Determination of Low Level Hydrocarbon Impurities in Propylene Using the Agilent 6820 Gas Chromatograph

Importance of the Topic

The precise measurement of trace hydrocarbon impurities in propylene is crucial for maintaining catalyst activity and polymer product quality. Even low-level contaminants such as alkynes and dienes can poison polymerization catalysts and affect reaction rates. Industrial specifications often require impurity levels below 10 ppm to ensure consistent performance in polypropylene manufacturing.

Objectives and Study Overview

This work presents a robust gas chromatography method for quantifying low-level C1–C5 hydrocarbon impurities in high-purity propylene. The goals are:

• To achieve baseline separation and repeatable detection at the 1 ppm level.
• To demonstrate simultaneous quantitation of major (near 100 %) and minor (ppm) components in a single run.
• To follow industry guidelines such as ASTM D2712 for trace hydrocarbon analysis.

Methodology and Instrumentation

Samples were analyzed on an Agilent 6820 GC equipped with a heated six-port gas sampling valve, a split/splitless capillary inlet, and a flame ionization detector (FID). A 50 m × 0.53 mm HP-Al2O3 PLOT “M” column provided high efficiency and resolution for C1–C5 isomers. The inlet temperature was set to 175 °C, with split ratios of 15:1 for standard runs and 4:1 for enhanced sensitivity at 1 ppm. The oven was programmed from 40 °C to 190 °C at 4 °C/min. Helium served as carrier and makeup gas. Automated control and data acquisition were handled by the Agilent Cerity Networked Data System for Chemical QA/QC.

Instrumentation Used

• Agilent 6820 Gas Chromatograph
• 6-port heated gas sampling valve (80 °C, 0.25 mL loop)
• Split/splitless inlet with adjustable ratio
• HP-Al2O3 PLOT “M” column (50 m × 0.53 mm × 0.25 µm)
• Flame ionization detector (300 °C)
• Agilent Cerity NDS for Chemical QA/QC software

Results and Discussion

The method reliably detected 10 ppm levels of 18 hydrocarbon impurities with relative standard deviations below 2.5 %. Baseline separation was achieved for most components, with minor overlaps appearing on the tail of the large propylene peak. The full dynamic range capability of the GC allowed accurate quantification of both major (propene at ~99 %) and trace (1 ppm) peaks in a single injection without manual range changes. A tenfold dilution experiment confirmed detection down to 1 ppm for key impurities such as iso-butane, n-butane, propadiene, and acetylene, all showing excellent signal-to-noise ratios.

Benefits and Practical Applications

• Enables routine QA/QC monitoring of propylene purity in petrochemical production.
• Meets or exceeds sensitivity requirements of ASTM D2712 for hydrocarbon trace analysis.
• Simplifies workflow by combining high-level and low-level quantitation in a single run.
• Reduces instrument setup time through digital full dynamic range acquisition.

Future Trends and Potential Applications

Advances may include coupling selective detectors or mass spectrometry for non-hydrocarbon contaminants, further column optimization for tighter isomer separation, and real-time online monitoring of polymer plant feed streams. Integration with process analytics and automated sampling systems promises even greater control over monomer quality.

Conclusion

The described GC-FID method on the Agilent 6820 system achieves sensitive, repeatable detection of trace hydrocarbon impurities in propylene at levels down to 1 ppm. Its full dynamic range digital acquisition and high-efficiency PLOT Al2O3 column make it an effective tool for petrochemical quality control and research applications.

References

• ASTM D2712: Standard Test Method for Hydrocarbon Traces in Propylene Concentrates by Gas Chromatography.
• Firor R., Trace Level Hydrocarbon Impurities in Ethylene and Propylene, Agilent Technologies, Publication 5965-7824E.