Determination of total fluorine, chlorine, bromine, and sulfur in liquefied petroleum gas by pyrohydrolytic combustion ion chromatography

Significance of the Topic

Liquefied petroleum gas (LPG) is a widely used energy source and chemical feedstock. Even trace levels of halogenated compounds and sulfur species can foul downstream catalysts, reduce product quality, and lower commercial value. Robust, accurate, and fast determination of total fluorine, chlorine, bromine, iodine, and sulfur in LPG is therefore essential for quality control, regulatory compliance, and process optimization in petrochemical and energy industries.

Objectives and Study Overview

This study aimed to develop and demonstrate an automated method for the fast and reliable determination of total halogens (F, Cl, Br, I) and total sulfur in n-butane LPG. The technique combines pyrohydrolytic combustion to convert organic halides and sulfur compounds into inorganic anions, followed by ion chromatography (IC) with suppressed conductivity detection. The method covers single and multiple combustions, allowing calibration from a single LPG standard.

Methodology and Instrumentation

• Sample Introduction and Combustion
  • AQF-2100H Automatic Combustion Unit with ABC-210 boat controller, GI-260 gas injector, HF-210 furnace, and GA-211 gas absorption unit.
  • LPG aliquots (30 µL) injected under argon (60 mL/min) into a dual-quartz pyrolysis tube heated at 800 °C (argon) then 900 °C (O₂ and water vapor), converting organics to HX and SOx gases.
  
• Absorption and IC Separation
  • HX and SOx absorbed in 10 mL of 50 mg/L hydrogen peroxide (1000 mg/L hydrazine added for iodide determinations).
  • 100 µL of the absorption solution injected into a Thermo Scientific Dionex Integrion HPIC system.
  • Separation on a Dionex IonPac AG20 guard and AS20 analytical columns (2 × 250 mm) at 35 °C, using an electrolytically generated KOH gradient (10–38 mM over 14 min; return to 10 mM by 20 min) at 0.375 mL/min.
  • Suppressed conductivity detection with an ADRS 600 suppressor (2 mm, 36 mA).
  
• Calibration and Data Processing
  • Calibration by incremental combustions of a single LPG standard (2–18 mg/kg and 15–135 mg/kg ranges), avoiding multiple external standards.
  • Linear or quadratic fits achieved (r² > 0.998).
  • MDLs (5–10 µL loop injections) estimated: 17 µg/kg (F), 280 µg/kg (Cl), 36 µg/kg (S), 1000 µg/kg (Br), 4000 µg/kg (I).
  

Key Results and Discussion

• Blank Characterization
  • n-Butane LPG without additives showed no detectable F or Br, and sub-ppm Cl and S, confirming low background.
  
• Calibration Performance
  • Excellent linearity and reproducibility (RSD <2%).
  • Recoveries (85–107%) for F, Cl, Br, and S in two LPG standards (2 mg/kg and 15 mg/kg additive levels).
  • Iodine required peroxide/hydrazine absorption, yielding 79% recovery and r² = 0.9986.
  
• Sample Analysis
  • Long-term LPG (1-year old tank) with nominal 15 mg/kg F, Cl, S showed recoveries of 88% (F), 85% (Cl), and 99% (S).
  • Nine-combustion injections of 2 mg/kg standard reproduced target concentrations with RSDs ≤2%.
  

Benefits and Practical Applications

• Comprehensive matrix removal by pyrohydrolysis ensures accurate quantification of all halogens and sulfur, even in complex hydrocarbon matrices.
• Incremental combustion calibration reduces standard consumption and simplifies workflow.
• Automated coupling of combustion and IC delivers high throughput (≈22 min per run) with minimal manual handling.
• MDLs in the low µg/kg range enable ultratrace analysis suitable for strict regulatory requirements and catalyst protection.

Future Trends and Opportunities

• Integration with mass spectrometric detection for speciation of halogenated and sulfur compounds post-combustion.
• Miniaturization of combustion modules for field-deployable CIC analysis in processing plants.
• Expansion of the method to other challenging fuel and petrochemical feedstocks (e.g., biofuels, shale gas condensates).
• Application in environmental monitoring for simultaneous determination of halogens and sulfur in soil, water, and air particulate samples.

Conclusion

An automated pyrohydrolytic combustion ion chromatography method was established for total halogen and sulfur determination in LPG. The approach achieves fast, accurate, and sensitive analysis with simplified calibration using a single standard. The method is robust against matrix interferences and meets stringent quality requirements for energy and petrochemical applications.

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