)
and reaction times ranging from 1 to 8 h. In the second step, a trans-
esterification was performed with methanol and NaOH. In that study,
the best condition for esterification was 4% catalyst, 70 °C and 4 h of
reaction. At the end of the two steps, 99.56% esters were obtained. In
another study, Lam and Lee [125] applied a SO42-/SnO2-SiO2 catalyst in
the transesterification of WFO (FFA content of 2.54% and water content
of 0.162wt%), evaluating the effect of the methanol/ethanol mixture
on the reaction. A mixture of methanol to ethanol to oil molar ratio of
9:6:1, 6 wt% catalyst, 150 °C and 1 h of reaction gave an 81.4% in es-
ters. The WFO conversion with a heteropolyacid catalyst using experi-
mental design was applied by Talebian-Kiakalaieh et al. [126]. In that
study, when the optimal condition was a reaction time of 14 h, tem-
perature of 65 °C, a methanol to oil molar ratio of 70:1 and 10wt%
catalyst, there was an FFA conversion of 88.6%.
Alhassan et al. [127] used nanoparticles of sulfated zirconia doped
with manganese sulfate as a catalyst in the transesterification of WFO
(FFA content of 17.5%), and the main parameters of the reaction were
evaluated. Ester yield of 96.5% was achieved at 180 °C, 600 rpm, 3 wt%
catalyst and a methanol to oil molar ratio of 20:1. The catalyst was
reusable for up to six times without any loss in catalytic activity. In
addition, Tran et al. [128] used sulfonated carbon as the catalyst for
WFO (acidity of 2.7mg KOH g−1). The catalyst mass ranged from 5 to
15% (by weight of oil), with a temperature 90 to 150 °C, a reaction time
0.5 to 6 h, and a methanol to oil molar ratio of 9.35:1. The optimal
conditions for that reaction were a temperature of 110 °C, a reaction
time of 2 h, 10 wt% of catalyst. Those conditions allowed for a yield of
89.6% in esters, and in that reaction, the WFO did not receive any type
of pre-treatment. Ahmad et al. [129] obtained an char-based acidic and
applied in the WFO (6 wt% of FFA) catalysis. The optimum conditions
were 6 wt% catalyst, methanol to oil molar ratio of 9:1, 65 °C for
130min, yielding 96% in esters. After being reused for 5 times, the
catalyst still showed a ester yield of 81%.
4.2.3. Enzymatic
The production of biodiesel via enzymatic catalysis has been con-
sidered an ecologically correct and efficient route. The process has
several advantages, including conversion of feedstock of low quality
(high FFA content) and lower energy consumption. However, this type
of catalysis has not been used on an industrial scale because of its high
enzymatic cost, which is much higher than basic homogeneous catalysis
in terms of alcohol deactivation of enzymes and conversion efficiency
[75,130].
The immobilized Candida lipase was studied by Chen et al. [131] as
a catalyst in the biodiesel production from WFO (acidity of 143.64mg
KOH g−1). The effects of lipase percentage, solvent, water, temperature
and reaction mixture flux were investigated. The higher yield condition
(91.08% of esters) was composed lipase to hexane to water to oil of
25:15:10:100, at 45 °C in a flow of 1.2ml min−1. The authors further
reported a yield in esters loss of∼16% that after 100 h of continuous
reaction. Maceiras et al. [132] used Novozym 435 lipase to catalyze
transesterification of WFO (0.04 wt% water and acidity of 1.35mg KOH
g−1). The optimal condition was 10% enzyme (based on oil weight), a
methanol to oil molar ratio of 25:1, 4-hour reaction time held at 50 °C,
yielding 89.1% in esters. Furthermore, the Novozym 435 lipase was
used by Azócar et al. [133] in order to catalyze transesterification re-
actions with percentage of oil ranged from 0 to 100%, temperature of
35 to 55 °C, enzyme concentration of 3–15% (relative to the oil) and the
methanol to oil molar ratio of 1.5:1 to 4.5:1. The optimal condition was
methanol to oil molar ratio of 3.8:1, 45 °C, 15 wt% of lipase and 100 wt
% oil, for 12 h at 200 rpm, obtaining approximately 100% esters.
Gharat and Rathod [66] also used Novozym 435 lipase in the catalysis
of WFO (FFA content of 2.402%) under the influence of ultrasonic ra-
diation with dimethyl carbonate (DMC). That study evaluated stirring
conditions without ultrasonic radiation, radiation without agitation and
radiation plus agitation in addition to parameters such as temperature,
stirring speed, enzymatic loading and the DMC to oil molar ratio. They
reported that both shaking and ultrasonic radiation increased the yield
in esters 86.61%. Razack and Duraiarasan [134] used two bacterial
species to produce lipase, which worked withthe catalyst in the in-
teresterification of WFO (acidity of 1.68mg KOH g−1). The catalyst
mass varied from 1.5 to 2.5 g, with an methyl acetate to oil molar ratio
of 10:1 to 14:1, a temperature of 30 to 40 °C and a reaction time of 48 to
72 h. The optimum condition was 2 g of catalyst, methyl acetate to oil
molar ratio of 12:1, 60 h of reaction at 35 °C, yielding 93.61% in esters.
4.2.4. Ionic liquid
The first application of ionic liquids (ILs) in biodiesel synthesis was
performed by Wu et al. [135]. ILs are salts that are in the liquid phase at
temperatures below 100 °C, have low volatility, excellent chemical and
thermal stability and high catalytic activity. They have the advantages
of being reused and easily separated from the reaction medium. Ionic
liquids are also capable of simultaneously promoting FFA esterification
and transesterification of triacylglycerols [9,136]. Their main dis-
advantages are related to the high temperatures used with some ionic
liquids and the possibility of deactivation due to the glycerol formed
[77].
