ASTM D8098: Analysis of Permanent Gases in C2 and C3 Hydrocarbon Products by Gas Chromatography and Pulsed Discharge Helium Ionization Detection

ASTM D8098Introduction

The production of polymers requires a high grade of ethylene and propylene building blocks. To determine the purity of polymer grade ethylene and propylene Scion Instruments has developed a configuration that determines the trace impurities of permanent gases in C2 and C3 matrices. This configuration will provide results according to ASTM D8098. This test method covers the determination of hydrogen, oxygen, nitrogen, methane, carbon monoxide, and carbon dioxide in the parts per billion mole (nmol/mol) to parts per million mole (μmol/mol) range in C2 and C3 hydrocarbons.

The instrument configuration consists of 2 channels. The first channel consists of a non O2 adsorbing pre-column and a molecular sieve column. The pre-column allows the complete elution of hydrogen, oxygen, nitrogen, methane and carbon monoxide onto the analytical column. All components heavier than these components (including CO2 and heavier) are backflushed to vent. The second channel consists of 2 porous polymer columns which separate a composite “air” peak (H2/O2/N2/ CO) from methane and CO2. All components heavier than CO2 are backflushed to vent. Column dimensions are optimized for the combination of both channels onto one PDHID (PDD) detector. Sample introduction is performed by the Scion Instruments integrated Gasifier. This device is capable of handling LPG samples of C2 through C4, effectively reducing the pressure resulting from gasifying the liquefied sample. GC control and data handling was performed using Scion Instruments Compass CDS™.

System Overview

System overview - Permanent Gases

Instrumental Settings

Integrated Gasifier 120°C
Valve Oven 50°C
GC Oven 80°C isothermal

for 18.50 minutes
Column Flows 15ml/min

UHP Helium
Detector PDHID 120°C

Chromatography

permanent gases in Helium ethylene sample

Area Repeatability (n=20) 2 ppm permanent gas test mix

Air CH4 CO2 H2 O2 N2 CH4 CO
Mean 5008965 3393477 2652170 482111 1911297 2242025 3731809 1781911
Std Dev 2688.4 12845 9044 1530 2020 6512 12683 6580
Rsd % 0.05 0.38 0.34 0.32 0.11 0.29 0.34 0.37

Method Detection Limit LOD-LOQ calculation

Air CH4 CO2 H2 O2 N2 CH4 CO
Mean 5008965 3393477 2652170 482111 1911297 2242025 3731809 1781911
Std Dev 2688 12845 9044 1530 2020 6512 12683 6580
Rsd % 0.05 0.38 0.34 0.32 0.11 0.29 0.34 0.37
Amount test mix PPM 2 2 2 2 2 2 2
LOD PPB 20 20 17 6 16 19 20
LOQ PPB 68 61 58 19 52 61 67

Conclusion

As the results clearly show, this system is perfectly suited for the analysis of trace impurities in ethylene matrices. Detection limits are at ppb level, depending on the type of component. Due to the use of a gasifier and the backflush configuration, matrices of C3 and C4 could also be analyzed without difficulty.

The Scion Instruments D8098 configuration is perfectly suitable for the determination of permanent gas impurities in ethylene and propylene matrices at ppb levels. The use of robust industry standard inert materials, coupled with purged MS leak specification valves and inert preconditioned columns with welded ends ensures a robust instrument ideal for the olefin producing and the polymer industry.

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