Determination of Various Properties of Biodiesel Fuels Based on Methyl Ester Composition
Jason Freischlag
Chem 403 Dr. Porter
4/30/2014
AbstractBiodiesel is an alternative fuel source produced from vegetable oil, animal fat oil, or
used cooking oil. In theory the usage of biodiesel seems to have tremendous advantages
however it does not come without its disadvantages; including a higher cloud point which leads
to engine filters becoming clogged at temperatures around 0 oC. This leads to biodiesel/diesel
blends that reduce the dependency on fossil fuels while maintaining some of the advantages of
biodiesel.
Using simple separatory techniques on a feedstock yields organic oil comprised of a
triglyceride mixture. Many methyl esters may result from this process with the only difference
being the length of the carbon chain and the degree of unsaturation in this chain. Although
there are many molecules the four most predominant are palmitic acid, stearic acid, lineolic
acid, and oleic acid.1
This research compared biodiesel synthesized from waste vegetable oil, rapeseed oil,
canola oil, and regular petroleum diesel. The main properties examined in order to determine
fuel efficiency were heat of combustion and cloud point. The results support the notion that
biodiesel fuel made from canola oil is the best form of biodiesel2 among those tested due to its
high heat of combustion and theoretical low cloud point.
IntroductionBiodiesel is different from diesel in that biodiesel is produced from renewable
resources while diesel is produced from petroleum oil, a fossil fuel. Biodiesel fuel has great
potential as it is much cleaner than typical diesel fuel and can reduce the huge dependence of
the world on fossil fuels.3 Biodiesel is composed of multiple different methyl esters, most
notably palmitic acid, stearic acid, lineolic acid, and oleic acid.1
Figure 1. Structures of Methyl Esters Composing Biodiesel
Biodiesel can be synthesized from feedstocks by their transesterification with an
alcohol. Transesterification, is the process of exchanging the R group of an ester with the R
group of an alcohol; thereby resulting in a different ester and a different alcohol. This process
is an Sn2 reaction that can be catalyzed with either an acid or a base.3 Acid catalyzing results
in a protonated carbonyl of the ester which makes it a better electrophile for attack by the
alcohol whereas base catalyzing deprotonates the alcohol making it an excellent nucleophile.
Transesterification is a useful process in that it allows for bulk quantities of biodiesel to be
easily produced from triglycerides with minimal lab preparation.2
Biodiesel fuel has been receiving major attention as a potentially viable green
alternative to gasoline and traditional fossil fuels. While biodiesel is unlikely to completely
replace traditional gasoline there is no doubt in its potential to lessen the dependence of
industrialized societies on fossil fuels. Because biodiesel simply refers to diesel made from
renewable vegetable oil or animal fat it follows that there are many forms of biodiesel. Each
form of biodiesel is a mixture of various hydrocarbons and methyl esters which causes the
chemical properties of each form to vary. In order to evaluate the applications of biodiesel it is
essential to first establish which properties accurately describe a mixtures efficiency as a fuel.
Figure 2. Transesterification
Another possible method for producing biodiesel is the pyrolysis reaction.
