Speight J.G. Natural Gas: A Basic Handbook
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76
Chapter
3
Composition
and
Properties
50
0
-50
-250
Carbon
Number
Figure
3-7
Carbon number and flash point
of
natural gas hydrocarbons
(up to octane,
CJ-Il&.
The flash point
of
a petroleum product is also used to detect contami-
nation.
A
substantially lower flash point than expected
for
a product
is a reliable indicator that a product has become contaminated with a
more volatile product, such as gasoline. The flash point is also an aid
in establishing the identity
of
a particular petroleum product.
A
further aspect
of
volatility that receives considerable attention is the
vapor pressure
of
petroleum and its constituent fractions. The
vapor
pressure
is the force exerted
on
the walls of a closed container by the
vaporized portion
of
a liquid. Conversely, it is the force that must be
exerted
on
the liquid
to
prevent it from vaporizing further
(ASTM
D323).
The vapor pressure increases with temperature for any given
gasoline, liquefied petroleum gas, or other product. The temperature
at which the vapor pressure of a liquid, either a pure compound
of
a
mixture of many compounds, equals
1
atmosphere
(14.7
psi, absolute)
is designated as the boiling point of the liquid.
The flammable range is expressed by the
lower explosive limit
(LEL)
and the
upper explosive limit
(UEL).
The
lower explosive limit
is the con-
centration
of
natural gas
in
the air below which the propagation
of
a
flame will not occur
on
contact with an ignition source. The
lower
explosive limit
for natural gas is
5%
by volume
in
air and, in most
cases, the smell
of
gas would
be
detected
well
before
combustion
con-
ditions are met. The
upper explosive limit
is the concentration
of
nat-
3.2
ProDenYes
77
ural gas
in
the air above which the propagation of a flame will not
occur on contact with an ignition source. The natural gas
upper
explosive
limit
is
15%
by volume in air.
Explosions caused by natural gas leaks occur a few times each year.
Individual homes, small businesses, and boats are most frequently
affected when an internal leak builds up gas inside the structure. Fre-
quently, the blast will be enough
to
significantly damage a building
but leave it standing. Occasionally, the gas can collect in high enough
quantities
to
cause a deadly explosion, disintegrating one or more
buildings
in
the process.
In
any form, a minute amount
of
odorant that has an obvious smell is
added to the otherwise colorless and odorless gas,
so
that leaks can be
detected before a fire or explosion occurs. Odorants are considered
non-toxic in the extremely low concentrations occurring
in
natural
gas delivered to the end user.
3.2.5
Behavior
An ideal gas is a gas in which all collisions between atoms or mole-
cules are perfectly elastic and
in
which there are
no
intermolecular
attractive forces.
An
ideal gas can be characterized by three variables:
absolute pressure,
P;
volume,
V;
and absolute temperature, T. The rela-
tionship between them is called the
ideal
gas
law:
PV
=
nRT
=
NkT
where
n
=
number of moles
R
=
universal gas constant
(=
8.3145
J/mol
K)
N
=
number
of
molecules
k
=
Boltzmann constant
(=
1.38066
x J/K
=
8.617385
x
lo-'
eV/K)
k
=
R/N,
NA
=
Avogadro's number
=
6.0221
x /mol
78
Chapter
3
Composition and PropelZies
The ideal gas law can arises from the pressure of gas molecules col-
liding with the walls of a container. One mole
of
an ideal gas at stan-
dard temperature and pressure occupies
22.4
liters. However, natural
gas is a non-ideal gas; it does not obey the ideal gas law but obeys the
modified gas law:
PV
=
nZRT
where
P
=
pressure
V
=
volume
T
=
absolute temperature (degree Kelvin,
OK)
Z
=
compressibility
n
=
number of kilo-moles of the gas
R
=
gas constant
For example,
if
all other factors remained constant, when the volume
of
a certain mass
of
gas is reduced by
SO%,
the pressure would double
and
so
on.
As
a gas, it would expand to fill any volume it is in. How-
ever the compressibility, Z, is the factor that distinguishes natural gas
from an ideal gas. For methane,
Z
is
1
at
1
bar, but decreases to
0.85
at
100
atmospheres, both at
25°C;
that is, it compresses to a smaller
volume than the proportional relationship.
3.2.6
Compression
and
Expansion
When gas is compressed, work is done and thus
it
gets hotter. It is
therefore necessary to cool gas during or after compression. Equally
when it
is
expanded, adiabatically (without heat being added) it gets
colder. This latter phenomenon is used to cool gas during treatment
to remove liquids.
Gas is difficult
to
store
in
the gaseous state outside the reservoir to
provide flexibility
of
supply. Only
a
small amount
of
flexibility is pro-
vided by the high-pressure
gas
in pipelines, known
as
linepack.
3.2
ProDerties
79
3.2.7
Liquefied Natural Gas
If
gas is produced at lower pressures than typical sales pipeline pres-
sure
(700
to 1,000 psi), it is compressed to sales gas pressure
(Mokhatab et al., 2006, Chapter
8).
