4.13 What is a typical composition?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
4.13 What is a typical composition?
There seems to be a perception that all gasolines of one octane grade are
chemically similar, and thus general rules can be promulgated about "energy
content ", "flame speed", "combustion temperature" etc. etc.. Nothing is
further from the truth. The behaviour of manufactured gasolines in octane
rating engines can be predicted, using previous octane ratings of special
blends intended to determine how a particular refinery stream responds to
an octane-enhancing additive. Refiners can design and reconfigure refineries
to efficiently produce a wide range of gasolines feedstocks, depending on
market and regulatory requirements. There is a worldwide trend to move to
unleaded gasolines, followed by the introduction of exhaust catalysts and
sophisticated engine management systems.
It is important to note that "oxygenated gasolines" have a hydrocarbon
fraction that is not too different to traditional gasolines, but that the
hydrocarbon fraction of "reformulated gasolines" ( which also contain
oxygenates ) are significantly different to traditional gasolines.
The last 10 years of various compositional changes to gasolines for
environmental and health reasons have resulted in fuels that do not follow
historical rules, and the regulations mapped out for the next decade also
ensure the composition will remain in a state of flux. The reformulated
gasoline specifications, especially the 1/Jan/1998 Complex model, will
probably introduce major reductions in the distillation range, as well as
changing the various limits on composition and emissions.
I'm not going to list all 500+ HCs in gasolines, but the following are
representative of the various classes typically present in a gasoline. The
numbers after each chemical are:- Research Blending Octane : Motor Blending
Octane : Boiling Point (C): Density (g/ml @ 15C) : Minimum Autoignition
Temperature (C). It is important to realise that the Blending Octanes are
derived from a 20% mix of the HC with a 60:40 iC8:nC7 ( 60 Octane Number )
base fuel, and the extrapolation of this 20% to 100%. These numbers result
from API Project 45, and are readily available. As modern refinery streams
have higher base octanes, these Blending Octanes are higher than those
typically used in modern refineries. For example, modern Blending Octane
ratings can be much lower ( toluene = 111RON and 94MON, 2-methyl-2-butene
= 113RON and 81MON ), but detailed compilations are difficult to obtain.
The technique for obtaining Blending Octanes is different from rating the
pure fuel, which often requires adjustment of the test engine conditions
outside the acceptable limits of the rating methods. Generally, the actual
octanes of the pure fuel are similar for the alkanes, but are up to 30
octane numbers lower than the API Project 45 Blending Octanes for the
aromatics and olefins [52].
A traditional composition I have dreamed up would be like the following,
whereas newer oxygenated fuels reduce the aromatics and olefins, narrow the
boiling range, and add oxygenates up to about 12-15% to provide the octane.
The amount of aromatics in super unleaded fuels will vary greatly from
country to country, depending on the configuration of the oil refineries
and the use of oxygenates as octane enhancers. The US is reducing the levels
of aromatics to 25% or lower for environmental and human health reasons.
Some countries are increasing the level of aromatics to 50% or higher in
super unleaded grades, usually to avoid refinery reconfiguration costs or
the introduction of oxygenates as they phase out the toxic lead octane
enhancers. An upper limit is usually placed on the amount of benzene
permitted, as it is known human carcinogen.
