Dual Channel Analysis of
Refinery Gas: Refinery Gas
System B
Varian Application Note
Number 36
Brian Thompson
Varian Chromatography Systems
Key Words: Refinery Gas, Unsaturated Hydrocarbons, Hydrogen Sulfide, Carbonyl Sulfide, PLOT Alumina Column
Introduction
Experimental
A single-channel analysis of refinery gas is described
in Application Note #34 (Refinery Gas System A). In
that method, a single thermal conductivity detector
(TCD) is used with argon carrier to obtain good
sensitivity and linearity for hydrogen. Because of the
low sensitivity of the TCD for carbon oxides against
argon carrier, they are converted to methane and
detected by flame ionization. Sulfur gases also are not
detected well enough by the TCD against argon; and
since they are not detectable by FID, they are not
measured at all.
Two 3400s are needed for concurrent operation of the
three detectors. GC #1 (Figure 1), with TCD and FID,
provides measurement of hydrogen (and helium if
present) against argon carrier on the TCD, and
hydrocarbon separation on the PLOT column with
helium carrier. GC #2 (Figure 2), with TCD only,
separates and detects oxygen, nitrogen, carbon
monoxide, carbon dioxide, hydrogen sulfide and
carbonyl sulfide using helium as carrier. Peaks are
also produced for hydrogen, methane, ethene and
ethane, but these are not needed for quantitation on
this GC.
The dual-channel method described here utilizes two
TCDs, the first with argon carrier for best performance
on hydrogen, the second with helium carrier for
detection of all other required elemental and inorganic
gases, including the common sulfur compounds.
Hydrocarbons from C1 to about C7 or C8 are
separated on a PLOT alumina column and detected by
FID. If desired, a 4-port valve may be installed to
permit backflushing of heavier components.
Figure 1. GC #1
With two chromatographs, the column systems
needed for the separations are simpler than that used
in System A.
Figure 2. GC #2
For the hydrogen and hydrocarbons on GC #1, two
aliquots of sample are injected simultaneously by
means of the 10-port valve, one into a molecular sieve
for the hydrogen, the other via a capillary injector into
the PLOT column for the hydrocarbons. After elution
of oxygen, the molecular sieve is backflushed to
remove the remainder of the sample and the detector
output is switched to the FID for the hydrocarbons
(Figure 3).
A similar parallel system is used in GC #2. The 10-port
valve injects two aliquots simultaneously, one into a
molecular sieve, the other via a delay column, into a
porous polymer. The molecular sieve elutes oxygen,
nitrogen, methane and carbon monoxide in less than
three minutes. After the carbon monoxide peak, the 8port valve is switched to backflush the heavy
components from the molecular sieve. This valve
switch also reverses the position of the porous
polymer in time to receive the sample from the delay
column. The porous polymer separates carbon
dioxide, hydrogen sulfide and carbonyl sulfide from the
elemental gases and light hydrocarbons for detection
by TCD (Figure 4). At the end of the analysis, after
elution of COS, the 8-port valve is switched back to its
standby (initial) position. Heavier components are
backflushed off the porous polymer to vent while it is in
standby.
If both chromatographs are operated at the same time
a dual-channel data system is required for data
acquisition, with an appropriate program to combine
the results.
Molecular Sieve
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Helium
Hydrogen
Oxygen
Methane
Ethane
Ethene
Propane
Cyclopropane
Propene
Is o b u ta n e
Propadiene
n-Butane
trans-Butene-2
Butene-1
Is o b u te n e
cis-Butene-2
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
2,2-Dimethylpropane
Isopentane/Methylacetylene
n-Pentane
1,3-Butadiene
Pentene-1
cis-Pentene-2
Methylcyclopentane
Cyclohexane
2-Methylpentane
3-Methylpentane
n-Hexane
Methylhexanes
n-Heptane
Benzene
C9s
C8s
Figure 3. Refinery System B (GC#1)
Molecular Sieve
Porous Polymer
1
2
3
4
5
6
7
8
9
10
11
12
Hydrogen
Oxygen
Nitrogen
CH4
CO
O2+N2
CH4
CO2
Ethene
Ethane
H2S
COS
Figure 4. Refinery System B (GC#2)
These data represent typical results.
For further information, contact your local Varian Sales Office.
GC36:0894R