Prefractionator for Reliable Analysis of the Light Ends of Crude Oil and other Petroleum Fractions

Significance of the Topic

The analysis of light hydrocarbon fractions from wide boiling range mixtures such as crude oil is critical for process optimization, product valuation and quality control in the petroleum industry. Traditional capillary columns can be damaged by high molecular weight residues and heavy ends that elute beyond the target analytes, leading to long run times, column degradation and increased maintenance. A prefractionation technique based on precolumn backflush enables high-resolution, time‐optimized separations of the C4–C12 fraction while protecting the analytical column and detector from heavy contaminants.

Objectives and Study Overview

This application note describes the development and implementation of a capillary-only precolumn backflush system with midpoint pressure control using Capillary Flow Technology (CFT) on an Agilent 7890A GC. The primary objectives are:

• To achieve reliable, high-resolution analysis of the C4–C12 fraction in crude oil samples.
• To demonstrate a general backflush method using a wide boiling range polymer standard (Polywax 500).
• To optimize backflush timing as a function of carbon number for precise cut‐off between light and heavy fractions.

Methodology

The system configuration utilizes a multimode inlet (MMI) as the precolumn inlet and an auxiliary EPC channel to control a purged union that directs flow either to the analytical column or to vent. Two capillary columns are connected in sequence:

• Precolumn: 2 m × 0.32 mm deactivated retention gap
• Analytical column: 100 m × 0.25 mm × 0.5 µm DB‐Petro

Temperature and pressure programs are set to allow analyte elution from the precolumn into the analytical column up to a defined carbon number, after which a rapid inlet pressure drop triggers backflush to vent. Backflush times are fine‐tuned using standard hydrocarbons or polymer standards and correlated to carbon number via polynomial or linear regression.

Instrumentation Used

• Agilent 7890A Gas Chromatograph
• 7693A Multimode Inlet (MMI)
• Capillary Flow Technology Purged Union (p/n G3186-60580)
• Auxiliary EPC channel for flow control
• DB‐Petro capillary column (100 m × 0.25 mm × 0.5 µm)
• Deactivated retention gap column (2 m × 0.32 mm)
• Flame Ionization Detector (FID)

Main Results and Discussion

Backflush timing was optimized for both crude oil samples and a Polywax 500 standard. A strong linear correlation (R2 > 0.98) between backflush time and carbon number was established, allowing prediction of cut‐off points: BF_time = (Carbon_number – 5.56)/3.68. Typical settings yield a 1.3 min backflush time for C12. Overlay of multiple crude oil injections demonstrated retention time repeatability better than 0.002 min and stable baselines, confirming complete removal of heavy ends and minimal carry‐over. Fine‐tuning of backflush times ensures sharp division of polyethylene fragments at even carbon numbers.

Benefits and Practical Applications

• Protects analytical columns from irreversible contamination and extends column life.
• Reduces overall analysis time by eliminating the need to elute heavy fractions.
• Improves resolution and quantitation of light-end hydrocarbons (C4–C12).
• Compatible with various detectors including FID and mass spectrometry.
• Applicable to routine crude oil characterization, fuel additives, biodiesel quality control and other petrochemical analyses.

Future Trends and Opportunities

Advances in electronic pneumatic control, software‐driven method optimization and CFT devices will further simplify backflush method development. Integration with automated classification software (e.g., DHA) and coupling with simulated distillation protocols can provide comprehensive characterization workflows. Exploration of novel stationary phases, thinner films and microcapillary architectures may enhance sensitivity and selectivity for specific hydrocarbon classes.

Conclusion

Capillary-only prefractionation with midpoint backflush on the Agilent 7890A GC provides a robust solution for analyzing light ends in complex petroleum matrices. This approach delivers high-resolution separations, efficient column protection and reproducible results. The methodology offers flexibility for a wide range of petrochemical applications and sets the stage for future enhancements in automated GC workflows.

References

1. Agilent Technologies. Flow Calculator software. www.agilent.com/chem/flowcalculator (2009).