Page 110 - Synthetic Fuels Handbook

96 CHAPTER THREE

Isoparaffins have higher octane numbers than the corresponding normal isomers, and the
octane number increases as the degree of branching of the chain is increased. Olefins have
markedly higher octane numbers than the related paraffins; naphthenes are usually better
than the corresponding normal paraffins but rarely have very high octane numbers; and
aromatics usually have quite high octane numbers.
Blends of n-heptane and isooctane thus serve as a reference system for gasoline and
provide a wide range of quality used as an antiknock scale. The exact blend, which matches
identically the antiknock resistance of the fuel under test, is found, and the percentage of
isooctane in that blend is termed the octane number of the gasoline. For example, gasoline
with a knocking ability which matches that of a blend of 90 percent isooctane and 10 percent
n-heptane has an octane number of 90.
With an accurate and reliable means of measuring octane numbers, it was possible to
determine the cracking conditions—temperature, cracking time, and pressure—that caused
increases in the antiknock characteristics of cracked gasoline. In general it was found that
higher cracking temperatures and lower pressures produced higher octane gasoline, but
unfortunately more gas, cracked residua, and coke were formed at the expense of the vol-
ume of cracked gasoline.
To produce higher-octane gasoline, cracking coil temperatures were pushed up to 510°C
(950°F), and pressures dropped from 1000 to 350 psi. This was the limit of thermal crack-
ing units, for at temperatures over 510°C (950°F) coke formed so rapidly in the cracking
coil that the unit became inoperative after only a short time on-stream. Hence it was at
this stage that the nature of the gasoline-producing process was reexamined, leading to the
development of other processes, such as reforming, polymerization, and alkylation for the
production of gasoline components having suitably high octane numbers.
During the manufacture and distribution of gasoline, it comes into contact with water
and particulate matter and can become contaminated with such materials. Water is allowed
to settle from the fuel in storage tanks and the water is regularly withdrawn and disposed
of properly. Particulate matter is removed by filters installed in the distribution system.
(ASTM D 4814, App. X6).
Oxygenates are carbon-, hydrogen-, and oxygen-containing combustible liquids that
are added to gasoline to improve performance. The addition of oxygenates gasoline is not
new since ethanol (ethyl alcohol or grain alcohol) has been added to gasoline for decades.
Thus, oxygenated gasoline is a mixture of conventional hydrocarbon-based gasoline and
one or more oxygenates. The current oxygenates belong to one of two classes of organic
molecules: alcohols and ethers. The most widely used oxygenates in the United States
are ethanol, methyl tertiary-butyl ether (MTBE), and tertiary-amyl methyl ether (TAME).
Ethyl tertiary-butyl ether (ETBE) is another ether that could be used. Oxygenates may be
used in areas of the United States where they are not required as long as concentration limits
(as refined by environment regulations) are observed.
Of all the oxygenates, methyl tertiary-butyl ether is attractive for a variety of technical
reasons. It has a low vapor pressure, can be blended with other fuels without phase sepa-
ration, and has the desirable octane characteristics. If oxygenates achieve recognition as
vehicle fuels, the biggest contributor will probably be methanol, the production of which is
mostly from synthesis gas derived from methane.
The higher alcohols also offer some potential as motor fuels. These alcohols can be
produced at temperatures below 300°C (572°F) using copper oxide-zinc oxide-alumina
catalysts promoted with potassium. Isobutyl alcohol is of particular interest because of its
high octane rating, which makes it desirable as a gasoline-blending agent. This alcohol
can be reacted with methanol in the presence of a catalyst to produce methyl tertiary-butyl
ether. Although it is currently cheaper to make isobutyl alcohol from isobutylene, it can be
synthesized from syngas with alkali-promoted zinc oxide catalysts at temperatures above
400°C (752°F).