One of the most important chemical characteristics of FAME is the structure and composition of the fatty acid methyl ester groups, which will be determined by the fatty acid components of the raw material used in the production process. Pictorially, we can see a difference if we look at the structures of two different fatty acids below – one a saturated fatty acid and one an unsaturated fatty acid – the saturated fatty acid has a ‘straight chain’ of carbon atoms (circled in red) whereas the unsaturated fatty acid has, in this case, two ‘double bonds’ present in the hydrocarbon chain. These are circled in green and result in ‘kinks’ in the hydrocarbon chain:
unsaturated fatty acid methyl ester components of FAME. However, the addition of antioxidants to the neat FAME prior to blending was found to prevent sediment formation. Vitamin E is a natural antioxidant which would appear to prevent the occurrence of such unwanted oxidation reactions.
Myristic Acid – A saturated fatty acid
FAME problems Linoleic Acid – An unsaturated fatty acid
FAME species which are composed of a relatively high proportion of saturated fatty acid methyl esters, for example, palm oil derived FAME (denoted PME) will in general be relatively stable to unwanted degradation reactions, but will have poorer cold temperature performance. FAME species which are composed of a relatively high proportion of unsaturated fatty acid methyl esters, for example, soybean oil derived FAME (denoted SME), will display markedly different behaviour, typically having improved cold temperature properties in comparison to PME, but being less stable to degradation reactions. The reason for the improved cold-temperature behaviour displayed by the FAME species high in unsaturated fatty acid methyl esters is that for every double bond in the hydrocarbon chain and associated kink in the molecule, the individual molecules cannot pack as closely together thereby reducing intermolecular forces and correspondingly decreasing the melting point. However, it is the presence of the double bonds in the unsaturated FAME species that infers the greater degree of instability upon the molecules – the double bond sites render the molecule prone to oxidative degradation.
The presence and composition of other chemical constituents are also important. For example, FAMEs with high levels of vitamin E are thought to be more stable to unwanted oxidative degradation reactions. Recent research reported in Biodiesel magazine has identified the formation of sediments in stored B5 and B20 blends. Analysis of the sediment components suggests that they originate from the oxidation of
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Water contamination: A major problem with regard to the carriage of FAME by sea is the issue of water contamination. FAME is a hygroscopic material, which means that is will absorb water from its surrounding environment, including the atmosphere. This renders FAME very sensitive to water contamination. The current maximum allowable water content in the European EN 14214 and American ASTM D6751 FAME standards is 500 mg/kg, although often selling specifications are lower (300 mg/kg being a typical maximum water content on a sales specification), reflecting the high potential for water pick-up in this material.
Unlike most conventional diesels, in which any un- dissolved water present will generally settle out over a period of time, FAME can hold water in suspension up to relatively high levels (above 1000 mg/kg). Apart from the fact this will render the cargo off-specification for water content, the presence of water can promote unwanted hydrolytic reactions, breaking down the FAME to form free fatty acids, which can again affect certain specification parameters for the material – such species are corrosive and may attack exposed metal surfaces. Additionally, once a certain threshold level of water content is reached, water can separate out from the FAME, forming a separate (and potentially corrosive) free water phase. The possibility of phase separation occurring is greater for blends of FAME and conventional diesel.
The presence of a FAME/water interface provides ideal conditions for the promotion of unwanted microbiological growth, which may in turn lead to filter blocking and corrosion problems. Certain publications have referenced the greater degree of biodegradability of FAME as a positive factor when dealing with environmental spillages. This is indeed correct, but by the same token this factor means that FAME is
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