ASTM distillation curves

Usually, product specifications for a crude distillation unit are expressed in terms of the products 15/5 or ASTM distillation curves. The prediction of a product 15/5 distillation is accomplished simply by blending the quantities of the pseudo components in the stream so as to form a true boiling point, 15/5 equivalent, distillation curve. This curve can then be converted to an ASTM type distillation using an empirical method. Figure 5 illustrates how a typical ASTM curve compares to the 15/5 curve for the same material. 

Boiling-point basis Volumetric average boiling point, which is equal to graphical integration of the differential ASTM distillation curve (Van Winkle s exact method ). 

Note that Fig. 4-18 can be used to relate the slopes of any pair of distillation curves. Thus, an ASTM distillation curve with a slope of 6.32 (reading from left scale) will have the following slopes when distilled by other methods (reading on vertical line) ... 

Convert one ASTM distillation curve into another... 

To extend the applicability of the characterization factor to the complex mixtures of hydrocarbons found in petroleum fractions, it was necessary to introduce the concept of a mean average boiling point temperature to a petroleum cut. This is calculated from the distillation curves, either ASTM or TBP. The volume average boiling point (VABP) is derived from the cut point temperatures for 10, 20, 50, 80 or 90% for the sample in question. In the above formula, VABP replaces the boiling point for the pure component. 

The normalized distillation curves, TBP, ASTM D 86, and ASTM D 1160, provide a way to judge the quality of a fractionation performed on petroleum cuts. 

Because of the time and expense involved in conducting laboratoiy distillation tests of all three basic types, it has become increasingly common to use empirical correlations to estimate the other two distillation curves when either the ASTM, TBP, or EFN- curve is available. Preferred correlations given in the API Technical Data Book—Petroleum Refining (op. dt.) are based on the work of Edmister and Pollock [Chem. Eng. Prog., 44, 905 (1948)], Edmister and Okamoto [Pet. Refiner, 38(8), 117 (1959) 38(9), 271 (1959)], Maxwell Data Book on... 

FIG. 13-86 Relationship between ASTM and TBP distillation curves. (From VF C. Edmistet Applied Hydrocarbon Thermodynamics, -ool. 1, 1st ed., 1961 Gulf Publishing Company, Houston, Texas. Used with permission. All tights reserved.)... 

As discussed by Nelson (op. cit.), virtually no fractionation occurs in an ASTM distillation. Thus, components in the mixture do distill one by one in the order of their boiling points but as mixtures of successively higher boiling points. The IBP, EP, and intermediate points have little theoretical significance, and, in fact, components boiling below the IBP and above the EP are present in the sample. Nevertheless, because ASTM distillations are quickly conducted, have been successfully automated, require only a small sample, and are quite reproducible, they are widely used for comparison and as a basis for specifications on a large number of petroleum intermediates and products, including many solvents and fuels. Typical ASTM curves for several such products are shown in Fig. 13-102. 

FIG. 13-84 Comparison of ASTM, TBP, and EFV distillation curves for kero-sine. 

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