Comprehensive multidimensional gas chromatography for the analysis of Fischer-Tropsch products

Van der Westhuizen, Katriena Elizabet (2011-12)

Thesis (PhD)--University of Stellenbosch, 2011.

Thesis

ENGLISH ABSTRACT: The analysis of Fischer–Tropsch–derived (FT–derived) synthetic crude and derived products is very challenging because of the highly complex nature of these products. In this study, the use of comprehensive multidimensional gas chromatography (GCxGC) with time-of-flight mass spectrometry (TOF-MS) and flame ionisation detection (FID) was investigated for the analysis of these products and the technique was found to be invaluable for the analysis of these complex mixtures. The compositions of FT synthetic crude, produced at low temperature (LT–FT) and high temperature (HT–FT) processes were compared and the effect that changes in FT reaction temperature has on product formation was investigated. Results for conventional onedimensional GC (1D-GC) and GCxGC were compared. It was found that conventional 1D–GC does not have sufficient peak capacity to separate the thousands of compounds in the HT FT products. GCxGC provides a huge peak capacity of tens-of-thousands to separate highly complex mixtures. Structured chromatograms, where groups of compounds with similar properties are grouped together, aid in peak identification. Moreover, sensitivity at low microgram per milliliter levels is obtained. These attributes enabled accurate analysis of various complex feed and product streams in the FT refinery, and also various final fuel products. The use of GCxGC alone was demonstrated, and also combined with high performance liquid chromatography (HPLC), supercritical fluid chromatography (SFC) and nuclear magnetic resonance (NMR) when even more separation power was needed. HPLC–GCxGC enabled the separation of alkene and cyclic alkane compound classes in oligomerisation products. These compound classes have similar mass spectra, elute in adjacent regions and co–elute even to some extent on the GCxGC contour plot, making differentiation difficult. SFC is a good replacement for HPLC for these applications because it does not use solvents as mobile phases. CO2 is easily evaporated after the separation and does not interfere with the GCxGC separation of the analytes. SFC is also a very good technique to separate the compound classes of alkanes, alkenes, aromatics and oxygenates, and is therefore highly complementary to GCxGC. The combination of GCxGC with NMR data was also found to be very valuable for the identification of branched alkane isomers in LT–FT diesels. GCxGC provides excellent separation of individual compounds but the identification of isomers (except for mono–methyl branching) is difficult because the mass spectra of most of these isomers are similar and not all compounds are in the mass spectral libraries. NMR, on the other hand, is able to distinguish between the individual types of branched isomers but has limited separation power for the complex mixtures. By combining the two techniques, the best of both was obtained. The study found GCxGC to be invaluable for the analysis of the highly complex FT–derived products, while its combination with other techniques such as HPLC, SFC and NMR provided even more separation power.

AFRIKAANSE OPSOMMING: Die hoogs komplekse samestelling van sintetiese ru–olie en afgeleide produkte, afkomstig van Fischer–Tropsch (FT) sintese, bied groot uitdagings aan die analis. Die studie het die gebruik van GCxGC met ’n TOF-MS en FID bestudeer vir die analise van FT produkte en het bevind dat die tegniek van onskatbare waarde is vir die analise van die hoogs komplekse mengsels. Die samestellings van produkte van lae- en hoë-temperatuur FT prossesse is vergelyk en die effek van ’n verhoging in die reaksie–temperatuur op die produk samestelling is ondersoek. Resultate vir 1D–GC and GCxGC is vergelyk en dit was duidelik dat 1D-GC nie naastenby voldoende piekkapasiteit het om al die komponente van die produkte wat tydens die hoëtemperatuur prosses gevorm word, te kan skei nie. Die GCxGC se piekkapasiteit daarteenoor is in die orde van tienduisende wat die skeiding van hoogs komplekse mensels moontlik maak terwyl die tegniek hoogs gestruktureerde kontoerplotte verskaf wat help met identfikasie van komponente. Die tegniek is verder ook baie sensitief en kan komponente op lae μg/mL vlakke waarneem. Hierdie eienskappe het akkurate analise van verskeie FT produkstrome moontlik gemaak. Die kombinasie van GCxGC met HPLC, SFC en KMR het selfs meer skeidingskrag verskaf waar nodig. HPLC–GCxGC het die skeiding van alkene en sikliese alkane moontlik gemaak. Hierdie komponent klasse se massaspektra is feitlik dieselfde en terselfdertyd elueer die twee groepe reg langs mekaar, en oorvleuel soms selfs tot ’n mate, op die GCxGC kontoerplot, sodat dit moeilik is om daartussen te onderskei. SFC is ’n goeie alternatief vir HPLC in meeste toepassings aangesien die tegniek net CO2 gebruik, wat maklik verdamp by kamertemperatuur en nie oplosmiddels gebruik wat se pieke steur met die van die laekookpunt komponente op die GCxGC kontoerplot nie. Skeidings van die komponentgroepe alkane, alkene, aromate en oksigenate is moontlik met SFC en daarom komplimenteer dit die GCxGC skeiding goed aan. Die kombinasie van GCxGC met kern–magnetiese resonansie (KMR) is van waarde gevind om die verskillende tipes vertakkings in ’n lae-temperatuur FT diesel te identifiseer. GCxGC verskaf uitstekende skeiding van individuele komponente maar die identifikasie van die verskilende isomere, behalwe vir die mono-metiel vertakkings, is moeilik aangesien die massaspektra van baie van die komponente soortgelyk is en die komponente nie in die massa spektrum–biblioteke voorkom nie. KMR, aan die ander kant, kan tussen die individuele vertakkings onderskei maar het beperkte skeidingskrag vir komplekse mensels. Deur die twee tegnieke te kombineer is die beste van albei tegnieke bekom. Die studie het bevind dat GCxGC van onskatbare waarde is vir die analise van die komplekse sintetiese FT produkte terwyl die kombinasie met ander tegnieke soos HPLC, SFC and KMR selfs meer skeidingskrag verskaf.

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