Controlling the Sulfur Content in Ultra Low Sulfur Diesel (ULSD) Fuels

Importance of the Topic

Sulfur emissions from diesel combustion form acidic compounds and contribute to environmental damage, corrosion and adverse health effects. Stricter international regulations on sulfur content in ultralow sulfur diesel (ULSD) require analytical methods capable of rapid, accurate and reproducible monitoring at parts per million levels.

Objectives and Overview

This study aims to develop a reproducible process control routine for quantifying sulfur in diesel fuel blends using energy dispersive X-ray fluorescence spectroscopy (EDX). A five-point calibration curve is established and applied to both standard reference materials and blended samples to assess method performance.

Instrumentation Used

Shimadzu EDX-720 energy dispersive X-ray fluorescence spectrometer equipped with 1, 3, 5 and 10 mm collimators, a helium purge kit, vacuum kit, autosampler and CCD camera for sample positioning.

Methodology and Instrumentation

• Minimal sample preparation: diesel poured into 10 mL X-Cell cups with 4 µm Ultralene film.
• Instrument parameters: 15 kV, 1000 µA, 10 mm collimation, aluminium primary filter, Si(Li) detector, helium purge at ≈2 mL/min.
• Calibration using NIST SRM 2724b and Analytical Services reference fuels across 0–28 ppm sulfur.
• Measurement protocol: 120 s purge followed by 300 s live counting per sample, total analysis time ≈7 min.

Main Results and Discussion

The low-range calibration showed excellent linearity with an accuracy index of 0.2311. A broader concentration range required dual calibration curves to compensate for matrix nonlinearity. Reproducibility tests comparing dynamic repositioning and static measurements yielded average sulfur concentrations of 6.71 ppm (all data) and 6.88 ppm (excluding outliers), corresponding to a 4.1 % error relative to the nominal 7 ppm value. Process control charts demonstrated clear specification and control limits, with automated flagging of out-of-specification samples.

Advantages and Practical Applications

• Rapid analysis (<7 minutes) with minimal consumables and no acid digestion.
• High precision and reproducibility suitable for production line screening and refinery quality control.
• Compact bench-top footprint and low maintenance requirements.

Future Trends and Applications

Advances in detector technology and software algorithms may enhance sensitivity and automation. Integration of multivariate calibration and online sampling could enable real-time monitoring of sulfur in diverse fuel matrices. Expanding ED-XRF to other regulated elements offers a broader analytical capability in petrochemical quality assurance.

Conclusion

ED-XRF using the Shimadzu EDX-720 provides an effective, low-cost screening approach for quantifying low ppm sulfur in ULSD. The method offers rapid, reproducible results with simple sample handling and is readily adaptable for process control environments.

References

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