Petroleum Fractions

In the chemical industry, we deal with compositions (mole fractions). In petroleum refining, we deal with boiling point ranges. For example, suppose that we take a sample of heating oil and place it in a heated container at atmospheric pressure. The temperature at which the first vapor is formed is called the "initial boiling point." This corresponds to the bubblepoint of a mixture of specific chemical components. If we continue to heat the sample, more and more material is vaporized. The "5% point" is the temperature at which 5% of the original sample has vaporized. Liquid volume percents are traditionally used. The "95% point" is the temperature at which 95 liquid vol% of the original sample has vaporized. The "final boiling point" is the temperature at which all of the liquid disappears. This is somewhat similar to the dewpoint of a mixture of specific chemical components. Heating oil has a 5% point of about 460°F and a 95% point of -620°F.

There are three types of boiling point analysis: ASTM D86 ("Engler"), ASTM D158 ("Saybolt") and true boiling point (TBP). The first and second are similar to the boiling of vapor as described in the previous paragraph. In the third, the vapor from the container passes into a packed distillation column and some specified amount is refluxed. Thus the third analysis exhibits some fractionation, while the first and second are just single-stage separations. The ASTM analysis is easier and faster to run. The TBP analysis gives more detailed information about the contents of the crude.

Figure 11.1 gives typical boiling curves for a light naphtha stream. The curve in Figure 11.1a is a TBP curve and that in Figure 11.1b, an ASTM D86 curve. The abscissa shows the volume percent distilled; the ordinate, temperature. Note that the initial and final parts of the curves are quite different because of the fractionation that occurs in the TBP distillation. The 50% boiling point is almost the same (249 and 243°F). Table 11.1 compares the results of these two methods.

L. Nelson, Petroleum Refinery Engineering, 4th ed., McGraw-Hill, 1958.

Figure 11.2 gives typical curves for a crude oil. Note the very wide range of the boiling points from -100 to 1600°F. The TBP curve is wider than the ASTM curve. Table 11.2 gives numerical values of these curves. The 50% points are similar, but both ends of the curves are different.

These curves are obtained by using the Plot Wizard. As we will demonstrate in the next section, after the steady-state program has run, click on Results Summary in the Data Browser. Then click Plot on the top toolbar and select Plot Wizard. The window shown in Figure 11.3a opens. Clicking the Dist Curve and hitting Next open the window shown in Figure 11.3b. The stream of interest and the type of curve are selected. Clicking Next and then Finish produces a plot.

Figure 11.2 gives typical curves for a crude oil. Note the very wide range of the boiling points from -100 to 1600°F. The TBP curve is wider than the ASTM curve. Table 11.2 gives numerical values of these curves. The 50% points are similar, but both ends of the curves are different.

These curves are obtained by using the Plot Wizard. As we will demonstrate in the next section, after the steady-state program has run, click on Results Summary in the Data Browser. Then click Plot on the top toolbar and select Plot Wizard. The window shown in Figure 11.3a opens. Clicking the Dist Curve and hitting Next open the window shown in Figure 11.3b. The stream of interest and the type of curve are selected. Clicking Next and then Finish produces a plot.

TABLE 11.1 Comparison of Boiling Point Methods for Naphtha

Vol% Distilled

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