Published: Jan 2010
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STABILITY GENERALLY REFERS TO THE RESISTance of a fuel to changes in its chemical quality, while cleanliness is a parameter relating to the physical presence of contaminants: most notably water and sediment, but also degradation products such as gums, asphaltenes, or polymerized products that can form following production of a fuel. There are significant differences in cleanliness and stability between light fuels (distillates) and residual fuels, and very different test methods are required. Also, while these would appear to be unrelated properties, the presence of contaminants can exacerbate reactions resulting in a loss of storage stability. In like manner, products such as gum, asphaltenes, or polymerized products produced when a fuel degrades can adversely affect fuel performance and can contribute to other reactions seriously affecting its usefulness. Two types of materials can make a fuel less suitable or unsuitable for the intended use. Fuel contaminants include materials such as water and rust or other particulate matter introduced subsequent to fuel manufacture, and fuel degradation products form in fuel during extended storage as a manifestation of fuel instability. Whether degradation products are soluble or insoluble depends on many factors, which include the chemistry of the degradation products and also the chemistry and solubility of the fuel itself (e.g., aromatics or oxygenate content). In addition, instability of heavy, residual fuels can result from mixing different heavy fuels or diluents (cutter stocks) designed to reduce viscosity or improve handling. Insoluble degradation products can combine with other fuel contaminants to reinforce deleterious effects. Soluble degradation products, such as gums, can form due to complex interactions and oxidation of small amounts of olefins, and sulfur, nitrogen, and oxygen compounds present in some fuels. These soluble gums are less volatile than fuel and can carbonize during combustion. The formation of degradation products can be catalyzed by dissolved metals such as iron, zinc, and copper. The increasing diversity of products in today's market, ranging from highly refined gasoline, through aviation turbine fuel and ultra-low sulfur diesel fuel, to residual fuel oil as well as newer biofuels, such as ethanol and alkyl ester biodiesel (fatty acid methyl esters [FAME]), demands an equally diverse array of test methods. Consequently, many test methods have been developed over the years, with quite a few new methods having been introduced within the past decade for assessing the stability of petroleum products, biofuels, and blends of the two and determining their cleanliness.
Forester, David R.
Director of Technical Services, Power Service Products, Weatherford, TX
Giles, Harry N.
PetroStorTech LLC, Arlington, VA