Determining Sulfur Concentration in Petroleum Products

Sulfur concentration in petroleum products is a vital quality control parameter for oil refineries and distributors. The use of Ultra-Low-Sulfur Diesel (ULSD) is imposed by many countries other than North America and Europe. The maximum permissible sulfur content, depending on a region or country, can be as low as 10 ppm. XRF and EDXRF in particular is often used as a fast analysis technique for the accurate determination of sulfur in petroleum derivatives. The ASTM D4294 test method covers this type of analysis using the EDXRF technique.

Instrument

The ARL QUANT’X EDXRF spectrometer used for this application has a Silicon Drift Detector (SDD) and a 50 kV, 50 W silver target X-ray tube of the new generation. The sample is excited by the primary filtered radiation used by the ARL QUANT’X. The ARL QUANT’X is easily adaptable per application or element range using a set of nine specially designed filters that are particularly designed to optimize the peak-to-background for elements from F to Am. Analysis in a helium atmosphere can be performed using the helium flush option equipped in the instrument.

Sample Preparation

To measure the petroleum product, 3 grams of product is transferred into a 4-micron polypropylene film sealed sample cup with an outer diameter of 32 mm.

Excitation Conditions

The excitation conditions used to carry out the analysis are shown in Table 1. The background below the characteristic lines of sulfur is removed using a thin graphite filter, while keeping a salient part of the tube’s characteristic silver L-lines used to excite sulfur. A 9 kV excitation voltage is used while the tube current is automatically adjusted to get a 50% dead time. The absorption of the characteristic sulfur X-rays by air is eliminated using a helium atmosphere. For each sample, a total measurement time of 100 seconds (live time) is applied.

Table 1. Excitation condition used for the analysis of sulfur in petroleum products

Calibration

Di-n-Butyl Sulfide (DBS) is mixed in white mineral oil to prepare calibration standards as per ASTM D4294. ASTM D4294 recommends working with three concentration ranges including 0-1000 mg/kg, 0.01-1.00 mass %, and 1.0-5.0 mass %. In this article, focus is given to the concentration range of 0-1000 mg/kg. To cover the concentration range, the blank and a set of 10 standards are prepared.

The calculated concentrations against the given concentrations for 0-1000 mg/kg calibration range are shown in Figure 1. The value of R² is 0.99999 with a Root Mean Square Error (RMSE) of 1.45 ppm. The net sulfur peak intensity is linked to the sulfur concentration using a second order calibration curve.

Figure 1. Calculated versus given concentrations for the 0-1000 mg/kg S in oil calibration.

Validation and Repeatability

Fresh sample cups are arranged with three calibration stock solutions of 10 ppm, 120 ppm and 500 ppm. The repeatability of analysis is calculated by analyzing each sample five times. Analysis results are shown in Table 2. Results show that both precision and accuracy are outstanding.

Table 2. Validation and repeatability results for 3 samples over 5 repeats

Limit of Detection

Ten different sample cups with blank oil are arranged and measured to find out the limit of detection. The limit of detection proportionate to three times the standard deviation of the found concentrations equals 1.05 ppm in 100 seconds live time. If necessary, this limit of detection can be enhanced using a longer counting time. A counting time of 300 seconds enables achieving a limit of detection of 0.6 ppm.

Conclusion

The suitability of the ARL QUANT’X EDXRF spectrometer for the fast, precise and accurate analysis of low concentrations of sulfur in petroleum products is explained in this article. The limit of detection is adequate for reliable quality control of ultra-low-sulfur diesel (Tier 3).

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

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