ASTM D7157 Standard Test Method for Determination of Intrinsic Stability of Asphaltene-Containing Residues, Heavy Fuel Oils, and Crude Oils (n-Heptane Phase Separation; Optical Detection)
1. Scope
1.1 This test method covers a procedure for quantifying the intrinsic stability of the asphaltenes in an oil by an automatic instrument using an optical device.

1.2 This test method is applicable to residual products from thermal and hydrocracking processes, to products typical of Specifications D396 Grades No. 5L, 5H, and 6, and D2880 Grades No. 3-GT and 4-GT, and to crude oils, providing these products contain 0.5 % by mass or greater concentration of asphaltenes (see Test Method D6560).

1.3 This test method quantifies asphaltene stability in terms of state of peptization of the asphaltenes (S-value), intrinsic stability of the oily medium (So) and the solvency requirements of the peptized asphaltenes (Sa).

1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

2. Referenced Documents
2.1 ASTM Standards:
D396 Specification for Fuel Oils
D2880 Specification for Gas Turbine Fuel Oils
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D4870 Test Method for Determination of Total Sediment in Residual Fuels
D6560 Test Method for Determination ofAsphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products
D6792 Practice for Quality Management Systems in Petroleum Products, Liquid Fuels, and Lubricants Testing Laboratories

3. Terminology
3.1 Definitions:
3.1.1 For definitions of some terms used in this test method, refer to Terminology D4175.

3.1.2 asphaltenes, n - (rarely used in the singular), in petroleum technology, represent an oil fraction that is soluble in a specified aromatic solvent but separates upon addition of an excess of a specified paraffinic solvent.
3.1.2.1 Discussion - In this test method, the aromatic solvent is toluene and the paraffinic solvent is n-heptane.

3.1.3 compatibility, n - of crude oils or of heavy fuel oils, the ability of two or more crude oils or fuel oils to blend together within certain concentration ranges without evidence of separation, such as the formation of multiple phases.
3.1.3.1 Discussion - Incompatible heavy fuel oils or crude oils, when mixed or blended, result in the flocculation or precipitation of asphaltenes. Some oils may be compatible within certain concentration ranges in specific mixtures, but incompatible outside those ranges.

3.1.4 flocculation, n - of asphaltenes from crude oils or heavy fuel oils, the aggregation of colloidally dispersed asphaltenes into visible larger masses which may or may not settle.

3.1.5 peptization, n - of asphaltenes in crude oils or heavy oils, the dispersion of asphaltenes to produce a colloidal dispersion.

3.1.6 stability reserve, n - in petroleum technology, the property of an oil to maintain asphaltenes in a peptized state and prevent flocculation of asphaltenes.
3.1.6.1 Discussion - An oil with a low stability reserve 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 stability reserve are likely to maintain asphaltenes in a peptized state and not lead to flocculation when blended together.

3.2 Definitions ofTerms Specific to This Standard:
3.2.1 intrinsic stability (S-value), n - of refinery residual streams, residual fuel oils and crude oils, an indication of the stability or available solvency power of an oil with respect to precipitation of asphaltenes.
3.2.1.1 Discussion - Since the equation defining S-value is S = (1 + Xmin), where X min is the minimum volume (in mL) of paraffinic solvent, n-heptane, to be added to 1 g of oil to result in flocculation of asphaltenes, the smallest S-value is 1, which means the oil is unstable and can precipitate asphaltenes without addition of any paraffinic solvent. A higher S-value indicates that an oil is more stable with respect to flocculation of asphaltenes. S-value by this test method relates specifically to toluene and n-heptane as the aromatic and paraffinic solvents, respectively.

3.2.2 inversion point, n - point in the n-heptane titration curve, where the onset of asphaltene flocculation leads to inversion of the light intensity.
3.2.2.1 Discussion - At the first stage of the addition of n-heptane to a dilution of specimen and toluene, light intensity increases through dilution. When asphaltenes start to flocculate, there will be a point where the increase in light intensity through dilution matches the light intensity decrease (inversion) as a result of coagulated asphaltenes obstructing the light beam.

3.2.3 Sa, n - the S-value of an asphaltene, which is the peptizability or ability of an asphaltene to remain in a colloidal dispersion.
3.2.3.1 Discussion - Sa can also be described as one minus the ratio of So to S. Sa is linked to the length and number of aromatic chains within the asphaltenes.

3.2.4 So, n - the S-value of an oil.
3.2.4.1 Discussion - So can also be described as the aromatic equivalent of the oil expressed as the ratio of the aromatic solvent to the aromatic plus paraffinic solvent mixture having the same peptizing power as the oil.

3.2.5 solvent aromaticity, n - of a binary mixture of a paraffinic and an aromatic solvent, the solvency power of the binary mixture.
3.2.5.1 Discussion - For the purpose of this test method, solvent aromaticity is defined as a ratio by volume of the aromatic solvent (toluene) to the paraffinic solvent (n-heptane).

3.3 Symbols:
FR = flocculation ratio
FRmax = maximum flocculation ratio
S = the intrinsic stability of an oil
Sa = the peptizability of an asphaltene
So = the peptizing power of an oil
Xmin = paraffinic solvent consumption of undiluted oil, in mL/g of oil