The low-level analysis of sulphur containing components such as Hydrogen Sulphide (H2S), COS (carbonyl sulphide) and mercaptans, in liquified petroleum gas (LPG), is challenging. First of all, the system has to be inert; stainless steel adsorbs H2S and other sulphur containing components. Secondly, the column used must be able to separate the components of interest. Although a highly selective pulsed flame photometric detector (PFPD) is used in sulphur mode, the bulk hydrocarbons tends to quench the PFPD signal.
The LPG type samples are injected as a gas via two gas sampling valves, in series. A micro-gasifier in front of the injection valves ensures a fully gaseous sample state. The complete sample path is Ultimetal deactivated ensuring an inert system preventing adsorption of the sulphur components.
If the bulk sample is mainly propane, H2S is analysed on the non-polar column from Channel A. The COS is analysed on the BOND Q column from channel B. The mercaptans can be analysed on both columns. However, if the bulk is mainly butane, the methyl mercaptan is analysed on BOND Q from channel B as it co-elutes with butane on the non-polar column of channel A. Analytical parameters of the analysis can be found in table 1.
SCION Instruments developed a two-channel configuration for this analysis with both channels being equipped with a PFPD, as shown in Figure 1.
Samples included a sulphur calibration standard in nitrogen. Bulk analysis was also performed on propane and butane
To validate the performance of the system, a calibration mixture was used. It is vital that the nitrogen must not quench either channel for sulphur components. Figures 2 and 3 show the chromatograms from the calibration standard on both channels.
The non-polar column from channel A shows co-elution of propane and COS. H2S and the mercaptans are very well resolved and perfectly placed for quantification. Figures 4 shows the chromatogram of the bulk propane analysis on channel A.
Analysis of sulphur components in bulk butane can be found in Figures 6 and 7. On the non-polar column in channel A, the bulk butane co-elutes with methyl mercaptan causing quenching of the PFPD. Methyl mercaptan is therefore analysed on the BOND Q column, on channel B.
The calibration mixture was analysed 15 times for validation. The validation data can be found in Tables 2 and 3, in the appendix. Figures 8 shows the repeatability from channel A.
The custom configured SCION 456 GC offers numerous benefits for the analysis of sulphur components in LPG. The micro-gasifier enables the direct coupling of an LPG stream to the GC, eliminating the need for sample pre-treatment. The Ultimetal sample path ensures a trouble-free analysis of sulphur containing components at low concentrations. Increased flexibility with regard to the different samples is achieved via a two-channel approach. Two different columns, each equipped with a PFPD detector, ensures excellent separation independent of the bulk components to analyse H2S, COS and mercaptans. Repeatability data shows that the system is perfectly suited for the analysis of these low level, sulphur containing components.