TBP distillation curve

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. 13-86 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.)... 

Regardless of the procedure used, certain initial steps must be taken for the determination or specification of certain product properties and yields based on the TBP distillation curve of the column feed, method of providing column reflux, column-operating pressure, type of condenser, and type of side-cut strippers ana stripping requirements. These steps are developed and ilhistrated with several detailed examples by Watkins (op. cit.). Only one example, modified from one given by Watkins, is considered briefly here to indicate the approach taken during the initial steps. 

Light-component analysis and the TBP and API gravity for the feed are given in Table 13-29. Representation of this feed by pseudocomponents is given in Table 13-30 based on 16.7°C (30°F) cuts from 82 to 366°C (180 to 690°F), followed by A1.1°C (TS T) and then 55.6°C (100°F) cuts. Actual tray numbers are shown in Fig. 13-114. Corresponding theoretical-stage numbers, which were determined by trial and error to obtain a reasonable match of computed- and measured-product TBP distillation curves, are shown in parentheses. Overall tray efficiency appears to be approximately 70 percent for the tower and 25 to 50 percent for the sidecut strippers. 

Figure 1 Typical crude TBP distillation curve and mid-percent gravity curve...

Make plots of the temperature in degrees Celsius AET as the ordinate (y—axis) versus the percentage mass and volume distilled as the abscissa (x—axis). These are the final TBP distillation curves. 

The feed to the fractionation system is often poorly defined in terms of actual components. We may only have an assay and associated bulk property measurements (such as density). We use the techniques discussed in Sections 1.1-1.3 to produce a complete TBP distillation curve and a density or specific gravity... 

We enter the distillation curve as shown in Figure 2.59. We must enter at least 5 points that include both the initial boiling point and the final point. For this model, we always enter as a 9-point distillation curve. It is also important to make sure that we choose the correct distillation type. As we noted earlier, we corrected all data to follow a D-86 curve. It is possible to use a combination of a D-86, D-1160, SimDist and TBP distillation curves in the assay input. The choice of distillation plays an important role when we create a pseudocomponent list because simulation software converts all distillation curves into a TBP basis. 

The properties of the different fraction recovered from atmospheric and vacuum distillations of a 13°API heavy crude oil are detailed in Table 1.10. With the volumetric yields, API gravity and sulfur content of each fraction, Figure 1.12 was constructed, in which the typical TBP distillation curve, API gravity curve, and sulfur curve for the three crude oils listed in Table 1.9 are shown. Heavier crude oils exhibit lower distillates yields, which have lower API gravity and possess higher content of sulfur. 

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