Trace analysis of permanent gases in ethylene and propylene hydrocarbon products

Significance of the Topic

Monitoring trace permanent gases in ethylene and propylene is critical for ensuring polymer quality, catalyst performance, and process safety. Even low concentrations of hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide, or methane can impact polymerization reactions and final product properties. Employing a highly sensitive detector such as pulse discharge helium ionization detection (PDHID) enables quantification down to sub-ppm or ppb levels, supporting stringent quality control in olefin production.

Study Objectives and Overview

This work outlines a capillary gas chromatography method coupled with PDHID, conforming to ASTM D8098, for determining trace permanent gases in C2 and C3 hydrocarbon streams. The goal is to achieve accurate and reproducible measurement of H2, N2, O2, CO, CO2, and CH4 in ethylene and propylene matrices at concentrations ranging from parts per million to parts per billion.

Methodology

A 0.5 mL sample loop collects either standard or process gas before injection. Valve 1 (10-port) introduces the sample onto Column 1, where most components elute onto Column 2. A timed backflush prevents heavy hydrocarbons from entering the detector. Valve 2 (6-port) selectively traps early-eluting gases, routing carbon dioxide through a restrictor while other gases proceed to Column 3. A temperature program (50 °C hold, ramp to 85 °C) and helium carrier gas (15 mL/min) enable baseline separation within approximately 7 minutes.

Instrumentation

The system comprises an Agilent 7890B gas chromatograph fitted with a PDHID detector, a 10-port sample valve with backflush capability, and a 6-port column switching valve. Internally purged valve rotors use helium to minimize air ingress. Separation is performed on two GS-Q porous polymer columns (50 m×0.53 mm, 6 µm) and one HP Molesieve 5A column (30 m×0.53 mm, 25 µm). Data acquisition and instrument control are managed via Agilent OpenLab CDS.

Main Results and Discussion

A representative chromatogram resolves six permanent gases in under 7 minutes with clear baseline separation. Precision was evaluated at four concentration levels in both ethylene and propylene matrices, yielding relative standard deviations below 3% for most analytes and recoveries between 90% and 115%. Linearity studies across replicate standards produced correlation coefficients (R2) above 0.99 for carbon dioxide, methane, and carbon monoxide, with slightly lower yet acceptable values for other gases. Outlier rates were minimal when samples were purged manually.

Benefits and Practical Applications

• High sensitivity and low detection limits support sub-ppm and ppb quantification.
• Rapid analysis cycle (<7 min) enhances laboratory throughput.
• Reproducible precision and strong linearity meet ASTM D8098 performance criteria.
• Adaptable to quality assurance, process troubleshooting, and impurity monitoring in olefin manufacturing.

Future Trends and Potential Applications

Advancements may include automated sampling systems to reduce contamination risk, integration of real-time online monitoring for process control, adoption of advanced data analytics for pattern recognition, and exploration of miniaturized or alternative ionization detectors for field applications.

Conclusion

The described GC-PDHID configuration provides a robust and efficient approach for trace permanent gas analysis in ethylene and propylene. It offers the sensitivity, precision, and linearity required by the polymer industry, aligning with ASTM standards and supporting enhanced quality control.

References

1. ASTM Standard D8098-17, Standard Test Method for Permanent Gases in C2 and C3 Hydrocarbon Products by Gas Chromatography and Pulse Discharge Helium Ionization Detector, ASTM International, 2017.
2. ASTM Standard D7915-14, Standard Practice for Application of Generalized Extreme Studentized Deviate (GESD) Technique to Identify Multiple Outliers in a Data Set, ASTM International, 2014.