APPLICATION NOTE
Analysis of Natural Gas and Natural Gas Liquids
Typically, natural gas is bought and sold on an
energy basis; therefore, composition analysis is
required to determine its heating value or BTU
content, in addition to other properties such as
specific gravity, compressibility, etc.
The Gas Processors Association (GPA) has
published methods for the analysis of natural
gas and demethanized hydrocarbon liquid
mixtures (Standards GPA 2261 and GPA 2177).
Components of interest include oxygen,
nitrogen, carbon dioxide, light hydrocarbons
and heavier hydrocarbons grouped by
backflushing. The grouped peak comprises
hexane and heavier components (C6+) in the
gas method (GPA 2261) and heptane and
heavier (C7+) in the liquid method GPA 2177.
Introduction
Experimental
Hydrogen sulphide, a component often present
in natural gas, is frequently of quantitative
interest. Although it is eluted as a well-defined
peak from the specified column, special
precautions may be needed to obtain
satisfactory accuracy.
The analytical system (Figure 1) provides the
requirements of both of these methods.
Three columns are used to accomplish the
single-channel analysis of natural gas: (1) a
molecular sieve for oxygen and nitrogen
separation, (2) a short stripper DC-200 column
for separating the hexanes- (or heptanes-) plus
fraction from the lighter components, and (3) an
analytical DC-200 column for the separation of
air, CO2, H2S and C1 through C5 hydrocarbons.
The 10-port valve allows two simultaneous
injections, one onto the molecular sieve column,
the other onto the short DC-200 column. The
columns are connected to a 12-port valve which
is plumbed to provide two functions: (1)
sequence reversal of the two DC-200 columns,
with backflush of the C6+/C7+ group from the
shorter stripper column to the detector, and (2)
backflush to vent of the molecular sieve.
The sequence of the DC-200 columns is
reversed as soon as the lighter components
through n-pentane pass from the short column
into the analytical column. The column system is
designed to elute oxygen and nitrogen from the
molecular sieve before the time required for the
sequence reversal. This ensures that the
permanent gases have been detected before
the 12-port valve is switched to backflush the
C6+/C7+ peak to the detector. The grouped
“heavies” peak is followed by the individual
peaks of the remaining components (Figure 2).
For chromatograms, please refer to the next
page.
AN084 Natural Gas System B
Advanced Projects & Custom Solutions Department
Figure 1. Flow diagram Natural Gas System B
Column 1
TCD Reference
TCD
front
Vent
EFC-24
(HE)
Sample OUT
SAMPLE LOOP
SAMPLE LOOP
1
2
3
4
5
6
7
8
9
10
11
12
V - 2
1
2
3
4
5
6
7
8
9
10
V - 1
Sample IN
V-3
NV-1
FID
middle
Column 2
Column 3
EFC-24
(HE)
APPLICATION NOTE
Chromatograms
AN084 Natural Gas System B
Figure 2. Natural Gas standard on 2nd TCD
Figure 3. Natural Gas standard on FID
APPLICATION NOTE
SCION Instruments
UK
Livingston Business Centre
Kirkton South Road, Livingston
West Lothian EH54 7FA
Scotland, UK
Phone +44 1506 300 200
The Netherlands
Amundsenweg 22-24
4462 GP Goes,
The Netherlands
Phone +31 (0) 113 287 600
AN084 Natural Gas system B
The Natural Gas system B hardware can be
upgraded with a number of options:
For natural gas liquids (GPA standard 2177), an
optional 4-port liquid sampling valve can be
installed. This injects the liquefied gas into the
DC200/500 columnset. The molecular sieve
column is not used since oxygen/nitrogen
separation is not needed on liquefied samples.
In those cases where the Helium and-/or
Hydrogen content needs to be quantified too,
an additional packed column channel can be
added. This consists of another
Injection+Backflush valve, packed columns and
TCD operated with nitrogen carriergas.
See figure 4.
Available options
Instead of adding a He/H2 channel *) a
dedicated low sulfur channel can be added too.
This channel consists of a GSV, S/SL or mini
gassplitter, capillary column and PFPD or SCD
detector.
This channel can provide detailed information
about the content of hydrogen sulphide,
carbonyl sulphide and mercaptans. See figure 5
below.
*) NOTE: A SCION Gaschromatograph can accommodate maximum 3 detectors.
So, only one additional channel can be added.
The dedicated Natural Gas Reporting Tool is a
powerful plug-in software package in the SCION
CompassCDS which allow calculations according
all common standards. The report can be
customized according the users’ wishes.
Software
Figure 4: Helium and Hydrogen analysis on 1st TCD
Peak ID
Compound Name
1
Helium
2
Hydrogen
Figure 5:
Sulfur components in Natural gas by means of PFPD
Peak ID
C o mp o und N ame
1
H2S
2
COS
3
M ethyl-M ercaptan
