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Analytical Instrumentation Fuel Analysis by Mid-Infrared Spectroscopy Dr. Roland Aschauer, R&D Director of Eralytics GmbH.
www.eralytics.com,
office@eralytics.com.
The analysis of fuels, especially gasoline and diesel automotive fuels, by a rapid, accurate, and easy-to-use method which can be applied also in the field is very important for the monitoring of fuel composition and quality at several points in the manufacturing process and distribution chain. Such a method is valuable in laboratories at refineries where fast, accurate results are needed, in small terminal and pipeline laboratories where it is not possible to perform all specified tests but monitoring fuel quality is necessary, at gas stations itself, and in mobile laboratories.
IR spectroscopy is the method of choice for such applications for several reasons discussed below.
Advantages of IR Spectroscopy The IR spectrum contains direct molecular specific information so that many parameters can be determined simultaneously from one measurement. The evaluation of the spectrum that leads to the final results can be performed automatically by microprocessors, so that the instrument can be automated. Thus IR instruments can be operated by personnel that have no background or training in analytical chemistry.
The measurement is very quick in comparison to other analytical methods, in the order of one minute per analysis.
The instruments to measure the IR spectrum can be made very compact, stable, and robust so that they are portable and may be used in harsh conditions and in the field. The IR analysers do not need any gas or other supply, as neither separation steps nor chemical reactions are required prior to the measurement.
No sample preparation is necessary, only an electrical power line is needed.
No other analytical technique combines all these advantages.
Principle of IR Spectroscopy
Infrared (IR) radiation is electromagnetic radiation with a wavelength between 0.7µm and 100µm. This range is further divided in the Near-IR (NIR) range (wavelengths 0.7µm to roughly 4µm), mid-IR (wavelengths 4µm to roughly 20 µm), and far-IR (wavelengths 20µm to 100 µm).
For analytical applications, mostly NIR and mid-IR spectroscopy are used.
In IR spectroscopy, IR radiation in the desired wavelength range is directed through the sample to be analysed, and the intensity of the transmitted radiation is measured as a function of wavelength. The process that leads to attenuation (absorption) of IR radiation by the sample is excitation of vibrations of molecules in the sample.
These vibrations can be excited only by radiation of wavelengths that match the energy difference between the ground and excited vibrational state. These wavelengths are different for different molecules; furthermore there are usually different vibrations in a given molecule. This is why IR spectroscopy allows for the simultaneous determination of many different substances in one sample by one measurement over a certain wavelength range.
If a vibration is excited by radiation of a specific wavelength, then the intensity of the IR radiation passing through the sample at that wavelength
decreases. The more molecules are present, the more vibrations can be excited, and the more the radiation decreases.
Therefore, the concentrations of many different substances can be determined quantitatively by IR spectroscopy.
Application to the Analysis of Fuels Gasoline and Diesel fuels are complex mixtures of many different hydrocarbons and hydrocarbon types. They contain alkanes (paraffins), alkenes (olefins) and aromatics. Moreover, driven in part by the desire to use renewable fuel sources, alcohols and ethers are blended into gasolines, and FAME from vegetable oils is very frequently a component in Diesel fuels. Additionally, additives like MMT to increase octane numbers in gasolines or 2-EHN to increase the cetane number in Diesel fuels are used. Small quantities (<0.1 %) of such additives have a significant effect on some fuel properties. In order to accurately determine the composition of complex mixtures, mid-IR spectroscopy can be very successfully applied to perform a quantitative analysis of gasoline and Diesel fuel.
In gasolines, it is important to know the concentrations of individual compounds, like
benzene, ethanol or MTBE, but also total concentrations of different hydrocarbon types, like olefins and aromatics or the oxygen content. With mid-IR spectroscopy, the lines of individual aromatics (like benzene, toluene, xylenes etc.) and oxygenates (like ethanol, methanol, MTBE, ETBE, etc.) can be resolved and used to calculate the concentrations of each compound, with no or minimal interference from other, chemically similar substances.
These individual lines as well as lines coming from certain hydrocarbon types (like olefins and aromatics) can then be used to calculate also the total concentrations of olefins, aromatics and oxygenates as well as the oxygen content of the gasoline sample.
Some additives like MMT which are present at concentrations typically only around 100 ppm can still be determined with mid-IR spectroscopy since the lines are very strong.
In Diesel fuels, the mid-IR spectrum allows to discriminate between single-nuclear and poly- nuclear aromatics. The FAME concentration can be measured with high accuracy using a very strong line. Unreacted vegetable oil and FAME have different mid-IR spectra, therefore with mid-IR spectroscopy, it is possible to detect if vegetable oil or FAME is present
Annual Buyers’ Guide 2011
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