Han et al. [137] prepared a Brønsted acidic ionic liquid with an
alkane sulfonic acid group as catalysts in the synthesis of biodiesel from
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
210
WFO (acidity of 100mg KOH g−1). The effects of the methanol to oil to
catalyst molar ratio, temperature and time were evaluated. Higher
temperatures increased yields and the best methanol to oil to catalyst
molar ratio was 12:1:0.06 for 4 h, obtaining 93.5% esters. In another
study, an acidic polymeric ionic liquid was used by Liang [138] for
simultaneous esterification and transesterification of residual oil
(acidity of 45mg KOH g−1). The effect of the methanol to oil molar
ratio with a fixed amount of catalyst and reaction time was in-
vestigated. The ester yield was close to 99% under the best operating
conditions. Another study, Yassin et al. [139] used various ionic liquids
of imidazolium chloride to catalyze the transesterification of WFO,
obtaining 97% of esters with a catalyst to oil ratio of 1:10 for 8 h and
55 °C. The catalyst presented good stability after 8 cycles of use, with
high yields. Three acid ionic liquids were evaluated by Ullah et al.
[140] in the pre-treatment to reduce the acidity of the residual oil
(acidity of 4.03mg KOH g−1 and 0.140wt% of water), before carrying
out a KOH catalysis. The effect of catalyst concentration (3–7.5 wt%),
temperature (80–180 °C), stirring (100–700 rpm), methanol to oil molar
ratio (3:1–18:1) and time (30–120min) were investigated in the es-
terification. BMIMHSO4 was found to be the best catalyst because of the
longer chain.
4.3. Noncatalytic with alcohol at pressurized conditions
The synthesis of biodiesel using an alcohol under pressurized con-
ditions and without the use of catalyst was initially proposed by Saka
and Kusdiana [26]. Due to the absence of catalysts, the method is tol-
erant to the presence of water and FFA, as reactions of hydrolysis, es-
terification and transesterification occur simultaneously. In addition,
the feedstock used may be of poor quality. The reactions are complete
in minutes and there is a decrease in the mass transfer limitations since
there is a better solubility between the phases (oil and alcohol) in re-
lation to the other methods, which raises reaction rates [26,141,142].
The main disadvantage of this method is high energy consumption
(2407 kW for 10,000 tons/year), due to high temperatures and pres-
sures used in the process, which increases the final cost of the produced
biodiesel [82,143].
Obtaining biodiesel from WFO with supercritical methanol without
a catalyst was studied by Tan et al. [144]. The reaction parameters
investigated were reaction time, temperature and the methanol to oil
molar ratio; the yield was compared to the yield of a refined oil. A yield
of 80% in esters was obtained under the best conditions for both the
residual oil and the refined oil. Gonzalez et al. [62] investigated the
production of biodiesel using WFO (acid value of 1.5 mg KOH g−1 and a
water content of 0.62 wt%), without a catalyst, but with methanol and
ethanol. Temperature, pressure, alcohol to oil molar ratio and addition
of water were explored by the researchers. Esters yield above 80% were
obtained for the two alcohols studied, though ethanol presented better
results. In the work of Ghoreishi and Moein [145], WFO (with FFA and
water content of 5.67% and 0.2, respectively) were used as the sub-
strate in the reaction with supercritical methanol and CO2 as co-solvent.
The main operational parameters were applied and 95.27% of esters
were obtained under the best conditions (methanol to oil molar ratio of
33.8:1, 271.1 °C, 23.1 MPa and 20.4 min of reaction time). Abdala et al.
[63] evaluated the production of biodiesel using WFO (acid value of
3.57mg KOH g−1 and water content of 0.03%) under supercritical
conditions without catalyst. The operating conditions of pressure
(20MPa), residence time (40min) and equal ethanol to oil mass ratio,
were kept constant, and the addition of water, co-solvent (n-hexane)
and ethyl esters at different temperatures were studied. The authors
reported that an increase in temperature, addition of 5 wt% water,
addition of 20 wt% n-hexane co-solvent and addition of up to 40 wt% of
ethyl esters are favorable to the process. The optimal conditions, which
include an ethanol to oil mass ratio of 1:1, 300 °C, 20MPa, 70min, and
20wt% co-solvent, yielded∼ 87% of esters. Aboelazayem [146] stu-
died the production of biodiesel with supercritical methanol withoutTa
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J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
211
catalyst using WFO (acidity of 0.8mg KOH g−1). An experimental de-
sign was applied to evaluate the influence of temperature, pressure,
residence time and the methanol to oil molar ratio. The highest yield in
esters (91%) was found with the following conditions: a methanol to oil
molar ratio of 37:1, 253.5 °C, 198.5 bar and 14.8 min of reaction. The
molar ratio and temperature had the most significant effect on the re-
action process. Fonseca et al. [147] employed the hydroesterification
route in a pressurized medium without catalyst; the method includes
hydrolyzing the triacylglycerols with subcritical water and subsequentesterification of the FFA with ethanol in a supercritical state to produce
biodiesel from WFO (1.63% FFA and 0.12 wt% water). In both stages,
the effect of temperature and residence time were evaluated. The
higher temperature of the hydrolysis (320 °C) obtained the highest FFA
(∼96%) in 12min. Regarding the esterification, 74% of esters were
obtained at 300 °C after 10min.
