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Analytical Instrumentation
Focus on XRF Technology
Easy Elemental Analysis of Heavy Fuel Oils Using Wavelength Dispersive X-ray Fluorescence
Robert Hergenrader, Manager, Petrochemicals and Analytical Service Labs, X-ray Elemental Analysis, Thermo Fisher Scientific, Ecublens, Switzerland
Heavy fuel oils are blended products based on the residues from various refinery distillation and cracking processes. They are composed from the highest boiling-point distillate fractions and non-boiling residuum of refined crude oils, resulting in viscous liquid products with a characteristic odor and requiring heating for storage and combustion. Heavy fuel oils are used as fuel for industrial heaters, boilers and engines in industrial plants, marine applications and power stations1
.
. Other terms commonly used to
describe heavy fuel oils include residual fuel oil, bunker fuel oil, industrial fuel oil, marine fuel oil and black oil. In 2008, 54% of the total global production of 530 million metric tons of heavy fuel oil was consumed in the marine fuel market to power the huge compression ignition engines of the world’s ocean-going ships2
Natural contaminants found in crude oil, such as sulfur, vanadium, nickel and iron (S, V, Ni, Fe) are largely tied up in complex non-volatile asphaltene and porphyrin molecules. As a result, these elemental contaminants remain and concentrate in the heaviest distillate fractions associated with heavy fuels. Refining process contaminants such as catalyst fines (Al, Si) also concentrate in these streams. In a high temperature, oxygen-rich combustion engine environment, the concentration and interaction of these variously abrasive and corrosive elemental contaminants can become virulent and highly damaging, reducing equipment service life by up to 80%3
. Likewise, heavy
fuel contamination from used oils including zinc, phosphorous and calcium additives (Zn, Ca, P) can affect oil viscosity and increase volatility, lowering fuel quality and causing safety and reliability problems such as ash fouling, slag and corrosion in engines. High asphaltene levels in particular make fuel oils unstable when stored and result in poor combustion.
New ISO Standard and Test Methods
Despite relatively inexpensive market prices, marine residual fuels must adhere to comprehensive production quality specifications which also guard against used oil dumping during storage. The International Organization for Standardization (ISO) is scheduled in summer 2010 to release the fourth edition of its ISO 8217 standard setting ever tighter specifications for organo-metallics and other contaminants in marine fuels4
.
As in previous editions, the ISO 8217:2010 standard describes ten total grades of distillate and residual fuels. Significant changes from the 2005 edition include a reduction of parts per million (ppm) allowances for elements aluminum and silicon (Al+Si) - indicative of trace catalyst fine contamination - designed to reduce the risk of abrasive particles
reaching the engine’s inlet. In addition, the 2010 edition reduces the limits for organo-metallic contaminant vanadium (V) for most grades, designed to limit post-combustion deposits.
The ISO 8217:2010 marine fuel standard specifically references wavelength dispersive X-ray fluorescence (WDXRF) as a preferred method for the analysis of sulfur, vanadium and nickel according to the ISO 14596:2007 and ISO 14597:1997 test methods. In addition, the WDXRF technique also provides advantages for easily measuring other organo- metallic contaminants in heavy fuel oils including aluminum and silicon from catalyst fine particles, and zinc, phosphorous and calcium from used oils (Table 1).
Wavelength Dispersive X-ray Fluorescence
The WDXRF technique provides many benefits for determining the elemental composition of a wide variety of samples, both solids and liquids. The technique provides significant advantages in overall speed of analysis owing to ease of sample preparation, which is performed in a non-destructive way and without the requirement for dilution. In addition, WDXRF achieves excellent spectral resolution, precision and stability from ppm to percentage concentrations across multiple elements. Traditional high power WDXRF analysis has long been a staple in petroleum laboratories due to the usefulness of both its full-range sequential elemental analytical capabilities for difficult problem-solving and ease of use for routine yet demanding petroleum quality analysis.
Shifting demands within the petroleum industry have necessitated corresponding changes to WDXRF technology and design. Recent advances in WDXRF technology allow new, lower power instruments to achieve impressively similar results to traditional high
Table 1: Elemental contaminants controlled by ISO 8217:2010 marine fuel standard
Element Sulfur (S)
Vanadium (V)
Aluminium (Al) + Silicon (Si) Zinc (Zn)
Phosphorous (P) Calcium (Ca)
Performance Factors
Corrosive wear, greenhouse emissions Corrosive wear, particulate emissions Abrasive wear
Used oil contaminant Used oil contaminant Used oil contaminant
†- ranges include lighter Distillate Fuel grades (low limits) to heaviest Residual grades (higher limits)
Limit (max) † 1.0-4.5 % m/m 50-450 mg/kg 25-60 mg/kg 15 mg/kg 15 mg/kg 30 mg/kg
power systems with significantly lower hardware and operating costs and less auxiliary support. The design challenge lies in developing such systems to meet ever-tightening regulatory standards. A study was performed to evaluate the suitability of advanced WDXRF technology for the analysis of heavy residual fuel oil according to typical international test protocols.
Experimental
The Thermo Scientific ARL OPTIM’X WDXRF analyzer was used for this study, equipped with a low power 50W rhodium target X-ray tube and Ultra Closely Coupled Optics (UCCOTM) technology greatly increasing X-ray efficiency. The instrument’s SmartGonio™ miniaturized goniometer operating sequentially covers elements at ppm levels in heavy fuels from Al (Z=13) to Zn (Z=30). The instrument was also configured with selected MultiChromator™ fixed channel crystals to enhance the performance on particular elements.
As mentioned, WDXRF provides a notable benefit of direct analysis of even highly viscous liquid samples with no dilution necessary. For this study, sample preparation involved simply pouring (with heating as necessary) fuel samples directly into Chemplex liquid
analysis cells sealed with 4 µm polypropylene (Spectrolene®) film. Samples were analyzed under a helium environment to eliminate air interferences; limits of detection were determined for 120 seconds counting time per element.
Results
The SmartGonio crystal and detector combinations used and sensitivities are shown in Table 2, along with some comparison of MultiChromator fixed channel sensitivities. The miniaturized goniometer sequential configuration provided extremely low limits of detection for virtually all contaminant elements in heavy fuels, well within the quality limits set by ISO 8217: 2010. The goniometer also provides spectral resolution ten times better than high-end energy dispersive XRF (EDXRF) instruments. For the lighter elements in particular, the additional MultiChromator fixed channel configurations with specially curved and focused crystals further improved sensitivity or could be used to reduce analysis time. The efficient low power system of the instrument does not require the same auxiliary water cooling as larger instruments, yet provides the good sensitivities shown along with full- range capabilities for high elemental concentrations such as percentage levels of sulfur.
June/July 2010
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