Elemental Analysis of Petrochemical Products

Arnd Bühler, Bruker AXS Alexander Seyfarth, Bruker AXS
Tags: oil analysis

Multielement analysis in the petrochemical industry is increasingly handled by X-ray spectroscopy (XRS). The popularity of this technique and instrumentation lies in the inherent advantages of using X-rays instead of light as the excitation source.

When relying on either the subsequent absorption or emission of light used as an excitation source, the atoms of the sample need to be in a free state. This means that the sample needs to be vaporized in order to be analyzed. When X-rays are used as the excitation source, the inner shells of the sample atoms are excited, producing characteristic energies (fluorescence radiation) that allow direct analysis of a solid or liquid sample. XRS is also referred to as X-ray fluorescence (XRF) spectroscopy.

XRF analysis of solid and liquid petrochemical samples is being used increasingly in recent years. An important reason for this trend is the possibility of direct analysis of undiluted liquid fuels and oils and the relatively simple preparation of samples. Liquid samples can be analyzed directly in disposable liquid cups, the bottom of which consists of a thin polypropylene film such as Mylar™ (Figure 1). Due to the absorption of the fluorescence radiation in the film, a useful liquid analysis via XRF is possible for elements only from sodium (Na) on up. Therefore, this excludes the common oil analysis element, boron.

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Figure 1. Sample Cups for XRF Analysis of Liquid Petrochemical Products

Instrumentation
XRF instrument capabilities have progressed dramatically, driven by advances in electronics, detectors, computing power and miniaturization. Just 15 years ago, XRF systems were big, cumbersome high-priced analyzers found in large quality-control laboratories operated by experienced spectroscopists. Today, the footprint of XRF instruments is greatly reduced and technology advances have led to smaller, more capable systems at lower costs.

Wavelength-dispersive XRF (WDXRF) employs a set of crystals positioned on a goniometer to sequentially detect and count each characteristic emission line allowing for the best resolution and sensitivity. Measurements of solid samples are normally made under vacuum as the absorption of X-rays in air renders impossible the analysis of elements which are lighter than titanium (Ti), atomic number 22. Analysis of liquids under vacuum is not feasible; therefore, a helium atmosphere is generally used.

Additional spectrometer features protect the system from accidental sample introduction, spills and leakages. Spectrometers with the latest technology automatically detect what kind of samples are going to be analyzed and automatically check that the right mode is used, such as helium atmosphere liquid samples. On Bruker AXS WDXRF analyzers, these combined system- and sample-protection technologies are called SampleCare™. These ensure exceptional uptime in tough multiuser production environments, such as refineries with 24/7 operation.

Applications
Along with spectrometer improvements for liquid analysis, refinements have been made to the universal precalibrated programs for "standardless" XRF quantitative analysis. The programs have been made more efficient for matrix correction by using variable alphas. These enhancements drove the development of a specialized, pre-calibrated program for full quantitative analysis of petrochemical products - Petro-Quant. Although the program has been optimized for quantitative analysis of liquid fuels and oils, it can also be used to analyze solid petrochemical products, such as greases, resins, polymers and plastics.

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Figure 2. In modern WDXRF spectrometers, a vacuum seal separates the sample and spectrometer chambers for safe and economic analysis of liquid fuels.

Sample Preparation
A petrochemical sample is measured in a disposable sample cup with a bottom made of a polymer film (Figure 1). Sample cups are assembled and covered with a film in a matter of seconds.

Today's sample cups are designed for reliability, stability and ease of use, as well as for spill and leakage safety. Weight of sample, diameter of cup and type of film used influence the analysis. The correct choice of film depends on the matrix of the petrochemical (for example, aromatic or aliphatic) to be analyzed. These factors have been taken into account in the development of the analysis program, allowing the use of different cups, films and sample weights to suit particular applications and methods.

