Literature Review on the Hydrolysis of Vegetable Oil to Fatty Acids for Biofuel Production

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Literature review on the hydrolysis of vegetable oil to fatty acids FOR BIOFUEL
PRODUCTION
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Introduction
The potential of the use of biofuel as an alternative renewable energy source has
drawn much attention across the globe. In view of its advantages, biofuel can replace fossil
fuels such as petroleum and natural gas in most applications (Jebur, 2017). Concerns about
the unsustainability of conventional sources of energy including environmental degradation
and their depletion have made it necessary to seek for alternative sources of energy. Biofuels
such as biodiesel can be generated from vegetable oils and animal fats. According to
Demirbas, and Karslioglu (2010), vegetable oils can be modified or processed using four
major methods to use them as biofuel. These methods include thermal cracking also known
as pyrolysis, blending or dilution with hydrocarbons, transesterification, and emulsification.
Factors such as the reaction temperature, presence of catalysts, the ratio of the moles of
glycerides to alcohol, the water content of fats or oils, and the reaction time affect
transesterification reactions. However, the use of biofuels faces a challenge due to its high
cost compared to conventional energy sources such as diesel. The high price of the feedstocks
used in biofuel production partly accounts for the high cost of biofuel. The feedstocks used in
the production of biofuels include edible vegetable oils such as coconut and soybean, non-
edible vegetable oils such as castor and neem as well as animal fats. However, the use of
edible vegetable oils raises a sustainability problem as a result of competition with food
sources. Waste cooking oils provide ideal feedstocks while at the same time solving the
problem of environmental degradation.
Conversion of vegetable oils into diesel fuels

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The use of vegetable oils as biofuels requires the improvement of their properties
through the reduction of viscosity, changing their cold flow properties and increasing their
volatilities. According to Ilham and Saka (2010), viscosity is the main property that affects
the use of vegetable oils as fuels. The most commonly used method is transesterification.
Another common path for the production of biofuel is the hydrolysis of vegetable oils
followed by methyl esterification reaction. Hydrolysis is the process that splits large fat or oil
molecules into smaller ones. It can be performed using catalytic or non-catalytic methods.
The catalysts employed include lipase, acid or alkaline-based catalysts (Zahan, and Kano
2018). If the acid-based catalytic method is used, free fatty acids react with methanol in the
methyl esterification process at the boiling point of methanol. It is also possible to process
methyl esterification at higher reaction pressure in a non-catalytic reaction using supercritical
methanol at a temperature of about 350 ℃.
Biofuel can be generated through transesterification by the use of enzymatic, alkali or
acid catalysts. Alkali and acid catalysts offer two major advantages over their enzymatic
counterparts. These include low cost and lower reaction time. Therefore, the production of
better quality biofuel and higher yields can be achieved by the use of alkali catalysts.
However, this process requires high grade feedstocks due to its sensitivity to foreign bodies
in the oils. A transesterification process involving two steps has also been studied with the
aim to improve the quality of the biofuel. The first step in this process is the esterification of
the oil using acid catalysts. The second step involves alkali-catalyzed transesterification.
However, this method faces several drawbacks which makes it unsuitable. These include low
yields and the necessity of high temperatures. The use of supercritical methanol in
transesterification without using catalysts has also been sought as an alternative process. This
method has the ability to produce higher yields and can handle highly impure feedstocks. A
major disadvantage associated with this method is its high cost. This is because it requires

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special engineering techniques to provide the strict pressure and temperature conditions
necessary. The other alternative is to optimize supercritical methanol transesterification
through hydrolysis. This method was successfully employed in the conversion of vegetable
oils into fatty acids with the subsequent conversion of the fatty acids into methyl esters
maintaining supercritical conditions (Bhuyan, Alam, Chu, and Seo 2017).
Non-catalytic production of biofuels
The conventional method for the production of biofuel is through alkaline-catalyzed
transesterification. This process requires highly pure oils. To produce biofuel from low-
quality oils as the feedstock, enzyme-catalyzed reactions can be employed. According to
Bhuyan, Alam, Chu, and Seo (2017), the use of enzymes offers several benefits including the
production of high purity glycerol as a by-product, elimination of saponification side
reactions and low cost processing. The transesterification method can be carried out non-
catalytically in two steps: esterification preceded by hydrolysis. Initially, hydrolysis is carried
out using sub- and supercritical water at a temperature ranging between 260 ℃and280 ℃ for
a reaction time of about 15 to 20 minutes. The yield obtained from this reaction is greater
than 97 %. In this step, vegetable oils are converted into free fatty acids. The second step
involves the conversion of the fatty acids into fatty acid methyl esters using ethanol. A
complete conversion is obtained at a temperature of about 300℃ and a reaction time of about
12 minutes (Luo, Xue, Fan, Li, Nan, and Li 2014). According to Avelar, Cassimiro, Santos,
Domingues, Castro, and Mendes (2013), lower reaction temperature is required in alkyl
esterification as compared to transesterification. As a result, this two-step reaction of
hydrolysis followed by esterification has several advantages including the elimination of
separation and diffusion problems, a decrease of reaction temperature and the tolerance of

