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Multidimensional Chromatographic Applications in the Oil Industry 381
14.2 GASES
The analyses of gases in the oil industry comprises the determination of the inert
gases (He, H 2 ,O 2 , Ar and N 2 ), low-boiling compounds (CO, CO 2 ,H 2 S, COS) and the
lower hydrocarbons, saturated and unsaturated, up to hexane. Some special samples,
such as natural gas, have to be analysed for low concentrations of higher-boiling
compounds (up to C 10 s) since such compounds have an important influence on the
calorific value and dew point.
For measuring the inert species, some of which are present in the majority of
gases, the thermal-conductivity detector (TCD) is often the detector of choice for gas
analyses. Since the TCD is a concentration detector and its sensitivity is lower than
that of mass-flow detectors such as the flame-ionization detector (FID), relatively
high concentrations of compounds in the carrier gas are needed. This means that
packed columns, with their high loadability, are still quite popular for such analyses.
Some of the analysis configurations in use enable the analysis of specific com-
pounds in gas samples, such as sulfur compounds in hydrocarbon gases and various
impurities in main compounds. Other configurations aim at the determination of all
of the different constituents of refinery gases. Various standardization organisations,
such as the American Society for Testing and Materials (ASTM), Institute of
Petroleum (IP), Universal Oil Products (UOP), Deutsches Institut für Normung
(DIN) and Gas Processors Association (GPA) (4–8) have published a number of
these configurations as standardized methods.
14.2.1 THE ANALYSIS OF TRACES OF SULFUR COMPOUNDS IN
ETHENE AND PROPENE
Ethene and propene are produced as bulk feedstocks for the chemical (polymer)
industry and therefore their purities are important parameters. In particular, H 2 S and
COS are compounds which may not only cause corrosion problems in processing
equipment, but also may have detrimental effects on the catalysts in use. Furthermore,
air pollution regulations issued by, among others, the US Environmental Protection
Agency (EPA) require that most of the sulfur gases should be removed in order to
minimize sulfur emissions into the atmosphere. Therefore, these compounds have to
be determined to the ppb level.
Several methods are available for the determination of sulfur compounds in refin-
ery gas streams. Figure 14.2 depicts a one-column system with column preflush and
a sulfur specific detector. In order to prevent adsorption of the acidic sulfur com-
pounds, a H 3 PO 4 -treated Carbopack phase is used in a Teflon column. The com-
pounds eluting in front of the H 2 S and COS are flushed to vent, after which the
second valve V2 is switched in order to direct the sulfur compounds towards the sul-
fur chemiluminescence detector (SCD). The remaining hydrocarbons, possibly co-
eluting with the sulfur compounds do not interfere, since the selectivity of the SCD
7
for sulfur to carbon is about 10 (9). An example of a typically resulting
chromatogram is presented in Figure 14.3, where the lower limit of detection is 50
ppb for both compounds.