Analysis of Sulfur and Trace Element Content in FAME

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Clyde Crawford
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AN APPLICATION REPORT FROM SPECTRO ANALYTICAL INSTRUMENTS

0 SPECTRO XEPOS (XEP05) Analysis of Sulfur and Trace

fuels and other biofuels play a growing role in the offer

Currently, fatty acid methyl ester FAME, mostly derived

is recycled vegetable oil derived from the food industry.

1 AN APPLICATION REPORT FROM SPECTRO ANALYTICAL INSTRUMENTS XRF-93, Rev. 0 SPECTRO XEPOS (XEP05) Analysis of Sulfur and Trace Element Content in FAME Introduction These days, renewable fuels and other biofuels play a growing role in the offer of automotive fuels. Currently, fatty acid methyl ester FAME, mostly derived from vegetable oil, is typically blended with regular diesel fuel in the 5-10 or 20% range. An alternative for the fairly expensive fresh vegetable oil is recycled vegetable oil derived from the food industry. An elemental analysis of the pure FAME, the used vegetable oil, the reprocessed vegetable oil; as well as of the biofuel blends is required to show compliance with actual specifications.

2 Analysis Sulfur and Trace Element Content in FAME Regulations and standards have made it increasingly important

quality. Only materials and products that conform with the respective specifications can be distributed.

It is, therefore, one of the preferred choices for elemental analysis at-line and in the lab.

analysis of Ca, K, Mg and Na in FAME, samples require a 1:1 dilution (50%) in kerosene (to reduce matrix effects)

EN16294-2012: describes the analysis of P in FAME, samples require a 1:3 dilution (25%) in kerosene and the use of

Sulfur is not part of these methods and is determined using different technologies.

This report describes a quick and economic alternative to quantify the sulfur content in FAME using ED-XRF.

monitoring further elements like P, Cl, Na, K, Mg, Ca, and many more elements which can be harmful for the car

2 2 Analysis Sulfur and Trace Element Content in FAME Regulations and standards have made it increasingly important for the manufacturers of FAME to monitor and test the elemental compositions of their products to ensure product quality. Only materials and products that conform with the respective specifications can be distributed. Energy-dispersive X-ray fluorescence (ED-XRF) spectrometry has proven itself to be a useful analytical technique for process control. ED-XRF is characterized by relatively simple sample preparation, ease of use and excellent precision. It is, therefore, one of the preferred choices for elemental analysis at-line and in the lab. Typically, this type of analysis is done using ICP-OES following the test methods EN : describes the analysis of Ca, K, Mg and Na in FAME, samples require a 1:1 dilution (50%) in kerosene (to reduce matrix effects) and no internal standard is used. EN : describes the analysis of P in FAME, samples require a 1:3 dilution (25%) in kerosene and the use of an internal standard is mandatory. Sulfur is not part of these methods and is determined using different technologies. The analysis requires some sample preparation and in-vestment in purchase and operating cost of the analyser. This report describes a quick and economic alternative to quantify the sulfur content in FAME using ED-XRF. Besides the analysis of the sulfur content, EDXRF equipment (depending on the analyzer performance), is capable of monitoring further elements like P, Cl, Na, K, Mg, Ca, and many more elements which can be harmful for the car engines. Recycled vegetable oils that cannot be used to be added into diesel fuel can be used as secondary burner fuel and an elemental analysis to show the absence of toxic elements is part of the quality control. This can also be done with the XRF equipment. In addition it would be of interest to at least check the samples for their oxygen content to make sure that the material used for blending in the diesel fuel is really based on FAME and not based on mineral oil.

Analysis Sulfur and Trace Element Content in FAME 3 Test Methods Several test methods are available that describe the use of ED-XRF for the analysis of sulfur in fuel. ISO 13032 is one of them.

using ED-XRF are limited to fuels with a discrete maximum amount of FAME or oxygen. In the example of ISO 13032 the maximum content of FAME is 10 % or 3.7 % of oxygen.

