Assay Distillation Types
Many feed stocks are characterized by assay analysis rather than by actual composition. Many different analysis methods are available in industry, each having its own advantages and disadvantages. Their suitability in any specific circumstance depends upon many factors. Several assay methods have become de facto standards in the energy business. PRO/II recognizes four types of laboratory distillations that may be used to supply assay data:
True Boiling Point (TBP) distillations are run in the laboratory with a fractionating still which is refluxed. TBP's correspond closely to a component by component separation of the sample and may even have "steps" in the temperature versus percent distilled curve where large volumes of a particular component occur. The curves are used directly to develop the average normal boiling points for the petroleum pseudocomponents. TBP distillations are normally reported by the laboratory on a 760 mm Hg basis, even though they are often run at reduced pressures (30 to 40 mm Hg) for the highest boiling portion of the mixture. The disadvantages of TBP distillations are twofold: they are time consuming and they are costly.
ASTM D86 is a laboratory test designed for gasoline and light distillates. A sample of the fluid is placed in a flask. Heat is applied and the material that boils off is condensed and collected in a receiver at laboratory pressure. The temperature versus amount collected is recorded. The test is fast, inexpensive to run, and repeatable within a few degrees Fahrenheit. The initial boiling point (IBP) is defined as the temperature at which the first drop of liquid is collected. The final boiling point or end point is defined as the maximum temperature reached during the distillation. The heaviest material in the sample usually remains in the flask and is reported as residue. The lightest material in the sample is not condensed and is reported as loss. Thus, this type of distillation is incomplete and does not account for the lightest or heaviest material in the sample. For this reason, the ASTM D86 distillation must be adjusted via a conversion method to a TBP basis before it may be used to define the average normal boiling points for the petroleum pseudocomponents. A correction should be applied to the reported temperatures for laboratory pressures other than 760 mm Hg. The laboratory may or may not pressure correct the reported values.
ASTM D1160 is a laboratory test designed for heavy distillates and residual streams. A sample of the fluid is placed in a flask and distilled at reduced pressures. The temperature versus amount distilled is recorded. The test is similar to the ASTM D86 in most respects. It is inexpensive to run and repeatable within a few degrees Fahrenheit. However, it fails to account for the lightest material in the sample which is reported as loss and the heaviest material in the sample which is reported as residue. For this reason, the test is adjusted via a conversion method to a TBP basis before being used to define the average boiling points for the pseudo-components. Because the test is run at low pressures (typically 1 to 10 mm Hg), the results approximate the TBP distillation very well for the middle and upper ranges of the distillation curve. In fact, the ASTM D1160 test is often used to extend the data from a conventional TBP distillation for high boiling mixtures such as crude oil since the lower still pressures attainable for ASTM D1160 tests allow more complete distillation of the heaviest materials in the sample. ASTM D1160 tests are normally reported by the laboratory on a 760 mm Hg basis.
ASTM D2887 is a simulated distillation test which uses chromatography to separate the component fractions. The test is designed to approximate the TBP still and results are usually very close to TBP results. Advantages of the method are threefold: inexpensive compared to TBP's, accurate and repeatable, and heavier materials may be separated and reported than from TBP stills. Results are normally reported on a weight basis at 760 mm Hg. PRO/II converts these curves to a TBP basis. The default method comes from the 1994 API Technical Data Book, but options allow using a correlation from the 1963 or 1987 edition of that Data Book. The conversion correlation was developed for gasoline and light distillates, and it becomes inaccurate for heavy stocks such as gas oils, crude oils, resids, etc. For heavy stocks, it is recommended that ASTM D2887 distillations be designated as TBP Distillations and used without conversion.
Distillation Basis and Pressure
The basis for laboratory distillations may be either liquid volume percent or weight percent. Default basis is liquid volume percent for TBP, ASTM D86, and ASTM D1160, and weight percent for ASTM D2887.
For distillations reported at pressures other than atmospheric (760 mm Hg), the pressure should be supplied. Appropriate correlations are used to convert the distillation temperatures as needed. See Assay Curves - Technical Information for additional information.
Correction for Cracking
ASTM D86 temperatures may be corrected for cracking using an API Technical Data Book correlation. The API Cracking Equation is used to adjust reported temperatures that are greater than 475 degrees Fahrenheit.
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