Speight J.G. Natural Gas: A Basic Handbook
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108
Chapter
4
Recovery, Storage, and Transportation
presence of
a
suitable catalyst) (Figure 4-2) (Speight, 1994 and refer-
ences cited therein; Davis, 2006 and references cited therein).
The Fischer-Tropsch process for natural gas is described by the fol-
lowing chemical equations:
CH,
+
950,
+2H2
+
CO
(4.1
1
(2n
+
1)H2
+
nCO +CnH2,,
+
nH,O
(4.2)
The mixture of carbon monoxide and hydrogen is called synthesis gas
or
spgus.
The resulting hydrocarbon products are refined to produce
the desired synthetic fuel. The utility of the process
is
primarily in its
role in producing fluid hydrocarbons or hydrogen from a carbon-
aceous feedstock such as natural gas or coal. Possible end products
include kerosene, naphtha, methanol, dimethyl ether (DME), alco-
hols, waxes, diesel fuel, and gasoline, with water or carbon dioxide
produced as
a
by-product. Dimethyl ether is also used as a transporta-
tion fuel or power generation fuel as well as a chemical feedstock for
the petrochemical industry.
Methanol is a gas-to-liquids option that has been in commission
since the mid 1940s. While methanol produced from gas was origi-
nally
a
relatively inefficient conversion process, optimized tech-
nology has improved the efficiency. Methanol can be used in internal
combustion engines as a fuel, but the current market for methanol as
a
fuel is limited, although the development of fuel cells for motor
vehicles may change this. Methanol is best used as
a
basic chemical
feedstock for the manufacture of plastics.
Other gas-to-liquids processes are being developed to produce clean
fuels, e.g. syncrude, diesel,
or
many other products including lubri-
cants and waxes, from gas but require complex (expensive) chemical
plant with novel catalyst technology.
4.3.8
Cas-to-Commodity
The components in natural gas, methane, ethane, propane, n-and
iso-butanes and pentanes are all useful in
their
own
right. The higher
paraffins are particularly valuable for
a
wealth
of
chemicals and
4.4
References
109
Figure
4-2
Conversion
of
gas
to
liquids
by
the Fischer Tropsch process.
polymer precursors such as acetic acid, formaldehyde, olefins, poly-
ethylene, polypropylene, acrylonitrile, ethylene glycol, etc., as well as
portable premium fuels, such as propane.
In
addition, methane can
be converted via syngas to methanol, ammonia, syncrude, lubricant,
or some precursor for chemicals manufacture, e.g., dimethyl ether
(DME),
urea, etc., and then used to make chemicals for export.
In
the gas-to-commodity concept, the gas is converted to thermal or
electrical power, which is then used
in
the production
of
the com-
modity, which
is
then sold, on the open market. It is the energy from
the gas, heat via electricity
or
direct combustion, and not the compo-
nents
of
the gas-to-liquids concept that
is
used. The gas energy is, in
essence, transported via the commodity.
4.4
References
Arnold,
K.
and Stewart, M.,
1999,
Surface Production Operations,
Volume
2:
Design
of
Gas-Handling Systems and Facilities,
2nd
edition, Gulf Professional
Publishing, Houston, Texas.
Materials, West Conshohocken, Pennsylvania.
ASTM,
2006,
AnnuaZ
Book
of
Standards,
American Society for Testing and
Ballard,
D.,
1965,
Hydrocarbon Processing.
44(4):
131.
Berecz,
E.
and Balla-Achs,
M.,
1983,
Gas
Hydrates,
Elsevier, Amsterdam, The
Brarrehaug,
A.
and
Gudmundsson,
J.S.
1996,
“Gas Transportation
in
Hydrate
Form,” Proceedings, EUROGAS
96,3541,
Trondheim, Norway, June
3-5.
Netherlands.
110
Chapter
4
Recovery, Storage, and Transportation
Cox, J.L.,
1983,
Natural Gas Hydrates: Properties, Occurrence and Recovery,
Butterworths, Woburn, Massachusetts.
Cranmore, R.G. and Stanton,
E.,
2000,
In Modern Petroleum Technology,
R.A.
Dawe (ed.), John Wiley and Sons Inc., Hoboken, New Jersey, Chapter
9.
Davis, B.,
2006,
Fischer-Tropsch Synthesis: Catalysts and Catalysis,
Elsevier,
Amsterdam, The Netherlands.
Fischer,
P.A.,
2001,
“Natural Gas: Part
8:
Monetizing Stranded Gas: How
Operators Will Bring ‘Worthless’ Gas to Market,”
World Oil,
222(11): 1.
Gaffney Cline and Associates,
2001,
GTL discussion paper, prepared for the
Gas to Liquids Taskforce, Australian Department of Industry, Science
&
Resources, Commonwealth of Australia, June.
