Analysis of Sulfur-Containing Compounds in Diesel and Residual Fuel Oil with Heart-cutting Multidimensional GC

Significance of the Topic

Monitoring sulfur in diesel and residual fuel oil is crucial for regulatory compliance and environmental protection. Sulfur compounds corrode engines and generate SOx emissions, driving stringent global limits. Detailed profiling of specific sulfur species informs refining strategies, ensuring effective hydrodesulfurization and consistent product quality.

Study Objectives and Overview

This study demonstrates a heart-cutting multidimensional gas chromatography (MDGC) approach using an Agilent 8890 GC system coupled with flame ionization detection (FID) and flame photometric detection (FPD Plus). The objective is to resolve and quantify sulfur-containing compounds, particularly alkylated dibenzothiophenes, in middle distillate (diesel) and heavy residual fuel oil (RFO) samples through targeted heart-cut transfers and calibration of 4,6-dimethyldibenzothiophene (4,6-DMDBT).

Applied Methodology

• Primary separation on a nonpolar DB-1ms Ultra Inert column.
• Heart-cut transfers via a capillary flow technology (CFT) Deans switch into a midpolar DB-17ht secondary column to reduce quenching in the FPD Plus and improve resolution of sulfur species.
• Backflush capability enabled by an electronic pneumatic control (PSD) module to remove high-boiling compounds without prolonged bake-out.
• Temperature-programmed multimode inlet (MMI) operations in split and splitless modes with helium as carrier gas.
• Calibration of 4,6-DMDBT over 1–100 ppm with linearized response (square-root transform) and determination of minimum detectable limit.
• Simulated distillation of RFO using a DB-HT Sim Dis column and polyethylene standard to assess carbon number distribution up to C70.

Used Instrumentation

• Agilent 8890 Series GC system with capillary flow technology (CFT) Deans switch.
• Agilent 7693A automatic liquid sampler and multimode inlet (MMI).
• Flame Ionization Detector (FID) and Agilent FPD Plus with separate thermal zones.
• Pneumatic switching device (PSD) for Deans switching and backflush.
• Columns: DB-1ms UI (30 m × 0.25 mm, 0.25 µm), DB-17ht (30 m × 0.25 mm, 0.15 µm), DB-HT Sim Dis (5 m × 0.53 mm, 0.15 µm).
• Consumables: Ultra Inert liners, tapered syringes, flexible metal ferrules; helium carrier gas; hydrogen, air, nitrogen for FPD Plus.

Results and Discussion

Heart-cut MDGC enabled clear separation of sulfur species in diesel (425 ppm total sulfur) and RFO (~2% sulfur). In diesel, a broad 18–24 min heart-cut revealed a homologous series of alkylated dibenzothiophenes on the secondary column detected by FPD Plus. RFO analysis employed backflush at >C36 to remove heavy interferences, followed by a narrow 20.1–20.4 min heart-cut to quantify 4,6-DMDBT. Calibration of 4,6-DMDBT exhibited strong linearity (R2 ≈ 0.99) and a minimum detectable limit of 2.5 pg S/sec. Simulated distillation characterized RFO carbon distribution from C10 to C70, validating method robustness for heavy fractions.

Benefits and Practical Applications

• Enhanced resolution of sterically hindered sulfur compounds that resist hydrodesulfurization.
• Reduced quenching in FPD Plus via midpolar secondary column.
• Efficient backflushing with fixed purge flow lowers carrier gas consumption and bake-out time.
• Quantitative detection across two orders of magnitude supports regulatory monitoring.
• Simultaneous hydrocarbon profiling and sulfur speciation for refining, quality control, and regulatory compliance.

Future Trends and Applications

Integration with mass spectrometry for structural confirmation, faster valve switching for near-real-time monitoring, novel stationary phases for improved selectivity, and automation for high-throughput analysis are expected. Emerging demands in biofuel blending and ultra-low sulfur targets will drive further refinement of MDGC workflows.

Conclusion

The Agilent 8890 GC combined with heart-cutting MDGC, FID, and FPD Plus offers a robust platform for separating and quantifying key sulfur species in diesel and RFO matrices. It meets stringent regulatory and process-control requirements by providing sensitive detection, efficient backflush, and comprehensive hydrocarbon characterization up to C70.

References

1. Molloy N. The IMO's 2020 Global Sulphur Cap—Impacts for Shipping Lines, Refineries and Bunkers. Platts, 2016.
2. U.S. EPA. Highway and Nonroad Diesel Fuel Sulfur Standards. 2016.
3. Firor R. Improved FPD for Dibenzothiophene Analysis. Agilent, Pub. 5991-1752EN, 2013.
4. Ma X; Sakanishi K; Mochida I. Hydrodesulfurization Reactivities of Sulfur Compounds. Ind. Eng. Chem. Res. 1996, 35, 2487-2494.