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Analytical Instrumentation
Focus on XRF Technology
Analysis of Low-Level Sulfur in Petroleum Products in Accordance with ISO 20884:04 and ASTM D2622-10
Taco van der Maten, Product Manager XRF, PANalytical B.V.
www.panalytical.com/axiosmaxpetro Email:
info@panalytical.com Tel: +31 546 534 444
Sulfur occurs naturally in crude oil, in concentrations typically ranging from 0.5 – 5.0 wt%. However, research has shown that the presence of sulfur in road fuels, at just a few hundred mg/kg, makes automobile pollution a major contributor to greenhouse gas emission and smog formation. Exhaust emissions of sulfur as sulfur dioxide and sulfate particulates directly contribute to the pollution
load. However, the main drive for reducing the sulfur level of fuel is to maximize the CO2 reduction potential of new fuel-efficient engine technology and to improve the efficiency of emission control devices, such as catalytic converters, reducing emissions of NOx and particulates.
In the United States the EPA Tier 2 regulations (2002) set an upper limit of 150 mg/kg for the sulfur content of road fuels. However, in the European Union even more stringent regulations are being set. The EU Directive 98/70/EC requires that, from 2005 onwards, the sulfur level in road fuels does not exceed 50 mg/kg (Emission Standard Euro IV). This is to be followed by Euro V, which requires a reduction of sulfur in fuel to a maximum of 10 mg/kg in 2009.
X-ray fluorescence spectrometry is used extensively for the analysis of sulfur in petroleum products. Simple sample preparation, high accuracy and precision, and good to excellent detection limits (0.2 – 1 mg/kg) are the principal reasons for this choice, and make it an excellent technique for production control.
International Standard Test Methods
Both ASTM 2622 and ISO 20884 methods are broadly analogous, employing mathematical matrix corrections, and are distinct from ISO 14596, which employs a Zr internal ratio correction method.
The latest 2010 version of ASTM D2622 is the most stringent test method in terms of repeatability and reproducibility.
In this study we have set up and made separate calibrations in accordance with the methods stipulated in both ASTM D2622-10 and ISO 20884:04 norms.
Preparation of Standards and Samples
Commercially available synthetic standards, prepared according to the test methods, were used for this study NIST Diesel SRM 2723a was used as purchased. Fifteen milliliters (15 ml) of sample material (standard or routine) were poured into 38 mm diameter disposable liquid cells, constructed using Mylar X-ray film.
Table 1. Analytical precision for sulfur analysis; repeatability of 20 consecutive measurements. Differences in the RMS and CSE values for the ISO and ASTM data in Table 1 can be attributed to different measurement times and different X-ray fi lm thickness.
(CSE) calculated in mg/kg is also shown in Table 1.
For the ISO method, 20 consecutive measurements of a sample demonstrate standard deviations better than 2 % relative at the 33 mg/kg level, e.g. 32.9 ± 0.6 mg/kg S, and this includes the error associated with preparing twenty different liquid cells.
The repeatability requirements of both test methods require that successive test results should not exceed certain limits more than one case in twenty. For samples with less than 60 mg/kg sulfur these limits are according to the equations below:
• ISO 20884:04 =
1.7 + (0.0248*mean(mg/kg)) • ASTM D2622-10 =
0.1462* mean(mg/kg)0.8015 AxiosmAX -Petro The AxiosmAX -Petro is especially
configured to meet the needs of XRF analysis in the petrochemical industry – today and in the future.
Offering the highest levels of reliability in this demanding environment, the AxiosmAX
-Petro is
a sequential XRF system for all XRF applications, from sulfur through catalysts to wear metals. It is simple to operate, and the compact design is easily integrated into today’s laboratories.
Precision and instrument stability The precision and repeatability of the AxiosmAX
Figure 1. Repeatability of sulfur measurements in diesel according to ISO 20884:04. Shaded area represents the precision limits set in the method.
-Petro is excellent. For comparison, the counting statistical error
The repeatability of measurements for samples containing 33 mg/kg and 8.3 mg/kg sulfur, together
with the repeatability limits for the ISO and ASTM methods are illustrated in Figures 1 and 2, respectively.
Repeated measurements on NIST diesel SRM 2723a for over 120 days have a maximum difference between two successive measurements of 0.5 mg/kg*.
The results demonstrate excellent longterm stability for the AxiosmAX
-Petro. Accuracy
The accuracy of the calibrations is illustrated in Figure 4. In this plot the calibration RMS value (0.23 mg/kg) is a statistical comparison (1 sigma) of the certified chemical concentrations of the standards with the
Sample preparation:
The key to analytical precision
Assembly of the liquid cells is critical to the precision of the method. Without careful attention it can easily become a significant source of error in the analysis.
Therefore, a special assembling tool is included in the AxiosmAX package.
-Petro
June/July 2010
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