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Structural investigation of asphaltenes from Saudi Arabian crude oils

Saleem, M. (1990). Structural investigation of asphaltenes from Saudi Arabian crude oils. (Unpublished Doctoral thesis, City, University of London)

Abstract

A structural investigation has been made of asphaltenes obtained from Saudi Arabian crude oils. For this purpose, asphaltenes precipitated from Arabian Light (AL) and Arabian Heavy (AH) crude oils by the use of different carbon number n-alkane solvents (nC5-nC10) were characterized in terms of elemental analysis, molecular weight determination (vapour pressure osmometry), thermal gravimetric analysis, pyrolysis gas chromatography, nonaqueous potentiometric titrations and infrared and nuclear magnetic resonance spectroscopy.

The higher carbon number alkane solvents (nC8— nC10) precipitated lower amounts of asphaltenes than those obtained from the lower carbon number alkane solvents (nC5- — nC7). These asphaltenes had higher molecular weights, higher concentrations of heteroelements and a higher degree of aromaticity than the lower alkane precipitated asphaltenes. Molecular weights of asphaltenes obtained from Arabian Heavy (AH) crude oil were determined by vapour pressure osmometry in five different solvents of varying dielectric constant (2.2 to 12.3). Lower molecular weights were recorded in the solvents with higher dielectric constant.

The asphaltenes were thermally reactive and released low molecular weight gases on pyrolysis at 350°C. The decomposition of asphaltenes at 520°C resulted in higher yields of gases, maltenes and coke and there was a marked tendency to concentrate nitrogen in the coke. The release of large amounts of oxygen-containing gases at the lower temperature (350°C) was attributed to the presence of carboxylic acid and ether- linkages on the periphery of the asphaltene structures. Retention of over 90% of the total nitrogen in the pyrolysis coke suggested that a major portion of the nitrogen functionalities belonged to those parts of the structure which tend to form coke.

Nonaqueous potentiometric titrations indicated the presence of very weak, weak and strong basic functionalities in the asphaltenes , which were due to indole, phenazine and pyridine groups, respectively. The proportion of these functionalities was higher in the higher alkane precipitated asphaltenes.

The data obtained from elemental analysis, molecular weight determination, and infrared and nuclear magnetic resonance spectroscopy were used to derive a series of structural parameters for an average asphaltene moiety. Gross differences in the structural parameters for asphaltenes precipitated from AH crude oil by the use of different alkane solvents were noted. These differences in the structures may possibly be responsible for their different behaviour on pyrolysis. The asphaltenes precipitated by the higher carbon number alkane solvents have higher numbers of aromatic and peripheral carbons, aliphatic carbon and aliphatic hydrogen. The substitution index is also higher for the higher alkane precipitated asphaltenes, whereas the aromaticity index, the condensation index, and the percentages of aromatic carbon, unsubstituted carbon, and nonbridge carbon are lower in the higher alkane precipitated asphaltenes.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QD Chemistry
Departments: School of Science & Technology
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