Page 106 - Synthetic Fuels Handbook

FUELS FROM PETROLEUM AND HEAVY OIL 93

hydrocarbon chain lengths range from C through C with a general hydrocarbon distribu-
l2
4
tion consisting of alkanes (4–8 percent), alkenes (2–5 percent), isoalkanes (25–40 percent),
cycloalkanes (3–7 percent), cycloalkenes (l–4 percent), and aromatics (20–50 percent).
However, these proportions vary greatly.
The majority of the members of the paraffin, olefin, and aromatic series (of which there
are about 500) boiling below 200°C (392°F) have been found in the gasoline fraction of
petroleum. However, it appears that the distribution of the individual members of straight-
run gasoline (i.e., distilled from petroleum without thermal alteration) is not even.
Highly branched paraffins, which are particularly valuable constituents of gasoline(s),
are not usually the principal paraffinic constituents of straight-run gasoline. The more pre-
dominant paraffinic constituents are usually the normal (straight-chain) isomers, which
may dominate the branched isomer(s) by a factor of two or more. This is presumed to indi-
cate the tendency to produce long uninterrupted carbon chains during petroleum maturation
rather than those in which branching occurs. However, this trend is somewhat different for
the cyclic constituents of gasoline, that is, cycloparaffins (naphthenes) and aromatics. In
these cases, the preference appears to be for several short side chains rather than one long
substituent.
Gasoline can vary widely in composition: even those with the same octane number may
be quite different, not only in the physical makeup but also in the molecular structure of
the constituents. For example, the Pennsylvania petroleum is high in paraffins (normal and
branched), but the California and Gulf Coast crude oils are high in cycloparaffins. Low-
boiling distillates with high content of aromatic constituents (above 20 percent) can be
obtained from some Gulf Coast and West Texas crude oils, as well as from crude oils from
the Far East. The variation in aromatics content as well as the variation in the content of nor-
mal paraffins, branched paraffins, cyclopentanes, and cyclohexanes involve characteristics
of any one individual crude oil and may in some instances be used for crude oil identifica-
tion. Furthermore, straight-run gasoline generally shows a decrease in paraffin content with
an increase in molecular weight, but the cycloparaffins (naphthenes) and aromatics increase
with increasing molecular weight. Indeed, the hydrocarbon type variation may also vary
markedly from process to process.
The reduction of the lead content of gasoline and the introduction of reformulated gaso-
line has been very successful in reducing automobile emissions Further improvements in
fuel quality have been proposed for the years 2000 and beyond. These projections are
accompanied by a noticeable and measurable decrease in crude oil quality and the reformu-
lated gasoline will help meet environment regulations for emissions for liquid fuels.
Gasoline was at first produced by distillation, simply separating the volatile, more valu-
able fractions of crude petroleum. Later processes, designed to raise the yield of gasoline
from crude oil, decomposed higher molecular weight constituents into lower molecular
weight products by processes known as cracking. And like typical gasoline, several pro-
cesses produce the blending stocks for gasoline (Fig. 3.18).
Thermal cracking and catalytic cracking, once used to supplement the gasoline supplies
produced by distillation, are now the major processes used to produce gasoline. In addi-
tion, other methods used to improve the quality of gasoline and increase its supply include
polymerization, alkylation, isomerization, and reforming.
Polymerization is the conversion gaseous olefins, such as propylene and butylene into
larger molecules in the gasoline range. Alkylation is a process combining an olefin and par-
affin such as isobutane. Isomerization is the conversion of straight-chain hydrocarbons to
branched-chain hydrocarbons. Reforming is the use of either heat or a catalyst to rearrange
the molecular structure.
Despite the variations in the composition of the gasoline produced by the various avail-
able processes, this material is rarely if ever suitable for use as such. It is at this stage of a
refinery operation that blending becomes important (Speight, 2007).