Table 4 presents those works in which biodiesel was produced from
WFO and analyses were conducted of certain specifications that were
needed in order to meet the required standards. According with this
table, it is possible to observe that homogeneous catalyzed reactions are
influenced by high FFA levels, reducing the final yield. The other
methods are little influenced by the presence of water and FFA.
4.4. Patent overview on biodiesel production from WFO
Table 5 presents the published patents in which WFO have been
employed as substrate of the reaction.
According to the data presented in Table 5, most patents that use
WFO as feedstock in biodiesel production were developed and pub-
lished in the last 10 years. Those various patents had different pro-
duction and different types of catalysis, including the methods de-
scribed above. In one patented study, Pan et al. [154] employed a two-
step method with solid and basic solid catalysts composed of inorganic
material, low cost and high catalytic efficiency, reducing production
costs. The acidic solid catalyst (1.5–4%) was added to the reaction
medium along with WFO and methanol, with a reaction time ranging
from 2 to 5 h at a temperature of 60 to 80 °C, to decrease the acidity of
the oil. The oily phase was collected and reacted with the basic solid
catalyst (also 1.5 to 4%); this reaction lasted between 1.5 and 2.5 h at
60 to 80 °C. In another study, Saidina [156] also employed an acidic
solid catalyst to reduce the acidity of the oil and then applied a basic
solid catalyst to complete the reaction. Solid acid and basic catalysts
have been used by other researchers due to their advantageous re-
activity with high acid oils, which may esterify and transesterify the
reaction either as a pre-treatment or in the reduction of the acidity of
WFO; these types of catalysis have been seen in the Chenglai and
Xiaona [160], Chuanfu et al. [168] and Li et al. [169] patents. The use
of enzymes as a catalyst is also presented in the patent presented by
Rethore et al. [165]. The enzymes that were used in that study were not
inhibited by FFA and water and esterified the FFA present and trans-
esterified the glycerides. In the system used, a membrane separator
separates the glycerine, alcohol and formed water; the glycerine can
then be burned for power generation to maintain the reaction.
5. Purification stage
Various standards must be observed in order to maintain quality
standards for the final product to be considered biodiesel. Biodiesel
must not be contaminated with impurities that could damage engines
by accumulating in the nozzles and forming incrustations that lead to
corrosivity. Such impurities come from unsaponifiable materials pre-
sent in the feedstock itself, including catalyst residues, water, glycerol
and excess alcohol from the reaction. When biodiesel is obtained from
WFO, other compounds will be present, such as polar compounds, di-
mers, mono and diacylglycerols and FFA [170]. Impurities from the
WFO, generally solid, are removed by centrifugation and/or filtration
before the biodiesel production reaction [171]. These impurities need
to be removed as a way to maintain the quality of biodiesel. There are
several methods for removing such impurities, such as membranes
[172], distillation [173] and wet washing and dry washing [174]. The
work sought to deepen the technique of wet washing, one of the main
techniques used in the industry today and the dry washing (adsorption)
that has been investigated in the last years.
5.1. Wet washing
Wet washing is the traditional method for removing impurities from
biodiesel. Although the efficiency of the wet washing process is proven,
the method has disadvantages, including the generation of con-
taminated liquid effluents, the considerable loss of product and the
formation of emulsions when biodiesel is produced from waste oils with
high water content and FFA [174].
Wet washing consists of adding a fixed amount of water with gentle
agitation to avoid the formation of an emulsion. The process is repeated
until the water is colorless, indicating the complete removal of im-
purities. The three main types of wet washing employ either deionized
water, an aqueous 5% phosphoric acid solution, and a mixture of an
organic solvent and water [159]. The presence of an acid in the wash is
beneficial in that it aims to neutralize the catalyst and decompose the
soap formed. After this process, the biodiesel is washed with water to
remove the remaining impurities [175].
Predojevic [30] investigated the purification of biodiesel obtained
from WFO using two washing methods: (a) use of a 5% phosphoric acid
Table 5
Published patents on biodiesel production using waste frying oils.
Patent Title Year Reference
CN 101696372 (A) Method for preparing biodiesel by solid acid-base two-step method 2010 [154]
JP 2010106065 (A) Method for promoting the use of waste cooking oil for biodiesel fuel 2010 [155]
MY 145698 (A) A process for producing biodiesel from waste cooking oil 2012 [156]
CN 102698813 (A) Method for preparing multifunctional solid superacid catalyst and method using waste cooking oil as raw material to synthesize
biodiesel
2012 [157]
CN 103421615 (A) Technology for producing biodiesel through illegal cooking oil or waste cooking oil 2013 [158]
CN 102925295 (A) Method for preparing biodiesel from waste cooking oil 2013 [159]
CN 103571637 (A) Method for preparing biodiesel from waste cooking oil 2014 [160]
CN 104164304 (A) Novel method for preparing biodiesel under catalysis of modified resin 2014 [161]
US 2014/318631 (A1) Methods and systems for converting food waste oil into biodiesel fuel 2014 [162]
CN 103627742 (A) Method for preparing biodiesel through conversion of waste cooking oil by utilization of immobilized lipase 2014 [163]
CN 104099184 (A) Method utilizing novel solid alkali catalysts for preparing biodiesel 2014 [164]
US 2015/0031097 (A1) Mobile processing systems and methods for producing biodiesel fuel from waste oils 2015 [165]
CN 105985868 (A) Method for producing novel environmental-protection biodiesel 2016 [166]
CN 105567436 (A) Method for preparing biodiesel by catalyzing high-acid-value waste cooking oil and detection method 2016 [167]
CN 107488519 (A) Method for preparing biodiesel through catalyzing waste cooking oil by magnetic carbon-supported acid-base 2017 [168]
CN 107513472 (A) Biodiesel preparation method 2017 [169]
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
212
solution and (b) use of hot distilled water. After the purification
treatment, the biodiesel followed the established norms on density,
kinematic viscosity, acidity and iodine concentration. The phosphoric
acid wash yielded 92% esters, while the hot distilled water yielded a
maximum of only 89%. In another study, Sabudak and Yildiz [148]
produced biodiesel with WFO and evaluated the use of hot water to
remove impurities. Despite removing impurities and increasing ester
yield at the end of the washing, the products were below the minimum
(96.5%) required to be classified as biodiesel. Hingu et al. [90] used a
5% phosphoric acid solution to purify biodiesel obtained from WFO.