Gudmundsson, J.S.,
1996,
Method for Production of Gas Hydrate for
Transportation and Storage. United States Patent No.
5,536,893.
Gudmundsson, J.S., Hveding,
F.,
and Brarrehaug, A.,
1995,
“Transport of
Natural Gas as Frozen Hydrate,” Proceedings,
Sh
International Offshore
and Polar Engineering Conference. The Hague, The Netherlands. June
11-16.
Volume
I:
282-288.
Gudmundsson, J.S. and Brarrehaug,
A.,
1996,
“Frozen Hydrate for Transport of
Natural Gas,” Proceedings, 2nd International Conf. Natural Gas
Hydrates, June
2-6,
Toulouse, pp.
415-422.
Gudmundsson, J.S., Andersson,
V.,
and Levik,
O.I.,
1997,
“Gas Storage and
Transport Using Hydrates,” Proceedings, Offshore Mediterranean
Conference, Ravenna. March
19-21.
Gudmundsson,
J.S.,
Andersson,
V.,
Levik,
O.I.,
and Parlaktuna,
M.,
1998,
“Hydrate Concept for Capturing Associated Gas,” Proceedings,
SPE
European Petroleum Conference, The Hague, The Netherlands. October
20-22.
Knott, D.,
1997,
Oil
&
Gas Journal,
June
23,
pp.
16-21.
Manning,
F.S.
and Thompson, R.E.,
1991,Oil
Field Processing
of
Petroleum,
Volume
I:
Natural Gas,
Pennwell Publishing Company, Tulsa, Oklahoma.
Max,
M.D.,
2000,
“Natural Gas in Oceanic and Permafiost Environments,”
Kluwer Academic Publishers, Dordrecht, The Netherlands.
Rojey,
A.,
Jaffret, C., Cornot-Gandolph,
S.,
Durand,
B.,
Jullin,
S.,
and Valais,
M.,
1997,
Natural Gas Production, Processing, Transport.
Editions Technip,
Paris, France.
Review,
January, Pages
38-39.
Butterworth Heinemann, Oxford, England.
Skrebowski, C.,
1998,
“Gas-to-Liquids or Liquefied Natural Gas?”
Petroleum
Speight,
J.G.,
1993,
Gas Processing: Environmental Aspects and Methods,
4.4
References
111
Speight,
J.G.,
1994,
The Chemistry and Technology
of
Coal,
Znd
edition. Marcel
Speight,
J.G.,
2002,
Handbook
of
Petroleum Product Analysis,
John Wiley
&
Sons
Speight,
J.G.,
2007,
The Chemistry and Technology
of
Petroleum,
4‘h edition.
Thomas,
M.,
1998, “Water into Wine: Gas-to-Liquids Technology the Key to
Thomas,
S.
and Dawe,
R.A.,
2003,
Energy-The International Journal,
28:
Dekker Inc., New
York.
Inc., Hoboken, New Jersey.
CRC-Taylor
&
Francis, Boca Raton, Florida.
Unlocking Future Reserves,”
Euroil.
May 17-21.
1461-1477.
History
of
Gas
Processing
Natural gas,
as
transported to domestic and industrial consumers, is
much different from the natural gas that appears at the wellhead.
Although the processing of natural gas is in many respects less com-
plicated than the processing and refining of crude oil, processing
(refining) natural gas is equally as necessary before sales. Natural
gas
used by consumers is composed almost entirely of methane (usually
>95%
by volume). However, natural gas found at the wellhead,
although still composed primarily of methane (usually
>
65%
by
volume), is by no means
as
pure
as
required by sales specifications.
Raw (impure) natural gas comes from three
types
of
wells:
1.
Crude oil wells
2.
Gas wells
3.
Condensate wells
Natural
gas
that
is
produced from crude oil wells is typically termed
associated
gas
and can exist separate from the crude oil in the reservoir
(free
gas),
or dissolved in the crude oil
(dissolved
gas).
Natural gas from
gas and condensate wells, in which there is little or
no
crude oil, is
non-associated
gas.
Gas wells typically produce only natural gas, while
condensate wells produce natural gas along with a liquid hydro-
carbon
condensate.
Whatever the source of the natural gas, once sepa-
rated from crude oil (if present), it commonly exists in mixtures with
other hydrocarbons; principally ethane, propane, butane, and pen-
tanes. In addition,
raw
natural gas contains water vapor
(H,O),
113
114
Chapter
5
History
of
Gas
Processing
hydrogen sulfide (H,S), carbon dioxide (CO,), helium (He), nitrogen
(N,), and other miscellaneous compounds.
