Achieving Precise and Accurate Sulfur Determination in Ultra-Low Sulfur Diesels, Biodiesels, and Jet Fuels Using Direct Injection Technology

Over the last few decades, there has been increasing regulatory pressure worldwide to reduce the maximum permissible levels of sulfur in fuels, in order to minimize environmental pollution and protect public health. This has triggered a need for analytical techniques offering low detection limits as well as optimal precision and efficiency. A total sulfur analyzer is able to accurately and cost-effectively determine low-level analysis of refinery products. This article discusses the benefits of a new technology for a direct injection technique in the routine analysis of sulfur content in common middle distillates such as ultra-low sulfur diesels (ULSDs), biodiesels, and jet fuels, in line with American Society for Testing and Materials (ASTM) requirements.

Determination of sulfur content in middle distillates

Petroleum fractions boiling in the range of 150 °C to around 350 °C include jet fuel, kerosene, and diesel fuel. These are commonly referred to as middle distillates and are often blended with cracked products from the hydrocracking and hydrogenation of heavy feedstocks. These cracked products are processed to remove sulfur and nitrogen and are then used to produce lighter, more valuable products. Lighter fractions are ideal for the production of downstream end products such as diesel and aviation turbine fuels.

Due to its environmental and health effects, the maximum permissible sulfur levels in diesel fuels are continually being lowered. In the European Union, the level is being reviewed and reduced and is now set at 10 mg/kg. Throughout much of the central Asian region, the sulfur content in diesel fuel has been reduced in recent years from 500 mg/kg to 350 mg/kg, with a further reduction to 50 mg/kg coming into effect in 2012. In the U.S.A., refineries are required to produce diesel fuel for use in highway vehicles with a sulfur content of no more than 15 mg/kg.1

    Figure 1 - Linear calibration line for the iPRO 5000 Series Total  Sulfur Analyzer.

Blending fatty acid methyl esters with petroleum diesel

To comply with government requirements mandating the use of fuel derived from renewable sources, manufacturers are now blending petroleum-derived diesel fuels with biodiesels, which are fatty acid methyl esters (FAMEs). Biodiesels contain little or no sulfur, and blending them with petroleum diesel reduces the overall sulfur concentration of the final diesel blend in addition to reducing its carbon footprint when compared to petroleum diesel. Typical blends generally include between 5% and 20% biodiesel, resulting in a fuel that can be used in most diesel engines without the need for modification. Certain manufacturer-approved engines are also capable of operating using 100% biodiesel.

Considering the above, it is necessary for fuel manufacturers to implement competent, powerful techniques for the analysis of low levels of sulfur (in the sub-mg/kg region) in ultra-low sulfur diesels, biodiesels, and jet fuels.

Modern sulfur analyzers

Traditionally, the introduction apparatus used for combustion-based trace elemental analysis has depended upon the boiling point of the sample. For light hydrocarbons, direct injection using a ceramic syringe or liquids module is common, while for heavy hydrocarbons, boat inlet introduction is often the technique of choice. Therefore, two introduction modules have been required by analysts to cover the range of sample boiling points analyzed. This need has meant that analyses take 5 min for light hydrocarbons and 12 min for heavy hydrocarbons, significantly impacting upon productivity.

The latest total sulfur analyzers feature an innovative direct injection design providing the capacity to analyze both heavy and light hydrocarbons without changing introduction modules. As a result, it is now possible to automatically quantify sulfur in hydrocarbons from μg/kg levels up to percent levels within just 3 min. Compared to the traditional methods, this is a dramatic reduction in the time spent from analysis to viewing results, which provides immediate cost savings and an increase in productivity.

Modern analyzers are capable of analyzing virtually any refinery sample, from raw crude to end product, while meeting the requirements of ASTM methods. They also feature a proprietary presoot detection technology, which ensures complete combustion that is soot-free. These capabilities make the instruments perfectly suitable for commercial testing laboratories requiring ASTM-compliant results quickly and cost-effectively. In addition, the emerging total sulfur analyzers offer exceptional precision and versatility, as well as reproducibility and repeatability of analysis, making them an ideal choice for high-performance applications such as those conducted by research and development centers.