3 Analysis Sulfur and Trace Element Content in FAME 3 Test Methods Several test methods are available that describe the use of ED-XRF for the analysis of sulfur in fuel. ISO is one of them. ISO 13032, Petroleum products Determination of low concentration of sulfur in automotive fuels Energy-dispersive X-ray fluorescence spectrometric method, ISO The currently available test methods using ED-XRF are limited to fuels with a discrete maximum amount of FAME or oxygen. In the example of ISO the maximum content of FAME is 10 % or 3.7 % of oxygen. The reason for this is that the oxygen has a matrix effect on the sulfur signal measured by XRF. To be able to use XRF for this type of analy-sis various approaches are possible. - Correction using defined correction values depending on the known oxygen content this requires a-priori knowledge of the oxygen content. - Correction using fundamental parameters based on the matrix, which has to be predefined also this requires an a-priori knowledge of the oxygen content. - Correction using backscatter information from the sample. This does not require any a-priori knowledge of the sample matrix but the backscatter correction must be based on a signal, which should be close to the analyte s X-ray fluorescence energy as otherwise additional effects occur. - Additional references to test methods are given in the annex of this report. For other elements of interest, the same effect is visible. Depending on the element, the influence on the results is different.

4 Analysis Sulfur and Trace Element Content in FAME Instrumentation SPECTRO XEPOS In a SPECTRO XEPOS analyzer, samples were excited by a forced-air-cooled 50 W endwindow X-ray tube combined with a

4 4 Analysis Sulfur and Trace Element Content in FAME Instrumentation SPECTRO XEPOS In a SPECTRO XEPOS analyzer, samples were excited by a forced-air-cooled 50 W endwindow X-ray tube combined with a doubly curved HAPG crystal for monochromatization and polarization of the primary tube spectrum. In addition, direct excitation using Pd-K and Co-K radiation can be used to optimize the excitation conditions for groups of elements. A high-resolution silicon drift detector (SDD) was used to collect fluorescence radiation from the sample. The highly stable spectral resolution of the SDD was < 130 ev for Mn Kα. Measurement parameters are given in Table 1. Table 1: Measurement conditions (Measurement time = clock time, live time is about 50% shorter than given measurement time) Element range kv/ma Excitation Meas. Time [s] Ag, Cd, Sn, Sb 49.5 kv, High Energy 180 Fe, Co, Ni, Cu, Zn, Mo, Pb 0.72 ma Pd 180 K, Ca, V, Cr, Mn 40 kv, Co 180 Mg, Al, Si, P, S, Cl 22.5 kv, 2.0 ma HAPG 180 Illustration 1: Calibration for S in mineral oil and biofuel with 12.5 % oxygen Sample Preparation The sample preparation for the analysis is very easy: 4 g of sample material is poured into a disposable XRF sample cup with an outer diameter of 32 mm. The analytical side of the sample cup is closed with a 4 µm thick polypropylene film. For additional safety, the sample cups are placed into a sample holder, which is closed with a second polypropylene film with the same thickness. Applications As explained above biodiesel blends according to specification EN 590 typically can be analyzed with a calibration prepared for diesel fuel. Using the same calibration for the analysis of a biofuel sample with 12.5 % oxygen would lead to a significant bias in the analysis result as can be seen from illustration 1. In many cases the oxygen content of the sample to be analyzed is not known, therefore the best option for an appropriate matrix correction is to determine the oxygen con-tent of the sample. Using XRF the direct determination of the oxygen content in fuel is not possible as the fluorescence energy of oxygen is very low and will not be detectable. The second approach is to use the backscatter information of the sample to indirectly determine the content of oxygen. The so-called Compton scatter signal will decrease with increasing O-content. To be able to achieve

Analysis Sulfur and Trace Element Content in FAME 5 Illustration 2: Calibration for O in fuels this accurately the energy of the Comptonscatter signal should be at relatively low energy as otherwise

