ASTM D7112 for stability and compatibility of heavy fuel oils and crude oils
ASTM D7112 Standard Test Method for Determining Stability and Compatibility of Heavy Fuel Oils and Crude Oils by Heavy Fuel Oil Stability Analyzer (Optical Detection)
1. Scope
1.1 This test method covers an automated procedure involving titration and optical detection of precipitated asphaltenes for determining the stability and compatibility parameters of refinery residual streams, residual fuel oils, and crude oils. Stability in this context is the ability to maintain asphaltenes in a peptized or dissolved state and not undergo flocculation or precipitation. Similarly, compatibility relates to the property of mixing two or more oils without precipitation or flocculation of asphaltenes.
1.2 This test method is applicable to residual products from atmospheric and vacuum distillation, from thermal, catalytic, 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 0.05 mass % or greater concentration of asphaltenes.
1.3 This test method is not relevant to oils that contain less than 0.05 % asphaltenes, and would be pointless to apply to unstable oils that already contain flocculated asphaltenes.
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 and health practices and determine the applicability of regulatory limitations prior to use.
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, Petroleum Products, and Lubricants
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance
D6560 Test Method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products
3. Terminology
3.1 Definitions:
3.1.1 For definitions of some terms used in this test method, such as crude oil, repeatability, reproducibility, and residual fuel oil, refer to Terminology D4175.
3.1.2 asphaltenes, n - in petroleum technology, molecules of high molecular mass, high carbon/hydrogen ratio, and containing hetero-atoms.
3.1.2.1 Discussion - Asphaltenes are generally found in crude oils and in heavy fuel oils containing residual fractions. Their presence is determined by their insolubility in alkanes such as n-heptane and solubility in aromatics such as xylene.
3.1.3 compatibility, n - of crude oils and of heavy fuel oils, the ability of two or more crude oils or fuel oils to be blended together within specified ratios without evidence of separation, such as flocculation or separation of asphaltenes.
3.1.4 flocculation, n - of asphaltenes in crude oils or heavy fuel oils, the aggregation of colloidally dispersed asphaltenes into larger, visible masses that may or may not settle.
3.1.5 stability reserve, n - of crude oils, heavy fuel oils, and residual streams containing asphaltenes, the property of an oil to maintain asphaltenes in a peptized (colloidally dispersed) state and prevent their flocculation when stored or when blended with other oils.
3.1.5.1 Discussion - An oil with a high stability reserve can be stored for a long period of time or blended with a range of other oils without flocculation of asphaltenes.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 aromatic solvent equivalent (xylene equivalent), SE, n - the lowest aromatic solvent (xylene) content, expressed as a volume %, in a mixture containing aromatic and paraffinic solvents (xylene and n-heptane) which, when mixed with oil, will not result in flocculation of asphaltenes. See flocculation ratio.
3.2.1.1 Discussion - SE is defined as FR5/1 multiplied by 100 %, as shown in Eq 2.
3.2.2 evaporation correction coeffficient, n - the rate of evaporation of aromatic solvent (xylene) from the sample cup, measured in grams per hour.
3.2.3 flocculation ratio (FR), n - the lowest aromatic solvent (xylene) concentration, expressed as a proportion of xylene to xylene plus n-heptane which, when mixed with an oil, will not result in flocculation of asphaltenes. See 15.1, Eq 1.
3.2.4 FR5/1, n - the flocculation ratio at a dilution of 5 mL of xylene and n-heptane solvent mixture to 1 g of oil.
3.2.4.1 Discussion - The ratio 5 to 1 is used internally by a number of oil companies involved with the stability and compatibility of heavy fuel oils and crude oils. This ratio is chosen so that a P-value of six represents an FR5/1 of zero.
3.2.5 insolubility number, IN, n - a crude oil blending model parameter which can be used to determine if blends of oils are compatible or incompatible. See solubility blending number.
3.2.5.1 Discussion - Insolubility numbers for individual oils are determined and calculated from the density of the oil, aromatic solvent equivalent value and volume of paraffinic solvent (n-heptane) that can be added to 5 mL of oil without asphaltene precipitation. The equations are given under Calculation of Results (see 15.2).
3.2.6 maximum flocculation ratio, FRmax, n - of asphaltenes in residual fuel oils and crude oils, the minimum required solvency power of a solvent mixture, expressed as a ratio by volume of aromatic solvent (xylene) to aromatic solvent plus paraffinic solvent (n-heptane) to keep the asphaltenes in an oil colloidally dispersed.
3.2.6.1 Discussion - FRmax is determined from a plot of flocculation ratios versus the oil concentration in solvent, extrapolated to infinite dilution of the sample at the y-axis (where (1/X) = 0. See Eq 3).
3.2.7 oil matrix, n - that portion of a sample of heavy fuel oil or crude oil that surrounds and colloidally disperses the asphaltenes.
3.2.7.1 Discussion - For purposes of this test method, an oil sample is considered to be composed of an oil matrix (sometimes called an oil medium) and asphaltenes.
3.2.8 P-value, n - of refinery residual steams, residual fuel oils and crude oils, an indication of the stability or available solvency power of an oil with respect to precipitation of asphaltenes.
3.2.8.1 Discussion - Since the equation defining P-value is P = (1 + Xmin), where Xmin is the minimum volume of paraffinic solvent, n-heptane, (in mL) needed to be added to 1 g of oil to result in flocculation of asphaltenes, the smallest P-value is 1, which means the oil is unstable and can precipitate asphaltenes without addition of any paraffinic solvent. A higher P-value indicates that an oil is more stable with respect to flocculation of asphaltenes. P-value by this test method relates specifically to xylene and n-heptane as the aromatic and paraffinic solvents, respectively.
3.2.9 Pa, n - the P-value of an asphaltene, which is the peptizability or ability of an asphaltene to remain colloidally dispersed.
3.2.10 Po, n - the P-value of an oil matrix. See oil matrix.
3.2.11 peptize, v - of an oil or cutter stock, to dissolve an asphaltene or to maintain an asphaltene in colloidal dispersion.
3.2.12 solubility blending number, SBN, n - a crude oil blending model parameter which can be used to determine if blends of oils are incompatible or compatible. See insolubility number.
3.2.12.1 Discussion - Solubility blending numbers for individual oils are determined and calculated from the density of the oil, aromatic solvent equivalent value, and volume of paraffinic solvent that can be added to 5 mL of oil without asphaltene precipitation. The equations are given under Calculation of Results (see 15.2).
3.2.13 step size, n - the volume in mL of each portion of n-heptane added to the stock solution in the course of the test procedure.
3.2.14 stock solution, n - a solution of a sample dissolved in a specific amount of xylene.
3.3 Symbols:
FR = flocculation ratio
FR5/1 = flocculation ratio at a dilution of 5 mL solution (xylene plus n-heptane) to 1 g of oil
FRmax = maximum flocculation ratio
IN = insolubility number
P = the P-value of an oil
Pa = the P-value of an asphaltene
Po = the P-value or peptizing power of an oil matrix
SBN = solubility blending number
SE = xylene equivalent, volume %
Xmin = n-heptane consumption of undiluted oil, in mL/g of oil