The ester content increased from 89.5 to 93.5% after washing. Distilled
water and tap water were studied in the purification of biodiesel ob-
tained fromWFO by Berrios et al. [176]. The results were similar for the
two washes, with an increase in water content and FFA. Soap, glycerol
and methanol were efficiently removed by the methods after two wa-
shes. Manique et al. [33] used a 1% phosphoric acid solution and wa-
shed the sample twice with hotwater. The method was efficient in
removing the impurities evaluated. However the water content at the
end of the process was higher than in crude biodiesel.
5.2. Dry washing
Dry washing is a technique that has been used regularly in industrial
plants. The process consists of the application of adsorbents, such as
Magnesol®, Amberlite®, Purolite® and activated carbon, which have
acidic and basic sites and attract polar substances, including glycerol
and alcohol [32]. The adsorption process is the accumulation on the
surface of a certain material (adsorbate) on the surface of a solid (ad-
sorbent) [177].
Magnesol® was developed specifically for biodiesel adsorption. It is
a synthetic commercial magnesium silicate and has shown good per-
formance in the removal of impurities [178]. Ionic exchange resins such
as Amberlite® and Purolite® are organic, insoluble and developed to
adsorb the maximum amount of the impurities found in crude biodiesel
[34].
In Sabudak and Yildiz, a Magnesol® and Purolite® ion exchange
resin was used to remove impurities from biodiesel obtained from WFO
[148]. The impurities were removed, and the ester yield after pur-
ification in the two-step transesterification process with acid-base cat-
alysis was above the minimum required by the standard. Furthermore,
silica gel with anhydrous sodium sulfate was used by Hingu et al. [90].
The percentage of esters was 94.5% after purification. In another study,
Paula et al. [170] produced biodiesel from WFO by conventional
transesterification and performed dry washing using bauxite (alu-
minum oxide compound), attapulgite and bentonite (silicon dioxide).
Parameters such as glycerol, water, acidity and soap were evaluated.
Bauxite stood out in the removal of glycerol, and three adsorbent ma-
terials were efficient in the removal of soap.
In similar research, Berrios et al. [176] used Magnesol® and ben-
tonite to remove biodiesel impurities from WFO. Both the adsorbent
agitation and dosage process were varied in adsorption assays to de-
termine their influences on the purification. The main parameters of
biodiesel standards were analyzed, with no effect on viscosity, density,
glycerides and ester content. Manique et al. [33] used WFO to produce
biodiesel and remove impurities with Magnesol®. These impurities,
such as glycerol, potassium, water, methanol, were removed. These
same authors tested the ashes of the rice husk as an adsorbent, obser-
ving that it efficiently reduces the acidity, water, methanol and gly-
cerol. Furthermore, activated carbons obtained from tea residues were
used by Fadhil et al. [179] for adsorption of impurities from biodiesel
obtained from WFO. The authors found that the activated carbon was
more efficient in removing impurities than simply washing with water
and silica gel; the quality of the biodiesel after purification was also
better. In another study, Farid et al. [180] carried out the adsorption
using a bundle-derived biosorbent in palm oil; the dosage of adsorbent
in the process was evaluated and the method was compared with
commercial adsorbents and washing with water. The use of 5% (wt/wt)
of biosorbent presented higher rates of removal of FFA, potassium,
water and a lower loss of esters when compared to other methods.
Using columnar coco coir fiber, Ott et al. [181] removed impurities
from biodiesel obtained from WFO. The authors evaluated the ability of
the adsorbent to remove soaps, methanol, FFA, calcium, magnesium
and glycerol. The material proved to be efficient in removing all im-
purities. Fonseca et al. [147] used sugarcane bagasse ash to remove FFA
and polar compounds from biodiesel produced from WFO. The re-
searchers investigated the effects of time (1–24 h), percentage of ad-
sorbent (2.5–15%) and the effect of successive beds on the removal of
impurities. The equilibrium conditions were found by using 12.5% ash
for 8 h. At the end of the 4th bed, ∼44% of FFA and 73% of polar
compounds were removed.
Agroindustrial waste can be an alternative to commercial ad-
sorbents, because it has high availability, low cost and good adsorption
capacity [182–184]. However, there are limited studies reported in the
literature and, for the most part, biodiesel is obtained from refined oils
[35,185,186,187,188].
In addition to the purification treatment after the reaction, a pre-
treatment can be carried out in the WFO prior to the reaction. As al-
ready mentioned, some impurities, such as solids, are removed by
centrifugation and/ or filtration prior to the production of the biodiesel.