5 Analysis Sulfur and Trace Element Content in FAME 5 Illustration 2: Calibration for O in fuels this accurately the energy of the Comptonscatter signal should be at relatively low energy as otherwise i.e. volume effects will affect the determination. As the SPECTRO XEPOS is equipped with an X-ray tube with thick binary Pd/Co alloy anode in principle there are two options to use backscatter information, one from the excitation using the characteristic radiation of Pd, the second using the characteristic radiation of Co. Due to the reasons listed above using the Cobalt excitation of the SPECTRO XEPOS, this determination is possible with good accuracy. The following illustration 2 shows the calibration of the measurements of samples with known oxygen concentration. The illustration demonstrates the sensitivity and the accuracy of this indirect determination of the oxygen content. Based on this the differentiation between material based on FAME or on mineral oil is easily possible. For the accurate determination of the trace element content the oxygen content determined with this procedure is used within a fundamental parameters program. The following table shows some examples. Table 2: Analysis results of S in fuel samples with different oxygen content Sample oxygen content S nominal in mg/kg S analyzed in mg/kg Diesel 0 % ± 0.1 Sample A 10.5 % ± 0.1 Sample B 11.0 % ± 0.1 Sample C 11.5 % ± 0.1 Sample D 11.5 % ± 0.1 Sample E 12.0 % ± 0.1

6 Analysis Sulfur and Trace Element Content in FAME As listed on the previous page, other trace elements can be determined simultaneously using the SPECTRO XEPOS.

6 6 Analysis Sulfur and Trace Element Content in FAME As listed on the previous page, other trace elements can be determined simultaneously using the SPECTRO XEPOS. The following table 3 gives detection limits for additional trace elements. Although there is currently no specified test method using XRF for the determination of these elements in biofuels the listed LODs clearly demonstrate that at least a screening for the content of these elements is possible using the SPECTRO XEPOS, even at low levels. Table 3: LODs in fuel using the SPECTRO XEPOS Element LOD in mg/kg Mg 23 ** Summary The SPECTRO XEPOS elemental analyzer clearly can be used for the analysis of S and other trace elements in biofuel. The determination of the oxygen-content helps to compensate for matrix effects efficiently, an a-priori knowledge of the oxygen content is not required. Al 5 Si 1 P 0.3 S 0.2 Cl 0.2 K 0.5 Ca 0.2 V 0.2 Cr 0.2 Mn 0.3 Fe 0.5 Ni 0.5 Cu 0.5 Zn 0.1 Mo 0.5 Ag 0.5 Cd 0.5 Sn 1.0 Sb 1.2 Ba 1.0 Pb 0.2 ** can be significantly improved if the samples are analyzed without the additional protection film

7 Analysis Sulfur and Trace Element Content in FAME 7 References Important references for specifications and test methods are listed below. EN 590, Automotive fuel- diesel- Requirements and Test Methods, CEN. EN 14214, Automotive fuels - Fatty acid methyl esters (FAME) for diesel engines - Requirements and test methods, CEN. ASTM D6751, Standard Specification for Biodiesel Fuel Blend Stock (B100) for Mid-dle Distillate Fuels, ASTM International. Energy dispersive X-ray Fluorescence spectrometry (EDXRF) is currently listed as one of the standard test methods for the elemental analysis of S content in EN 590 and ASTM D6751. Depending on the analyzer performance even lower levels of S can be monitored. UK Environment Agency: Quality Protocol Processed Fuel Oil (PFO), geho0211btmo-e-e.pdf Test methods, which describe EDXRF and polarization EDXRF for the analysis of S content are: ASTM D 4294, Standard Test Method for sulfur in Petroleum and Petroleum Products by Energy Dispersive X-Ray Fluorescence Spectrometry, ASTM International. ISO 20847, Petroleum products- Determination of sulfur content of automotive fuels- Energy dispersive X-ray fluorescence spectrometry, ISO. ASTM D 7220, Standard Test Method for sulfur in Automotive Fuels by Polarization X-Ray Fluorescence Spectrometry, ASTM International. ISO 13032, Petroleum products Determination of low concentration of sulfur in automotive fuels Energy-dispersive X-ray fluorescence spectrometric method, ISO For referenced ASTM standards, visit the ASTM website, or contact ASTM Customer Service at service@astm.org. For referenced CEN standards, visit the CEN website, For referenced ISO standards, visit the ISO website,

