PPE - BioFuels Acidity Poses Storage Risk
Biofuels are commonly classified into two main categories: bioethanol and biodiesel. Both are used commercially as blends with conventional oil based gasoline and diesel, with ratios of 90% gasoline/10% bioethanol being most common. Diesel blends are conventionally 5% biodiesel blends with 95% conventional oil based diesel. However, ASTM (American Society for Testing and Materials) specifications are now in place for a range of blends up to 20% biodiesel.
High Biofuel Fuel Acidity - Valve Seals Swelling Risk
Results of 12 month immersion tests of a range of elastomer polymers typically used in fuel-handling equipment show that the seals are prone to significant swelling leading to seal failure in valves and other equipment. The swelling is caused by increasing acidity of the biodiesel due to oxidation. Moreover, the presence of water contamination of the biodiesel was found to accelerate the rate of elastomer swelling.
The chemical nature of the Biofuels blends is significantly different from that of oil based gasoline or diesel. Bioethanol is a polar solvent and not compatible with many of the elastomer grades that are used with non-polar gasoline. The higher the blend ratio of ethanol the more significant the effect will be, leading to excessive swelling and a deterioration of sealing properties over time.
With Biodiesel the chemical compatibility situation is more complicated. The originating chemical structure generated from the biomass is that of a methyl ester. Over time and the inevitable exposure to atmospheric oxygen, biodiesel undergoes oxidation to produce carboxylic acids and lowering the fuel’s pH.
John Kerwin, head of materials technology, Precision Polymer Engineering, commented, “The research show that companies handling biofuels need to be aware of the increased acidity risk of biofuels on equipment seals, and either maintain their seals more regularly to check for signs of swelling or switch to alternative sealing materials such as peroxide-cured, fluoroelastomers.” The 12 month ethanol and biodiesel immersion tests by PPE have shown that:
• Conventional NBR elastomers can be used within their normal operating parameters for both conventional gasoline and gasoline / ethanol blends. However they suffer from significant swelling with biodiesel.
• Bisphenol-cured FKM elastomers which are prone to a reversal of the rubber curing process, should be replaced with peroxide-cured FKM for biofuel and biodiesel applications.
• The rate of swelling varies depending on the immersion conditions, for example aged fatty acid methyl ester is more aggressive than fresh fatty acid methyl ester.
Bioethanol is derived mainly from sugar cane and corn or maize. It is currently the most significant biofuel with production outstripping biodiesel by ten times. Biodiesel, on the other hand, is derived from a variety of sources. These include oils from rapeseed, sunflower, palm, and soya and animal fats mainly produced in Europe, which is also its major market.
Precision Polymer Engineering
Tel: 01254 295400
Published in Valve User Magazine Issue 18
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