In the last years, some researchers applied the adsorption method, in
which the materials are agroindustrial residues, to reduce the acidity of
the oil before the reaction. Ali and Anany [189] used sugarcane bagasse
ash to regenerate the quality of the WFO. Several parameters of the oil
were investigated. The acidity reduced from 0.88 to 0.15mg KOH g−1
after treatment. Bonassa et al. [190] also used sugarcane bagasse ash in
the treatment of WFO. Was applied an experimental design, in which a
reduction of 68% of the acidity was observed under the best conditions.
Rice husk was used by Schneider et al. [191] in the treatment of WFO.
At the optimal process conditions, 22.4 °C, 80.36 rpm and 1.61 g of
adsorbent, a 63% reduction of acidity was observed. In the work of
Miyashiro et al. [192] the adsorption potential of bentonite and su-
garcane bagasse clay was evaluated to reduce the FFA of the WFO. The
sugarcane bagasse clay had a greater reduction capacity (58%) than
bentonite (50%).
5.3. Patent overview on biodiesel purification
Table 6 presents patented biodiesel purification works, using dif-
ferent methods to remove impurities.
In relation to the purification of biodiesel obtained from WFO, there
are few reports, such as in the Takaka [193], Perchtoldsdorf [194] and
Gurski et al. [200] patents, which indicated that the subject is still
poorly investigated and of minimal interest in related research. In those
two reported patents, the authors employed the use of dry washing as a
purification method.
The use of water washing has declined in patent research. Most
studies have employed dry washing (adsorbents) as a means of pur-
ification and removal of impurities present after the reaction. The ad-
sorbent materials used are formed from different types of organic and
inorganic compounds. Munson et al. [195] reports a purification pro-
cess that uses a column packed with adsorbent material (silica, carbon,
clay, among others) to remove impurities, so that the eluted biodiesel is
ready for the recovery of the alcohol used in the process. The adsorbent
is then regenerated and reused. In another study, Sohling [197] pa-
tented an adsorbent containing approximately 40 wt% aluminum oxide
for purification of crude biodiesel. Furthermore, Wang et al. [205]
applied an adsorbent material composed of wood and molecular sieves
to remove glycerol from biodiesel. Melde et al. [206] used a meso-
porous organosilicon material to remove glycerol and detergents from
crude biodiesel. A pressurized system with carbon dioxide has also been
patented by Ndiaye et al. [202] for the purpose of purifying crude
biodiesel, in which 10 to 50% of CO2 is injected into the system after
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
213
the reaction. In this system, the pressure is then ramped up from 5 to
20MPa at ambient temperature, forming two phases, where the upper
phase would consist only of esters and alcohol and the lower phase
would only contain impurities.
6. Future perspectives
Biodiesel production, applying WFO as substrate, and purification
processes of the same biodiesel were described in this work. The use of
WFO have numerous advantages over refined oils, yet further studies
that evaluate the technique and limitations of WFO must be performed.
For these studies there is a need to evaluate: i) the identification of the
compounds generated during the frying process, ii) the analysis of
convertibilityof WFO and iii) an investigation of the best catalytic or
non-catalytic reaction methods. Regarding purification methods, the
dry washing process excels over the conventional process. However, it
is necessary to further investigate the proposed technique in relation to
lower cost materials and greater process efficiency. The analysis of
these points would be important in determining both the efficiency of
using WFO as a substrate in the production of biodiesel and whether the
dry washing process can be used as a substitute for washing with water
in the purification processes.
In 2013 two revision works were carried out regarding the use of
WFO for biodiesel production [6,8]. Talebian-Kiakalaieh et al. [6]
presented a perspective regarding the use of heterogeneous acidic and
basic catalysts, due to its main property being reuse in order to make
production of biodiesel more economical. In the work of Yaakob et al.
[8] the perspective was of an increase in the use of WFO in the pro-
duction of biodiesel, along with pretreatment processes that reduced
the water and FFA contents. The development of new catalysts that
improved the reaction process was also challenged. It was verified in
this review that, after 5 years, the development of heterogeneous cat-
alysts, both acidic and basic, occurred in order to improve their prop-
erties, especially with regard to reuse. Catalysts obtained from lower
cost sources, waste, have been investigated, and reuse studies have
shown a good stability of developed catalysts. WFO continues to be
studied by the scientific community over the last five years, in which
WFO pretreatment processes and post-reaction purification processes
have been the subject of research, in which studies of a simple and
inexpensive method have been presented, adsorption, which has been
shown to be efficient.
Acknowledgements
The authors are grateful to CAPES (process: 88887.286823/2018-
00) for financial support.
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
References
[1] Baskar G, Aberna Ebenezer Selvakumari I, Aiswarya R. Biodiesel production from
castor oil using heterogeneous Ni doped ZnO nanocatalyst. Bioresour Technol
2018;250:793–8. https://doi.org/10.1016/j.biortech.2017.12.010.
[2] Kirubakaran M, Arul Mozhi Selvan V. A comprehensive review of low cost bio-
diesel production from waste chicken fat. Renew Sustain Energy Rev
2018;82:390–401. https://doi.org/10.1016/j.rser.2017.09.039.
[3] Sander A, Antonije Košćak M, Kosir D, Milosavljević N, Parlov Vuković J, Magić L.
The influence of animal fat type and purification conditions on biodiesel quality.
Renew Energy 2018;118:752–60. https://doi.org/10.1016/j.renene.2017.11.068.