Natural gas processing (refining) consists of separating all
of
the var-
ious hydrocarbons and fluids from the pure natural gas, to produce
pipeline quality
dry natural gas (i.e., gas that meets a specified
analytical composition). Pipeline companies usually impose restric-
tions on the composition of the natural gas that is allowed into the
pipeline. Thus, before the natural gas can be transported, it must be
purified, and while the ethane, propane, butane, and pentanes must
be removed from the methane, they are by no means waste products.
These higher molecular weight hydrocarbons, once extracted, are
termed
natural
gas
liquids
(NGLs) and are used
in
other products. But
whatever, the composition
of
the raw gas, it must be processed before
transportation and use.
As
already noted (Chapter
1),
the history
of
natural gas extends into
antiquity, but the history
of
gas processing is somewhat more recent.
In
fact, the history
of
gas processing is, of course, carefully inter-
twined with the development
of
gas use and gas technology (Speight,
1994;
Murphy et al.,
2005
and references cited therein). Therefore,
any treatise relating to the history of gas processing must refer to the
evolution of the use and development of gas production technology.
This must include reference to the original commercial gas industry
that involved the production of gas from coal. It is from such an
industry that the modern gas processing industry evolved.
5.1
Coal
Gas
Coal gas is the gaseous mixture (mainly hydrogen, methane, and
carbon monoxide) produced by the destructive distillation (i.e.,
heating in the absence of air) of bituminous coal. Sometimes steam is
added
to
react with the hot coke, thus increasing the yield of gas; coal
tar and coke are obtained as by-products.
Thus, before natural gas (and by inference the natural gas industry),
there was coal gas (also called
town
gas)
(Sugg,
1884;
Stewart,
1958;
Higman and Van Der Burgt,
2003).
Town gas is a gas manufactured
gas from coal produced for sale to consumers and municipalities. The
terms
manufactured
gas,
syngas (synthetic natural gas, SNG), and
hygas are also common. Depending on the processes used for its cre-
ation, the gas is a mixture of different gases of different heat value,
i.e., hydrogen, carbon monoxide, methane and higher molecular
5.1
CoalGas
115
weight volatile hydrocarbons with small amounts of gases, such
carbon dioxide and nitrogen, which detract from the heat value
of
the mixture.
Prior to the development
of
natural gas supplies and transmission
in
the United States during
1940s
and
195Os,
virtually all fuel and
lighting gas was manufactured, and the by-product coal tars were an
important chemical feedstock for the chemical industries. The devel-
opment of manufactured gas paralleled that
of
the industrial revolu-
tion and urbanization.
By way
of
introduction to this important topic, coal was known to be
used
in
Britain four to five thousand years ago, where it has been
detected as forming part
of
the composition
of
funeral pyres.
It
was
also commonly used as a fuel in the later period of the Roman occu-
pation. Evidence
of
trade
in
coal is plentiful and shows that coal was
in common use as a source
of
fuel
in
dwellings.
Carbon forms more than
60%
by weight of coal and is dependent
on
coal
rank,
with higher rank coals containing less hydrogen, oxygen,
and nitrogen, until
95%
purity
of
carbon is achieved at anthracite.
Methane gas (coal-bed methane, Chapter
1)
is a dangerous component
of
coal, because
it
can cause coal-seam explosions, especially in under-
ground mines, and it may cause the coal to spontaneously combust.
Coal
gasification
traditionally refers to a process through which solid
coal is converted into a low-to-medium energy fuel gas. Coal gasifica-
tion has its origin in
1780,
when it was used to produce a product
referred to variously as town gas or blue gas. Coal gasification is the
process by which coal is converted into a fuel gas. The process was
first developed
in
about
1780
and was widely commercialized by the
early
1900s.
Before natural gas became widely available
in
the
1940s,
many North American and European cities used coal gas as a heating
and lighting fuel.
It
was referred to variously as
bZue
gas,
producer
gas,
water
gas,
town
gas,
or
fuel
gas
depending
on
the method
of
produc-
tion, gas properties, and gas use. Often, using the same low-pressure
mains for distribution, natural gas replaced fuel gas in most uses by
the
1950s
because of its greater heating value and lack
of
contaminants.
During the
1920s,
the chemical industry began to explore gasification
to synthesize chemicals, but it
was
not until World War
I1
that the
real potential for gasification was realized.
In
the face
of
scarce oil
116
Chapter
5
History
of
Gas Processing
supplies and for several years thereafter, gasification was used exten-
sively by Germany (as well as Britain and France)
to
produce liquid
fuels from coal.
In
the face
of
this increasing use of coal for gasifica-
tion purposes, tar was another product
of
coal gasification processes
and had to be separated from the gas before the gas was sent
to
the
consumer.
The first process used
to
gasify coal was the carbonization and partial
pyrolysis
of
coal. The
off
gases liberated
in
the high-temperature car-
bonization (coking) of coal in coke ovens were collected, scrubbed,
and used as fuel. Depending
on
the goal of the plant, the desired
product was either a high-quality coke for metallurgical use, with the
gas being a side product or the production
of
a high-quality gas with
coke being the side product. Coke plants are typically associated with
metallurgical facilities, such as smelters and blast furnaces, while gas
works typically served urban areas.