8 8 Analysis Sulfur and Trace Element Content in FAME Choosing an ED-XRF analyzer ED-XRF technology keeps getting better and better. Today s most advanced instruments can provide a quantum leap in performance, even over earlier topranked models. Look for the following benefits: High sensitivity and precision. In multi-element analysis of major, minor, and trace element concentrations, it s critical to maximize spectrometric sensitivity and precision. Example: the newest SPECTRO XEPOS analyzers combine exclusive new excitation technology with innovative detector and tube designs. These help deliver up to 10X greater sensitivity and 3X better precision than previous models. So users get fast, accurate analysis of a wide range of elements, from sodium to uranium. Long-term stability. Most ED-XRF analyzers shut down their X-ray tube between measurements. Unfortunately, resulting temperature variations can negatively affect repeatability and accurate readings. To ensure stability, look for an instrument that maintains constant tube power. Low detection limits. Lower limits of detection (LODs) improve performance with minor and trace element concentrations. The best new models combine high sensitivity with minimized backgrounds, achieving exceptionally low LODs for a wide range of elements. Consumables flexibility. Analysis in an inert-gas environment sometimes produces better results. But constant gas purging becomes expensive. SPECTRO XEPOS instruments offer low-volume helium purging only when required for light elements in liquids and powders, with a vacuum system for solid samples. Lower costs. Today, an advanced ED-XRF analyzer such as SPECTRO XEPOS exhibits significantly lower costs of initial investment and long-term ownership than wavelength dispersive X-ray fluorescence (WD-XRF) spectrometers. Yet it generally provides comparable performance for many applications. GERMANY SPECTRO Analytical Instruments GmbH Boschstrasse 10 D Kleve Tel: Fax: spectro.sales@ametek.com U.S.A. SPECTRO Analytical Instruments Inc. 91 McKee Drive Mahwah, NJ Tel: Fax: spectro-usa.sales@ametek.com CHINA AMETEK Commercial Enterprise (Shanghai) CO., LTD. Part A1, A4 2nd Floor Building No.1 Plot Section No.526 Fute 3rd Road East; Pilot Free Trade Zone Shanghai Tel.: Fax: spectro-china.sales@ametek.com Subsidiaries: u Hong Kong: Tel , Fax , spectro-ap.sales@ametek.com, u FRANCE: Tel , Fax , spectro-france.sales@ametek.com, u GREAT BRITAIN: Tel , Fax , spectro-uk.sales@ametek.com, u INDIA: Tel , Fax , sales.spectroindia@ametek.com, u ITALY: Tel , Fax , spectro-italy.sales@ametek.com, ujapan: Tel , Fax , spectro-japan.info@ametek.co.jp, u SOUTH AFRICA: Tel , Fax , spectro-za.sales@ametek.com, u SWEDEN: Tel , Fax , spectro-nordic.info@ametek.com. uspectro operates worldwide and is present in more than 50 countries. For SPECTRO near you, please visit AMETEK Inc., all rights reserved, subject to technical modifications C-17, Rev. 0 Photos: SPECTRO, Corbis, Getty Images, istockphoto Registered trademarks of SPECTRO Analytical Instruments GmbH : USA (3,645,267); EU ( ); IR China ( ); IR Japan ( ); SPECTRO : EU ( ); IR USA ( ); IR China ( ); IR Japan ( ); IR South Korea ( ); ical: EU ( ); USA (3,189,726); SPECTRO XEPOS : Germany ( ), USA (2,415,185)

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