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XRF Feature


21


The D7039 method (MWDXRF) is essentially a subset of D2622 (WDXRF) with some important distinctions. The excitation X-ray beam of a WDXRF instrument is polychromatic whereas the MWDXRF excitation beam is monochromatic. For both, the output of the X-ray tube comprises the characteristic energy of the target element and the Bremsstrahlung spectral energy associated with the production X-rays by electron acceleration in a vacuum tube. The target element is chosen for a characteristic X-ray just high enough in excitation energy to produce X-ray fluorescence of the element of interest (sulfur) but low enough to minimize background scattering.


WDXRF instruments aim the multi-energy beam at the sample and the resulting beam is typically collimated and aimed towards a diffraction crystal, where it is then diffracted to a detector. Acting as a filter, the diffraction crystal is selected and physically arranged to direct the characteristic X-rays of the element(s) of interest towards the detector. The detector sees a spectral background with distinct peaks associated with the element(s) of interest rising above the background.


Figure 2. US Crude Sulfur Content and Desulfurization Capacity – EIA.GOV. Total Sulfur Methodologies and Technologies


There are a number of different technologies available on the market for testing sulfur in liquid petroleum products, due to regulations and requirements around the world. Shown below is a table outlining the different relevant technologies and their correlating methods. Process analyzers based on these technologies typically correlate to the respective laboratory method, or in some cases may have a method of their own.


Table 1. Total Sulfur Methods


ASTM Method


D2622 D4294 D5453 Technology Range WD XRF ED XRF UVF Scope Fuel Types Figure 4. Polychromated (left) and Monochromated (right) Beams


3 ppm - 4.6 wt.% Diesel fuel, jet fuel, kerosene, other distillate oil, naphtha, residual oil, lubricating base oil, hydraulic oil, crude oil, unleaded gasoline, gasoline- ethanol blends, and biodiesel


17 ppm - 4.6 wt.% Diesel fuel, jet fuel, kerosene, other distillate oil, naphtha, residual oil, lubricating base oil, hydraulic oil, crude oil, unleaded gasoline, gasoline- ethanol blends, biodiesel, and similar petroleum products


1.0 - 8000 ppm


Liquid hydrocarbons, boiling in the range from approximately 25 to 400°C, with viscosities between approximately 0.2 and 20 cSt at room temperature. Including naphtha, distillates, engine oil, ethanol, FAME, and engine fuel such as gasoline, oxygen enriched gasoline (ethanol blends, E85, M85, RFG), diesel, biodiesel, diesel/biodiesel blends, and jet fuel.


D7039


MWD XRF 3.2 – 2822 ppm Gasoline, Diesel Fuel, Jet Fuel, Kerosene, Biodiesel, Biodiesel Blends, and Gasoline-Ethanol Blends


In this paper, we will discuss the performance and precision of the D7039 method using MWD XRF technology. This technique utilizes high performing doubly curved crystal (DCC) optics coupled with a low power X-ray tube creating a low maintenance, highly precise technology. MWD XRF is a simplified and highly robust X-ray technique which provides sub-1 ppm sulfur detection. An MWD XRF analyzer engine (Figure 3) consists of a low power X-ray tube, a point-to-point focusing optic for excitation, a sample cell, a second focusing optic for collection and an X-ray detector. The first focusing optic captures a narrow bandwidth of X-rays from the source and focuses this intense, monochromatic beam to a small spot on the sample cell. The monochromatic primary beam excites the sample and secondary characteristic fluorescence X-rays are emitted. The second optic collects only the characteristic sulfur X-rays and focuses them on the detector. The analyzer engine has no moving parts and does not require consumable gasses or high temperature operations. MWD XRF removes the scattered background peak created by the x-ray tube increasing the signal-to-background ratio (S/B) by a factor of 10 compared to conventional WD XRF technology. The S/B is improved by using the monochromatic excitation of the x-ray source characteristic line. Additionally, the focusing ability of the collection optic allows for a small-area x-ray counter, which results in low detector noise and enhanced reliability.


Figure 3. Typical MWD XRF Setup


The WDXRF technique has been accepted practice for measuring sulfur in petroleum liquids for many years. However, when regulations for highway diesel moved to less than 15 ppm at the point of use, mandated by the EPA in 2006, improvements to the analytical instruments and revision to the method was required in an effort to remain competitive with emerging techniques. Similar evolution of the UVF method has taken place while EDXRF has not yet established itself as a viable ultra-low sulfur measurement technique. MWDXRF, on the other hand, was developed specifically to address the need of refiners and petroleum distribution partners for a simple measurement technique, ideally suited for single element, ultra-low sulfur measurements.


Figure 5. Flourescent Signal Before DCC (left) and after DCC (right)


For both techniques, the detector can be a proportional counter and a pulse height analyzer is required. In the case of MWDXRF, the pulse height analyzer can consist of an integrated pre- amplifier/amplifier/ single channel analyzer, since only a single energy appears in the spectrum.


Value of Precision


American Society for Testing and Materials (ASTM) methods such as D7039 are required to include full precision statements that include a repeatability and reproducibility component. Repeatability (r) is the variation of two measurements within a 95% confidence interval taken on one instrument of the same sample under the same operating conditions. Reproducibility (R) is the variation of running the same sample at different test sites using similar equipment. The ASTM D7039 precision statement was updated in 2013 to include a repeatability (r) for all products of 0.4998 * X^0.54 and a reproducibility (R) for all products of 0.7384 * X^0.54. With process instrumentation, reproducibility becomes critical. If the process can be continuously and quickly monitored, variation can be identified and optimization can be handled. As compared with the other methodologies in Table I, D7039 offers superior reproducibility from 5-10ppm which is critical for the Tier 3 mandate. As seen below in Figure 6, MWDXRF provides better reproducibility at the Tier 3 levels of 10 ppm sulfur in reformulated gasoline. This R value can help a refiner justify the economics of installation quickly when process optimization can be achieved faster.


MWDXRF instruments, on the other hand, direct the excitation beam to a doubly curved crystal (DCC), selected and aligned such that the maximum beam flux is captured and only the characteristic energy of the target is diffracted towards the sample. Figure 4, below illustrates the Polychromatic (left side) and Monochromatic (right side) excitation beams. With the DCC, the monochromatic excitation is a highly focused, single energy beam incident for the sample. This in turn results in a cleaner fluorescence signal of the sample with far less scattering, which is then directed to another doubly curved crystal for selecting only the characteristic energy of the element of interest to be diffracted to the detector. The end result is a single energy peak with very little spectral background. This is what delivers a signal-to-background ratio improved by a factor of 10 over WDXRF. It also allows use of a much lower power X-ray tube. Figure 5 illustrates the impact of the second DCC.


JUNE / JULY • WWW.PETRO-ONLINE.COM


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