[4] Jamil F, Al-Muhatseb AH, Myint MTZ, Al-Hinai M, Al-Haj L, Baawain M, et al.
Biodiesel production by valorizing waste Phoenix dactylifera L. Kernel oil in the
presence of synthesized heterogeneous metallic oxide catalyst (Mn@MgO-ZrO2).
Energy Convers Manag 2018;155:128–37. https://doi.org/10.1016/j.enconman.
2017.10.064.
[5] Mansir N, Teo SH, Rashid U, Saiman MI, Tan YP, Alsultan GA, et al. Modified waste
egg shell derived bifunctional catalyst for biodiesel production from high FFA
waste cooking oil. A review. Renew Sustain Energy Rev 2018;82:3645–55. https://
doi.org/10.1016/j.rser.2017.10.098.
[6] Talebian-Kiakalaieh A, Amin NAS, Mazaheri H. A review on novel processes of
biodiesel production from waste cooking oil. Appl Energy 2013;104:683–710.
https://doi.org/10.1016/j.apenergy.2012.11.061.
[7] Associação Brasileira de Bares e Restaurantes (ABRASEL) 2011. http://www.
abrasel.com.br/noticias/684-290811-reciclar-oleo-e-garantia-de-lucro-nagrande-
bh-.html.
[8] Yaakob Z, Mohammad M, Alherbawi M, Alam Z, Sopian K. Overview of the pro-
duction of biodiesel from Waste cooking oil. Renew Sustain Energy Rev
2013;18:184–93. https://doi.org/10.1016/j.rser.2012.10.016.
[9] Farooq M, Ramli A, Naeem A. Biodiesel production from low FFA waste cooking oil
using heterogeneous catalyst derived from chicken bones. Renew Energy
2015;76:362–8. https://doi.org/10.1016/j.renene.2014.11.042.
[10] Atapour M, Kariminia HR, Moslehabadi PM. Optimization of biodiesel production
by alkali-catalyzed transesterification of used frying oil. Process Saf Environ Prot
2014;92:179–85. https://doi.org/10.1016/j.psep.2012.12.005.
[11] Vieitez I, Callejas N, Irigaray B, Pinchak Y, Merlinski N, Jachmanián I, et al. Acid
value, polar compounds and polymers as determinants of the efficient conversion
of waste frying oils to biodiesel. JAOCS, J Am Oil Chem Soc 2014;91:655–64.
https://doi.org/10.1007/s11746-013-2393-y.
[12] Mittelbach M, Enzelsberger H. Transesterification of heated rapeseed oil for ex-
tending diesel fuel. JAOCS, J Am Oil Chem Soc 1999;76:545–50. https://doi.org/
10.1007/s11746-999-0002-x.
[13] Ruiz-Méndez MV, Marmesat S, Liotta a, Dobarganes MC. Analysis of used frying
fats for biodiesel production. Grasas Y Aceites 2008;59:45–50. https://doi.org/10.
3989/gya.2008.v59.i1.489.
[14] Can Ö. Combustion characteristics, performance and exhaust emissions of a diesel
engine fueled with a waste cooking oil biodiesel mixture. Energy Convers Manag
2014;87:676–86. https://doi.org/10.1016/j.enconman.2014.07.066.
[15] Bouaid A, Vázquez R, Martinez M, Aracil J. Effect of free fatty acids contents on
biodiesel quality. Pilot plant studies. Fuel 2016;174:54–62. https://doi.org/10.
1016/j.fuel.2016.01.018.
[16] Manuale DL, Torres GC, Vera CR, Yori JC. Study of an energy-integrated biodiesel
production process using supercritical methanol and a low-cost feedstock. Fuel
Process Technol 2015;140:252–61. https://doi.org/10.1016/j.fuproc.2015.08.
026.
[17] Talukder MMR, Wu JC, Chua LPL. Conversion of waste cooking oil to biodiesel via
enzymatic hydrolysis followed by chemical esterification. Energy Fuels
2010;24:2016–9. https://doi.org/10.1021/ef9011824.
[18] Canakci M, Gerpen J Van. Biodiesel Production Via Acid Catalysis. Am Soc Agric
Table 6
Published patents on biodiesel purification.
