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)
12. Procedure
12.1 Test Procedure:
12.1.1 Place the titration cells containing the specimen solutions (9.3.4 and 9.3.6) into their slot positions in the measurement system.

12.1.2 Position the optical probes in the titration cells.

12.1.3 Place the dosing tubes of the titration apparatus into the titration cell. The position of the tube is such that droplets fall directly into the sample vortex so that mixing is optimal.

12.1.4 Switch on the magnetic stirrer and adjust the speed so that agitation is just visible on the surface of the sample.

12.1.5 Initiate the automatic test procedure according to the manufacturer's instruction manual. For further details, refer to Appendix X1.

12.1.6 n-heptane is added in steps of 0.05 mL at a rate of 0.05 mL/s. Addition of n-heptane is discontinued for 2 s between each step.

12.1.7 For dilutions of very stable samples (see Table 1), it is needed to pre-add an accurately known quantity of 7 mL of n-heptane by the automatic titration unit to ensure that the probe is fully immersed.

NOTE 2 - The addition of 7 mL of n-heptane is automatically performed by the apparatus when "pre-dilution" mode is selected by the operator.

12.1.8 The test is completed when either an inversion point is encountered (V para ) or when the total liquid content of the titration cell reaches a maximum of 50 mL. A report is displayed automatically at that time or earlier if requested by the operator.

NOTE 3 - The apparatus will stop automatically when the maximum limit of 50 mL liquid has been reached.

12.1.9 Draw a straight line through the three points (see Fig. 2) using the least squares fit equation. For a valid result the measure of goodness of fit (R-squared) should be = >0.98. If R-squared <0.98, discard the result and repeat the test with fresh dilutions from 9.3 onwards.

NOTE 4 - See Appendix X1 for detailed automatic test program and examples of the titration curve and regression report.

12.1.10 When, for any of the three dilutions, n-heptane has been added up to a total of 50 mL liquids in the titration cell without encountering an inversion, the sample is considered to be very stable. In such a case, the test shall be repeated from 9.3 onwards, using fresh dilutions as given in Table 1 under "Very stable samples".

13. Calculation of Results
13.1 The instrument automatically calculates the stability parameters, using the following equations:
Mtol = M1 - Moil
Varom = (Mtol)/Dtol
FR = (Varom)/(Varom + Vpara)
1/X = (Moil)/(Varom + Vpara)
where:
Moil = mass of oil, g (see 9.3.2),
M1 = mass of specimen plus toluene, g (see 9.3.4 and 9.3.6),
Mtol = mass of toluene in the mixture, g,
Varom = volume of toluene in the mixture, mL,
Dtol = density of toluene at 20 °C = 0.8669 kg/L,
FR = flocculation ratio,
Vpara = volume of n-heptane in the mixture at inflection point, mL (see 12.1.8), and
1/X = oil to solution ratio, g/mL.

13.2 From linear regression and extrapolation of FR versus 1/X (see Fig. 2), using the expressions given below, the following stability parameters are calculated:
S = 1 + Xmin
Sa = 1 - FRmax
So = (FRmax)S or So = (FRmax)(1 + Xmin)
where:
S = S-value of an oil,
Xmin = paraffinic solvent consumption of undiluted oil, mL/g of oil (at FR = 0),
Sa = peptizability of an asphaltene,
FRmax = maximum flocculation ratio (at 1/X = 0), and
So = peptizing power of the oil matrix.

The R-squared of the regression provides a measure for the validity of the test. The test results are acceptable if R-squared is equal to or greater than 0.98 (see 12.1.9).