Using XRF to Measure Low Levels of Sulphur in Petroleum Products According to ASTM D 2622-08

Sulphur in automotive fuels has been a source of air pollutants like sulphur dioxide and sulphates, which are poisonous. The international legislation has enforced that fuels should have very low residual sulphur concentrations in fuels, such as the actual limit of 30 ppm for diesel in the US. The EU directive 2003/17/EC has regulated that the maximum allowed level in the EU is 10 ppm. It is possible that future regulations may drive these limits down even further. Wavelength-dispersive X-ray fluorescence (WDXRF) analysis is one of the best methods available for precise and exact analysis of low sulphur concentrations in petroleum products. WDXRF instrumentation can be conveniently incorporated into industrial environments such as refineries and laboratories because of its simple sample preparation, reliability, ease-of-use and precise analytical results.

The S8 TIGER 1 K has a number of added benefits for the analysis of low levels of sulphur in petroleum products and satisfies the requirements of ASTM D 2622-08. This report will outline the analytical performance of the S8 TIGER and describe the advantages related to this analytical task.

S8 TIGER 1 K WDXRF Spectrometer

The S8 TIGER 1 K is the ideal solution for performing elemental analysis for the hydrocarbon processing industry.

The salient features of the S8 TIGER 1 K are listed below:

  • Traces of sulphur can be analyzed with minimal detection limits and high precision because of the optimized beam path.
  • High sensitivity for sulphur is achieved with the close coupling between X-ray tube and sample and the curved germanium analyzer crystal.
  • An additional external cooling device and compressed air is not required hence easy installation and reliable operation is ensured.
  • The S8 TIGER 1 K is optimized for highest instrument uptime, lowest cost of ownership and ultimate reliability.
  • The unique SampleCare feature of the S8 TIGER 1 K overcomes difficulties with previous WDXRF spectrometer models as it fully protects the spectrometer chamber and its components in case there is any spillage of samples.
  • The innovative vacuum seal separates the sample and spectrometer chamber and ensures complete dependability for petroleum users.
  • Fumes from diesel and gasoline are kept out to avoid damage to detectors, motors and crystals in the spectrometer chamber.
  • Helium consumption is also significantly reduced and hence the cost of operation, since helium flushing is only needed for the small sample chamber, while the spectrometer chamber is always in a vacuum state.

Samplecare for Safe and Reliable Analysis of Liquid Samples

The SampleCare technology of the S8 TIGER 1 K continuously protects the X-ray tube and all goniometer components from damages because of oil spillage, which might lead to inaccurate results or even stoppage of the system.

The S8 TIGER with SampleCare prevents this safely in four ways as given below:

  1. The contamination shield 1 acts as a protection for the X-ray tube while loading.
  2. The contamination shield 2 safeguards the goniometer during sample loading and helium flushing
  3. The DuraBeryllium tube shield safeguards the head of the X-ray tube during measurement.
  4. The vacuum seal with a high transmission window divides the sample and goniometer chamber during measurement. The diesel fumes and oils cannot spill out of the goniometer chamber, since this chamber is always in a vacuum state.

Just in case, anything goes wrong and the sample leaks out, droplets are collected in the reservoir. System components are well protected, easy to access and can be cleaned with minimal effort.

Standard and Sample Preparation

Calibration according to ASTM D 2622-08 was conducted with five standard samples prepared by using commercially available standards covering a concentration range from 0 – 50 ppm. The standards are traceable to the NIST reference sample SRM 1616 a – sulphur in kerosene. Seven grams of the standard or sample were pipetted into a liquid cell prepared with a 3.6 ìm Mylar film support as displayed in Figure 1. The samples were measured directly after preparation.

Quick, easy and simple sample preparation

Figure 1. Quick, easy and simple sample preparation

Measurements

Measurements were conducted on the S8 TIGER 1 K using the curved germanium crystal XS-GE-C, 0.46° collimator and 1 kW excitation with the settings 27 kV and 40 mA. The adjusted peak position of S Ká1 for the XS-GE-C crystal was set to a 2è value of 110.746°, the background position was set at 112.746°. The measurement time was 100s for the peak and 30s for the background.

Results

The calibration curve was determined from the intensity of the five standard samples by multiple regression. Calibration details are shown in Table 1, and the curve is shown in Figure 2.

Table 1. Calibration details for low sulphur in fuels

Peak [° 2è] 110.746°
Bkg [° 2è] 112.746°
Cal.Offs. [ppm] 0.5
Cal. Dev. [ppm] 0.1
r2 0.999987
LLD [ppm - 3ó, 100 s] 0.3

Calibration curve for ultra low sulphur in petroleum products covering a concentration range 0 – 50 ppm

Figure 2. Calibration curve for ultra low sulphur in petroleum products covering a concentration range 0 – 50 ppm

The detection limit is calculated according to the formula given below.

where
          m is the sensitivity of analyte in kcps/mass%
          Ib is the background intensity for analyte in kcps
          Tb is the counting time in seconds at the background angle

To test the method’s accuracy, one sample in the most important range below 10 ppm was measured 20 times. The results and the statistical parameters are shown in Table 2 and Figure 3.

Table 2. Precision data for sulphur in fuels

Sample S [ppm] Difference [ppm]
1 5.42
2 5.12 0.3
3 5.17 0.05
4 5.31 0.14
5 5.78 0.47
6 5.47 0.31
7 5.67 0.2
8 5.34 0.33
9 5.66 0.32
10 5.71 0.05
11 5.73 0.02
12 5.59 0.14
13 5.24 0.35
14 5.57 0.33
15 5.74 0.17
16 5.34 0.4
17 5.53 0.19
18 5.67 0.14
20 5.56 0.11
Mean value [ppm] 5.51
Abs. Std. Dev. [ppm] 0.20
Rel. Std. Dev. [%] 3.70

ASTM D2622: Precision test at 5.5 ppm S samples

Figure 3. ASTM D2622: Precision test at 5.5 ppm S samples

According to the standard ASTM D 2622-08 it is needed that the difference between two successive measurements stays at 19 out of 20 cases below 0.57 for a level of 5.5 ppm. This test limit is calculated as follows:

Repeatability (r) = 0.1462 and mean (c) = 0.8015

Conclusions

The analysis of low sulphur concentrations in automotive fuels according to ASTM D 2622-08 with the S8 TIGER 1 K is consistent, precise and accurate. The optimized excitation and the intensity optimized beam path with the curved germanium crystal, XS-GE-C enables a superior analytical performance. The precise and accurate determination of very low sulphur concentrations enables to achieve the production of low sulphur automotive fuels efficiently. The innovative vacuum seal and the SampleCare packages ensure that the S8 TIGER 1K is an indispensable tool for every refinery and testing lab providing protected and reliable analysis of liquid samples, highest instrument uptime and lowest cost of ownership.

This information has been sourced, reviewed and adapted from materials provided by Bruker X-Ray Analysis.

For more information on this source, please visit Bruker X-Ray Analysis.

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