Patent Title Year Reference
US 2008/0318763 (A1) System for production and purification of biofuel 2008 [193]
US 2009/0049741 (A1) Biodiesel purification method and system 2009 [194]
US 2009/0199460 (A1) Biodiesel purification by a continuous regenerable adsorbent process 2009 [195]
WO 2009/132670 (A1) Process for removing steryl glycosides from biodiesel 2009 [196]
WO 2010/057660 (A1) Aluminum oxide-containing adsorbents for the purification of biodiesel 2010 [197]
CN 102533442 (A) Method for purifying biodiesel 2012 [198]
WO 2012/004489 (A1) Method for purifying a fatty-acid alkyl ester by liquid/liquid extraction 2012 [199]
US 8192696 (B2) System and Process of Biodiesel Production 2012 [200]
US 2014/0000155 (A1) Process for purification of biodiesel and biodiesel obtained by sad process 2014 [201]
WO 2014094093 (A1) Method for separating and/or purifying biodiesel using pressurized carbon dioxide 2014 [202]
CN 104531350 (A) Biodiesel purification method 2015 [203]
WO 2016/098025 (A1) Process for the purification of biodiesel 2016 [204]
CN 104119948 (B) Method for removing impurities from biodiesel 2016 [205]
US 9777233 (B1) Sorbent design for improved glycerol adsorption 2017 [206]
EP 3305877 (A1) Process for purifying biodiesel 2018 [207]
J.M. Fonseca et al. Energy Conversion and Management 184 (2019) 205–218
214
https://doi.org/10.1016/j.biortech.2017.12.010
https://doi.org/10.1016/j.rser.2017.09.039
https://doi.org/10.1016/j.renene.2017.11.068
https://doi.org/10.1016/j.enconman.2017.10.064
https://doi.org/10.1016/j.enconman.2017.10.064
https://doi.org/10.1016/j.rser.2017.10.098
https://doi.org/10.1016/j.rser.2017.10.098
https://doi.org/10.1016/j.apenergy.2012.11.061
http://www.abrasel.com.br/noticias/684-290811-reciclar-oleo-e-garantia-de-lucro-nagrande-bh-.html
http://www.abrasel.com.br/noticias/684-290811-reciclar-oleo-e-garantia-de-lucro-nagrande-bh-.html
http://www.abrasel.com.br/noticias/684-290811-reciclar-oleo-e-garantia-de-lucro-nagrande-bh-.htmlhttps://doi.org/10.1016/j.rser.2012.10.016
https://doi.org/10.1016/j.renene.2014.11.042
https://doi.org/10.1016/j.psep.2012.12.005
https://doi.org/10.1007/s11746-013-2393-y
https://doi.org/10.1007/s11746-999-0002-x
https://doi.org/10.1007/s11746-999-0002-x
https://doi.org/10.3989/gya.2008.v59.i1.489
https://doi.org/10.3989/gya.2008.v59.i1.489
https://doi.org/10.1016/j.enconman.2014.07.066
https://doi.org/10.1016/j.fuel.2016.01.018
https://doi.org/10.1016/j.fuel.2016.01.018
https://doi.org/10.1016/j.fuproc.2015.08.026
https://doi.org/10.1016/j.fuproc.2015.08.026
https://doi.org/10.1021/ef9011824
Eng 1999;42:1203–10. https://doi.org/10.13031/2013.13285.
[19] Helwani Z, Othman MR, Aziz N, Fernando WJN, Kim J. Technologies for pro-
duction of biodiesel focusing on green catalytic techniques: a review. Fuel Process
Technol 2009;90:1502–14. https://doi.org/10.1016/j.fuproc.2009.07.016.
[20] Mbaraka IK, Shanks BH. Conversion of Oils and Fats Using AdvancedMesoporous
Heterogeneous Catalysts. J Am Oil Chem Soc 2006;83:79–91.
[21] Semwal S, Arora AK, Badoni RP, Tuli DK. Biodiesel production using hetero-
geneous catalysts. Bioresour Technol 2011;102:2151–61. https://doi.org/10.
1016/j.biortech.2010.10.080.
[22] Dizge N, Aydiner C, Imer DY, Bayramoglu M, Tanriseven A, Keskinler B. Biodiesel
production from sunflower, soybean, and waste cooking oils by transesterification
using lipase immobilized onto a novel microporous polymer. Bioresour Technol
2009;100:1983–91. https://doi.org/10.1016/j.biortech.2008.10.008.
[23] Shimada Y, Watanabe Y, Sugihara A, Tominaga Y. Enzymatic alcoholysis for
biodiesel fuel production and application of the reaction to oil processing. J Mol
Catal - B Enzym 2002;17:133–42. https://doi.org/10.1016/S1381-1177(02)
00020-6.
[24] Andreani L, Rocha JD. Use of ionic liquids in biodiesel production: a review.
Brazilian J Chem Eng 2012;29:1–13. https://doi.org/10.1590/S0104-
66322012000100001.
[25] Zhang P, Liu Y, Fan M, Jiang P. Catalytic performance of a novel amphiphilic
alkaline ionic liquid for biodiesel production : influence of basicity and con-
ductivity. Renew Energy 2016;86:99–105. https://doi.org/10.1016/j.renene.
2015.08.008.
[26] Saka S, Kusdiana D. Biodiesel fuel from rapeseed oil as prepared in supercritical
methanol. Fuel 2001;80:225–31. https://doi.org/10.1016/S0016-2361(00)
00083-1.
[27] Kusdiana D, Saka S. Effects of water on biodiesel fuel production by supercritical
methanol treatment. Bioresour Technol 2004;91:289–95. https://doi.org/10.
1016/S0960-8524(03)00201-3.
[28] Quesada-Medina J, Olivares-Carrillo P. Evidence of thermal decomposition of fatty
acid methyl esters during the synthesis of biodiesel with supercritical methanol. J
Supercrit Fluids 2011;56:56–63. https://doi.org/10.1016/j.supflu.2010.11.016.
[29] Karaosmanoǧlu F, Cıǧızoǧlu KB, Tüter M, Ertekin S. Investigation of the Refining
Step of Biodiesel Production. Energy Fuels 1996;10:890–5. https://doi.org/10.
1021/ef9502214.
[30] Predojević ZJ. The production of biodiesel from waste frying oils: A comparison of
different purification steps. Fuel 2008;87:3522–8. https://doi.org/10.1016/j.fuel.
2008.07.003.
[31] Brito JF, Oliveira Ferreira L, Silva JP, Ramalho TC. Tratamento da água de
purificação do biodiesel utilizando eletrofloculação. Quim Nova 2012;35:728–32.
https://doi.org/10.1590/S0100-40422012000400014.
[32] Atadashi IM. Purification of crude biodiesel using dry washing and membrane
technologies. Alexandria Eng J 2015;54:1265–72. https://doi.org/10.1016/j.aej.
