Article pubs.acs.org/IECR
Combined Comprehensive Two-Dimensional Gas Chromatography Analysis of Polyaromatic Hydrocarbons/Polyaromatic SulfurContaining Hydrocarbons (PAH/PASH) in Complex Matrices Thomas Dijkmans, Kevin M. Van Geem,* Marko R. Djokic, and Guy B. Marin Laboratory for Chemical Technology, Ghent University, Technologiepark 914, 9052 Zwijnaarde, Belgium S Supporting Information *
ABSTRACT: A new gas chromatographic method has been developed that is able to quantify polycyclic aromatic hydrocarbons (PAH) and polycyclic aromatic sulfur-containing hydrocarbons (PASH) up to four rings. The method combines the power of both flame ionization detection (FID) and sulfur chemiluminescence detection (SCD) in series on a single comprehensive twodimensional gas chromatography (GC × GC) system and provides mass fractions of compounds separated by carbon number n (CnHxSy) and class. In addition to PAH and PASH separation, the method is extended toward nonaromatic and monoaromatic (sulfur-containing) compounds (paraffins, naphthenes, monoaromatics, thiols, sulfides, disulfides, and thiophenes). The 95% confidence interval is doubled when a single injection technique is used instead of a more-accurate double injection technique. A flexible correction procedure that combines the advantages of the two-dimensional separation of GC × GC and its ability to easily define overlapping groups between the FID and the SCD chromatograms is applied. The method is validated using theoretical reference mixtures and is applied on three commercial gas oils with sulfur content from 0.16 wt % up to 1.34 wt %. The repeatability is good, with an average of 3.4%, which is in the same range as the much more expensive Fourier transform ion cyclotron resonance−mass spectroscopy (FTICR-MS) technique.
1. INTRODUCTION Crude oil is a complex mixture containing a wide variety of compounds such as alkanes, naphthenes, olefins, monoaromatics, and polycyclic aromatic hydrocarbons (PAH). In addition to these hydrocarbons, crude oil consists of a significant fraction of heteroatom-containing compounds, with sulfur being the principal heteroelement present in crude oils.1 Despite the low content of sulfur in light fractions (e.g., naphtha), sulfur can represent up to 6 wt % of the total elemental content in heavier fractions (e.g., vacuum gas oils).2 Sulfur-containing compounds found in crude oils include thiols, sulfides, thiophenes, benzothiophenes, dibenzothiophenes, and homologues. Benzothiophenes, dibenzothiophenes, and higher sulfur-containing ring systems are often called polycyclic aromatic sulfurcontaining hydrocarbons (PASH). These sulfur compounds can induce air pollution in the form of sulfur oxides (SOx) during combustion, promote corrosion and bad odor in fuels, as well as poison the catalyst.3 Detailed information on the distribution and the type of sulfur-containing compounds in middle distillates is essential for improving desulfurization technology.4 Also, for steam cracking, small quantities of sulfur compounds can drastically influence coke formation,5,6 and accurate quantification of the type and quantity of the sulfur containing compounds is crucial. Several standardized methods for the detection of PAH have been developed. ASTM standard ASTM-D6591 is a method to determine the aromatic hydrocarbon types in middle distillates via high-performance liquid chromatography with a refractive index detector.7 ASTM-D6379 also uses high-performance liquid chromatography with a refractive index detector, but is used for aviation fuels and petroleum distillates.8 ASTM-D5186 is a method for the determination of the aromatic content and © XXXX American Chemical Society
polynuclear aromatic content of diesel fuels and aviation turbine fuels via supercritical fluid chromatography.9 ASTM-D2425 is a test method for different hydrocarbon types in middle distillates by mass spectrometry.10 One of the main weaknesses of all the above methods is that they incorrectly quantify the amount of PAH when sulfur-containing compounds are present.7−10 Quantifying sulfur compounds is very laborious and timeconsuming, because of the complexity of sulfur compound isomers and the matrix consisting of an excess of hydrocarbons with similar properties. Standard methods for the quantification of the total amount of sulfur (ASTM standards ASTM-D262211 and ASTM-D545312) or specific sulfur compounds (ASTM standard ASTM-D550413) have been developed but, to the authors’ knowledge, no standard method is currently available that can quantify the sulfur-containing hydrocarbons separately. The latter also implies that there is no standard method available that could quantify both PAH and sulfur-containing hydrocarbons simultaneously. Next to ASTM methods, gas chromatography−mass spectrometry (GC-MS) methods have been developed for the detection and quantification of sulfurcontaining hydrocarbons . However, this technique is not purely selective toward sulfur compounds,14 because the hydrocarbon matrix fragmentation pattern will often interfere, because of the orders-of-magnitude difference in concentrations between these chemical structures.15 One of these methods is Robinson’s Special Issue: Alirio Rodrigues Festschrift Received: January 8, 2014 Revised: February 24, 2014 Accepted: February 25, 2014
