| dc.description |
High molecular weight (HMW) hydrocarbons (defined herein as C35+ compounds) are difficult to characterise
by conventional analytical methods. Very few studies have reported precise and reproducible quantification of
such compounds in fossil fuels. Nonetheless, such components have important effects on the physical and
biological fate of fossil fuels in the geosphere. For example, the phase behaviour of waxy gas condensates is
significantly affected by the varying proportions of HMW compounds. Similarly HMW compounds are amongst
the most resistant petroleum components to biodegradation. The current study reports the development of
reproducible quantitative high temperature capillary gas chromatography (HTCGC) methods for studying both
these aspects of the chemistry of HMW hydrocarbons. In addition those hydrocarbons which remain unresolved
when analysed by gas chromatography (so called unresolved complex mixtures UCMs) are also studied. UCMs
may account for a large portion of the hydrocarbons in many fossil fuels yet very little is known about their
composition. Knowledge of these compounds may be important in enhancing the prediction of phase behaviour.
Oxidative degradation and GC-MS is used to elucidate the types of structures present within the UCM.
The concentrations of C3S4h. ydrocarbons in two unusually waxy gas condensates from high temperature wells
in the North Sea were determined by HTCGC. The whole C, 5+
fraction comprised about 20% of the total
hydrocarbons and consisted of compounds with carbon numbers extending up to and beyond Coo. By paying
particular attention to sample dissolution and injection, good reproducibility and precision were obtained. For
example, for authentic n-C, to n-C60 alkanes a relative standard deviation of under 5% for manual injection,
linear response factors (1.01 Cm to 0.99 C6), and a linear calibration for 5 ng to 25 ng on-column were found.
Limits of detection are reported for the first time for HMW n-alkanes. The limits were found to be as low as
0.8 ng for Cto to 1.87 ng for C60. Tristearin is proposed as a suitable HTCGC internal standard for quantification
since the FID response factor (1.1) was close to that of the HMW n-alkanes and response was linear.
Importantly, when co-injected with the two waxy North Sea condensates, tristearin was adequately separated
from the closest eluting alkanes, n-C59 and n-C60 under normal operating conditions. Qualitative characterisation
of the HMW compounds in the waxy gas condensates and in synthetic wax blends (polywax 1000) using
HTCGC-EI MS and HTCGC-CI MS produced molecular ions or pseudo molecular ions for n-alkanes up to n-
C6o. The spectra of some HMW compounds contained fragment ions characteristic of branched compounds but
detailed characterisation was very limited.
This study has also shown, for the first time, the significance of the unresolved complex mixture in gas
condensatesU. CM hydrocarbonsa ccountedf or over 20% of the total hydrocarbons in a waxy North sea
condensateT. he condensatew as first distilled and the distillate UCMs isolated. Thesew ere found to be between
64 to 97 % unresolved after molecular sieving (5A) and urea adduction. The UCMs were oxidised using
CrO3/AcOHw hich produced5 -12% C02, and 55-83% dichloromethane-solublep roducts. Thus 65-94% of the
original UCMs were accounted for as oxidation products. The remainder were thought to be water soluble acids
which could not be determined in the presence of the AcOH reagent. Of the recovered oxidised products, 27-
81 % were resolved and these comprised mainly n-monocarboxylic acids (19-48 %). The average chain length
was found to be C12 indicating the average length of alkyl groups. Branched acids, ketones, ketoacids, ndicarboxylic
acids, branched dicarboxylic acids, lactones, isoprenoid acids, alkylcyclohexane carboxylic acids
and toluic acids accounted for the majority of the remaining resolved products. The distillate UCMs all showed
variations in amountso f productsb ut not in composition. Retro-structurala nalysis suggestedth at the UCM in
the gasc ondensatew as mainly aliphatic and branched.T he numbero f isomerso f simple brancheda lkaneso ver
the UCM molecular weight range (determined by cryoscopy) was calculated to be over 15000. Overall, oxidation
provided structural information for about half of the UCM.
HTCGC was also used to measure the biodegradability of HMW alkanes in a waxy Indonesian oil. Traditional
alkane isolation techniques (TLC and CC) discriminated against HMW compounds above C40 whereas adsorption
onto alumina in a warm cyclohexane slurry provided an aliphatic fraction still rich in HMW compounds and
suitable as a biodegradation substrate. A waxy Indonesian oil was subjected to 136 day biodegradation by
Pseudomonas fluorescens. Extraction efficiencies of over 90 % (RSD <5 %) were obtained for n-alkanes up to
C6o using continuous liquid-liquid extraction. Over 80 % of the oil aliphatic fraction was degraded within 14 days.
After 136 days only 14% of the original aliphatic fraction remained, yet surprisingly no decreases in the
concentrations of compounds above C45 were observed. However, the use of a rapid screening biodegradation
method proved conclusively that Pseudomonasfluorescens was capable of utilising n-alkanes up to C60 once the
bacteria had acclimated to the HMW alkanes. This is the first report of bacterial utilisation of an n-alkane as
large as C.. |
|