Production • Processing • Handling
Traditional techniques
Bomb combustion has traditionally been used for the analysis of sulphur in fuel. However, the method does not have a low detection limit and is only applicable to low volatility samples containing more than 0.1 per cent (1000ppm) of sulphur. In addition, it is a time consuming gravimetric method. Inductively coupled plasma optical emission
spectrometry (ICP-OES) is a sensitive and accurate technique applicable to the measurement of a large proportion of the elements in the periodic table. It has low detection limits for sulphur at approximately 10-15 µg/kg in petroleum products. However, there is some evidence that the efficiency of the method for the detection of sulphur in the plasma is dependent upon the species of the sulphur compounds in the sample. Tis may be caused by the different transportation efficiencies of the various species. In addition, the method is not currently capable of measuring nitrogen in fuels due to the large background associated with atmospheric nitrogen in the plasma. In order to address these shortcomings, a more accurate and reliable analytical technique is required capable of minimising safety concerns whilst maximising precision. X-ray fluorescence (XRF) is a powerful technique for sulphur measurement in fuels and is very quick and simple, particularly at the high levels of sulphur traditionally found in bunker fuel. XRF achieves fast multi-elemental analysis of a
wide variety of materials in a non-destructive way with little or no sample preparation but requires matrix matching for optimum accuracy. Te method is flexible and offers sequential and/or simultaneous analytical capabilities, although it is generally a more expensive solution. Tere are two
types of XRF technology; energy dispersive X-ray fluorescence spectrometry (EDXRF) and wavelength dispersive X-ray fluorescence spectrometry (WDXRF). Te principal difference is in the achievable energy (spectral) resolution. WDXRF typically provides working resolutions between 5 and 20eV whereas EDXRF systems typically provide resolutions between 150 and 300eV or more. While X-ray spectrometry, ICP-OES and bomb combustion are all excellent techniques in their own right, they all demonstrate certain limitations for this type of analysis. Te analysis of total nitrogen and total sulphur in bunker fuel by combustion and subsequent chemiluminescence/UV fluorescence detection is a highly competent alternative. Te total sample is combusted and analysed making sample homogeneity and matrix matching trivial considerations. In addition, repeatability and reproducibility are excellent with typical RSDs less than 2 per cent even at low concentrations. Tese instruments are also regulated by ASTM, DIN and ISO methodology for a wide range of sample types.
Experimental A total nitrogen (TN)/total sulphur (TS) analyser (TN/TS 3000, Termo Scientific) was used to measure total nitrogen and total sulphur content in a bunker fuel sample. Te system performs high temperature combustion analysis according to ASTM D5453 principles. A bunker fuel sample was received from a multinational oil company to assess the capability of the method to accurately measure both sulphur and nitrogen in the fuel. Te instrument was calibrated using nitrogen and sulphur standards prepared from pyridine and thiophene diluted in xylene over the range 0 to 6000mg/l (where the density of xylene is taken to be 0.86g/cm3
).
Fig. 2. Example of the nitrogen calibration line used.
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