Bromine number in pyrolysis gasoline

Significance of the Topic

Pyrolysis gasoline contains high levels of unsaturated hydrocarbons that compromise its use as a motor fuel. Monitoring unsaturation, expressed as the bromine number, is critical for quality control in petrochemical processes. Traditional titrimetric methods require low temperatures and reagents, increasing analysis time and cost. Near-infrared spectroscopy (NIRS) offers a rapid, reagent-free alternative that can accelerate decision-making in industrial and laboratory settings.

Objectives and Study Overview

This study aims to develop and validate a NIRS-based method for fast determination of the bromine number in pyrolysis gasoline. By comparing NIRS predictions with the standard chemical titration (ASTM D1159), the work evaluates accuracy, precision, and cost advantages. A dataset of 180 pyrolysis gasoline samples was used to build and test a predictive model.

Methodology and Instrumentation

Samples were measured in transmission mode over the 400–2500 nm range at 40 °C using disposable 8 mm glass vials. Spectra were collected on a DS2500 Liquid Analyzer and processed with Vision Air Complete software. Partial least squares regression was applied to correlate spectral features with reference bromine numbers obtained by titration under ASTM norms D8321 and D6122.

Used Instrumentation

• DS2500 Liquid Analyzer (Metrohm 2.929.0010): robust Vis-NIR spectrometer covering 400–2500 nm, temperature-controlled up to 80 °C.
• Disposable glass vials, 8 mm pathlength (Metrohm 6.7402.000).
• Vision Air 2.0 Complete software (Metrohm 6.6072.208) for data acquisition and model development.

Main Results and Discussion

The NIRS model achieved a calibration R² of 0.836, with standard errors of calibration and cross-validation of 1.84 and 1.89 bromine units, respectively. Correlation plots show good agreement between NIRS predictions and titration values across the sample set. The model demonstrates sufficient robustness for routine quality control.

Benefits and Practical Applications

• Analysis time reduced from 30 minutes to under 1 minute per sample.
• No chemical reagents or sample cooling required, eliminating hazardous waste.
• Annual running costs drop from approximately $12,533 to $1,125 for a throughput of 10 analyses per day.
• Capability to determine additional quality parameters such as diene value in the same measurement.

Future Trends and Potential Applications

Integration of NIRS with automated sampling robots and flow-through cells can further boost productivity. Expansion of spectral databases and chemometric techniques will improve prediction accuracy and extend applications to other unsaturation indices. Adoption in continuous online monitoring for real-time process control is a promising avenue.

Conclusion

The NIRS approach for bromine number determination in pyrolysis gasoline delivers rapid, reliable results without reagents or extensive sample preparation. It meets ASTM requirements and offers substantial cost and time savings, making it an attractive tool for industrial quality assurance and research laboratories.

References

• ASTM D1159 – Standard Test Method for Bromine Number of Hydrocarbon Resins and Similar Materials.
• ASTM D6122 – Standard Test Methods for Determination of Total Aluminum in Lubricating Oils by Graphite Furnace Atomic Absorption Spectroscopy.
• ASTM D8321 – Practice for Use of Near-Infrared Multivariate Quantitative Analysis Procedures.
• Metrohm Application Note AN-NIR-094: Fast determination of bromine number without chemicals.