Analysis of Trace Permanent Gas Impurities in Ethylene or Propylene by GC

Importance of the Topic

Trace levels of permanent gases in ethylene or propylene feeds can poison catalysts and alter polymer properties during production. Reliable quantitation of CO, CO2, H2, N2, O2/Ar and CH4 at ppb levels supports quality control in petrochemical and polymer industries and meets regulatory standards.

Objectives and Overview

This study presents a single injection gas chromatographic method using an Agilent 8890 GC coupled with a pulsed discharge helium ionization detector to detect trace permanent gas impurities in ethylene or propylene. The method aims to achieve low detection limits, strong linearity and reproducible quantitation in compliance with ASTM D8098 17 and T CIESC 0021 2022 standards.

Methodology

Pure ethylene or propylene serves as carrier and diluent, purified further by a helium purifier to eliminate background air. A dynamic dilution system generates calibration standards from approximately 50 ppb to low ppm concentrations. A custom valve configuration replaces isolation valves with a single injection approach. Valve switching sequences direct early eluting hydrogen and air components through column 1 before backflushing heavier analytes to columns 2 and 3 for further separation. Data collection runs at 5 Hertz over an 8.5 minute isothermal analysis at 60 °C.

Instrumentation

• Agilent 8890 gas chromatograph fitted with one 10 port valve and two 6 port valves
• Pulsed discharge helium ionization detector for high sensitivity
• Columns: 30 m HP PLOT Q 40 micron film; 30 m HP MOLSIEVE 50 micron film; 0.8 m fused silica tubular column
• Helium purge chamber to minimize baseline noise by preventing air ingress
• Dynamic dilution via pneumatic control module for accurate standard generation

Main Results and Discussion

Chromatograms demonstrate clear separation of six analytes with hydrogen eluting first, followed by CO and methane, argon coeluting with oxygen, nitrogen and CO2. Minimal detection limits below 20 ppb were achieved for all target gases except argon oxygen and nitrogen which were slightly affected by chemisorption and ambient background. Calibration curves from 50 ppb to 5 ppm exhibit excellent linearity with correlation coefficients above 0.995 for major analytes. Repeatability studies show area RSD below 1 percent for CO2, H2, CH4 and CO across six replicates.

Benefits and Practical Applications

• Single injection workflow reduces analysis time and risk of valve induced baseline drift
• Low ppb detection meets stringent industry standards for polymer feedstock purity
• Robust helium purge chamber maintains stable baseline for trace analysis
• Simultaneous quantitation of six permanent gases increases laboratory throughput

Future Trends and Possibilities

Implementation of alternative molecular sieve phases could resolve argon oxygen coelution and improve quantitation accuracy. Integration of inline purification modules may further lower background interference. Expansion of this approach to additional reactive permanent gases or to on line process monitoring can enhance real time quality control in olefin production.

Conclusion

The Agilent 8890 GC PDHID configuration provides a sensitive, linear and reproducible method for quantifying trace permanent gases in ethylene or propylene. By achieving detection limits below 20 ppb and meeting ASTM and T CIESC standards, this approach supports rigorous quality assurance in petrochemical processes.

References

1. ASTM D8098 17 Standard Test Method for Permanent Gases in C2 and C3 Hydrocarbon Products by Gas Chromatography and Pulse Discharge Helium Ionization Detector
2. T CIESC 0021 2022 Standard Test Method for Permanent Gases in Polymer Grade Ethylene and Propylene by Gas Chromatography and Helium Ionization Detector