Calibration and Validation
To create the petrochemical precalibrations included in the program, various sets of certified standard oils from manufacturers such as Conostan and Analytical Services were used. The standards were measured with the S4 series of WDXRF systems to establish a universal petrochemical base calibration, including the determination of any potential matrix interference and spectral line overlap.

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Figure 3. XRF Liquid Sample Preparation

Instrumentation
S4 PETRO analyzers include a WDXRF spectrometer and Petro-Quant, thereby offering a complete solution for the analysis of liquid and solid petrochemical samples. Available in 1 kW and 4 kW configurations, the analyzer systems optimize sample throughput and cost of operation by using a set of three crystals, two collimators and a choice of different detectors.

Calibration
The analysis program includes calibrations for 26 elements, sodium to lead. Two different calibrations are available for sulfur and chlorine, one each for trace amounts (up to 1,000 ppm) and one each for amounts in the percentage range (>1,000 ppm). Table 1 summarizes the most important calibration parameters for each element: concentration range, residual standard deviation, sensitivity, detection limit and standard deviation of repeatability for 25 µg/g and 500 µg/g samples.

Use
S4 PETRO systems provide a set of analysis methods that employ the analysis program calibration and measure 26 elements.

Using the operator-friendly software interface, a nonchemist can easily perform the analysis. In less than 20 minutes for routine scans, the results are displayed and stored in the GLP/GMP compliant database. Remote query of the database and data transfer to any LIMS are possible when the system is integrated into a computer network.

Screening of fresh and used lubrication oils with one out-of-the-box method like Petro-Quant allows the analyst to concentrate on the interpretation of the data. The ability to analyze wear metal elements, lubrication elements, as well as new elements such as potassium, all with one calibration, eliminates the bias of separate methods, which can lead to false positives when potentially interfering elements are not measured. Measuring suspect elements one at a time enables accurate and bias-free results.

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Figure 4. Bruker AXS Petrochemical Analyzers

Fuel and Lubricant Round-robin Results
During a European round-robin study, gear oil, base oil and a heavy fuel oil samples were sent to study participants. The results obtained by Bruker AXS instrument users (Castrol Austria and OMZ) who employed and reported the Petro-Quant base calibration values for all samples are shown in Tables 2 and 3.

The elements listed are the ones reported in the round-robin study for the respective sample types.

The values reported by the users match the expected certified values. This shows that with the calibrated analysis program, the user can save time and effort on calibrations right away.

In addition to the versatile settings which can be applied in the user's own methods, the analysis lines can be combined with user calibrations. This comes in handy when no reference materials are available on-site for an element found in the oil. The settings can also be used for methods where in-house reference materials are available.

Table 1. Element and Concentration Ranges of Calibrations

Conclusion
As a universal multielement analysis system for oils, Petro-Quant represents a new level of performance for XRF. The traceable, predefined methods enable laboratories which previously outsourced their wear metal and lube oil analyses to perform the analyses on-site within a short time.

Analytical labs use Petro-Quant on the S4 Series of WDXRF analyzers to lease solutions to their clients. Production operators can perform the analysis, and the analyzer itself can be validated remotely and checked via WEBEX™. Petro-Quant and S4 Series systems perform efficiently in sites that develop and analyze lubrication oils as well as in those that manufacture cars and machinery.

Table 2. Results for Gear Oil

About the Company
Bruker AXS Inc. is a developer and provider of advanced X-ray tools for industrial elemental and process control applications.

Since the early 1950s, the XRF and XRD instrumentation has developed to include systems and solutions from hand-held to high-end systems.

In 1999, Petro-Quant was introduced as Oilquant, a program used primarily for wear metal and lubrication oil analysis. Castrol and Porsche were among the first customers to use the original program. The traceable unified method now provided by Petro-Quant for the 26 elements important to petrochemical analyses gives bias-free results superior to ICP.

The company offers seminars, webinars and learning materials to anyone interested in increasing their knowledge of XRF and its many applications. Visit www.bruker-axs.com for information about upcoming events.

Table 3. Results for Heavy Fuel Oil