Determination of Sulphur Gases in Natural Gas

Significance of the Topic

Effective monitoring of sulfur compounds in natural gas is critical to ensure product quality, protect downstream catalysts, prevent corrosion and eliminate malodorous emissions. Low levels of sulfur compounds can damage processing equipment, poison industrial catalysts and violate regulatory limits. Implementing a robust analytical method supports both environmental compliance and operational efficiency.

Objectives and Study Overview

This application note illustrates the use of the ASTM D6228 standard method for quantifying hydrogen sulfide (H2S), carbonyl sulfide (COS) and methyl mercaptan (CH3SH) in natural gas. The main goal is to achieve complete separation of these sulfur gases from methane and other major gas components, while ensuring sensitive, selective and equimolar detection irrespective of sulfur compound identity.

Methodology

The study employs capillary gas chromatography coupled with a pulsed flame photometric detector (GC-PFPD). Key steps include:

• Sample introduction via an automated six-port Valco sampling valve equipped with a 250 µL deactivated metal loop.
• Separation on a SCION-1 capillary column (60 m × 0.53 mm × 5 µm PDMS stationary phase).
• Oven temperature program: 30 °C (1 min), ramp 15 °C/min to 200 °C, then 20 °C/min hold for 5 min.
• Carrier gas: helium at 6.1 cm3/min.
• PFPD conditions: flame temperature 250 °C, hydrogen flow 13.4 cm3/min, air flows of 17.3 cm3/min and 9.7 cm3/min for Air 1 and Air 2, respectively.

Used Instrumentation

• Gas chromatograph with automated injection valve (six-port Valco).
• SCION-1 capillary column, 60 m length, 0.53 mm ID, 5 µm PDMS film.
• Pulsed flame photometric detector with delayed emission monitoring and optimized gate delay and width.

Main Results and Discussion

Under the optimized conditions, hydrogen sulfide, carbonyl sulfide and methyl mercaptan are fully resolved from methane within a 10-minute run time. The PFPD delivers an equimolar response to each sulfur compound with a detectivity of 1 pg S/s and selectivity of 10^6 S/C. Retention time precision over four consecutive injections is outstanding (≤0.005 min for H2S and COS, 0.004 min for CH3SH), well within the ASTM requirement of 0.05 min.

Benefits and Practical Applications

• High sensitivity and selectivity for trace sulfur analysis in complex gas matrices.
• Equimolar response simplifies calibration and quantitation across different sulfur species.
• Robust retention time repeatability ensures reliable routine monitoring.
• Configurable system allows integration of chemiluminescence detection per ASTM D5504 if required.

Future Trends and Opportunities

Advances in stationary phase chemistries may further improve separation efficiency for emerging sulfur species. Integration of automated sample preparation and real-time data processing can boost throughput and facilitate online monitoring. Coupling GC-PFPD with additional detectors or mass spectrometry could expand the analytical scope to other trace contaminants in natural gas streams.

Conclusion

The described GC-PFPD method meets ASTM D6228 guidelines for sulfur gas analysis in natural gas, offering complete separation from methane, high sensitivity, equimolar detection and excellent precision. This approach provides a reliable tool for quality control and regulatory compliance in gas production and processing operations.

Reference

1. ASTM D6228 – Standard Test Method for Determination of Sulfur Compounds in Natural Gas by GC-PFPD.
2. ASTM D5504 – Standard Test Method for Determination of Total Reduced Sulfur Compounds in Natural Gas by GC with Chemiluminescence Detection.