15% n-paraffins RON MON BP d AIT
n-butane 113 : 114 : -0.5: gas : 370
n-pentane 62 : 66 : 35 : 0.626 : 260
n-hexane 19 : 22 : 69 : 0.659 : 225
n-heptane (0:0 by definition) 0 : 0 : 98 : 0.684 : 225
n-octane -18 : -16 : 126 : 0.703 : 220
( you would not want to have the following alkanes in gasoline,
so you would never blend kerosine with gasoline )
n-decane -41 : -38 : 174 : 0.730 : 210
n-dodecane -88 : -90 : 216 : 0.750 : 204
n-tetradecane -90 : -99 : 253 : 0.763 : 200
30% iso-paraffins
2-methylpropane 122 : 120 : -12 : gas : 460
2-methylbutane 100 : 104 : 28 : 0.620 : 420
2-methylpentane 82 : 78 : 62 : 0.653 : 306
3-methylpentane 86 : 80 : 64 : 0.664 : -
2-methylhexane 40 : 42 : 90 : 0.679 :
3-methylhexane 56 : 57 : 91 : 0.687 :
2,2-dimethylpentane 89 : 93 : 79 : 0.674 :
2,2,3-trimethylbutane 112 : 112 : 81 : 0.690 : 420
2,2,4-trimethylpentane 100 : 100 : 98 : 0.692 : 415
( 100:100 by definition )
12% cycloparaffins
cyclopentane 141 : 141 : 50 : 0.751 : 380
methylcyclopentane 107 : 99 : 72 : 0.749 :
cyclohexane 110 : 97 : 81 : 0.779 : 245
methylcyclohexane 104 : 84 : 101 : 0.770 : 250
35% aromatics
benzene 98 : 91 : 80 : 0.874 : 560
toluene 124 : 112 : 111 : 0.867 : 480
ethyl benzene 124 : 107 : 136 : 0.867 : 430
meta-xylene 162 : 124 : 138 : 0.868 : 463
para-xylene 155 : 126 : 138 : 0.866 : 530
ortho-xylene 126 : 102 : 144 : 0.870 : 530
3-ethyltoluene 162 : 138 : 158 : 0.865 :
1,3,5-trimethylbenzene 170 : 136 : 163 : 0.864 :
1,2,4-trimethylbenzene 148 : 124 : 168 : 0.889 :
8% olefins
2-pentene 154 : 138 : 37 : 0.649 :
2-methylbutene-2 176 : 140 : 36 : 0.662 :
2-methylpentene-2 159 : 148 : 67 : 0.690 :
cyclopentene 171 : 126 : 44 : 0.774 :
( the following olefins are not present in significant amounts
in gasoline, but have some of the highest blending octanes )
1-methylcyclopentene 184 : 146 : 75 : 0.780 :
1,3 cyclopentadiene 218 : 149 : 42 : 0.805 :
dicyclopentadiene 229 : 167 : 170 : 1.071 :
Oxygenates
Published octane values vary a lot because the rating conditions are
significantly different to standard conditions, for example the API Project
45 numbers used above for the hydrocarbons, reported in 1957, gave MTBE
blending RON as 148 and MON as 146, however that was partly based on the
lead response, whereas today we use MTBE in place of lead.
methanol 133 : 105 : 65 : 0.796 : 385
ethanol 129 : 102 : 78 : 0.794 : 365
iso propyl alcohol 118 : 98 : 82 : 0.790 : 399
methyl tertiary butyl ether 116 : 103 : 55 : 0.745 :
ethyl tertiary butyl ether 118 : 102 : 72 : 0.745 :
tertiary amyl methyl ether 111 : 98 : 86 : 0.776 :
There are some other properties of oxygenates that have to be considered
when they are going to be used as fuels, particularly their ability to
form very volatile azeotropes that cause the fuel's vapour pressure to
increase, the chemical nature of the emissions, and their tendency to
separate into a separate water-oxygenate phase when water is present.
The reformulated gasolines address these problems more successfully than
the original oxygenated gasolines.
Before you rush out to make a highly aromatic or olefinic gasoline to
produce a high octane fuel, remember they have other adverse properties,
eg the aromatics attack elastomers, may generate smoke, and result in
increased emissions of toxic benzene. The olefins are unstable ( besides
smelling foul ) and form gums. The art of correctly formulating a gasoline
that does not cause engines to knock apart, does not cause vapour lock in
summer - but is easy to start in winter, does not form gums and deposits,
burns cleanly without soot or residues, and does not dissolve or poison the
car catalyst or owner, is based on knowledge of the gasoline composition.
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