The use of multidimensional GC techniques for the analysis of complex petrochemical products

Van der Westhuizen, Rina (2005-12)

90 leaves on CD format, preliminary i-ix pages and numbered pages 1-81. Includes bibliography, list of figures in color to pdf format (OCR).

Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2005.

Thesis

ENGLISH ABSTRACT: The composition of petrochemical products obtained from Fischer Tropsch (FT) technologies is of the highest complexity possible and may contain thousands of components. Chemicals produced from FT feedstocks often contain trace level contaminants that can poison catalysts or that affect product performance in down-line processes. Single dimension GC analysis of these mixtures provides incomplete information because of lack of separation power. This study evaluates the separation power of heart-cut GC-GC, comprehensive GCxGC and sequential GC-GC for three selected challenging petrochemical applications. The fundamental theoretical aspects of the techniques are discussed. Oxygenates are removed as far as possible in C10 – C13 alkylation feedstocks, used in the production of linear alkyl benzenes, because the oxygenates may have deactivating effects on some expensive alkylation catalysts. Residual oxygenates may still be present and can consist of hundreds of components. Detection of individual components at ng/g levels is required. Heart-cut GC-GC is used to illustrate the separation and enrichment power for oxygenates in an alkylation feedstock. The stationary phase in the first dimension column was selected to provide separation of the oxygenates from the hydrocarbons in a relatively narrow window. The oxygenate fraction is then enriched by repeated injections and collection on the cryotrap. After sufficient enrichment, the trap is heated and the oxygenates are analysed on the second dimension column. Comprehensive GCxGC and Sequential GC-GC are compared for the separation and analysis of the oxygenated chemical component classes in the alkylation feedstock, before removal of oxygenates. Cyclic alcohols can occur in detergent alcohols produced from FT feedstocks. These cyclics are regarded as impurities because they affect the physical properties of the detergents. The cyclic and noncyclic alcohols in a narrow C12 – C13 detergent alcohol distillation cut have similar boiling points and polarities, and separation of individual components is thus difficult to achieve. Comprehensive GCxGC and sequential GC-GC are evaluated for the separation of the alcohol component classes. The study shows that both approaches provide component class separation but the high resolving power of the second column and the optimal chromatographic operating conditions of sequential GC-GC provide better separation of the individual components. The study illustrates the immense power of the three multidimensional GC techniques namely heart-cut GC-GC, comprehensive GCxGC and sequential GC-GC. The three multidimensional GC techniques each have their own advantages, disadvantages and unique applications and should be used as complementary rather than as competitive analytical tools.

AFRIKAANSE OPSOMMING: Fischer Tropsch (FT) petrochemiese produkte is van baie hoë kompleksiteit en kan uit duisende komponente bestaan. Chemikalië afkomstig van dié voerstrome bevat soms spoorhoeveelhede onsuiwerhede wat deaktiverend op kataliste kan inwerk of wat die werkverrrigting van finale produkte kan beïnvloed. Enkeldimensie GC analises van die komplekse mengsels is meesal onakkuraat as gevolg van geweldige piekoorvleueling. Die studie evalueer die skeidingsvermoë van drie multidimensionele tegnieke, Heart-cut GC-GC, Comprehensive GCxGC en Sequential GC-GC vir geselekteerde petrochemiese toepassings. Die fundamentele teoretiese aspekte van die tegnieke word bespreek en drie analitiese toepassings word beskryf. Oksigenate word so ver moontlik verwyder uit C10 – C13 paraffien-voerstrome, wat gebruik word in die vervaardiging van liniêre alkielbenzene, aangesien dit deaktiverend kan inwerk op alkileringskataliste. Die oorblywende oksigenate kan uit honderde komponente bestaan sodat analise van individuele komponente tot op lae ng/g vlakke nodig is. Heart-cut GC-GC word gebruik om die skeiding en verryking van die oksigenate in die alkileringsvoerstroom te illustreer. Die stationêre fase in die eerste-dimensie kolom is so gekies dat skeiding tussen oksigenate en koolwaterstowwe verkry word. Met herhaalde inspuitings verhoog die oksigenaat-konsentrasie op die cryo val en - na voldoende verryking - word die val verhit en die oksigenate geanaliseer op die tweede dimensie kolom. Die skeiding en analises verkry met Comprehensive GCxGC en Sequential GC-GC word vergelyk vir die chemiese klasse-skeiding van die alkileringsvoer (voor verwydering van oksigenate). Sikliese alkohole kan voorkom in detergent-alkohole vervaardig vanaf FT voerstrome. Dit word as onsuiwerhede beskou aangesien dit die fisiese eienskappe van die finale produkte beïnvloed. Die sikliese en nie-sikliese alkohole se kookpunte en polariteite is baie naby aanmekaar sodat skeiding van individuele komponente moeilik verkry word. Comprehensive GCxGC en Sequential GC-GC word evalueer vir die skeiding van die alkohol. Die studie toon aan dat albei die tegnieke skeiding gee van die chemiese komponent-klasse maar dat die hoë-resolusie tweede-dimensie kolom en die optimisering van die experimentele kondisies van die Sequential GC-GC sisteem beter skeiding van individuele komponente gee. Die uitsonderlike skeidingsvermoë van die drie multidimensionele tegnieke, Heart-cut GC-GC, Comprehensive GCxGC en Sequential GC-GC word geïllustreer in die studie. Elke tegniek het sy eie voordele, nadele en unieke toepassings en die drie tegnieke behoort as komplementêre eerder as kompeterende tegnieke gebruik te word.

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