Abstract
Plastic-waste pyrolysis oils contain large amounts of linear, branched, and di-olefinic compounds. This makes it not obvious to determine the detailed group-type composition in particular to the presence of substantial amounts of N-, S-, and O-containing heteroatomic compounds. The thorough evaluation of different column combinations for two-dimensional gas chromatography (GC × GC), i.e., non-polar × polar and polar × non-polar, revealed that the second combination had the best performance, as indicated by the bi-dimensional resolution of the selected key compounds. By coupling the GC × GC to multiple detectors, such as the flame ionization detector (FID), a sulfur chemiluminescence detector (SCD), a nitrogen chemiluminescence detector (NCD), and a mass spectrometer (MS), the identification and quantification were possible of hydrocarbon, oxygen-, sulfur-, and nitrogen-containing compounds in both naphtha (C5–C11) and diesel fractions (C7–C23) originating from plastic-waste pyrolysis oils. Group-type quantification showed that large amounts of α-olefins (36.39 wt%, 35.08 wt%), iso-olefins (8.77 wt%, 9.06 wt%), and diolefins (4.21 wt%, 4.20 wt%) were present. Furthermore, oxygen-containing compounds (alcohols, ketones, and ethers) could be distinguished from abundant hydrocarbon matrix, by employing Stabilwax as the first column and Rxi-5ms as the second column. Ppm levels of sulfides, thiophenes, and pyridines could also be quantified by the use of selective SCD and NCD detectors.
Highlights
Disposal and upgrading of plastic waste is one of the main challenges of the 21st century [1,2]
The performance and suitability of different column combinations were evaluated by comparing the chromatographic resolution obtained for each analysis by calculating the bi-dimensional resolution of the internal standard, each detected compound, and the intraclass bi-dimensional resolution [27,54]
The results reveal that the group-type separation is improved remarkably for diolefins, iso-olefins, and α-olefins and for n-paraffins, iso-paraffins, and monoaromatics, whereas the separation of naphthenes is deteriorated but still acceptable compared to the normal-phase combination
Summary
Disposal and upgrading of plastic waste is one of the main challenges of the 21st century [1,2]. It has been shown that pyrolysis oil obtained by pyrolysis of post-consumer High-Density Polyethylene (HDPE) bears fuel properties similar to crude oil [13,14] These pyrolysis oils are complex mixtures of n-paraffins, iso-paraffins, olefins, diolefins, iso-olefins, naphthenes, and aromatics with a wide carbon number distribution (C9–C25). It contains a relatively low abundance of heteroatom compounds such as oxygen-, nitrogen-, and sulfur-containing compounds, as well as halogens and metals [14,15,16,17,18]. The compositions thereof vary widely depending on the plastic source used and the operating conditions applied (e.g., temperature, pressure, and residence time) [19,20,21]
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