2015.08.005.
[33] Manique MC, Faccini CS, Onorevoli B, Benvenutti EV, Caramão EB. Rice husk ash
as an adsorbent for purifying biodiesel from waste frying oil. Fuel 2012;92:56–61.
https://doi.org/10.1016/j.fuel.2011.07.024.
[34] Faccini CS, Da Cunha ME, Moraes MSA, Krause LC, Manique MC, Rodrigues MRA,
et al. Dry washing in biodiesel purification: a comparative study of adsorbents. J
Braz Chem Soc 2011;22:558–63. https://doi.org/10.1590/S0103-
50532011000300021.
[35] Alves MJ, Cavalcanti ÍV, de Resende MM, Cardoso VL, Reis MH. Biodiesel dry
purification with sugarcane bagasse. Ind Crops Prod 2016;89:119–27. https://doi.
org/10.1016/j.indcrop.2016.05.005.
[36] Proctor A, Palaniappan S. Adsorption of soy oil free fatty acids by rice hull ash. J
Am Oil Chem Soc 1990;67:15–7. https://doi.org/10.1007/BF02631381.
[37] Rovani S, Censi MT, Pedrotti SL, Lima ÉC, Cataluña R, Fernandes AN. Development
of a new adsorbent from agro-industrial waste and its potential use in endocrine
disruptor compound removal. J Hazard Mater 2014;271:311–20. https://doi.org/
10.1016/j.jhazmat.2014.02.004.
[38] Imtenan S, Varman M, Masjuki HH, Kalam MA, Sajjad H, Arbab MI, et al. Impact of
low temperature combustion attaining strategies on diesel engine emissions for
diesel and biodiesels: a review. Energy Convers Manag 2014;80:329–56. https://
doi.org/10.1016/j.enconman.2014.01.020.
[39] Shah S, Sharma S, Gupta MN. Enzymatic transesterification for biodiesel produc-
tion. Indian J Biochem Biophys 2003;40:392–9. https://doi.org/10.1039/
C6RA08062F.
[40] Berrios M, Martín MA, Chica AF, Martín A. Study of esterification and transes-
terification in biodiesel production from used frying oils in a closed system. Chem
Eng J 2010;160:473–9. https://doi.org/10.1016/j.cej.2010.03.050.
[41] Su CH. Kinetic study of free fatty acid esterification reaction catalyzed by re-
coverable and reusable hydrochloric acid. Bioresour Technol 2013;130:522–8.
https://doi.org/10.1016/j.biortech.2012.12.090.
[42] Santana HS, Tortola DS, Reis ÉM, Silva JL, Taranto OP. Transesterification reaction
of sunflower oil and ethanol for biodiesel synthesis in microchannel reactor: ex-
perimental and simulation studies. Chem Eng J 2016;302:752–62. https://doi.org/
10.1016/j.cej.2016.05.122.
[43] de Mello BTF, Gonçalves JE, de Menezes Rodrigues G, Cardozo-Filho L, da Silva C.
Hydroesterification of crambe oil (Crambe abyssinica H.) under pressurized con-
ditions. Ind Crops Prod 2017;97:110–9. https://doi.org/10.1016/j.indcrop.2016.
12.014.
[44] Hasan MM, Rahman MM. Performance and emission characteristics of biodie-
sel–diesel blend and environmental and economic impacts of biodiesel production:
a review. Renew Sustain Energy Rev 2017;74:938–48. https://doi.org/10.1016/j.
rser.2017.03.045.
[45] Knothe G, Razon LF. Biodiesel fuels. Prog Energy Combust Sci 2017;58:36–59.
https://doi.org/10.1016/j.pecs.2016.08.001.
[46] Fjerbaek L, Christensen KV, Norddahl B. A review of the current state of biodiesel
production using enzymatic transesterification. Biotechnol Bioeng
2009;102:1298–315. https://doi.org/10.1002/bit.22256.
[47] Knothe G. Biodiesel and renewable diesel: a comparison. Prog Energy Combust Sci
2010;36:364–73. https://doi.org/10.1016/j.pecs.2009.11.004.
[48] Ambat I, Srivastava V, Sillanpää M. Recent advancement in biodiesel production
methodologies using various feedstock: a review. Renew Sustain Energy Rev
2018;90:356–69. https://doi.org/10.1016/j.rser.2018.03.069.
[49] Processamento de Matérias-primas. 2017.
[50] BiodieselBR. Óleo de fritura usado 2018.
[51] Bril’kov M, Falck-Ytter AB, Strætkvern KO. Evaluation of methods for reducing the
ash content of waste frying oil processed to biofuel oil. Fuel Process Technol
2015;134:487–93. https://doi.org/10.1016/j.fuproc.2015.03.008.
[52] Koh E, Surh J. Food types and frying frequency affect the lipid oxidation of deep
frying oil for the preparation of school meals in Korea. Food Chem
2015;174:467–72. https://doi.org/10.1016/j.foodchem.2014.11.087.
[53] Safari A, Salamat R, Baik OD. A review on heat and mass transfer coefficients
during deep-fat frying: determination methods and influencing factors. J Food Eng
2018;230:114–23. https://doi.org/10.1016/j.jfoodeng.2018.01.022.
[54] Martínez-Yusta A, Guillén MD. Deep-frying food in extra virgin olive oil: a study
by1H nuclear magnetic resonance of the influence of food nature on the evolving
composition of the frying medium. Food Chem 2014;150:429–37. https://doi.org/
10.1016/j.foodchem.2013.11.015.