Petroleum Products - Average Boiling Points

Boiling point: T he 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

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Petroleum Products - Average Boiling Points

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*(T_10% + T_30% + T_50% + T_70% + T_90% )   from D86

WABP = 0.2*(T_10% + T_30% + T_50% + T_70% + T_90% )   from D2887

Slope = (T_90% - T_10% )/80

Naphta - D86 and D2887 Analysis

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)