Page 104 - Synthetic Fuels Handbook

FUELS FROM PETROLEUM AND HEAVY OIL 91

The principal constituent of natural gas is methane (CH ). Other constituents are par-
4
affinic hydrocarbons such as ethane (CH CH ), propane (CH CH CH ), and the butanes
3
3
2
3
3
[CH CH CH CH and/or (CH ) CH]. Many natural gases contain nitrogen (N ) as well as
2
3
3 3
2
3
2
carbon dioxide (CO ) and hydrogen sulfide (H S). Trace quantities of argon, hydrogen, and
2
2
helium may also be present. Generally, the hydrocarbons having a higher molecular weight
than methane, carbon dioxide, and hydrogen sulfide are removed from natural gas prior to
its use as a fuel. Gases produced in a refinery contain methane, ethane, ethylene, propyl-
ene, hydrogen, carbon monoxide, carbon dioxide, and nitrogen, with low concentrations of
water vapor, oxygen, and other gases.
Liquefied Petroleum Gas. Liquefied petroleum gas (LPG) is the term applied to certain
specific hydrocarbons and their mixtures, which exist in the gaseous state under atmo-
spheric ambient conditions but can be converted to the liquid state under conditions of
moderate pressure at ambient temperature. These are the light hydrocarbons fraction of
the paraffin series, derived from refinery processes, crude oil stabilization plants and natu-
ral gas processing plants comprising propane (CH CH CH ), butane (CH CH CH CH ),
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3
2
3
3
2
2
isobutane [CH CH(CH )CH ] and to a lesser extent propylene (CH CH=CH ), or butylene
2
3
3
3
3
(CH CH CH=CH ). The most common commercial products are propane, butane, or
3
2
2
some mixture of the two and are generally extracted from natural gas or crude petroleum.
Propylene and butylenes result from cracking other hydrocarbons in a petroleum refinery
and are two important chemical feedstocks.
Mixed gas is a gas prepared by adding natural gas or liquefied petroleum gas to a manu-
factured gas, giving a product of better utility and higher heat content or Btu value.
The compositions of natural, manufactured, and mixed gases can vary so widely, no
single set of specifications could cover all situations. The requirements are usually based
on performances in burners and equipment, on minimum heat content, and on maximum
sulfur content. Gas utilities in most states come under the supervision of state commissions
or regulatory bodies and the utilities must provide a gas that is acceptable to all types of
consumers and that will give satisfactory performance in all kinds of consuming equip-
ment. However, there are specifications for liquefied petroleum gas (ASTM D1835) which
depend upon the required volatility.
Since natural gas as delivered to pipelines has practically no odor, the addition of an
odorant is required by most regulations in order that the presence of the gas can be detected
readily in case of accidents and leaks. This odorization is provided by the addition of trace
amounts of some organic sulfur compounds to the gas before it reaches the consumer. The
standard requirement is that a user will be able to detect the presence of the gas by odor
when the concentration reaches 1 percent of gas in air. Since the lower limit of flammability
of natural gas is approximately 5 percent, this 1 percent requirement is essentially equiva-
lent to one-fifth the lower limit of flammability. The combustion of these trace amounts of
odorant does not create any serious problems of sulfur content or toxicity.
The different methods for gas analysis include absorption, distillation, combustion, mass
spectroscopy, infrared spectroscopy, and gas chromatography (ASTM D2163, ASTM D2650,
and ASTM D4424). Absorption methods involve absorbing individual constituents one at
a time in suitable solvents and recording of contraction in volume measured. Distillation
methods depend on the separation of constituents by fractional distillation and measurement
of the volumes distilled. In combustion methods, certain combustible elements are caused to
burn to carbon dioxide and water, and the volume changes are used to calculate composition.
Infrared spectroscopy is useful in particular applications. For the most accurate analyses,
mass spectroscopy, and gas chromatography are the preferred methods.
The specific gravity of product gases, including liquefied petroleum gas, may be deter-
mined conveniently by a number of methods and a variety of instruments (ASTM D1070,
ASTM D4891).