The boiling point of a pure compound in the liquid state is defined as the temperature at which the vapor pressure of the compound equals the atmospheric pressure or 1 atm. The boiling point of pure hydrocarbons depends on carbon number, molecular size, and the type of hydrocarbons (aliphatic, naphthenic, or aromatic) as discussed in Lesson 1. Figure 2.1 shows the boiling points of n-alkanes as a function of carbon number.
Complex mixtures such as crude oil, or petroleum products with thousands of different compounds, boil over a temperature range as opposed to having a single point for a pure compound. The boiling range covers a temperature interval from the initial boiling point (IBP), defined as the temperature at which the first drop of distillation product is obtained, to a final boiling point, or endpoint (EP) when the highest-boiling compounds evaporate. The boiling range for crude oil may exceed 1000 °F.
The ASTM D86 and D1160 standards describe a simple distillation method for measuring the boiling point distribution of crude oil and petroleum products. Using ASTM, D86 boiling points are measured at 10, 30, 50, 70, and 90 vol% distilled. The points are also frequently reported at 0%, 5%, and 95% distilled. ASTM D1160 is carried out at reduced pressure to distill the high-boiling components of crude oil. As an alternative method, distillation data can be obtained by gas chromatography (GC), in which boiling points are reported versus the weight percent of the sample vaporized. This test method described in ASTM D2887 is called simulated distillation (SimDis).
Average boiling points are useful in predicting physical properties and for characterization of complex hydrocarbon mixtures. The key here is to represent a mixture of compounds with a range of boiling points by a single characteristic boiling point. Since this is a formidable task, there are five different “average boiling points” that are used in different correlations. They are:
1, 2, and 3 can be defined for a mixture of n components as:
where ABP is is expressed as VABP, MABP, or WABP and xi is the corresponding volume, mole, or weight fraction of component i, and Tbi is the normal boiling point of component i. Cubic average boiling point (CABP) and Mean Average Boling Points (MeABP) can be calculated as follows.
For petroleum streams, volume, weight, or mole fractions of the components are not usually known. In this case, VABP is calculated from standard distillation (ASTM D86 Method) data, and empirical relationships (charts, or equations) are used to calculate the other average boiling points.
Here is the procedure:
Equation 1 (Ts are ASTM D86 temperatures for 10, 30, 50, 70, and 90% volume distilled, respectively):
Along with VABP, the slope of the ASTM D86, SL, is used for converting VABP to other average boiling points.
Equation 2:
The following empirical equations can, then, be used to obtain the temperature difference (ΔT) between VABP and other average boiling points (ABP) [2] :
Equation 3:
Equation 4:
Equation 5:
Equation 6:
and
Equation 7:
The temperature unit used for VABP, SL, and ΔT in these correlations is Kelvin.
The following script can be used to calculate VABP, MeABP by entering the distillation temperatures in the table.
You may also use the charts in Figure 4.1a and Figure 4.1b (p. 39) of your textbook [3] to obtain MeABP and MABP, respectively, from VABP. Note that the slope of the distillation curve used in those charts refers to True Boiling Point (TBP) distillation (not to ASTM distillation), and it is calculated as (T70% -T10%)/60.
[3] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 4, p.39.