INTRINSIC STABILITY OF ASPHALTENE CONTAINING OILS: ASTM D7157
EXPLANATION
This test method describes a sensitive method for estimating the intrinsic stability of an oil. The intrinsic stability is expressed as an S-value. An oil with a low S-value is likely to undergo flocculation of asphaltenes when stressed (for example, extended heated storage) or blended with a range of other oils. Two oils each with a high S-value are likely to maintain asphaltenes in a peptized state and not lead to asphaltene flocculation when blended together. This test method can be used by petroleum refiners to control and optimize the refinery processes and by blenders and marketers to assess the intrinsic stability of blended asphaltene containing heavy oils. This test method is applicable to residual products from thermal and hydrocracking processes, to products typical of Specifications ASTM D396 Grades No. 5L, 5H, and 6, and ASTM D2880 Grades No. 3-GT and 4-GT, and to crude oils, providing these products contain 0.5 m % or greater concentrations of asphaltenes (see Test Method ASTM D6560). This test method quantifies asphaltene stability in terms of state of peptization of the asphaltenes (S-value), intrinsic stability of the oil medium (So) and the solvency requirements of the peptized asphaltenes (Sa).
TEST SUMMARY
This test method uses an integrated analytical measurement system with an optical probe for the detection of asphaltene precipitation from a toluene solution of the sample. Three test specimens are dissolved in three different quantities of toluene. The three specimen/toluene solutions are automatically and simultaneously titrated with n-heptane to cause precipitation of the asphaltenes. The optical probe monitors the formation of flocculated asphaltenes during the titration. Flocculated asphaltenes will alter the detected light intensity. Start of flocculation is interpreted when the optical probe detects a significant and sustained decrease in rate-of-change of the light intensity. A computer routinely calculates the stability parameters and subsequently the intrinsic stability of the oil from the added n-heptane at the inversion point, the mass of specimen, and the volume of toluene, for the three specimen/toluene solutions.
INTERFERENCES - High content of insoluble inorganic matter (sediment) will cause some interference in this test method. In this case, the insoluble matter shall be removed by filtration according to Test Method ASTM D4870. Also, free water in the oil can cause difficulties with the optical detector and should be removed by any suitable means (for example, centrifugation) prior to testing.
TEST PRECISION
Only preliminary precision data are available at the moment based on one laboratory. Further data will be collected through a round robin to obtain proper precision estimates.
Repeatability: for S 0.19; for Sa 0.031; and for So 0.12.
This test method has no bias because the results of the test are defined only in terms of this test method.