In
the early years
of
manufactured gas plant operations, the goal
of
a
utility gas works was to produce the greatest amount
of
highly illumi-
nating gas. The illuminating power
of
a gas was related
to
the amount
of soot-forming hydrocarbons (illuminants) dissolved in it. These
hydrocarbons gave the gas flame its characteristic bright yellow color.
Gas works would typically use oily bituminous coals as feedstock, and
these coals would give
off
large amounts of volatile hydrocarbons
into the coal gas, but would leave behind a crumbly, low-quality coke
not suitable for metallurgical processes. Coal or coke oven gas typi-
cally had a heat value (CV) between
250
and
550
Btu/ft3.
Gas clean-up varied from process (Davidson,
1913)
to process, but
in
many processes volatile matter driven from the coal mass passed
upward through cast-iron
goosenecks
into a common horizontal steel
pipe (called the collecting main), which connected all the ovens
in
series. This unpurified
fouZ
gas
contained water vapor, tar, light oils,
solid particulates (coal dust), heavy hydrocarbons,
and complex
carbon compounds. The condensable materials were removed from
the exhaust gas
to
obtain purified coke oven gas.
As
it
left the heating
chamber, the gas was initially cleaned with a weak ammonia spray,
which condensed some tar and ammonia from the gas stream. This
liquid condensate flowed down the collecting main until it reached a
settling tank. Collected ammonia was used
in
the weak ammonia
spray, while the rest was pumped to
an
ammonia still. Collected coal
tar was pumped to a storage tank to await sales or use as a fuel.
5.1
Coal
Gus
117
The remaining gas was cooled as it passed through a condenser and
then compressed by an exhauster. Any remaining coal tar was
removed by a tar extractor, either by impingement against a metal
surface or collection by an electrostatic precipitator (ESP). Further
amounts
of
ammonia were removed by passing the gas through a sat-
urator containing a
5
to
10%
solution
of
sulfuric acid, where the
ammonia reacted with sulfuric acid
to
form ammonium sulfate,
which was crystallized and removed.
The gas is further cooled, resulting in the condensation
of
naphtha-
lene. The light oils were removed in an absorption tower containing
water mixed with
straw
oil
(a high boiling petroleum fraction). The
straw oil acted as an absorbent for the light oils, and was later heated
to release the light oils for recovery and refinement. The last cleaning
step was the removal of hydrogen sulfide from the gas that was usually
achieved
in
a scrubbing tower after which the gas was suitable for use
as fuel for the coke ovens, other plant combustion processes,
or
sold.
Coal gas was used for lighting, heating, and cooking.
In
1802,
Wil-
liam Murdock installed gas burners at James Watt's (steam engine)
factory, near Manchester.
In
1812,
gas lighting was used in the streets
of
London.
In
England objectors to gaslight argued that it was
destroying the whaling industry, because demand for whale oil for
lighting was declining.
Coal gas was produced using a retorting operation
in
which the coal,
inside airtight retorts, was heated by a coke furnace (Figure
5-1).
Air
was forced through the coke to produce carbon monoxide, which
burned around the retorts. The temperature was high enough to drive
off all volatiles over a period
of
hours. Tar and water (with dissolved
ammonia) formed in the air-cooled condensers. These liquids were
pumped to the liquid separator. The gaseous impurities were
hydrogen cyanide (HCN,
0.1%
by volume), hydrogen sulfide
(H,S,
1.3%
by volume), and carbon disulfide (CS,,
0.04%
by volume). The
gas purification section of the plant produced gas suitable for heating
and lighting. The gas was scrubbed to remove the last traces
of
ammonia.
Carbon disulfide was removed by reaction with hydrogen using a
nickel catalyst
in
an atmosphere
of
hydrogen:
CS,
+
ZH,
+ZH,S
+
C
118
Chapter
5
History
of
Gas
Processing
Figure
5-1
Schematic
of
a coal
gas
plant.
CS,
+
4H, +2H,S
+
CH,
Next, the gas was passed through iron
(111)
oxide removing hydrogen
sulfide:
Fe,O,
+
3H,S +Fe,S,
+
3H,O
The hydrogen cyanide was removed (as Prussian blue):
Fe,O,
+
6HCN -+2FeC3N,
+
3H,O
Prussian blue, a valuable pigment, was washed out with water. The
remaining material was sold to sulfuric acid plants:
2Fe,S3
+
90,
-+2Fe,0,
+
6S0,
The liquid separator produced ammonia solution and coal tar. Nearly
all the ammonia was converted into fertilizer (ammonium sulfate).