Oxidation stability of diesel fuel during storage

Abstract

Over the last 25 years, the diesel fuel has undergone major changes in its composition, which have fundamentally affected its long-term storage possibilities. An oxidation stability is a main parameter characterising the storage of the diesel fuel and it is strongly affected by a diesel fuel composition. The oxidation stability decreases in a line saturated hydrocarbons – aromatic hydrocarbons – unsaturated hydrocarbons. The mandatory use of FAME as a biocomponent of the diesel fuel negatively affect its oxidation stability. The FAME contain unstable double bonds C=C and their mixtures with mineral diesel fuel cannot be storage for a long time. The use of antioxidants with FAME has not effect in the long time storage. A recommended usable life of diesel fuel with FAME accor-ding to ČSN 65 6500 is from 1 to 3 months according to the FAME content. However, in some cases it is ne-cessary to store diesel fuel for a long time. State material reserves or back-up diesel generators are examples, when the diesel fuel is stored for the long time. The oxidation stability of the fuel expresses a resistance to an oxygen action. The oxygen, which is dissolved in the fuel, attacts molecules of the fuel and various oxidation products create. Hydroperoxides are the primary oxidation products. Secondary oxidation reactions give aldehydes, ketones, karboxylic acids and insoluble deposits. The oxidation products negatively affect the diesel fuel properties. Besides the composition the oxidation stability of the fuel is negatively affected by a high temperature,a high content of dissolved oxygen, an UV radiation and a presence of metals with a catalytic effect. An overview of methods used for the measuring or the observing oxidation stability was prepared in the article. It means not only the oxidation stability measuring but also a measuring of the content of different oxidation products which are related to the various oxidation degree . The aim of the article was to summarize the possibilities of a predicting the storage time of the diesel fuel and to propose a procedure for the monitoring and the predicting its longterm durability. Only one standardized storage test according to the ASTM D4625 can be found in the literature. The test is based on the storage of 400 ml of the diesel fuel at 43 °C for periods of 4, 8, 12, and 24 weeks. After aging for a selected period, a sample is analyzed for insolubles. The correlation of the test results is: a week at 43 °C is roughly equivalent to a month of the storage at the temperature of 21 °C. The test is a time and material consuming and the correaltion was determined for diesel fuels made up to 1990´s. The new shorter storage test based on the standard test according to ASTM D4625 was proposed to predict diesel storage stability. The temperature was rised and the the time was shortened up to one month. During the test short laboratory analyses can be used for monitoring oxidation of the diesel fuel for example: the oxidation stability by PetroOxy, the peroxide number, the oxidation index by infrared spectroscopy, the acid number, the bromine number or the antioxidant content. The sample consumption is about 100 ml according to the selected analyses.