Transport of sales gas is done at
high pressure to reduce pipeline diameter. Pipelines may operate at
very high pressures (above
1,000
psig) to keep the gas
in
the dense
phase, thus preventing condensation and two-phase flow. Compres-
sion typically requires
two
to three stages
to
attain sales gas pressure.
As
stated previously, processing
may
be
done after the first
or
second
stage, prior to sales compression.
Compression is used
in
all aspects of the natural gas industry,
including gas lift, reinjection
of
gas for pressure maintenance, gas
gathering, gas processing operations (circulation
of
gas through the
process or system), transmission and distribution systems, and
reducing the gas volume for shipment by tankers or for storage. In
recent years, there has been a trend toward increasing pipeline-
operating pressures. The benefits of operating at higher pressures
include the ability to transmit larger volumes
of
gas through a given
size of pipeline, lower transmission losses due to friction, and the
capability to transmit gas over long distances without additional
boosting stations. In gas transmission, two basic types of compres-
sors are used:
reciprocating
and
centrifigal
compressors. Reciprocating
compressors are usually driven by either electric motors or gas
engines, whereas centrifugal compressors use gas turbines or electric
motors as drivers.
When natural gas is cooled to about -160°C (about -260°F) at atmo-
spheric pressure, it condenses to a liquid (liquefied natural gas; LNG).
One volume of this liquid takes up about 1/600th the volume
of
nat-
ural gas. Liquefied natural gas weighs less than one-half that
of
water,
actually about 45% as much. Liquefied natural gas is odorless, color-
less, non-corrosive, and non-toxic. When vaporized
it
burns only in
concentrations
of
5% to 15% when mixed with air. Neither liquefied
natural gas, nor its vapor, can explode
in
an unconfined
environment. Because liquefied natural gas takes less volume and
weight, it presents more convenient options for storage and
transportation.
The task of gas compression is
to
bring gas from a certain suction
pressure to a higher discharge pressure by means of mechanical work.
ideal processes: isothermal, isentropic, and polytropic compression.
The actual compression process
is
often compared
to
one
of
three
80
Chapter
3
Composition and Properties
Isothermal compression
occurs when the temperature is kept constant
during the compression process. It is not adiabatic because the heat
generated in the compression process must be removed from the
system. The compression process is
isentropic
or adiabatic reversible
if
no
heat is added to
or
removed from the gas during compression, and
the process is frictionless. The
polytropic compression
process is, like the
isentropic cycle, reversible but it is not adiabatic. It can be described
as an infinite number
of
isentropic steps, each interrupted by isobaric
heat transfer. This heat addition guarantees that the process will yield
the same discharge temperature as the real process.
3.2.8
Environmental
Properties
The environmental issues regarding the use
of
natural gas are dis-
cussed in Chapter
8,
but a brief mention of such properties is also
warranted here. However, to fully evaluate the environmental effects
of
natural gas, the general properties
of
the constituents (Table
3-6)
must also be considered
in
addition to the effects
of
the combustion
properties.
Currently, natural gas represents about one quarter
of
the energy con-
sumed in the United States, with increases in use projected for the
next decade. These increases are expected because emissions
of
green-
house gases are much lower with the consumption
of
natural gas rela-
tive to other fossil fuel consumption. For example, natural gas, when
burned, emits lower quantities
of
greenhouse gases and pollutants per
unit
of
energy produced than other fossil fuels. This occurs
in
part
because natural gas is fully combusted more easily and in part because
natural gas contains fewer impurities than any other fossil fuel. How-
ever, the major constituent
of
natural gas, methane, also contributes
directly to the greenhouse effect through venting or leaking
of
nat-
ural gas into the atmosphere (Speight,
2005).
Purified natural gas (methane) is the cleanest
of
all the fossil fuels.
The main products
of
the combustion
of
natural gas are carbon
dioxide and water vapor. Coal and petroleum release higher levels
of
harmful emissions, including a higher ratio of carbon emissions,
nitrogen oxides
(NO,),
and sulfur dioxide
(SO,).
Coal and fuel oil also
release ash particles into the environment, substances that do not
burn but instead are carried into the atmosphere and contribute to
pollution. The combustion
of
purified natural gas,
on
the other hand,
releases very small amounts
of
sulfur dioxide and nitrogen oxides,
3.2
Properties
81
Table
34
General Properties
of
the Constituents
of
Natural Gas
Up
To and Including n-octane
(C8H18),
as well as Toluene, Ethyl
Benzene, and Xylene
Vapor Boiling Ignition Flash
Mo'ecular
Density Point Temperature Point
"C "C
"C
Weight Gravity
air
=
,
Methane 16 0.553 0.56 -160 537 -22
1
Ethane 30 0.572 1.04 -89
515
-135
Propane 44 0.504 1.50 -42 468 -104
Butane 58 0.601 2.11
-1
405 -60
Pentane 72 0.626 2.48 36 260 -40
Hexane 86 0.659 3.00 69 225 -23
Benzene 78 0.879 2.80 80 560
-1
1
Heptane 100 0.668 3.50 98 215
-4
Octane 114 0.707 3.90 126 220 13
~ ~~ ~~ ~
Toluene 92 0.867 3.20 161 533
4
Ethyl benzene 106 0.867 3.70 136
432
15
Xylene 106 0.861 3.70 138 464
17
virtually
no
ash or particulate matter, and lower levels of carbon
dioxide, carbon monoxide, and other reactive hydrocarbons.
