VARIATION IN COMPOSITION OF SUBFRACTIONS OF PETROLEUM ASPHALTENES

Boscan asphaltenes were precipitated from the crude oil using mixtures of toluene and heptane at temperatures of 24, 50, and 80 degrees C. A nother process of extracting solid n-heptane asphaltenes (24 degrees C ) using the same solvent systems and temperatures was also investigate d. Asphaltene yield is different by the two processes at similar condi tions although both increases in temperature or toluene content lead t o lower solid yield. This way the asphaltene continuum was investigate d from incipient flocculation to total n-heptane precipitated asphalte nes. The asphaltenes were analyzed using elemental analysis, fluoresce nce spectroscopy, vapor pressure osmometry, and HPLC size exclusion ch romatography. The distribution of porphyrins was also measured using a bsorbance spectroscopy. The asphaltenes were found to follow a regular trend in elemental ratios, H/C and N/C, indicating increased aromatic ity and nitrogen content as the solid yield decreases. Also the molecu lar weight was seen to increase. Size exclusion chromatograms and fluo rescence spectra were found to be different comparing solids from the two separation processes. This indicates that the asphaltene fractions obtained by extraction of solid asphaltenes are altered relative to a sphaltenes obtained by ordinary precipitation. The porphyrin concentra tion was found to diminish rapidly with solid yield decrease in both p recipitation and extraction experiments, the latter fractions, however , containing significantly less asphaltenes indicating an adsorption s tep in the coprecipitation of porphyrins. Soluble fractions were found to exhibit relatively low molecular weights and an apparent lack of i ndications of association up to a point of solubles exceeding 50% of t he total asphaltenes. The-latter have implications for the further und erstanding and experimental investigation of the associating nature of asphaltenes such as concentration effects during analytical character ization.