True Boiling Point TBP

True Boiling Point (TBP) jL Simulated distillation (gas chromatography) ... 

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. 

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. 

Data from a true boiling point (TBP) distillation test provide a much better theoretical basis for characterization. If the sample contains compounds that have moderate differences in boiling points such as in... 

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.

In addition to the whole crude oil tests performed as part of the inspection assay, a comprehensive or full assay requires that the crude be fractionally distilled and the fractions characterized by the relevant tests. Fractionation of the crude oil begins with a true boiling point (TBP) distillation using a fractionating column with an efficiency of 14-18 theoretical plates and operated at a reflux ratio of 5 1 (ASTM D-2892). The TBP distillation may be used for all fractions up to a maximum cut point of about 350°C atmospheric equivalent temperature (AET), but a low residence time in the still (or reduced pressure) is needed to minimize cracking. 

One of the most important physical parameters dehning these products is their boiling range distribution (ASTM D-86, ASTM D-1078, ASTM D-2887, ASTM D-2892, IP 123). However, this method is a low-efficiency, one-theoretical plate distillation, and, although it has been adequate for product specihcation purposes, true boiling point (TBP) data are also required (ASTM D-2887, ASTM D-2892). 

Testing Materials (ASTM) Or in more elaborate, refluxed true boiling point (TBP) stills. Complex mixtures have been handled successfully for years in the petroleum industry, using empirical methods such as those of Packie23 and Hough)and et al.23 Such methods have been summarized and converted to stepwise computational procedures by Van Winkle 24 they may be regarded as approximate methods. 

A distillation curve with initial and final boiling point. It is converted to a true boiling-point (TBP) curve ... 

This equipment contains no external feed or withdrawal mechanism, other than the withdrawal of distillate at the outlet from the reflux tube. In principle, distillation is continued until the vat is exhausted - i.e. until all the liquid contained in the vat has gone. The study presented below is the theory of the true boiling point (TBP) curve used by American Petroleum Engineers. This curve represents the variations in the temperature of the distillate in the vapor state at the top of the column as a function of the fraction of the vat s contents having been vaporized. If the column is equivalent to 30 theoretical plates and if the reflux rate is around 30 (i.e. very high), the TBP appears as a succession of platforms, each corresponding to a pure component. 

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