Pyrolysis is the thermochemical decomposition of organic compounds in the absence of
oxygen. This process has gained increasing attention as an efficient method for the creation of
biofuels. Pyrolysis is useful because it can convert different feedstocks into useable fuels in a
simple process that is the same regardless of stock, thereby wasting as little energy as
possible. Pyrolysis of feedstock into biodiesel is similar to the epoxide chemistry that is used to
convert crude oil into petroleum. Both processes require a final reduction step in order to
create the diesel or biodiesel fuel.1
The determination of biodiesel can be made using gas chromatography with a flame
ionization detector because biodiesel consists mostly of carboxylic acids and their methyl ester
derivatives.3 A flame ionization detector is suitable because it is extremely sensitive to organic
molecules and also offers the advantage of not being destructive. 3
Several properties can be used to determine the efficiency of biodiesel as a fuel
but some important properties include cetane index, cloud point, pour point, heat of
combustion, acid value, and iodine value. The cetane index of a fuel is a measure of its ignition
quality and indicates the ability to ignite spontaneously in the combustion chamber. The higher
the cetane index, the shorter the ignition delay and the easier the fuel is to ignite; the higher
the cetane index, the better the ignition quality of the fuel. The cloud point is the temperature at
which a fuel begins to form wax crystals. A lower cloud point is a good property in a fuel
because it can withstand colder environments before crystals start to form which very easily
clog up filters. Cloud point is directly attributed to the saturated methyl ester content because
saturated fats solidify faster than unsaturated fats. The pour point is the temperature at which a
fuel loses its flow characteristic. A lower pour point indicates a better fuel for similar reasons to
that of the cloud point. As the temperature of a fuel approaches the pour point it will no longer
be able to flow. The Heat of combustion is defined as the energy released per mol of fuel when
the fuel is burned. This value is one of the most important properties of fuel and is the basis for
defining fuel efficiency. A large heat of combustion means more energy is released and thus a
more efficient fuel source than that with a lower heat of combustion. Acid value refers to the
mass of potassium hydroxide, in milligrams, that is required to neutralize one gram of a
chemical substance and is useful in determining the number of carboxylic acid functional
groups on a molecule. Acid value is measured by dissolving a known amount of sample in an
organic solvent and titrating it with a solution of known concentration of KOH with
phenolphthalein indicator. The Iodine value is the mass of iodine in grams that can be
consumed by 100 g of a substance. This is a way to measure unsaturation of a molecule as
the carbon carbon double bonds react with the iodine compounds. The higher the iodine value
the more double bonds present in a molecule.**
This research will address many topics involved in the determination of biodiesel fuel
efficiency including which properties best describe a viable fuel, how these properties relate to
fuel efficiency, which methods are best to determine the quantitative values of these properties,
and which feedstocks are most currently used in biodiesel production.
One similar study used several chemical properties to determine a best fuel
between sewage sludge and canola oil.1 The properties compared included cloud point, pour
point, specific gravity, viscosity, distillation range, cetane index, flash point, heat of combustion,
water and sediment, ash, carbon residue, and elemental analysis. Pyrolysis products were
also analyzed using gas chromatography, IR spectroscopy, and
13C
NMR. The cloud point,
spectra, and chromatograms all suggested that these sources may be able to provide a
capable replacement to diesel fuel.All fuel properties were determined using the American
Society for Testing and Materials (ASTM) standard methods including cetane index (D 976),
cloud point (D 2500), pour point (D 97), and heat of combustion (D 240). Each test was run on
sludge lipid and canola oil biodiesels as well as regular diesel fuel.1
The cetane index of both the biodiesels were found to be higher than that of regular
diesel indicating that biodiesel is actually better at igniting than diesel fuel. The cloud point of
both biodiesels was higher than the diesel counterpart which indicates that biodiesels are not
as applicable for colder weather climates. The pour point of both biodiesels was greater than of
regular diesel. The heats of combustions for both biodiesels were comparable to that of
biodiesel and indicate either could be used as an efficient alternative. IR of both biodiesels
showed a weak carbonyl peak at 1720 cm-1 which is indicative of a carboxylic acid. 13C NMR of
both products showed exclusively hydrocarbon signals with a touch of residual solvent toluene.
