Page 389 - Multidimensional Chromatography
P. 389

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.
   384   385   386   387   388   389   390   391   392   393   394