Distillation true-boiling-point curves

The physical properties (normal boiling points, densities, and molecular weights) of the 34 pseudo components selected to represent the true boiling-point curves of the feed, distillate, and sidestreams are presented by Cecchetti et al.8 On the basis of these data, a feed composed of 34 pseudo components and having the molar compositions and total flow rate shown in Table 4-13 was selected by Cecchetti et al. The specifications for the theoretical analogue column are also given in this table. 

Assay analyses of whole crudes are done by combining an atmospheric and vacuum distillation run. These two runs when combined will provide a TBP (True Boiling Point Curve). While these batch distillation methods are labor intensive, taking between three to five days, they allow the collection of distillation fractions that can be utilized for testing. While each of the distillations techniques have been standardized by ASTM, cut schemes tend to mimic Refinery product classifications and there is no standardization of the individual inspection formats. Each corporation tends to perform both physical and chemical testing that best meet the needs of their refining operations and product suites. 

The physical distillation data from the D 2892 and D 5236 distillation runs are used to create a True Boiling Point Curve (TBP). These two curves will have a disparity in the area corresponding to the overlap between the 15/5 and Hivac distillation runs. This artifact is due to the Hivac still not being at optimum conditions at the early stages of the run. Once a TBP curve has been created property measurements can then be curve fit. 

The design methods considered for multicomponent mixtures in Chap. 9 were based on a limited number of definitely known components. In some cases, the mixtures are so complex that the composition with reference to the pure component is not known. This is particularly true of the petroleum naphthas and oils which are mixtures of many series of hydrocarbons, many of the substances present having boiling points so close together that it is practically impossible to separate them into the pure components by fractional distillation or any other means. Even if it were possible to determine the composition of the mixture exactly, there are so many components present that the methods of Chap. 9 would be too laborious. It has become customary to characterize such mixtures by methods other than the amount of the individual components they contain, such as simple distillation or true-boiling-point curves, density, aromaticity (or some other factor related to types of compounds), refractive index, etc. 

It has been found possible to use true-boiling-point curves to dehne the compositions for distillation calculation. A fraction distilling over a narrow range is taken as an individual component. Thus the fraction coming over as distillate between 39 and 40 per cent in curve B of Fig. 11-2 might be considered as a component, the boiling point of which, at the pressure at which the distillation was carried out, being... 

If the distillate during a true-boiling-point distillation were to be divided into two fractions at some convenient point. A, corresponding to the temperature h, and rimple distillation and true-boiling-point curves obtained for the two fractions, the results would resemble the... 

The true-boiling-point curves for the feed, the distillate, and the residue are given in Fig. 11-5. The curves for the liquids sampled from the plates are given in Fig. 11-6. 

Fig. 19-9. True-boiling-point curves of distilled charge stocks except curve 6, which contains some residual material. (See Table 19-15 for kinds of materials.)...

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. 

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. 

FIG. 13-85 Average true-boiling-point distillation curves of crude oils. From W. E. Edmister, Applied Hydrocarbon Thermodynamics, vol. 1, 1st ed., 1961 Gulf Publishing Company, Houston, Texas, Used with permission. All rights reserved.)... 

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... 

Geddes, R.L. Computation of petroleum fractionation—estimation of A.S.T.M. distillation curves from true boiling-point distillation analyses. Ind. Eng. Chem. 1941, 33, 795-801. 

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