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Examining fuel stability

Hydrocarbon Engineering,


Both crude oil based and renewable feedstock based fuels are prone to degradation. More importantly, the chemical reactions, the antagonists and the stabilisers are the same. This article discusses the impact that very small amounts of metal (iron and copper) may have on fuel stability.

The Details

At a very high level, metal contamination such as copper and iron catalyse the oxidative breakdown. In addition to metal contamination, everyone storing any fuel monitors oxidative contact with air, ambient and storage temperatures, water content, and storage time/duration. Often a fuel will have a number of intermediate transfers between the production plant and the final customer tank including railcar, road car and ship transfers, all of which increase the possibility of air, metal, moisture and bacteria contamination.

The oxidation of any fuel is a chain reaction involving the formation of peroxy (ROOH) and free radicals (R·). It requires very little heat to begin and once initiated, it continues to promote further oxidation. Fuel stabilisers either stop the chain reaction or decompose the peroxide. The chain reaction can be stopped by either providing an electron acceptor or donor.

It is important to note that the reaction is not reversible. Fuel additives can and do impact the rate of reaction and when used properly will result in a very stable fuel.

Blends and biodiesels

Gasoline/oxygenate blends, although initially free of corrosive components, develop acidity in storage, particularly over extended periods of time. The corrosion inhibitor functions by substantially different mechanisms when performing as a corrosion inhibitor in the gasoline oxygenate blends than when in fuel oil where water bottoms are the primary problem. As an inhibitor in fuel oil water bottoms, the amine component forms a polar salt with the organic acid inhibitor and acts to transport the inhibitor into the water phase. As an inhibitor in gasoline/oxygenate blend systems, the amine component acts by neutralising acidic impurities in the oxygenate, thus allowing the organic acid inhibitor to be effective.

Biodiesel based fuels, whether derived from seed, plant or animal, are chemically different from crude oil based gasoline and diesel. Virgin oils that are fresh from the seed crusher are likely to be low in free fatty acids and subject to the pretreatment stages. The fatty acid profile of the feedstock will have an important effect on overall stability with feedstocks high in polyunsaturates tending to be less stable, such as animal fats, soybean, cottonseed and jatropha. Distilled seed based biodiesel has fewer tocopherols (natural occurring antioxidants) retained in the product than biodiesel produced in other processes and tend to be less stable. Biodiesel produced from hydrogenated feedstocks tends to be more stable.

Bio based fuels are very susceptible to destabilisation from low level metal contamination.

Metal deactivators inhibit the catalytic effects of such ions, especially copper, retarding the formation of gummy residues (e.g. gels containing copper mercaptide). Even concentrations of copper as low as 0.1 ppm can have detrimental effects.

Metal deactivators also function to prevent corrosion in pipelines, storage tanks and vehicles. They attach to the metal surface, preventing corrosive agents from interacting with the surface.

This degradation can be prevented through the use of antioxidants such as ortho alkylated phenols, phenylenediamines, alkylenediamines (diethylenetriamine, triethylenetetramine, etc.), and alkylamines (diethylamine, tributylamine, ethylamine). Other useful additives include gum inhibitors such as N-substituted alkylaminophenols and colour stabilisers such as N-(2-aminoethyl) piperazine, N,N-diethylhydroxylamine, and triethylenetetramine.

Conclusion

Fuel stability additives are available in a variety of blends and dilutions, from blends of phenolic and aminic antioxidants and sweetening agents for gasolines, to blends of antioxidants and metal deactivators. Fuel additives can and do impact the rate of reaction and when used properly will result in a very stable fuel.

Author: SBZ Corporation

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Read the article online at: https://www.hydrocarbonengineering.com/gas-processing/26052010/examining_fuel_stability/


 

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