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Jul-2019

Reduce conjugated diene blockage in fuel refiners with the automated fuels analyzer

The Automated Fuels Analyzer from VUV Analytics is quickly becoming the new industry standard for hydrocarbon analysis, combining the power of gas chromatography–vacuum ultraviolet (GC-VUV) spectroscopy with the VUV Analyze™ software platform and ASTM D8071 methodology.

Alex Hodgson
VUV Analytics
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Article Summary
In the gasoline range, compounds of each PIONA class have spectral features that distinguish them from other PIONA classes. Absorbance in the 170-200 nm region increases with double bond equivalency, especially as π bonds are added. This gives VUV a distinct advantage over mass spectrometry when differentiating olefins from naphthenes. Even though they have the same double bond equivalency, the π bond on the olefin gives a nice spectral feature around 180 nm that the naphthene does not. In mass spectrometry, olefins and naphthenes have the same molecular mass and thus overlapping ion fragments, making them more difficult to distinguish – a task that becomes nearly impossible when they co-elute.

A certain subset of olefins that are particularly troublesome to refiners are conjugated diolefins (CDOs), which are also called conjugated dienes. These compounds polymerise in high enough concentrations, which in hydrocarbon streams can gum up pipes and subsequently require a refinery to shut down to clean out the blockage. CDO levels must be kept below a threshold to prevent this polymerisation from happening.

One of the original methods for measuring CDO content is UOP-326, which was developed in 1965 and uses maleic anhydride as a dienophile. Excess maleic anhydride is added to the fuel sample, and some amount of the maleic anhydride is consumed in a Diels-Alder reaction with the CDOs. The remaining maleic anhydride is converted to maleic acid, which is then measured by colorimetric titration.

Although UOP-326 is still used today, it has several drawbacks. The method takes over 3 hours, whether it is performed manually or automatically. Certain nucleophiles like alcohols and thiols (which are commonly found in or added to fuels) also react with maleic anhydride, positively skewing values. On the flip side, some sterically hindered diolefins like 2,5-dimethyl-2,4-hexadiene do not react at all, which negatively skews values. Because of this lack of selectivity, the method is only semiquantitative and cannot give qualitative information, like which diolefin species are present.

More recently, a wide variety of CDO measurement methods have cropped up, including derivatised GC with mass spectrometry/NCD, HPLC, SFC-UV, NMR, near-IR, and voltammetry. VUV can identify and quantitate CDOs because of the power of the absorbance spectra. These absorbance spectra demonstrate distinct spectral features above 200 nm, which means they will stand out from saturates, olefins, and mono-aromatics (Figure 1). Five CDO species were spiked into a gasoline matrix and run in duplicate under D8071 conditions. Focusing in on two of the more baseline-separated CDOs, they give a nice linear response (r2 > 0.99) from 1% down to 0.05% for C7+ and 0.02% for C6 (Figure 2).

As for the lightest CDO, 2-methyl-1,3-butadiene (also known as isoprene), it was actually detectable down to 0.01% despite co-eluting with several major analytes. This example really showcases the power of spectral deconvolution, specifically time interval deconvolution via VUV Analyze™ Software. Isoprene co-elutes with a paraffin (pentane), which absorbs out until about 160 nm, and an olefin (trans-2-pentene), which has a nice spectral feature around 180 nm but falls off around 210 nm. Thus, the 220 nm spectral feature of isoprene can never be confused with either of these compounds, allowing for high quantitative accuracy (Figure 3).

It doesn’t get much simpler than the Automated Fuels Analyzer with from VUV Analytics to obtain not only the normal PIONA quantitation with speciation of BTEX, select oxygenates, and di-aromatics, but also when obtaining speciated quantitation of CDOs.
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