Natural gas has
no
known toxic or chronic physiological effects (that
is, it is not poisonous), but it is dangerous insofar as an atmosphere
rich in natural gas will asphyxiate humans and animals. Exposure to a
moderate concentration of natural gas may result in a headache or
similar symptoms due to oxygen deprivation, but it is likely that the
smell (through the presence of the odorant) would be detected well in
advance
of
concentrations being high enough for this to occur.
In
the natural gas and refining industries (Speight,
ZOOS),
as in other
industries, air emissions include point and non-point sources. Point
sources are emissions that exit stacks and flares and, thus, can be
monitored and treated. Non-point sources are
figitive
emissions
that
82
Chapter
3
Composition and Properties
are difficult to locate and capture. Fugitive emissions occur
throughout refineries and arise from, for example, the thousands of
valves, pipe connections, seals in pumps and compressors, storage
tanks, pressure relief valves, and flanged joints.
While individual leaks are typically small, the sum of all fugitive leaks
at a gas processing plant can be one
of
its largest emission sources.
These leaks can release methane and volatile constituents
of
natural
gas into the air. Companies can minimize fugitive emissions by
designing facilities with the fewest possible components and connec-
tions and avoiding components known to cause significant fugitive
emissions. When companies quantify fugitive emissions, this pro-
vides them with important information they can then use to design
the most effective leak-repair program for their company. Directed
inspection and maintenance programs are designed to identify the
source
of
these leaks and to prioritize and plan their repair
in
a timely
fashion.
A
reliable and effective directed inspection and maintenance
plan for an individual facility is composed
of
several components,
including methods of leak detection, a definition
of
what constitutes
a
leak, set schedules and targeted devices for leak surveys, and
allowable repair time.
A
directed inspection and maintenance program begins with a base-
line survey to identify and quantify leaks. Quantification of the leaks
is critical because this information is used to determine which leaks
are serious enough to justify their repair costs. Repairs are then made
only to the leaking components that are cost effective to fix. Subse-
quent surveys are then scheduled and designed based
on
information
collected from previous surveys, permitting operators to concentrate
on the components that are more likely to leak. Some natural gas
companies have demonstrated that directed inspection and mainte-
nance programs can profitably eliminate as much as
95%
of gas losses
from equipment leaks.
3.3 References
ASTM,2006,
Annual Book
of
Standards,
American Society for Testing and
Mokhatab,
S.,
Poe, W.A., and Speight,
J.G.,
2006,
Handbook
of
Natural Gas
Speight,
J.G.
(ed.),
2005,
Lunge’s
Handbook
ofchemistry,
16”
Edition,
McGraw-
Materials (ASTM), West Conshohocken, Pennsylvania.
Transmission and Processing,
Elsevier, Amsterdam, The Netherlands.
Hill, New York.
3.3
References
83
Speight, J.G.,
1993,
Gas Processing: Environmental Aspects and Methods,
Speight, J.G., 2002,
Handbook of Petroleum Product Analysis,
John Wiley
&
Sons
Speight, J.G.,
2005,
Environmental Analysis and Technology for the Refining
Speight, J.G., 2007,
The Chemistry and Technology
of
Petroleum,
4th
Edition,
Butterworth Heinemann, Oxford, England.
Inc.,
Hoboken, New Jersey.
Industry.
John Wiley
&
Sons
Inc., Hoboken, New Jersey.
CRC-Taylor
&
Francis, Boca Raton, Florida.
Part
II
Gas
Processing
CHAPTER
4
Recovery, Storage, and
Transportation
4.1
Recovery
Natural gas is recovered (extracted) from the reservoir through a
well
(a drilled structure that is not a natural occurrence). However, extrac-
tion
of
natural gas through a well leads to decrease
in
pressure
in
the
reservoir that, in turn, will lead to decreased rates
of
gas production
from the reservoir.
Once a potential natural gas reservoir has been located, the decision
of
whether or not to drill a well depends
on
a variety
of
factors,
not
the least
of
which is the economic characteristics
of
the gas reservoir.
After the decision to drill has been made, the exact placement
of
the
drillsite depends on a variety
of
factors, including the nature
of
the
potential formation to be drilled, the characteristics of the subsurface
geology, and the depth and size
of
the target deposit. During this
time,
it
is also necessary for the drilling company to complete all the
necessary steps
to
ensure that it can legally drill in that area. This usu-
ally involves securing permits for the drilling operations, establish-
ment
of
a legal arrangement
to
allow the natural gas company
to
extract and sell the resources under a given area of land, and a design
for gathering lines that will connect the well
to
the pipeline.
If the new well does in fact come in contact with a natural gas reser-
voir, the well is developed to allow for the extraction of the natural gas
and is termed a
development
well
or
productive
well.
At this point, with
the well drilled and hydrocarbons present, the well may be completed
to facilitate its production
of
natural gas. However,
if
the exploration
team was incorrect
in
its estimation
of
the existence
of
marketable
87