STABILITY AND COMPATIBILITY OF HEAVY FUEL OILS AND CRUDE OILS BY OIL STABILITY ANALYZER (OPTICAL DETECTION): ASTM D7112
EXPLANATION
Automatic determination of stability parameters using a light back-scattering technique improves accuracy and removes human errors. In manual testing, operators have to visually compare the oil stains on pieces of filter paper to determine if asphaltenes have been precipitated. Refinery thermal and hydrocracking processes can be run closer to their severity limits if stability parameters can be calculated more accurately. This gives increased yield and profitability. Results from this test method could be used to set a standard specification for stability parameter for fuel oils. The compatibility parameters of crude oils can be used in crude oil blending in refineries to determine, in advance, which crude oil blends will be compatible and thus can be used to minimize plugging problems, unit shut downs, and maintenance costs. Determination of crude oil compatibility parameters also enables refineries to select crude oil mixtures more economically. This test method measures stability and compatibility parameters, and determines stability reserve on different blends for particular applications to optimize the blending, storage, and use of heavy fuel oils. Users of this test method would normally use stability and compatibility parameters to determine stability reserve of residual products, fuel blends, and crude oils. However, the interpretation of stability, stability reserve, and compatibility is heavily "use dependent", and is beyond the scope of this test method.
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. This test method is applicable to residual products from atmospheric and vacuum distillation, from thermal catalytic, 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 0.05 m % or greater concentration of asphaltenes. It would be pointless to apply this test method to unstable oils that already contain flocculated asphaltenes.
TEST SUMMARY
Stability and compatibility parameters are determined by titration and optical detection of precipitated asphaltenes. A stock solution is prepared and three different mixtures of the sample oil plus xylene are titrated with n-heptane to cause precipitation of asphaltenes. The titrated mixture is continuously circulated through an optical detector which detects precipitated asphaltenes by back-scattering of visible light. The amount of oil, xylene, and n-heptane are used to calculate stability parameters: solvent equivalent, P-value, and FR5/1. If the density of a crude oil sample is known, then the compatibility parameters (SBN and IN) of the crude oil may also be calculated.
INTERFERENCES - Free water present in the oil can cause difficulties with the optical detector and should be removed by centrifuging prior to testing. Solid particles such as coke or wax particles, mud, sand, or catalyst fines in the oil will not affect the optical detector or interfere with the results.
TEST PRECISION
Only limited amount of in-house data from three laboratories are published in the test method. A full interlaboratory study will be undertaken to determine the true precision. This test method has no bias because the results of the test are defined only in terms of this test method.
