INSTABILITY OF MIDDLE DISTILLATE FUELS BY PORTABLE SPECTROPHOTOMETER: ASTM D6748
EXPLANATION
Storage stability depends on complex interactions. It varies with feedstock type and source, and the processing used. The rate of degradation may not change uniformly with temperature. Chemical reactions may lead to a change in color followed by the formation of soluble gum and insoluble sediments which may overload the filters, and plug nozzles and injectors. This test method provides a rapid procedure using a portable apparatus and allows the test to be carried out on site or in the laboratory to give a result within 35 min. The potential beneficial effects of stability additives in fuels may not be recognized by this test method. Therefore, the actual storage stability of middle distillate fuels with stability additives may not be correctly indicated by these test results. The unstable reactive products (phenalenes and phenalenones) detected by this test method may be present in fuels containing catalytically cracked or straight run materials and can affect the potential instability of the fuel. If this test method is used by any party for a rapid assessment of stability, it is the responsibilities of the parties involved to decide whether or not this procedure yields meaningful results.
This test method rapidly determines phenalenes and phenalenones in middle distillate fuels, including marine, automotive, heating, and gas turbine fuels such as those specified in Specifications ASTM D396, ASTM D975, ASTM D2069, and ASTM D2880. This test method is applicable to both dyed and undyed fuels at all points in the distribution chain from refinery to end-user. It is not applicable to fuels containing residual oil. This test method is suitable for testing samples with a relative absorbance of up to 5.00 absorbance units.
TEST SUMMARY
A 5 mL volume of middle distillate fuel is mixed with an equal volume of an immiscible reagent solution (a proprietary methanol solvent containing a Lowry-Bronsted acid). A second reagent solution (proprietary solution containing potassium permanganate and an oxidizing agent) is then added, the new blend mixed and allowed to settle for 30 min for two phases to separate. The top darker reagent layer is then placed in the portable spectrophotometer and the relative absorbance of near infrared and visible light, at fixed single wavelengths is measured instantaneously and automatically, and reported in absorbance units.
When mixed with oil samples containing phenalenes, the second oxidizing reagent oxidizes the phenalenes to phenalones which are subsequently converted to colored indolphenalene salts by the acidic first reagent. The spectrophotometer measures the absorbance of light caused by the colored salts while eliminating the effect of the initial color of the fuel. The light absorbance is a measure of the concentration of the colored salts that are formed.
TEST PRECISION
Repeatability: 0.0876(x + 0.2)AU
Reproducibility: 0.1799(x + 0.2)AU
Where x is the average of results being compared.
Since there is no accepted reference material suitable for determining the bias of the procedures in this method, bias cannot be determined.