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dx.doi.org/10.1021/ie5000888 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Table 1. Measured Elemental (C, H, and S) Composition of the Three Gas Oils by FLASH 2000 Elemental Analyzer and Using the GC × GC Composition GC × GC
Elemental Analysis gas oil
carbon (wt %)
hydrogen (wt %)
sulfur (wt %)
carbon (wt %)
hydrogen (wt %)
sulfur (wt %)
A B C
85.6 ± 0.3 85.7 ± 0.3 86.5 ± 0.3
13.0 ± 0.2 13.4 ± 0.2 13.3 ± 0.2
1.34 ± 0.02 0.84 ± 0.02 0.16 ± 0.01
85.3 85.4 86.2
13.4 13.7 13.6
1.30 0.83 0.16
method,16 which is a mass spectrometric procedure that can determine up to 21 compound types in petroleum aromatic fractions including different types of PAH and PASH. It has been reported that this method fails when thiols and sulfides are present, since these compounds tend to be distributed along the saturated types.16 To solve some of these problems, sulfurselective detectors are widely applied because they allow a straightforward quantification of sulfur-containing hydrocarbons. Several types of sulfur detection methods exist, such as sulfur chemiluminescence detection (SCD),17,18 atomic emission detection (AED)15,19,20 and flame photometric detection (FPD).21 The main advantage of SCD and AED detectors is that the response is more or less linear and equimolar to the amount of sulfur,22 which bypasses the need for calibration of these detectors. In contrast, the response of the FPD detector is nonlinear, not equimolar22,23 and has a limited dynamic range, making calibration very time-consuming. In addition, the FPD exhibits quenching of the sulfur response from coeluting hydrocarbons, which increases the detection limit. The newest generation FPD method, the so-called “pulsed flame photometric detection” (pFPD) process, allows one to overcome the reduced sensitivity and equimolar response but still exhibits quenching of the sulfur response, a nonlinear increase of the signal with the concentration23 and a limited dynamic range. With the rise of GC × GC, nowadays, all these selective detectors can be coupled to two-dimensional (2D) techniques such as GC × GC-MS,24−28 GC × GC-FPD,29 GC × GCpFPD,30 GC × GC-AED,31 and GC × GC-SCD,3,4,32−34 but all the previously mentioned advantages and disadvantages still hold. The acquisition rate of the AED detector is also too slow to be used quantitatively in GC × GC-AED31 without artificially increasing peak width in the second dimension, thus decreasing the resolution. SCD has also been successfully coupled to other comprehensive techniques, such as LC-GC-FID-SCD,35 TLC-GC-FIDSCD,36 and SFC-GC × GC-SCD34 to identify various sulfur containing compounds in hydrotreated gas oils.36 Obviously when using selective detectors, only information about the sulfur compounds is obtained and no information regarding both the PAH and the PASH distribution is obtained separately. Next to chromatographic techniques, purely spectroscopic techniques (such as Fourier transform ion cyclotron resonance− mass spectroscopy (FTICR-MS)) are also available.1,27,37 The advantage of the latter is that it can identify the elemental composition, double-bond equivalents (rings plus double bonds to carbon), and carbon number, based on ultrahigh-resolution and accurate mass measurements.38 However, a big disadvantage of FTICR-MS is its high cost, which prohibits its widespread availability and routine use.39 To overcome the limitations of the currently available methods for analyzing sulfur-containing fractions, a new characterization method that gives quantitative information about the PAH and PASH distribution per carbon atom must be developed. The method proposed in this work combines the
sensitivity and straightforward calibration of a FID device and SCD equipment with the increased separation power of an GC × GC. Quantitative information about the content of PAH and PASH compounds can be obtained in a single run or two separate runs. The GC × GC results in structured ordering of peaks in the 2D chromatogram without wrap around (i.e., the roof-tile effect) if the modulation time, column length, and carrier gas flow rate are properly selected.40 The method also allows to determine the elemental composition and validation of other analytical techniques (e.g., elemental analysis).
2. EXPERIMENTAL SECTION 2.1. Samples and Chemicals. Analytical gases (helium, nitrogen, hydrogen, and air) were provided at a minimum purity of 99.99% (Air Liquide, Belgium). Heptane, decane, dodecane, hexadecane, toluene, styrene, naphthalene, bromobenzene and 1,2-benzodiphenylene sulfide were purchased from Sigma− Aldrich with a minimum purity of 99%. Phenanthrene, fluoranthene, 1-pentanethiol, thiophenes, benzothiophene, 3methylbenzothiophene, dibenzothiophenes, and 3-chlorothiophene were purchased from Sigma−Aldrich with a minimum purity of 98%, and 1-decanethiol was purchased from Sigma− Aldrich with a minimum purity of 96%. Gas oils A, B, and C were supplied by the Total refinery in Antwerp, Belgium. The elemental composition of gas oils A, B, and C was determined using a Flash EA2000 (Interscience, Belgium) equipped with both a TCD and FPD detector. For sulfur concentrations of >5000 ppm, the TCD detector was used, while for sulfur concentrations of