No carbonyl signal was seen for either biodiesel NMR confirming that both biodiesel reactions
were run to completion. Gas chromatography of the both biodiesels resulted in a fairly uniform
hydrocarbon chain length distribution of chains of 7 to 17 carbons. The Chromatogram of
diesel shows a similar distribution to that of the biodiesels, only lacking the 7 to 9 carbon length
chains.1 This research shows that all the physical properties associated with biodiesel are
intertwined and that the difficulty in interpreting these results comes mostly in understanding
the relationships between the values. According to this experiment biodiesel shows great
potential as an alternative fuel source as it mirrors many of the key chemical properties of
diesel. Furthermore, the differences between sewer sludge and canola oil biodiesels were
negligible in most cases.1
One thermodynamic study used various fatty acid methyl esters to establish a prediction
model for the cloud point of actual biodiesel made from different feedstocks based on their
chemical composition.3 This study found that the cloud point of a biodiesel mixture could be
determined by the amount of saturated fatty acid methyl esters regardless of composition of
unsaturated esters. Biodiesel naturally borrows chemical characteristics from the oils and fats
it was made from. Saturated fats have high melting points which means the biodiesel produced
from these fats would likewise have a high melting point. This is a severe concern during cold
weather as freezing gas could destroy an engine. Unsaturated fats have a lower melting point
but at the same time are more likely to undergo oxidation. This oxidation of fuel can cause
engine damage as well so a balance between these two properties is desired; for this reason
biodiesel is usually synthesized from semi-drying oils like rapeseed and soybean oils.
Base catalyzed transesterification was used to synthesize biodiesel fuel from sunflower
oil, coconut oil, rapeseed oil, soybean oil, olive oil, peanut oil, beef tallow, and palm oil. Each of
the products were classified based on weight percent of various fatty acids present. Based on
the data from this experiment it can be seen that these biodiesels consist mostly of palmitic
acid, stearic acid, oleic acid, and linoleic acid. These four methyl esters were chosen to make
standards in various molar ratios for use as a model for predicting the behavior of biodiesel
fuel.3
The range of recorded cloud points went from 270 K to 290 K which is a noticeable
difference in a crucial temperature range when dealing with natural environments.
Furthermore, it was found that cloud point could be predicted based on molar ratios using
known enthalpy values, assuming a eutectic system. The data supports the notion that cloud
point is based primarily on the ratios of saturated esters; unsaturated esters have a negligible
effect on cloud point because once one component of a mixture begins to crystallize the entire
solution becomes cloudy. Oxidation and polymerization of the fuel caused by unsaturation can
be remedied by using mono-unsaturated esters as opposed to those with higher degrees. This
experiment shows that there is a tool to accurately estimate fuel properties and a way to
determine optimal fatty acid methyl ester composition.3
Another study examined the viability of honne oil feedstock as a potential renewable
diesel alternative.4 Based on the information presented in this article honne oil offers a viable
alternative that matches many of the desired properties of diesel fuel when used in a biodiesel-
diesel blend. Cetane number was calculated using the formula CN = 46.3 + (5458)/SV 0.225*IV where SV is the saponification value and IV is the iodine value.
One study determined new formulas for calculating heating values of fuels by their
proximate analysis data.5 Proximate analysis resulted in values for weight fraction of moisture,
volatile matter, fixed carbon, and ash as set up by ASTM method E870-82. It was also found
that the yield of volatile materials can be increased by increasing the heating rate and the
temperature of the pyrolysis process. Heating values of fuels were measured using a bomb
calorimeter and ASTM method D2015.5
Food industries naturally produce substantial amounts of inedible waste. Processing
these unwanted byproducts can result in a product of value with potential as a biodiesel fuel.
Tomato seed is the major byproduct of tomato paste manufacturing and shows potential as a
raw material capable of producing biodiesel. One study separated tomato seeds from pulp and
dried them using a silica gel desiccant. Oil was then extracted from the seeds using hexane
and a soxhlet extractor and purified by rotary vacuum evaporation. Fatty acid composition was
determined using gas chromatography. A flame ionization detector was used with helium
carrier gas. 16 fatty acids were present in the oil according to GC results but the same four as
previously mentioned were predominant, with linoleic acid comprising 56% of the oil. Iodine
value is the measure of unsaturation of a fuel and the value found for tomato oil (124) is well
within the recommended range (80-145).6
SynthesisMethanol was combined with sodium hydroxide to form sodium methoxide. A three
necked round bottom flask was set to stir in a water bath with one neck equipped with a
thermometer, the middle neck a condenser, and the last neck a rubber stopper so that
reactants could be added. Feedstock was added to the round bottom flask then the sodium
methoxide solution was slowly added to prevent emulsion. This solution was heated and
stirred for an hour before being removed and allowed to set overnight. This solution was then
transferred to a seperatory funnel and both layers were extracted. The biodiesel has a lower
density than the glycerol byproduct so it was expected that the bottom layer would have been
the waste glycerol but because the reaction was run with excess methanol which mixed with
the glycerol layer and made the bottom layer in actuality the biodiesel product. This product
was thoroughly filtered. Analysis of each biodiesel product is as follows.
