Conversion of Vegetable Oils into Aviation Fuel Research Paper 2022

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CONVERSION OF VEGETABLE OILS INTO AVIATION FUEL
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SECTION 1

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Stage 1: hydrothermal hydrolysis of vegetable oils
Hydrolysis reactions can be performed using heat (thermally) as liquid-liquid
reactions or as gas-liquid reactions utilizing superheated steam. Another method
involves the use of lipolytic enzymes at room temperature. Biodiesel can be
produced via two routes beginning with vegetable oils or fats. In the first
process, the reaction occurs in two stages. The vegetable oil/fat is first converted
into fatty acids through hydrolysis and then the fatty acid is converted to the fuel
via esterification. The second method which is the conventional method involves
transesterification of vegetable oils into biodiesel in the presence of alkali-based
catalysts (Demirbas 2008).
Stage 2: Hydrothermal decarboxylation of fatty acids
Decarboxylation is a process in which a carboxyl group is eliminated from a
molecule. The thermal decarboxylation of fatty acids at moderate temperatures
less than 400 degrees Celsius produces a low yield of hydrocarbons which then
means it is necessary to use a catalyst. Hydrothermal decarboxylation of fatty
acids at moderate temperatures (less than 400 °C ) gives low yields if no catalyst
is used (Heimann, Karthikeyan, & Muthu 2016). This indicates that a catalyst is
necessary to drive the reaction and to boost the yield. Commonly used catalysts
are metals such as platinum Pt, nickel Ni and palladium Pd (Demirbas 2008).
Stage 3: Hydrothermal cracking of long-chain alkanes (in the presence of CO2)
Cracking is a chemical process in which a large hydrocarbon molecule is broken
down into smaller molecules. For instance, the hydrocarbon C15 H32 is broken
down into ethane, propene, and octane as shown below:

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C15 H32 → 2C2 H 4 +C3 H6 +C8 H18
In hydrothermal cracking, high temperatures as high as 750 °C are used
(Boyadjian & Lefferts 2018). The pressure used is also very high (in the range of
70 atm) (Boyadjian & Lefferts 2018). It is a combined process of both thermal
and catalytic cracking. The major steps in hydrothermal cracking include:
i) Thermal reactions break down the carbon-carbon (C-C) bonds of the alkane
into free reactive radicals.
ii) This is followed by the dissociation of the free radicals.
iii) The free radicals are then converted into hydrocarbon molecules through
catalytic hydrogenation.
The two reactions (thermal and catalytic) occur in parallel and the thermal
reaction dominates at higher reaction temperature range.
Stage 4: Hydrogenation of alkenes to make biojet fuel (biokerosene)
Hydrogenation involves the addition of hydrogen atoms to carbon-carbon double
bonds in the case of alkenes. In order to meet the Jet A1 specification, the double
bonds in the alkenes must be saturated (Demirbas 2008). The overall effect of
the addition of hydrogen is the elimination of the double bond in the alkene.
Despite the fact that the reaction is exothermic, the activation energy is quite
high and the reaction cannot take place under normal conditions. Therefore, a
catalyst is normally used to lower the activation energy and to speed up the
reaction. A commonly used catalyst is aluminium (iii) oxide Al2 O3.