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feedstocks that have high free fatty acid and water content (Pandey 2011). Bhuyan, Alam,
Chu, and Seo (2017) successfully converted low quality edible oils into biofuel using
subcritical hydrolysis process with the subsequent esterification of the resulting fatty acids.
They achieved almost 90 % free fatty acid content through subcritical hydrolysis at a
temperature of about 275 ℃ for about 45 minutes without employing any catalyst. Both
subcritical and supercritical conditions were employed in the second step which was a
catalyzed esterification process.
The effect of water content on hydrolysis reactions
Enzymatic reactions are significantly affected by the water content due to the
formation of hydrogen bonds which are basic for the maintenance of the conformations of the
enzymes. Consequently, water strongly influences the catalytic activity stability of catalysts
such as lipase (Ondul, Dizge, Keskinler and Albayrak 2015). The yield from the reaction is
dependent on the interfacial area. This area can be increased by adding certain quantities of
water. However, if excess water is used, the transesterification yield decreases as the
hydrolysis reaction is promoted. According to Ondul, Dizge, Keskinler, and Albayrak (2015),
the optimal water content present in the reaction medium is dependent on the enzyme type,
the medium and can vary greatly.
One step hydrolysis and esterification to produce biofuel
Most studies have investigated the reaction processes under supercritical conditions
but few have focused on one step conversion of triglycerides into fatty acid methyl esters
under subcritical conditions. A study by Satyarthi, Srinivas, and Ratnasamy (2011) showed
that hydrolysis followed by alkyl esterification process takes place at a faster rate compared
to the process of transesterification. Furthermore, they showed that the reaction temperature

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was considerably lower in this process than in the transesterification process. This suggests
that a single step hydrolysis process with subsequent esterification could be a desirable
method of biofuel production. Darla, Hymavathi, Garimella, and Balijepalli (2014) employed
alkaline-catalyzed vegetable hydrolysis process at a temperature of about 180 ℃ for a period
of about 6 to 10 hrs and a pressure of about 1 Mpa. They achieved a yield of 97 %.
The reaction of hydrolysis followed by methyl esterification can be represented by the
following chemical formula,
R−COOH + R4 OH ⇔ R−COO−R4 + H2 O
Where R−COOH is the general formula for the free fatty acids, R4 OH is an alcohol and
R−COO−R4 is an ester.
Conclusion
Several methods that can be used in the production of biofuel have been identified.
Transesterification is the main technique used to convert vegetable oils or animal fats into
fatty acids and subsequently into biofuel. The choice of an efficient process that uses low-
quality feedstocks while simultaneously having the capability to avoid denaturation of the
yield is a difficult task. The susceptibility of biofuels such as biodiesel to undergo
denaturation at high temperatures calls for the use of catalytic treatments at lower
temperatures. It was also established that single step hydrolysis conversion of vegetable oils
into fatty acids has not been widely investigated. Therefore, more studies and experiments
need to be conducted in this field to establish the competitiveness of the process.

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References
Avelar, M.H., Cassimiro, D.M., Santos, K.C., Domingues, R.C., de Castro, H.F. and Mendes,
A.A., 2013. Hydrolysis of vegetable oils catalyzed by lipase extract powder from dormant
castor bean seeds. Industrial crops and products, 44, pp.452-458.
http://www.academia.edu/download/44981067/
Hydrolysis_of_vegetable_oils_catalyzed_b20160422-2885-h4kp3f.pdf
Bhuyan, M.S.U.S., Alam, A.H.M.A., Chu, Y. and Seo, Y.C., 2017. Biodiesel production
potential from littered edible oil fraction using directly synthesized S-TiO2/MCM-41 catalyst
in esterification process via non-catalytic subcritical hydrolysis. Energies, 10(9), p.1290.
https://www.mdpi.com/1996-1073/10/9/1290/pdf
Darla, Hymavathi, Garimella, and Balijepalli. (2014). BIODIESEL PRODUCTION FROM
VEGETABLE OILS: AN OPTIMIZATION PROCESS. 4. 2277-4807.
https://www.researchgate.net/publication/
301557017_BIODIESEL_PRODUCTION_FROM_VEGETABLE_OILS_AN_OPTIMIZAT
ION_PROCESS
Demirbas, A. and Karslioglu, S., 2010. Biodiesel production facilities from vegetable oils and
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https://www.tandfonline.com/doi/abs/10.1080/009083190951320?
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Ilham, Z. and Saka, S., 2010. Two-step supercritical dimethyl carbonate method for biodiesel
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Jebur, M.G., 2017. Evaluating One-Step Catalytic Free Method Including Hydrolysis,
Esterification, Transesterification, and Degradation Reactions to Produce Biodiesel from
Soybean Oil.
http://scholarworks.uark.edu/cgi/viewcontent.cgi?article=3923&context=etd
Luo, H., Xue, K., Fan, W., Li, C., Nan, G. and Li, Z., 2014. Hydrolysis of vegetable oils to
fatty acids using Brønsted acidic ionic liquids as catalysts. Industrial & Engineering
Chemistry Research, 53(29), pp.11653-11658.

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https://pubs.acs.org/doi/abs/10.1021/ie501524z
Ondul, E., Dizge, N., Keskinler, B. and Albayrak, N., 2015. Biocatalytic production of
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https://www.intechopen.com/books/biofuels-status-and-perspective/biocatalytic-production-
of-biodiesel-from-vegetable-oils
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Satyarthi, J.K., Srinivas, D. and Ratnasamy, P., 2011. Hydrolysis of vegetable oils and fats to
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https://www.sciencedirect.com/science/article/abs/pii/S0926860X10002322
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