Methods and resultsGC will be used to identify methyl ester composition in the biodiesel. A flame ionization
detector will be used because of cost efficiency as well as specificity to these molecules. A
bomb calorimeter can be used to find the heat of combustion. Based on the information
obtained in the literature review it is estimated that there will be a correlation between the
unsaturation degree and the values of these fuel properties. Biodiesel offers some positive
benefits while regular diesel comes with others. The future of fuel comes in understanding the
best relationship between these two. Also based on these reviews it can be seen that there is a
methodical approach to understanding how the chemical composition of biodiesels affect their
properties and that transesterification is a common mode of synthesis when dealing with
biodiesel. It can be observed that the most common feedstocks used to produce biofuel are
rapeseed oil and olive oil because they produce a good combination of desirable physical
properties. The cloud points of the biodiesel products from rapeseed oil and olive oil are lower
than the competing versions that were tested. Bomb calorimetry is the standard means of
measuring heat of combustion, although there are multiple standard methods available to
follow. HPLC offers a method for determining the chemical composition of the biodiesel fuels.
Titration is a possible means of finding acid value, saponification value, and iodine value. Table
1 below shows the composition of multiple biodiesels.
Table 1. Composition of multiple oils
The HPLC Data, although not complete, shows that the biodiesel formed from waste oil
consists of multiple components but since the waste oil biodiesel is roughly 60% oleic acid it
was expected that the chromatogram from the waste oil biodiesel in figure. 3 should show at
peak at 5.5 minutes similar to the chromatogram for oleic acid in figure 4.
Figure 3. Waste oil Biodiesel Chromatogram
Figure 4. chromatogram of olaic acid
Heat of combustion, ΔH, is defined as the energy released per mole of fuel once burned
and is one of the most important quantities when describing fuel. A large heat of combustion
means more energy is released and thus a more efficient fuel source. A bomb calorimeter was
used in all calculations for ΔH.
Using the fact that the ∆𝐻𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 = -3230 kJ/mol for benzoic acid and
∆𝐻𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 ≈ ∆𝐸𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 + 𝑅𝑇∆𝑛𝑔𝑎𝑠𝑒𝑠
1)
From the above combustion it can be seen ∆𝑛𝑔𝑎𝑠𝑒𝑠 is five. Considering the two moles of
benzoic in combustion reaction ∆𝐸𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 = −6472𝑘. 𝑞𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 = −𝑞𝑐𝑎𝑙𝑜𝑟𝑖𝑚𝑒𝑡𝑒𝑟 and 𝛥𝐸𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 =
𝑞𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 for benzoic acid ΔT is 2.706.
qcalorimeter = Ccal ΔT
2)
Using data from benzoic acid Ccal was found to be 2392 kJ/ oC
Table 2. Determination of delta H for multiple feedstocks
Conclusion
These results would indicate that biodiesel synthesized from canola oil would make the
best fuel due to the lower composition of saturated fatty acids that give it a low cloud point and
the high heat of combustion which ensures the combustion is efficient enough to be viable.
These results are far from conclusive though. The composition is not known for each of the
biodiesel products making it difficult to make conclusions about the cloud point without further
analysis. Also the HPLC data was skim as only two trials were run.
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