EXPLANATION
This procedure describes a rapid and sensitive method for estimating the stability reserve of an oil. The stability reserve is estimated in terms of a separability number, where a low value of separability number indicates that there is a stability reserve within the oil. When the separability number is between 0 to 5, the oil can be considered to have a high stability reserve and asphaltenes are not likely to flocculate. If the number is between 5 to 10, the stability reserve in the oil will be much lower. However, asphaltenes in this case are notlikely to flocculate as long as the oil is not exposed to any worse conditions, such as storing, aging, and heating. If the separability number is above 10, the stability reserve of the oil is very low and asphaltenes will easily flocculate, or have already started to flocculate.
This test method can be used by refiners and users of oils, for which this test method is applicable, to estimate the stability reserves of their oils. This test method is not intended for predicting whether oils are compatible before mixing, but can be used for determining the separability number of already blended oils. However, oils that show a low separability number are more likely to be compatible with other oils than are oils with high separability numbers.
This test method covers the quantitative measurement, either in laboratory or in fields, of how easily asphaltene containing heavy oils diluted in toluene phase separate upon addition of heptane. The test method is limited to asphaltene containing heavy fuel oils, such as those in Specification ASTM D396, Grades 4, 5, and 6, Specification ASTM D975, Grade No. 4-D, and Specification ASTM D2880, Grade Nos. 3-GT and 4-GT. Refinery fractions from which such blended fuels are made also fall within the scope of this test method.
TEST SUMMARY
Dilution of oil with toluene followed by addition of heptane causes asphaltenes to flocculate, and the oil to phase separate. The rate of the phase separation is determined by measuring the increase in transmission in the sample from the bottom of the test tube to the top (or a portion thereof) over time. The oils are first diluted with toluene in ratios that depend on the oil type. Two mL of the oil/toluene solution is mixed with 23 mL of heptane. Seven mL of this is transferred into a glass vial that is inserted into an optical scanning device. The change in light transmission through the glass vial containing the above mixture is recorded by scanning the vial vertically with the optical scanning device. One scan is run every 60 s for 15 min. An average of the transmittance is calculated from 1125 readings at 0.04 mm intervals along the glass vial, starting 10 mm above the bottom of the vial and continuing up to 55 mm for each scan. The separability number from 16 scans is calculated and reported.
TEST PRECISION
Following precision was obtained in a preliminary study:
Reproducibility of the test method is not yet determined.
Bias of this method cannot be determined since no accepted reference materials are available suitable for this test method.