Experimental: Analysis of sulfur content

A direct injection technique was used for the routine analysis of sulfur content in common middle distillate fuels. A Thermo Scientific iPRO 5000TM Series Total Sulfur Analyzer fitted with a Thermo Scientific jetPRO direct spray injector (Thermo Fisher Scientific, Cambridge, U.K.) was used to measure the total sulfur content of three ASTM Crosscheck sample types, namely ULSDs, biodiesels, and jet fuels. Sulfur detection was conducted using ultraviolet fluorescence in accordance with the ASTM D54532 test method.

ASTM D5453

This test method can be used to determine sulfur in process feeds and finished products, while also serving as a tool for purposes of regulatory control. It covers the determination of total sulfur in liquid hydrocarbons containing less than 0.35 % (m/m) halogen(s) and boiling in the range from approximately 25 to 400 °C, with viscosities between approximately 0.2 and 20 cSt (mm2/S) at room temperature. Samples containing 1.0–8000 mg/kg total sulfur can be analyzed using this test method.

Three separate interlaboratory studies (ILS) on precision, and three other investigations that resulted in an ASTM research report, have determined that the ASTM D5453 test method is applicable to naphthas, distillates, engine oil, ethanol, and FAMEs. It is also applicable to engine fuels such as gasoline, oxygen-enriched gasoline, diesel, biodiesel, diesel/biodiesel blends, and jet fuel. Estimates of the pooled limit of quantification (PLOQ) for the precision studies were calculated, with values ranging between less than 1.0 and less than 5.0 mg/kg.

Table 1 - Sulfur signal peak areas obtained using an iPRO 5000 S analyzer—Three injections of 25 μL sample volume were performed for each of the ASTM samples

Methods

Calibration of the iPRO 5000 S analyzer was achieved using sulfur standards that were produced by dissolving a suitable thiophene-based compound in xylene. Several standards covering the range of 0–50 mg/kg were prepared by serial dilution of stock standards and were then used to provide calibration of the sulfur detector. An iPRO 5000 S analyzer was used in conjunction with a Thermo Scientific AI3000 II autosampler to directly inject samples into the furnace.

Data acquisition and peak analysis were fully automated using an intuitive software platform (Thermo Scientific NSX Visual Software). System parameters were set by this platform according to the selected ASTM D5453 preloaded method and automatically resulted in fully ASTM-compliant system parameters. All samples except the jet fuel were injected directly into the iPRO 5000 S analyzer without dilution, and measurements were made in triplicate. The jet fuel sample required dilution to bring it into the concentration range of the calibration line.

Table 2 -Comparison between results obtained using an iPRO 5000 Series S analyzer with those obtained from the ASTM Crosscheck Programs—The mean value and standard deviation reported are shown

Results

Presented in Figure 1 is the sulfur calibration line for the iPRO 5000 S analyzer. The calibration fit shows excellent linearity, with R2 = 0.9996.

The individual peak areas for each repetition and for each one of the ASTM samples tested are shown in Table 1. The direct spray injector ensured efficient sample introduction and resulted in a typical analysis time of 3 min per replicate. In Table 2, the results obtained in this study are summarized and compared to the results obtained from the ASTM Crosscheck Programs.

Conclusion

The need for accurate and reliable sulfur monitoring in petroleum products is increasing at a fast pace, triggered by stringent global regulations imposing lower maximum permissible levels of sulfur in fuels and the need to protect expensive catalytic exhaust emission control devices. Emerging technology has overcome the requirement to use two introduction modules for the precise analysis of sulfur in light and heavy hydrocarbons, offering a new design of injector module that improves the accuracy and precision of results, while reducing analysis time to within 3 min compared to the boat inlet method. In addition, advanced total sulfur analysis technology can determine the sulfur content of common middle distillates by direct spray injection using ultraviolet fluorescence in accordance with ASTM D5453. Experimental results demonstrate that this technique analyzes sulfur with high precision and accuracy and produces results that are in complete agreement with those obtained from ASTM Crosscheck Programs.

References

1. Regulatory Impact Analysis: Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements—EPA420-R-00-026 (2000); United States Environmental Protection Agency.
2. ASTM D5453–09 Standard Test Method for Determination of Total Sulfur in Light Hydrocarbons, Spark Ignition Engine Fuel, Diesel Engine Fuel and Engine Oil by Ultraviolet Fluorescence.

Angela Seipel is Application Specialist, Thermo Fisher Scientific, Mercers Row, CB5 8HY Cambridge, U.K.; tel.: +44 (0) 1223347400; e-mail: analyze@thermofisher.com.

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