Crude Distillation Curves

In crude distillation, there are thousands of different compounds present having a virtually continuous spectrum of boiling points. It would be impractical to consider each of these compounds in describing the crude or designing the equipment to process it. Instead the crude is treated as if it were composed of a manageable number (< 50) of pseudo components. These are defined by dividing the crude distillation curve into a series of adjacent boiling cuts. 

The most important curve is the TBP distillation, properly defined as T = f (% volume or weight). Figure 8.4 shows the distillation curves for an Arabian Light crude. The chart is used to obtain yields for the different cuts as a function of the selected distillation range. 

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. 13-85 Average tnie-boiling-point distillation curves of crude oils. (Prom VP E. Edmister, Applied Hydrocarbon Thermodynamics, vol. I, 1st ed, 1961 Gulf Publishing Companif, Houston, Texas, Used with peirrussion All nghts leseived.)... 

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. 

Several methodologies can be used to identify not only crude or refined product type, but also the brand, grade, and, in some instances, the source crude. The petroleum industry has yielded conventional methods for the characterization of refined products. The simplest is the routine determination of API gravity and development of distillation curves where NAPL is present. More sophisticated methods include gas chromatography and statistical comparisons of the distribution of paraffinic or n-alkane compounds between certain C-ranges. With increased degradation and decomposition, the straight-chain hydrocarbons ( -alkanes) become less... 

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

Figures 10 and 11 show some examples of model predictions of the feedstock characterization of crude distillation residues. Figure 10 compares model predictions with the experimental distillation curves of three Arabian atmospheric residues. Figure 11 shows the model ability in predicting the aromatic carbon content and the H/C of different feeds in comparison with some NMR data. A more detailed description and discussion of this residue characterization is reported elsewhere (Bozzano et al., 1995, 1998).

Figure 1. Distillation curves for selected tar sand products and Wilmington crude...

With the aid of the columns and other apparatus described it is possible to separate liquid mixtures continuously at normal and reduced pressures as well as at small overpressures (chap. 5.4.5) as sharply a.s in batch distillation. This is clear from the distillation curves of the products obtained in the continuous distillation of a mixture of crude fatty acids in the C4 to Ci range (Fig. 168). A further example shows that by the use of control devices it was possible to separate a mixture of phenols into its main components with great constancy. Fig. 169 illustrates the results of the first separation in this sequence, which was made between the ortho- and Twcfa/pom-cresol fractions. 

Fig. 4-1 Distillation Curves of Crude Oil (Landau Field, Germany) Curve 1 Simulated Distillation by TG750 Curve 2 Boiling Curve DIN 51 751...

The new analytical developments appeared during the last decade are mostly focused on a faster determination of the distillation curve (TBP). HTSD is widely accepted as one of the best alternatives. Originally developed as an extension of conventional GC simulated distillation (ASTM D2887/D5307), it provides better information on the crude oil back ends. The technique is currently under study to become an ASTM standard, overcoming the problems related with reproducibility of front and heavy end information. 

Normally, liquid and gaseous hydrocarbons are a mixture of many different hydrocarbons. For example, gasoline consists of a mixture of 40 different hydrocarbons. Most liquid hydrocarbons like gasoline, kerosene, diesel, and so on are derived from crude oil by distillation or cracking processes each type is characterized by its distillation curve. The distillation curve is obtained by slowly heating the crude so that each hydrocarbon element vaporizes and condenses. The more volatile component is vaporized first. 

Figure 2.1 shows the TBP and atmospheric pressure EFV curves for a typical crude oil. Also illustrated is the planned yield of three distillate products for a process under study. Figure 2.2 shows the ASTM, TBP and atmospheric EFV distillation curves for a typical petroleum fraction. 

In this part will be discussed methods for estimating the yields and properties of the various products which can be produced off the atmospheric tower. The general method of attack will be to determine distillation properties of the desired products and apply these to the whole crude TBP curve in order to estimate volumetric yields. Volumetric yields are then used to obtain product gravities, molecular weights and other properties from the crude assay. There are many ways to define a product slate, but most of these are based on specifying certain ASTM distillation tern-... 

The final material balance superimposed on the whole crude TBP curve is shown by Figure 3.4. This is for a typical operation producing as basic products asphalt and two lube stocks. The other three distillates are yielded only as determined by the lube cuts. The following procedures were used to develop the overall material balance. 

L.S. Ott, B.L. Smith, T.J. Bruno 2008. Advanced distillation curve measurements for corrosive fluids Application to two crude oils. Fuel 87 (13-14), 3055-3064. 

The recommended distillation method for crude oils up to cutpoint 400 C (752 F) AET is Test Method D 2892. This test method can be used for heavy crude oils with initial boiling points greater than 1S0 C (302 F). However, distillation curves and fraction qualities obtain by these methods are not comparable. 

---