Determination of Metals in Petroleum Fractions using MP-AES

Significance of the Topic

The accurate quantification of metals such as vanadium, nickel, iron, calcium, sodium, and molybdenum in heavy petroleum fractions is crucial for refining process optimization, catalyst management, and environmental compliance. Conventional techniques rely on acid-assisted digestion and costly plasma or flame methods. Microwave Plasma Atomic Emission Spectroscopy (MP-AES) represents a robust, cost-effective alternative, particularly when coupled with simplified sample preparation.

Objectives and Study Overview

This study aims to evaluate MP-AES for direct dilution analysis of crude oil residues in comparison with wet ash digestion followed by ICP-OES. Key goals include establishing method detection limits, accuracy, precision, and assessing matrix effects across diverse feedstock samples.

Methodology and Instrumentation

• Sample Preparation: Direct dilution in o-xylene with matrix modifier, dispersant, and scandium internal standard; comparison with wet ash digestion for select elements.
• Calibration: Multielement standards (V, Ca, Fe, Ni, Na, Mo) at 1–10 mg/kg; blank and continuing calibration verification using S21 standard and NIST Trace Elements in Fuel Oil CRM.

Used Instrumentation

• Agilent 4200 MP-AES (or 4210 MP-AES) with nitrogen plasma generated on-site by Agilent 4107 nitrogen generator.
• MicroMist nebulizer, double-pass glass cyclonic spray chamber, External Gas Control Module, SPS 4 autosampler.
• MP Expert software for automatic wavelength recommendation, nebulizer flow, and background correction settings.

Main Results and Discussion

• MDLs and LOQs ranged from 1.61–9.40 µg/kg and 5.37–93.97 µg/kg respectively, demonstrating sufficient sensitivity for petroleum residues.
• Quality control recoveries for CCV and CRM samples were within ±10 % (±12 % for vanadium), with RSD values below 4.7 %.
• Direct dilution results correlated well with wet ash digestion for vanadium and nickel (ratios 1.00–1.14), while iron showed a consistent low bias due to possible insoluble particulates.
• High sample throughput achieved with dilution factors between 1:10 and 1:200; sodium quantification limited in highly diluted samples.

Benefits and Practical Applications

• Eliminates time-consuming acid digestion steps, reducing preparation time and contamination risk.
• Uses nitrogen from air, avoiding flammable gases and lowering operational costs.
• Suitable for high-volume laboratories and remote or field locations.
• Enables rapid feedstock assays for refinery decision-making and quality control.

Future Trends and Applications

• Expansion of MP-AES to on-site crude oil evaluation and environmental monitoring.
• Integration with automated sample handling and data analytics for real-time process control.
• Further investigation of particle-bound metals and sample dispersion strategies to improve iron accuracy.
• Development of standardized MP-AES protocols in petroleum assay standards.

Conclusion

MP-AES combined with direct dilution in organic solvents offers a reliable, efficient approach for multielement analysis of heavy petroleum fractions. While vanadium and nickel can be accurately quantified, iron analysis may require digestion methods to address particulate insolubility. The method’s simplicity and cost-effectiveness support its adoption in refinery quality control and research laboratories.

References

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• Poirier L. et al. Preparation Methods for Metals in Petroleum Fractions by MP-AES. Energy Fuels, 2017, 31, 7809–7815.
• ASTM D5708-12e1, D5863, D7876; ASTM International.