Petroleum asphaltenes -: Part 1 -: Asphaltenes, resins and the structure of petroleum

The definition of the nonvolatile constituents of petroleum (i.e., the asphaltene constituents, the resin constituents, and, to some extent, part of the oils fraction insofar as nonvolatile oils occur in residua and other heavy feedstocks) is an operational aid. It is difficult to base such separations on chemical or structural features. This is particularly true for the asphaltene constituents and the resin constituents, for which the separation procedure not only dictates the yield but can also dictate the quality of the fraction. For example, the use of different hydrocarbon liquids influences the yield by a considerable factor. The technique employed also dictates whether or not the asphaltene contains coprecipitated resins. This is based on the general definition that asphaltene constituents are insoluble in n-pentane (or in n-heptane) but resins are soluble n-pentane (or in n-heptane). The results of structural studies of asphaltene constituents are moving away from the older ideas that asphaltene constituents contained large polynuclear aromatic systems and there are a variety of functional types that also play a role in asphaltene behavior. The stability of petroleum is dependent upon the molecular relationships of the asphaltene and resin constituents and the balance with the other constituents of petroleum. Thus, the stability of petroleum can be represented by a three-phase system in which the asphaltene constituents, the aromatic fraction (including the resin constituents), and the saturate fraction are in a delicately balanced harmony. Various factors, such as oxidation, can have an adverse effect on the system, leading to instability or incompatibility as a result of changing the polarity, and bonding arrangements, of the species in crude oil.