True distillation curve

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. 

The mathematical solution of Eq. (A.15) is tedious. An elegant graphical solution has been proposed by Stichlmair and Fair [1]. The occurrence of a reactive azeotrope is expressed geometrically by the necessary condition that the tangent to the residue (distillation) curve be collinear with the stoichiometric line. Such points form the locus of potential reactive azeotropes. In order to become a true reactive azeotrope the intersection point must also belong to the chemical equilibrium... 

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.)... 

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. 

To design complex distillation columns, multicomponent methods are used. The true boiling curve is replaced by an approximate stepwise representation as a shown in Figure 12.18. Each step represents a pseudocomponent with a boiling point as indicated and a fraction of the total feed mixture based on the length of the horizontal portion of the step. 

Distillation Curve Type, we use the drop-down menu to select True boiling point (liquid volume basis). In the Bulk gravity value, the number 31.4 is entered in the API gravity box. 

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

Most of the investigations of the relation between the composition of liquid and of vapour have been carried out at constant temperature, but in practice a liquid is almost always distilled under constant pressure. Brown, however, whose distillations were carried out in the usual manner, found, in the case of carbon tetrachloride and carbon disulphide, that when a mixture was boiled the composition of the vapour was independent of the pressure under which ebullition took place, and, if this were generally true, a curve constructed from results obtained at constant temperature could be used to ascertain the vapour composition in a distillation under constant pressure. It is, however, to be noticed that the ratio of the vapour pressures, even of two closely related liquids, is not the same at different temperatures, and if the relation xJX2 = PiWJP W is really true for such liquids, PJP would be a constant for a distillation at constant temperature, but would vary slightly if the distillation were carried out under constant pressure. 

True boihng point curves are not usually determined directly by batch distillation because achieving complete fractionation in the laboratory is impractical. Instead, standardized batch distillation tests are condncted nnder closely defined conditions. Several snch tests are specified by the American Society for Testing and Materials, inclnding procedures D86, D1160, and D2887 (ASTM, 1990). These distillation curves are converted to TBP curves using methods documented by the American Petroleum Institute (API, 1983). 

Relative Bias—There exists a bias between the empirical results of distillation properties obtained by this test method and the true boiling point distillation curve obtained by Test Method D 2892. The amount of relative bias between the two test methods has not been determined. 

The test method details procedures for the production of a liquified gas, distillate fractions and residuum of standardized quality on which analytical data can be obtained, and the determination of yields of the above fractions by both mass and volume. From the above information a graph of temperature versus mass-percent distilled can be produced. This distillation curve corresponds to a laboratory technique which is defined at 15/5 (15 theoretical plate column, 5 1 reflux ratio) or TBP (true boiling point). 

Fio. 4-3. Average true-boiling-point distillation curves of crude oils. 

Tnie-boiling-point Apparatus. This equipment is used to obtain a so-called true-boiling-point distillation curve (see Figs. 4-3 and 4-9). An equipment that accomplishes a good degree of fractionation is termi d true-boiling-point equipment. 

Fig. 4-20. Correlation of Hempel and true-bo3ing-point distillation curves (based mainly on crude oils). (Oil Gas J.)...

Fig. 7-9. True-boiling-point and ASTM (dotted) distillation curves of topped crude oils produced by flashing at 400, 550, and 650 F. Oil Gas J.)...

Experimental Vaporization Curves. When such complex materials as gasoline and petroleum fractions are dealt with, the application of the aforementioned equilibrium laws is cumbersome. Furthermore, the component analyses of these heavy oils cannot be easily obtained and even if such analyses are-availalUe, accurate vapor-pressure or equilibrium data for the compounds or fractions contained in them are not always available. At present most equilibrium relations are obtained by determining experimental flash-vaporization curves or by computing such curves from the empirical relationships discussed in Chap. 4. Empirical flash curves can be estimated from true-boiling-point or ASTM curves, and with less accuracy from Hempel or Saybolt distillation curves. 

Fio. 16-21. True-boiling-point distillation curves of gasolines produced in a four-plate tower using various reflux ratios. Note the curve for infinite reflux and infinite plates. Oil Gas J.)... 

The approximate condensation and true-boiling>point distillation curves of the products are shown in Fig. 17-16, and a diagram of the system is shown in Fig. 17-17. nce this example is aimed at an illustration of principles, the molecular weights, specific heats, and latent heats have not been corrected to the latest values shown in this edition. 

Distillation curves for FCC feedstock can be limited. Because of the nature of the feedstock, complete true boiling point (TBP) analysis without D-2887/SimDist methods is frequently not possible. Many refiners still use a limited D-1160 distillation method to obtain some information about the distillation curve. Table 4.9 shows a typical D-1160 analysis for a heavy FCC feedstock. 

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. 

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