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Average boiling point

Definition, explanation and examples of calculation of various types of average boiling point of petroleum products and other mixtures of hydrocarbons: VABP, MABP, WABP, CABP and MeABP

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Boiling point: The temperature at which a liquid turns into a gas

A mixture of different compounds boils over a certain range of temperature, reflecting the boiling point of each specific compound present in the mixture. For many purposes, it is suitably to calculate an average boiling point (ABP) of mixtures.

Then, the boiling point of each compound is weighted with regard to the fraction of the compound compared to the whole blend. There are several types of fractions that can be calculated: The mole fraction, the weight fraction and the volume fraction, and the type of fraction must be specified when the term average boiling point is used.  There is also a cubic average boiling temperature and a mean average boiling temperature. The definitions of each type of average boiling point are given by the formulas:

See also Average boiling point from gravity and molecular weight

Example: "Gasoline" mixture

Gasoline consists of branched paraffins, cycloparaffins and aromatics with carbon numbers typically from C4 to C12.

A "simple" gasoline could then contain a mixture of iso-pentane, 3-methylpentane, iso-octane, meta-xylene and hexylcyclohexane. The table below shows calculations of the different kinds of average boiling points of a mixture containing the given volume fractions of the five compounds

For full table - rotate the screen!

Compound Molweight Density@15°C Boiling point Xvol #mol Xmol Weight Xw
g/mol g/cm3 °C cm3/cm3 mol mol/mol g g/g
Iso-pentane 72.2 0.620 28 0.1 0.00086 0.13 0.062 0.084
3-methylpentane 86.2 0.660 63 0.2 0.00153 0.23 0.132 0.180
Iso-octane 114.2 0.690 99 0.3 0.00181 0.27 0.207 0.282
Meta-xylene 106.2 0.860 139 0.2 0.00162 0.24 0.172 0.234
Hexylcyclohexane 168.3 0.808 225 0.2 0.00096 0.14 0.162 0.220
MABP,  °C WABP,  °C VABP,  °C CABP,  °C MeABP,  °C
109.3 123.6 117.9 106.5 107.9

It can be seen that the WABP is biased against the densest components, while the MABP is biast against the lightest components.

However, a real gasoline contains hundreds of different compounds with individual properties, and a similar calculation is not possible to performe due to the complexity and lack of precise analytical tools.

The way to get past this restriction is to use a distillation curve obtained using a real distillation, as ASTM D86, or a gas chromatographic analysis, as ASTM D2887, to calculate VABP or WABP and then, use agreed correction factors to find the other kinds of ABPs.

The table below shows results from a D86 and a D2887 analysis of a naphtha sample, and the calculated VABP and WABP and the slopes for the distillation curves, using the formulas:

VABP = 0.2*(T10% + T30% + T50% + T70% + T90%)   from D86

WABP = 0.2*(T10% + T30% + T50% + T70% + T90%)   from D2887

Slope = (T90% - T10%)/80

Fraction of sample recovered D86 D2887
Vol% (D86) or Wt% (D2887) °C °F °C °F
IBP 121 85
5 129 104
10 132 115
20 134 123
30 136 129
50 143 144
70 153 160
80 157 167
90 163 176
FBP 183 197
VABP 145 294
WABP 145 293
Slope, °C/% recovered 0.39 0.76
Slope, °F/% recovered 0.70 1.37

Correction factors are found by using the figures below. Note! The figures are using °C or °F, and results from D86.

The narrower the cut of the oil fraction (smaller slope), the smaller is the correction.

The figures show that the correction from a VABP of 294°F to MABP is -6°F for a sample with a slope of 0.7°F/% recovered. That gives a MABP of 288°F.

The correction from a VABP of 294°F to CABP is only -1.5°F for a sample with a slope of 0.7°F/% recovered. (To see more detailes, right hand click on the figure and choose view image)

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