TBP Curves

Table 4.16a Coeff exam dents for conver pie applied to a pt ting a TBP curve Hroleum cut (Riaz into an ASTM D 86 curve and an i s method). ...

Initial portion of the TBP curve of a Saharan crude oil (Note the discontinuities due to the presence of aromatics benzene B, toluene T, xylenes X). 

A crude-oil assay always includes a whole crude API gravity and a TBP curve. As discussed by Nelson (op. cit., pp. 89-90) and as shown in Fig. 13-85, a reasonably consistent correlation (based on more than 350 distillation curves) exists between whole crude API gravity and the TBP distillation curve at 101.3 kPa (760 torr). Exceptions not correlated by Fig. 13-85 are highly paraffinic or naphthenic crude oils. 

FIG. 13>96 Comp arison of computed TBP curves with plant data for the example of Fig. 13-94. 

Important properties of petroleum and its fractions are measured by standardized procedures according to the API or ASTM. A particularly distinctive property is the true boiling point (TBP) curve as a function of the volume percent distilled under standardized conditions. Figure 13.19 is the TBP curve of a whole crude on which are superimposed curves of products that can be taken off sidestreams from a main distillation column, as in Figure 19.21. As samples of the distillate are collected, their densities and other properties of interest also are measured. The figure with Example 13.14 is of such measurements. 

In earlier years, API set up calculation procedures to convert these more easily run ASTM D86 and D1160 boiling point curves to the sought TBP curve data. This book presents a unique program, named ASTM4, which receives ASTM curve inputs, both D86 and D1160 data, and converts them to TBP data. API 3A1.1 and API 3A2.1 methods are... 

Feed analyses in terms of component compositions are usually not available for complex hydrocarbon mixtures with a final normal boiling point above about 38°C (100°F) (n-pentane). One method of handling such a feed is to break it down into pseudocomponents (narrow-boiling fractions) and then estimate the mole fraction and K value for each such component. Edmister [Ind. Eng. Chem., 47,1685 (1955)] and Maxwell (Data Book on Hydrocarbons, Van Nostrand, Princeton, N.J., 1958) give charts that are useful for this estimation. Once K values are available, the calculation proceeds as described above for multicomponent mixtures. Another approach to complex mixtures is to obtain an American Society for Testing and Materials (ASTM) or true-boiling point (TBP) curve for the mixture and then use empirical correlations to construct the atmospheric-pressure equihbrium flash vaporization (EFV) curve, which can then be corrected to the desired operating pressure. A discussion of this method and the necessaiy charts is presented in a later subsection Petroleum and Complex-Mixture Distillation. 

Figure 13.19. True boiling point (TBP) curve of a crude oil, with superimposed TBP curves of five fractions into which it is separated by a typical fractionating system like that of Figure 15.20. The separations are not sharp, with as much as 50°F difference between the end point of a light product and the initial of the next heavier one. It is common to speak of the gap between the 95 and 5% points rather than the end points.

The true boiling point and specific gravity variation with the volume percent distilled are found by standard ASTM procedures. In the present case, the smooth TBP curve is replaced by a stepped curve of eleven pseudo components characterized by their 50% boiling points and specific gravities. Their molecular weights are... 

---