Asphaltene

Asphaltenes are molecular substances that are found in crude oil, along with resins, aromatic hydrocarbons, and alkanes (i.e., saturated hydrocarbons). [Mullins, OC et al Editors, Asphaltenes, Heavy Oils and Petroleomics, Springer, New York (2007) 700 pages, 23 Chapters, 46 contributing scientists.] [ [http://tigger.uic.edu/~mansoori/Asphaltene.Molecule_html Asphaltene] ]

Asphaltenes consist primarily of carbon, hydrogen, nitrogen, oxygen, and sulfur, as well as trace amounts of vanadium and nickel. The C:H ratio is approximately 1:1.2, depending on the asphaltene source. Asphaltenes are defined operationally as the n-heptane (C7H16) insoluble, toluene (C6H5CH3) soluble component of a carbonaceous material such as crude oil, bitumen or coal. Asphaltenes have been shown to have a distribution of molecular masses in the range of 400 u to 1500 u with a maximum around 750 u. All techniques are now roughly in accord, including many different mass spectral methods (ESI FT-ICR MS, APPI, APCI FIMS, LDI) and many different diffusion techniques (time-resolved fluorescence depolarization (TRFD), fluorescence correlation spectroscopy (FCS), Taylor dispersion). This result deviates substantially with previous conventional wisdom, the old and incorrect view is promulgated on many popular web sites putatively illustrating asphaltene molecular structures. Aggregation of asphaltenes at very low concentrations (in toluene) led to aggregate weights being misinterpreted as molecular weights with techniques such as VPO or GPC. The chemical structure is difficult to ascertain, due to the complex nature of the asphaltenes, but has been studied by all available techniques including X-ray, elemental, and pyrolysis GC-FID-GC-MS. However, it is undisputed that the asphaltenes are composed mainly of polyaromatic carbon i.e. polycondensed aromatic benzene units with oxygen, nitrogen, and sulfur, (NSO-compounds) combined with minor amounts of a series of heavy metals, particularly vanadium and nickel which occur in porphyrin structures. Furthermore, asphaltene rotational diffusion measurements show that small PAH chromophores (blue fluorescing) are in small asphaltene molecules while big PAH chromophores (red fluorescing) are in big molecules. This implies that there is only one fused polycyclic aromatic hydrocarbon (PAH) ring system per molecule. Very recent fragmentation studies by FT ICR-MS strongly support this 'island' molecular architecture refuting the 'archipelago' molecular architecture.

Asphaltenes are today widely recognised as soluble, chemically altered fragments of kerogen which migrated out of the source rock for the oil, during oil catagenesis. Asphaltenes had been thought to be held in solution in oil by resins (similar structure and chemistry, but smaller) but recent data shows this is incorrect. Indeed it has recently been suggested that asphaltenes are nanocolloidally suspended in crude oil and in toluene solutions of sufficient concentrations. In any event for low surface tension liquids such as alkanes and toluene, surfactants are not necessary to maintain nanocolloidal suspensions of asphaltenes.

The nickel to vanadium contents of asphaltenes reflect the pH and Eh conditions of the paleo-depositional environment of the source rock for oil (Lewan, 1980;1984) and this ratio is therefore in use in the petroleum industry for oil-oil correlation and for identification of potential source rocks for oil (oil exploration)

The word "asphaltene" was coined by Boussingault in 1837 when he noticed that the distillation residue of some bitumens had asphalt-like properties.

Asphaltenes in the form of distillation products from refineries are used as "tar-mats" on roads.

Heavy oils, tar sands and biodegraded oils (as bacteria can not assimilate asphalten [e] s, but readily consume saturated hydrocarbons and certain aromatic hydrocarbon isomers - enzymatically controlled) contain much higher proportions of asphaltenes than do medium-API oils or light oils. Condensates are virtually devoid of asphaltenes.

They are of particular interest to the petroleum industry because of their depositional effect in production equipment such as tubulars in oil wells. In addition, asphaltenes impart high viscosity to crude oils, negatively impacting production. The variable asphaltene concentration in crude oils within individual reservoirs creates a myriad of production problems.

External links

* [http://www.aiche-chicago.org/symposium06/wiehe.pdf Article regarding asphaltene fouling by Irwin A. Wiehe]

References


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