1.2. Chemical composition of heavy oils

As described earlier the composition of heavy oils is dependent on the source crude oil composition. Since this is unique and varies from source to source and geographical origin, the chemical composition of heavy oils cannot be defined with absolute precision.

If deasphalted oil and petroleum distillates are separated during refining, a more complex, higher molecular weight, residual fraction known as asphaltic bitumen remains. This fraction, along with a variety of other petroleum oils and residues will become fuel oils.

Classification of samples into various fractions by using precipitation by solvent treatment is a widely-used method for assessment of petroleum stock characteristics. None of the fractions defined contain a single molecular structure but a class of structures with a range of molecular weights and polarity characteristics that cause the fraction to be insoluble in other compounds.

The asphaltic bitumen components of petroleum are traditionally separated by a paraffinic solvent, n-heptane, into (Tort and Bernasconi (1995)) :

  1. A soluble maltene portion, containing high molecular weight waxes (C30 - C90), washed out of precipitated asphaltene with hot n-heptane. Further solvent addition separates this fraction into saturates (with n-heptane), aromatics (with benzene) and resins (with methanol/benzene/trichloroethylene).

  2. A precipitated asphaltene (sometimes called "hard asphalt"), wax-free material insoluble in large quantities of n-heptane, but soluble in hot benzene. This fraction normally represents 19 % of the initial oil mass.

The structure and proportion of asphaltenes vary with the source of crude oil. The asphaltene fraction separated by heptane is characterised by a large molecular weight (1 - 2×103 or 2 - 16×103 according to different authors and depending on temperature and the polar strength of solvent), whereas maltenes (saturates, aromatics and resins) have smaller molecular weights (200 - 400 or 500 - 1,200, according to different authors).

The colloidal nature of petroleum bitumens has been clearly established. Phenomena occurring in this colloidal system involve the aromatic high molecular weight asphaltenes being peptised by the maltenes and the resins contained in the latter. Interactions between asphaltene-maltene-resin are deemed to occur by means of strong hydrogen bonding between many of the components. The presence of phenols, carboxylic acids, esters, ketones and basic nitrogen components reinforce hydrogen bonding.

Resins (the petroleum fraction which is soluble in pentane and easily absorbed on surface-active materials such as Fuller's earth) also act as co-solvents for asphaltenes and other hydrocarbon fractions.

As a result of these considerations a dynamic equilibrium has been suggested:

Asphaltenes can be reduced by hydrogenation to lower molecular weight oils (maltenes, resins...). The opposite procedure, ie oxidation, will shift the dynamic equilibrium towards asphaltene formation. Thus asphaltenes can be formed by air blowing from resins, aromatics and maltenes. Also heating with sulphur causes the same effect as it produces dehydrogenation. The addition of strong acids into oil could cause precipitation of asphaltenes.

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Pollutant formation and interaction in the combustion of heavy liquid fuels
Luis Javier Molero de Blas, PhD thesis, University of London, 1998
© Luis Javier Molero de Blas