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Oct-2017

Avoid sulphur measurement bias from particulate settling in crude oil

Petra MAX™ delivers advanced D4294 sulphur analysis in addition to 12 elements from P to Zn including Ni, V, and Fe.

Kyle Kuwitzky and Jon Dunphy
XOS
Article Summary
This robust benchtop analyser complies with ASTM D4294 and ISO 8754 for measuring sulphur in hydrocarbons. Petra MAX is powered by HDXRF, utilising XOS patented doubly curved crystal optics coupled with a high-performance silicon drift detector and an intense monochromatic excitation beam. This industry-leading technology reduces background noise and increases signal-to-noise output, enabling low detection limits and high precision without the need for consumable helium gas, a vacuum pump, or extensive sample preparation.

Background
Test methods for measuring sulphur content, like ASTM D4294 and ISO 8754, have become critical for assessing the value of crude oil. These methods utilise X-ray Fluorescence (XRF) analysis, and with any method, it is important to consider the interferences inherent within the analysis technique. The ASTM D4294 standard test method references matrix effects as a known interference, which may influence the sulphur measurement in crude oil and produce bias results.
 
Challenge
Crude oil presents unique challenges for ASTM D4294 analysis. While sulphur containing compounds in crude oil are primarily comprised of organosulphur compounds that remain homogeneous in hydrocarbons, interfering elements like Ca, Cl, and Fe are commonly present as particulate solids that settle to the bottom of a sample over time. These particulate solids can absorb the X-ray signal and reduce the concentration of sulphur reported. While many D4294 instruments (traditional XRF) can correct for interfering elements, particulate settling in crude oil can create challenging scenarios. Graph A demonstrates how particulate settling in a Western Canadian crude blend increases the concentration of chlorine over time. Because the concentration is changing, it is not possible to accurately correct sulphur due to interference from chlorine.

Particulate settling in crude oil has shown to cause underreported sulphur measurements by as much as 40%. Such a significant error can cause misclassification of sour crude oil as sweet crude oil. With global regulatory trends lowering sulphur levels in refined products from diesel to marine fuel, underreporting sulphur may cause refiners to miscalculate the costs associated with processing incoming crude oil. Because D4294 instruments (traditional XRF) take their measurement from the bottom of the sample, particulate settling occurs at the focal point of the analysis rendering the analyser’s automatic interference correction, ineffective.
 
Solution
Many D4294 analysers are designed with the X-ray detector focused on the bottom of a sample cup where particulate settling occurs, as depicted in Diagram 1. Since particulates settle over time, it is difficult to obtain accurate sulphur measurements due to the changing concentration of interferences. To combat the effects of particulate settling in crude oil, Petra MAX delivers a new, innovative sample chamber that rotates the sample on its side, providing a clear measurement window for more accurate results. See Diagram 2.
 
Particulate settling studies
To evaluate the effects of particulate setting, crude oil samples were obtained from three North American refineries. The samples were received in five-gallon drums and then stored in one-litre containers. The iron concentration for each sample was used to estimate the degree of particulate settling. Table 1 shows a summary of the iron concentration and particulate classification for each sample.
 
Traditional xrf vs. Petra max
To study the effects of particulate settling on sulphur measurements, a crude oil sample was analysed using a traditional XRF analyser and Petra MAX. Refer to Diagrams 1 and 2 for sample introduction methods. The following sample analysis procedure was performed using both methods:
•    A particulate-free certified reference standard of 2 wt% S in mineral oil sample was measured for 100-seconds to check instrument accuracy
•    One-litre bottles of crude oil were shaken vigorously, and samples were prepared and measured immediately for 100-seconds

Measurements were repeated 5 times with a 5-second pause in-between. The data was collected and compiled to evaluate the effect of particulate settling on sulphur analysis.

Results
2% S in mineral oil - no particulates

In order to test the accuracy of each sample introduction method, a particulate-free certified reference standard of 2% sulphur in mineral oil sample was analysed using both traditional XRF and Petra MAX. Results for both methods demonstrate excellent accuracy. No particulates were present, and all measurements were within 1% of the known sulphur value and met the repeatability requirements for ASTM D4294 and ISO 8754. These results show that in the absence of particulate settling, both sample introduction methods provide accurate results.

Crude A - high level of particulates
Results for Crude A, containing a high level of particulates, are shown in Graph 1.  While the traditional XRF results show a rapid drift in sulphur concentration due to particulate settling, the results from Petra MAX remain stable for each repeat measurement. This demonstrates that, even with high levels of particulates, Petra MAX delivers accurate and precise sulphur measurements in crude oil for ASTM D4294 and ISO 8754 methodology.
 
Crude B - medium level of particulates
The results for Crude B, containing a medium (common) level of particulates, are shown in Graph 2. In this crude oil sample, the drift in sulphur concentration for traditional XRF analysis is much less than in Crude A. However, there is a 12% lower sulphur concentration reported by the traditional XRF analysis than Petra MAX, demonstrating that even medium levels of particulate settling still impact the reported sulphur concentration. Petra MAX delivers stable results over the five repeat measurements of Crude B.

Crude C - low level of particulates
The results for Crude C, containing a low level of particulates, are shown in Graph 3. These results demonstrate that when particulate settling is low, both the traditional XRF and Petra MAX methods show agreement in reported sulphur concentration. This confirms that particulate settling is the cause for underreported sulphur concentrations with traditional XRF analysis. 

Results summary
Table 2 shows a summary of the total sulphur drift results of all three crude oil samples from the first to the fifth 100-second measurement, after sitting for 500 seconds. Results from the particulate-free reference standard samples are also included.
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